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#11
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On Wed, 28 Jan 2004 01:43:36 UTC, "Bill Daniels"
wrote: : Remember the old spin-the-hammer trick from freshman physics? It seems : solid objects don't like to spin around their long axis - they prefer to : spin about their shortest. Actually, the physics says that objects are stable when rotating around the axes corresponding to the greatest and least moments of inertia. For a glider I'd expect yaw to be the highest moment of inertia and pitch to be the lowest. Howver, that ignores aerodynamic effects, which I'd expect to be much more significant. Ian -- |
#12
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On 28 Jan 2004 11:17:41 GMT, "Ian Johnston"
wrote: On Wed, 28 Jan 2004 00:56:12 UTC, (Tim Shea) wrote: : After 4 or so rotations, the nose seemed to float up and the rotation : *seemed* to slow considerably. I remember thinking that this is cool! : Kind of like floating. When it was time for the recovery I applied the : control inputs I'd been taught (as specified above) and much to my : surprise, nothing different happened.....for a long time. I estimate : that we completed another 5+ rotations nose high before it broke, : rolled over and recovered. I am told that a Junior oscillates nose up and down while spinning, and that recovery is much much snappier nose down. That's all in the POH. A Junior has three different spin behaviours depending on cockpit load. IIRC the oscillation occurs with a light pilot. I'm in the middle group (180 with a chute) and it recovers automatically after just over 2 rotations, even with the controls still fully crossed. The last half rotation gets really slow. I was a bit annoyed. Having just done Silver height and wanting down in a hurry, I was after 3 turns and was intending to come most of the way down in a 3-turn - recover - spin the other way sequence. Still, I repeated the experiment in the other direction and with recovery by merely centreing the controls and got consistent recovery after just over 2 rotations. Be sure to read the POH before attempting more than one rotation. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#13
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I'm in the middle group (180 with a chute) and it recovers
automatically after just over 2 rotations, even with the controls still fully crossed. The last half rotation gets really slow. I was a bit annoyed. Having just done Silver height and wanting down in a hurry, I was after 3 turns and was intending to come most of the way down in a 3-turn - recover - spin the other way sequence. Still, I repeated the experiment in the other direction and with recovery by merely centreing the controls and got consistent recovery after just over 2 rotations. I quite frankly find this a quite scary post! I do hope you allowed suficient height above the start of your 1000m gain. I take it you are also one of the 'glider pilot hero' types? Still, i'm sure you impressed everyone at the bar afterwards.......... So, what was wrong with a more conventional 'rapid decent', you know, the one that uses full airbrake circling in sink, or sideslipping with full airbrake. |
#14
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I have been told two things:
1. That the Reynolds number applicable to a glider changes with altitude, 2. That the Reynolds number affects the stall/spin behaviour and recovery. I can remember an anecdote (I am vague as to who or when) about a K21 at Aboyne. The glider was at about 20,000 ft., and the crew wanted to get down for the next pupil. They put it into a spin (which it entered without difficulty), and then held the spin without moving the controls. It span down to about 7,000 ft. and then self-recovered without any change in control position. Heights are QNH. I don't understand Reynolds numbers, but I know it matters; and it would seem not only to designers. It might well be that some gliders more easily spin and are harder to recover at height. I would expect this to apply at altitudes above about 7,000 ft. QNH, above which height they are almost certainly not test flown. W.J. (Bill) Dean (U.K.). Remove "ic" to reply. "Chris Nicholas" wrote in message ... Reading of these brave souls who have been testing the Puchacz spinning characteristics with multi-turn spins at high altitudes reminds me of an anecdotal rumour which reached me about at least one other glider type, and I think also some power aircraft, which similarly misbehaved until many turns/it got lower. It made me wonder a few things: At the heights people here have been writing about - 10 to 17 thousand feet - what is the true airspeed at which it enters the spin on command and how does that differ from the lower altitude airspeed used for certification tests? Bear in mind also one poster's comments that a glider does not instantly cease forward motion and go instead into vertical motion with a rotational component - in the absence of infinite forces, the first is subject to some deceleration taking time and space, and the second some vertical acceleration taking time and height. Similarly, what is the true vertical velocity at onset and when stable in the spin? What is the ratio of those two velocities compared with the ratio at test air densities? Does the rotation rate remain identical, whether at height (lower air densities) or at lower altitude (higher density)? Does all that have an effect on true angle of attack? Could such things account for high altitude spins when fully developed requiring more turns to recover? I wonder if the people who conduct these high altitude tests were in a regime not tested by the maker or the certification test pilots such as Chris Rollings? Chris N. |
#15
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On 28 Jan 2004 13:42:00 GMT, Pete Zeugma
