Recently there was a lot of discussion about the pros and cons of balancing stabilators on jet models. Local modeler Jim Drever (turbine F-18 flier) got in touch with Blaine Beron-Rawdon of Evision Design (http://members.cox.net/evdesign/).

He makes the "Plane Geometry" and "Linkage Design" software that many of us used (highly recommended by Bob Parks of the RC Jet List)

Here is what he said:

1.) 1. We can't think of a good reason to avoid mass balancing a control surface similar to the full-flying stab on an F-18. There may be a perception by some that the extra weight will have an adverse effect on the responsiveness of the elevator. This is positively not true for almost all systems. The most important element in the control system inertia is the armature of the servo. Inertia goes as RPM squared. With a typical servo being geared close to 100:1, the armature has a 10,000:1 disadvantage compared to the output arm, and if the output arm is hooked to the control surface at a 4:1 ratio, this ratio jumps to 160,000:1 ratio. Meaning: The control surface has to have on the order of 160,000 times as much mass moment of inertia as the servo armature in order to equal the armature's effect on servo responsiveness. Conclusion: The armature dominates and the effect of almost any control surface's inertia is insignificant. We have confirmed this in recent relatively sophisticated experiments using model airplane servos.

2.) It is important to pivot the full-flying control surface slightly ahead of the quarter chord of the exposed surface mean aerodynamic chord. Someplace around 20% is typical. If you pivot it behind the quarter chord there will tend to be a divergent moment on the surface that drives it to greater deflection as deflection is increased. On the other hand, if the pivot is too far forward, hinge moments and the load on the servo may be excessive. 20% gives a safe margin from 25% to account for errors and aerodynamic funny business.

3.) It is very important to use nearly all the travel of the servo in driving the control surface through its full range. That is, make sure that you have the most favorable gearing in driving the stabilizer. This increases the stiffness of the system. Concerns regarding the frequency response of the surface (or the angular rate) are misplaced if the control surface is hand-flown. The frequency response of high quality servos is much greater than the hand-eye combination and is sufficient even at full throw. This may not be so true if a flight control computer or pitch rate gyro is used.

4.) There are two basic forms of elasticity in an F-18 type model that are of concern, and there are two types of mass balance that address these two forms. The most likely concern is elasticity and free play (slop) in the servo/control linkage system. In recent testing of very high quality model airplane servos including Volz metal-case, metal-gear servos, we found that there is significant free play around neutral. Furthermore, the control amplifier gain schedule near neutral is relatively soft. These two effects combine to give a relatively rubbery feel to the control surface near neutral. Note that for full size airplanes such as the F-18 the surfaces are controlled by hydraulic actuators that are extremely rigid by comparison.

If the primary free play is located in the servo and the linkage, then the location of the mass balance is relatively unimportant. I would suggest that a good location is at the leading edge of the root of the stabilizer. This mass will counter the potential for pitch flutter driven by wholesale stabilizer oscillation arising from free play in the linkage.

5. Another form of elasticity in the system arises from torsional flexibility in the stabilizers themselves. Even if the servos and linkage were perfectly rigid and slop-free, the stabilizers could flutter if their balance point was aft of the elastic axis and if the stabilizers were limber in torsion. A way to resolve this is to mass balance out near the tip of the stabilizer. Of course, this presents a problem on the F-18 in that the tip of the stab is behind the pivot axis due to sweep. A solution would be to mount a mass-balance boom at the tip and weight the tip of this boom well ahead of the pivot axis.

A possibly more attractive solution to this problem is to make sure that the torsional flexibility of the stabilizer is insignificant relative to the freeplay and elasticity in the control system.

Since your model will not be flying at high subsonic speeds, it may be possible to thicken the stabilizer airfoils. This can have strong effect on torsional rigidity. Alternatively, or additionally, the stabs can be made stiffer by the addition of a composite skin. This is ideally a bi-woven cloth oriented at +/- 45 degrees.

OK, so here are the conclusions:

1.) We can't see any good reason to not mass-balance a full-flying stabilizer on a very fast model jet. This appears to be a very worthwhile risk-reduction with no significant adverse effect.

