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1. 07:53 AM - Reducing power on take off versus thrust line. (Peter Cowan)
2. 08:53 AM - Re: Reducing power on take off versus thrust line. (John Hauck)
Message 1
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| Subject: | Reducing power on take off versus thrust line. |
This topic came up some months back and there was quite a good analysis of
the forces involved so I was surprised when John mentioned that after going
to his higher horsepower engine he seemed to be okay with having to reduce
power to complete a normal take off.
Surely this is a condition that is abnormal and one we would not want to
have in our aircraft, especially if it is avoidable. And it is!
I was experimenting with this myself last March and subsequently notes from
Boyd young and Richard Swiderski in April did a pretty good job of
analyzing the forces involved.
I had recently purchased a Beaver pusher that was designed for a 503 or 582
but had a 912 80 in it. Both the owner and the builder had experienced this
problem of requiring power reduction in order to rotate and their
conclusion was, like Johns, that it was just excess power doing it and
there was no alternative. Both suggested I should just go back to lower
power.
This came as a surprise and a challenge to me. Like the previous posters
who provided a sound analysis of the forces involved, I finally discovered,
with the primary source being a model airplane design site, that providing
upthrust on a high, aft mounted engine was exactly the reverse of what was
required.
My one circuit flight with this additional positive thrustline was almost
more than I could handle in terms of requiring a huge amount of back
pressure to stay airborne. A more positive thrustline actully produced DOWN
THRUST.
After learning how things actually worked and putting in close to minus
six degrees down on the thrustline, the plane has become a jem in handling
with full power all through the takeoff roll AND NO STICK FORCE change
going from low-power to high-power and back in level flight.
I do plan to add a little more down in order to get a nose UP on power
application and improve the trim it needs now.
Please tell me if I am missing something.
Peter
Message 2
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| Subject: | Reducing power on take off versus thrust line. |
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