Flutter

[bd32322]

Feel free to explain your answer

[takair]

One of the factors in establishing Vne is flutter. Vne, for our aircraft, is indicated airspeed.

[Jamie]

Can't be anything other than true, unless someone can explain how the thing fluttering can see the airspeed indicator.

[bd32322]

I am hoping for a 50/50 split on this one and no googling please

[M016576]

It's indicated airspeed. That's the speed that the plane "feels". True airspeed is an adjustment of calibrated (indicated) airspeed to give you an accurate measure of speed in relation to the air mass, but at the speeds all our aircraft operate at compressability of air isn't a factor. All our aircraft cares about is the dynamic pressure on its control surfaces / airframe... And that's what's measured by the Airspeed Indicator (ie, indicated airspeed). That is why all our limitations are written in indicated airspeed as well. If we needed to factor in faster speeds, our limitations would be written in IMN. The one place where almost all aircraft use the ASI is for landing gear and flaps extention speeds... As those are typically all speeds where the compressability of air isn't a factor.

[carqwik]

It's true airspeed.  Flutter is a function of resonant frequency which is a function of the speed at which the molecules of air pass by a surface and excite the resonance.  That speed is true airspeed.

[M016576]

Can't be anything other than true, unless someone can explain how the thing fluttering can see the airspeed indicator.

It's a matter of what the air speed indicator is measuring, vs what you are calculating for TAS. The aircraft can't see the ground, the air or the airspeed... It just knows pressure (ie force)

Edit: upon reviewing brown's link to the vans description- I am wrong. It's the TAS not IAS for flutter.

[RJBrown]

Go to Vans website and read his explanation,http://www.vansaircraft.com/pdf/hp_limts.pdf

[M016576]

Deleted- see next one

[M016576]

Go to Vans website and read his explanation,http://www.vansaircraft.com/pdf/hp_limts.pdf

I stand corrected- I must have slept through that day of class!

[bd32322]

Damn! You guys are a knowledgable bunch!

Yeah I found it hard to believe it was TAS too ...

[M016576]

Try again, M016576.

Flutter is a function of true airspeed. I know this because I have read several articles about the danger of turbocharging homebuilts and flying them higher than the manufacturer intended. Most notably by Dick VanGunsven. But I can't explain the science behind it from memory.

Jim

I stand corrected!

[takair]

In my own defense, it can be said that on a STOCK Mooney, staying within our limitations.....including indicated Vne.....and a well maintained aircraft, you should not see flutter. Modify the airplane or have slop in the controls and all bets are off. Get into jets and the Vne is adjusted as to the TAS/environment. By the way, good article, but that's cheating.

[aviatoreb]

Unless you are a flight engineer designing an airplane - we don't need to worry about if flutter is TAS or IAS or whatever. (But as the VANS guy points out, a homebuilder is a flight engineer...).

The flight engineers in a certified airplane did the work for us. If you stay below Vne and within certified altitude, you will not flutter. End of story.

I have read, repeating from memory, but I cannot confirm that certifying an airplane flights involve demonstrating that the airplane is free of flutter to some speed - 130% of Vne within the flight envelope. So Vne is marked in IAS, so 196IAS on my airplane. But the higher you go, the higher your TAS at a given IAS. And the physics of flutter cares about TAS. But the pilot can only see IAS and certified ceiling for my M20K is FL24. So they resolve that by demonstrating free of flutter at max certified altitude at Vne and I believe at 130% of Vne.

That's how the FAA mind works - the one little red line on your airspeed indicator is all you need to know.... Punchline - stay below Vne and within ceiling and you are good to go, from flutter perspective....in smooth air..yadyada or you then if in rough air you have yellow lines, green lines, Va and more to worry about.

High aspect wings by the way are more likely to flutter at a given speed and given stiffness/build. High aspect means long and skinny wings. Long and skinny wings are most often used in glider applications - also wind turbine applications....

Here is a great (scary) video of flutter in a glider. Those guys go to FL30 and get some terrific speeds in those long skinny wings.

Flutter by the way is mathematically due to a phenomenon called "Hopf Bifurcation" which is where so many oscillations you see in nature arise, from biology, epidemiology, structures, electronics,....

Bridges and buildings can flutter too by the way.

[N601RX]

A Mooney has a lot of natural resistance to flutter built into them with the push/pull tub control system.  As long as the rod ends are good, there isn't any slop in the system.  Planes that use pull/pull cables are more prone to flutter especially if the cables are not tight.

[aviatoreb]

A Mooney has a lot of natural resistance to flutter built into them with the push/pull tub control system.  As long as the rod ends are good, there isn't any slop in the system.  Planes that use pull/pull cables are more prone to flutter especially if the cables are not tight.

 

That is not correct.  Flutter comes from the structure (how strong the build) and the shape (length to width ratio of the wing).  The rods or cables, whichever, are not enough part of the strengthening of a wing to noticably change the flutter speed.  As I said, even tall buildings or bridges can flutter.  Mooney wings are built strong which is why we part of why we have decently high Vne.  The nice control rods to lend to better control feel and less delay in how we actuate our controllers - which means ailerons and so forth - so better "road feel".

