Hi,

I previously posted a comment and video about a test-hop (maiden) I helped a fellow modeler with. The airplane looked and flew GREAT, and while the gear had a little trouble retracting reliably on the ground, they performed flawlessly in the air.

Yesterday, we flew the airplane another two flights. The first ended in a touch-n-go (we fly from a short runway and with 0 head-wind, I went longer than I'm used to) followed by a super-grease landing. The second flight, unfortunately, ended in disaster.

On the second pass after the takeoff, I was climbing at about 10 degrees with full-throttle (K-170) when the starboard stab began fluttering. By the time my eye perceived the problem, the oscillation was EXTREME. No exaggeration... the trailing edge of the stab was flapping about 10" up and 10" down. Frankly, I couldn't understand how it was staying on the plane; or how the aft-end of the plane was staying together at all. I chopped the power immediately and began the fight to regain control of the plane. With a lot of cross-control, I did manage to get the jet turned around and away from the highway it was headed for, and ended up dumping it in the marsh.

The airframe was totaled, but the equipment inside was OK. The stab linkage was snapped at the ball-link. The question now is whether the failure at the linkage caused the flutter or if flutter caused the failure at the linkage. My own thought is that the flutter/oscillation was a pretty clear indication that the stab is not 'dynamic balanced' or inherently stable at flight speed.

Thoughts?

On such a large surface, mass balancing is pretty essential. At certain flight modes/speeds/parameters if the surface isn't mass balanced you result in the 'tail wagging the dog!' I would be sure that the linkage broke as a result of dynamic flutter.

I'd recomend that if you do this again you mass balance the stabs at the root.

Cheers,
Rob

Where on the stab's MAC is the pivot located?

On most full-size jets, the pivot is located at about 20% of the MAC. If the pivot is aft of that at or near 25% of the MAC, the stab will start to 'wander' if the control is not mass-balanced.

I flew a model of MiG-29 that had the pivot incorrectly located and the stabs did not want to stay centered in flight. Since it was a small-ish model, the control imbalance was not able to overpower the servo, so this was not an issue.

You can test this. Remove the stab and attach it to test fixture that will allow the stab to rotate freely. Then, have someone drive you down the freeway and stick the test fixture out the window into the slipstream. You'll see immediately whether the stab is dynamically balanced.

Then, mass-balance the control and repeat the test. You should see a dramatic change in how the control behaves.

Dan

Please correct me if I'm wrong but I thought mass balancing helped with static balance and not dynamic balance. How would mass balancing stop flutter?

My opinion is that the linkage broke. You had flights with rock solid high speed passes without any problems (DEC 8th video) and one good flight before the flutter. If the stab was not balanced, or if it was prone to flutter, it would have shown up then.

Plus the immediate and violent flutter indicates a catastrophic failure of a part, IMO.

have to agree with Andy, something let loose on ya, i'd say the ball link snapped before the flutter.

+1

Beave

Hi,

I have to admit, I've always been very confused on the mass-balancing thing...to the point where I'm not even sure of the accurate definition of it. I've read lots of conflicting notions on it, and sometimes the conflicting notions have compelling evidence to back them up. Either way, those kinds of computations only work when you can plug in known variables in the design. I'm of the opinion that in most cases, even the manufacturers have no idea what those variables are, so it's kind of a pig-in-a-poke.

I do know that I had a servo arm pop off on a Y/A F-18 (the small one) rendering the stab a free-floating plane. I lost control of the airplane because it would roll when I applied up-elevator, but at full throttle and all the way to the ground, the stab appeared (visually) to mirror what the other one was doing. It was basically weather-vaning. We only figured out what caused the crash after the fact because the aft end was undamaged and the servo screw was MIA (actually, it was on my building table). At speeds great and small, that stab was rock-stable when not connected to a servo. That stab was not balanced on the pivot (if that's what mass-balance means), so I always assumed it meant that that particular stab was inherently stable or dynamic balanced.

If the stab was not dynamically balanced close to the MAC you would have seen this problem before now. Sounds like a linkage let go. Dantley flew his Mig-29 for several flights with a P-200 and never had any issues with flutter so I dont think they have the pivot point wrong. Static balancing wont help flutter, most full scale jets do not have statically balanced stabs, why should ours?

Sorry you are wrong. Static balancing is there exactly to help prevent flutter. Full size jets have irreversible hydraulic controls. Our models do not.
The old Byron F16 was a classic example. They where nortorious for rudder flutter, to the point that many people glued them in place. But a balanced rudder did not flutter.

