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#21
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 2, 6:13*am, Andy wrote:
On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I cut a corner or two on my post Andy. The only glider wing flutter video I've seen looks like an interaction between the main wing bending and tortional elasticities - wing bends up, gains a little angle of attack and bends/twists more as a result until the restoring force gets big enough to bring it back down (or the wing breaks). This creates a symmetric wing flapping kind of flutter. I would think if the ailerons were significantly involved you'd be more likely to see something asymmetric, or more tortional motion - which I'm sure can occur under some set of circumstances. The reason I picked this mode of flutter was also its likely the one most affected but water ballast which should up the natural frequency in bending mostly. Just educated guesses on my part, but that was my logic. 9B |
#22
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 2, 4:31*pm, wrote:
On Jan 2, 6:13*am, Andy wrote: On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I cut a corner or two on my post Andy. The only glider wing flutter video I've seen looks like an interaction between the main wing bending and tortional elasticities - wing bends up, gains a little angle of attack and bends/twists more as a result until the restoring force gets big enough to bring it back down (or the wing breaks). This creates a symmetric wing flapping kind of flutter. I would think if the ailerons were significantly involved you'd be more likely to see something asymmetric, or more tortional motion - which I'm sure can occur under some set of circumstances. The reason I picked this mode of flutter was also its likely the one most affected but water ballast which should up the natural frequency in bending mostly. Just educated guesses on my part, but that was my logic. 9B Interesting to note that the damage I've seen in practice has been two cases of elevator flutter with folks pushing too fast at high altitude. I suspect that ignoring speed restrictions at ~18k' was the cause. I believe the mylar seals were in good condition in both cases and presumably not a factor. There are alligators in these here swamps... be careful where you tread. Darryl |
#23
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 2, 6:40*pm, Darryl Ramm wrote:
On Jan 2, 4:31*pm, wrote: On Jan 2, 6:13*am, Andy wrote: On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I cut a corner or two on my post Andy. The only glider wing flutter video I've seen looks like an interaction between the main wing bending and tortional elasticities - wing bends up, gains a little angle of attack and bends/twists more as a result until the restoring force gets big enough to bring it back down (or the wing breaks). This creates a symmetric wing flapping kind of flutter. I would think if the ailerons were significantly involved you'd be more likely to see something asymmetric, or more tortional motion - which I'm sure can occur under some set of circumstances. The reason I picked this mode of flutter was also its likely the one most affected but water ballast which should up the natural frequency in bending mostly. Just educated guesses on my part, but that was my logic. 9B Interesting to note that the damage I've seen in practice has been two cases of elevator flutter with folks pushing too fast at high altitude. I suspect that ignoring speed restrictions at ~18k' was the cause. I believe the mylar seals were in good condition in both cases and presumably not a factor. There are alligators in these here swamps... be careful where you tread. Darryl- Hide quoted text - - Show quoted text - Thanks for all the interesting replys. I think the comment about "live" air is the most interesting. It is difficult to detect a 30 percent change in sink rate when the air is moving up and down at up to 1500 feet per minute. Looking at flight logs most people fly pretty fast at high altitude. I guess if you have it (energy) you might as well spend it? Speaking of flutter. I believe that slop in control connections can contribute to the on set of flutter. Bill Snead 6W |
#24
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 2, 5:40*pm, Darryl Ramm wrote:
Interesting to note that the damage I've seen in practice has been two cases of elevator flutter with folks pushing too fast at high altitude. I suspect that ignoring speed restrictions at ~18k' was the cause. I believe the mylar seals were in good condition in both cases and presumably not a factor. There are alligators in these here swamps... be careful where you tread. ASW-19 gliders were required to have an elevator modification that substantially reduced the chord. The mod was required to prevent flutter. Just one data point that suggests control surface flutter may be more of a factor than primary structure in setting Vne. Andy |
#25
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 2, 7:13*am, Andy wrote:
On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I think the flutter mode which occurs first may change with altitude, the generation of glider, and wear, excluding the pilot induced mode. Since the optimization of structures for operating under 6000m, I would suspect dynamic flutter to occur first at lower altitudes, but elastic flutter to occur first at higher altitudes, say above 8-9000m, as the center of pressure shifts. Dynamic pressures are more directly related in IAS, rather than TAS. Elastic modes are related to TAS. IIRC, spar placement in modern designs is not as resistant to elastic twisting at higher altitudes. Frank Whiteley |
#26
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 3, 9:02*am, Frank Whiteley wrote:
On Jan 2, 7:13*am, Andy wrote: On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I think the flutter mode which occurs first may change with altitude, the generation of glider, and wear, excluding the pilot induced mode. Since the optimization of structures for operating under 6000m, I would suspect dynamic flutter to occur first at lower altitudes, but elastic flutter to occur first at higher altitudes, say above 8-9000m, as the center of pressure shifts. *Dynamic pressures are more directly related in IAS, rather than TAS. *Elastic modes are related to TAS. IIRC, spar placement in modern designs is not as resistant to elastic twisting at higher altitudes. Frank Whiteley- Hide quoted text - - Show quoted text - You should be able to do something structurally to reduce the bending/ tortional coupling. NASA built the X-29 with a carbon fiber wing that had forward sweep to show exactly that. Forward sweep has always been known to have performance and handling advantages in transonic jets, but "structural divergence" kept designers away from it in practice. http://www.nasa.gov/centers/dryden/n...-008-DFRC.html Excerpt: "Construction of the X-29's thin supercritical wing was made possible because of its composite construction. State-of-the-art composites permit aeroelastic tailoring, which allows the wing some bending but limits twisting and eliminates structural divergence within the flight envelope (i.e., deformation of the wing or breaking off in flight)" The past few generations of composite sailplanes would appear to have greater aeroelastic stability by virtue of swept back leading edges and (perhaps) spars that are further back in the chord. Here is the sailplane wing flutter video I was referring to: http://www.youtube.com/watch?v=kQI3AWpTWhM You can see the flutter is symmetric with several waves from tip to tip. It looks to me like you can see the twist increase at the tip as the wing deflects upward - there may also be some aileron involvement, but from the frequencies involved I would think this is secondary to the main flutter mode. In reflecting on this a bit I recall that control surface flutter is typically at much higher frequencies (often described by pilots as making a buzzing sound). While this may destroy the control surface itself or the hinges and control circuits, it seems unlikely that it would activate the resonant frequency of the associated primary structure (wing, horizontal/vertical stab). That's not to say that losing you elevator is any less cause for concern than losing your wing. I think wing flutter by design occurs at the lowest airspeed. By virtue of the smaller forces on control surfaces it should be easier to damp out control surface flutter mechanically - unless your control circuits are out of spec. Going back to the original question about water ballast, it would appear that ballast might help damp out or delay the onset of the bending/twisting flutter mode - although in the video the amount of deflection isn't that great where the ballast tanks would be located so who knows how favorable an effect it would be. I'm not totally sure, but it kind of feels sensible to me. 9B |
#27
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Minumum Sink Rate/Best L/D at 17,000 feet ?
wrote:
Here is the sailplane wing flutter video I was referring to: http://www.youtube.com/watch?v=kQI3AWpTWhM You can see the flutter is symmetric with several waves from tip to tip. When I pause the video, I can see one tip is up while the other tip is down. Isn't that asymmetric flutter? -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4 * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
#28
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 3, 12:39*pm, Eric Greenwell wrote:
wrote: Here is the sailplane wing flutter video I was referring to: http://www.youtube.com/watch?v=kQI3AWpTWhM You can see the flutter is symmetric with several waves from tip to tip. When I pause the video, I can see one tip is up while the other tip is down. Isn't that asymmetric flutter? -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4 * * * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org Your definition is, of course, correct Eric. I looked to me like once the flutter was established it was symmetric. I'll take another look. I think the symmetry or assymmetry may be aside to the main points of the discussion as it isn't clear to me that it would necessarily indicate anything one way or the other on the issue Andy raised about control surface interaction and certainly not on the ballast question. 9B |
#30
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Minumum Sink Rate/Best L/D at 17,000 feet ?
On Jan 3, 9:02*am, Frank Whiteley wrote:
On Jan 2, 7:13*am, Andy wrote: On Jan 1, 4:52*pm, wrote: One interesting experiment would be to deflect the wings on the ground and release them - with and without water - and measure the difference in the frequency of the oscillations. That would be of interest if the flutter limit speed was set by primary wing structure, *Is it, or do the control surfaces flutter first. In my experience in transport aircraft flight test the flutter testing is always done with maximum allowable free play in control linkages. Do glider manufacturers do that, it not, does flutter speed reduce as control links wear? Andy I think the flutter mode which occurs first may change with altitude, the generation of glider, and wear, excluding the pilot induced mode. Since the optimization of structures for operating under 6000m, I would suspect dynamic flutter to occur first at lower altitudes, but elastic flutter to occur first at higher altitudes, say above 8-9000m, as the center of pressure shifts. *Dynamic pressures are more directly related in IAS, rather than TAS. *Elastic modes are related to TAS. IIRC, spar placement in modern designs is not as resistant to elastic twisting at higher altitudes. Frank Whiteley- Hide quoted text - - Show quoted text - In looking again at the only glider flutter video I have it appears that there is control surface involvement. But wing bending and tortional masses and elasticities are also part of the equation. This raised a question in my mind - if you experience wing flutter doe it matter whether you hold onto the stick or let it go? Holding on to the stick would provide some damping of control deflections. If it does matter, which should you do? My instincts say hold on, but that may just be my personality at work... 9B |
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