Physics Of Flutter

[mattijs321]

I just started a phD at the unniversity of Brussels engineering department.
The subject is flutter. More precise...a non linear system identification approach to flutter.

The people currently in charge of the flutter research and the windtunnel are at some congres and wont be back for the following week.

I work at the electronics department and have little training in aerolasticity and aerodynamics.

The people at my department asked me to study the very basics of the physical side of things and extract an equivalent model of a system that is able to flutter (or suffers from).

Seeing i cant reach the people at the mechanics department for now...i was wondering if i could get some help from you guys.

What i need is a good url, book, basicly anything that can help me understand the basics and help me find (the simplest) model.

All help appreciated!

[Rodney]

As you have no doubt found out by now, everything will flutter if properly stimulated. To find out at what frequency, have the subject piece mounted as it will be in use then instrument it with accellerometers being careful so that the accellerometer mounting does not change the mass or stiffness of the subject piece, excite the subject with a single impulse and measure the frequency at which the vibrations occur. That is usually the prime frequency at which the subject will flutter if given sufficient stimulus. I think you will find this a very interesting and suprising subject to experiment with, I know I did.

[macboffin]

As you have no doubt found out by now, everything will flutter if properly stimulated. To find out at what frequency, have the subject piece mounted as it will be in use then instrument it with accellerometers being careful so that the accellerometer mounting does not change the mass or stiffness of the subject piece, excite the subject with a single impulse and measure the frequency at which the vibrations occur. That is usually the prime frequency at which the subject will flutter if given sufficient stimulus. I think you will find this a very interesting and suprising subject to experiment with, I know I did.
Aileron, rudder, or elevater flutter can indeed lead to interesting and surprising results, usually culminating in an expensive hole in the ground!
Ask me how I know!

[Bill Ervin]

A simple model for your study might be a flag waving in the wind. If the wind is "just right" you may even be able to count the oscillations/period.

[yoyoML]

Wouldn't the simple model be just a mass-spring-damper oscillator? Are you familiar with it? Probably you extend the spring force from k*x to k_1*x+k_2*x^2+k_3*x^3+...., and the damping term from bv to b_1*v+b_2*v^2+... Well you get the idea.

edit: Ok, you're in electronics, so think of it as an LRC circuit...

[Mike the Snake]

I've encountered really high speed flutter with sailplanes while Dynamic soaring.

It seems most model planes have a speed at which they will flutter, but that it is caused IMO by linkages and surfaces that aren't stiff enough.

We had one type of sailplane that was fluttering almost to the MPH at 255mph.
The flutter began during the highest G-load.

What I think was going on is the wing flying fast at high AOA, and a separation bubble is forming on top at the TE. This bubble forms and sucks the surface upwards, and if there's any flex or slop, the surface will be sucked up. The bubble then collapses, releasing the surface. If conditions are right, this happens over and over at a frequency.
The key to eliminating flutter on models I think comes from making the surfaces stiff and rigid enough, and having strong stiff linkages so no movement can happen.

Wing skin deformation where the servo is glued/mounted is the cause of a lot of flutter as well, because it allows movement/flex of the control surface. On my DS planes, I always capture the servo in the wing with balsa blocks.

In a wind tunnel situation, I would think that frequency of the flutter could be modulated by dampening.

An RC car shock mounted to the control horn would be perfect. The silicone oil used in RC car shocks comes in many viscosities and is used for tuning.

With a lot of dampening one might be able to view really low frequency flutter maybe?

[mattijs321]

Wouldn't the simple model be just a mass-spring-damper oscillator? Are you familiar with it? Probably you extend the spring force from k*x to k_1*x+k_2*x^2+k_3*x^3+...., and the damping term from bv to b_1*v+b_2*v^2+... Well you get the idea.

edit: Ok, you're in electronics, so think of it as an LRC circuit...

that is exactly what im looking for right now, i don't know though how to account for the aerodynamics.

