On Nov 25, 9:19*am, Oliver Arend wrote:

But why does the torque have to be produced by the engine in direct
drive?
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Hahahahahahaaaa.... (wheez!)

Sorry Oliver, but that's just so funny.... I forget that most
subscribers to this list are not familiar with 'real' aircraft
engines, ALL of which incorporate some form of speed reduction.
'Real' airplane engines are those big round things with all the
jugs... or those sleek narrow things tucked into the nose of a P-51 or
an Me-109.

'Smalll' aircraft engines.... typically those below 550cid or there
abouts are an EXCEPTION and tend to use direct drive. All the rest of
the world uses 'real' engines... or did, until Frank Whittle got
someone to listen to his 'crazy' ideas.

But as to the core of your question, the reason we don't find speed
reduction units on small engines (*) is their weight. The smaller the
engine, the greater the 'overhead' for a PSRU (Prop Speed Reduction
Unit).

(*) But there are exceptions. Continental made a geared A-85... Just
look for a 'G' in the Type Number. I'm not familiar with any others
but Lycoming and Franklyn probably made them.
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Couldn't weight and space be saved by using a high-revving,
small displacement engine (such as a car or even motorcycle engine)
with a reduction gearbox?
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That is correct in theory, especially if the engine or 'power unit' is
water-cooled. Rotax has recently earned a 1500 TBO for the 1300cc
power unit of their 912 engines, although the TBO of the gear
reduction unit remains around 850 hours. (Maybe a Rotax mechanic can
jump in here and give us the actual numbers.)

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I'm aware that a reduction gearbox will add
weight (but not that much?), complexity and failure modes, and that
transmitting the forces created by the prop to the airframe could be
an issue.
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First off, much of the 'issue' is bureaucratic, in that the FAA
requirements for a type certificate will presently cost about a
quarter of a million dollars to satisfy. That is, over and above your
cost of development, the PAPERWORK will add another quarter-mill to
the pot. Given the market, it simply doesn't make good economic sense
to put that kind of money into ANY aspect of 'General Aviation'
today.

But that doesn't mean small, modern engines such as you've described
aren't out there. For some nice examples of modern light-airplane
engines, take a peek behind the propeller of any of the RPV's and tell
me what you see :-) There are also some shoe-box size turbines
slinging props that beg to be bolted to a KR or similar. In the 50
to 70hp range, the hot section is out of a GPU or APU. As the United
States digs it's self deeper into Third World status we'll see more of
these units appear as surplus... if we don't sell them all to
China :-)

-R.S.Hoover