rmh

Rodneys info was understandable -but does not give a correct picture to models such as we fly.
I kept harping on light planes because they approximate our setups.
Ireally get irritated at the guys who work on 747's and then try to explain problems with balsa and glass models -based on flying cattlecars .
my models are oft times dissmised as abberations -unlike "normal aircraft.--
Therefor - my results/observations are simply not correct.
Some of you guys should note that this is a model forum and tailor your inputs accordingly
Grrrrrruffff!

Squid

Thanks for your input Brian, I was able to extract the information pertinent to the problems were are currently experiencing with our models and will factor it in when applying fixes accordingly. I for one and glad to have input from any angle and find the 'non relevant' parts very interesting non the less. Then again, I watch all the aviation programs on Discovery and have closely watched the A380 maiden recently, and any behind the scenes info is most welcome, even from their competitors!

Dick, your models are not aberrations, its all avaiation and its all good! We all have to flight the "Its only a toy" mentality sometimes,
now drop, stay...good boy!!!

Spoiler

"Alastair is correct in that mass balancing can solve some natural frequency issues but its effect is simply that of a "larger" force being needed to induce the same resonant displacement. Qualifier: this is not necessarily true in all cases. Most structures resonate at very low frequencies, thus if you can lower the natural frequency below the excitation frequency, air loads, or possible engine vibes then this will fix the problem"

This makes sense and confirms that mass balancing can cure flutter. Remeber we are working with an existing design that prevents unlimited alterations. If I were and F90 designer I'd have specced dual MG servos for each surface - that'd be sure to stiffen things up a la Boeing...

alasdair

NFOOTE
Thanks for the input.
As you work at Boeing on flight controls you should be able to find somebody who really knows about Flutter and mass balancing. As you know, the 747, 757, 767 and 777 do not have mass balance. Their controls are hydraulic. However, the hydraulic controls on the 737 had manual back up, operated by cables, and it had mass balances. Ask somebody what the mass balances were for. Ask what happens when a mass balance falls off.!!! I am sure I once read a safety report about that. Incidentally the 737 ailerons also have internal aerodynamic balance.

From my standpoint Rodney is still quite wrong. He contradicts not only my model flying experience but also every aerodynamics book that I have read on the subject of flutter.

Maybe it is a language thing. Maybe in the USA you can use the term Aerodynamic Flutter for any bit of aeroplane that wobbles around a bit. But in my Degree course in Aeronautics it was very specific.
Aerodynamic Flutter is a driven vibration. It depends on the masses and stiffness of the wing and ailerons etc and the displacement of the masses from the flexural axes and it also depends on AIRSPEED, which is the term Rodney has omitted. I remember (from many years ago) that the analysis evaluated the work done by the forces in the system as a function of airspeed.
The result was that above a certain airspeed the net work done was positive, that is , energy was extracted from the airflow and so the flutter was self perpetuating. Below the flutter speed it was damped out.

What Rodney seems to be concerned with is Resonant Vibration. That too depends on masses, stifness and some form of stimulus. He has not mentioned airspeed. If airspeed is not involved it is NOT FLUTTER.
Alasdair

Paternguy

My Feedback: (1)

How about a practical real world solution, as opposed to throwing around theories that apply to full scale (which don't always apply when scaled down). I always wick the balsa on my ailerons with ca to stiffen them (requires a little bit of extra sanding) and I always seal the hinge gap with clear packaging tape, I have never experienced flutter of any kind on my models.

I believe this to be a function of lazy people buying cheap pre-built airframes. just my 2 cents.

Build your own birds and this wont happen nearly as often.

mr_matt

My Feedback: (10)

Here is how I do it. Not ideal as you really want the mass distributed evenly over the leading edge of the movable part, but these composite structures produce such a stiff surface I seem to be able to get away with it.

The last plane I used it on went pretty fast (can't say on here due to AMA stuff), before I used these mods I fluttered my rudder/vertical stab 3 times, saved the plane each time fortunately, very scary

TManiaci

My Feedback: (6)

All,

This is an awesome discussion guys.

Intro... Mechanical Engineer, flying RC for less than one year. Several patents on the wall. Expertise in mechanical design and innovation. Out-of-the-box thinker...

