Aileron Balancing

I have a question regarding aileron balancing and would like to bounce my understanding off the collective wisdom of the forum.

I think that ailerons can be balanced dynamically and/or statically.

1. Dynamic balance is done to reduce pilot force input requirements and is achieved by having some area of the control surface in front of the hinge arm.

2. Static balance is done to reduce the possibility of flutter by moving the aileron c of g closer to (or forwards of?) the hinge arm and is achieved with some ballast forward of the hinge arm.

3. A 100% statically balanced aileron would have a c of g on the hinge line.

4. A 100% dynamically balanced aileron would have as much area in front of the hinge arm as behind it. This would mean that the pilots input would only need to overcome system friction and no aero forces.

Are my statements 1-4 correct? Are there other ways of achieving 1? Is 4 potentially dangerous as it verges in unstable control surfaces? I.E. if it were 101% balanced the pilot would have to juggle to prevent the controls going to full deflection.

thanks for your help

 

Some say 100% static balance, some say a little less.
You don't want 100% dynamic balance as you described, just enough to lighten the control if needed. It doesn't take the same area on both sides of the hinge to be 100% dynamically balanced, except when you are not moving and there is no wind. The wind(airspeed) is acting like hydraulic power steering without a pressure regulator. The more wind, the more assist you get.
Remember it also adds a little drag.

 

The term Dynamic Balance implies that this is involved with some sort of interaction with movement - perhaps Aero Balance or Aerodynamic Balance are more accurate terms You appear to understand Static Balance. So, let's edit the first three:

1. Aero balance is usually done to reduce pilot force input requirements and is achieved by having some area of the control surface in front of the hinge arm. There are other methods for achieving static balance.

2. Static balance is done to reduce the possibility of flutter by moving the aileron c of g closer to (or forwards of?) the hinge arm and is achieved with some ballast forward of the hinge arm.

3. A 100% statically balanced aileron would have a c of g on the hinge line.

Now 4, well, right concept but very wrong on execution. Aero balance gets complicated in a hurry.

First let's get the concept straightened out. If you put the hinge at 50% C', the ailerons would try to deflect hard over, called aileron snatch. This is because the lift produced at the aft end of all airfoils decreases pretty much linearly and toward zero as you go towards the trailing edge. Look it up in the v/V or (v/V)^2 plots on any airfoil. The lift looks like a triangle, and the centroid of lift on things like ailerons is about 33% C'. All other things having no effect, if you put the hinge line about 1/3 back along an aileron, it would have zero restoring moment.

There are other things in there too.

If you were to simply make the profile of the aileron a slice from the wing and put the hinge at 33% C', when you deflect the aileron, the part forward of the hinge protrudes into the airflow and adds more moment towards deflection. This is another type of aero balance added on top of what you already have and is capable of producing aileron snatch if too large. It can be tuned by changing the radii at the forward corners, narrowing the width of the forward portions, or rounding the whole thing over;

You can do what a number of aerobatic ships and the last of the muscle powered big airplanes did. Round off the part of control surfaces ahead of the hinge, which reduces the additional balance due to deflection into the air flow. This reduces but does not eliminate the issue.

Next up is that lift is distributed pretty much elliptically span wise, so the lift at the tip gets pretty darned small as you move towards the tip. A sheilded balance horn at the tip of wing is mostly to get mass balance at min weight while giving some aero balance when deflected. An unshielded balance horn will reduce on center feel, while a shielded horn tends to have better on-center feel but can have non-linear feel as deflection proceeds.

Extending the balance horn forward allows less weight to do the job. When the balance horn is at the wingtip, it does tend to have less aero balance than if the same area and arm effects were further inboard. Moving it inboard will increase the aero balance effect of the same horn. Placing the balance horn at the wing tip MAY also require more structure weight in the aileron as it is at one end so all of the moment from the surfaces has to be carried to one end. Min gage issues and designing ailerons to stiffness tends to make this last one moot in our little airplanes.

Most of us like to have some on-center feel for controls, and then nicely increasing control forces as the surface is deflected. The on-center feel allows you to put a hand on the control and not deflect it, and keeps the control from floating, which will make the airplane wander. The entire force vs deflection curve that starts low (but not on zero) and monotonically grows allows us to feel control surface is deflection. Aileron snatch is the opposite, with forces to hold a certain deflection decreasing and then going opposite the direction of travel, and needs to be avoided. Worst case is aileron lock, where it snatches, then has such high forces that you can not return the control to neutral... Hope you are wearing a parachute and can get out if this ever happens.