wrote: I'm in the middle group (180 with a chute) and it recovers automatically after just over 2 rotations, even with the controls still fully crossed. The last half rotation gets really slow. I was a bit annoyed. Having just done Silver height and wanting down in a hurry, I was after 3 turns and was intending to come most of the way down in a 3-turn - recover - spin the other way sequence. Still, I repeated the experiment in the other direction and with recovery by merely centreing the controls and got consistent recovery after just over 2 rotations. I quite frankly find this a quite scary post! I do hope you allowed suficient height above the start of your 1000m gain. I was at 5300 ft when I decided to come down and to practise spinning on the way, ending the last spin at about 2500 ft. That's quite low enough for me: I won't deliberately initiate a spin under 3000 ft. So, what was wrong with a more conventional 'rapid decent', you know, the one that uses full airbrake circling in sink, or sideslipping with full airbrake. Not as much fun. Besides I hadn't spun the Junior for a while and thought I needed the practise. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#16
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Shawn Curry writes:
Anyone else spin the Puch for more than three turns? What happened (obviously you survived)? I've heard that some other aircraft also have a flatter spin mode that after several turns that is hard to recover from. Any knowledge of why this happens? (Now where's my copy of Stick and Rudder?) The Pitts S-1S is the one for this. Get it spinnong, then let the controls train and open the throttle. It will go into a FAST flat spin, picking the nose up to above the horizon. If you are not carfull and precise in the recovery you can inadvertantly flick it into a spin in the other direction, or into an outside spin. Not ever heard that it is hard to recover from though. -- Paul Repacholi 1 Crescent Rd., +61 (08) 9257-1001 Kalamunda. West Australia 6076 comp.os.vms,- The Older, Grumpier Slashdot Raw, Cooked or Well-done, it's all half baked. EPIC, The Architecture of the future, always has been, always will be. |
#17
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W.J. (Bill) Dean (U.K.). wrote:
I have been told two things: 1. That the Reynolds number applicable to a glider changes with altitude, 2. That the Reynolds number affects the stall/spin behaviour and recovery. I can remember an anecdote (I am vague as to who or when) about a K21 at Aboyne. The glider was at about 20,000 ft., and the crew wanted to get down for the next pupil. They put it into a spin (which it entered without difficulty), and then held the spin without moving the controls. It span down to about 7,000 ft. and then self-recovered without any change in control position. Heights are QNH. I don't understand Reynolds numbers, but I know it matters; and it would seem not only to designers. It might well be that some gliders more easily spin and are harder to recover at height. I would expect this to apply at altitudes above about 7,000 ft. QNH, above which height they are almost certainly not test flown. W.J. (Bill) Dean (U.K.). Remove "ic" to reply. So I'm a test pilot every time I fly from my home field at 7,500 ft MSL? |
#18
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On Wed, 28 Jan 2004 14:12:44 UTC, "W.J. \(Bill\) Dean \(U.K.\)."
wrote: : I don't understand Reynolds numbers They are simply a way of comparing flow patterns round gliders in different situations. If two gliders have the same Reynolds number then the flow round them will look the same - the stream lines are the same shape - regardless of the velocity. It's given by density * velocity * characteristic length / viscosity. So if you go to a high altitude (- low density) you need a higher speed for the same flow pattern [1]. Or if you make a 50% scale model (- smaller length) you also need a higher speed. But beware - Reynolds numbers can be used to compare smaller parts of aerodyamics as well. For example, separation of a boundary layer typically occurs at a particular Reynolds number based on distance from the leading edge of the wing, frontal drag from the fuselage will depend on the Reynolds number based on the mean diameter of the fuselage and so on. This is one of the main reasons people find Reynolds numbers confusing ... Ian, with his Fluid Dynamics Lecturer's hat on. [1] Yes, I know viscosity must change with pressure and density, but I can't offhand remember how! -- |
#19
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On Wed, 28 Jan 2004 15:51:28 UTC, Martin Gregorie
wrote: : On 28 Jan 2004 13:42:00 GMT, Pete Zeugma : wrote: : So, what was wrong with a more conventional 'rapid : decent', you know, the one that uses full airbrake : circling in sink, or sideslipping with full airbrake. : : Not as much fun. Coo. You've been and gone and said it now. "Fun", eh? Ian |
#20
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Shawn Curry wrote "So I'm a test pilot every time I fly from my home
field at 7,500 ft MSL?" I suspect you may be, if you go in for multi-turns spins at much above that sort of altitude. That's the point I was asking about. What do you think? More conventional high flying is probably within the range of Reynolds numbers that correspond with tests, provided you don't push the envelope at the edges. Also the difference in density and RN is not great from 7,000 to 7500 feet. If you know all this, of course, you can educate me by telling me the answers. If you don't - . . . back to your own question, I think, or perhaps an aerodynamicist could tell us both (and any others who may be interested). The higher you go, of course, the more difference it makes. As pointed out in other threads, if you go high enough, you stall at the same speed as flutter onset, which leaves no usable envelope at all. In my earlier post about true velocities/IAS/density/AoA/rotational speed etc., as I don't know if everyone realises their tie up with Reynolds numbers, I deliberately didn't refer to RN. Few (certainly not me) would know off by heart the formulae, even if they have heard of the things, or how the other factors and RN change with height. I did, however, presume that all post bronze or equivalent people will have done some reading on true vs IAS, flight envelopes, etc.. and might therefore appreciate that the geometry of a spin, effectiveness of control surfaces, and rotational aspects, high up could be different from lower down. Chris N. |
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