A question for experienced designers/aerodynamacists:

I have built and flown my third own design twin boom pusher plane. All 3 are very similar in dimension and proportion.1.7m span, 1.3m length, 1.3kg weight, Clark-Y airfoil.

However from the previous experience I determined that I needed about 3° decalage to prevent "tuck-under" (pitch down with increasing speed) in a dive test (no power, nose down 45°, release sticks, see what plane does). The others would gently pull out of the dive which was desirable.

Currently on this one my CG is at 26% from leading edge at the root, (straight leading edge wing) but I still am getting mild "tuck-under", it's expectedly worse if I move the CG back. If I go to 25% (feels a bit nose heavy) then it's almost right as then it flies just straight, I haven't yet tried further forward. The others flew happily at 28-30% CG.

Because of the decalage there's quite a strong pitch up moment with full throttle, but motor thrustline is 0°,0° as it was on the others.

So I want to lessen the pitch up on full throttle AND stop the tuck-under in the dive test AND possibly move the CG to ~27%.

What can I change to get this right?

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Hi

it looks like the problem is due to the increasing effect of higher speed in the froont section of the fuselage, try rounding it so its not operating as a forward elevator, Also I think your elvator is too narrow to be effective. a wider elevator with less movement is much more efficient and stable. try both

Dwgsparky

ahh, but the main problem is during a dive when it speeds up, so then the front angle (my camera window) "should" push the nose up which is not happening.


The horizontal stab and elevator is larger than it appears in the photo, it is 400mm x 200mm (800cm²) with the elevator 50mm wide (200cm²).

I would see if the airframe is deflecting under load.

the tail looks small.. did you do a Tail Volume Coeeficient verification?

Increasing throttle increases the airflow over the tail.. so I am not s'prised there is a throttle-pitch couple'ing

Also... do a "MAC" investigation on the wing... see if the CG is at a 25 to 30'ish percent location on the MAC...

 

horizontal tail looks kinda small to me....

Put a camera on the stab over the weekend, no noticable deflection. Will post the video shortly.

Upthrust on the motor has seemed to cure most of the ills, now need much less down trim to maintain gentle climb with power which has eliminated the increasing dive in the dive test.

Wing area is 4620cm², stab is 800cm²

CG is 26%

Thanks Paul, the plane is sorted now. The booms had carbon tow/cyano on the upper and lower surfaces during the dive tests in the video.

The main cure was that the plane was being trimmed with a lot of downtrim for a full power climb but the pitch-up under power was actually caused by a poor motor thrustline, then the new downtrim caused the tuck-under.

So in reality there wasn't actually a problem except for getting the motor to push the nose down under full power.

There's a flying video here:

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