Kmot

My Feedback: (24)

When applying tape/monokote to a hinge line to seal it off to prevent flutter, do you cover both sides of the control surface hinge line or just one side?

campbec

Hi Kmot,

You only need to cover one side, usually the underside. Fully flex the control surface and affix the tape or money coat. Don't forget to secure the edges with clear nail polish etc, to stop it lifting in the air flow or ingress of fuel residue.

Cheers,

Colin

MinnFlyer

My Feedback: (4)

And don't do it to prevent flutter - it won't

It should be done if you have a large gap that is preventing adequate control response - OR - to equalize the response of the two ailerons.

I used to always seal my gaps, now I never bother.

rajul

My Feedback: (58)

Crease the strip of covering you're using along the middle line so that it forms a v shaped section. Then it will be easier to iron it down in the hinge gap.

Kmot

My Feedback: (24)

Thanks gents! [sm=thumbup.gif]

W4RCF

I can tell you that with a little electric ARF, sealing the elevator hinge line can be CRUCIAL. I almost lost my 300-E by not following directions and when I sealed it with dermo-plast tape, all problems went away! elevator was flexing due to extremely small rod that connects the two halves. The tape is virtually clear and lasts very well. Happy Flying!

Fred

dbacque

My Feedback: (3)

That's like saying that motorcycle riders shouldn't wear a helmet because they won't prevent deaths.

No, it won't prevent flutter. But it will help. There are many things that contribute to flutter. And reducing each one of them as much as possible will help to prevent flutter.

But the good news is that something as simple as sealing control surface gaps does many wonderful things for your aircraft. It reduces the throws required. It reduces drag. It makes the airfoils more effective and more consistent, especially when surfaces are at high deflection. And as an added bonus, it helps to reduce the risk of flutter.

When I've got over a grand and countless hours invested in an airplane, I find that the mere minutes required to seal the gaps to be well worth the effort.

Dave

NCIS

If you have Model Aviation's Sept 06 issue turn to page 67 and it tells you all about hinge sealing and how to do it. It also tells you how the leakage of air affects the plane. It does matter a lot and people that don't must not fly large scale 3D aircraft.

Gibbs

MinnFlyer

My Feedback: (4)

debacque, Did you know that sealing a hinge gap can CREATE flutter?

Flutter is caused by aerodynamic forces. Sealing a closed hinge gap MIGHT reduce it, but then, opening a sealed hinge gap might reduce it also.

By the same token, doing either just might do nothing. this is more likely the case.

Do not rely on the hinge gap to affect flutter. Keep your linkages tight, and use servos that are appropriate for the application and you won't have to worry about it.

dbacque

My Feedback: (3)

As I said, there are many things that contribute to flutter. And you're right, proper linkages and servos are the first step. But not the only step. And I never said to rely only on sealing the surfaces to reduce flutter. I said to reduce all contributing factors as much as possible.

Air leakage through a control surface gap changes the pressure differential between the two sides of the surface. Sometimes this happens in an oscillatory fashion. When this oscillation sets up, it can lead to flutter if the control system is not strong enough to resist the forces. So the point is that while a strong control system and stiff surfaces are important in keeping a pressure oscillation from turning into flutter, doing anything you can to help eliminate the possibility of aerodynamic oscillation is also a good idea.

Sealing a hinge gap can create flutter? I don't buy it.

Honestly, the most important reason to seal gaps is improved aircraft performance. But if you don't care to waste your time on such things, that's cool by me.

Dave

MinnFlyer

My Feedback: (4)

Then I suggest you study aerodynamic forces before you give an opinion as fact.

dbacque

My Feedback: (3)

I have studied aerodynamics. And I didn't just state an opinion, I gave an explanation. You're the one who stated opinions with no explanations.

I explained that air flowing through a control surface gap can cause changes in air pressure above and below the control surface and that some times these changes in pressure can oscillate. This produces a force that tries to move the control surface back and forth with the oscillation. If it hits a resonant frequency and the control system isn't up to the challenge, this can help initiate aerodynamic flutter.

Now, please explain how a sealed surface can cause flutter instead of just stating YOUR opinion as fact. Then explain why an oscillating pressure wave on the control surface won't impart an oscillating moment on the surface.

Dave

Newc

For what it's worth, I also would like to see the explanation for

MinnFlyer

My Feedback: (4)

Ok kids, let's go to school.

First, make a flag by gluing a piece of paper to a stick. Now hold it out the window of a moving car. There is no gap, yet the flag waves.

Why???

There is no gap to create an oscillating force!

This is impossible!!!

The flag waves due to aerodynamic forces.

Now, EVERYTHING in the world vibrates at a certain frequency - Right?

Ok, so now make your flag by hinging a stiff piece of wood to your stick (let's call this a rudder) and seal the gap. It is very possible that at some precise speed, and depending on the shape of your stick, and the relative humidity, and the phase of the moon, etc. that the rudder is going to flutter - right?

