zetor

My Feedback: (2)

I don't know where exactly this should be posted but figured this was the place. I am wondering what the function of stabilizers on control surfaces are for. The name suggests that it will stabilize. People I've talked to at the field tell me that they serve to increase control surface area. Is this the purpose of them?

FHHuber

do you mean the extensions forward of the hinge line on some rudders and elevators? (rarely on ailerons But I've seen them there too.)

If that is it they are aerodynamic balancing areas... they help reduce the possiblity of flutter and take some load off of the servo.

They aren't really to increase the control surface area... you can do that just by having the hinge line a little further forward. This can increase the area by the same amount and in some cases is easier.

zetor

My Feedback: (2)

Yes that's exactly what I'm talking about FH. I knew there must have ben some sort of stabilization function or they wouldn't be called stabilizers. I was thinking stability in terms of aircraft stability and I couldn't make sense of it. The airplane is a pacific edge by pacific aeromodelers and it does have the stabilized ailerons. I guess the purpose is to have an offset hinge line which would help prevent flutter.

FHHuber

The fixed portion of the tailplanes are the stabilizers. The elevator is hinged to the horizontl stabilizer. The rudder is hinged to the fin which is also the vertical stabilizer.

zetor

My Feedback: (2)

Yes I know what a verticle and horizontal stab are. What I meant was the stabilizers on the control surfaces Rudder, elevator and ailerons). I suppose I shouldn't call them stabilers but what would the name be for those extensions on the control surfaces.

BMatthews

You all missed the name for the extensions that are being referred to. I believe they are called aerodynamic balance tabs. Aerodynamic because, as Huber said, they reduce the load on the servo or the pilot's arms depending on what is moving the control.

Often they are used to hide weights that are called balance or anti flutter weights. Anything that can move the balance point of the surfaces towards the hinge line is a good thing.

On our models there really is no need for them. I have no idea why they are so popular with the 3D design crowd. Our servos easily have the power to move the surfaces and they don't put weights in them to assist with reducing flutter. So I have no idea why they use them. Style I guess.

And if you haven't figured it out from all this yet they do not have any stabilizing function. That is for the fixed surfaces as stated.

SST

My Feedback: (2)

When you get into the really large models, there is a real value to aerodynamically counterbalancing the control surfaces to reduce loads on the servos. MikeSell and I recently completed a practice plane for his son Taylorcraft1947's SAE cargo competition. The plane was built to the class's design specs, so there could be a chance to practice with a similar handling aircraft prior to competition day. Anyway, I digress...The (24 inch[sorry...not 40 inch...had a case of H.I.A.]) rudder and elevator were made with these counterbalancers to allow the use of standard size (and consequently lighter) servos. Every effort was made to build the plane light, so more cargo weight could be added. Competition ends today, and we're anxiously waiting for the phone call with the results...

rmh

The aerodynamic counterbalances are quite useful on aerobatic models .
On some ARFS they are simply styling effects.
On my 1/3 and large scale - I use them on the ailerons ends-for adding counterweight ( Igo for 25% cg on the aileron- .
On the rudder they really do unload the servos better - -on some designs - they are too effective and unless a lot of rudder holding power is used - they will cause tail wag as they self servo. seen it , fixed it, etc.
In some cases - a flat dam, attached to the rudder trailing edge is needed to act as a drogue - to keep the rudder from self servoing easily. This is a case of toooo much aero counterbalance area.
On elevators - on my tiny flat foam models -they also reduce elevator load - the servos are tiny and a simple flat hinged plate just takes too much power-for holding against a full prop blast at very low speeds.
On some of the large models - ganged servos having in excess of 300-400 in ounces of torque - will not hold against air loads.
That is why the 3D guys use the assists.
My next slide please ----

MikeSell

My Feedback: (2)

My son just completed competition in SAE Aerodesign east. Their plane lifted the heaviest payload (over 30 lbs). It was servoed with HS81 servos on rather large control surfaces. The aerodynamic counterbalances made that possible. The gross weight of the airpalne was over 50 lbs and it was powered by a .60 but controlled by micro servos, one per surface.

BMatthews

Ah, it's good to see that there are still some oddball extremists out there....

Thanks for setting me straight on the aero models Dick and congrats on your efforts Mike.

dyrbr_d

What exactly do you mean you go for 25% cg on the aileron? You add enough mass in front of the hinge line that it balances 25% back from the hinge line?

When designing what is a rule of thumb that makes the counter balances enough without going to too much? What percentage of the total control surface area?

FHHuber

The counterbalance area becomming too large a percentage of the total control surface area will create a problem with the surface wanting to deflect more than what is commanded by the servo. Depending on the shape of the moving surface and the fixed surface, this can occur earlier, but it will always happen by the time you hit 30%. Its better to have the balance area considerably smaller than that to prevent the resulting "wagging" (its not quite like flutter, its slower.)

