Description
During the energy crisis of the mid 1970’s Moulton B. (Molt) Taylor decided to turn to a small, easy-to-build, light plane which focused on maximum cruise speed and efficiency. The result is the Taylor Mini-IMP.
The Mini-Imp is a two-seat version of the single-seat Imp designed in 1975. It features an unusual configuration with a pusher prop aft of an inverted V-type tail assembly and cantilevered high wing that folds back for towing and storage. The wing is the latest NASA design with spoiler and flaps. The retractable gear is the tricycle spring-legged type. A con-trollable propeller is available. Power is provided by any engine from 60 to 115 hp with the 60-hp Franklin, 60-hp Limbach VW, 70-hp Tur-bo Revmaster VW, 100-hp Continental or 115-hp Avco Lycoming modification being the most common. The aluminum and fiberglass Mini-Imp requires a minimum of tools to construct, and all hard-to-build parts are available. It offers unequalled safe flyability and stabili-ty, positive spiral stability, limited acrobatic capability (stressed to 9 Gs), and good fuel economy (3 1/2 gph).
The Model “C” version of the Mini-IMP is the long nose version which was developed after the prototype was flown with the Limbach converted VW engine. The Model “C” is powered with the Continental O-200 engine (100 HP at sea level at full throttle). The Model “C” also incorporates a larger baggage compartment and the nose is lengthened 12 inches so that the pilot sits one foot further forward of the main bulkhead. This lengthening of the nose required the installation of an additional vertical fin on tip of the tail boom giving the “long nose” Mini-IMP an inverted “Y” tail configuration. This addition was used instead of lengthening the tail boom to accommodate the longer nose length of the design rather than lengthen the shaft and to accommodate the further aft placement of the propeller (with its weight effect on the CG).
The O-200 powered Mini-IMP requires the use of a different propeller and due to the increased weight of the engine a heavier landing gear is used. The drawings indicate several other areas of change for the Model “C” needed to accommodate the higher power and resulting performance increases. These include heavier side frame members and a different nosewheel installation. When ordering “kits” be sure to advise of your preference in this regard. The same set of drawings is used to cover either version of the Mini-IMP (the long nose or the short nose).
It is practical to use the long nose version with the big baggage compartment if the builder intends to use the turbocharged Revmaster engine and controllable propeller. However, if the normally aspirated Revmaster engine and a fixed pitch propeller (or other such VW conversion) is to be used, the short nose configuration should be used.
A prototype of the Model “C” has been flown extensively and its improved performance with the higher power is evident. Flight tests of the O-200 powered Mini-IMP “C” have shown a cruise speed of approximately 175 MPH at 4000 foot altitude at 75% rated power. Climb speeds of better than 1500 FPM are initially available at full throttle. These performances are obtained at approximately 1000 pounds gross weight. Exact performance to be obtained with any engine combination is of course dependent on the power level the builder wants to pull from his engine/propeller installation. The O-200 Model “C” requires approximately the same takeoff and landing run as the VW versions, with exact performance dependent on temperature, altitude, and gross weight.
The Mini-IMP Aircraft Company has been formed to keep the aircraft design available, to provide builder support and to further promote this wonderful aircraft design.
Specifications:
Engine: Continental O-200, 100 hp
Hp range: 8-125
Speed max: 200 mph
Cruise: 170 mph
Range: 500 sm
Stall: 45 mph
ROC: 1500 fpm
Take-off dist: 600 ft
Landing dist: 600 ft
Service ceiling: 20,000 ft
Fuel cap: 13 USG
Weight empty: 700 lb
Gross: 1000 lb
Height: 4 ft
Length: 16 ft
Wing span: 27 ft
Seats: 1
Cockpit with: 26 in
Landing gear: retractable nose wheel
Engine: 60-hp
Gross Wt. 800 lbs
Empty Wt. 500 lbs
Fuel capacity 12+ USG
Wingspan 25’
Length 16’
Cruise 150+ mph
Stall 48 mph
Climb rate 1200 fpm
Takeoff run 800 ft
Range 500 sm
INTRODUCTION
With the ever increasing emphasis on energy conservation and the high cost of “store-bought” lightplanes making fun flying more and more prohibitive, the need for a low cost, easy-to-build, high performance, sophisticated “homebuilt” lightplane has become very apparent. The difficulty with most sophisticated homebuilts in the past has been that they usually have been very complicated, high powered, and have involved years of time to construct. The Mini-IMP is a “second-generation” version of the original Taylor “IMP” two place homebuilt. This original version proved to be too complicated, and too costly to build for the “average” homebuilder. However, since the IMP configuration offered great opportunity for further simplification, and had the potential of giving performance equal or better than some of the other “mini” designs being offered, it was decided to scale the IMP configuration down to single place, design it to accommodate a great variety of available engines, and further simplify its construction.
Construction of the Mini-IMP is basically all metal. However, in order to obtain the desired aesthetic and aerodynamic shape, the designer has incorporated a few fiberglass parts which are quickly and easily assembled over the basic structure. These parts are easily removed for inspection and maintenance. The entire aircraft can be “stripped” down for access to any and all components in a few minutes. All of the formed fiberglass components are available at modest cost. This assures the builder that his Mini-IMP closely conforms to the original prototype in performance as well as shape and size.
The Mini-IMP has proven to be a poor “aerobatic” design. This is mainly due to the fact that the propeller blast does not impinge on the tail surfaces. Thus, you cannot “blow the tail around” as is necessary for many aerobatic maneuvers. Limited aerobatics such as rolls, loops, wing-overs, etc. can be performed nicely. However, it is basically NOT an aerobatic design.
ADDITIONAL MISCELLANEOUS INFORMATION
It is impossible to anticipate all of the questions, which might be asked by potential Mini-IMP builders. However, we will try to cover a few items here that we have found to be of interest from the many people who have contacted us regarding the project. First, we do not recommend the use of SAFOAM in the fuel tank (see the article in Sport Aviation, Dec. 1972). We DO NOT recommend the installation of fuel tanks any larger than the one shown in the drawings, although it would be entirely possible to seal up additional areas of the wing during initial construction in anticipation of larger tankage if desired. The very high cost of the Continental O-200 engine makes it probable that this engine would cost (new) more than the rest of the entire airplane and its equipment. However, the Mini-IMP is structurally suitable for use of this engine (100 HP). The components listed in this brochure are the only ones we have available. We do not offer a complete kit of material at this time but are investigating the possibility of offering a “turn key” package in the future if demand warrants and the financial realities allow. We do not anticipate offering a factory built certified model commercially although the forthcoming “Light Sport Aircraft” category may enable a modified version of the Mini-IMP to be built and sold directly to the flying public. The Mini-IMP could be fitted with a jet engine although this would be very expensive, and it is not felt that the overall performance and range possible with a jet engine would be desirable except for very special purposes. The Mini-IMP could be trailered for short distances on its own gear and it was anticipated originally that we would incorporate this ability. However, experience with the prototype has shown that the very small tires which are needed in order to make landing gear retraction practical do not lend themselves to extended road towing. Further, the spring leg gear used in the Mini-IMP does not lend itself to lengthy ground towing due to wheel geometry changes when the wings are folded with attendant potential excessive tire wear. Therefore, the Mini-IMP should be towed on its own trailer.
