If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
W.J. (Bill) Dean (U.K.). wrote:
It is clear that some types are marginal in recovering from an inadvertent spin entry without exceeding limits, especially if the entry is from an accelerated stall. An ETA broke up recovering from a deliberate stall/spin entered for certification purposes. This is exactly the point: Certificated gliders can always be recovered from a spin without exceeding the limits, otherwise they wouldn't have been certificated. The ETA wasn't certificated and broke up during a test flight. It won't get certificated before this issue is fixed. Stefan |
#22
|
|||
|
|||
Bert Willing wrote:
Nonsense. Exceed the structural g-load limit and you fly without wings. Not so much fly as.... plummet. Tony V. :-) |
#23
|
|||
|
|||
On Fri, 26 Mar 2004 14:00:08 -0000, "W.J. \(Bill\) Dean \(U.K.\)."
wrote: I once saw the wreck of an ASW20C just after it crashed at about 200 knots. It turns out that the stability and control characteristics of that type are such that above a certain speed it is impossible to recover from a dive even with the stick on the back stop; this is because the wing twists. pretty interesting case - I had expected that the 20 (and 20C) would loose its wings due to aileron/flap flutter at this speed. It is clear that some types are marginal in recovering from an inadvertent spin entry without exceeding limits, especially if the entry is from an accelerated stall. An ETA broke up recovering from a deliberate stall/spin entered for certification purposes. The eta case is particularly interesting: Afaik it did not break up because it exceeded Vne (in fact it lost its tail at "only" about 200 kp/h), but because it exceeded the yellow line with fully deflected rudder (from the spin recovery) and the resulting torque broke the tail boom that was not designed for this case of load (fully deflected rudder at this speed). Bye Andreas |
#24
|
|||
|
|||
W.J. (Bill) Dean (U.K.). wrote:
Unless you are the designer, you don't know what was the limiting factor which sets the limit speed. I once saw the wreck of an ASW20C just after it crashed at about 200 knots. It turns out that the stability and control characteristics of that type are such that above a certain speed it is impossible to recover from a dive even with the stick on the back stop; this is because the wing twists. are YOU the designer of the ASW20C ??? -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#25
|
|||
|
|||
W.J. (Bill) Dean (U.K.). wrote:
If you get it wrong and have to take one of the risks, I am told that you should centralise the ailerons, then pull however hard is necessary not to exceed VNE, and make sure the brakes stay shut. No. You should "pull however is necessary but not exceeding the maximum g permitted near VNE (usually it is reduced to 4 g's at that speed, even without airbrakes). And certainly not "pull hard" because a even very small force backwards on the stick is likely to cause higher than expected g's ! If your speed is going to exceed VNE within this manoeuvre, you should stop or reduce pulling and apply full airbrakes. At any dive angle up to 45° this prevents the glider to exceeding VNE, and you have time to recover pulling gently (under 2 g's). This of course supposes that there is sufficient ground clearance... -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#26
|
|||
|
|||
Stefan wrote:
Don Johnstone wrote: I am not sure that enough G could be pulled at speeds below VNE to cause serious catastrophic failure as there is a margin between the placarded limit and the design limit of most gliders. IIRC, JAR requires a safety margin of 1.5 Va (maneuvering speed) is generally accepted as the highest speed you can make full control deflections without exceeding the flight limits. If there is a 1.5 safety margin, a speed only 22% higher would allow you to exceed the design limits. On my ASH 26 Va is 99 knots, so this speed would be 121 knots, 25 knots _below_ Vne. I am sure enough G can be pulled at speeds below Vne to cause serious catastrophic failure. Further, At Vne (47% higher than Va), you could pull 2.2 times the flight limits! -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#27
|
|||
|
|||
I would tend to side with Don here.
