Page 1 of 1

Elevator Trimming, model with flaps

PostPosted: Mon Jul 25, 2011 6:30 am
by takilara
I have just gotten my first airplane (I have been flying helicopters for several years).
The plane is a Multiplex FunCub with flaps.
This weekend I flew it for the first time. (First time ever with a plane)

My question is regarding trimming of the elevator/CG/downthrust.
The motor is quite powerful for the plane, but then also quite heavy, and i struggle to achieve the CG stated in the manual

If i have the elevator set "flush", the model will dive if i am flying with raised flaps. It will fly straight at certain speeds if i have lowered the flaps.

I have then used the trims on the Tx to have it fly straight when i have raised flaps, however this means the elevator has some throw "by default". The trim was about 15 notches on my Hitec Optic 6. I have not measured the angle/throw of the Elevator in this position.

Flying with the elevator like this works, however i am concerned that this is not the best way to get it to fly straight, and that it might have negative effects on stuff later.
Also I have to fly VERY slow for it not to try to climb with neutral elevator with flaps lowered.
Coming from helicopters, i prefer to have everything as mechanically straight as possible before adjusting on the radio.

I would assume that it should be possible to achieve a trim that would include flush elevator causing the plane to fly straight at some reference speed with raised flaps. And straight at another (lower) reference speed with lowered flaps.

Options I can think of:
  • Move CG further back by adding weight to tail (probably the best option)
  • Reduce motor downthrust
  • Add Flaps->Elevator mixing that would change elevator neutral position depending on flaps position
  • "Finalize" the elevator trim i have now, by moving the links, and then zeroing the Tx, accepting the angle on the Elevator.

Any suggestions or ideas will help me greately!

Re: Elevator Trimming, model with flaps

PostPosted: Mon Jul 25, 2011 8:04 am
by RCModelReviews
Most aircraft will show pitch changes when flap is added.

In a low-wing aircraft, lowering the flaps will frequently cause a nose-down pitch, in a high-wing aircraft the plane will often pitch up with flap -- although this is not always true.

Most people do a bit of mixing so that the elevator automatically adjusts to compensate for the pitch change.

However, you can't rely on elevator mixing alone to compensate.

The amount of pitch change with flap will vary depending on the aircraft's speed so when you initially deploy flap (and the model is still flying quickly), the pitch change will be more pronounced than once the model has slowed due to the extra drag the flaps create.

So, mix in a little up/down elevator to compensate for the pitch if you want but be aware that you'll still have to *fly* the model by using the sticks ;-)

Re: Elevator Trimming, model with flaps

PostPosted: Mon Jul 25, 2011 8:50 am
by takilara

I am aware that i will experience pitch changes when i deploy the flaps. However I was hoping that after the speeds slow down, the model would then "stabilize", so that zero stick giving level flight at some reference speed.

I have proportional flap control, so i was hoping that while graduately deploying flap, while reducing airspeed would require adjustments only during speed reducion phase, and not after speed has "settled" for the two scenaries.

I'm basically concerned that having a "non flush" elevator as default (for level flight without flaps) is a problem for other scenarios..