The empty weight of the prototype Mini-IMP came out to 520 pounds, including the radio, instruments, Flexidyneä full of flow charge, and engine full of oil. This is with the long wing (25 ½ foot one piece unit). With the slightly shorter folding wings, the weight will be about the same, with the weight of the fold fitting, etc. making up for the slightly shorter wing length. The folding wings will result in the overall length being exactly the same as the fuselage. Thus, the wing tips will fold aft to be even with the tip of the propeller spinner. Flight tests indicate that it may not be desirable for the conservative pilot to build the Mini-IMP with shorter wings since they would inevitably result in higher takeoff and landing speeds. It is recognized that some builders will want to experiment with very short wings either after having built the long wings to get some experience with the aircraft, or at the outset of construction. It is obviously desirable to keep the weight of the Mini-IMP as low as possible. The addition of an electric system with radios, starter, alternator, battery, etc. can be expected to add at least 50 pounds to the weight of the aircraft. While the basic structure of the Mini-IMP has been designed for 1000 pounds gross weight with a 6 g ultimate load factor (4 g limit), it is apparent that the basic structure has actually come out somewhat stronger than that. However, static tests to determine actual ultimate capacities are not anticipated although, as mentioned, the wing attach fittings were tested to 14 g before they failed and the tail surfaces had taken no deformation or permanent set at the 9 g loading. Higher weights may necessitate the use of thicker landing gear legs, but the present 7/16 inch landing gears are entirely adequate for 900 pounds gross.
CONCLUSION
The Mini-IMP is modern, sophisticated, high performance lightplane. It is extremely easy to fly, it has exceptional stability and maneuverability without being the least bit “touchy” or sensitive. It is extremely comfortable, has room enough and performance enough to carry heavy, big, or tall pilots. It has a very big baggage space in the O-200 powered model, and the VW engined version has a baggage space adequate for most short trips. The range possible with your version of the Mini-IMP is dependent on the tankage you incorporate in it when you build it. The drawings cover how to increase fuel capacity if you want it. If you have additional questions do not hesitate to write to us. To assist us in answering your questions include a self-addressed envelope and list your questions far enough apart so that we can reply and return them to you. If we are delayed in getting your answers back to you it is only because there are apparently going to be a lot of Mini-IMPs flying around one of these days.
Do not expect the Mini-IMP (or any other design) to be all things. We have tried to convey to you some idea of why the design has been arranged as it has. We do want to point out that the Mini-IMP is now a well proven configuration which does meet the design expectations which were laid down for it when it was originally conceived. We are fully aware of the capabilities and limitations of other designs which might be considered comparable. We are also fully informed on the many types of materials which might be utilized to construct an aircraft of this type. At this time we feel that the Mini-IMP represents an optimum design for someone who wants an aircraft with its capabilities. It is an ideal “first project” for someone building his own aircraft. The design is as simple to build as any design with comparable performance. There is no such thing as a low cost aircraft. However, the Mini-IMP is as inexpensive to build as any other design of equal capability and at this point we do not know how to make it any better nor has anyone else been able to come up with any suggestions to that end.
Fuselage
The Mini-IMP fuselage has 3 main bulkheads. The front, or “nosewheel”, bulkhead carries the nosewheel, rudder pedals and nosewheel “box”. The rear of this “box” carries the instrument panel and the floorboards attach to its sides. The instrument panel is so arranged as to have a center panel and two side panels in order to give more room for radios and instruments. These side panels and center panel attach to two basic structural angle panels which form the structural sides of the fuselage, as well as the arm rests, seat support, and mount for the flight controls and landing gear control system. A modest baggage space behind the seat (which tilts forward for access) provides the structural attachment to the main bulkhead.
The second main bulkhead carries the wing above it, the engine behind it, and the main landing gear beneath it. The wing is attached with two basic fittings which have been statically tested to 14 g loading (at 900 pounds gross weight). Even then, these fittings did not fail catastrophically, but merely bent slightly. Since the engine, pilot, fuel, and landing gear are all attached to this single component, the basic structural simplicity of the Mini-IMP is obvious.
The top of the engine compartment aft of the main bulkhead is a tension web which takes the normal down loads on the tail. This web is suitably strengthened for negative loads which might be imparted in aerobatic flight. The tail “boom” consists of a flat-wrapped sheet metal cone structure which uses 2 identical skin halves. The tail skin together with sheet metal channels at top and bottom, as well as 4 standard angle extrusions to stiffen the cone, carry the thrust loads of the propeller, and provide adequate attachment of the inverted “V” tail surfaces. The two tail surfaces are attached to the tail cone through aluminum plate attachment fittings which in turn bolt to a semi-bulkhead for the front stabilizer spar and a ring bulkhead for the rear stabilizer and elevator-hinge spar.
There are no welded fittings other than the engine mount (which is available pre-manufactured). While the entire aircraft is designed for an ultimate load factor of 6 g’s, it is apparent that the structure will obviously accommodate even higher loading, although this might be very difficult to accomplish, even deliberately.
Tail
The Mini-IMP employs an inverted “V” tail configuration to improve flight performance and simplify construction as well as reduce weight and cost. This arrangement also facilitates the use of the tail propeller since the tail of the aircraft must be high enough off the ground to give adequate propeller clearance when the aircraft is rotated for either takeoff or landing. The original wind tunnel tests, radio controlled model tests and computer studies showed that the inverted “V” tail also contributed greatly to improved stability, better flight control and maneuverability, and reduced external noise from the aircraft. The inverted “V” arrangement imparts a favorable rolling movement to the aircraft in flight when the rudder is applied, whereas conventional tails or upright “V” tails cause the aircraft to tend to roll in the opposite direction from the direction of rudder application. Further, the relatively low positioning of the inverted tail surfaces tend to cause the nose of the airplane to raise as the aircraft is rolled, and this imparts very desirable “spiral stability” into the flight characteristics. When the Mini-IMP is put in a turn and the controls are centered, the aircraft will come out of the turn and fly straight and level. In fact, given sufficient room, the Mini-IMP will return to level flight from practically any attitude other than being inverted, and recovery from a vertical turn to level flight requires less than 180 degrees of turn – “HANDS OFF”. The tail propeller of the Mini-IMP imparts directional and pitch stability far in excess of what is found in conventional lightplanes, and the Mini-IMP tend to hold course and altitude (the latter when properly trimmed) far better than most lightplanes.
