Flutter - which aircraft are susceptible?

Morne

Morne

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Morne
In a recent thread someone mentioned about how flutter at higher altitudes, even below indicated Vne, can be rather bad for certain aircraft. My question is, "Which aircraft are susceptible to flutter below indicated Vne?"

If someone wants to give a layman's definition of flutter, that'd be great, but I understand the gist of it.

Also, is it certain types of airfoils that are more susceptible to it than others? Or certain wing loadings? What are the bricks on this paved road?
 
It's mostly a function of the aggregate stiffness of the component (wing structure et al) and the arm between the center of twist and the center of lift. If said component is excited at one of its harmonic frequencies and that's part of it's flight envelope, watch out. It could be as benign as vibration of consequence only on fatigue life, or it could be catastrophic diverging amplitude self-oscillation that leads to almost-immediate failure.

Bottom line, don't cross the red line, otherwise you're a test pilot. If an aircraft exhibits the latter kind of flutter inside of the operating envelope, somebody got some esplaining to do to the FAA. :D
 
Somewhere on the internet there is a video of a Vari EZ experiencing flutter on its canard. Very interesting video. Impressively, the plane held together. You can see dramatic flexing of the canard on video shot from the ground. That illustration is probably one of the best.

I thought flutter was generally a control surface issue, not necessarily related to the airfoil? I have limited knowledge on the subject, but I believe it occurs typically due to problems with control surface balancing and other control systems issues (slack in cables or bushings, etc.). I believe the weights you see on ailerons & elevators are there primarily to prevent flutter by changing the resonant frequency of the control surface (half guessing).
 
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Morne said:
In a recent thread someone mentioned about how flutter at higher altitudes, even below indicated Vne, can be rather bad for certain aircraft. My question is, "Which aircraft are susceptible to flutter below indicated Vne?"

If someone wants to give a layman's definition of flutter, that'd be great, but I understand the gist of it.

Also, is it certain types of airfoils that are more susceptible to it than others? Or certain wing loadings? What are the bricks on this paved road?
Click to expand...

Here is a good video on flutter:

http://www.youtube.com/watch?v=qpJBvQXQC2M

The second part is interesting showing flutter destroying the Tacoma Narrows bridge.
 
Bonanzas.

Under the FLUTTER section of the Beechcraft Single Engine (Piston) Safety Information guide, the following information is provided in case excessive vibration in the controls is encountered:

"If an excessive vibration, particularly in the control column and rudder pedals, is encountered in flight, this may be the onset of flutter and the procedure to follow is:

1. IMMEDIATELY REDUCE AIRSPEED (lower the landing gear if necessary).

2. RESTRAIN THE CONTROLS OF THE AIRPLANE UNTIL THE VIBRATION CEASES.

3. FLY AT THE REDUCED AIRSPEED AND LAND AT THE NEAREST SUITABLE AIRPORT.

4. HAVE THE AIRPLANE INSPECTED FOR AIRFRAME DAMAGE, CONTROL SURFACE ATTACHING HARDWARE CONDITION/SECURITY, TRIM TAB FREE PLAY, PROPER CONTROL CABLE TENSION, AND CONTROL SURFACE BALANCE BY ANOTHER MECHANIC WHO IS FULLY QUALIFIED."
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http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001208X07676&ntsbno=CHI97IA122&akey=1
 
You'd think you'd get any aircraft to flutter under the right conditions.
 
DavidWhite said:
Vans says not to put turbocharged engines in their planes due to flutter.
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I'm surprised the builder can't just throw a few extra pop rivets in it if they want to turbo charge or put turbine power in it?
 
They could. Some RV folks have bumped the gwt up to 2850 from 2700 by strengthening stuff. Vans doesn't recommend it.
 
dell30rb said:
That's interesting, cause the other day I was thinking a turbo RV would be a pretty sweet ride
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I thought the same thing. I have the NA RV-7A up to 17,500' without any problems, so I thought a turbo'd RV would be the bees knees. There are several discussions on VansAirforce.net discussing flutter at higher altitudes where the turbo would get its optimal usefulness.
 