For designs such as the Fokker Dr1's rudder... you are kind of hozed... the area ahead of the hinge line is very close to 30% of the total rudder area (and there is no fixed fin) Level flight above 65% throttle would always cause my Triplane's tail to wag at about 3 side-side oscillations per second. (despite going to kevlar pull-pull, and upgrading the hinges and putting in a servo rated double the torque from the "standard" one listed as appropriate in the kit instructions...)

The difference between the wag and flutter is obvious once you have seen both. Flutter is a VERY rapid oscillation and will make a "buzzing" noise. The model may track straight durring flutter. The wag is slow enough that the model can be seen to wave its tail or wobble its wings (depending on which surface is involved). There is no noize with the wag. Flutter can destroy the servo and can even rip the airframe apart. The wag is not going to destroy the servo or the airframe. (unless the servo has a weak gear...)

rmh

The 25% design note, was a concession to being balanced at the hinge line - difficult and heavy on most models.
somewhere in that range has seemed to be best to prevent the ailerons from ever getting into a "flutter".
Why?
The inertia of the aileron is lower.
On elevators - it is best to hold about the same setup -25%
On rudder - same
Funny thing on rudders - If the total vertical area is "undersized"- such as the DR1.
the tail can wag, as the model simply yaws till the load is peaked - then swing back -then repeats--the rudder can be locked solidly - makes no difference.
Also - on some designs - notably- the old KAOS - the location of the canopy can start the same wag. the air is turbulant from the canopy -and the model starts the same rythmic wag from side to side -
We proved this with small paper cups, taped to the top of a fuselage-
really -
The location of the cups could easily control the wag.

Rodney

I've done a few experiments with having part of the control surface forward of the hinge line. My limited testing has proven to me that you do not want to exceed having 10% of the surface ahead of the hinge line or you will get severe hunting around neautral; i.e. the surface will constantly hunt for neautral and cause a tail wag or oscillation. Up to 10% though definately helps take a load off the servos. As to flutter, has little or no effect unless they are flimsy which will then cause flutter to occur more quickly. Do not confuse airodynamic balance with static balance. For static balance you can just add weight ahead of the hinge line, for airodynamic balance you need surface area ahead of the hinge line. Neither has much effect on flutter as flutter is caused by a surface going into self resonance and is strictly a function of hinge slop, soft controls (flimsy push rods) and structural stiffness. Every thing will flutter given the proper stimulus, the secret to good flight surfaces is to keep that resonant frequency out of the range at which we will be using them. Hinge gap and airodynamic balance is not part of the formulae.

Forgues Research

My Feedback: (7)

Boost tabs is the name of the game.

Roger

rmh

Boost tabs do work - however - where is you info re: How much the tab changes the efficiency of the deflected surface?

Forgues Research

My Feedback: (7)

For all practical purposes, the amount of area the boost tab takes away from the control surface is neglegible concerning our models.

I have been using boost tabs on all my large ones with great success.

For instance, on my last Extra (33%), I had enough servo power not to need them,but on the other hand, after 3 15 minutes flights of aerobatics, (not 3D) I checked the battery voltage and it was as if I had just taken the battery off the charger., So one can use also smaller battery, smaller servos etc.

Roger

FHHuber

While the DR1's tail wag could partially be the Dutch Roll effect so well described by Mr Hanson (but he didn't say it was Dutch Roll)

The Dutch Roll is typically accompaied by a noticeable roll in addition to the yaw. (which is why its called Dutch Roll)

In the case of THIS triplane... there was no visible roll accompaning the yaw oscillations... and the rudder itself was observed to be flexing. Thus.. I can't assign all of it to the phenomenom he appropriately mentioned.

Something that may be interresting: You can get Dutch Roll at some combination of power, airspeed and angle of attack with just about any prop driven aircraft. Most designs... its found at low airspeed, high power and relatively high (but not at stall) AOA. (Such as my Tiger 60's full throttle appx 45 deg takeoff angle from short field to clear a 50 ft obstical ASAP. This is the only time it shows with this model.)

***************

The information on the "Boost Tabs" is Great. Best explaination of them that I have seen.

rmh

The so called "Dutch Roll",can easily be designed out of our models.
Since many of our models are copies of a full scale subjects, or aproximations of em, we have relatively the same moment arm relationships, etc..
IF-- you shorten the span , the problem is reduced .
IF you increase the tail volume the problem is reduced -sometimes.
IF you reduce drag in the forward portions of the airframe and increase it in the aft sections - the problem is reduced.
IF you could take a simple arrow and slide the feathers forward along the shaft - you should get into this same situation at some point.
Realistically, the problem in the models appears to be (to me), a situation where the dynamic forces occurring in each wing panel , are more than the tail group can easily stabilize.
All of my own fiddling bears this out.
Sorry - my engineering speak isn't working --but I am certain someone has a formula (similac?) for all of this .