All tail control surfaces are fitted with full piano-type hinges, which assures high strength attachment and less air leakage through the hinge lines. Flight control surfaces are also fully balanced to assure minimum danger of flutter and reduce friction in the control system. The tail surfaces have been static tested to 6 g ultimate (at 1000 pounds gross) without taking permanent “set”.
The tail surfaces of the Mini-IMP are fully adjustable, so that pitch trim of the aircraft can be easily accomplished without drag-inducing trim tabs, or force-relieving (also control displacing) bungee installations. This arrangement further reduces drag of the aircraft when the flaperons (full span ailerons) are collectively displaced UP into the “cruise” position, which produces an attendant nose up trim. This trim effect is completely eliminated through re-trimming of the tail surfaces in a “nose down” direction which further reduces down load on the tail with a resulting reduction in effective wing loading. This reduction in tail drag also results in further reduction of the induced drag of the wing resulting in a very efficient cruise configuration. Tail trim is accomplished by a manual control, which is positioned at the pilots left side where it is “at hand” without excessive motion of the pilots left hand. The lift lever is also positioned adjacent and just above the trim lever along with engine throttle so that the pilot need not move his arm from the arm rest on the left side to move any of these three controls.
The 2-element tail not only provides excellent stability and control, but it also provides suitable structure below the tail boom, which serves as an adequate bumper to prevent possible inadvertent striking of the propeller. This is particularly important when handling the unloaded aircraft such as when pushing it into a hanger or loading it on its trailer. It is virtually impossible to strike the propeller when flying or with a pilot sitting in the cockpit. The 2-element tail is also lighter than a 3-element tail and this compensates for the weight of the propeller and shaft at the rear of the aircraft.
Landing Gear
The Mini-IMP features retractable landing gear to improve performance. The nosewheel gear of the Mini-IMP is constructed of cut and drilled aluminum extrusion and tube. Aluminum plate is cut on a metal cutting bandsaw to provide bearing mounts and nylon flanged bushings are used for mount bearings. The nosewheel is equipped with a shimmy damper, which is easily constructed as part of the nosewheel fork. The nosewheel leg is a heat treated, and bent spring steel rod which bolts in to an aluminum mount block. An assist spring, not only helps retract the nosewheel, but also provides over center locking of the gear in the DOWN position. A nosewheel UP lock is provided to assure that the gear will not be displaced during aerobatic maneuvering. The nosewheel retracts forward into the fiberglass cone ahead of the nosewheel bulkhead. Retraction and extension on the prototype is mechanical through a single lever on the left side of the cockpit. A grip on the lever provides both up and down locking and unlocking, and gear retraction requires a single motion of about 12 inches travel and is almost “instantaneous”. The main gear is retracted mechanically through cables, which run to suitable chain sprockets, which rotate the main gear outward and up into wheel wells on the bottom of the wing outboard of the fuel tank. The main gear legs lay in recesses on the side of the fuselage and pass in front of the air ducts to the engine in such a way as to provide straightening vanes for the cooling air. The gear system is also equipped with both mechanical and electrical gear position indicators so that the pilot can be assured that the gear is not only where he wants it, but is also locked there.
The prototype was originally fitted with a fully manual, mechanical retraction system. However, the aircraft can be fitted with a pneumatic or hydraulic retraction system to power the main gear. With these systems the nose gear continues to be mechanically actuated, but the main gear in lifted by a power cylinder which in turn is powered by a pump. Either gear retraction system can be fitted and it is entirely a matter of personal preference as to which system is the more desirable. The nosegear can also be fitted with powered retraction if desired and several builders have done this.
The main wheels are fitted with extremely effective disc brakes, which are individually controlled by toe pedals. The system includes a parking brake arrangement, which employs a bypass valve so that inadvertent “brake lock-out” cannot be accomplished. Ground steering is easily accomplished through use of full castering nosewheel and differential brake steering. The full swivel nosewheel permits pushing the aircraft backward with out nosewheel impairment to motion to the rear as is common with most nosewheels.
The Mini-IMP is fitted with nosewheel doors which fair nicely into the nose of the aircraft while the main wheels are fitted with backing plate covers which serves to streamline the main wheels when they are retracted into the bottom of the wing. With the fully manual (unpowered) gear retract system, a portion of the main wheel backing covers is hinged and actuated by the gear retract grip so that the pilot can deflect the hinged portion of the backing plates to obtain a “flap-air-servo” effect to assist in retracting the landing gear. The nose gear is tied-in to these systems so all three wheels are easily actuated. All wheels are mechanically locked in the UP positions and main gear is mechanically LOCKED in the DOWN position. The nosegear is locked down due to an over-center positioning of the actuator. With the pneumatic or hydraulic systems, the nosegear is operated exactly like to manual system but the main gears are moved with the power cylinder and are BOTH locked in the UP and DOWN positions mechanically with the power system only doing the actual gear movement. With the power system, the gear has a “free-fall” emergency provision in the event of pump failure. The electric pumps only operate while the main gear is in motion.
Flight evaluations have shown that nosewheel doors are extremely desirable and the drawings show a suitable nosewheel door installation. The nosewheel itself has been changed in the prototype from the original 5 inch wheel (the same as the main wheels) to a 4 inch wheel to further lighten the nosewheel installation and facilitate retraction and nosewheel door operation.
The special geometry of the Mini-IMP landing gear is such that, despite the use of rather small wheels and tires, the gear moves slightly AFT under loading. This gives the main gear an effective “TRAILING ARM” characteristic, which tends to compensate for the small wheels by giving them an effectively larger rolling radius. The extreme smoothness of the prototype landing gear (which has been obtained without resort to complicated oleos, dampers, etc.) indicates that this extremely simple system is most effective. The nose wheel is equipped with an adjustable shimmy damper, which can be built in a few minutes. Extensive testing has shown that the prototype exhibits absolutely no tendency for nosewheel shimmy.
