Spoiler

Am busy building mass balancers onto the ends of my Funtana 90 ailerons in an effort to cure the flutter problem this design has if overspeeded. I have read an article regarding full size homebuilts and it appears that overbalancing ie using more counterbalance weight than necessary may eliminate flutter completely.

Anyone ever done this beofre or have any resources thay can refer me to. [8D]

Rodney

Unfortunately, mass balancing has little or no effect on eliminating flutter, in fact in some rare cases, it may even cause it. Flutter is best solved by changing the rigidity (stiffness) of the aileron itself and, in some cases by where the horn is attached to the aileron. You want the horn to be at about 1/3 or 2/5 ths of the span of the aileron. Above all, have all slop out of the hinges, control horns (no oversized holes or wallowed out Zbends) and pushrods and make sure the pushrods are very stiff. If the ailerons are driven by a single servo with wire control rods to each half, any spring or give in these can contribute to the onset of flutter. The only cure for that is to increase the diameter of the wire so that there is no spring back if the surfaces are pushed on.

Bax

My Feedback: (11)

You also have to make sure the control surface is stiff and not flexing in flight. You can have the finest control system with no slop, rigid pushrods, gapless hinges, and still get flutter if the control surface is too 'bendy'

danny03

Rodney, could you clarify something for me? You mentioned that the control horn should be placed 1/3 to 2/5 on the aileron. Is this measurement from the inboard going out or from the outboard going in? Thanks for the clarification..

alasdair

I am afraid that Rodney is quite wrong. Mass balancing is the only sure way of eliminating flutter. All full size aircraft from about the nineteen tweties onwards had mass balances if they had cable operated controls. However the mass balances are usuall applied in a single lump, not distributed evenly along the surface. Also, the control input is usually applied at a single point.

For these reasons the control surfaces have to be as rigid as possible in addition to being mass balanced. Also, the flutter speed is a function of the flexibility of the airframe, the wing and aileron if we are discussing aileron flutter. The stiffer the wing and aileron and control rods then the higher the speed at which flutter starts. If the structure is stiff AND mass balanced then flutter speed can be infinite.

I had a model whose ailerons fluttered. I stiffened the ailerons, the pushrods, cut off and fixed the tips, reshaped the aileron ends, and it increased flutter speed a few mph. But it still crashed. Next I added a single mass balance half way along and it would stand a full power dive.

Ever hear of a fun fly that suffered flutter? Ever read that you can only use full power going straight up?
I mass balanced all the controls on my fun fly and it too would stand a full power dive.

mas balance does the job, but not in isolation.

rmh

Yup - a balance of say - 20% to 0% is the best fix.
I hardly ever seal hinge lines and also don't add $400.00 worth of servos and linkage to try and control bad aileron setups .
After much testing -I am now ready to go back to ONE servo to drive both ailerons up to 35% models.
The setup will be cf pushrods to ball end connections on a L shaped bellcrank each end.
Flaperons are a feature I never use and differential is no problem on light models .
Just use equal throws .
Believe it or not -this is what full scale aerobats use- and you can roll em with two fingers of pressure.
On our bigfoamies (up to 400sqinches and 15 ounces ) we can go full speed with no aileron flutter -with one puny 6 gram Bluebird servo .
and roll instantly
We used single aileron servos for years on early pattern models -the ONLY problem was when using those lousy strip aileron torque rod setups
Worthless.

Darryl Usher

I think that the people that feel holding the surface with a servo to prevent
flutter is lost. Have you ever thought of the flutter load while driving down the road with the plane in the back? Flutter is the relation between the wing and aileron, not many flaps flutter. Some real aircraft have balance weight in the wing to change it's frequency. I you put the weight at the tip of the aileron then the aileron must be more ridged than if it was spread out. Check my fastest 180HP in the world and the real airplane at bottom.
www.acsip.com/~dgu
Darryl

Rodney

I have to dissagree with Alasdair. I spent many years in the flight testing of real aircraft and missiles and can guarantee that everything will flutter given the input that will excite that structurer's natural frequency. The only way to eliminate flutter is to make sure that the natural frequency of any of the components lie outside the range of stimulus. You do that by structurally designing the component parts so that their natural frequency lie outside that range. Mass balance has nothing to do with that unless, in the process of adding it, you have changed the stiffness of the structure. The reason for having the inputs at an odd division of the span is to make sure that the control inputs have not inadvertently created a stimulus at a harmonic that is resonant with both halves (if the control horn were centered), just using the laws of probability to make the possibility of induced flutter a little less. Where mass balance helps, especially on ailerons and elevators, is to take the shock loads off the servos during high G impacts (read that hard landings) allowing the hinges to take the full force of the stresses and not stressing the push rods or linkage.

rmh

OK-then why do most (if not all-Ihave not seen all of them) light aircraft have heavy counterbalance forward of /at hinge line.
It does nothing for aerodynamic balance.
Anything (almost ) can be made to go resonant-sure -but leaving the C/G of the aileron at it's trailing edge-simply invites destruction.
I make all of my own aileron designs as stiff and as light as possible - then balance to 20%-or further forward.

