Flapping bi-wings

[plug&fly]

I just got a multiplex Gemini. It is a biplane. It flew very well until...I crashed into a zebra crossing pole. The wings were dislocated but I have since repaired it. Now tragically, the wings flaps violently whenever I reach a certain airspeed. When I dive or push the throttle, the wings flap so vigorously, I killed 4 aileron servos already. The wings feel solid, no soft spots and the ailerons and the wing have no gaps in between. It is a mystery to me why the entire four wings flap. Can any of the gurus help? Please, otherwise I have to junk this beauty.

[mave]

Alamak Bro, what happen?

Got change propeller to see if it helps?

I just got a multiplex Gemini. It is a biplane. It flew very well until...I crashed into a zebra crossing pole. The wings were dislocated but I have since repaired it. Now tragically, the wings flaps violently whenever I reach a certain airspeed. When I dive or push the throttle, the wings flap so vigorously, I killed 4 aileron servos already. The wings feel solid, no soft spots and the ailerons and the wing have no gaps in between. It is a mystery to me why the entire four wings flap. Can any of the gurus help? Please, otherwise I have to junk this beauty.

[msc]

The *#$!* red post at the end of the runway hit the wings of my Formosa. It took me a while to do the repair. I have not flown it yet.

Time to petition for the removal of all vertical posts along the runway??

MS

[Sunstorm]

Looks like this is fast becoming a 'I hit the pole at PFW' thread.

I hit the signboard too with my cessna trainer while coming in for a landing.

[msc]

Looks like this is fast becoming a 'I hit the pole at PFW' thread.

I hit the signboard too with my cessna trainer while coming in for a landing.

Except in my case, I am very sure that the post hit my plane!!

[mave]

ha...ha...

Except in my case, I am very sure that the post hit my plane!!

[plug&fly]

I am asking for expert aerodynamics help and ....

all you guys can talk about is the lamppost...thanks guys...but I am trying to solve a problem here...

[msc]

all you guys can talk about is the lamppost...thanks guys...but I am trying to solve a problem here...

Hi Chegu, we are talking about the solution to the root cause of the problem.

If the lamp post didn't hit your plane, then your wings will not be damaged in the first place, right?
Many of us have the same problem here...and the solution is clear.

MS

[Sunstorm]

Reinforce with carbon or wood?

That's the solution to the Zagi flap.

Cos maybe after the crash, the internal structure of foam has softened?

Guessing. Shots in the dark.

Back to lamp post and signage.

Should make marks there everytime a plane strikes.

[renold]

Is there any flyer who have not hit any post at all when flying in PFW after say, a year or more. Then this flyer must be No. 1.

[collectivepitch]

Just guessing only.
Maybe it resonance at that particular speed after the crash.
Change a different prop size and see if it helps.

Lamppost.....

[joe yap]

Chekgu,

High speed flutter is caused by aft CG of the flying surfaces along with sloppy linkages.

All large unbalanced control surfaces are prone to this but stiff linkages can delay it from happening. After some hard arrivals, slops inflicted can aggrevate it. This is exactly the same phenonmenon as those push trolleys with castering wheels. If you push them fast enough, they'll start to oscillate violently. To solve this, first, take out the control slops. To eliminate the problem, you can balance the control surface.

To balance the control surface means bringing the CG of it to the hinge line. If done properly, the control surface can even stay horizontally on its own without the control linkages connected. Some aileron designs have extension at the tip and go beyond the hinge line. A small weight is attached to the leading edge of the extension to bring the CG to the hinge line.

[leonlio]

Chekgu,

High speed flutter is caused by aft CG of the flying surfaces along with sloppy linkages.

All large unbalanced control surfaces are prone to this but stiff linkages can delay it from happening. After some hard arrivals, slops inflicted can aggrevate it. This is exactly the same phenonmenon as those push trolleys with castering wheels. If you push them fast enough, they'll start to oscillate violently. To solve this, first, take out the control slops. To eliminate the problem, you can balance the control surface.

To balance the control surface means bringing the CG of it to the hinge line. If done properly, the control surface can even stay horizontally on its own without the control linkages connected. Some aileron designs have extension at the tip and go beyond the hinge line. A small weight is attached to the leading edge of the extension to bring the CG to the hinge line.

Just for everyones interest... some of the real World War II planes designs counter-weights to reduce control surface flutters.

In this case its the elevators... notice the small counter-balancing weight attached to the top/bottom of the elevator?

Chekgu, I think for your case its still the slop issue. Remember Biplane = 4 aileron surfaces!

Waiting for you to get it fix to do Gemini Formation...

...till then...
Cheers,
Leon

[plug&fly]

Ok Thanks, Joe

But does this phenomena involves the entire wing fluttering or just the control surfaces? In this case, all four wings flutter. Could it be that the incidences of the top and bottom wings aren't the same. I measured it, the te and le distances are equal. I noticed that the wing root connections were weakened and the wings may rock about 0.5mm.

[joe yap]

Chekgu,

The flutter usually starts from the control surfaces. But if the wing structure is not rigid enough, it will try to cancel out the control surface movements by flexing in the opposite directions. Thus, the wings can also seen as fluttering.

In fact, some full size pilot do encounter 'aileron reversals' particularly at higher airspeeds and less rigid wing structures. When an aileron moves down, the wing will tend to twist downwards and when it moves up, the wing will tend to twist upwrds, creating opposite roll effect.

Sounds funny right? But remember that servo tabs on control surfaces do exactly the same thing,isn't it? When an elevator servo tab is deflected downwards, it'll push the TE of the elevator upwards, deflecting the elevator upwards. The net result is that the airplane pitches its nose up.

What is happening to your wings is that they keep doing this in a numerous cycles within a very short period of time. Thus, giving you this superb Ricky Martins gyrations.