thailazer

I've been having a lot of fun slamming my Tiger II around the sky since I built it a few months ago, and a few times on full-throttle somewhat downhill runs I have heard what sounded like an engine/prop warble. Well today, on yet another windy flying day, it happened again and I could visibly see the ailerons vibrating in the setting sun. Off the throttle, and I landed right away.

There is some play on the torque rods of course, but the Futaba 3001 servo is solid. Glue joints and hinges are fine, and the wing is built to the stock plan with good epoxy bonds on the aileron support tubes/associated parts. Looking at the linkages, the torque rods seem a bit long in my opinion, and I am wondering what else could be contributing to my problem. Would appreciate any suggestions folks would have to resolve the flutter, as it WILL cause failure ultimately if I don't fix it.

I am using an OS .35 AX on it so it is not over-powered. Have had a lot of models but this is my first experience with flutter besides witnessing a fellow flyer's P-38 with aileron flutter just before I was dodging pieces of it rolling by me! Don't want that to happen to my sweet Tiger II.

Dr1Driver

The Tiger II can be a very aerobatic plane. I believe you answered your own question when you mention the play in the torque rods. Torque rods are a terrible way to actuate a control surface and have natural play built in. The rods themselves twist, inviting flutter. If possible, I'd retro-fit individual servos in each wing panel, with direct. straight line rods to the ailerons. Use the shortest servo arm and longest aileron control horn you can and still achieve the movement you desire. If the problem continues. you can try counterbalances at the ends of the ailerons. Sometimes cutting the end of the aileron back at a 45 degree angle helps, too. Eliminating flutter is not an exact science, rather a "cut and try" exercise.

Good luck with it.

AllTheGoodNamesAreTaken

My Feedback: (180)

Where is the play in the torque rods?

prgonzalez

I totally agree with Dr1Driver, retro-fit the wings with individual servos in each wing panel, been there and done that to solve the problem.

Your other solution for you not to modify the plane and avoid flutter is to reduce the max speed of the model, don't fly that fast.

thailazer

All_Taken.....The play in the rods is simply the torsion or twist they allow due to their own material. In other words, I can push on the ailerons and see them move while the other end of rod (on the servo end inside the fuselage) does not. The rod itself twists allowing the play and contributes to the flutter.

Dr1.....Thanks for those comments and I appreciate it. Will consider the separate servo idea as that would be the best solution.

thailazer

Prgonzales..... I hear what you are saying. Hard to do when one is having fun and you get that grin on your face for a fast downwind pass! For now, I will have to limit the speed just to keep the airplane together.

Just had a thought about using spades, but that would require a whole lot more experimentation than using dual servos.

Lnewqban

Thailazer,

Check this old thread out:

http://www.rcuniverse.com/forum/m_91...tm.htm#9193591

Note counterbalancers in pictures.

thailazer

LNE, Thanks for that! JohnBuckner has some good ideas there, and with his experience I am sure they work.

Looking at the wing on my Tiger II, adding a spade/counter-balance would be the easiest thing for me to do, so going to give it a try. I can just trim the inboard side of the aileron and epoxy on a ply support that will allow the spade to be forward of the hinge point, just like John's counter-weight.

gsoav8r

My Feedback: (6)

Sometimes just sealing the hinge gaps can fix flutter problems.

thailazer

After looking at the flutter issue, I've found out a lot, and here are some things I can do:

-Add damping (or loss) to the system. (things like dash-pots or simple friction plates)
-Move the CG of the aileron forward. (this is why counter-balances work)
-Reduce mass of the aileron. (hard to do)
-Add dominant force vectors. (wing mounted servos or spades)

I've got some experience in stability analysis and the spring action of the aileron torque rod is my major problem. It adds energy to the system, works the opposite of damping, and allows the flutter to start and maintain. That spring action puts the system on the wrong side of the S-plane stability-wise.

Didn't want to add wing-mounted ailerons at this point but vow to add them to every fast ship I build from now on! And, since I have never tried spades, am going to go that route because they look cool! It will accomplish moving the CG of the aileron forward as well as adding a dominant force vector. (actually aerodynamically counterbalancing the aileron forces required somewhat, and I am hoping the positive feedback does not allow the aileron torque rod to load up in torsion.)

This was surprisingly easy to accomplish with 1/8" ply, and I had them mounted in under two hours.

