A Peculiar Characteristic - Any Ideas?

[Tony65x55]

Hi Folks, I am flying a homebuilt AP plane, conventional design. It is primarily built of fan fold foam and has 622 sq/inches of wing area. It is quite light, coming in at about 22 oz empty and carrying a camera payload of up to seven ounces. As you can imagine, it's a floater. I have two wing set-ups for it, a Clark Y section with full span flaperons, huge differential on the ailerons and no dihedral and a second wing for light air days, a single surface 4-40 section with 8 degrees dihedral supported by struts. This second wing flies on simple rudder / elevator only and gives very smooth, slow video for those calm days. Power is a STModels brushless outrunner turning a GWS 9050 HD prop.

Now that I've given you a bunch of detail about the aircraft let me state that generally, it flies really nicely but it has one strange peculiarity. I am at a loss to figure it out and I onder if any of you aerodynamics gurus can help me out?

On occasion, as the plane comes out of a gentle turn, it begins to dive. Shallow dive at first but progressively steepening. It will not recover from the dive on its own but a gentle application of elevator easly corrects it. It seems a little more prevelant following a right handed turn but it will still do it on a left.

In all other flight attitudes the plane flies hands off and lets me concentrate on getting good pics but as you can imagine, at very high altitudes when perspective is limited and the plane is a speck in the sky, you don't really notice the dive until it becomes steep.

Any ideas what might cause this strange behaviour?

Tony

[Tom Harper]

What's the difference in incidence between the wing and stab?

[Tony65x55]

Thanks for the quick response Tom. The wing should be at +2 degrees.

[Tom Harper]

You might try increasing it a tad and moving the CG forward a little. Then observe any change.

I'd go slow, sounds like it doesn't need much.

[Tony65x55]

Terrific, I'll give that a shot and report back on the results. Thanks,

Tony

[Ollie]

Your model is Spiral Unstable. You need more dihedral or flying skill or gyro. See:
http://members.cox.net/evdesign/pages/spiral_stability_gyro.html

"Models with low dihedral will require aileron deflection to avoid rolling in. The alternative to aileron deflection is unacceptably large yaw angles. Aileron deflection is typically provided by the pilot as a learned response. A tricky aspect of this is that the more the airplane is banked, the more aileron deflection is required, so constant adjustment is required."

[Tom Harper]

Tony,

Ollie makes a good point.

I assumed the problem you described was with the rudder only dihedral wing. Is there a difference between the two wings? Do they both exhibit the same problem?

[Tony65x55]

Thanks for the comments guys. I would have thought the plane was spirally unstable too, but it is not rolling in. The dive commences after the roll out and is perfectly level. That is to say, the plane is not spiral diving, Just coming down wings level. It begins as a shallow dive and slowly increases until it is very steep. It will not correct on its own but as I said before, a little up elevator brings it right out immediately. When the plane is relatively low to the ground and easily referenced visually, recovery is so fast that unless you know what it did, you would not detect it. No problem...super easy to fly. The problem comes when you are very high (taking pictures) and small pitch changes are harder to detect. By the time you see the dive, it has already steepened. It also does it intermittently, say 25% of the time.

The problem only happens with the undercambered (rudder only) wing. I'm inclined to think it may be incidence related as the LE of that wing is 1/4" thinner than the Clark Y wing. It may be that is causing the UC wing to sit at a lower AoA. While you are in the turn, you are holding a small amount of up elevator, as you roll level, you release the up elevator to return to it's trimmed flight condition. Could it be that the quick decrease in AoA, combined with too small a degree of positive incidence, is causing the wing to tuck under?

Thoughts?

Tony

[Sparky Paul]

You have failed the "dive test"! :)
The pitching moment of the undercambered wing pushes the nose down, you have insufficient horizonal dihedral and/or c.g. too far aft.
If you let it go long enough, the plane might just tuck unrecoverably, or the wingtips may touch on the top.

[Tom Harper]

Tony,

I think you've got it. The dihedral wing sits at a lower angle of incidence. Shim the LE a bit and check the CG.

[Tony65x55]

Thank again guys. Great advice. I wil raise the LE and advise.

Tony

[mlbco]

Tony,

Here's my 2 cents worth of advice regarding your problem. I think your plane is falling victim to a form of aeroelastic divergence that is well known in aircraft with flexible structures that have highly undercambered wings. As the plane speeds up the wing twists to lower incidence (because of airfoil camber and increased dynamic pressure) and the decelage between the wing and tail is effectively lowered. If the structure is flexible enough this process continues resulting in a divergent dive or possibly wing failure. The problem won't go away until you torsionally stiffen the wings with an improved strut configuration or perhaps a kingpost and a lot of rigging wire, like on a hang glider or ultralight.


Steve

[Tony65x55]

Steve, you may be bang on. I do get high speed flutter on the traling edge if I push the wing hard. I just don't fly this particular wing fast and haven't had a problem, at least in straight and level flight. There is only one strut per wing and the TE may be billowing up as air pressure increases. That would certainly cause a decrease in AoA. I'll try an incidence increase first (because it's easiest) and if the problem persists, I'll try increasing the wing's torsional stiffness.