Given a choice between exceeding Vne or the placard 'g' loading, I would go for pulling hard every time. From what I can remember, the glider manufacturer comes up with a 'Design Dive Speed', Vd, which is then reduced (c.10%) to give a 'Never Exceed Speed', Vne. It is important to note that this margin is there to cope with things such as ASI position and calibration errors. When you fly a glider at an _indicated_ speed of Vne, you might actually be nearer Vd and the realm of the test pilot. Flutter is a big problem and I am told that it is usually the determining factor in the calculations used to produce Vd. Remember this is for a new glider, with factory mass-balancing and pristine seals on the flying surfaces. On the other hand, the airframe 'g' limits have: (a) a regulatory margin of 1.5, often much greater by the time the glider is made (it musn't be below 1.5. so ends up being 1.6+) and (b) been promulgated for max. flight-mass (leading to an even bigger margin if you are below this). To use an example, the K-21 has a Vne of 280Kph/151Kts and declared flight manual 'g' limits of +5.3/-4.0. That makes Vd around 166Kts. The design 'g' limits work out as 8/-6 and it is unlikely to fail at that point. As Bill Dean has mentioned, some gliders lose a lot of their control authority over Vd. You may find yourself flying faster and faster and unable to do much about it. Also, the faster you get, the more likely you are to overstress the airframe with control inputs (and I don't just mean the elevator). I agree with other posters about the use of airbrakes; either pull the brakes OR pull hard but NOT BOTH. The Nimbus-4 which broke up near Minden was handled in this way. In most modern gliders you should be able to pull 6g+ without breaking any bits off them. The rise is drag is high, and will pretty certainly arrest any further speed build-up and bring the nose up quite smartly. Also you won't get so close to the ground, if this is a factor. |
#28
|
|||
|
|||
Don Johnstone wrote:
2. Pulling excess G can damage the airframe, however the damage is likely to be far less than the damage caused by flutter. Both may cause the same damage : loosing the wings (and both have) I am not sure that enough G could be pulled at speeds below VNE to cause serious catastrophic failure Below manouevreing speed, no. Above it (and below VNE) yes ! I stand to be corrected but I suspect it is unlikely that 8G could be attained at an airspeed less than VNE. If 8 g is the extreme limit this includes a safety margin and the real limit is what is placarded (in most gliders it's about 5 or 5.5 g). And this limit is only valid at manouevering speed, at VNE it is much lower. yes 8 g may be attained well below VNE (if stalling speed is 70 km/h, you may exceed 8 g at 198 km/h, as lift depends on square of speed.) At 270 km/h you'd reach 14.9 g !!! 3. Airframe flutter can occur at less than VNE. The likelyhood of flutter increases dramatically above VNE and the severity increases with the speed. Nonsense. Flutter cannot appear under Vc, a design speed that is just above VNE. Not all modes of flutter are catastrophic (it depends of the damping) but most are explosive. At any speed above flutter speed. The N really does stand for never. Yes, but the G limits are *never* to be exceeded too, even if nobody has thought to call it GNE. It is exactly the same. Will airbrakes effect the recovery from a spin, I don't know yet, That was the question. Thus if you don't know, please don't reply ! -- Denis R. Parce que ça rompt le cours normal de la conversation !!! Q. Pourquoi ne faut-il pas répondre au-dessus de la question ? |
#29
|
|||
|
|||
That is not the issue Jean. I am talking about pulling the airbrakes before
the rotation of the glider has stopped. This not in the manual of course. The idea is to avoid a high speed with the glider at a pitch angle of something like 60° directly after it has stopped rotation. Testing what happens when I do the whole thing with my new rather heavy low drag Ventus-2cxT is an invitation to others to call me an "idiot". Probably I will do that myself too. For me it is more like an "if then" case. While instructing I have done tens of spins with students with a ASK-13, but that's easy of course and does hardly apply to what can happen to modern gliders. Thanks Karel, NL "Jean" schreef in bericht ... Why don't you check your glider's flight manual ? Jean "K.P. Termaat" a écrit dans le message de ... Yesterday evening I talked with a friend about avoiding excessive speed when recovering from a spin in a modern low drag glider with the somewhat larger span. He came up with the idea of pulling the airbrakes when still recovering from the rotating mode. I am not sure this can be done without disturbing the recovering action or without hurting the glider. Any comment will appreciated. Karel, NL |
#30
|
|||
|
|||
Edward Downham wrote:
I would tend to side with Don here. Given a choice between exceeding Vne or the placard 'g' loading, I would go for pulling hard every time. Does "pulling hard" mean: a) pulling hard on the stick, or b) pulling "high Gs"? There is a big difference! To use an example, the K-21 has a Vne of 280Kph/151Kts and declared flight manual 'g' limits of +5.3/-4.0. My manual, as do many, show a _reduced_ allowable G load at increasing speeds, going from 5.3 G (Va) to only 4 G at Vne. That makes Vd around 166Kts. The design 'g' limits work out as 8/-6 and it is unlikely to fail at that point. "Unlikely"? How about gusts or simultaneous control deflections (say, a bit of rudder or aileron)? Have you talked to a Schleicher designer (or any designer) about this, or are you speculating? -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Avoiding Shock Cooling in Quick Descent | O. Sami Saydjari | Owning | 32 | January 21st 04 04:32 AM |
Avoiding gliders | Stefan | Piloting | 16 | August 6th 03 05:44 AM |