Obviously even a first order system can oscillate if you make it...but what would be the simplest model that could be said to have atleast some of the basics in commom with real flutter of any kind?

I hoped to find a book or pdf about the subject as it'd probably start by giving the reader a very simple electronic or mechanical system.

Any pointers?

Any people in the field?

[mattijs321]

Aileron, rudder, or elevater flutter can indeed lead to interesting and surprising results, usually culminating in an expensive hole in the ground!
Ask me how I know!


I've made wings myself that fluttered enormously too. Ive never heard balsa crack THAT hard midair ;)

[MarkusN]

The literature list of this: http://elib.dlr.de/14115/ should give you some pointers.

As for a simple model: Take torsional flutter of a high aspect ratio wing.

On a conventional wing section with negative pitching moment, the aerodynamic forces always have the effect of increasing any change in pitch. I.e. the nose of the wing/aircraft goes up, lift increases and moves farther forward, thus increasing the pitching up movement. The stabilator conteracts that and keeps the aircraft as a whole stable. However, the lift moving forward induces a torsional deformation in the wing. Which causes the tip to pitch up more, which makes the lift move farther forward... If you induce this change quickly enough, you can start torsional flutter. Or, if the pitching up is violent enough, you can reach the point where the tip stalls and lift goes away, leaving you with the twisted wing, which of course violently snaps back. There you have your non-linear case.

Add control surfaces with mass to the equation and you have more flutter cases. Add wing sweep, yet more (torsion and bending overlap). Every plane has a basketful of potential flutter cases. The question is at which speed they are induced.

[yoyoML]

that is exactly what im looking for right now, i don't know though how to account for the aerodynamics.

Obviously even a first order system can oscillate if you make it...but what would be the simplest model that could be said to have atleast some of the basics in commom with real flutter of any kind?

I hoped to find a book or pdf about the subject as it'd probably start by giving the reader a very simple electronic or mechanical system.

Any pointers?

Any people in the field?

My undergraduate textbook was "Classical Dynamics of Particles and Systems, Marion Thornton", where oscillation was chapter 3. Although you really should understand the simple oscillator first, this book won't be sufficient after a short while.

I guess you'll have to go through stages like:
Rigid body with forces functions of current position/velocity.
Rigid body with forces functions of the history of position/velocity. (bubbles breaking away, for example)
Non-rigid body.

I think you can ask "markdrela" on the forums. He's really professor Mark Drela of MIT Aero & Astro, so he'll have some answers.

[Ralph Walton]

Hi I 've never seen anyone approach the condition of flutter with a simple premise. - to make control surfaces that are aerodynamically stable rather than unstable. Take an arrow, stable . i believe that arrow like projections could be put onto the trailing edges of our control surfaces, and by trial and error a compromise could be found between flexible enought to allow control responses and rigid enought to prevent flutter. Mig jets got around the problem with large lead blocks in the leading edges of the wings. How do birds do it?
Go for it
Its a great project.

[markdrela]

The subject is flutter. More precise...a non linear system identification approach to flutter.
The people at my department asked me to study the very basics of the physical side of things and extract an equivalent model of a system that is able to flutter (or suffers from).
What i need is a good url, book, basicly anything that can help me understand the basics and help me find (the simplest) model.
Flutter is fairly easy to understand or explain:
http://en.wikipedia.org/wiki/Aeroelasticity#Flutter
but extremely complicated to quantify. Theodorsen was the first to quantify and accurately predict it for the "simple" case of a 2D airfoil with vertical and pitching motions:
http://naca.central.cranfield.ac.uk/reports/1935/naca-report-496.pdf
As you can see, it's not simple at all. As a minimum, you need several ingredients in any such analogy:

1) A dynamical system with at least two degrees of freedom. Some examples:
* bending + twisting (e.g. wing bend/twist flutter)
* bending + aileron deflection (e.g. wing bend/aileron flutter)
* twisting + aileron deflection (e.g. wing twist/aileron flutter)
* bending + airplane pitching (e.g. swept-wing pitch/bending flutter, or "Zagi flutter")

2) A source of energy which can be tapped (e.g. the airstream, tapped via airloads)

3) The tapping of the energy critically depends on the phasing between the two degrees of freedom. In bend/aileron flutter, for example, the phase between the wing bending and resulting aileron deflection critically depends on whether the aileron is "nose heavy" or "tail heavy" on its hinge. So the aileron mass balance very strongly affects whether or not the system flutters.


