3D-MAN

My Feedback: (4)

Fellow R/Cers,
I have noticed with different 3-D and aerobatic aircraft such as The Edge 540, Extra 300's, Sukhoi's and Lasers all have different stab and wing positions. Most recent designs seem to place the horizontal stabilizer above the wing. My question is why???? Does the horizontal stabilizer prove more effective above the wing placement? Is this done to improve elevator response? Does it have any impact on the aircraft during rolling or tumbling manuevers? My main question is: In the overall design of aerobatic aircraft, what in your opinions is the best set up? Even with the wing, above the wing or maybe even under it???? Any feedback would greatly be appreciated... Hope to hear from you all very soon.


Thanks,
3D-MAN

Ben Lanterman

This is discussed with respect to radio control models in the "horizontal stabilizer dihedral" discussion and the "stab positioning" although not a rock solid answer based on data was evident.

For models and a configuration that has to spend an equal amount of time upright and upside down it would seem that the tail should be at the same level as the wing. With everything at zero and a very stable CG it would require holding a little back stick when upright and a little forward stick when upside down but in equal amounts both directions. Then when the airplane went knife edge and the pilot released elevator there wouldn't be a pull to the canopy or belly. The airplane would be equally responsive upright and inverted. A nice machine.

But it gets to be a pain holding a little elevator all the time though so the flier tends to trim to upright 1g with the elevator. Then when he pulls an up line or down line or go knife edge the airplane would probably pull toward the canopy. To keep the down lines straight he moves the CG aft. This requces the control required to do a 1 g flight and also straightens the knife edge some and lessens the amount of control to do inverted.

Then the flliers thinks that perhaps if I can position the stab in the wings downwash at the right location maybe I can get some self trimming effects when I go inverted or at least in the up and down lines, maybe knife edge. Then the juggling begins. I tell a good story (might even be true) about this in the "horizontal stabilizer dihedral" discussion.

It is all a big balancing act brought about by the compromise of having upright flight accomplished with a lower pilot load.

With respect to manned aerobatic machines maybe the same process occurs.

I would think a higher tail position also would be desirable to remove the tail from the major effects of the wing downwash generated during positive g maneuvers. It would provide a more linear stability and control response. There would be practical limits on how high you could position the tail based on structural weight required for the loads vs desirable qualities in stability and control response.

However whatever would be a benefit in positive g maneuvers would seem to be a detriment in negative g maneuvers so I am not very sure that it is a valid argument. Perhaps it is because the higher g maneuvers are positive instead of negative. I don't know the answer but maybe it will stir someone to contribute some truth in the area.

I have wondered about this too and it would be interesting to hear from someone that has had contact with the designers of some of the airplanes you mentioned. It would be interesting if the expected characteristics for the chosen tail position actually were realized.

Failing any of the above just some goo speculation would be OK.