[N601RX]

Flutter comes from the stiffness of a system and the natural resonate frequency of it, not strength of it.  Wing flutter is caused by the undampened bouncing of the aileron at it resonate frequency. The stiffness of the pushrod system will help dampen it and the springiness of loose control cables will allow it to increase in amplitude and frequency until it tears itself apart .

 

Counterbalance is the generally accepted way to prevent it.

[bd32322]

Flutter comes from the stiffness of a system and the natural resonate frequency of it, not strength of it.  Wing flutter is caused by the undampened bouncing of the aileron at it resonate frequency. The stiffness of the pushrod system will help dampen it and the springiness of loose control cables will allow it to increase in amplitude and frequency until it tears itself apart .

 

Counterbalance is the generally accepted way to prevent it.

 

 

I wasn't trying to instill doubt into the safety of airframes. I just thought that theoretically it was curious that flutter is dependent on TAS and not IAS. Your Mooney will not experience airframe flutter unless you go outside its limits.

 

Having said that - according to your explanation the Tacoma narrows bridge should have never violently oscillated and crashed. It had very strong steel supports.

 

Propellers can also induce flutter although they are very strongly attached and balanced to the engine .. that's why experimental folks prefer to use known engine prop combinations if they value their lives because the airframe engineers on certified aircraft have done the design work of which props are flutter free with which engines.

 

I think you are thinking of flutter as the dictionary would describe it - like you said motion of something that is out of balance. The correct term according to this thread should be resonance. Wikipedia explains a bit of that if you search for resonance.

[bd32322]

Unless you are a flight engineer designing an airplane - we don't need to worry about if flutter is TAS or IAS or whatever. (But as the VANS guy points out, a homebuilder is a flight engineer...).

The flight engineers in a certified airplane did the work for us. If you stay below Vne and within certified altitude, you will not flutter. End of story.

I have read, repeating from memory, but I cannot confirm that certifying an airplane flights involve demonstrating that the airplane is free of flutter to some speed - 130% of Vne within the flight envelope. So Vne is marked in IAS, so 196IAS on my airplane. But the higher you go, the higher your TAS at a given IAS. And the physics of flutter cares about TAS. But the pilot can only see IAS and certified ceiling for my M20K is FL24. So they resolve that by demonstrating free of flutter at max certified altitude at Vne and I believe at 130% of Vne.

That's how the FAA mind works - the one little red line on your airspeed indicator is all you need to know.... Punchline - stay below Vne and within ceiling and you are good to go, from flutter perspective....in smooth air..yadyada or you then if in rough air you have yellow lines, green lines, Va and more to worry about.

High aspect wings by the way are more likely to flutter at a given speed and given stiffness/build. High aspect means long and skinny wings. Long and skinny wings are most often used in glider applications - also wind turbine applications....

Here is a great (scary) video of flutter in a glider. Those guys go to FL30 and get some terrific speeds in those long skinny wings.

Flutter by the way is mathematically due to a phenomenon called "Hopf Bifurcation" which is where so many oscillations you see in nature arise, from biology, epidemiology, structures, electronics,....

Bridges and buildings can flutter too by the way.

 

 

Holy cow! That's some video! Hopefully that's a test pilot with a parachute on his back

[KSMooniac]

Some of what has been written above it true, and some is close but not quite correct.

 

Flutter happens in a variety of ways, and to a variety of structures.  Buildings, bridges, road signs, etc. are all susceptible to it, but since we're just concerned with airplanes I'll try to stay in that sandbox.  Airplanes can experience flutter due to engine/prop/engine mount combinations (likewise turbofan engines+pylons), as well as flying surface flutter either due to the structure of the wing/stabilizer alone, or in combination with a movable control surface.  The math gets really tricky (and sadistically fun for a few of us!) because there are a variety of inputs into the equation, and these inputs come from a variety of engineering disciplines.  

 

Looking at a wing, you can have flutter due to vertical movement or due to twist (torsion), or both combined.  This may or may not be dependent on control surface movements or defects/wear in the control system.  Ingredients that go in to the flutter analysis include the wing stiffness (both in bending and torsion), structural damping characteristics, control surface effectiveness and damping, and the aerodynamic response of the wing at various speeds and angles of attack.  Imagine a wing at a speed and at an angle of attack... add a vertical gust to the scenario and you get some wing response that normally we feel as a bump that quickly damps out and the wing returns to it's state before the gust.  Now, in a flutter situation, that gust might start a reaction that causes a vibration in the wing that doesn't damp out, or in fact grows to the point the wing fails in bending or torsion.  The gust can lead to a higher angle of attack which leads to more air load until the wing stalls or the structural response forces it to return to a lower angle of attack.  Or it can keep feeding the situation until things break, and it can happen very, very quickly.  Control surfaces can induce this phenomena, but it can also happen without their involvement.