I'm aware of any non-aero mechanical or electrical analogies with all these ingredients.

[NX-687]

Hi I 've never seen anyone approach the condition of flutter with a simple premise. - to make control surfaces that are aerodynamically stable rather than unstable. Take an arrow, stable . i believe that arrow like projections could be put onto the trailing edges of our control surfaces, and by trial and error a compromise could be found between flexible enought to allow control responses and rigid enought to prevent flutter. Mig jets got around the problem with large lead blocks in the leading edges of the wings. How do birds do it?
Go for it
Its a great project.

How did you know that migs had lead blocks in their wings? , just curious, , gee not good for wing loading

[vintage1]

Hi I 've never seen anyone approach the condition of flutter with a simple premise. - to make control surfaces that are aerodynamically stable rather than unstable. Take an arrow, stable . i believe that arrow like projections could be put onto the trailing edges of our control surfaces, and by trial and error a compromise could be found between flexible enought to allow control responses and rigid enought to prevent flutter. Mig jets got around the problem with large lead blocks in the leading edges of the wings. How do birds do it?
Go for it
Its a great project.


Arrows are not stable at very high speeds.

In general stability over a given range is possible, not not over every possible range.

[Neil Stainton]

Thanks for posting the above Mark.

I guess bridge self-destruction when it is caused by soldiers marching in step (not wind) would be an example of non-aero causation of a similar phenomenon.

Neil.

[markdrela]

I guess bridge self-destruction when it is caused by soldiers marching in step (not wind) would be an example of non-aero causation of a similar phenomenon. No. That phenomenon is "resonance", not flutter. The key differences are:

Resonance:
Energy is provided by an oscillating force, or moment, or voltage, etc. The resonating object oscillates exactly at the force's frequency. The resonance is strongest when the forcing frequency and object's natural frequency are nearly the same.

Flutter:
Energy is provided by a steady source, e.g. the freestream flow. The fluttering object itself determines the oscillating flutter frequency. This frequency can often be quite different from the object's natural frequency in the absence of the energy source.

In math/dynamics jargon...
Resonance is a forced response problem
Flutter is a natural response problem or eigenvalue problem.
They are related in some ways, and very different in other ways.

[Neil Stainton]

Thank you for the explanation of the difference between flutter and resonance.

Neil.

[vintage1]

Not sure Mark is exactly correct: The flutter is caused by alternating vortex shedding and does indeed amount to an oscillating force. The elasticity of the air, and the hingeing plus the
mass of the surface gets you the resonant frequency as well.

[markdrela]

Not sure Mark is exactly correct: The flutter is caused by alternating vortex shedding That's not a good way to describe it. The vortices are simply a necessary by-product or a side effect of the oscillating air pressure forces on the wing or bridge deck. Sort of like trailing tip vortices are a side effect of a finite lifting wing. It's hard to make the case that tip vortices cause lift. Let's not get into that argument. :rolleyes:

And in any case, the shed vortices during flutter are part of the overall naturally-oscillating aero/mechanical wing system. There is no oscillating external force, in contrast to the case of resonance.

And just to add to this:
The classical airfoil flutter analysis of Theodorsen can be done with the shed vortices either taken into account, or simply ignored. In most typical applications, there is little error in the flutter-prediction results if the vortices are ignored (this is called the quasi-steady aero approximation). This convincingly shows that vortex shedding is not an important part of classical wing flutter.

[MarkusN]

Not a usual case in airplane aerodynamics, but karman-vortices behind a flagpole are a possible exception to the phenomena stated above. They are present even if the pole stands rock-solid, and if they have just the right frequency they may cause resonance.