Canopy
The canopy is a shaded or clear, Plexiglasä unit with excellent optical characteristics so that there is a minimum of distortion to forward vision despite the very acute vision angle through it in the forward direction. Two side “opera” type windows permit the pilot to observe the main gear in the extended position. The Plexiglasä canopy can be fitted precisely to the fuselage through the use of patterns which are included in the drawing set. The canopy on the prototype opens to the left on a sealing piano hinge and is restrained from opening too far with a restraint cable. A low-drag installation is obtained by using a removable hex “T” wrench through a hole in the canopy for the outside access handle. The inside handle actuates two hooks which pull the canopy down snug with an over-center motion of the handle to provide inside security and prevent inadvertent canopy opening in flight. The canopy frame is assembled with blind rivet construction. The Plexiglasä is held in the frame with RTV sealant and mechanical restraint in such a way as to prevent inadvertent blow-out. Two pins on the canopy frame guide and align the right edge of the canopy to assure flush alignment on the outside at all times.
Cockpit
The cockpit of the Mini-IMP is entered by merely stepping aboard over the edge, which is only 20 inches high, onto the floorboard where it is possible to stand erect without having to stand on the seat. A hand grip on top of the instrument panel visor is sufficiently strong to permit you to lower yourself into the foam and vinyl upholstered semi-reclining bucket seat. The fuselage is 26 inches wide at the pilot’s elbows which gives plenty of room inside even for a very large person. Seating is extremely comfortable with plenty of leg room for even very tall people, and the seating is wide enough for pilots up to 250 pounds weight. Permissible Center of Gravity travel is sufficient for even very large pilots as well as very small individuals.
The primary flight control is located on the structural angle that forms the right arm rest. The handle is slightly inclined to the left when in the neutral (aileron) position. This permits the pilot to easily rest his right hand against his leg in flight to steady any inadvertent motion of the flight control. The flight control system is a combination of rods for the ailerons and ruddervator mixer with cables running to the tail surfaces from the mixing system. The trim system permits instant trim adjustment and the trim handle position provide indication. There is no “feed-back” between the rudder pedals and the elevator motion of the flight control as is so common with some “mixer” systems which are used for “ruddervators” in some aircraft nor is there any “feed-back” from the elevator trim system in the aileron motion of the flight control. The rudder pedals are equipped with toe brakes and a parking brake pull handle is located on the instrument panel to the right. The rudder pedals are the “hanging” type which leaves the floor boards clear and prevents any possibility of fouling the pedals with anything dropped on the floor. The ignition switch and carburetor heat controls occupy the left hand semi-panel. Engine instruments are located on the center semi-panel between the pilot’s legs, with flight instruments on the main panel ahead of the pilot. The fuel shut off is located on the main bulkhead to the left behind the pilot. The main bulkhead is equipped with a large removable access panel which gives complete access to the magnetos and starter for any service or adjustment, making access to the engine complete on all sides.
The cockpit of the Mini-IMP is fitted with a specially designed semi-reclining fiberglass bucket seat, which is upholstered with deep urethane foam and covered with a durable vinyl cover. The seat folds forward for easy access to the baggage space behind it and is locked in the seated position with two Camlocä fasteners. The seat is fitted with shoulder harness as well as the safety belt. The shoulder harness ties directly to the wing attachments and engine mount cluster so that there is no danger of the shoulder belts tearing out in an emergency or crash landing
The instrument panel of the Mini-IMP is large enough to accommodate up to 8 standard size (3 1/8” diameter) instruments. Other panel arrangements are possible. Vacuum powered instruments are practical since it is easy to mount the driving venturi aft of the cooling fan so that they are not outside and otherwise impair performance. Radio antennae can be installed inside the nose cone or the optional composite vertical fin. The prototype radio installation exhibits excellent performance with the internal antenna. Miniature instruments and radios now available would permit easy installation of complete IFR capability if desired, although it should be pointed out that the Mini-IMP was not originally designed for IFR type flying.
The cockpit is fitted with a fresh air vent control on the left of the instrument panel. The vent gets its air from the nosewheel area without external drag producing openings. The rear of the cockpit can be fitted with a sound blanket, which effectively shuts out engine noise transmitted forward from the engine spaces aft of the rear cockpit bulkhead. The Mini-IMP prototype has not been fitted with a cabin heater. It is anticipated that it would be necessary to include an electrically powered blower in order to move warm air ahead into the cockpit area, and this necessitates the installation of an electrical system with its attendant complication and weight.
Engine Compartment
The engine space behind the main bulkhead is large enough to permit installation of a great variety of available engines. Access to the engine from the sides is provided by removing the left and right engine doors which are retained by means of Camlocä fasteners. These “doors” also provide the air scoops for directing cooling air over the engine. The engine MUST be fitted out with a cooling fan. The fan is positioned in the bulkhead aft of the engine compartment and draws the hot air from below the engine out into the tail cone where it is exhausted out through a suitably screened vent at the top of the tail cone, well forward so as to avoid any turbulence on the flight control surfaces. Although the prototype is not equipped with an oil radiator, this can be installed if operation in extremely hot climates is anticipated. The prototype has proven to extremely cool running, and continuous operation at up to 1500 RPM on the ground without forward velocity does not result in exceeding operating temperature limits for either the cylinder heads or the oil in mild climates.
It is not necessary to stop the engine of the Mini-IMP with an idle cutoff provision. This is possible due to the use of the Flexidyneä which isolates the engine from propeller inertia, which causes most engines to require this shut-down procedure. Just turn the ignition switch OFF and the engine stops just as it does in an automobile.
Although the engine of the Mini-IMP is directly behind the pilot, the engine mount and structure of the aircraft has been designed in such a way that the engine could only be thrown into the pilot under circumstances of a head-on crash which would not be survivable even if the engine were ahead of the pilot. The structure and engine mount are further designed to absorb any crash impact that would otherwise be survivable. The side rails of the basic metal structure have been designed to provide adequate protection to the pilot in the event of a wheels-up landing, although such a landing probably should be made on a grass surface if at all possible.
Engine Compatibility
The Mini-IMP was originally designed to accommodate the two cylinder Franklin 60 HP engine. It is anticipated that some builders will want to install these engines which are extremely smooth running and are fully certificated. These engines are equipped with full electric starting, as well as other modern features. Although these engines were not available for many years after the production rights were sold into the former Soviet bloc, they are once again available and are quite satisfactory.
Since the original concept of the aircraft was to obtain maximum cruise speed and efficiency, smaller engines in the 50-75 HP range are preferred for the short nosed aircraft. However, the aircraft has been designed so as to permit the installation of aircraft engines up to and including the Lycoming O-235. The preferred engines include the Continental O-200/IO-240, Lycoming O-235, the various Volkswagen conversions, the Jabiru 2200/3300 and the Rotax 911/912. The Mini-IMP engine installation is excellent for use of various modified engine conversions since there are no propeller loads introduced into the engine crankshaft by the shaft installation. The shaft arrangement provides a flexible coupling as well as a slip spline so that engine motions on their rubber mounts as well as thermal expansion of the structure do not impair the engine operation or installation. Only “torque” is taken from the engine. Further, the shaft system embodies a cooling fan (part of the kit along with the shaft system) which assures proper and adequate cooling of any of the various buried engine installations. With such provisions, the various “converted” engines are much more likely to give satisfactory life and operation.