So John, you are telling me that if you statically balance a stab with the pivot point at 31% MAC that your stab will not flutter? If so then you are wrong! It doesn't matter how much you balance your stab on the ground if it is inherently aerodynamically unstable in the air. I guess pretty much 90% of the models out there should be fluttering then since that's about the percentage of planes flying that do not have static balanced stabs. Feel free to continue to balance yours though, to each their own, it certainly wont hurt anything.

I'm by no means a mechanical engineer but to me the only thing static balancing does is lighten the load on the servo at non flying speed.

Agree 100% KC.

I bought a set of moulds of a Mig 29 in 1992 from a Russian guy who attended the German Jet Masters. The drawings,supposedly masterd from the full size, showed the pivot of the flying stab raked backwards along the mean average chord. This made mass balancing very simple with very little weight needed to balance the tailerons. I flew it for about 8 years and although it was d/f it had a surprising turn of speed. I used Futaba 9303 servos which I believe produce about 7 kg of torque. I never suffered any flutter problems. I have since seen Mig 29s with the pivot point at right angles to the fuselage centre line and I have often wondered how much load is imposed on the servo and the linkage. I don't kow which is correct but I thought I would contribute. Ron.

So on the 29, the tube is coming out at an angle, balance the same way? I would have to put a TON of lead on the leading edge to get this thing even remotely balanced. Skymaster Mig-29 owners, did you balance the stabs?

exacly...including hard landings or even taxing on a bad runway. Mass static balancing it takes this loads of the servo gears/links/mounts

..my 2c

Sorry about your loss. What kit was the Mig?





John

Ian, et al,

The only AE PhD I personally know agrees with you wholeheartedly, and even points out that balance on the pivot can even cause problems in certain situations. Hydraulics are not the reason why it's ok not to in full scale, since Pipers have no hydraulics and are not balanced on the stabs either...

Deleted. Not worth the bother. I,ll get my coat!

David,

With great respect in return, I started my comment with an clarification that I have no education or pretense of a firm understanding of these matters... so you are reiterating my lack of understanding rather than revealing it. May I suggest you consider losing the patronizing 'from-on-high' tone?

Why not enlighten? It's one thing to point out that others don't have your superior knowledge, but another entirely to help. What, for example, is the difference between mass-balancing and balancing on the pivot point when referring to a full-flying stab. I have a feeling that most people think they're one-and-the-same. Static balance at the pivot point as a rule has been thoroughly debunked to my satisfaction (even with my clearly limited capacity to understand) by the AE people I know.

I'm all ears.

Ok,now I'm a little confused and nervous...lol. I recently bought a YA F-15 as my first jet and noticed a buzzing from the servos(8411) on the stab.I disconnected the linkage and found that the stabs are very "tail" heavy. I wasn't sure if this was by design or if they needed to be balanced on the pivot. Would the solution be to balance them or use a more heavy duty servo (8711) or both?
Typically I fly Warbirds so a flying stab is new to me.I would hate to learn by trial and error on this one because the learning curve could be very costly.I have watched several vids of Shaun's F-15 and some others and they all looked fine.Could this be a problem only on the Mig (I hope).
I will be paying close attention to this thread in hopes of a solution!
Signed,
John the Newbie Jet Pilot

John, you could move to an 8711 if you like but I don't think its necessary. The stabs on a yellow F-15 are fairly small so 8411's should be fine, not to mention the F-15 is hardly a speed demon. My Skymaster F-15 flew fine with similar servos on the stabs and it is ever so slightly larger. Once the plane is in the air there will be no buzzing as the airflow across the stab will neutralize the weight of the stab. I am sure I am using all the wrong terms and most likely David will point that all out but I prefer "simple" terms. Just imagine if you had something like an F-18 which has a ton of weight behind the pivot point of the stab, they make even 8711's buzz.

Comparing the flight characteristics and load dynamics of our models to full scale jets isn't really a good comparison. Everything between the two as it relates to how the flight surfaces are actuated is mechanically different. At best one would be guessing the comparable effects without any hard facts to prove your point.

But Ian, Dantley's Mig-29 was from SM and the one Shaun was flying was from JL. Not to start a brand war, but Ian, (as you are well aware) the JL F-20 had some Stab balancing issues apparently that led to some problems and crashes. Frankly, it sounds more like a linkage failure than a balance issue in this particular case, but of course Murphy's Law says it would have to happen to a JL product so the 800lb elephant in the room becomes: "If they had poorly balanced stabs leading to disaster on their F-20, is it possible that the same issue occurred on their Mig-29?".

I'm not sayin', I'm just sayin'

Consider the pot stirred!

Shaz

+3