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#21
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#22
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Mark Navarre wrote:
As speed increases, the nose will come up (away from vertical) even if you don't want it to, and even with full forward trim, it would still require an honest push on the stick to maintain that attitude. Not true with all gliders. Several modern racing ships will, when trimmed within CG limits for best climb performance, "tuck" or pitch down with increasing airspeed, resulting in a large outside loop or exceeding VNE without pilot input. The crossover speed for this in my own ship is about 115 kts, above which slight back pressure is needed to maintain speed. Which glider is that? I thought a pitch-up tendency with increasing speed was a certification requirement? And what is the CG position for this to happen? Is this "feature" mentioned in the flight manaul? Sounds kind of scary. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#23
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Nimbus 3 and 4 and Duo Discus brakes do not meet the
'speed limiting in a 45 degree dive' requirement, but do achieve that in a 30 degree dive. That's why they are non-aerobatic. At 15:18 31 March 2004, W.J. \bill\ Dean \u.K.\. wrote: NO. This is thoroughly misleading. HISTORICAL. When the first gliders with good (for the day) performance were built, it was found that the good performance made them difficult to land. So they were fitted with spoilers as a landing aid. Then pilots started to cloud fly, and some lost control in cloud and overspeeded and overstressed their gliders, which broke up. This was countered by developing and fitting speed-limiting airbrakes (DFS, e.g. Weihe and Slingsby Sky, and Schempp-Hirth). These were intended to be speed limiting in a true vertical dive. In the U.K. it was a requirement that the glider was test flown to prove that at max. all up weight in a vertical dive Vne was not exceeded, I understand that the Slingsby Skylark series all passed this test. Note that max. manoeuvring and rough air speeds WOULD be exceeded. Later, it was found that with higher wing loadings, thinner wing sections and higher aspect ratios it became practically impossible to fit true speed limiting brakes (in the sense that Vne would not be exceeded in a true vertical dive at max. a.u.w.). The first U.K. built gliders for which this applied were, I believe, some at least of the Slingsby Dart series. Also, if the rules were relaxed life would become a lot easier for the designer, because it would save weight and cost. So the rules were relaxed, and 'Speed limiting' came to mean 'In a dive at X degrees', usually I understand of 45 degrees. TODAY Most gliders today, including I believe all those built in Europe, are designed to JAR 22. See: Joint Aviation Authorities, Europe. http://www.jaa.nl/ , JARs – Section 1 – JAR-22 http://www.jaa.nl/section1/jars/445499. pdf . The relevant clause is: 'JAR 22.73 Descent, high speed 'It must be shown that the sailplane with the airbrakes extended, will not exceed VNE in a dive at an angle to the horizon of: '(a) 45° when the sailplane is approved for cloud flying and/or aerobatics when certificated in the Aerobatic or Utility Category; '(b) 30° in other cases. '[Ch. 5, 28.10.95]' Some modern gliders, including some being built today, probably still have true speed limiting brakes by the strict old definition given above; my guess is that these would all be gliders with trailing edge brakes or braking flaps such as the early Pik 20; but this would not necessarily be true for all gliders with such brakes. Some gliders were built with tailchutes, either in an attempt to comply with the old strict requirement, or because it was necessary if they were to comply with the relaxed rule. I have always understood that the Janus was fitted with a tailchute to be speed limiting in a 45 degree dive at max. a.u.w. with full water ballast. At what dive angle would a Duo-Discus with full brakes go through Vne? I would be astonished if this is more than 45 degrees, it may very well be 30 degrees. So if in a spin recovery, or for any other reason, you are diving at a very steep angle your air-brakes are unlikely to save you from exceeding Vne. I am sure they won't in the Nimbus 3/4 series; it was not a requirement for certification. W.J. (Bill) Dean (U.K.). Remove 'ic' to reply. 