Engines must be equipped with electric starters since hand cranking through the propeller is not easily accomplished although it can be done in an emergency. The airplane will sit level on the ground (without dropping on its tail) without the pilot being aboard or without special parking provision or ballast. It can be equipped with turbocharged engines such as the Revmaster 2100D. This permits high altitude flight and exceedingly attractive cruise speeds on relatively low power and low fuel consumption. There is adequate space in the baggage compartment behind the seat for a portable oxygen system if high altitude flight is to be accomplished.
Numerous builders have indicated their interest in possible installation of various models of the Continental engine. Since the Mini-IMP must have an electric starter, this means that engines from the C-75 up to the O-200 would be usable. New TCM O-200 engines are available as well as factory rebuilt engines. Cores are plentiful and parts are available too. (cylinders, crankshafts, etc). Another option would be the Teledyne Mattituck Services TMX-200 experimental engine. They are assembled from new TCM components and run on the test cell prior to shipping. Essentially the TMX engine is a new “uncertified” O-200 engine. Needless to say the cost is significantly less than a new certified engine. More information is available at www.mattituck.com. Since the Lycoming O-235 engine is quite similar to the O-200, the designer began constructing a prototype Mini-IMP with the O-200 engine in order to develop drawings which are applicable to both the Continental and the Lycoming engines. The drawings set includes coverage of these installations as well as the Limbach (and similar Revmaster VW converted engines). These versions of the Mini-IMP require modifications to the basic structure as well as mechanical arrangement of the airplane. The O-200/O-235 installation is applicable to any of the other Continental/Lycoming engine installations from the C-65 up. These new prototypes also permit us to develop full electrical installations for the airplane and all of this information is included in the drawing file.
The O-200 prototype is about 150 pounds heavier and requires additional fuel tankage which is provided for in the drawings. These prototypes provide even greater performance than the conservative performance being obtained from the Limbach-powered prototype.
Builders contemplating the construction of copies of the Mini-IMP should make necessary decisions concerning what engine they intend to use early in their program so that necessary structural differences can be accommodated in their project to start with. Due to increased engine weight, the Model “C” requires the use of a longer nosecone, heavier landing gear, and other changes which must be provided at the outset. When ordering drawings be sure to indicate your anticipated engine type so that we can ship you the appropriate parts. We do not recommend that you attempt to install the O-200 in the shorter Model “L” fuselage or that you attempt to use the lighter Model “L” landing gear with the heavier Model “C” engine and tankage. However, to obtain the best possible performance you should not use the heavier parts with the smaller engines either.
We have many inquiries regarding the use of engines like the Mazda rotary engine. There have been some successful conversion/installations in the Coot amphibian and in the RV-4. If this is your area of interest, we recommend that you contact the engine vendor directly and purchase a turn-key, well-proven package from them. We do NOT recommend that the individual builder attempt to develop his own unproven engine installation. This is a time consuming and costly process, which usually results in frustration and “no airplane”.
Drive Train
The Mini-IMP is equipped with a tubular, aluminum drive shaft system designed to transmit the torque loads from the engine to the propeller and to prevent torsional feedback from the propeller to the engine. The driveshaft system consists of several components, each of which has an important and specific purpose. Attached directly to the engine output shaft is a “Flexidyneä ” dry-fluid coupling made by Dodge Manufacturing Company of Greenville SC, (no connection with Dodge automobiles). This coupling is essentially a “soft start” and torque-limiting slip-clutch intended for use in electrically powered industrial equipment, such as de-barkers or conveyer belts. The purpose of the Flexidyneä in the Mini-IMP drive train is to act as a torque-limiting device which slips only during initial engine start and when the engine is accelerating or decelerating through narrow RPM ranges that are prone to have torsional resonance feedback. At all other engine speeds the Flexidyneä is “locked up” and operates with no slippage, and thus no power loss or heat buildup.
Flexidyneä couplings are manufactured in various sizes, depending on the load to be driven. Molt originally used the 8C size for the VW-powered model. In 1979 Dodge Manufacturing discontinued the 8C coupling and replaced it with a new model, designated 75C. At that time Molt began to have his own Flexidyneä housings cast in the 8C size, although his molds for these castings have not been found. We are investigating the suitability of the larger 9C coupling, since it is still in production and it appears to be appropriate for use in the Mini-IMP with the addition of a simple adapter plate. Professor Ed Lesher used a size 9C coupling in his record setting, O-200 powered, “Teal”. One source for the 9C (other than the kit from the Mini-IMP Aircraft Company) is Motion Industries, a distributor of power transmission equipment, which has over 250 retail outlets across USA. Do not attempt to contact Dodge Manufacturing directly, since they do not desire to have any dealings with aircraft builders in order to avoid potential product liability judgments.
Aft of the Flexidyneä is a cooling fan to draw air across the engine during ground operation. (In contrast to tractor-type aircraft, pushers require the use of an auxiliary fan to cool the engine until airborne.) Aft of the cooling fan is a slip spline joint. This allows the shaft to move slightly fore and aft with relation to the engine, and also allows the shaft to be removed without disassembling the entire drive train. Next comes a “REX” brand flexible disk coupling, made from a series of thin metal disks bolted together between two flanges. This coupling accommodates small angular misalignments that inevitably occur with engine movement, and it tolerates temperature changes better than other couplings.
The remainder of the drive shaft system consists of the aluminum shaft itself, the propeller flange and associated bearings. The Mini-IMP uses a thrust bearing at the propeller flange, thereby transmitting the propeller thrust loads into the tail cone, rather than into the engine crankshaft, which was not designed for this type of loading. Drawings for construction of the drive shaft system are included in the set of plans, in case experienced builders wish to do it themselves.
