Possible Trim Tab Flutter at 135kt

[dick747]

There are many forms of flutter. Control surface flutter normally does not involve any twisting of the control surface. Usually just deflection of the control surface against the control system or the supports.

[FlyingMonkey]

An article on aerodynamic flutter I read in Kitplanes magazine said that flutter requires forces acting in more than one axis.

[FlyingMonkey]

From the Kitplanes article:

 

Balancing control surfaces requires care and, often, special tools to properly support the surface and yet allow it to move with little friction. Here, the expected weight of the paint to come is temporarily applied to help get the basic balance right. Final balancing must be done after the paint is applied.

 

Last month we started a look at aeroelasticity. As we saw, airplanes are not perfectly rigid. No matter how stiff the structure is, it does distort under the influence of aerodynamic forces. And structures do not just deflect; they can also vibrate or oscillate.

 

Flutter is a sustained or increasing oscillation of the airplane’s structure that is driven by aerodynamic forces. Sometimes flutter appears as a sustained limit cycle oscillation that maintains a steady amplitude, but does not damp out. Such a flutter is often referred to as “buzz.” It is undesirable because of the cyclic loads it puts on the airframe, which can cause failures due to fatigue or working fasteners loose. In general, having a sustained aerodynamically driven oscillation in the airframe structure is considered unacceptable for safe operation of the airplane.

 

In more severe cases, the flutter oscillation does not reach a limiting steady-state amplitude, but grows rapidly once the oscillation begins. This situation is extremely dangerous, because if the building flutter is not stopped quickly, it will cause catastrophic structural failure of the airframe.

Mechanism

 

A structure has multiple degrees of freedom, which lead to multiple modes of deflection/vibration. For example, on a cantilever wing the first two structural modes are wing bending, where the wing bends up or down like a diving board, and wing torsion, where the wingtip twists relative to the root. Each structural mode has a natural frequency at which it vibrates when it is excited by a force. The natural frequency is determined by the stiffness of the structure and its mass and inertia. In some modes, where the aerodynamic force that arises from deflection acts like a centering spring (aileron deflection, for example), the frequency is also a function of airspeed.

 

For flutter to occur, two conditions must be met. First, two modes must have the same natural frequency. When the natural frequency of two modes is the same, the motions of the modes couple, allowing one mode to directly affect the other. The second condition for flutter is that deflection in one mode causes forces that drive a deflection in the second mode. For example, wing twist changes the lift of the outer portion of the wing, which in turn changes the bending moment on the wing. Accordingly, a change in wing twist (one mode) causes a change in wing bending (other mode). If the natural frequencies of the first wing-bending mode and the first wing torsional mode are the same, there is a potential for flutter.

Other factors affecting the onset of flutter are the amount of damping in the system and the magnitude of the driving aerodynamic forces. Damping absorbs energy while opposing the motion of the structure. An example of this is friction in a control-surface hinge or linkage. When the surface moves, the friction opposes the motion and absorbs some of the energy trying to make the surface move. The faster the surface moves, the more energy the friction dissipates. Because it tends to reduce the motion of the structure, damping delays flutter onset.

 

The aerodynamic forces that drive the deflections increase as a function of the dynamic pressure, which is proportional to airspeed squared. The higher the airspeed, the higher the aerodynamic force, and the more energy the aerodynamic forces can provide to drive the structural oscillation.

 

 

Mass-balance weights can help increase flutter margins.

Flutter Onset

 

As airspeed increases, two things happen: The magnitude of the aerodynamic forces on the airplane increase, and the natural frequency of some of the deflection modes changes, typically increasing with increasing airspeed. If the natural frequency of the right two modes come together at an airspeed that is high enough for the aerodynamic forces to overpower the damping in the system, the conditions for flutter exist.

 

Full article:

 

http://www.kitplanes.com/issues/27_3/designers_notebook/Structural_origins_of_flutter_9130-1.html?zkPrintable=true

[dick747]

This article describes what you might call conventional flutter particularly of a wing. Mass distribution, stiffness and aerodynamics all play a role. Swept wings in particular have coupled bending torsion modes that can be susceptible to flutter. Flutter does not require "two modes with the same natural frequency" as the article states. I have seen leading edge slat flutter involving single axis deflection of the slat basically resulting from inadequate stiffness of the support arms. I have also seen loss of most of a vertical tail that involved not only the tail but single axis rotation of the rudder and interaction with the rudder actuation system.

