Trimming Out a Symmetric Airfoil Setup

[Lon Enloe]

I'll ask my question in a nutshell first, then give you all the background:

Is it possible to trim out an aircraft with a symmetric airfoil for "hands-off" flight both right-side-up and inverted?

For the background, I built myself an aircraft with a symmetric airfoil (Eppler 168) and no dihedral--I want to learn aerobatics and want something that does as well upside-down as right-side up, outside loops as inside. All the thrust lines and tail surfaces are lined up square, for the same reason. It's been pretty much of a success so far, but...

I also want something that doesn't fall out of the sky if I itch my nose or glance over at someone else's plane at the field, so that's how I trimmed it out. Again, so far, so good, but...

I'm getting more comfortable flying inverted, doing horizontal eights around the field, but I can't leave it alone for even a moment when it's upside-down--it starts drifting toward the ground. I have to give it down-elevator all the time to keep it level.

I know it's got up-elevator trim when I trim it so I can go hands-off when it's upright. One part of me says that this is normal--a symmetric airfoil has to sit a some positive angle of attack to generate lift, right? So, if the setup puts it at this angle of attack when upright, it's all wrong when it goes upside down, and having to dial in the down-elevator is just the price you pay for not having to touch the elevator when it's upright.

Another part of me says I've messed something up 'cause I don't know nearly as much aerodynamics as I should.

So which side of me is right?

Thanks,

Lon

[Sparky Paul]

You need the "up" when upright... therefore you'll need "down" when inverted.
It may be just slight thumb pressure on the stick, but it will be there.

[Lon Enloe]

OK, good, so I'm not crazy--the solution to my porpoising up and down when inverted is practice, and that I'm willing to do. I just didn't want to think I was fighting a losing battle--and knowing that will probably make things easier. Thanks!

[vintage1]

It IS possible to have the model equally able to fly hands off in either orientation, BUT it involves a bit of a rearward CG and you may find that makes the model VERY twitchy on the elevator.

Large tails an/or long tail moments help a lot with this.

In gherenral though mots pwople trim so that its nutrally srtable one way up - i.e. if put in a climb, stays there, if put in a dive, stays there - and accept the small forward stick needed to maintian inverted stability.

[Al M]

As I remember we used to trim the Kwik-Fli II by moving the cg aft until it didn't need any down elevator when inverted. It flew with no change in pitch trim upright or inverted. I don't think it used a lot of elevator travel.

[Andy W]

That's exactly how I set the CG on my Diablotin - just moved the CG back until it maintained level flight both inverted and level..
..a

[vintage1]

...Bearing in mind you two guys are pretty experienced stick twiddlers...:D

[Andy W]

True.. hopefully, a beginner wouldn't be trying to trim a model for neutral CG.. :)
..a

[Rodney]

Two of my planes will fly "hands off" either right side up or upside down with no trim changes required. They are both bipes, the smaller Lazy Ace and the Phaeton 40--both semisymetrical airfoils. Neither are abit twitchy under any circumstances. One of the most critical (after proper CG location) is aileron position. Both planes have ailerons on the lower wing only and both required that the ailerons be slightly up at neautral trim. If you have a computer radio and each aileron on its own channel, try the flap mix to be able to move both ailerons in the same direction at once and then try various positions for inverted and right side up level flight. you will probably find a position that will allow you to maintain level flight hands off whether upright or inverted.

[Lon Enloe]

Thanks, all, for the excellent suggestions--I'll tweak the CG first, then play with the neutral point on the ailerons (I do have a computer radio and have a flaperon setup on the ship in question already). I'm assuming that we're talking relatively small amounts of "up" trip on the ailerons, not so much that they become spoilers, yes?

[Rodney]

Thanks, all, for the excellent suggestions--I'll tweak the CG first, then play with the neutral point on the ailerons (I do have a computer radio and have a flaperon setup on the ship in question already). I'm assuming that we're talking relatively small amounts of "up" trip on the ailerons, not so much that they become spoilers, yes?
Correct, just a few degrees, usually only 2 or 3 degrees.