 

And yes, it is a function of TAS, and it was correctly mentioned above just stay below Vne within the published envelope and you should be fine, unless there is something wrong with the plane.  The flutter margin will be higher at Vne down low compared to Vne at the service ceiling.  We enjoy a very robust airframe in the Mooney world, while others do not.  

[aviatoreb]

Some of what has been written above it true, and some is close but not quite correct.

 

Flutter happens in a variety of ways, and to a variety of structures.  Buildings, bridges, road signs, etc. are all susceptible to it, but since we're just concerned with airplanes I'll try to stay in that sandbox.  Airplanes can experience flutter due to engine/prop/engine mount combinations (likewise turbofan engines+pylons), as well as flying surface flutter either due to the structure of the wing/stabilizer alone, or in combination with a movable control surface.  The math gets really tricky (and sadistically fun for a few of us!) because there are a variety of inputs into the equation, and these inputs come from a variety of engineering disciplines.  

 

Looking at a wing, you can have flutter due to vertical movement or due to twist (torsion), or both combined.  This may or may not be dependent on control surface movements or defects/wear in the control system.  Ingredients that go in to the flutter analysis include the wing stiffness (both in bending and torsion), structural damping characteristics, control surface effectiveness and damping, and the aerodynamic response of the wing at various speeds and angles of attack.  Imagine a wing at a speed and at an angle of attack... add a vertical gust to the scenario and you get some wing response that normally we feel as a bump that quickly damps out and the wing returns to it's state before the gust.  Now, in a flutter situation, that gust might start a reaction that causes a vibration in the wing that doesn't damp out, or in fact grows to the point the wing fails in bending or torsion.  The gust can lead to a higher angle of attack which leads to more air load until the wing stalls or the structural response forces it to return to a lower angle of attack.  Or it can keep feeding the situation until things break, and it can happen very, very quickly.  Control surfaces can induce this phenomena, but it can also happen without their involvement.

 

And yes, it is a function of TAS, and it was correctly mentioned above just stay below Vne within the published envelope and you should be fine, unless there is something wrong with the plane.  The flutter margin will be higher at Vne down low compared to Vne at the service ceiling.  We enjoy a very robust airframe in the Mooney world, while others do not.  

 

Yes!

 

And every airfoil has a critical flutter speed. It is a critical transition meaning below that speed, disturbances will naturally decay to a stead state although deflected in the case of a wing, state, and above that speed they will natural build away from that steady state which still exists but is not unstable.  But they may build to a limit cycle.

 

And flutter is a Hopf Bifurcation phenomenon.  That is a nonlinear bifurcation phenomenon.

 

The nasty danger of flutter is that if you cross it in smooth air, then the steady state is still a steady state but now it is unstable, but you wouldn't know....until you continue to go faster, and the steady state is still unstable but more so, and you still don't know, but then you hit the slightest puff of turbulence and your wings fall off - poof - very fast.  

 

Having said that - according to your explanation the Tacoma narrows bridge should have never violently oscillated and crashed. It had very strong steel supports.

 

Theoretically EVERY air foil, speaking broadly to include bridges, buildings, propellors and so forth, will pass through the flutter phenomenon at some critical speed.  The question is what is that speed.  It can be predicted to some degree theoretically for not too complicated structures analytically, for a bit more complicated structures by computational simulation, but when it is important you go to a wind tunnel. These days we have wind tunnels for airplanes, other wind tunnels for buildings and so forth.  You don't just put up a $1B building in NYC these days and hope the wind around all the other buildings around it doesn't do something bad.  You test it and test it in relationship to the other building around. 

 

Tacoma Narrows was build in 1940 I think. The source of flutter was entirely unknown as far as I know at that time, and this kind of testing was not the norm.  And that bridge, Galloping Gertie, she fell down....

 

 

I think you are thinking of flutter as the dictionary would describe it - like you said motion of something that is out of balance. The correct term according to this thread should be resonance. Wikipedia explains a bit of that if you search for resonance.

 

Flutter is not a resonance phenomenon.  Resonance is a linear phenomenon.  Hopf bifurcation is a nonlinear bifurcation phenomenon.  They are similar in spirit in terms of the outcome but the details is completely different.   Critical phenomenon like this can be quite dramatic in that they do not build gradually in intensity but can be more like a light switch - below the critical value, no problem, above the critical value - you are having a very bad day.

 

[bd32322]

Interesting to think that rockets going into space have to be designed especially carefully given the TAS they reach while passing through the atmosphere !!

 

Our mooney speeds pale in comparison.

[jetdriven]

That maximum dynamic pressure is called "Max-Q", which was the reason the space shuttle reduced power for a few moments. After a few more thousand feet of altitude, they would throttle back up.

Jets aren't so much TAS limited as they are Mach limited. Funny things happen when parts of the airframe break the speed of sound as well.

[carqwik]

Interesting link discussing aerodynamic flutter:

 

http://www.cs.wright.edu/~jslater/SDTCOutreachWebsite/aerodynamic_flutter_banner.pdf

[smccray]

Some of what has been written above it true, and some is close but not quite correct.

 

[/snip]

 

The last of your aeronautical engineering posts is how I knew the answer to this poll without using google.