'Arnold Pieper' wrote in message . com... John, The airbrakes were designed not only to be used for approach and landing, but also to avoid reaching VNE. Look at your glider's POH and check what is the maximum speed to deploy the airbrakes, and what becomes the VNE with them deployed. In most modern design gliders, the airbrakes can be deployed up to VNE, and they will prevent the glider from reaching VNE when fully opened. The airbrakes are designed for this purpose. Once the airbrakes are opened and will prevent you from going over VNE, there's no need to pull at anything even close to the design limit G. Spin training therefore, is the best way to ease this fear and learn how to pull without overstressing the airframe. AP 'John Galloway' wrote in message ... Through the contributions to the avoiding VNE thread runs the theme of the difficulty of avoiding overspeeding and/or overstressing some modern designs in accidental spin recovery. This is made more difficult than in older composite gliders because they had a little more drag and a little more (fortuitous) margin in the g limits. Is it not blindingly obvious that there is a need for an emergency drag device that does not reduce the G limits of gliders? Clearly if we all handled the recovery from inadvertent spins etc perfectly all would be well but equally clearly that does not always happen and it is a shame to lose pilots in this situation. As the Phoebus pilot pointed out a tail chute is ideal for this - providing that it can be made to actuate and jettison reliably. (I found the design used on the Kestrel particularly good and I never once had a failure for landing use.) On the other hand they are expensive and inconvenient to replace and there are several ways that they can fail. So can anyone think of a better idea than a chute? The best I can come up with is some sort of flush fitted rectangular-with the-long-edge-horizontal rear hinged airbrakes (like old fashioned automobile suicide doors) located on the fuselage sides somewhere in the region below or below/behind the wings. If they opened to about 45 degrees with a spring actuator (and limited by sliding metal stays that hinge/attach to the front of the panel and whose inner ends slid along in runners) then they would provide a lot of drag without any deep internal mechanism (such as wing airbrakes have). Once they have done their job the rear end of the brakes could be released by a spring loaded mechanism similar to the front end so that the brakes would then instantly spring to as position set out from and parallel to the fuselage so that there would be very little drag - only that provided by the stays at both ends and the brake panels edge on to the wind. That configuration would be good enough to fly home with. It would only be possible to reset these brakes on the ground and they would not replace conventional wing airbrakes for approach control - although they could have a secondary use for emergency approach control. I am envisaging something the could be included in new designs although there does not seem to be any obvious reason why such a device could not be retrofitted as a fairly major modification. The contours of the brake panels would be specific to the individual fuselage type but the mechanism could be generic. The assembly would be fairly shallow and complete within itself apart from e.g. a cable release attachment. I am not advocating a technical solution to this problem in place of spin recovery practice but I do think that there must be something that the combined intellects of the gliding community can come up with other than observing that if we get into that particular overspeeding/steep attitude condition we are stuffed. Anyone got any simpler or better ideas? I am definitely not an engineer. John Galloway |
#24
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I think positive static stability means that this doesn't
happen. The only certified aircraft I'm aware of that tucks in pitch is the Learjet 20/30/50 series, but this happens at transonic speeds as the center of lift moves aft. The aircraft is certified with a stick puller to ensure you never get there. In most aircraft the phugoid mode is pretty lightly damped and has a period on the order of tens of seconds, so the speed can build up a bit before the nose comes up. Are you sure that's not what you're experiencing? The only other thing I can think of is a c.g. out of limits, so it might be worth double checking your weight and balance if your aircraft behaves this way. At 06:18 01 April 2004, Eric Greenwell wrote: Mark Navarre wrote: As speed increases, the nose will come up (away from vertical) even if you don't want it to, and even with full forward trim, it would still require an honest push on the stick to maintain that attitude. Not true with all gliders. Several modern racing ships will, when trimmed within CG limits for best climb performance, 'tuck' or pitch down with increasing airspeed, resulting in a large outside loop or exceeding VNE without pilot input. The crossover speed for this in my own ship is about 115 kts, above which slight back pressure is needed to maintain speed. Which glider is that? I thought a pitch-up tendency with increasing speed was a certification requirement? And what is the CG position for this to happen? Is this 'feature' mentioned in the flight manaul? Sounds kind of scary. -- ----- change 'netto' to 'net' to email me directly Eric Greenwell Washington State USA |
#25
