High engined plan trim question

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Boo - 16 Feb 2009 13:04 GMT

Hi,

I just came across a video of the Saro "Cutty Sark" twin engined amphibian plane
at <http://www.youtube.com/watch?v=g3pBO5kjTbo> and the question occurred to me
what planes with engines as high mounted as this one are like to fly ? ISTM
there must be a huge nose-down with power trim problem. In particular, landing
must have been tricky with the reduced power giving a nose up trim right at the
time you are near the stall and I wondered what the original designers did to
counteract that ?

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Boo

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Anyolmouse - 16 Feb 2009 14:38 GMT

> Hi,
>
[quoted text clipped - 5 lines]
> time you are near the stall and I wondered what the original designers did to
> counteract that ?

The engine thrust is offset accordingly. I have a power pod that is
used for temporarily converting a 2M sailplane to electric power. (also
have a 1/2A glo engine one) Both are set up in a tractor configuration
with a positve engine thrust line. I.E., the engine points up in
relation to the fuselage center line. Here is a good read on it:
http://www.djaerotech.com/dj_askjd/dj_questions/upthrust2.html

Notice he points out that if the engine is placed on, or very nesr, the
CG the effect is not as pronounced.

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Anyolmouse

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Lee B. - 16 Feb 2009 18:40 GMT

Both the lateral and verticle CG location enters the question. Granted, the
engine can be located on the lateral CG -- it can not be located on the
verticle CG. The moment produced by the engine can not be reduced with down
thrust to a significant degree. A 10 degree down thrust -- which would be
VERY large -- would reduce engine moment by about (sine of 10 degrees) 17
percent. The result is that, during cruise flight the airplane. when trimed
for hands off flight must use down trim to counter the engine. When thrust
is reduced the moment produced by engine thrust must be replaced by down
elevator -- or be retrimed to lower the nose. For a pilot this is a natural
thing and not a big deal.

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Model Forum / Radio Controlled / Air Models / February 2009

High engined plan trim question