I brought this whole discussion up in the "Funtana 90!" thread several months ago, but it was way too technical to draw interest. See the launch of the discusstion here:

http://www.rcuniverse.com/forum/m_16...86/key_/tm.htm (post 2575)

The thing I think you guys touched on that I believe is on track, is that of the wing and aileron acting as a "system". If you go back in the F90 thread, there is a photo sequence that was clipped frame-by-frame (see upload) from a video of a flutter failure on the F90. If you look very closely, you can make out the wing twisting (resonance) just before catastrophic failure.

There is so little post-mortem evidence to see, but this is pretty valuable. you can see the WING is occilating, probably initiated and excited by an aileron flutter.

rmh

If you still have a good 2H pencil- here is some "engineering "work you might try:
Make up some quality 90 degree bellcranks and get some decent small ball joints.
Place aileron servo inside fuselage - and drive each aileron at two points (aprox 2/5th from each end) Most importantly - interconnect ailerons
This setup will provide maximum resistance to flutter -for least amount of power applied .
It doe not replace proper aileron design or good static balance -but it will reduce air loadings to servo(s).
Dont believe it ?
look at any decent full scale aerobat.
For a larger model --- place one aileron servo at inboard edge of each aileron ---then drive an additional point at other end of aileron with push rod.
Interconnect the two alierons thru fuselage.
Same idea -just less bits n pieces.
And what ever you do -do not use torque rods!
In these sizes - you simply end up with a device which can easily go resonant.

Rocketman_

I have seen the wings on contest type free flight models flutter and explode in flight. Those wings have no ailerons.
So, doesn't that mean that if a wing with ailerons flutters as system it makes no difference how fat your push rods are, how powerful your servos are, how strong the gears are, or how much gap sealing you do?

Spoiler

After a supreme lack of motivation I finally finished the mass balancers. They are untested as yet but I can already hear the improvement. The servos nolonger grumble under the weight of the ailerons as the mass balancers help them keep those barn doors level.

jfitter

Here's some practical modelling experience on the flutter issue. I've read all the posts and the subject has been talked all around in circles and I'm sure lotsa people are quite confused. I provide simple and practical expalanations to club members when they ask about flutter and the solutions do work.

Flutter requires a driving force. It is a result of oscillating pressures at the wing trailing edge - an alternating vortex stream. This is the same driving force which causes flexible steel power poles to sway from side to side in a strong wind. The frequency of vortex shedding increases with increasing airspeed.

On a wing the result is oscillating torsional forces (wing twisting). On a control surface, the surface is forced to flap up and down against it's eleastic restraints. Forget the wing for now and consider the control surface. When the frequency of the oscillating force (airspeed dependent) matches one of the natural frequuencies of the control surface/linkage system, then resonance will occur. What is resonance? In simple terms, the energy from the airstream is admitted freely to the control surface/linkage system with little if any resistance - the result is usually very spectacular. The control surface motion effects the wing in interesting and complicated ways, mostly resulting in wing structural failure - quickly.

How do we fix it? Well we can't. It will always happen - at some speed. What we want is for resonance to occur at some speed outside the expected flight envelope, ie. a speed that we will never attain. This is most conveniently a very high speed, especially a speed that the airplane cannot possibly attain. At high speed the vortices are shed at a high rate. A solution is to make the natural frequencies of the control surface/linkage system high so they match the vortex shedding rate only at high speed.

To increase the natural frequencies of the control surface/linkage system the following need to be done;

1. Increase control surface stiffness to weight ratio.
2. Reduce the moment of inertia of the control surface by redistributing the mass more evenly about the torsional axis (or axis of rotation - hinge line).
3. Reduce the mass of the linkages.
4. Reduce the moment of inertia of rotating linkage components.
5. Increase the stiffness of the linkage components.

Other things that work;

1. Remove all free play. Free play does not change the resonant frequency but it reduces the effectiveness of any dampening.
2. Make trailing edges thin. This reduces the magnitude of the driving forces by reducing the size of the vortices.
3. Make wings stiff in torsion and bending. This delays the effects of resonance (the "interesting and complicated" stuff I referred to in para 3).
4. Make the tail boom stiff in torsion (to resist rudder effects).