Starting
As stated earlier, engines must be equipped with an electric starter. The Limbach 1900 Volkswagen engine installed in the prototype Mini-IMP has been equipped with a special starter, which is shown in the drawings. This starter can be adapted to other engines since it basically replaces the electric starters used on most engines. The starter uses an “ECLIPSE” type starter drive gear and the unit is fitted with a “follow-thru” type drive unit which prevents the starter from disengaging until the engine has actually started. These are standard Ford-type starter pinion drive units and are easily obtainable. The Flexidyneä effectively decouples the drive shaft and propeller from the engine so that the inertia of the propeller is not available to assist in cranking as is common with most aircraft engines. This necessitates the use of some special form of starter that will have sufficient power to turn the engine through compression without propeller inertia to help. Many years of experience with this problem in the AEROCAR “Flying Automobiles”, designed by the designer of the Mini-IMP, have shown that aircraft engines can be easily started without propeller inertia if dual impulse couplers are installed and if the impulse couplers are set up with sufficient “lag” so that they do not fire the spark plugs until the engine has passed top dead center slightly. The starter shown in the drawings for the Limbach (or other VW conversion) provides sufficient torque for this purpose, and any engine fitted into the Mini-IMP MUST have the magnetos equipped with suitable impulse coupler to give the delayed firing necessary to avoid “kicking” caused by slow turning during starting. The starter must be specially designed and set up to provide higher torque for cranking than would be possible with any standard starter. The drawings provide suggestions in this regard as far as how to modify standard starters to get the desired torque capability. It should be mentioned that standard aircraft type Bendixä magnetos can be equipped with impulse couplers with sufficient “lag” for this requirement However, they must be specially ordered depending on the degree of spark advance normal for the engine so as to assure that the engine will NOT fire before top dead center during cranking. Slickä magnetos have the “lag” of their impulse couplers adjustable and work nicely due to this capability, although they must be properly adjusted before attempting to crank the engine in a Mini-IMP. Thus, if the engine is timed 30 degrees BTC, the impulse coupler MUST be set to give at least 30 degrees of lag (or possibly even a degree or so of advance can be set for the mags 28 or 29 deg. BTC), so that the 30 degree “lag” for the impulse couplers will positively assure timing SLIGHTLY after TDC during cranking. With these provisions, no particular difficulty will be experienced despite the fact that the propeller is decoupled from the engine during cranking.
Propeller
The tail position of the Mini-IMP propeller is extremely desirable. Not only does it move the propeller with its noise and vibration as far away from the pilot as possible, but it also permits the entire aircraft to “fly” in undisturbed air. Further, the flight controls of the aircraft are not influenced in their effectiveness by the propeller slipstream. Thus, the aircraft will (for all intents and purposes) stall at the same airspeed either power ON or power OFF. The absence of slipstream not only reduces vibration and fatigue of both the aircraft structure and the pilot, but it also greatly reduces the drag of the aircraft which is flying in unaccelerated air, as contrasted with the high velocity air that goes over the fuselage and tail of conventional aircraft. While the propeller itself may be operating in somewhat disturbed air due to the passage of the fuselage ahead of it, and thus have a slightly lower efficiency, the overall “system” of the aircraft/propeller arrangement is more efficient with the tail propeller.
Aircraft designed with the “tail” propeller must have special provisions in a number of ways to permit attaining proper use of the propeller. The number of tail surfaces closely ahead of the propeller greatly influences design in this regard, and the simple two-surface arrangement of the inverted “V” tail of the Mini-IMP is felt to be optimum. It is essential the propeller not intercept two surface “wakes” simultaneously, and that the wake of those surfaces closely ahead of the propeller are kept in the proper angular relation to the propeller so as to minimize vibration and excitation of both the propeller and the shaft system. The design of the Mini-IMP has drawn heavily on over 25 years of experience by the designer in working with this arrangement. The smooth operation of the prototype and the almost complete lack of “distress” noise from the propeller of the aircraft indicate that the system and arrangement as shown in the drawings provides optimum performance and capability for the design. The thrust loads developed by the propeller are carried directly into the tail cone by a rubber mounted thrust bearing which is self-lubricated, and the minute bending loads due to propeller inclination and asymmetrical disc loading of the propeller are carried into the shaft and forward to the engine where they are resisted by the engine mounts themselves in slight sideways displacement. These forces are known to be there and are provided for as are motions due to thermal expansion, etc. Years of experience and testing have gone into the system as it is provided in the Mini-IMP, and the arrangement has been fully tested for FAA certification and approved for unlimited life in the fully certificated AEROCARs.
Although the tail position for the propeller does subject the propeller itself to some hazards due to debris being thrown into the propeller by the airplane, it should be recognized that there is no slip stream to pick up dirt and gravel. The only real dangers are from objects picked up by the tires. To this end, the Mini-IMP should be equipped with “slicks” with wide profiles so as to preclude possibilities in this regard. The drawings and Bill of Materials call out suitable tires for the use.
In an aircraft such as the Mini-IMP, which has a bare minimum of cross section area (this is dictated by the size of the pilot in the semi-reclined position), and which has the least number of tail surfaces to create drag, and which utilizes the latest technology as far as wing design is concerned to get best lift and least drag, the only remaining consideration which can effect performance are the landing gear and the propeller. The Mini-IMP incorporates a fully retracting landing gear, so this leaves the propeller as the only thing over which much further control is possible as far as getting improved performance is concerned. Since it is obvious that the propeller is probably the only factor over which the builder has much control, it is apparent that the selection of the proper propeller is very important. The prototype is initially being flown on a hand-carved wood unit which is obviously a compromise with its fixed pitch. However, it is desirable to have a propeller which would at least permit the Mini-IMP owner to select the obtainable performance under some circumstances. Thus, at the time of this writing, a Warnke ground-adjustable propeller has been ordered so that the propeller can be adjusted to obtain optimum performance with some compromise both for takeoff/climb and cruise. This propeller will permit the builder to set the unit so that he can favor either the takeoff/climb condition or the cruise condition if he so desires. Propeller manufacturers all over the world are being contacted in an effort to find one that might be interested in building a mechanically controllable propeller for the Mini-IMP’s, and it is hoped that eventually we will have something like this available since the propeller is so important as far as the ultimate performance of the aircraft is concerned. With the original propeller, the Mini-IMP is easily airborne in less than 800 feet and “cruise” in the 150 MPH range is obtainable without exceeding the recommended 3200 RPM cruise speed for the Limbach engine. Higher cruise speeds are obtainable, but at the expense of exceeding recommended engine limits. Performance with other engines will be determined in large measure by the propellers with which they are fitted, and whether engine limits are exceeded. It is a well-known fact that Formula One racers greatly exceed published engine limits to get their 240 MPH plus speeds. Whether the Mini-IMP builder wants to do this is entirely up to him. The Mini-IMP closely approximates Formula One racers in weight and equivalent flat plate area as far as drag is concerned, and has a retractable landing gear which Formula One racers do not have. However, in a effort to give it “pussy cat” characteristics, the designer has given it sufficient wing area to give approximately 45-50 MPH takeoff and landing speeds, although the GA(PC)-1 wing does permit the pilot to effectively “de-camber” the wing to some extent and reduce trim drag during cruise. It should be recognized that if a hand carved wood propeller is going to be fitted on your Mini-IMP, and you are going to be using some engine other than the one fitted on the prototype, you may well have to go through several propellers before you find the optimum diameter and pitch.