Rotaryphile

Somebody should tell all those misguided full-scale aircraft manufacturers that they are wasting time, weight, money, and drag on their mass balancers.

I have found that carefully applied mass balancers can cure almost all cases of flutter. Overbalancing a control surface can be worse, much worse, than no balancer at all particularly if it is attached at the tip, since it can, but moving the control surface in the opposite direction in response to random excitation by rough air, actually excite a higher harmonic, which almost invariably happens at high speed, and is usually extremely destructive. I made that mistake, and the airplane was virtually destroyed in the air - just a huge cloud of pieces raining down, and the thing was flying straight and level at full throttle at the time, no "G" load at all, other than that caused by normal air turbulence. Live and learn - experimentation with high performance models certainly keeps you humble.

Spoiler

Intersting replies and some contradictory advice. To Rodney, if mass balancing does nothing for flutter, why is it found on various full size planes?

On the Funtana 90, i have noticed that the ailerons are very large and have a different structure to the wing i.e they are cross braced and are stiffer IMO than the wing structure which in typical fun fly fashion has relatively few ribs. I wonder if this contributes to the problem on the F90, which for those that havent read the threads, is a flutter incident at moderate speed leading to catastrophic failure of, usually, the right wing. On my plane the flutter stripped both servos and lead to some cracking and interior damage to the wing ribs. The ailerons and hinges showed no damage at all...

The mass balance idea is to stave off the ailerons tendency to flutter thus increasing the Vne at which flutter may occur making the plane more tolerant of higher speeds.

Squid

Rodney, yourself and Alisdair both have valid points. If I can expand on one of your comments:
>>"Mass balance has nothing to do with that unless, in the process of adding it, you have changed the stiffness of the structure."
I feel it would be more accurate to say "resonant frequency characteristic" instead of "stiffness".

This resonant frequency characteristic is a function of both stiffness AND inertial mass around the axis of flutter, ie. by adding mass to the object (mass balance or otherwise) it increases its inertial mass (resistance to twist), effectively lowering its resonant frequency. It shouldnt matter whether you lower or raise the resonant frequency as long as it is out of the range of the frequency of the excitation force being applied.

When applying this fix, it is important to locate the mass as far from the centre of twist as possible to be effective.

The Aileron and wing should also be looked at as a system rather than independently and they act upon and influence each other.

After witnessing Spoilers wing failure, I was facing my F90 maiden with some trepidation, so I made up some weights on the ends of dowels glued onto the ends of the wings facing forward and some facing backward from the ailerons as well. It wasnt pretty, but I got 2 flights out of it before the flutter set in on the third flight on a slight overspeeding. I acutally saw the wing rather than the aileron flutter this time. IMO the wing is under built. If it had been build stiffer using less rib spacing and cross bracing (as per the ailerons), flutter could have been designed out. In fact Im going to have to apply this mod retrospectively as all the wing ribs broke along the lightening holes. The weight addition should be minimal and the stiffness increase 'maximal'! F90 designer please take note!

rmh

You guys are flying them too fast .
It is that simple
The design was intended to be a slow speed tumble bug -
I did one for a magazine review using a gas engine - worked well andfound that speed of say - up to 30 mph is all that is necessary for the maneuvers .
The aileron servo mount is quite iffy -to be candid but I had no structural problems and used using JR811 servo throughout ,on 6 Volts .
The Harrier by Seagull models is a similiar example - these will come apart if speed is much above basic maneuvering speed .
These things were done to appeal to th craze of "I want it to hover".
Weight is crucial on these so integrity for fast flying was sacrificed.
I place the blame for most of these failures squarely on the customer who will not hold the speed down to the intended range.

Spoiler

Dick, its quite clear we must have flown them too fast - too fast for the design of the ailerons/wing that is. The tail doesent flutter at the speeds we have flown them at, just the ailerons. Its crazy that the design allows flutter at the airspeeds we attained. I don't dispute the purpose of the plane just the margin for error built into the design. I have never had flutter on any of the ARF's or kit built planes I have built. My Tower Uproar handles full power dives without any, as did my H9 Twist. Sure they are both small planes in comparison but the designs have no quirks and flutter simply doesnt occur even at insane speeds.

