Building all moving stabs on my SU-27. Original plans called for elevator, but I wanted scale. They will function as 'ailerons' coupled with flaperons. Two questions: (1)What is the formula for determining the pivot point and (2) should they be 'nose' heavy or 'tail' heavy? I know this has been discussed before but can't find anything in the new RCU search. Bill S.

The pivot point for least load on the servo(s) should be no further aft than 25% mac of the surface. Balancing a little forward of the pivot aids in streamlining under load, without adding too much load on the servo.
Find the mac using the normal procedure used for finding it on a swept wing.

Hi Bill,

Place the pivot point in the scale location. It is located at ~20% MAC. It is unnecessary to balance the stab. This subject has been covered numerous times in the jet forum.

If you insist on balancing, balance the stab at 25% MAC. Yes, I know this is aft of the pivot point, but that is what you want. You definitely do not want the stab balanced at or ahead of the pivot point.

As an example, you'll notice that in all the static shots of all the jets when the hydraulics system is relieved of pressure, does the trailing edge droop or the leading edge? In all cases, the trailing edge droops which says that the stab is balanced aft of the pivot point.

Same for the Piper airplanes with full-flying stabs. Pivot at ~20%, balanced aft of pivot point. (confirmed by Piper owner/drivers)

Dan

ailerons on full scale puddlejumpers are balanced and linked together and there is no hydraulic/etc.
Just the armstrong setup.- a whole different ballgame.
The 20-25 % is for our uses - the good setup
and on flying stabs ditto.

Not talking about ailerons. Talking about full-flying stabs.

Most full-flying stabs are NOT balanced on the pivot point. In fact, I can't think of one that is. Can you?

Maybe I posed the question the wrong way. Assuming the pivot point is on the scale 20% MAC, would-or should- the stab be 'balanced' at that point with no linkage connected. That is, would it 'sit' level. Indeed, I recall most jet stabs hang tail heavy. I assume full size, and models too, are attempting to put the least strain on the actuator, or servo. Maybe there are other reasons aerodynamically for their design and balance-or being purposely weighted to hang aft.

wsmalley, as long as you keep the pivot point around 20-25% you should be fine. The stab probably will not balance at this point but shouldnt cause you any problems. Make sure your linkages are good and no slop.

I just muddied the waters -- Dan Savage answer is good advice.
More mud--- It is easy to really screw up counterbalanced (aerodynamic and static stuff )
Example - the rudders on some models have the top counterbalance increased in size -in an effort to make the rudder more effective . The "3D" boys use this.
It does - BUT in some cases the rudder will not hold center as the counterbalance tries to snatch -or self servo and cause tail wag.
a flying stab, can really cause problems if it is done in this manner (too much area ahead of the hinge point.
The STATIC balance - I mentioned was in referrence to wings w/ simple hinged ailerons and as DanSavage mentioned not really a requirement of the flying stab /stabilator stabilon(?)
what the hell do you call them

Let's see: all moving stabilizer, taileron- only I guess if they function as ailerons, elevon, stabilator, I was confused too. Maybe someone can clear this factoid up once and for all. Reminds me of the discussions on leading edge flaps and slats. Even 'tech' writers use the terms interchangeably. Slats go forward and allow air to pass between for high AOA. Flaps hinge down and change the chord-right?

I just checked and the scale pivot location for the Su-27 stab is 21% MAC.

I'm not an aeronautical engineer, nor do I play one on the internet. But, as I understand it, it has something to do with the static margin of balance. With the stab balanced aft of the pivot point, it wants to "rotate" around it's own CG. Since the pivot is located ahead of the CG, it physically prevents this "rotation" from occurring, thus preventing the surface from fluttering. Or something like this.

I used to have a more scientific explanation, but can't seem to put my fingers on it right now. Charles at Yellow Aircraft has explained this, too and says that it is unnecessary to balance the stabs on any of his jets. Says you can if you really want, but it really doesn't matter. Says that all you're doing is relieving the servo load on the ground.

It's not that they're purposefully balanced tail heavy. Rather, the CG is aft of the pivot point, so they naturally hang down once the hydraulic pressure is removed.

('leading edge devices'?)

Dick,

I understand what you're describing and you're right about hinged controls like ailerons, elevators and rudders. I agree that those should be static balanced.

I know what I call them and have heard many different names including elevons, tailerons, flying tails, flying stabs and even ailervators. (Yes, Futaba does use that term in the manual to one of their radios, or so I'm told)

I just call them elevons because I'm too lazy to say taileron.

Still confused! Was looking through an old book I've had, "RC Ducted Fans-How to build and fly your own jet successfully" by Frank Fanelli. Under 'Control surface balance' -" With a stabilator, weight is added.....in the root of the control surface at its leading edge. In this case, the required balance doesn't mean that the surface remains level at its pivot point. The leading edge should actually droop five degrees down to eliminate any flutter." Which is right-weighted fore or aft?

The c.g. ahead of the pivot point, which should be at or ahead of the 25% mac, is likely to be destabilizing. The aerodynamic forces on the surface will be fighting the inertial force.
I'd think an arrangement which "auto-streamlines" the surface would take precedence over a static balance. If there's a LOT of unbalance so the surface droops trailing edge down under no flight loads, then I'd do something to change that, so it is less off-balance, but not to the point where it's overbalanced nose-down. Kinda t.e. down statically, which doesn't load the servo all that much, but maintains the streamlining situation.

Keep in mind also that the pivot on the full scale stab has to be placed with supersonic loading in mind. Aerodynamic centers generally shift aft when the flow becomes supersonic. I expect that a pivot placed to minimized subsonic servo loads will result in high supersonic loads and vice versa. Probably not a concern at less than full scale.