Building Time
Most potential builders are interested in “how long” it will take them to complete the construction of any homebuilt design they are contemplating. This is very difficult to answer since we have no way of knowing the degree of your skills, experience, equipment, facility, etc. It is impossible for anyone to give you an exact estimate of how long it will take you to put a Mini-IMP together. It depends entirely on how fast you work, how long you take to talk things over with the multitude of friends that are bound to drop in to see your project, and how long it takes you to get all of the parts and materials together. If you are handy with tools and you have good facilities and working space, you should be able to build your Mini-Imp in approximately 1500-2000 hours. If you are not handy with tools, find difficulty in reading drawings, and find it impossible to follow instructions, it will take you longer. This compares with an estimated 10,000 hours to build a Falco or 1,500 hours to assemble the kit of a typical fixed-wing, fixed-gear aircraft. The longest documented amount of time taken was 5000 hours by Seńor Bernardo de Sousa Dantas of Sao Paulo, Brazil. He made many changes and also performed his own conversion on the plentiful VW engine. You will probably take much less time. When people quote so many years to build an aircraft, they mean that they chose to spread the number of hours required over that many years, for whatever reason. If you have a limited amount of time available, consider subscribing to the newsletter, to see if there are good projects for sale that you can complete. However, we want to give you every reason to succeed, and the Mini IMP Aircraft Company is available to give you advice, help you find parts, and assist you. It must be remembered that since there are dozens of people wanting more information, it helps to make your letters or phone calls worthwhile. Please don’t ask questions that are already answered in the drawings or instructions, and enlist the help of experienced friends wherever possible. If you do have to write, be sure to send a self-addressed stamped envelope along with your specific questions listed on a separate sheet. If you call on the phone, have your questions in mind. The best method however, is to use the Mini-IMP Builders Email discussion group to send your inquiry to all participating builders. See the information in the Email section of this information package.
Building the Mini-IMP
Probably the second most repeated question we get concerning the Mini-IMP has to do with “how hard is it to build?”
Molt tried to do everything possible from a design standpoint to make it as easy to build as something like this can be. He tried to incorporate detail drawings of every little piece. He also came up with “assembly” drawings of how the pieces go together. Molt detailed everything and dimensioned everything so that there can be little question as to how the individual parts are made. It should be recognized that since each builder is going to be making only a single example of the Mini-IMP he is going to have to develop his own assembly methods, provide the necessary alignment etc., so that things all fit once he gets them made. This obviously entails the need to drill and fit many things together upon assembly. In a factory, parts are made on master jigs so that all of the holes will align when assemblies are put together. With a “homebuilt” we jokingly say that our “master jig” amounts to throwing the parts in the air and nailing them together before they hit the ground. Seriously, you have to do a lot of planning to be sure that you are going to be able to make things fit and assemble together as indicated on the drawings. This is probably the most difficult part of building any homebuilt airplane. It is very easy to spoil a part just because you didn’t anticipate how it was going to be fit together with some other part. However, you should recognize that you do not build the whole airplane AT ONE TIME. It is built ONE PIECE AT A TIME, and there are no parts in the Mini-IMP that are hard to build. Builders should build the wings first. This is the most time consuming part of the Mini-IMP to construct.
The designer had wide experience with the “Coot” light amphibian homebuilt and worked with dozens of “first builders” who had never built anything like an airplane before. This experience enabled him to design the Mini-IMP with even more consideration for the total amateur builder. The full size patterns enable you to quickly lay out parts which might otherwise be difficult to make. All bends of the sheet metal are straight. A detailed “shearing guide” permits you to take your sheet metal to a local sheet metal house and have it all sheared at one time. From there on it is more a matter of cutting off little pieces, bending them sometimes and drilling holes and bolting or riveting things together. Some small parts are made by merely cutting them out of the proper material with a band saw, and then sanding the edges smooth on your belt sander, drilling some holes, and the parts are ready to be assembled. We have lined up suppliers who have complete lists of things like the “aircraft hardware”. These are things like the bolts, nuts, washers, pulleys, cable, fittings, etc. which are “aircraft grade parts”. There are many aircraft supply houses in the country and you can find their ads in “Sport Aviation” which is the monthly magazine for the Experimental Aircraft Association (EAA). If you don’t belong to the EAA, we suggest you join immediately.
There are only a couple of welded assemblies in the Mini-IMP. These include the engine mount and main gear pivots which are available pre-made. These parts are all detailed in the drawings. There are some things which take very costly tooling to build. The shaft system is particularly critical, and we do not recommend that you attempt to build this yourself or have it built by anyone other than an experienced machinist. Materials and components of the shaft system are obviously critical. The instructions that come with the drawings cover the installation, alignment, balance, and maintenance of the shaft system in detail so that you can perform those operations yourself. Things like wheel alignment, brake installation, control adjustment, etc. are covered in the instructions. We feel that if you have even modest mechanical talent you should have no difficulty building a Mini-IMP even if you have not tried something like this before. We might suggest that if you can find a nearby chapter of the EAA that you join them since such groups usually have several experienced homebuilders in their membership and these people are usually only too happy to assist you with anything that might be giving you problems.
The photo set, which is included in the drawing set can be a great aid to a “first time builder”, particularly if you are not experienced in reading drawings although our drawings are very detailed. Further, Mini IMP Aircraft Company is available on the phone (your nickel please) to discuss any problems, direct you to suppliers, etc. As Molt liked to say it “WE WANT TO GIVE YOU EVERY REASON TO SUCCEED”.
General Construction
The years of experience that the designer of the Mini-IMP acquired with homebuilders have shown that the ideal homebuilt design should require no welding. It should require a minimum of tools, and it should not require many costly machined parts. While it is impossible to design an aircraft with out some of this complication, the Mini-IMP has been designed to keep these requirements to a bare minimum. We are investigating the possibility of offering a complete “kit” of materials. At this time only those items indicated in this information package are available. However, we do plan to further investigate the possibility of making arrangements with other outside suppliers to have kits of such items as aircraft hardware and all aluminum available for those builders that may find it difficult to locate suitable parts and materials easily.
