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#11
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Roy Bourgeois a écrit :
My my own experience (32 years soaring , 2000 hrs, active CFI-G) leads me to doubt that the pilot reaches for the dive brake while plummeting down in a dive. More likely, he over stresses by a too dramatic pull out during which an un commanded dive brake pop out occurs that destroys the wing. Because of the location of the dive brakes on the Nimbus 2 and 4 (they are quite far inboard) - this creates loads that would not be found on a 15m ship pulling the same G load. It is not unlikely that speed be very close to VNE increasing and in such a case most pilots would extend the airbrakes if not all. That's what airbrakes are for. The wisest manouever in such case would be to extend the airbrakes as soon as the glider is nose down, that would avoid the need for too much g-load on recovery. Note that in the case of an uncommanded pop out the airbrakes do *not* increase the loads on the wing because they *decrease* the g-load - only for the same g-load factor the wing root bending stress is increased. Therefore, if there is a pop-up and the pilot does not pull more to get the same g-load again, there should be no increased risk. -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#12
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I have never flown a glider of more than about 20 metres span (ASW 17,
Jantar 2). However, I am in the process of asking for other pilots' experience. One such pilot's reply includes the following: From your experience of big Nimbi do you think there is merit in Stan's ideas? "Only to the extent that you fly a big glider like a big glider, not like a Spitfire. You adjust all your flying to the fact that you are flying a big ship. For instance to adjusting your thermalling technique to gradual bank angle changes rather than spirited use of controls to roll in and out of thermals. "I have always said about large span gliders, of whatever make, that you regard them like flying a 747 Jumbo. Not like an agile 15m glider. "For instance, in a very turbulent thermal such as what you often get over power stations, I often am reluctant to put on over about 15 degrees of bank. Yes, fifteen degrees. I do not wish to be sucked in to a "loss of control" situation like Ivans and Engen or the Levers. Which particular brand of Nimbus do you fly? "4DM. I am entirely happy with it but I fly it like the big glider that it is." I would like to add the following: On the question of use of air-brakes, it would appear that they were not opened in the case of the Spanish accident. In the case of the Minden accident, there was correspondence on Rec. Aviation Soaring when the report was published by others who had experience of the inadvertent deployment of the brakes at high speed in turbulence; the suggestion was that deployment may have been uncommanded. In addition, Dick Johnson posted that with the Minden accident there may have been a problem with the oxygen system undiscoverable after the crash, such as oxygen tanks filled with Nitrogen (not entirely unknown I am afraid). It is also notable that whereas there seems to have been no attempt to bale out with the Minden accident, with the Spanish accident both pilots did bale out and deploy there parachutes, but unfortunately one of them had his canopy foul of the wreckage which brought him down with it. W.J. (Bill) Dean (U.K.). Remove "ic" to reply. "Bill" wrote in message oups.com... I posted the message below on the thread, "Nimbus 4DT accident 31 July 2000 in Spain." I am posting it again for better visibility. Stan Hall presented his analysis of the Nimbus-4DB accident in Minden, NV, 1999. Stan's analysis is scary, to say the least. The article, Probing for the Smoking Gun, was reprinted in the Soaring Association of Canada's free flight, 2/04. Go to the link below. Click on free flight on the side bar. Go to free flight back issues - 2004 - issue 2. Down load the PDF file. http://www.sac.ca/ Bill Feldbaumer 09 |
#13
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In the case of the Minden accident, there was correspondence on Rec. Aviation Soaring when the report was published by others who had experience of the inadvertent deployment of the brakes at high speed in turbulence; the suggestion was that deployment may have been uncommanded. I had an unintentional airbrake opening on my ASW24. I asked other ASW 24 owners on our Yahoo group if anybody experienced the same thing. Apparently I was the only one, so far. Very scary indeed. I installed a lock to prevent it from happening again. The flight was through many vertical gust for many miles. On one very powerful one it let go. Regards Udo |
#14