[vintage1]

That's not a good way to describe it. The vortices are simply a necessary by-product or a side effect of the oscillating air pressure forces on the wing or bridge deck. Sort of like trailing tip vortices are a side effect of a finite lifting wing. It's hard to make the case that tip vortices cause lift. Let's not get into that argument. :rolleyes:

And in any case, the shed vortices during flutter are part of the overall naturally-oscillating aero/mechanical wing system. There is no oscillating external force, in contrast to the case of resonance.

And just to add to this:
The classical airfoil flutter analysis of Theodorsen can be done with the shed vortices either taken into account, or simply ignored. In most typical applications, there is little error in the flutter-prediction results if the vortices are ignored (this is called the quasi-steady aero approximation). This convincingly shows that vortex shedding is not an important part of classical wing flutter.

Well you may be right in that. To me it would seem to be simply a case of too much negative feedback with enough lag causing oscillation.

As with all such systems the answer is to add damping (stiffness) or change the mass (balance weights) or force (move the hnge line) to take the system out of resonance.

[daboz]

Not sure Mark is exactly correct: The flutter is caused by alternating vortex shedding and does indeed amount to an oscillating force. The elasticity of the air, and the hingeing plus the
mass of the surface gets you the resonant frequency as well.

I would assume that more mass decreases the resonant frequency. Curious, if the mass of the control surface determines the resonant frequency can you create the right mass ratio to get a resonant frequency that for some instances can overcome flutter with a half or quarter of the flutter frequency? I.e. destructive interference? Wondering if tuning a control surface with weight for certain frequencies is possible. Could this be combined with critical damping or over damping to help reduce oscillations? Bottom line does an increase in control surface mass help flutter but increase the forces induced when the flutter occurs?

Do high speed aircraft use stiffness and nothing else?

[MarkusN]

Do high speed aircraft use stiffness and nothing else?
Mass balancing is definitely also used. (Lowering both resonance frequency and the mass-deflection coupling.)

[daboz]

I DS (dynamic soar) gliders and what seems to kill speed is flutter. The Opus that I mostly DS has been measured with a radar gun at 292mph. It's extremely stiff, has a 78" wing span, and weighs about 95 to 110oz or just under 7lbs. The record for any glider DSing is 305mph.

A standard Opus begins to flutter at about 250-260 or so creating huge amounts of drag, destroying the strongest of servos and killing any hopes of faster speeds. Some of the modifications that have positive results are Aluminum tubes under the flap and aileron wipers used as torsion stiffeners. I've also tried carbon rods both of these methods increase the flutter speed and allow the plane to attain higher speeds. I have placed steel bars in place of the wipers but crashed that plane before getting it over 200. If flew great at the slower speeds the crash was purely pilot error.

I'll be building a new Opus in a week or so and I could really use some insight or help, any suggestions could help. I would be glad to post any “real world” results here! Speeds are measured with a radar gun and a laptop computer.

???, should I weight balance the control surfaces with steel bars instead of Al. tubes and what would be a good approach. Would it make sense to add the weight evenly along the leading edge of the Control surface or concentrate more of the weight out at the extremities where the flutter seems to begin.

Thank you very much for any information.

Chris

DS explanation: http://www.hsl.org.au/articles/ds.pdf

Video of Joe Manors airplane the current record holder @ 305mph. This is a 160” 47lb monster. Doesn’t look like it but I think it got over 250. Too see this in person is amazing! http://www.rcgroups.com/forums/showthread.php?t=659039&highlight=160

[vintage1]

mass, to work, should be as far from the hinge line as possible.
You are trying to increase the moment of inertia.

If the surface is not torsionally stiff, try and distribute it along e.g. the trailing edge..


Is the problem MAINLY on ailerons?

[hul]

try and distribute it along e.g. the trailing edge.

don't think you want to do that. This will increase tendency to flutter.
Add the mass in front of the hinge line to bring the CG of that surface to the hinge line.

