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#1
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Grob Twin Astir Tailshaking
On a recent thermalling flight with a friend in the front seat I commenced a
high speed run from 3000 feet to lose height and demonstrate glider flight at higher speed. I trimmed out for 100kts and flew a straight course until down to about 2000ft where I then turned left 90 degrees. This took the glider beneath some late afternoon cumulus that was still providing lift. Immediately the glider began fishtailing rapidly giving the impression the horizontal tail surface was moving side to side by a few inches. It sure got my attention and I immediately opened the airbrakes and raised the nose to reduce speed to about 60 kts. I honestly felt there was some chance of structural failure.It gave another shudder and then settled down. I asked the previous owners of the glider, another gliding club, if they had experienced this phenomenon but nobody had. However another of our instructors had a similar experience back in February and again late last year. The engineer is not too concerned as he has not found anything loose or obviously amiss that might cause this tail oscillation. It does seem to be rough air related as I have carried out loops in this glider where I have dived to 105 knots with none of the tail shaking evident. Our glider is the fixed gear G103 Twin Astir. I wonder if anyone else has experienced this. I cant find ant relevant service bulletins that address the matter. |
#2
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Grob Twin Astir Tailshaking
my old club in the UK had an Astir which exhibited
the same tail shaking at high speed. the problem was the 'banjo' joint where the tail met the fuselage. unfortunately I don't have any other details except that it was at the Lakes gliding club back in '94 or there abouts. I'm sure they'd remember. At 06:06 11 January 2007, Peter wrote: On a recent thermalling flight with a friend in the front seat I commenced a high speed run from 3000 feet to lose height and demonstrate glider flight at higher speed. I trimmed out for 100kts and flew a straight course until down to about 2000ft where I then turned left 90 degrees. This took the glider beneath some late afternoon cumulus that was still providing lift. Immediately the glider began fishtailing rapidly giving the impression the horizontal tail surface was moving side to side by a few inches. It sure got my attention and I immediately opened the airbrakes and raised the nose to reduce speed to about 60 kts. I honestly felt there was some chance of structural failure.It gave another shudder and then settled down. I asked the previous owners of the glider, another gliding club, if they had experienced this phenomenon but nobody had. However another of our instructors had a similar experience back in February and again late last year. The engineer is not too concerned as he has not found anything loose or obviously amiss that might cause this tail oscillation. It does seem to be rough air related as I have carried out loops in this glider where I have dived to 105 knots with none of the tail shaking evident. Our glider is the fixed gear G103 Twin Astir. I wonder if anyone else has experienced this. I cant find ant relevant service bulletins that address the matter. |
#3
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Grob Twin Astir Tailshaking
my old club in the UK had an Astir which exhibited
the same tail shaking at high speed. the problem was the 'banjo' joint where the tail met the fuselage. unfortunately I don't have any other details except that it was at the Lakes gliding club back in '94 or there abouts. I'm sure they'd remember. At 06:06 11 January 2007, Peter wrote: On a recent thermalling flight with a friend in the front seat I commenced a high speed run from 3000 feet to lose height and demonstrate glider flight at higher speed. I trimmed out for 100kts and flew a straight course until down to about 2000ft where I then turned left 90 degrees. This took the glider beneath some late afternoon cumulus that was still providing lift. Immediately the glider began fishtailing rapidly giving the impression the horizontal tail surface was moving side to side by a few inches. It sure got my attention and I immediately opened the airbrakes and raised the nose to reduce speed to about 60 kts. I honestly felt there was some chance of structural failure.It gave another shudder and then settled down. I asked the previous owners of the glider, another gliding club, if they had experienced this phenomenon but nobody had. However another of our instructors had a similar experience back in February and again late last year. The engineer is not too concerned as he has not found anything loose or obviously amiss that might cause this tail oscillation. It does seem to be rough air related as I have carried out loops in this glider where I have dived to 105 knots with none of the tail shaking evident. Our glider is the fixed gear G103 Twin Astir. I wonder if anyone else has experienced this. I cant find ant relevant service bulletins that address the matter. |
#4
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Grob Twin Astir Tailshaking
Peter,
check the rudder hinges and the ribs that holds the hinges. Michael |
#5
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Grob Twin Astir Tailshaking
Forget that....