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I think we have to accept that there are occasions when the finding of an accident report should be 'I don't know but........' In the case of the Nimbus 4 accident(s) there are so many factors which 'may' have occurred that a definitive conclusion is almost impossible, the best that anyone can do is speculate, and this applies to most glider accidents where there are no living witnesses. It is very easy to attribute the cause to human factors and in many cases this may be true but there is always the possibility that there is an unknown factor. What I have taken from these reports is that it would appear that there is a point at which a situation becomes irrecoverable and this point maybe reached quicker in big wing gliders thatn in small ones and the response must be to act to prevent the situation ever arising. To me this means that care should be taken to never spin or enter a spiral dive which may mean smaller bank angles and higher airspeed in turbulent conditions even if this means that some performance is lost. There are plenty of examples of problems with aircraft getting into irrecoverable situations, The DH Chipmunk was just one such example. Modern aircraft, designed for performance at the expense of handling such as the SEPECAT Jaguar are know to be almost irrecoverable if they depart from flight. In the case of the Jaguar this was discovered during testing and the pilot had the option of departing the aircraft, survived and was able to tell the story. While not wishing to digress the discussion too much we have a similar situation involving the Puchaz where there have been several fatal spin ins. It has been found in most cases that the pilot must have mishandled the controls and this may indeed be the case however this has to be speculative. It is not beyond the realms of possibility that there are some loading conditions and other circumstances which make recovery from the spin impossible, we cannot know for certain as there are no living witnesses to these events. While this may be thought unlikely the DH Chipmunk is an example of this and several pilots died before the true problem was realised and anti spin strakes were added. Test flying can never duplicate every loading condition or minor difference in construction and I believe it is very dangerous to assume, that in the absence of any other cause, the pilot must have been at fault. Perhaps the lesson to be learned from all this is that we know less that we think we do and 'exploring the envelope' can bring us closer to disaster. It is a matter of personal choice as to whether we accept this risk. In this context Stans assessment has cogent arguments but must be speculative, we simply will never know. Not knowing is propbably the most difficult thing that we as human beings have to accept. DAJ ASW17 401 At 00:00 08 July 2005, W.J. \bill\ Dean \u.K.\. wrote: I have never flown a glider of more than about 20 metres span (ASW 17, Jantar 2). However, I am in the process of asking for other pilots' experience. One such pilot's reply includes the following: From your experience of big Nimbi do you think there is merit in Stan's ideas? 'Only to the extent that you fly a big glider like a big glider, not like a Spitfire. You adjust all your flying to the fact that you are flying a big ship. For instance to adjusting your thermalling technique to gradual bank angle changes rather than spirited use of controls to roll in and out of thermals. 'I have always said about large span gliders, of whatever make, that you regard them like flying a 747 Jumbo. Not like an agile 15m glider. 'For instance, in a very turbulent thermal such as what you often get over power stations, I often am reluctant to put on over about 15 degrees of bank. Yes, fifteen degrees. I do not wish to be sucked in to a 'loss of control' situation like Ivans and Engen or the Levers. Which particular brand of Nimbus do you fly? '4DM. I am entirely happy with it but I fly it like the big glider that it is.' I would like to add the following: On the question of use of air-brakes, it would appear that they were not opened in the case of the Spanish accident. In the case of the Minden accident, there was correspondence on Rec. Aviation Soaring when the report was published by others who had experience of the inadvertent deployment of the brakes at high speed in turbulence; the suggestion was that deployment may have been uncommanded. In addition, Dick Johnson posted that with the Minden accident there may have been a problem with the oxygen system undiscoverable after the crash, such as oxygen tanks filled with Nitrogen (not entirely unknown I am afraid). It is also notable that whereas there seems to have been no attempt to bale out with the Minden accident, with the Spanish accident both pilots did bale out and deploy there parachutes, but unfortunately one of them had his canopy foul of the wreckage which brought him down with it. W.J. (Bill) Dean (U.K.). Remove 'ic' to reply. 'Bill' wrote in message oups.com... I posted the message below on the thread, 'Nimbus 4DT accident 31 July 2000 in Spain.' I am posting it again for better visibility. Stan Hall presented his analysis of the Nimbus-4DB accident in Minden, NV, 1999. Stan's analysis is scary, to say the least. The article, Probing for the Smoking Gun, was reprinted in the Soaring Association of Canada's free flight, 2/04. Go to the link below. Click on free flight on the side bar. Go to free flight back issues - 2004 - issue 2. Down load the PDF file. http://www.sac.ca/ Bill Feldbaumer 09 |