Hans

[MarkusN]

Yep. The idea is to get an aileron that is mass balanced, i.e. that does not tend to deflect under inertia loads.

Actually the whole concept of adding mass is controversial. Mass balancing counteracts the oscillation-leads-to-deflection-which-in-turn-amplifies-oscillation issues. OTOH, adding mass lowers the resonance frequencies, something you don't want to do. It's not like this is a machine shaft that can run supercritical. Going over the speed of flutter usually means destruction.

[biber]

BTW, the Swiss Logo Team did manage to overcome their problems with flutter by massballancing only the rudder.
It seems to have turned out that the rudder was the cause of flutter and disintegrated airframes they had during highspeed dives.

Smart ways to prevent flutter (that is not just to double the carbon) is something the DS guys should learn to deal with. ;)

biber

[Ralph Walton]

How did you know that migs had lead blocks in their wings? , just curious, , gee not good for wing loading

It was on a tv programme (Discovery I think) From memory They said the mig 15 had a tendancy to flap its wings at certain speeds and it was cured by putting 20 kg's of lead in the leading edge of each wingtip (not good for payload)
Other migs (and I believe many other planes) had solutons like weights in front of the ailerons(mass balancing?) and tip weights on full flying horistabs.

I'm curious to know if the flutter of model plane control surfaces is the same phenomenon ,( taking into account the very different Re numbers), that fullsize planes have.

I'm guessing it is, just the frequencies would be different.

Early cars used to bounce around like pogo sticks until the shock absorber was discovered. Is there some way of dampening these control surface oscilations, eg shock absorbing pushrods?

[daboz]

Is the problem MAINLY on ailerons? We think so, and flaps, its really hard to tell with these speeds. The rudder is out because most of us either fly Vtails, or planes without rudders. Some of the planes that do have rudders are modified and the rudder is fixed. The control surfaces as far as model planes go are torsionally stiff. The Opus I fly is made to DS and is all high quality carbon with layers placed at 45 degrees to each other. Please excuse my layman’s terms.

Yes MarkusN going over the speed of flutter quickly disassembled one of my favorite planes. The flutter seemed to start in the ailerons or flaps then the whole plane basically vibrated apart in what seemed like a very noisy instant. Does the problem with lowering the resonance frequencies have a point of diminishing return, i.e. can the frequency be lowered so much with a heavy flap that it can actually help?

RalphW: Many of us add lead in fuse or wing ballast tubes for speed retention and penetration through the shear. I know a lot of guys are adding lead near the wing tips, (center cord) and that seems to help with stability. As you said it may be better to add the weight to the LE instead of mid cord! Perhaps the LE weight could help with the flutter and also give the stabilizing effect whilst penetrating shear and turbulence.

I’ve also thought of critically, or over dampening control surfaces with shocks. I know the model car industry makes some nice little shocks for cars. I was thinking of rods at the two midpoints between the control rod and the extremities of the wings but I thought this would add to the parasitic drag, another speed killer.

I am going to add the steel bars to the LE of the flaps and ailerons again. I may also add some weight to the LE of the outside wing. AND more carbon at the TE of the surfaces and ?????

A problem with these modifications is the cost of the plane. The Opus runs about a thousand dollars without the guts. Needless to say a crash at over 200mph doesn’t leave spar parts so I’ll be adding weight in small increments and analyzing the results.

Thank you!!! :D

[biber]

All the modern gliders (fullscale that is) have almost all controll surfaces mass ballanced to some extend.
Rudders are mostly fully ballanced (on LS8 e.g.).
Almost any other control surfaces have lead on their LE just ahead of the hinge.
And even the wings themselves have lead in their LEs on some glider types
(I know that for at least the ASH 25 and the ASW 27).
And one version of the LS 3 is well known as the 'LS Blei'
(which in german pronaunciation sounds quite similar, like 'LS dry' and 'LS bligh'),
as it hat full span flaperons that caused severe flutter issues
that had to be solved with heaps of lead attached to the front of the flaperons.
That wings got quite heavy over that.

