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Ercoupe project flies unstable in pitch and yaw

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28 Jan 2024 21:06 #1 by Raymond Knox
Replied by Raymond Knox on topic Ercoupe project flies unstable in pitch and yaw
5 degrees down is almost twice the correct 3.5 degrees

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28 Jan 2024 12:55 #2 by Ronald Raty
Replied by Ronald Raty on topic Ercoupe project flies unstable in pitch and yaw
www.ercoupe.info/uploads/Main/drawings/a-654.jpg

I'm pretty sure I am reading this drawing correctly, 3-1/2 to the right, 5 down. That raises an interesting conundrum; which is correct?

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27 Jan 2024 18:25 #3 by Harold Johnson
Replied by Harold Johnson on topic Ercoupe project flies unstable in pitch and yaw
where did you find that drawing? I looked on ercoupe info and could not find it. The drawing that I am referring to is in the maintenance manual and clearly says 3-1/2 degrees down and 5 degrees right. The trim tab has been inspected.

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27 Jan 2024 12:00 - 27 Jan 2024 12:01 #4 by Ronald Raty
Replied by Ronald Raty on topic Ercoupe project flies unstable in pitch and yaw
According to Assembly Drawing 415-31913, down thrust is 5 degrees down from the airframe centerline. That may be within your margin of error, depending on how the angle was measured. And it doesn't include compressed lower engine mount rubber bushings which would increase the engine angle.

A tail too low can cause the plane to take off before you tell it to. The top of the vert stab should be about 75" above the level ground when static.

A mis-adjusted or seized trim tab can cause a surprisingly high AOA on departure which looks/feels like jumping into the air.

These are two things that are easy to check, that I haven't seen anyone mention yet.
Last edit: 27 Jan 2024 12:01 by Ronald Raty. Reason: grammar

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26 Jan 2024 09:18 #5 by Matt Gunsch
Replied by Matt Gunsch on topic Ercoupe project flies unstable in pitch and yaw
The Ercoupe, by design, is almost impossible to get out of CG. all the seats and fuel tanks are in a straight line across the plane, your only way to move it aft is baggage.
You didn't leave your A&P in the tail cone did you?

A&P, IA, PPSEL
too many years GA and Warbird Maint
Check out the Ercoupe Discussion group on facebook

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25 Jan 2024 20:31 #6 by Harold Johnson
Replied by Harold Johnson on topic Ercoupe project flies unstable in pitch and yaw
Checked thrust line today and found that I have 6.5 degrees of down thrust! I don't understand why too much down thrust would cause the plane to want to jump off of the runway, but could see why it might be touchy in pitch.
We are checking the weight and balance this weekend to rule out an aft cg before doing anything with the engine mount.
If any of you can explain why too much down thrust would make the airplane want to pitch up, I am very interested as it seems counter intuitive.Any other suggestions appreciated.

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