Van isn't the manufacturer. He's just the designer and parts builder. the RV builder is the manufacturer (and test pilot, etc....).
 
flyingron said:
Van isn't the manufacturer. He's just the designer and parts builder. the RV builder is the manufacturer (and test pilot, etc....).
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That is true, except for the LSA RV-12. You become a licensee to built the plane. ;)

However, most RV builders follow the plans and respect the designers limitations on HP, engine selection, and air speed restrictions.
 
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Alexb2000 said:
I'm surprised the builder can't just throw a few extra pop rivets in it if they want to turbo charge or put turbine power in it?
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They can, it is called a Harmon Rocket. :hairraise: ;)

They will lite your hair on fire. :yes:

But that does not change VNE. ;)
 
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Alexb2000 said:
I'm surprised the builder can't just throw a few extra pop rivets in it if they want to turbo charge or put turbine power in it?
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It's not that simple. Aeroelastic effects are not merely only a function of component strength and stiffness. The time-dependent interactions of aerodynamic loading as a sub-function of component twist (which straight mickey mouse aerodynamics effectively dismiss at "low airspeed" by putting a ZERO on the equation) are such that more extensive flight testing is required in order to ascertain whether or not any diverging modes exist in the flight regime being attempted. Modifying the component in any way changes the harmonics of it, thus new divergence points need to be uncovered. Sometimes such change is not of consequence, other times it is indeed relevant.

I'd hate to be the guy who broke the Vne barrier flutter free just to discover I didn't take into account any and all control surface interactions to the equation and so I go to bank and poof goes the flapping wings in the rear view mirror lol. Is the extra 20 minute flight time savings worth that? Not to me. I'd love to do this kind of work for Uncle Sam... for money and a decent retirement and life insurance policy. For my recreational pursuit, meh let paid people do the legwork for me.

But hey, it's experimental. One is legally free to push the envelope. Wear a chute and bring your magic charms to hang by the compass. :D
 
Geico266 said:
That is true, except for the LSA RV-12. You become a licensee to built the plane. ;)

However, most RV builders follow the plans and respect the designers limitations on HP, engine selection, and air speed restrictions.
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That leaves me out.....;):wink2:
 
hindsight2020 said:
It's not that simple. Aeroelastic effects are not merely only a function of component strength and stiffness. The time-dependent interactions of aerodynamic loading as a sub-function of component twist (which straight mickey mouse aerodynamics effectively dismiss at "low airspeed" by putting a ZERO on the equation) are such that more extensive flight testing is required in order to ascertain whether or not any diverging modes exist in the flight regime being attempted. Modifying the component in any way changes the harmonics of it, thus new divergence points need to be uncovered. Sometimes such change is not of consequence, other times it is indeed relevant.

I'd hate to be the guy who broke the Vne barrier flutter free just to discover I didn't take into account any and all control surface interactions to the equation and so I go to bank and poof goes the flapping wings in the rear view mirror lol. Is the extra 20 minute flight time savings worth that? Not to me. I'd love to do this kind of work for Uncle Sam... for money and a decent retirement and life insurance policy. For my recreational pursuit, meh let paid people do the legwork for me.

But hey, it's experimental. One is legally free to push the envelope. Wear a chute and bring your magic charms to hang by the compass. :D
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I was just being sarcastic and I thought the joke was obvious. Now I'll actually have to find out more about the Harmon Rocket (with a few extra pop rivets).

I agree they haven't printed enough money for me to test flutter limits in my barn built bird. Even with my bobble head Jesus statue on the glare shield.
 
Flutter is a HARD calculation. It's a dynamical problem. Stiffness is one of the variables, but there are oodles of others. If you want to fix it, you'll need a good FEM program, a good FEM model of your aircraft (good luck with that), a mechanical engineer, and a wind tunnel. If you half-ass it, you're likely to die. It's quite serious.

Somewhere on YouTube, there is a flutter test on a 747. Watch that before considering any experiment along these lines.
 
MAKG1 said:
Flutter is a HARD calculation. It's a dynamical problem. Stiffness is one of the variables, but there are oodles of others. If you want to fix it, you'll need a good FEM program, a good FEM model of your aircraft (good luck with that), a mechanical engineer, and a wind tunnel. If you half-ass it, you're likely to die. It's quite serious.