The task of balance is thus several fold:

If you are fast enough to need mass balance, you do it:

Aileron forces should be put into harmony with the other control forces. There are a variety of rules on the apparent forces for each axis. Ailerons should be lightest, and yet, they tend to be heaviest unless we do some aero balance and/or use very small aileron chords;

The forces should be progressive - that is higher as airspeed goes up, higher as the deflection increases, higher as response increases;

No force reversals are desired. Aerobatic pilots sometimes put in huge efforts to get their spades and thus control feel right over the whole range.

Have fun making all of that work...

Billski

 

One more: Any control that requires almost no effort to move (too much dynamic balance) can result in overstressing the structure much too easily. Force feedback gives the pilot an intuitive idea of the stress he's placing on the airframe, and low force could imply that little stress is being generated. Dangerous.

 

Aero balance requires extension beyond the wingtip. Like they do the aero balance on the tips of elevators and rudders.

 

Frise ailerons?

 

A Frise aileron creates drag, by having its leading edge deflect into the airflow below the wing, that counters adverse yaw.


BJC

 

Thanks all. A bit of reading to be done to get through that lot!

 

Other factors to consider when designing aerodynamically balanced ailerons are centering force / breakout force (a little is desirable, a lot is not) and force required for deflection. A decreasing force requirement per degree of deflection is generally undesirable, and if it goes negative, leads to the previously mentioned aileron snatch. (Check out rudder lock for another interesting aerodynamic condition.) Ailerons on modern aerobatic airplanes are designed to seal the leading edge gap at full deflection to maximize effectiveness at full deflection.

Also note that mass balancing the ailerons is done to address aileron flutter; the wing that the aileron is attached to is another issue.


BJC

 

Back in the early 70's I was contest flying in the then poplar Fun Fly event. Takeoff and climb and max turns in a spin in 30 seconds--- then high speed past pass the judges at 25' and start rolling for max rolls in 30 seconds -- then most loops in front of judges in 30 seconds --- spot landing.
My design-- 48" span, 576 sq" of area, weight 3 lbs. Powered by a racing Super Tiger .61 running lots of nitro. Hold model straight up and let go and it would accelerate going straight up and out of sight in about 10 sec for the most turns in a flat spin and about 1/3 throttle. Need at least 60 turns.
Rolls- need a min of 60 rolls in 30 seconds-- Large barn door ailerons for max arm. Having trouble with the aileron servos stripping gears from the airloads. ( no metal servo gears back at that time). I had flat wing tips and extended the aileron past the wing tip and forward to about the main spar location. That was about 25% area of the aileron ahead of the hinge line. Stopped the stripping of the gears, but the next weakest point showed up.
When making the low pass at high speed past the judges to start the rolls you have to start climbing while rolling and then level off ( l like left rolls the best) and rolling in a circle. When leveling off and going to a slight high airspeed the wings starting failing ahead of the barn door ailerons. Just couldn't take the twist. I had to extend the balsa sheeting covering on the wings toward the tips that added weight that I didn't need for the vertical climb in the spin part of the flight.


Added --- Used the same design for Open Pylon racing. Except for a 8% airfoil instead of the 15% airfoil. Also a lot smaller control surfaces with much less throw on the surfaces.
Moral of the story-- Tread softly.

 

https://aviationglossary.com/frise-type-aileron/

[FONT=&quot]Engineer Leslie George Frise (1897-1979) developed an aileron shape which is often used due to its ability to counteract adverse yaw. The Frise aileron is pivoted at about its 20% chord line and near its bottom surface. The leading edge of the aileron is bluntly rounded so that when the aileron is deflected up (to make its wing go down), the leading edge of the aileron dips into the airflow beneath the wing and adds significant drag to that wing. The resulting drag causes the aircraft to pivot (turn) in the desired direction.[/FONT]
[FONT=&quot]The leading edge also gives a servo assist to the stick force – the moment of the leading edge in the airflow helps to move the trailing edge. The down-moving aileron also adds energy to the boundary layer by the airflow from the under-side of the wing that scoops air by the edge of the aileron that follows the upper surface of the aileron and creates a lifting force on the upper surface of the aileron aiding the lift of the wing. That reduces the needed deflection angle of the aileron.[/FONT]
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