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#1
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When I made my original post on this subject my contention
was quite simple, if faced with the choice of exceeding VNE or pulling to avoid exceeding VNE and overstressing the glider I would choose the latter. I would hope that I would never get to the situation where I had to do either and if I keep my wits about me I never will. I stand by what I said but this was not an original thought, it was the advice of someone who knows a great deal more than me. An earlier posting said 'Also, I find it a bit strange that some here feel that it is possible to over-G a sailplane to damage, but not destruction. It seems like a fine point to me and there are several examples of unlucky souls who have misjudged the point'. Of course it is possible but I accept it is purely a matter of luck. I never made the above statement, I did say that overstress may cause serious damage, but flutter is much more likely to be catastrophic. With some gliders there is such a large margin between placarded limits and the forces the airframe will withstand that overstressing is definitely the lesser of 2 evils. The Grob Acro is a perfect example of this. One of the Acros delivered to the RAF in the UK in the 80's was given to Slingsbys to test on a rig. After the 'normal' testing (The wing spigot problem was discovered in this test and I was told that had this failed in flight the airframe would probably have stayed in one piece). Following this attempts were then made to break the glider but despite every effort the only thing that broke was the test rig, the glider never did. There is no such margin for the onset of flutter. Not all gliders have the strength of the Grob I would agreed but there is still a margin of some sort. The way the whole thing was explained to me was that pulling excess G may break the glider, in particular it may cause damage to the wing/fuselage fixing but this damage is not necessarily total (See above re wing spigot problem). The damage caused by flutter is much more likely to cause total failure not only of the wing but other aerodynamic surfaces as well (the tailplane and fin) and a glider without a tailplane/fin is not where I would want to be. I am told that the weakest point on any glider is the fuselage just in front of the fin. Again I am told that when an aircraft breaks up in flight the cause is almost invariably flutter, the result of overspeed, whether this is preceeded by overstress causing loss of control or not. There has been a lot of use of words such as rubbish and other derogatory terms. You do not have to agree with me but I would suggest that perhaps you might be better researching and then posting a cogent argument why I am wrong instead of just gainsaying. (Remember the Python :-) UK joke) I stand by my original post, faced with the choice of exceeding VNE or pulling too much G I would chose the latter as the lesser of two very great evils. |
#2
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Don, I agree with you completely. You have made the points which I have
been trying to put. W.J. (Bill) Dean (U.K.). Remove "ic" to reply. "Don Johnstone" wrote in message ... When I made my original post on this subject my contention was quite simple, if faced with the choice of exceeding VNE or pulling to avoid exceeding VNE and overstressing the glider I would choose the latter. I would hope that I would never get to the situation where I had to do either and if I keep my wits about me I never will. I stand by what I said but this was not an original thought, it was the advice of someone who knows a great deal more than me. An earlier posting said 'Also, I find it a bit strange that some here feel that it is possible to over-G a sailplane to damage, but not destruction. It seems like a fine point to me and there are several examples of unlucky souls who have misjudged the point'. Of course it is possible but I accept it is purely a matter of luck. I never made the above statement, I did say that overstress may cause serious damage, but flutter is much more likely to be catastrophic. With some gliders there is such a large margin between placarded limits and the forces the airframe will withstand that overstressing is definitely the lesser of 2 evils. The Grob Acro is a perfect example of this. One of the Acros delivered to the RAF in the UK in the 80's was given to Slingsbys to test on a rig. After the 'normal' testing (The wing spigot problem was discovered in this test and I was told that had this failed in flight the airframe would probably have stayed in one piece). Following this attempts were then made to break the glider but despite every effort the only thing that broke was the test rig, the glider never did. There is no such margin for the onset of flutter. Not all gliders have the strength of the Grob I would agreed but there is still a margin of some sort. The way the whole thing was explained to me was that pulling excess G may break the glider, in particular it may cause damage to the wing/fuselage fixing but this damage is not necessarily total (See above re wing spigot problem). The damage caused by flutter is much more likely to cause total failure not only of the wing but other aerodynamic surfaces as well (the tailplane and fin) and a glider without a tailplane/fin is not where I would want to be. I am told that the weakest point on any glider is the fuselage just in front of the fin. Again I am told that when an aircraft breaks up in flight the cause is almost invariably flutter, the result of overspeed, whether this is preceded by overstress causing loss of control or not. There has been a lot of use of words such as rubbish and other derogatory terms. You do not have to agree with me but I would suggest that perhaps you might be better researching and then posting a cogent argument why I am wrong instead of just gainsaying. (Remember the Python :-) UK joke). I stand by my original post, faced with the choice of exceeding VNE or pulling too much G I would chose the latter as the lesser of two very great evils. |