2.) Make the stabilizers relatively rigid in torsion.

3.) Gear the elevator servo so that the full servo motion is used in making full elevator motion. Strive for a rigid, slop-free linkage and use a high-quality servo.

4.) Mass balance the surface about the pivot axis with a weight near the leading edge root.

5.) Pivot the control surface near to 20% of the exposed MAC.


Regards,

Thanks Matt

Dang Matt,,,,,,,,,,, Now I have to go back to school. I got what you said,,,,, between this and the meeting I was at last weekend with Steven Hawking's I had to take an ADVIL. Thank you.

DaveR

Matt,


Thank you. That's some impressive info. How did you get those guys to do such specific tests for you so fast? What were the 'tests' exactly? Did they charge you anything for those tests? If not, I've got a bunch more they can do!

Again, I talked to Charles at Yellow, and again, he says it's 'Bullsh**t that sounds like science'. He again pointed out that of all the world's 100+ fullsize aircraft that have full-flying stabs (including mine), the number of them that have the stabs statically balanced at the pivot point is right around ZERO. So, now my question is what do we modellers know that no designer or engineer of fullsize aircraft seems to know? All the BS about 'it doesn't apply because of supersonic/subsonic' or 'those are hydraulic and these are pulley' aside, common sense tells me that if it were best to mass balance on the pivot, there would be at least a few examples of this flying around in the fullsize world. They aren't THAT different, are they?

These are guys that do this stuff for a living, and happen to do modeling as well. They did the tests for their own benefit not mine.

It sure does......

LOL @ Matt's Avitar

Yeah, some of the top scientific minds of the day wrote scholarly papers proving that women and Blacks were physiologically unsuitable for pilot-training, but we see where that went....

So, if your buddies did similar tests on fullsize aircraft, would they come up with the same summations, that it's better to mass-balance at the pivot? That's really the only question I have left--the one I keep asking, why the difference in the 'real world'? Would you mind answering that? I think once we get that sorted out, I can figure out whether or not to balance my stabs. Thanks.

Sorry I just can't get any clearer than that post. Reread number 4


There are two basic forms of elasticity in an F-18 type model that are of concern, and there are two types of mass balance that address these two forms. The most likely concern is elasticity and free play (slop) in the servo/control linkage system. In recent testing of very high quality model airplane servos including Volz metal-case, metal-gear servos, we found that there is significant free play around neutral. Furthermore, the control amplifier gain schedule near neutral is relatively soft. These two effects combine to give a relatively rubbery feel to the control surface near neutral. Note that for full size airplanes such as the F-18 the surfaces are controlled by hydraulic actuators that are extremely rigid by comparison.




And I am not sure I agree that no planes use mass balancers, you might want to check on that one, I spent one summer of my youth machining the mass balancers for the L-1011.

PS I just did a search on "mass balanced" stabilators and got several hits of full size planes with mass balancers.

PPS I am an engineer by trade, engineering by definition is "applied science". Not sure what the alternative is if you want to design something.

Matt,

You can't get any clearer than that, huh? Dang. Number four doesn't address my question a bit. Let me see if I can break it down a bit. If the reason it's best to mass balance a hornet stab on the pivot on a model, but NOT on the real one is because the real one uses a rigid hydraulic system, then wouldn't they use a lighter system if they could by mass-balancing the stabs on the pivot? My Cherokee isn't using a rigid hydraulic system, and the stab is not mass-balanced on the pivot, it's very tail heavy (until it aerodynamically balances itself when I take off...). Also, an L-1011 uses a full-flying stabilator?? I don't mean for trim, I mean in flight? If so, why is there a big ol' elevator AND an elevator trim tab?

ATFQ, please...

I just looked up what ATFQ means, real nice....


First I never said the L-1011 used a mass balanced stab, I said it had mass balancers, I think the ones I made were for the wing, maybe the ailerons.

Second, if you are going to read the post but dismiss the content I don't know how to help you. Most full size fighters fly supersonic. In supersonic flight, the aerodynamic center moves very far aft, therefore they need massive irreverisble hydraulic actuatirs for this (read number 2 about where to put the pivot). A supersonic fighter cannot put the pivot that far forward as in supersonic flight the control forces would be too high.