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"Eric Greenwell" wrote in message ... Mark Navarre wrote: As speed increases, the nose will come up (away from vertical) even if you don't want it to, and even with full forward trim, it would still require an honest push on the stick to maintain that attitude. Not true with all gliders. Several modern racing ships will, when trimmed within CG limits for best climb performance, "tuck" or pitch down with increasing airspeed, resulting in a large outside loop or exceeding VNE without pilot input. The crossover speed for this in my own ship is about 115 kts, above which slight back pressure is needed to maintain speed. Which glider is that? I thought a pitch-up tendency with increasing speed was a certification requirement? And what is the CG position for this to happen? Is this "feature" mentioned in the flight manaul? Sounds kind of scary. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA Though it was certainly not a "modern racing ship" my old Lark IS28 would go neutral pitch stability at about 90 knots and become very nose heavy above that even with negative flaps. Bill Daniels Nimbus 2C |
#26
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#27
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Bill Daniels wrote:
"Eric Greenwell" wrote in message ... Mark Navarre wrote: As speed increases, the nose will come up (away from vertical) even if you don't want it to, and even with full forward trim, it would still require an honest push on the stick to maintain that attitude. Not true with all gliders. Several modern racing ships will, when trimmed within CG limits for best climb performance, "tuck" or pitch down with increasing airspeed, resulting in a large outside loop or exceeding VNE without pilot input. The crossover speed for this in my own ship is about 115 kts, above which slight back pressure is needed to maintain speed. Which glider is that? I thought a pitch-up tendency with increasing speed was a certification requirement? And what is the CG position for this to happen? Is this "feature" mentioned in the flight manaul? Sounds kind of scary. Though it was certainly not a "modern racing ship" my old Lark IS28 would go neutral pitch stability at about 90 knots and become very nose heavy above that even with negative flaps. Was that a common experience, or just your particular IS28? I'm wondering if there might be a rigging error. None of the gliders I've owned has exhibited that; in fact, all of them ran out (or required nearly full) of forward trim nearing Vne. THese gliders are the Ka-6E, Std Cirrus, Libelle H301, ASW 20 C, ASH 26 E, Blanik. The Cirrus had a particularly noticeable concave under section on the elevator to induce this behavior. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#28
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On Thu, 01 Apr 2004 07:58:46 -0800, Eric Greenwell
wrote: Though it was certainly not a "modern racing ship" my old Lark IS28 would go neutral pitch stability at about 90 knots and become very nose heavy above that even with negative flaps. This behaviour of a neutrally instable aircraft is typical for a CG that is out of range. Was that a common experience, or just your particular IS28? I'm wondering if there might be a rigging error. None of the gliders I've owned has exhibited that; in fact, all of them ran out (or required nearly full) of forward trim nearing Vne. Just like you I also have never flown an aircraft that did not hve positive pitch stability. Bye Andreas |
#29
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I don't know where you guys get these things from, but this behaviour means
these gliders should have their certifications cancelled. I've never flown anything with these characteristics. "Mark Navarre" wrote in message ... From: "Arnold Pieper" Date: 3/31/2004 1:16 PM Pacific Standard Time Message-id: m Maybe true if "near vertical", however, staying at a near vertical dive is something that requires a conscient effort. The glider won't just stay there on its own. To remain in a 90 degree vertical dive requires a significant amount of forward stick force and concentration. As speed increases, the nose will come up (away from vertical) even if you don't want it to, and even with full forward trim, it would still require an honest push on the stick to maintain that attitude. Not true with all gliders. Several modern racing ships will, when trimmed within CG limits for best climb performance, "tuck" or pitch down with increasing airspeed, resulting in a large outside loop or exceeding VNE without pilot input. The crossover speed for this in my own ship is about 115 kts, above which slight back pressure is needed to maintain speed. - Mark Navarre 2/5 black ace LoCal, USA - |
#30
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You're affraid of something that
"Todd Pattist" wrote in message ... "Arnold Pieper" wrote: Don't be affraid to open the airbrakes at high speed, believe me, the manufacturer is just a little smarter than that. I'm not afraid to open them at high speed, I'm afraid to lower the G-limit when I need G-s to recover. I've seen too many accident reports where structural failure was attributed to overstressing with the brakes open. You have to be careful at high speed just because they tend to jump out more easily, so, have a firm hand on it. Real aerobatic training (as opposed to some occasional loops) will clarify a lot of this. Although it's been long time, my training was by a national glider aerobatic champion. Todd Pattist - "WH" Ventus C (Remove DONTSPAMME from address to email reply.) |
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