#3
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Non-catastrophic may happen if you have a structure which has a plastic
behavious prior to rupture. Ironically, you don't have that with "plastic" gliders. You might well enconter that you can pull more g's because the designer has put lots of margins, and nothing will happen But if *something* happens, you're wings are simply gone on a GRP/CRP ship. The idea that you'll get away with some sort of damage and land the ship is, hm, fairly naive. But to the initial question: If you are going to exceed Vne in a dive, you can chose between putting your joker on a good spacing between Vne and flutter speed, or put your joker on a pessimistic design margin and a well crafted serial number. There is actually no way to tell the answer beforehand. But pulling the airbrakes would be fairly suicidal. -- Bert Willing ASW20 "TW" "W.J. (Bill) Dean (U.K.)." a écrit dans le message de ... Don, I agree with you completely. You have made the points which I have been trying to put. W.J. (Bill) Dean (U.K.). Remove "ic" to reply. "Don Johnstone" wrote in message ... snip The way the whole thing was explained to me was that pulling excess G may break the glider, in particular it may cause damage to the wing/fuselage fixing but this damage is not necessarily total |
#4
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Bert Willing wrote:
Non-catastrophic may happen if you have a structure which has a plastic behavious prior to rupture. Ironically, you don't have that with "plastic" gliders. You might well enconter that you can pull more g's because the designer has put lots of margins, and nothing will happen But if *something* happens, you're wings are simply gone on a GRP/CRP ship. The idea that you'll get away with some sort of damage and land the ship is, hm, fairly naive. But to the initial question: If you are going to exceed Vne in a dive, you can chose between putting your joker on a good spacing between Vne and flutter speed, or put your joker on a pessimistic design margin and a well crafted serial number. There is actually no way to tell the answer beforehand. I agree with Bert. To imagine Don's advice to be suitable for all gliders is too ignore the huge differences in design and materials. For example, the flexible, fiberglass wing of ASW 20 probably means it has a greater strength reserve because of the extra material needed to control flutter, while the stiffer carbon wing in the ASW 27 might give it the reverse margins. Consider the Standard Cirrus with it's relatively thick fiberglass wing: where are it's margins the greatest? And, it appears the 25 m gliders may have special problems. Until you have discussed the design of your _particular_ glider with it's designer, you are simply speculating about the dangers of overspeeding versus overloading. Even the designer may not know, if the glider hasn't been tested to flutter! And if you damage the structure during a high G pull-up, what do you suppose will happen to the speed at which flutter occurs? You may now have damaged glider experiencing flutter! Fortunately, this situation seems to rare. Personally, I have never encountered it in 4500 hours of soaring, not even an incipient spin. Here is more speculation: I think the reality is most pilots that have the problem will use Don's method out of reflex, not training or conscious choice. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#5
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Bert Willing wrote:
But to the initial question: If you are going to exceed Vne in a dive, you can chose between putting your joker on a good spacing between Vne and flutter speed, or put your joker on a pessimistic design margin and a well crafted serial number. There is actually no way to tell the answer beforehand. But pulling the airbrakes would be fairly suicidal. I suppose you meant "pulling the airbrakes while pulling too hard" ??? As Eric noticed it, the allowed G-loading at VNE in ASH26 (for example) is 4 G without airbrakes, and a very close 3.5 G with airbrakes. Thus in most cases it will be *safer* to pull airbrakes (including close to the ground, if the dive angle is high). -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#6
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Yes. However, judging g-loads with the seating position in modern gliders is
difficult - especially if you run on 100% adrenaline. -- Bert Willing ASW20 "TW" "Denis" a écrit dans le message de ... Bert Willing wrote: But pulling the airbrakes would be fairly suicidal. I suppose you meant "pulling the airbrakes while pulling too hard" ??? As Eric noticed it, the allowed G-loading at VNE in ASH26 (for example) is 4 G without airbrakes, and a very close 3.5 G with airbrakes. Thus in most cases it will be *safer* to pull airbrakes (including close to the ground, if the dive angle is high). -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#7