Check your parachute has had a fresh repack!!! Is it too late to ask for your money back? Al Michael Huber wrote: Peter, check the rudder hinges and the ribs that holds the hinges. Michael |
#6
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Grob Twin Astir Tailshaking
Peter
I second Michael's rudder statement. I have a Grob 103 C Twen Acro III and the rib that is glued to the vertical skin by the upper hinge came unglued. You can check this by putting pressure on the right side of the rudder at the upper hinge point and see if the vertical fin moves. The glider had five hours on it at the time. Ken (KP) On Jan 12, 12:30*am, "Michael Huber" wrote: Peter, check the rudder hinges and the ribs that holds the hinges. Michael |
#7
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Grob Twin Astir Tailshaking
Peter,
You may have experienced low frequency rudder flutter. I have seen a 2 cycle per second rudder flutter at 100 knots in a Duster. Recommend you check the rudder balance to see if it is within factory specs. If it did flutter, the hinges and drive rib may be damaged as well. have a good look at the fin spar with the rudder off. JJ At 06:06 11 January 2007, Peter wrote: On a recent thermalling flight with a friend in the front seat I commenced a high speed run from 3000 feet to lose height and demonstrate glider flight at higher speed. I trimmed out for 100kts and flew a straight course until down to about 2000ft where I then turned left 90 degrees. This took the glider beneath some late afternoon cumulus that was still providing lift. Immediately the glider began fishtailing rapidly giving the impression the horizontal tail surface was moving side to side by a few inches. It sure got my attention and I immediately opened the airbrakes and raised the nose to reduce speed to about 60 kts. I honestly felt there was some chance of structural failure.It gave another shudder and then settled down. I asked the previous owners of the glider, another gliding club, if they had experienced this phenomenon but nobody had. However another of our instructors had a similar experience back in February and again late last year. The engineer is not too concerned as he has not found anything loose or obviously amiss that might cause this tail oscillation. It does seem to be rough air related as I have carried out loops in this glider where I have dived to 105 knots with none of the tail shaking evident. Our glider is the fixed gear G103 Twin Astir. I wonder if anyone else has experienced this. I cant find ant relevant service bulletins that address the matter. |
#8
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Grob Twin Astir Tailshaking
John Sinclair wrote: Peter, You may have experienced low frequency rudder flutter. Can the rudder have low frequency flutter without the rudder pedals following. I assume not, so were the pedals moving? Andy |
#9
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Grob Twin Astir Tailshaking
JJ is spot on! I've been through this myself in a Concept 70.
Early certification guidelines indicated that balancing the rudder was not necessary on aircraft with a Vne below a certain speed. Of course the guidelines were written for aircraft with steel tube or monocote aluminum construction. GRP structures behave differently. In the case of my Concept 70, the rudder was grossly out of balance. It took almost 12 ounces of lead ahead of the hinge line to balance the rudder. That was the short term fix. About two years later, while refinishing the glider, I had a new rudder built out of glass, carbon, and kevlar. Plus I had the fin spar inspected and, as it turned out, it and the hinges needed repair. We stripped the old rudder and found at least 4 layers of finish on it. BTW, how serious is rudder balance? On a V-tail Bonanza the rudder being out of balance by the weight of a silver dollar (1 ounce) on the rudder trailing edge is enough to cause flutter and catastrophic structural failure at speeds below the yellow arc. It is not something to play with. As to the rudder pedals moving, you would be surprised how much the rudder cable can stretch. Since most glider rudder systems are cable driven at the rudder regardless if push rods are used up front, unless you are on the rudder pedal real hard, you might not notice the pedal moving until the rudder deflections get rather large. Since the control surface extracts energy from the airflow, the only way to deal with flutter in flight is to slow down. Right Now! Don't wait and don't think you can control it with control inputs from the cockpit. You have to get rid of the active energy in the situation. Dave Andy wrote: John Sinclair wrote: Peter, You may have experienced low frequency rudder flutter. Can the rudder have low frequency flutter without the rudder pedals following. I assume not, so were the pedals moving? Andy |
#10
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Grob Twin Astir Tailshaking