Maybe that is why they do not balance them, they already have these fantastically stiff actuators.

As I said before, do a google search on "mass balanced" stabilators I found full size with balancers that are not mach 2 fighters

Not a flame, just a suggestion to re-read the posted material, specifically number 2.

He's not saying that the stab should be static mass balanced at the pivot point.

Dan

===================================

2.) It is important to pivot the full-flying control surface slightly ahead of the quarter chord of the exposed surface mean aerodynamic chord. Someplace around 20% is typical. If you pivot it behind the quarter chord there will tend to be a divergent moment on the surface that drives it to greater deflection as deflection is increased. On the other hand, if the pivot is too far forward, hinge moments and the load on the servo may be excessive. 20% gives a safe margin from 25% to account for errors and aerodynamic funny business.

Dan,

No flame taken. The point of this thread was to back up his position that the stabs on that plane should be balanced on the pivot. That's where the whole deal started.

Matt,

If you read my post, I was talking about the absence of fullsize planes with FULL FLYING stabs that are balanced on the pivot, the way you say we should on models. I don't know why you mentioned the L1011 if you weren't talking about that. Anyway, ATFQ wasn't meant seriously, or offensively. It's just us guys talking, right? That's cool you're an engineer. I'm just a dumb cop myself, but I still want to know why if balancing on the pivot is good, why nobody does it to fullsize aircraft--supersonic, sub-sonic or otherwise. Can't just be the rigidity of the control system (F18), because like i said, the control system on a PA-26 isn't so rigid. The stab is tail heavy but I notice that when there's air moving over it, it lightens up and 'balances' aerodynamically. That's what Charles is saying the F18 stabs do. Now, if I went over and added a bunch of lead to the leading edge of the stab on the Cherokee so that it balanced on the pivot, would I be creating a problem? If no, WHAT'S THE DIFFERENCE? (why didn't Piper--or any other mfgr.--balance them) If yes, WHAT'S THE DIFFERENCE (why isn't it beneficial on the Cherokee, but it's beneficial on the 1/7th F18? I'm not trying to be a jerk, I just want a straight answer. Since you seem to be the expert, I'm asking you.

Thanks for clarifying the ATFQ, no offense taken.

As for me being an engineer, I am not an aeronautical or mechanical engineer, so please do not think I am an expert, I am not. I am a dumb electrical engineer.

I will try to get a better answer to your direct question.

BTW here are links to planes with balanced stabilators, I looked for about 15 seconds and found 2:

http://www.libertyaircraft.com/libertyxl2/specs.php

http://www.jodel.com/g-aydz.htm

I should have been more clear about to whom I was referring when I wrote "he".

I was referring to Beron-Rawdon, not Matt.

I, for one, would be curious to find out if the stabs of full-size fighters aren't actually mass balanced as Charles claims or if they're simply balanced aft of the pivot point as Beron-Rawdon recommends, which makes them appear to be unbalanced. I don't know either way, but I think this information would have a bearing on this discussion.

You mention that the stab on your Cherokee isn't mass balanced on the pivot, but does that mean it isn't mass balanced at all? Also, where on the stab's MAC does your Cherokee's pivot lay? Is it 20%, 23%, 25%, 30%, etc.?

I did a brief online search for information about the Cherokee stab balance and found the following on the Cherokee 235/236 Owners Group Web-Board:

advice on buying a dakota
--John Brooks (qqvepy)

"...Look in the tail cone, from the baggage compartment. This area holds the battery, the stabilator weight, and control cables. See if the battery box has been kept clean and corrosion free. If not, look underneath the box for structure corrosion. Also from the baggage area, shine a bright light at the stabilator balance weight. No corrosion allowed, shiny black paint is what you want to see! You can tell a lot about the plane by snooping around the interior..."

Stabilator Balance (Discusses how to balance the stabilator both on and off the airplane)

Naturally, this web site is dedicated to Dakota owners, but I can't imagine that the Dakota, which is basically a Cherokee with a tapered wing and a more powerful engine, uses a statically balanced stab and the Cherokee doesn't.