#15
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Earlier, Bill wrote:
...Stan Hall presented his analysis of the Nimbus-4DB accident in Minden, NV, 1999... One aspect of Hall's report with which I will take issue is what is meant by a "45-degree bend" in the wing. The report shows a diagram that depicts a 45-degree bend as the wing bend at which the pilot must looks upwards 45 degrees from the lateral axis to see the wingtip. I don't doubt that that might be what witnesses reported seeing. What I do doubt is whether that is what the factory meant when they said that under static test their wing deflected about 45 degrees (46.5, to be exact) at 8g. When I was doing deflection calculations for the wing spar in my HP-24 project, it seemed that the most useful measures of deflection were the angular deflection at any point on the wing, and ultimately the total accumulated angular deflection at the wingtip. This figure compares 45-degree bends as defined by Stan Hall and by me: http://www.hpaircraft.com/misc/nimbus_bend.GIF I'm not privy to what definition of deflection the Schempp-Hirth factory used in saying that their Nimbus wing would bend 46.5 degrees at 8g. However, if their definition is similar to mine, and what witnesses saw matched Stan's definition, it would mean that the aircraft was seen at a loading substantially greater than 8g. Also, I will submit that witnesses often over-report wing bending, and that photos often seem to exaggerate it. What happens is that the witness or camera viewpoint is rarely very close to the aircraft longitudinal axis, and the oblique angle of the view tends to "compress" the wings laterally, exaggerating the dihedral and wing bend. Thanks, and best regards to all Bob K. http://www.hpaircraft.com |
#16
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I have not flown a Nimbus 4D, but have several hundred hours experience in
its' early predecessor the Nimbus 2. A couple of points that might be relevant to this accident: 1) With flapped gliders there is usually a positive flap limiting speed, often way below Vne. If you are thermalling you are likely to be in a positive flap setting, so if a loss of control occurs that leads to a spiral dive, it is very easy to exceed the flap limiting speed and risk twisting the wings off. 2) With large span gliders, opening the airbrakes causes the wingtips to bend up. We fitted a second paddle to our Nimbus 2 airbrakes and I did the test flight in which I was supposed to fly up to Vne (135 knots) with the brakes extended. By 95 knots the wings were bending up so much that I started to fear for the structure and didn't go any faster! I resolved that should I ever lose control of the speed, I would slow it down by pulling g rather than opening the airbrakes. It also had a tail-chute that could be deployed in extremis. I can only recommend that the first action in any loss of control situation should be to select neutral or negative flap, and then sort things out. Derek Copeland |
#17
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Derek Copeland a écrit :
2) With large span gliders, opening the airbrakes causes the wingtips to bend up. We fitted a second paddle to our Nimbus 2 airbrakes and I did the test flight in which I was supposed to fly up to Vne (135 knots) with the brakes extended. By 95 knots the wings were bending up so much that I started to fear for the structure and didn't go any faster! I resolved that should I ever lose control of the speed, I would slow it down by pulling g rather than opening the airbrakes. I don't understand your choice ! if there is a risk at high speeds, the best choice to avoid it is to avoid these speeds, and that's what the airbrakes are for, aren't it ? Pulling g's after loosing control is the best way to break any aircraft... -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#18
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Denis a Nimbus 4 does not have airbrakes. It has spoilers. These are
two different animals alltogether |
#20
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A Nimbus 4 has airbrakes.
And as for different animals: A glider certified under JAR22 needs to have means which limit a dive to 45 deg at vne. Whatever you call them, you can use them to limit your speed and deploy at any speed up to vne. -- Bert Willing ASW20 "TW" a écrit dans le message de news: ... Denis a Nimbus 4 does not have airbrakes. It has spoilers. These are two different animals alltogether |
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