Fortunately, these things are easy to do at the small scale of model aircraft. High stiffness to weight ratios come with the territory. Of course, despite nature being on our side, there is always one person in every club who manages to get it wrong, to the considerable amusement of the other club members as his model transforms itself into a balsawood shower.

On my jets and pattern models I use the following;

Ailerons - Titanium pushrods with MK ball race links both ends. Aluminium servo arms. Digital high speed servos. Ailerons are lightweight, stiff, and have many hinges (at least 5). Trailing edges are inlaid hardwood and faired to 1/2mm.
Elevator - CF tube - large diameter, thin wall - titanium threaded ends, MK ball race links, etc. (same as for ailerons).
Rudder - same as elevator or cables with tensioning system.

With these I cannot get any flutter. If I do then the next move is to consider mass balancing (reduction of control surface moment of inertia).

Spoiler

Jfitter, very interesting stuff and a nice clear explanation. In my case I have stiffened up the linkages and done the mass balance thing as an experiment. I hope I am right...

Darryl Usher

There is one thing I would question. The first item #3 keep the mass of the linkage low. The linkage is part of the balance system and if it were infinity large we would have no flutter. This is why we interconnect the two ailerons to reduce flutter.
Darryl Usher

jfitter

Darryl - to a point you are correct. If the linkage system were infinitely large then it would also be infinitely heavy and there would of course be no flutter. You could not drive the control surface either, which is a problem, and the aeroplane would be a tad heavy for takeoff, which is a bigger problem!!!

By making the control linkage system heavy you are reducing the resonant frequency of the system. If it is heavy enough then resonance will not occur at any speed above the stall, ie. after takeoff. This is not a proper solution. You would need massive servos to get the desired control response, and there are still no guarrantees that resonance will not occur at some harmonic (multiple of the fundamental resonant frequency).

The proper solution is to make the linkages light and stiff. There are no resonant modes below the fundamental, so if the fundamental occurs above the airplane's maximum speed then it can be assumed that no resonance will occur below this speed.

Keep in mind that the system under consideration here is EVERYTHING that moves with the control surface. You MUST keep the moments of inertia low, the masses low, the stiffnesses high, and ZERO free play.

For ailerons, before you mass balance then try the following;

1. Short, light, stiff pushrods (titanium - aluminium is lighter but it WILL fail by fatigue eventually).
2. Ball race rod-ends.
3. Lightweight servo arms.
4. Light weight horns on the ailerons (and use aluminium screws - from MicroFasteners).
5. Split the ailerons if they are large ie. 4 instead of 2 and use smaller servos.
6. DO NOT join ailerons together, especially left to right. This is very bad and can result in a particularly destructive resonant mode (destructive = spectacular, show stopper!!!). Ailerons should be driven by independent servos.

Ailerons need to be stiff in torsion because they are driven from a single point. Torsional flex modes contribute to the elasticity of the entire control system and therefore flutter. Using two links to the surface may help but is offset by the increased live mass of the two linkages. A better solution is to just increase the torsional stiffness. You could build the entire aileron on a lightweight, thinwall carbon fibre tube. This would provide for very high torsional stiffness. The structure behind the tube is just a passenger so make it very light indeed. The LE of the aileron would then be circular, which is what we want anyway for a good wing seal.

Horn placement should be to prevent additive torsional vibration modes, ie. not in the centre and not at 1/3, or 1/4, or any other low value integral fraction, but keeping each section as short as possible. Just off centre, say 5/11 would be a good place. Keep in mind that driving the ailerons from adjacent servos increases the moment of inertia of the wing and so reduces the crispness of the aileron response. Driving the torque tube from the wing root may be a viable option.

PS. For readers unfamiliar with the term Moment of Inertia.

For linear motion - the MASS of an object is that property of an object which resists linear acceleration (linear motion).
For rotary motion - the MOI of an object is that property of an object which resists angular acceleration (rotary motion).

Linear: Force = Mass x Accelration
Rotary: Torque = MOI x angular acceleration

High MOI = Weights concentrated at tips
Low MOI = Weight concentrated near centre of rotation

I hope this clarifies a few points. If anyone is helped by this discussion then it is worth the trouble.
Cheers all.

ptxman

My Feedback: (2)

Interesting stufff, thanks

- do you have any pics or hardware description of how you are using titanium? I dont thing procurring & threading rod blanks is for te average Joe?