The Mini-IMP has been designed to be built with a bare minimum of special tools. Thus, a drill press with variable speed, a metal-cutting band saw with variable speed, and a bench sanding belt are the only power tools necessary other than usual hand tools. A variable speed 3/8” chuck electric hand drill (or equivalent) is necessary. A propane torch for brass brazing is desirable, although this work can be easily done by some job shop. A variety of high speed drills are needed, and these are listed in the drawings. Good quality sheet metal snips are required. A hand riveter is required, and if the aircraft is to be flush riveted, a hand dimpling adapter for the riveter is needed. There are a few long sheet metal members that must be sheared. These can be done by a job shop, or can be hand done and the edges reworked and filed to get the desired smooth edges. There are some members which must be bent, but these can be bent over the edge of suitable planks. Drawings on how to do this are included in the drawing set. Other than these special considerations, the builder needs a good work bench, a heavy vice, and the usual hand tools found in any hardware store. The Mini-IMP can be built in a single car garage. Wooden jigs to support the structure during construction can be made from 1 X 4 or 2 X 4 material available at any lumber yard. The drawing set lists all tools and hardware needed for construction. Outside contractors are being lined up to supply many of the hard to build assemblies. Although none of these are really complicated or expensive, it is recognized that some builders would rather buy as much as possible and still stay legal within the FAA requirement that they do 51% of the work. This requirement is easily met when building the Mini-IMP since there just isn’t too much that is needed other than putting it together. The basic structural material for the Mini-IMP is 2024-T3 aluminum sheet. There is no heat treating of the aluminum required. The only heat treated parts in the aircraft are the main landing gear legs and the nosewheel leg. These components are available.
Although there is some fiberglass component trimming necessary in the construction of the Mini-IMP there is absolutely no fiberglass lay-up or fabrication necessary, with none of the fumes, mess, and other problems such as skin allergies (necessitating work gloves, etc.).
Performance and Load Factors
The performance of the Mini IMP compares favorably with other aircraft in its size and weight class. It is similar is size and weight to several Formula One race planes and has performance capabilities that approach that level while maintaining “pussy cat” handling qualities, many “creature comforts”, and slow takeoff and landing speeds.
Many inquiries are received regarding the “gliding characteristics” of the Mini-IMP. We have found that the Mini-IMP (at zero thrust – idle power) will glide approximately 20 to 1. This amounts to a rate of descent of approximately 400 feet per minute, at 90 miles per hour.
The Mini-IMP has been designed to an ultimate load factor of 6 g (4 g limit). This safety margin does permit one to overload the aircraft for normal operations. Design gross weight is 1000 pounds (500 Empty plus 500 Usable). The structure is adequate for this weight (at the 6 g ultimate) thus for normal operations the aircraft can be loaded over 800 pounds gross and still be adequate for “Normal Category” operations. (such as additional fuel or baggage). In these instances the FAA requirement of a rate of climb ten times the stall speed (500 FPM for a 50 MPH stall speed) set the gross weight limits. With the baggage space virtually on the Center of Gravity and with the fuel tanks on the Center of Gravity, the Mini-IMP is not too sensitive to various weight loadings. The trim provision is adequate for a great range in pilot weights although it is not adequate for the modifications required for two passengers. The GA(PC)-1 wing section used on the Mini-IMP has a very considerable trim change effect as the airfoil is varied for the possible flight configuration that the pilot might desire, and a trimmable horizontal tail is incorporated in the design to accommodate these effects. Baggage up to 100 pounds can be easily accommodated without prohibitive trim effects. With heavier pilots, such baggage loads do result in longer takeoff runs before the aircraft can be rotated, particularly with heavy fuel loads since there is no propeller slipstream blowing over the tail surfaces to assist in “rotation” for takeoff.
We get many inquiries regarding the suitability of the Mini-IMP for operations from rough fields and unprepared runways. Due to the requirement for retracting the wheels into the wings we were unable to include larger diameter tires on the Mini-IMP since then they could not be retracted completely so as to give the performance we desired from low-powered engines. These small tires do limit the “roughness” of the runways from which the Mini-IMP can be suitably operated. The “spring-leg” landing gear has proven to be suitable for rough area taxi operations, but take off and landing in rough areas should be avoided. The low taxi speed thrust available from the low-powered engines used in the Mini-IMP do not permit taxiing over very large clumps of grass, runway joints, etc., or getting started on uneven ground. In order to get the desired high speed cruise capability, the compromises in propeller selection makes the low speed thrust available for takeoff and acceleration very limited. A large clump of grass can effectively slow the acceleration of the Mini-IMP on takeoff from an unprepared surface. Although, the takeoff speed for the Mini-IMP is not nearly as high as some of the other “mini” designs now available, we do NOT recommend the Mini-IMP for operation from short unprepared runways.
The propeller blades are virtually stalled during initial takeoff acceleration due to the high pitch and low RPM when using a propeller of sufficiently high pitch to give the desired high cruise speed possible with the design. We now have drawings for a more costly, light-weight, controllable, all-metal propeller suitable for the Mini-IMP which greatly assists with this problem. This propeller also enables us to take advantage of various “turbo” installations, which are now available for use with some of the VW conversions etc. Thus we are able to control these propellers to provide low pitch for taxi, takeoff, and climb, and then control them for higher pitch for cruise, and for higher altitude operation where the turbo let us still pull full rated sea level power from the engines. These turbo engines are NOT to be rated at any higher power than they are designed for (for sea level operation), but the turbo lets you maintain these power levels at high altitude where the unassisted engine rapidly loses power as altitude is gained. Such engines are not “blown” but are merely “maintained”. Do not be confused by claims that the turbocharging will increase the power of non-turbocharged engines. While this might be possible, it is NOT desirable to try to pull more than normal rated power from most modified engines and thus develop more power than these engines are rated for. Various gearing systems are necessary to let some engines turn at higher speeds than are suitable for propellers. Such engine conversions must be properly designed so as to avoid exceeding the designed engine speed limitations of the engine. While excessively high RPM will result in more power, it also results in a quick loss of engine life and durability. Further, most converted engines do not have sufficient heat rejection.
The Mini-IMP design takes advantage of the very latest state of the art. The GA(PC)-1 airfoil is one of the most recent (and best) to come from NASA. It was especially designed for use on aircraft of the “homebuilt” type where the builders are relatively inexperienced, and the people who fly the machine are usually not professionals. Thus, things like surface smoothness are not as critical as with some other available wing sections. Also, the flight characteristics of an aircraft with such an airfoil section are not as demanding as they can be with some of the so-called laminar airfoils. The stall is exceedingly gentle, and the wing has excellent high lift capability without the use of complicated flaps, etc.
The fuel economy and thus operational expense of the Mini-IMP is outstanding. These features and capabilities have been obtained through the use of new materials, the latest airfoil, and unique design. However, these capabilities can not be retained if the design and configuration are modified to any great extent. Any extension of capabilities (such as a two-place arrangement) could only be realized by making the design bigger and using more power. This would mean an entirely new aircraft if these capabilities are to be retained in a two passenger configuration. This also means more weight, more structure, more complication, and more cost.