I don't fly this wing that often as it is rarely calm enough where I live. I usually fly the Clark Y and it is very stable.

Tony

[JRuggiero]

A Slow Stick will behave exactly as mlbco/Steve describes. If a Slow Stick builds up too much speed in a dive, you can observe the wing tips twisting to a lower incidence setting. Cut the power and pull out gently.

Jim R

[BMatthews]

Thank again guys. Great advice. I wil raise the LE and advise.

Tony

Tony, assuming the model you're working on here is the one in the picture and that you're reffering to the rear wing then what you need is to raise the TRAILING edge of the rear wing to get the model to be stable. What's happening is your undercamber is acting like down elevator.

Another option would be to bend up some reflex into the curved dihedral wing's airfoil.

[peterangus]

mlbo [post 12] has the most-likely explanation; wing torsional deflection.

Can you replace the single struts with V-struts? Might make sufficient improvement.

The phenomenon is well known to the glider fraternity; refered to as "tuck under". In an extreme case full up elevator will not pull out from the dive; the only escape then is to apply down elevator, and bunt out. Quick thinking, strong nerves, and some altitude, are all required.

[Sparky Paul]

I've lost a couple-three planes to flutter-induced tuck.. one coming down off the towline, with both wingtips a blur.
Open structures are really susceptible to this, with sheeting/shearwebbed wings least susceptible.. other than sheeted foam.

[Tony65x55]

Tony, assuming the model you're working on here is the one in the picture and that you're reffering to the rear wing then what you need is to raise the TRAILING edge of the rear wing to get the model to be stable. What's happening is your undercamber is acting like down elevator.

Another option would be to bend up some reflex into the curved dihedral wing's airfoil.

No, the plane I'm referring to is conventional configuration. The little canard flies great.

Tony

[Tony65x55]

I think you guys have got it. Flutter induced tuck sure sounds like the beast. The plane is pretty big and slow so I'm sure a V-strut will fix the problem right up. I wasn't aware this phenomenon was well known to glider pilots. Thanks to all of you. I will post the results.

Tony

[ghoti]

Tom suggested your plane is tail heavy. I suggest the opposite. When nose drops in a turn, plane is nose heavy. If it raises during a turn it is tail heavy, right? This is probably too simple an answer!
fish

[JRuggiero]

Vee struts can cure speed-induced wingtip tuck, as I found out on my GWS Pico Cub J-3F. The stock configuration calls for only a single wing strut, but that's not enough if you set up the airplane for higher power for outdoor flying. At the resulting higher weight, building up speed in the dive will twist the wing, thus calling for a second strut, making it a Vee.

Jim R

[Tony65x55]

Tom suggested your plane is tail heavy. I suggest the opposite. When nose drops in a turn, plane is nose heavy. If it raises during a turn it is tail heavy, right? This is probably too simple an answer!
fish

Fish, the plane isn't dropping it's nose in the turn, it starts after the wings level out. During the turn it's fine. I think the plane may be experiencing a small increase in airspeed due to the lesser lift in a bank and when the wings level and the full lift is restored, the plane's momentum ,translated into a slightly higher airspeed, starts the TE fluttering process, causing the plane to tuck under. I have added an extra pair of TE struts to 'er and I'm waitiing for the gale force winds to subside enough to test it out. It's a pretty light air flyer.

Tony

[BMatthews]

If it's the thin wings that are doing this then I suspect the twist issue is the key. Adding support struts to control any twisting will soon answer the question.

That avatar model is a cutie. Is there a thread about it somewhere?

[Tony65x55]

Problem solved. You guys are awesome! I increased the AoA by 1/4" and added a set of rear struts to the wing. No more tucking at any speed. I turned all directions and tried to replicate the tuck every way I could think of but...NOTHING!! It flew stable as a baby's rocker in some pretty impressive winds. Recovered from all artificial attitudes all on it's own. I even took some great aerial photos. Thanks to everybody for their valuable suggestions (I love RC Groups)

Bruce, the plane is called the Funky Flyer and the thread is here:

http://www.rcgroups.com/forums/showthread.php?p=3587055&highlight=Funky+Flyer#post3587055

Lots of people have built one and they fly very nicely. I'm about to build a larger outdoor version and I will post the build thread in the Foamies section.
___________

Thanks again to all,

Tony

[peterangus]

The improvement is due entirely to the V struts.

The change to the wing rigging angle has no significant effect. It does not increase the wing angle of attack. It merely alters the attitude of the fuselage in flight. It does add a small negative angle to the fixed part of the tailplane, but the effect of that will be cancelled when you adjust the elevator trim to obtain level flight.

[BMatthews]

Thanks for that link Tony. My thoughts are drifing towards some sort of Crimson Skies kind of model based on this layout...

[Tony65x55]

Enjoy. Some folks have built a larger park flyer version and it stops traffic. Flies well apparently. I've only flown the indoor version although it flies pretty good in light winds.

Peter, I concur. The rear struts made the difference. Any increase in AoA I experienced was probably trimmed out.

Tony