- I have not heard of the magic number integral fraction horn placement thing before. Can you elaborate on that? I always thought it was a support distance thing; better to place the horn at the mid span to divide the load bearing control surface in half, kind of like a statics problem to minimize bending stress

- what about if the aileron is tapered, does the formula vary according to proportionate lifting area?

- many 170 mph+ high G F5D type electric pylon models (7-8% thickness airfoils, 3 - 5" span, composite bageed CF skin type construction) often tape the outer portion of the aileron to mitigate flutter. Ive had first hand success with that. No tape = flutter, tape (or not even bothering to cut the outer aileron opening gap) = no flutter. IOW it reduces teh aileron to a variation of surface twisting. Unlike jets & dynamic soaring models, its very diffficult to get good quality outboard servos to physically fit in these wings, although its being done more now with semi-decent 8mm thick servos & enclosed molded covers yo smoothen teh airflow bumps. Anyway, can you describe what you think is going on with this 'constrained on one end' approach in the context of the principles you've described.

TManiaci

My Feedback: (6)

PTXMan,

I'll take a shot at that explanation... others feel welcome.

We are talking about the "natural frequency of ocillation" in a system. The natural (or inherent) vibration frequencies are in a escallating order, where you have "focal points" for the vibration which work out to be natural divisions of the length of the member. So, the lowest natural frequency of a member is 1/2 (center), then the endpoints (I think), then the next order is 1/3 and 2/3 (two focal points), then 1/4, 1/2 and 3/4 length, and so on. (sorry, can remember the exact order of divisions, been too long). To avoid excitation of the member, you try to connect at some high-order point (or points) such that it would take a very high frequency (speed) to excite the member into resonant vibration. By connecting at 5/11 or some other odd division of the member's length, you make it far more difficult to excite the member into resonance.

So, this is a sound engineering effort to take a preventative measure to prevent flutter, as we know that the initiation of a flutter event is from some excitation force "matching" a natural frequency. If you avoid the match points (harmonic, or frequencies in harmony), you avoid the flutter. This is why we like a very rigid structure (aileron & wing) because the natural freqiency of vibration goes up with a more rigid member.

Spoiler

As I posted on the F90 forum, the mass balanced F90 has flown successfully without flutter. Unfortuneatly the fix included upgraded HD control horns so the experiment doesnt really prove that the balancers are the solution, maybe part of it at best. No signe of any strange quirks as a result of the balancers design, which was questioned by at least one person who thought they would induce turbulence. So far so good...

jfitter

Titanium rods - your pretty brave if you machine them yourself. Special skills are needed, beyond what most modellers have. For short pushrods, such as aileron rods, I use r/c car suspension components available at most hobby shops. The rods come in many different lengths and have left and right threaded ends with a square section in the middle. I screw the MK ball race links onto these rods.
For longer rods, specialist pattern product suppliers sell kits consisting of 2 carbon rods and 4 titanium rod ends with 2-56 or 4-40 threaded ends. Again, screw the MK ball race links to these.

TManiaci does a fair explanation of horn placement so I won't go over this again.

The placement of the horn IS effected by aileron taper. The maths is quite simple - plenty of guys on this forum can work it out and give you the answers.

F5D aileron flutter - my initial suspicion is that the tape on the outer aileron half is dampening the oscillations, ie. it is more a mechanical solution than an aerodynamic one. How about some discussion - see what others think.

One way to test it is to provide for friction free gap sealing on the aileron outer end. If there is no flutter then the solution is aerodynamic and if the flutter persists then the solution is mechanical.

BFoote

The mass balancing on the 737 47 57 67 on the cables was done to keep the stick force down for the pilot and to dampen the cables vibrational natural frequencies so the guides on the cables would not have excessive wear. As it is the cables still need tightening every 500 to 3000 hrs or so. Sorry about late date of this post, but just checked back in. Reason for manual backup is because no one system failure will result in a catastrophic crash of an airplane, thus all systems must have an independant backup which cannot have a common mode failure between systems.