Or are you saying that this is impossible?

Ok, if you agree that this rudder CAN indeed flutter (And you should) then you can also see that while there must be certain critical conditions to MAKE it flutter, changing any of those conditions can also stop it.

Unsealing the gap will change those conditions. It will no longer flutter AT THOSE CONDITIONS.

But of course, flutter can NOW occur at other conditions, such as a different speed, humidity, etc, etc.

It's like a guitar string: Tune it to an "A" an it will vibrate at 440hz. IF you play an "A" on another instument, the guitar string will magically vibrate by itself (These BTW are aerodynamic forces acting upon the string). Now tune the string a little sharp, say 450hz Now if an "A" is played by another instrument, the string will NOT vibrate. However, play a note at 450hz and the string will again vibrate.

THEREFORE: Sealing a gap is NOT a good method of preventing flutter. It won't. All that does is move the flutter to another condition (Like another speed).

IF you have a gap, and you want more positive control, seal the gap.

IF you are looking to prevent flutter, make sure your connections are solid and your servos are strong enough.

dbacque

My Feedback: (3)

Absolutely! As I said, there are many things that contribute to flutter and we need to minimize all of them.

I never said that a gap was the only thing that causes flutter. But it can contribute to it. Doesn't always, but it can. And I explained one situation where it can.

Yes flutter can occur with a sealed gap. And it can occur with an unsealed gap. But the question is if the gap can help induce the flutter.

Will sealing gaps prevent all flutter? No. But it can help to prevent some cases of flutter. Combine that with solid control systems, eliminating flex in flight surfaces and control surfaces, braces when required, counter balances and other techniques and you can prevent most cases of flutter.

But you made the statement that sealing a gap can CAUSE flutter. Please give us an explanation of how this can happen. What aerodynamic force associated with a sealed hinge is going to cause flutter that wouldn't occur with an unsealed hinge.

Dave

da Rock

Don't need an explanation as I've seen it happen.

Guy at the field has a BIG ONE. Got gaps. It used to fly like a big one with gaps. Guy decides to make the airplane fly better and seals the gaps. First flight, flutter. What caused it? Sealing the gaps obviously. The theory? Who knows. It happened.

BTW, one of the first things they teach you about flutter, is that EVERYTHING has a flutter speed, even a bowling ball. Flags have been used forever by target shooters to indicate the wind speeds. When the flags starting being made out of lighter, slicker fabric they suddenly indicated the wind speed differently. Their flutter characteristics changed simply from better airflow or somesuch. So change the airflow over anything and it's flutter speed changes. ANY change can cause flutter. At a different speed.

Lighten up. This isn't a debate. It's still a discussion.

da Rock

It won't if the surface is constrained with sufficient strength to resist the oscillating forces.

And furthermore, if the oscillating forces were being degraded by the disruptive effects of blowthrough at the hingeline, and that disruptive effect is removed, the oscillating force might, just might then be able to flutter the surfaces. But probably wouldn't on model airplanes unless the rigging to that surface was sloppy.

MinnFlyer

My Feedback: (4)

Case closed

dbacque

My Feedback: (3)

Not obvious and not closed.

This is anecdotal evidence at best, not an explanation of how sealing a gap can induce flutter. There could have been other things done, even unintentionally, when the plane was worked on that caused the flutter. And even if sealing the hinge lines in this case did contribute to the flutter, that doesn't mean that leaving gaps open is the best practice.

You still have not answered how a gap can "CAUSE flutter". So I have forwarded the question to a friend who has a masters in aerospace engineering and 35 years experience designing aircraft, flight simulators and missiles for NASA and the military. He is out for a few days but when I hear back from him next week I will post his reply.

Dave

MinnFlyer

My Feedback: (4)

Read again. I DID explain how sealing a hinge gap can induce flutter.

darock PROVED that sealing a gap can induce flutter.

Ok, I give up. I tried

Kmot

My Feedback: (24)

Okay, so now instead of fluttering at 100MPH my plane will flutter only at 200MPH or better. Kewl. [sm=thumbup.gif]

MinnFlyer

My Feedback: (4)

Or, maybe 98mph

Kmot

My Feedback: (24)

lol...

aerowoof

this is one of those theory vs real life scenarios.the mail purpose for sealing a hinge line that has a large gap ie to increase the effectiveness of the deflected control surface.if you have a flutter problem then there are other things to do also.counterbalance the control surface,stiff linkages proper number of hinges.with all the reports of flutter and the widespread use of ca hinges,I wonder if there is a correlation between them and larger than recommended power plants contributing to the problem as it was not a common problem years ago.there were instances of flutter but were easily tracked down to sloppy linkages or improper hinging

rajul

My Feedback: (58)

Another factor in flutter is the cg of the control surface itself. In full scale aircraft, weights are sometimes added to the control surface to bring its cg closer to the trailing edge of the wing/elevator/vert fin