Being forwarned about the F90 we took precautions - better than std servos, taking care with throttle control, linkages etc. and still we fluttered. There's no margin for error or varying prop pitch to set a motor up etc. Thats bad.

Also, looking at the Seagull Harrier, its smaller than the F90 but lists a 90 as minimum size. I'm wiling to bet it won't exhibit the same flutter characteristics at all. The wing is very different and I bet the ailerons are lighter than the F90's.

I am very interetsed to see how my balancers affect the flutter problem altho with a non stock control horn it will be impossible to say which mod worked.

Squid

Dick are you telling me that a Moki 1.35 with a 8 pitch prop running rich at 1/3 throttle on a 10 degree descent into the landing circuit is too much for the plane? Im shocked! But not surprised really looking at the minimalist build of the plane... Ive got some 17X5 APCs on the way to slow things down, and I promise i'll excersise my left thumb a bit more next time! I guess our flight performance requirements should have included medium speed flight as well when looking for a 3Der. I still feel a certain amount of extra engineering in the wing dept, is required to allow boneheads like me to at least get a couple of flights in before planting it!

Its odd how my Uproar 40 just loves to dive at full throttle (Irvine 40) in combat, and even my Green Models 72" LAser 200 nicely overpowered with an OS1.60 regularly survives an 80% throttle dive on strafing the runway with an 18X8 apc....

Oh well, whats a nice replacement 3Der for a Moki1.35 that wont break up in mid air??

rmh

Glad to see you guys are candid about how you fly em -
I did a Harrier for 3D Flyer -and Ireally liked it '.
Sold it to a good friend - who promptly lost it due to the tail group leaving it in mid flight .
Why?
he thought it was not going very fast -and relative to what he was used to - it wasn't going fast .
The big BUTT--tho is that for great 3D performance -weight must be absolutly minimal.
The kit mfgrs took a deep breath and produced models superior to th faster "3D" models -simply because they are lighter.
The result ?
flutter failure as these models ar totally unforgiving on this issue.
My own first shot at a super high power 3D model/pattern design stayed together using a tuned exhaust ZDZ40 on a 1300 sq in airframe at 11 lbs.
again BUT- wide open throttle was never used in level flight or a dive.
Power was spectacular and wing loading at 19 oz ft made for good 3D .
Our current electric stuff (no pun) is now way down on wing loading and using flat plate wings -all foam and wing loading is under 5 ozs per sq ft.-on 400 sq inches (AUW of 15 ozs inc batts.)
these will not flutter easily as the foam has such low weight to strength and stiffness.
But they will flutter -- just not come apart.
My larger stuf uses built up ailerons (see pics ) of very light full sheeted ailerons -typically tapered 2-1 from root to tip -which gives an extremely stiff aileron.

Squid

Yea Dick, I guess we work hard and play hard, which makes it a bit tough on our planes but thats the way we like it! Maybe when I retire, I'll settle down with a nice Sig Rascal and potter round the sky.

What we both didnt mention was that the motors were brand new on these planes and as such, were running coarser props to keep the load on (the GMS/Webra tend to richen up in flight otherwise, the Moki transitioning after only 2 tanks is also a bit poor) and need to keep the revs up a bit to reduce the chances of flameout. This obviousely makes it even trickier to stay within the recommended speed range.

We could laugh or cry about this, but at the end of the day, its all about having fun, keeping it safe and taking responsibility for ones actions, however insane they may appear to be at the time! Each new plane I build/fly is a new learning experience, sometimes in an unexpected area such as this F90 flutter saga. Im actually enjoying the challenge of increasing the flight envelope for this thing even though the test flights are fairly disasterous at this stage, all good stuff!!!

Anyway, dont wanna get too philosophical, lets go FLY!!!!

PS thats a nice plane Dick, wing reminds me of my ol Electrostreak, kinda looks 'right'...!

Rodney

Very good and pertinent points Squid. Flutter is a very complex subject because its cause and effects are so very varried and is hard to comprehend unless you spend many hours in the study and measurements of cause and effect. Someones comments above on the fact that the main surface could be the origin of flutter is also correct; i.e. wing design could be the cause of what is perceived to be aileron flutter. With out extensive measurements on any individual case, it would be very difficult to pinpoint the exact cause. As to the comment of "Why do full scale designers mass counter balance some surfaces?", the reason is to make it physically less demanding on the pilot. Ailerons tend to droop and elevators also if not counter balanced requireing in some case excessive stick force to overcome at low speeds. It also releives shock loads on the control linkage durring high G forces. And, yes the fact still remains the Mass Balance does nothing directly to eliminate or minimize flutter.

mr_matt

My Feedback: (10)

Then why are rudders mass balanced in full size so often?