The glider recently had its 7000 hr inspection carried out and the engineer has assured me the rudder balance was checked and corrected at the time. Here is what he has come up with to date. Quote: Banjo joint?? – Is he talking about the vertical spar that runs down the inside of the tail and bells out to the shape of the fuselage right at the back of the fin, that the rudder is attached to…. This is ‘the’ main load bearing thing that carries the horizontal and vertical tail loads. The forward structural pick-up for the tailplane is a shorter spar that goes part-way down the fin (from memory). The glue joints of the fin spar were visually inspected when we had the rudder off for the 7000 hr inspn and subsequently tap testing was done in this area. If there was a problem here then I am quite confident we would have found it during the inspn, or at least during subsequent inspections of the area where I have put fairly high side-loadings on the tail to try to find the problem. I cannot say categorically that the structure is perfect, only that there is no internal noises apparent when the fin is side-loaded in an oscillatory manner, and the fuselage/fin torsional damping appears to be O.K. Also done at the 7000 hr inspn was a weight and total moment check of the rudder while off, which was found to be out of limits and corrected. I am open to suggestions as to what else can be the cause. It would ease our minds to find and fix the problem. An interesting piece of information I have found relates to a known phenomenon called ‘tail-snaking’ which was defined as a snaking / twisting of the tail unit, but not flutter of the control surface as such. The original article related to large RC model gliders in competition at high speed and another article made mention of (full size) aircraft manufacturers using an add-on ridge on both sides of a control surface at the T.E to prevent boundary layer separation shift fore and aft on alternate sides of the surface. It is also known that the trailing edges of control surfaces should not be rounded but kept abrupt as this is an area that can cause problems. (the T.E of the rudder on MW is not rounded.. J). Below is a bit on the subject and is bound to help you sleep. Snaking Oscillations Another stability problem that was quite common in airplanes of the period around WW II was a tendency for a continuous small-amplitude lateral oscillation in straight and level flight. This problem was called "snaking" and its cause was quite mysterious. Among the explanations offered were response of the normal lateral oscillation of the airplane to continuous small-amplitude turbulence, periodic flow separation from the wing root that affected the vertical tail, or nonlinear aerodynamic characteristics of the wing or tail surfaces for small changes in angle of attack. One explanation, which will be discussed subsequently, was the unsteady lift characteristics of the vertical tail at low frequencies. While some of these explanations may have had some influence in rare instances, the true explanation was first given by George Schairer of the Boeing Company in an analysis of this problem on the Boeing 314 flying boat, one of the China Clippers. He pointed out that at small angles of sideslip, the rudder had a tendency to float against the relative wind, which caused the airplane to swing around and yaw in the opposite direction. Friction in the rudder system, however, held the rudder in this position as the airplane swung through zero sideslip. On reaching a sideslip in the opposite direction, the rudder hinge moments would eventually break through the friction force and the cycle would be repeated in the opposite direction. Thus, energy was fed into the oscillation by the rudder, which caused the oscillation to build up to an amplitude where this energy equaled that removed by the inherent damping of the airplane. On learning of this explanation, efforts were made to verify it. A convenient test airplane was the Fairchild 22 on which an experimental all-moveable vertical tail had been installed. This type of tail surface was an invention of Robert T. Jones and had the advantage that hinge moments due to angle of attack and due to deflection could be adjusted separately with changes in the hinge location and tab gearing. The tests were made covering a range of conditions and friction values, and the validity of the theory was established (ref. 4.8). The question arises as to how such an apparently obvious control motion could have escaped detection. The explanation is that because of the relatively low damping of the Dutch roll oscillation, the rudder motion required to sustain a constant-amplitude oscillation is only a small fraction of the amplitude of the yaw or sideslip. For example, in a typical snaking oscillation of plus or minus two degrees of sideslip, the rudder motion required might have been only plus or minus two-tenths of a degree. This small motion was less than the sensitivity of control position recorders used at that time, and [31] this motion could be absorbed by stretch in the control cables without being felt at the pilot's rudder pedals. Another little-known aspect was the tendency of the rudder to float against the relative wind at small sideslip angles. Most control surfaces float with the relative wind at larger sideslip angles. In typical wind-tunnel tests, measurements had been made at increments of angles of attack or sideslip of five degrees, and as a result, the small changes in characteristics at very small values of angle of sideslip were not detected. In addition to flight tests, theoretical studies were made to explain and quantitatively predict the oscillation. These studies are discussed in a subsequent chapter. I and our CFI had a quick check yesterday of the rudder and all we found was a small amount of play in the top hinge bearing. The components themselves seem well connected to the structure. |
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