Somewhere on YouTube, there is a flutter test on a 747. Watch that before considering any experiment along these lines.
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Just google..... Zenith 601XL flutter.

http://www.nashero.com/Home/related...ontsbreportonch-601xl6crasheswith10fatalities
 
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Even better, watch the YouTube video of the Airbus A-380. Let's talk pucker.
 
Alexb2000 said:
I was just being sarcastic and I thought the joke was obvious. Now I'll actually have to find out more about the Harmon Rocket (with a few extra pop rivets).

I agree they haven't printed enough money for me to test flutter limits in my barn built bird. Even with my bobble head Jesus statue on the glare shield.
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I don't care if it rains or freezes as long as I got my plastic Jesus.....
 
I'd like to put a caveat on the Bonanza being susceptible. The Bo's are a high performance aircraft, that have been extremely popular, and also extremely modified. The margin between safe flight and airfoil flutter is possibly lower than on other planes of the similar performance, but are still completely acceptable on conforming aircraft.

Sadly, many Bonanzas have been maintained, or modified such that the planes are no longer conforming. The FAA, and the type club found a significant number of planes with with out of balance control surfaces. Also, there were a number of planes with out of rig cable tension. These two combined defects which are common GA maintenance items reduce the margin of protection for the Bonanza.

On the plus side, the Bonanza was built to handle the stress of Utility category loading up to full gross weight, unlike other planes in the same group. Downside of that is the Bonanzas are often overloaded. a mixed bag it would seem.
 
Morne said:
In a recent thread someone mentioned about how flutter at higher altitudes, even below indicated Vne, can be rather bad for certain aircraft. My question is, "Which aircraft are susceptible to flutter below indicated Vne?"
Click to expand...
The simple answer is "most". WRT the potential for flutter, the airspeed of interest is true airspeed yet the "official" Vne for most NA airplanes is given as a CAS or IAS value. There is a margin in that the certification regs require testing at something like 110% of Vne but AFaIK for NA airplanes there's no altitude specified in the FARs for such testing. For a given CAS, TAS increases about 2% for each 1000 ft of altitude so if the dive testing was performed at 5000 MSL that 10% margin is all used up if you push Vne above 10,000. I suspect that the FAA is relying on the expectation that NA airplanes generally don't operate near Vne at high altitudes but with turbocharging to maintain available HP, high CAS at high altitude is not only possible, but even likely and for that reason most turbo'd airplanes come with a POH requirement to reduce the CAS Vne as you climb high.

But if you take a lightly loaded C-182 to it's absolute ceiling at 15,000-20,000 ft and push over into a "full power" dive you can easily reach the published Vne and will be seriously risking a flutter event.
 
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bartmc said:
I didn't want to say it but, yep.

Check this wentworth eBay ad out.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=140906185292

For some reason, that comforts me :dunno:

There's also an NTSB on one, happened somewhere around spokane that is an interesting read.
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I know this plane, and it was not damaged by flutter. The plane was flown through the edge of a t-storm. At front CG, with only a pilot or pilot and passenger in front, with near full fuel there is a pretty high negative pressure point on the tail.

A number of years back, the Bonanza V tail airframes from mid-1951 forward were the subject of a serious AD. The AD provided for cuffs on the front of the ruddervator. without the stiffening cuffs, the tail on this plane would have deformed down and flat in front, and the plane would have experienced a high-G pitch up, possibly failing the main wing.

The evidence here is that although the plane may have been conforming, it flew into very serious turbulence and came out the other side to make a landing, sacrificing the fuselage in the effort. This tells me that the design of the plane is as robust or more than anything flying today.

<edit; in case anyone is wondering, the plane can never fly again. There is other stress damage not visible in the pics. It will need to be parted out.>
 
docmirror said:
I know this plane, and it was not damaged by flutter. The plane was flown through the edge of a t-storm. At front CG, with only a pilot or pilot and passenger in front, with near full fuel there is a pretty high negative pressure point on the tail.

A number of years back, the Bonanza V tail airframes from mid-1951 forward were the subject of a serious AD. The AD provided for cuffs on the front of the ruddervator. without the stiffening cuffs, the tail on this plane would have deformed down and flat in front, and the plane would have experienced a high-G pitch up, possibly failing the main wing.