Brian

TGDF

My Feedback: (3)

To try and answer the original question, 1) If you know that the plane is prone to flutter at high speeds, you may be just beating yourself over the head trying to fly it faster.

2) Three books I've just bought after returning to the library the 8 books I borrowed are: "Simplified AircrAft design for homebuilders" by Dan Raymer, "Model aircraft aerodynamics" by Martin Simons and "Basics of R/C Model Aircraft Design" By Andy Lennon.
All three discuss flutter, though not in huge detail. Interestingly they all relate mass balancing to flutter.

3) Better yet, get a beginning electronics book and read the part about resonant circuits. Circuits of resistance combined with capacitance or resistance combined with inductance. The math is extremely simple. Then go out and look at the spring, shock absorber system attached to each wheel of your car. That's a mechanical version of a resonant circuit.

Now the deal is, flutter is caused by an aircraft surface being inadvertently designed as a resonant system. Its designed in such a way that under certain circumstances, (in Spoiler's case excess speed) energy gets added to the system at the resonant frequency. And just like when a fool keeps adding just a little push to a child's swing at the swing's resonant frequency, the child goes higher adn higher until the swing wraps around the bar or the kid shoots out of the arc or whatever.

In that same way, the control surface picks up more and more energy from the airflow and buzzes louder adn louder until the part/plane fails.

The ONLY 2 cures for this are to change the resonant frequency (strictly speaking, you broaden the frequency range where the system will NOT resonate) or remove the energy input or alter the input frequency.

Mass Balancing is just that. You balance the mass of the surface around the hinge. While one way to address flutter, it needs to be done based on proper consideration of the principals behind the problem.

Making the parts stiffer or the linkages tighter also, while being valid ways to address the problem, if not done in combination with proper understanding of the underlying theory, you may just be altering the resonant frequency. The plane might just self destruct at a higher/lower speed.

Anyone who's seen film of the Tacoma Narrows bridge knows strength or stiffness ALONE is meaningless. It was a huge, modern suspension bridge that rocked itself to pieces because the WIND reached the right speed. It had nothing to do with gusting. The turbulence around parts of the structure resonated at a certain wind speed. It rocked the bridge back and forth until it ripped itself appart. Until you see a good demonstration of resonance, you can't believe how little power it takes to destroy something if that little bit of energy is just added at the resonant frequency.

Sorry, I can't be of more practical help. Like I said, the electrical math is easy so its easy to learn an understanding of what's going on. Understanding a spring/shockabsorber combination gives better insight into mechanical resonant systems. You're looking for the opposite of a band pass filter in electronics. A combination of weight and or stiffness (shock absorber type of stiffness) should do it, but how to find the correct combination to dampen a frequency range, I couldn't tell you.

Ben Diss

My Feedback: (23)

Here's some good reading:

http://www.iitk.ac.in/infocell/Archi...1/flutter.html
http://www.dfrc.nasa.gov/Education/O...t/nasflut.html
http://www.nasa.gov/centers/dryden/p...ain_H-2077.pdf

...and some entertaining videos:

http://www.airandspacemagazine.com/A...47Flutter.html
http://www.airandspacemagazine.com/A...A6Flutter.html
http://www.airandspacemagazine.com/A...C5Flutter.html
http://www.airandspacemagazine.com/A...PWFlutter.html
http://www.airandspacemagazine.com/A...TCFlutter.html
http://www.airandspacemagazine.com/A...FM/Hammer.html

TGDF

My Feedback: (3)

I just love people who make it easy for me to enlarge my library! Keep it coming!!

3dd

My Feedback: (6)

mass balanceing is for one thing only and that is to lessen the load on the controling devices being a servo or a control yoke or what ever.remember that faster puts too much pressure on the control serface to move it easy without mass balancers

Rodney

3dd, you are confusing mass balance with airodynamic balance. Mass balance is simply having the mass equally distributed about the hinge point so that any acceleration forces do not rotate the surfaces about that hinge point. Airdynamic balance is what makes the airload forces equal at all speeds, i.e. the controling of the air flow force both in front of and behind the control surfaces hinge point.

TGDF

My Feedback: (3)

Oops! Now I really feel stuipd. I didn't see there was a second page of postings before I wrote this. jfitter and others have it right.