One should be aware of the fact,
that sometimes a more of carbon doesn't do the trick,
while less carbon and ballast in the right places can do it.
Keep the control surfaces as light as possible
and balance them correctly is sometimes the better way,
than beefing it all up.
That's certainly one reason, why fullsize control surfaces often use kevlar.
It's a very light fibre!
They often have only one layer of quite thin kevlar, 4mm rohacell, and then again kevlar.
Nothing that could stand up again even light beating and not at all repair friendly.

biber

[Ralph Walton]

Are our control surfaces just too big?

I've experienced that the larger the aileron the more likely it is to flutter.

Heres an example of my supposition. I've been lead to believe, 22% of an RG15 chord is the "right" chord length for ailerons.

I tried 2 different percentage chords in profilli on an RG 15 airfoil. The black line shows a flap(or aileron) 22% of chord, diflected 3 degrees. Red line shows similar flap thats 11% diflected 6 degrees. Results are similar.

Would the flying characteristics be similar?

Should we be looking at smaller ailerons/control surfaces that can be diflected further (than we are used to) to provide control at low speed,and less flutter at high speeds?

[daboz]

it looks a lot like the Opus problems is flutter in the tail sections due to fuse. flex. A new record was set the other day for the Opus, 298mph with no flutter. The build incorporated a carbon sock glassed around the fuse.

[Mike the Snake]

I think mass balancing is not practical on our small models, and that flutter is overcome (usually) by making the airframe and surfaces and servos/linkage all rock solid.

If it can't move it won't flutter.

Shortening the chord of the surfaces results in different response at different speeds. Same chord, shorter length I think is better, as response is more consistent from slow to super fast.



The Opus flutter is very interesting, as I originally thought it was the wing, but apparently it's the entire tail fluttering from the boom twisting.

What would cause the v-tail flutter, or what would cause the twisting forces?

Could it be that maybe one elevator had more throw than the other? Wouldn't this twist the boom and possibly cause the start of the oscillations?

I would think the normal forces at high speeds would only want to push the boom down from the tail wanting to counter the pitching moment of the wing.

I've talked at length to many people about the boom flex on the MC3, and everyone has a different version of how it affects flight.

Some say you run out of elevator response as speeds go really high. I have experienced this on my MC3, where above 250 I have to go back up to high rates.

Some say the tucking as the plane goes faster is the boom flexing. I have also experienced this on my MC3.

Could it be that the entire V-tail on the MC3 is too small?

My MC3 did have lots of extra tow in the boom, and never fluttered.

I still wonder if the first two explanations aren't simply a CG or incidence issue.

[MarkusN]

What would cause the v-tail flutter, or what would cause the twisting forces?
This is a kind of flutter that might be corrected by mass balancing. In fact, the very type that was mentioned above in the statement about the Logo-Team.

For the moment ignore what caused initial twist, just look at what the control surface does as the boom twists and stores deformation energy. The v-tail is slowed down. Inertia of the flap makes it go further and deflect against the sense of the tail deformation. This accelerates the tail back, adding to the previously stored elastic energy in the boom. And so on ad infinitum.

Now any external disturbance can cause the initial deformation. When airspeed induced and mass resonance frequencies coincide you get flutter.

Mass (and aerodynamic) balancing of the V-tail flaps may help.

[daboz]

Mass (and aerodynamic) balancing of the V-tail flaps may help.

Mike I’m doing this on my current build, should be interesting. I think that an eloquent solution may be better than brut force because if the surface wants to move it will find a weak point and move. If the forces aren’t there that should be better than simply overpowering them and could only help... I’ve got four eighth inch dia. bars for each control surface and aluminum eighth inch thick plate for the V tail mods.