I thought the reason for balancing was to ensure the CG of the movable surface was more inline with the hingeline. THis ensures that any resultant force (acting on the movable surface from the fixed surface) at the hinge line, would not induce a moment in the movable surface that could possibly amplify the initial movement in the fixed surface?

Isn't this energy coupling between the fixed and the movable surface REQUIRED for flutter (at least this typical flutter mode) to occur?

rmh

As I recall -ailerons in full scale, light aircraft are coupled -the mass balance has nothing to do with flight control effort.
spades however, do -
elevators droop because they are not hooked up like ailerons .

alasdair

dick Hanson (& mr_Matt and others) correct
Rodney wrong!
Mass balancing is to prevent flutter (almost wholly)
Aerodynamic balancing is to reduce pilot effort.
Rodney, why do you suppose they mass balance rudders just like aileron and elevators? Same reason, Flutter. Read a book about it.
Alasdair

Spoiler

Thanks for the replies, Guys! Yes, we do like to throw the planes around, but in the case of the F90's, we were both taking it very easy, or so we thought. No yank and bank stuff at all. Dick I see from the pics that you do build the ailerons very stiff, the difference seems to be that the size of the aileron is waaaay smaller than the just about, 1/3 wing area ones on the F90. I have included some fuzzy shots of my mass balancers. Any comments welcome

Rodney

Alasdair, I've done a little more than read a book about it. I worked in flight test instrumentation for many many years. That is where I found out mass balance does little or nothing to prevent flutter. To prevent flutter, you must change the modulus of elasticity, mechanically change the coupling forces, stiffen or lighten the surfaces, remove the stimulus plus many other factors. Again, mass balance alone (unless you change one of the other factors in constructing the mass balance) does NOTHING to eliminate flutter. Check with a good airodynamict for more info.

NFOOTE

I work in Flight Controls at Boeing, and as an avid Aero Enthusiast have learned all that I can, and also used my head. There are sevearl factors that Rodney and Alasdair are correct on.

2 forces are acting on the moveable surfaces, ailerons, rudder, flaps, spoilers, horizontal stabilizer, trim tabs etc.

The first force creating a moment at the hinge line is the mass of the object itself.
The Second force is the air moving over the surface creating moments, drag etc which vary depending on the airspeed.

On manual aircraft (almost all aircraft made untili FBW Flight-by-wire aircraft) the surfaces and controls were set with the pilot in mind so that the stick force could actually move the surfaces.

ON Flutter. Flutter is directly proportional to the stiffness of the structure, Stiffness of the controling mechanism, Velocity of flight, and mass of the surface.

The natural frequency of an object is directly proportional to its mass and geometry.

Rodney is correct in saying that if the structure is sufficiently stiff so that all the natural frequencies are out of the operational range or harmonics which are almost just as bad then no counter balance is needed. In fact if you add weight then it can simply cause different harmonics/natural frequencies to be induced.

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.

Full sized aircraft, 2-seaters etc, are mass balanced because most planes are designed for 50lb stick force max and 80 for rudder... there are FAR's about maximum allowed stick force/rudder pedal force. My numbers are close I think but not correct. This is also why you see Z rudder tops and horizontal stabilizers, because they act to counter the moment needed to move said surface.

For instance on all of your full scale commercial aircraft such as 787,777, A350, A380 they don't mass balance anything, but rather design the structure to be sufficiently stiff. IN otherwords the attachment spar and the controling mechanisms. Saves weight thus more efficient airplane.

The quintessance(<<Spelling?) of this principle is observed in variable pitch constant speed propellers where the propeller has to balance the aerodynamic forces, mass of the blade, and be very stiff for most efficient acceleration of the propelled mass of air. In this case the stiffness of the propeller blade often overides any natural frequencies that may be induced by engine vibrations. Off subject, but the principle still applies in a rather obfuscative measure to be sure.

Brian Foote
PS flaps on small planes are usually not counter balanced becsaue they are designed for one operating airspeed (generally), thus you can balance the aero forces and the weight, but this is usually discarded since anymore most private aircraft have a servo mechanical device deploying the flaps or a deploy assist.

NFOOTE

Oh forgot one thing that Rodney said. Yes, put your control horns etc at anything other than 1/2 or 1/3 span of the aileron because the lowest frequencies induced in a structre are 1/2 wavelength of structure followed by 1/3 wavelength nodes.

Brian
thus the 2/5 is a good option