Photos below. Test flights this coming week.

grbaker

My Feedback: (29)

Agree!!!

Lnewqban

thailazer,

I hope the test flights show it works; however, it is not a common solution.
Are you sure your concept of spade working against the flutter is correct?

Copied from http://en.wikipedia.org/wiki/Aileron#Aileron_spades:

“Aileron spades
These are flat metal plates, usually attached to the aileron lower surface, ahead of the aileron hinge, by a lever arm. They reduce the force needed by the pilot to deflect the aileron and are often seen on aerobatic aircraft. As the aileron is deflected upward, the spade produces a downward aerodynamic force, which tends to rotate the whole assembly so as to further deflect the aileron upward. The size of the spade (and its lever arm) determine how much force the pilot needs to apply to deflect the aileron.�

Check these threads out:

http://www.rcuniverse.com/forum/m_88...tm.htm#9091497

http://www.rcuniverse.com/forum/m_47..._2/key_/tm.htm

http://www.rcuniverse.com/forum/m_74...tm.htm#7400111

Copied from http://www.airspacemag.com/flight-to...side-Down.html

“AILERON SPADES
These shovel-shaped surfaces, rigidly mounted on arms forward of the ailerons, provide “aerodynamic balance,� reducing the effort needed to roll the airplane. Aerobatic airplanes need aerodynamic balances because their control surfaces are large and their speeds are sometimes high. When the ailerons are neutral, the spades are aligned with the airstream and do nothing. But when an aileron is deflected upward, for example, its spade tips downward. Air presses against it, helping the aileron along, just as the weight of a small person on one end of a teeter-totter helps a larger person at the other end push off the ground. The farther the aileron is deflected, the larger the force supplied by the spade. Aerobatic pilots describe spades as akin to power steering.
“Spade design is a black art,� says airshow pilot Patty Wagstaff. “You see all kinds of shapes and all sizes, depending on the airplane. Akro pilots are always tweaking them to get the control feel just right—not too light and not too heavy. I’ve flown without spades, and it was like driving a Mack truck.�"

What I am trying to say is that flutter happens around neutral position of the aileron and spade, where the reactive force of the airstream does not much against either one.
However, weight of the body (aileron in this case) is always present, and it is the only thing that leads to resonance (a vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system), under the action of disturbing turbulence on the aileron, wing, etc.

gmohr

I wound up with flutter in my Tiger 2 also. The reason was that I didn't "harden" the wood that surrounds the torque tube with CA where it is inserted in the aileron. This was back in 1999 when I didn't know jack about such things. I suspect your slop is from the wood, not the torque tube iteself.

The real solution is to use separate aileron servos, as has been suggested.

Regards,
George

Gray Beard

My Feedback: (-1)

Sealing the hinge gap often helps. Also making sure you are using 4-40 rods and not 2-56. Counter balancing. There shouldn't be any slop or bend in your controls. I couldn't see how you had the controls set up in your photos but the spades looked cool! I have never tried them before.

thailazer

LNEWQBAN, those are good links. I am pretty familiar with spades on full scale aircraft, having had friends that used them on their ships. It reduces stick force and also reduces the loads on the control linkages. That was my basis for trying them as the positive feedback would reduce the torsion in the torque rods.

Test flights were good, and it took a lot more speed to induce the flutter, and it now occurs at a much lower frequency due to the mass of the wood I expect. It was fun to try this, but I am going to dual ailerons so I can fly the ship any way I want to.

GMOHR, Thanks for your post. I was curious if anyone else had this problem as most folks use a lot more power than I have in mine.

thailazer

Flutter problems gone. Put in dual servos and flew today with some vertical dives at full power....something I rarely do but I wanted to see if I could induce the flutter. No problems at all and had a lot of fun flying fast and low! The wind curse continues though as it was blowing about 20 mph with gusts to 30 when we left. I have yet to have three easy landings in a row with this airplane due to winds. Winds were light when we arrived at the field and they grew steadily as we launched. Very happy about getting rid of the flutter problems.

prgonzalez

[sm=thumbup.gif]

Glad you got rid of that problem.

Lnewqban

Great!

Doog-meister

Have an OS .46 in my Tiger II and fly pretty much full-throttle. Dual aileron servos, and never a flutter issue. Glad this route worked for you.

Just as a matter of personal preference, I put dual aileron servos in anything with a .45 or larger engine.