Mini-IMP Aircraft Company
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.
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.
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.
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