Flutter modes can be simple or very complex involving multiple axis deflections.

[FlyingMonkey]

This article describes what you might call conventional flutter particularly of a wing. Mass distribution, stiffness and aerodynamics all play a role. Swept wings in particular have coupled bending torsion modes that can be susceptible to flutter. Flutter does not require "two modes with the same natural frequency" as the article states. I have seen leading edge slat flutter involving single axis deflection of the slat basically resulting from inadequate stiffness of the support arms. I have also seen loss of most of a vertical tail that involved not only the tail but single axis rotation of the rudder and interaction with the rudder actuation system.Flutter modes can be simple or very complex involving multiple axis deflections.

Hmm...I guess my question is how can you tell from watching flutter what forces are acting on it? It's not like there are big arrows showing the forces involved. Just because something moves in one direction, does not mean the only force causing the movement is in that axis.

[IrishAl]

This is probably the most famous example of flutter in the world.

 

Even today, engineers can't agree over the precise cause, so it's a pretty complex subject.

 

https://www.youtube.com/watch?v=xox9BVSu7Ok

 

The bridge didn't have the option of changing the wind speed, but - mercifully - pilots do.

[floats]

Hey all...in the the last couple of weeks I have noticed that at higher speeds my CTSW has some airframe vibration that feels like fast buffeting.  It starts to come on about 125-130 knots indicated, and by 135 knots indicated it's very strong and downright unnerving, shaking the whole airframe.

 

At first I thought the wheelpants might be shaking, so I removed them.  The problem did not go away.  Afraid this might be some type of flutter, I mounted a GoPro pointing to the tail and did some runs up to 135 KIAS.  Below is a video of one of the runs.  The video has a little shake to it, but at about 12sec into the video, it looks to me like I'm seeing trim tab flutter developing.  The trim tab seems to flap up and down pretty strongly until I reduce speed toward the end of the video, then it settles back down.

 

The Vne of the CTSW is 145kt, so I don't think I should be seeing flutter of any aerodynamic surface below that speed (or actually below Vne + 10% safety margin).  Has anybody else see this before?  What might cause it, slack in the trim cables or something else?

I I can't find the video

[FlyingMonkey]

We got the stabilator back on the airplane yesterday.  While the plane was down we also did the annual, so both the stab re-install and annual were signed off by the A&P.  The airplane should be done with scheduled maintenance in a few days, once I change the oil (which is due in 8 hours) and change tires (again...my airplane is still eating tires quickly, that will have to get addressed at some point).

 

Once I fly it around the pattern again to remember how to fly (it's been six weeks!), I'll slowly start edging up to higher speeds and report my findings. 

[Runtoeat]

Hi Andy. Take a couple of pics when you get the stab back on.

[FlyingMonkey]

It's on, Dick.  Let me know what if you want pics of specific areas/items.

[Runtoeat]

Just wondering if any areas of the stab look different.  If it's the same externally, no need for photos.  Thanks. 

[FlyingMonkey]

Just wondering if any areas of the stab look different.  If it's the same externally, no need for photos.  Thanks. 

 

The same externally.  They reinforced the internal structure of the stab, I think the same as yours.  There was some talk of shortening the trim tab, but the factory didn't think it was necessary and I really don't like the look of the hacked up trim tab the service instructions showed.

 

One change I noted was that on the bottom of the stab, instead of bolus tape to seal the gap, they used tape on the stab side that holds a still silicone rubber strip that seals over the gap and is free on the trim tab side.  I'll take a pic of it if that's hard to visualize.

 

Oh, and they replaced the hinges, so now my stab has the heads of the drilled out rivets rattling around inside.  You can't hear them with the stab on, but they sure made a lot of noise moving the stab around the hangar.  Unfortunate, but I don't think there'd be any way to get them out without cutting the stab open.