Dan

Dan,

Flown the Dakota, the stab isn't balanced on the pivot. Charles never said it isn't balanced! He said it isn't balanced on the PIVOT! Let's not get the water too muddy here, we've been talking about balancing on the pivot, not some mysterious location on the stab. Nobody said there isn't a balance point, or that they don't balance these things when they build them. We've been talking about balancing on the stab. Now, the guys that have been saying we should balance the stabs on the YAC hornet, where were you saying to balance it? So now are we agreed that fullsize planes with full-flying stabs aren't balanced on the pivot at least?

It would appear that your last statement is the case, ie. that the stabs are balanced, they just arent' balanced on the pivot.

If I were to hazard a guess, I would say that they're probably balanced at 1/4 chord (25%) of the MAC with the pivot at somewhere about 20% MAC.

And this makes sense to me.

Where flying stabs are concerned, there's always going to be a balance point. The question is, where is it balanced? Is it 20%, 23%, 25%, 30%, 35%, 50%, etc.?

For the record, with regards to the balancing/not balancing debate, I've never taken a position either way because I'm not an aeronautical engineer, nor do I play one on the internet.

Dan

The Cherokee stabilator also uses a large anti-servo tab. This adds a damping effect that would have an impact on the flutter response about the pivot axis.
Roger

Dan,

Gotcha.

Matt,

Those two planes you showed look like they're mass-balanced. Are they balanced on the pivot? Doesn't look like it. The second one is clearly drooping at the trailing edge. Remember, he said there aren't any fullsize airplanes with full-flying stabs that are balanced on the pivot. I'll keep retyping that if I have to! So, I'll bet you a six-pack neither of those two planes have full-flying stabs that don't drop aft-end down when you let go of the yoke (with the control lock removed, of course). Again, this is the core of the issue. Remember, everyone was talking about whether or not to balance the F18 stabs on the pivot!

Roger,

I think that tab is a mechanical trim tab, designed to use air pressure to force the surface one way or the other to eleviate pilot strain in holding trim.

The Cherokee anti-servo tab does function as a trim tab, but when the stabilator is moved, the tab moves further, generating an aerodynamic force which opposes the stabilator motion, making it an anti-servo tab.
Roger

Nope unfortunately a gentleman on this thread confused it a bit. When I say balanced it means on (or close to) the pivot, because that is where any force from the rest of the plane will act on the stab, at the pivot.

If the CG of the stab is at the pivot, then any force at the pivot cannot generate a (torquing) moment. That is what you are trying to avoid, in order to help flutter resistance.


But it does not have to be right on the pivot to help, if a balance weight helps move the CG closer to the pivot that is better than having it further away.

OK I pulled Bob Parks away from his work on the Mars plane to weigh in on this. A lot of guys on this board know who he is, for those who don't, trust me he is one of the best aeronautical engineers I have ever met.

I asked him 2 things:

1.) Comment on the original post by Blain. His response "I agree, Blaine knows his stuff".

2.) Second question, why don't all planes have mass balancers on the stabilator?

His answer:


<<The mass balance is to suppress flutter. Do not confuse this with the
tail heavy stick "feel" of the Cherokee on the ground. Mass balance is one of many ways to prevent flutter, and mass balancing has the obvious disadvantage of adding weight in a part of the airplane you don't want to add weight to.

Proper mass balance will raise the flutter speed, all else being equal. For many of the full scale examples, they may not have mass balances, but what they do have is a lot of design analysis and flight test to prove that they will not flutter within the placard limits. (and many general aviation airplanes with full flying tails HAVE had flutter problems in flight test that had to be fixed)

There are lots of full scale airplanes with mass balances. For some very obvious examples, look at some of the Soviet jet fighters where they have tip balances in booms ahead of the LE.

Note that the one piece, full span tail on the Cherokee is a very
different matter than the split tails on the jets.. the split tails have
a few more ways to flutter, so more care is needed.

Summary, a -18 with mass balances will have a flutter speed that is higher than one that does not have the mass balances. If you have really good servos and good linkages, (and you make sure they are not wearing out), and you don't go too fast, then its fine. If its not really that good, and you go too fast, the tails come off and you crash.

bob>>


There you go guys, I have given all I have to this discussion!!