The evidence here is that although the plane may have been conforming, it flew into very serious turbulence and came out the other side to make a landing, sacrificing the fuselage in the effort. This tells me that the design of the plane is as robust or more than anything flying today.

<edit; in case anyone is wondering, the plane can never fly again. There is other stress damage not visible in the pics. It will need to be parted out.>
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I wasn't in the plane when whatever happened, happened so I dunno. If one were to experience tail flutter in a Bonanza, the results would look similar to the plane in that ad. While the damage is troubling, the fact he got it and himself on the ground in one piece makes me feel better about the construction of the ruddervators.

The first iteration of that eBay ad, wenworth was selling it as a package deal where they supplied the parts to get it up and running again. :hairraise:

EDIT: Here's the NTSB I was referring to http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001208X07676&ntsbno=CHI97IA122&akey=1
 
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bartmc said:
I wasn't in the plane when whatever happened, happened so I dunno. If one were to experience tail flutter in a Bonanza, the results would look similar to the plane in that ad.
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That's true, but not due to the flutter. When the ruddervator in the Bonanza goes into flutter, it's generally in a descent, with full power up near Vne. The pilot who put it in that condition in the first place then hauls back on the yoke, and creates the same kind of stress that heavy turbulence would cause, which is also why the cuffs are needed. The deformation of the aft fuselage and the ruddervators is caused by extreme down forces, not the flutter.

Since the ruddervators are made of mag, when they flutter seriously they usually shred the attachment rivets from the front torque tube or the break the ruddervator actuator horn which is the subject of another serious AD in the older airframes. Best we know from a few that have broken up in flight, the ruddervators on the busted ones were out of balance due to excess paint. The subject of a serious strict compliance with the Beech service instructions on keeping the control surfaces very light, and balancing them carefully before installation and after service/repair/rebuild.
 
DavidWhite said:
If the manufacturer of my airplane told me not to do something though, but I had the option to, I wouldn't do it.
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Beech will tall you not to do the excaliber conversion on any of the 50-series. Do you really think the wichita original was better in any way ?
 
dell30rb said:
That's interesting, cause the other day I was thinking a turbo RV would be a pretty sweet ride
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There are a few turbocharged RVs out there. The biggest issues are keeping them cool, fitting all the exhaust plumbing under the cowl and around the engine mounts, and the extra weight of the installation.

An RV really doesn't need a turbocharged engine anyway, the performance is quite good with normally aspirated engines.

How quickly do you need to reach Vne anyway? B)
 
dell30rb said:
That's interesting, cause the other day I was thinking a turbo RV would be a pretty sweet ride
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Yeah it would! I just found this little article explaining why it's a bad idea :( :

https://www.vansaircraft.com/pdf/hp_limts.pdf

Long story short, turbo-normalizing an engine allows the engine to maintain sea level manifold pressure at altitude so that the engine can produce rated power. The problem with that is when you're at altitude your TAS can read substantially greater than IAS, so, at altitude, Vne on your airspeed indicator can be greater than what the aircraft was is able to handle flutter-wise. If you have a non-turbo-normalized engine you don't have to worry about exceeding the "flutter speed" because the engine won't make enough power to let you go that fast (that's how it was designed), but if you turbo-normalize the engine and you're making a bunch of extra power at altitude, you could potentially find yourself exceeding the flutter speed and that could potentially cause problems...

That's a pretty lame explanation, I know. If you want a better explanation it's in the article :)
 
Every mechanical system has a fundamental frequency. This can be heard by plucking a guitar string. Plucking the tail wire on a Super cub.

The prop goes around at 2400 rev/min. Two blades so double it. Divide by 60 seconds per minute and you get 80 cycles per second. That is an E2 on the piano. That vibrating air goes back to the horizontal stabilizer. You don't want the horizontal stabilizer wire to have a fundamental frequency of 80Hz.
 