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Don
First - I agree entirely that you are contemplating which of two evils you should perpetrate having got yourself into an untenable situation. But having got there you need to at least have considered what you should do in the event. I doubt there would be much time for deliberation. I suggest that the correct action depends on the aircraft to some degree, but that flutter is much more damaging to the structure than moderate overstress in most cases since it introduces large cyclical and localised loads on the structure in addition to whatever G load the aircraft is exposed to. First generation glass, before the finite element analysis programs allowed the designers to design to the limit is probably much safer to over stress than overspeed. Similarly the latest carbon designs seem to have G limits imposed by the JAR22 deflection limits rather than ultimate strength. Presumably these aircraft have huge strength reserves. For interest look at the wing test on the DG1000. When I asked Schempp-hirth about the possibility of flutter damage in an incident where a Std Cirrus had made a loud chattering noise on a high speed pass, they replied that it would be unlikely to have been flutter. This because they did not think it likely that the aircraft would remain controllable due to control system damage in the event of flutter. In inspection we found that the noise came from an airbrake cap that had lost some tension in the retention springs. It was sucking slightly open and banging against the sides of the slot as the pilot pulled up. Over one G, close to Vne and soft springs combined to allow a millimeter or so of play. The noise was disconcertingly loud from the ground, we thought there might be a glass-fibre confetti shower. I'd take a gamble on the Cirrus's wings handling more Gs than the manual said if my life depended on it. Conversely I take great pains not to get even near that point in a 32 year old glider. |
#8
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Bruce Greeff wrote:
First generation glass, before the finite element analysis programs allowed the designers to design to the limit is probably much safer to over stress than overspeed. Similarly the latest carbon designs seem to have G limits imposed by the JAR22 deflection limits rather than ultimate strength. Presumably these aircraft have huge strength reserves. For interest look at the wing test on the DG1000. I agree that *some* earlier, 15m designs may have a good safety margin in overstress, mostly those in glassfiber (more flexible). But not *all*, and certainly not modern open-class gliders. I recall what I posted before, because there are facts from NTSB and manufacturer data, which I think are more reliable than some honorable but ill-based opinions expressed in this thread, and which nobody here have contested yet: the link (on Minden Nimbus 4 accident) : http://www.ntsb.gov/NTSB/brief.asp?e...12X19310&key=1 (...) The report quotes that the G limit for the Nimbus 4 at VNE is 3.5 g *only* (compared to 5.3 g at Va) and the design "safety margin" is between 1.55 to 1.75. Thus even on a plane in perfect condition, and if the manufacturer made no mistake, it *will* break between 5.4 and 6.1 g at VNE (even without airbrakes) -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#9
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Earlier, Bruce Greeff wrote:
...Similarly the latest carbon designs seem to have G limits imposed by the JAR22 deflection limits rather than ultimate strength... I'll certainly agree that composite sailplane structure is bounded more by stiffness than by strength. However, I've spent my lunch hour searching JAR22 and I can't find anything that codifies deflection limits. The closest thing I found seems to be: : JAR 22.305 Strength and deformation : (a) The structure must be able to support : limit loads without permanent deformation. At : any load up to limit loads, the deformation may : not interfere with safe operation. This applies in : particular to the control system. : with respect to the sailplane. Do you know of other relevant JARs that specify maximum structure deflection in quantifiable terms? I'm not trying to nitpick or anything, I just want to make sure I'm not missing something important. Thanks, and best regards to all Bob K. http://www.hpaircraft.com |
#10
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Bob Kuykendall wrote:
Earlier, Bruce Greeff wrote: ...Similarly the latest carbon designs seem to have G limits imposed by the JAR22 deflection limits rather than ultimate strength... I'll certainly agree that composite sailplane structure is bounded more by stiffness than by strength. I've been told that is more likely true for fiberglass construction, but not so likely to be true for carbon fiber construction, because of the great differences in material characteristics, such as stiffness. So, it might correct to argue that a glass fiber sailplane has a "substantial" G loading margin, but not correct for the carbon fiber sailplane. And the bounds might be quite different for a 15 meter glider and a 25 meter glider, or a thick wing trainer and a thin wing racer. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
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