So let me see if I got this straight.......

1. It's always better to static balance the full-flying stab on the pivot. [though there isn't a single example of this in fullsize aircraft that anyone can think of...(and no, Matt, I aint talking about some Russian STOL plane with a couter-balance barbell sticking out in front of the aileron--I'm talking about a full-flying stab that's balanced on the pivot)]

2. It's to reduce chance of flutter and servo stress. [though nobody's ever seen the stab on one of these F18's flutter when unbalanced, even on the smaller one with a RAM 750 at full throttle and only 90 oz. servos on the tail (mine)]

3. The aeronautical engineer guy who designed the tail on this model, and all the guys who've ever designed ANY fullsize plane with a full-flying stab, are wrong about balancing on the pivot [wait, this doesn't count because a.) Cherokees have straight, constant-chord stabs; b.) military jet stabs aren't balanced on the pivot because they're supersonic (that rule seems to apply to the subsonic ones, too AND that rule ignores the fact that those planes spend less than 1% of their air time in supersonic flight); c.) military jet stabs aren't balanced on the pivot because they use hydraulics (?????); d.) military jet stabs aren't balanced on the pivot because they use computer control (???)]

Seems like if you take the common sense approach, balancing the stab on the pivot doesn't make any sense. If it did, you'd see it happening in something other than this one particular model. Every time someone comes up with a contradicting example, like the Cherokee (and every other plane on Earth with a full-flying stab), someone just says, 'well, that's different'. How can they all be different? I'm convinced that stabs are balanced. Mass balanced or whatever, but I haven't seen anything (sensible anyway) that says they should be balanced on the pivot. I think it's ridiculous to ignore the fact that no airplanes, jet or otherwise, supersonic or otherwise, computer-controlled or otherwise, hydraulic or otherwise, have full-flying stabs that are balanced on the pivot.

Charles (aeronautical engineering PHD) was done with this discussion when he wrote in the instructions not to balance it on the pivot. Shawn was done with this discussion when it turned into a bashing of the kit. Matt says he's done (I'm glad, too. No offense, Matt, you seem like a nice guy, but you were starting to make me dizzy with the circles you started going around in). Now, I'm done cuz I can't get any straight answers. I think I'm gonna stick with the PHD and all the real-world examples on this one. Thanks!

First of all...I'm not an aeronautical engineer. These are just my observations from having an interest in full scale and model aviation.

I do not believe that there is a clear answer here. Different types of aircraft, (I'm talking full scale) flying at different speeds with different methods of control surface actuation, will all vary in relation to static balancing---If balanced at all. The full scale F-18 does not require static balancing on the stab pivot point due to the THOUSANDS of pounds of pressure that the hydraulic actuators can provide. Small private planes vary in their balancing due to their speeds, loads, etc.

Our models are completely different animals. On a model F-18 we do not have the same type of control surface actuation and force available to us. We rely on servos with their combination of torque and mechanical advantage in the linkage to overcome flight loads and flutter. One PROVEN method of decreasing the potential for flutter in our models is to balance the stabs on the pivot point. The pivot point, in most cases on our models, is located on the point of the center of pressure on any given stab. Balancing the stabs, especially in our larger jets, is very important in my opinion. The stabs own weight has a force of its' own. In flight, the servo must overcome this force plus any other forces incurred. Depending on the speed, oscillations can develop. Go fast enough, those oscillations turn into full blown flutter. In the early days of jet modeling I've witnessed first hand the horrible outcome due to failing to balance the stabs on a Byron F-16. In some cases, the oscillations alone can strip servo gears without flutter ever raising its ugly head. By then it could be too late. Your $8000 jet is a smokin' hole in the ground and you don't have a clue as to why.

Comparing what is done in full scale aviation vs jet modeling, in this case, is comparing apples to oranges. The bottom line is this....You can take your chances by not balancing the stabs. If you feel comfortable with the servo and linkage taking on all of the flight and static loads---Don't balance. As for me, I'm going to make my servos lives a little easier and balance the stabs.

Flame suit on---Fire away!!!

Kevin