I know, I know its a guess, but it’s interesting

[MarkusN]

...may be better than brut force because if the surface wants to move it will find a weak point and move. If the forces aren’t there that should be better than simply overpowering them and could only help...
Don't underestimate the effect of a stiff airframe. While it will not prevent flutter, it will move the speed at which it occurs. Shift it far enough out of your envelope and you have avoided flutter.

[Mike the Snake]

In this case, the elevators aren't fluttering. It is the whole V-tail.

The linkages and rigidity of most MC3 tails I've seen are slop free, and the tail is more than stiff enough. Probably there IS some stored energy in the surface that ends up in deflection while fluttering as you stated, as the surfaces are not light, but I just wonder if it's not an aerodynamic irregularity that's yanking the whole tail over, like uneven throws, or tail halves not having the same incidence.

The MC3 is supposed to bolt together at zero/zero. I've measured mine, and they seemed to be 1 degree positive decalage (1 degree negative tail incidence="UP"), but one tail half was about 1/2 degree off.

Daboz, I'm not sure if adding steel rods inside the LE of the surfaces can be placed forward enough to have any effect other than adding weight. The surfaces are pretty heavy, and will need a lot of weight to balance on, or forward of the hingeline.

I don't think the flutter on the MC3 is stemming from a surface flapping.

Tube modding, big servos, proper linkages, and proper servo mounting effectively eliminate the surface flutter possibilities on the MC3.

Now the weak link seems to be the boom on older versions. Newer versions with the reinforced boom shouldn't flutter with a good build I would think.

Obviously reinforcing everything so it can't move fixes the issue.

Maybe tube modding the elevators might help things, as the surfaces are driven from one end? Once flutter is induced, I can see some possible inertial torsional twist at the outer end of the elevators, maybe?.

I think the "boom bending issue" of the MC3 is showing it's head in different ways as more people are now going faster with them.

Airframe aside, the build for DS cannot be understated. It is much like offroad RC car racing, where if there IS a weak link, it shows itself immediately on the track.

Everything must be overkill, or work in perfect concert.

The MC3 wing, tail, and surfaces are stiff enough for 300 without modification. The flutter encountered was boom twist on non-reinforced booms.

For models, I think stiffer and stronger is the way to ultimate speed and no flutter.

The MC3 has almost every base covered, stiff, strong, and with a good build, goes almost 300 so far.

Now that the booms are reinforced, I think the limiting factor on the MC3 will be the build.

I had one plane that fluttered due to the wing skin deforming where the servos were glued. All linkages were slop free, but when force was applied to the surfaces they deflected quite a bit, and square deformations formed on the top skin where the servos were glued.

Things like this are easily overlooked, as they really don't come into play on normal builds. The forces above 200 are incredible.

.

[daboz]

Don't underestimate the effect of a stiff airframe. While it will not prevent flutter, it will move the speed at which it occurs. Shift it far enough out of your envelope and you have avoided flutter.

agreed I still add all the stiffness modifications. Just want to balance it too. And as mike said its the whole boom and vtail.

[daboz]

For models, I think stiffer and stronger is the way to ultimate speed and no flutter.

Totaly agree! But I'm also going to "TRY" and balance this one...

We'll see..

[Mike the Snake]

Sounds good, go for it.

I think the steel rods will also have the effect of "tube modding" the surfaces at the same time.

[daboz]

Sounds good, go for it.

I think the steel rods will also have the effect of "tube modding" the surfaces at the same time.


yea totally and the radar gun!! by the way I've already flown one like this up to about 2bills and it was fine... AND I also do what you do with the cutting em short about 1.5 inches of the tips for wing/control surface induced dihedral.

looking forward to flying with you some day and I value your input on these things

Thanks

Chris

[daboz]

the plane went 280mph on its first day second flight! My last highest speed was 247 this one seemed to be much smoother and quicker to speed. I think the weight mods help prevent flutter but who knows I also stiffened up all the surfaces as well. The bad news is I have to build another one to further experiment. Had a radio glitch and she left, looping until she disappeared in the distance! Ouch! This hobby is very humbling.