A/P_Mechanic said:
Yeah it would! I just found this little article explaining why it's a bad idea :( :

https://www.vansaircraft.com/pdf/hp_limts.pdf

Long story short, turbo-normalizing an engine allows the engine to maintain sea level manifold pressure at altitude so that the engine can produce rated power. The problem with that is when you're at altitude your TAS can read substantially greater than IAS, so, at altitude, Vne on your airspeed indicator can be greater than what the aircraft was is able to handle flutter-wise. If you have a non-turbo-normalized engine you don't have to worry about exceeding the "flutter speed" because the engine won't make enough power to let you go that fast (that's how it was designed), but if you turbo-normalize the engine and you're making a bunch of extra power at altitude, you could potentially find yourself exceeding the flutter speed and that could potentially cause problems...

That's a pretty lame explanation, I know. If you want a better explanation it's in the article :)
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That's still not a mechanical problem - it's a pilot problem. There are a bunch of flight controls in the cockpit that, if used incorrectly, can result in the disassembly of the airplane while airborne. The throttle is just one of those controls. The pilot is responsible for manipulating ALL the controls in a manner consistent with keeping the airplane within its flight envelope. Building an airplane that is capable of exceeding that envelope simply means the pilot has to be more vigilant about paying attention and not crossing that line.

In other words - you have to be a PILOT, not a passenger, if you want to go there. You don't get to just sit there and watch the pretty clouds go by.

Most business jets are quite capable of exceeding their Vne, even while climbing significantly, especially in the lower altitudes. Pilot vigilance is the only thing that separates them from an NTSB report. They know the dangers and operate accordingly.
 
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coloradobluesky said:
Every mechanical system has a fundamental frequency. This can be heard by plucking a guitar string. Plucking the tail wire on a Super cub.

The prop goes around at 2400 rev/min. Two blades so double it. Divide by 60 seconds per minute and you get 80 cycles per second. That is an E2 on the piano. That vibrating air goes back to the horizontal stabilizer. You don't want the horizontal stabilizer wire to have a fundamental frequency of 80Hz.
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What if you run at something other than 2400 RPM? How do they account for the entire range of RPM?
 
When working with a Beech instructor, I was demonstrated a graveyard spiral in my -35 - let's just say that sucker speeds up VERY quickly with the nose pointed anywhere near "down" and that spiral tightens upon itself about as quick...

By the way, in the really early 35's, the rudder doesn't flutter - but the wings do disappear somewhere past Vne... at least that is what I'm told and ... well ... I'll take the guru's word for it ...
 
ateamer said:
What if you run at something other than 2400 RPM? How do they account for the entire range of RPM?
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Well, you try to keep the natural frequency of structures away from the range of any known exciting frequencies being generated. Easier said than done because there are so many parts, and lots of them move! :eek:

An example: a number of folks experience cracking in the weld of the step on A models. That part is a 'wire' hanging out in the wind subject to being 'twanged' by engine vibration, airflow turbulence vibration, the combination of both, etc. which cause high stress in the weld. In this case, the step is excited near enough it's natural frequency (whatever it is) to generate high stresses. After enough twanging, the repetitive high stresses cause the weld to crack.
Flutter would be if the step was excited right at it's natural frequency - then the stresses would be a LOT higher, and the failure would occur a LOT quicker.

Wings, tail surfaces (landing gear, antennas, etc) are all 'wires in the wind' subject to the same phenomenon. The frequency/forces have been shown to be OK, below VNE for the airframe. The speed of the air is a major factor in the frequency and magnitude of the 'twanging' force. Staying below VNE is a really really good idea :rolleyes:
 
DGlaeser said:
An example: a number of folks experience cracking in the weld of the step on A models. That part is a 'wire' hanging out in the wind subject to being 'twanged' by engine vibration, airflow turbulence vibration, the combination of both, etc. which cause high stress in the weld. In this case, the step is excited near enough it's natural frequency (whatever it is) to generate high stresses. After enough twanging, the repetitive high stresses cause the weld to crack.
Flutter would be if the step was excited right at it's natural frequency - then the stresses would be a LOT higher, and the failure would occur a LOT quicker.
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That's resonance caused by an external excitation.

Flutter is an aero-elastic phenomena that is self exciting and is typically triggered by a constant air flow - not engine vibrations. pulsations from propellers, etc.

http://youtu.be/6ai2QFxStxo
 
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