[Tom Baker]

 

 

One change I noted was that on the bottom of the stab, instead of bolus tape to seal the gap, they used tape on the stab side that holds a still silicone rubber strip that seals over the gap and is free on the trim tab side.  I'll take a pic of it if that's hard to visualize.

 

 

 

There never should have been Bolus tape on the bottom of the stab. The factory had at least 2 different ways of putting seals on the tab, and neither one included Bolus on the bottom.

[FlyingMonkey]

There never should have been Bolus tape on the bottom of the stab. The factory had at least 2 different ways of putting seals on the tab, and neither one included Bolus on the bottom.

 

Mine never had tape on the bottom, but I thought I had seen Roger recommend tape top and bottom.

[Roger Lee]

Some CT's came with nothing on the bottom gap, some came with the plastic strip Andy is talking about. That may last for years. On the ones that have nothing or the strip came off there is nothing wrong with using Bolus tape on the bottom to seal that gap. Many have had that like that for 8 years. Doesn't hurt or affect anything and seals that gap just as the plastic strip does. The top gaps came with cloth tape or plastic stripping to. 1" Bolus tape does a great job of sealing that gap also. When applied correctly both should last at least two years or more. It only takes a few minutes to apply. When applying the top tape make sure the stab is in the full down position and when applying the bottom tape the stab is in the full up position. This will keep the tape from stretching and relaxed while in use.

I must know at least 30 or more CTpeople using Bolus tape for these gaps, probably more US and worldwide.

 

If you want to leave it open then no problem. I prefer to have it sealed. FD sealed some and others they didn't.

[CT4ME]

i wonder if sealing can add a bit of  speed...???  most of the "speed mods" i've seen over the years included sealing those gaps...

[Runtoeat]

The factory tape installed on the bottom of the stab consists of a thin strip of a "bolus" type of tape that is adhered to a strip of mylar type plastic.  The tape is attached to the stab and the mylar rubs the trim tab.  This functions similar to the gap seals made from aluminum found on GA.

[FlyingMonkey]

The factory tape installed on the bottom of the stab consists of a thin strip of a "bolus" type of tape that is adhered to a strip of mylar type plastic.  The tape is attached to the stab and the mylar rubs the trim tab.  This functions similar to the gap seals made from aluminum found on GA.

 

That's what they did to mine.  The strip that you call mylar looks to me like still silicone plastic/rubber, but I'm not a plastic materials engineer...  

 

Before I sent the stab to FD it had nothing on the bottom of the gap, it was open.  Sealing that *has* to be worth 10 knots, right??  

 

BTW, has anybody used bolus tape on the gaps where the gear legs meet the fuselage, and between the gear legs and the wheel pants?  Those areas look kind of "draggy". 

[Roger Lee]

The plastic strip on the bottom is stock FD.

[sandpiper]

 

 

BTW, has anybody used bolus tape on the gaps where the gear legs meet the fuselage, and between the gear legs and the wheel pants?  Those areas look kind of "draggy". 

 

All the time. Came new that way.

[Roger Lee]

Hi Andy,

 

This is one of those personal preference items. Some tape them some not.

[FlyingMonkey]

Perfect, smooth weather today. I took the plane up for its second flight. I slowly increased speed up to 140kt indicated, 145kt true. There was none of the previous vibration at those speeds, so I'm guessing the problem is solved. I will post here if any of these problems occur again.

[Runtoeat]

HI Andy.  I think we're all relieved.  I'm thinking it's the looseness in the system that is the culprit but we'll probably never know for sure.  Did you get up to speed by using a shallow dive without power or did you have power on during the high speed?

[FlyingMonkey]

HI Andy.  I think we're all relieved.  I'm thinking it's the looseness in the system that is the culprit but we'll probably never know for sure.  Did you get up to speed by using a shallow dive without power or did you have power on during the high speed?

Hey Dick, because of our conversation I made sure to do the test in a shallow dive adjusting to maintain ~5500rpm.

 

My A&P suspects the possibility of a marginal factory stab balance, and that the rebalance done after the work was complete might have helped as much as the structural reinforcement.

[Runtoeat]

Great!  Lots of good flying weather coming up!