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#161
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Lancair crash at SnF
On Apr 27, 8:18*am, Bertie the Bunyip wrote:
I'm still not sure what you mean when you say the aircraft has to accelerate. Are you saying that turning downwind will cost more in acceleration than any other type of turn? No, that's not what I'm saying. Cheers |
#162
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Lancair crash at SnF
WingFlaps wrote in news:167e4f31-1a4a-4bfa-98d0-
: On Apr 27, 8:18*am, Bertie the Bunyip wrote: I'm still not sure what you mean when you say the aircraft has to accelerate. Are you saying that turning downwind will cost more in acceleration than any other type of turn? No, that's not what I'm saying. OK Bertie |
#163
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Lancair crash at SnF
In rec.aviation.student Bertie the Bunyip wrote:
45 degrees isn't enough unless you have a very nice gliding airplane or are starting from a good height to begin with. A 45 degree bank will give you the minimum turn radius and thus minimum altitude lost for any heading change done with a steady state turn. Are you suggesting that there are more... interesting techniques which can be used to turn around, or just that sometimes you are doomed? -- Michael Ash Rogue Amoeba Software |
#164
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Lancair crash at SnF
In rec.aviation.student WingFlaps wrote:
On Apr 27, 8:22?am, Michael Ash wrote: In rec.aviation.student WingFlaps wrote: On Apr 27, 6:04?am, wrote: ? ? ? ? Lots of people had the impression you were talking about the dreaded downwind turn, with all the talk about the energy required to accelerate to maintain airspeed. The energy required, as pointed out in a much earlier post with several very good references, is so tiny that it's not worth fooling with at all. Perhaps you could put a number on that? Could you try a gliding turn with stopwatch and altimeter and compare that to a straight glide? In the optimal 45-degree-banked turn the load factor will be about 1.4. Your best glide speed and min sink speed will increase by the square root of that, or 20%. The glide angle remains the same if you increase your airspeed appropriately, so your sink rate will also increase by 20%. So instead of 650fpm you'll be coming down at 780fpm. At 78kts (65kts best glide speed from previous post plus 20%) and a 45 degree bank you're making a circle a bit over 500ft across which will take you 13 seconds to complete half of. The extra sink rate from the turn will therefore cost you 30 feet over what you would have experienced in a straight glide for the same amount of time. You'll also lose about 80 feet to accelerate from 65kts to 78kts. But you'll gain this back at the end, so as long as the end of your turn ends at a reasonable height it can be ignored. The numbers will, of course, vary between aircraft but it would appear that the extra energy loss due to the turn itself isn't all that significant. If 30 feet is the difference between making it and not making it you probably should not be turning around in the first place. I make the turn diameter bigger than that using the formula rad=(knots^2)/(11.26 x tan(bank)) (assuming it's right) or about 1080'? You're right, I gave the radius, not the diameter, but worded it as though it were the diameter. My 13 seconds is based on v/(pi*r) though so it ought to be correct. On a side note, these equations are generally vastly more comprehensible if you leave out unit conversions altogether. Turn radius in any circle caused by acceleration is v^2/a, and here the acceleration is 9.8m/s^2*tan(bank). Anyway, the result is the same, I just find it easier. Moving on.... So, what would you consider the minimum height taking decision time into account and a 225 degree turn followed by a 45 to line up back on the runway? I really haven't a clue. I don't fly these things and thus don't have the experience to comment on this. I've heard that you need to be quite high to be reasonably safe doing it. The physics only gives you a raw minimum. You need a hefty safety margin on top of that, plus knowledge of your personal ability to perform close to the ideal, obstacles which may modify your options, and other such things. Wind, density altitude, engine performanc, aircraft weight, and other such things will all contribute as well to change the answer. My personal decision height for the analogous glider launch emergency is *usually* a shade under 200 feet. Conditions, performance, obstacles, and landing opportunities ahead can all modify this value. The physics would tell you that the altitude required is around 35% of your wingspan, because the energy from the extra speed you carry is theoretically sufficient to zoom slightly, make the turn at min sink speed, and roll out lined up with the adjacent taxiway at the same altitude you started. (If you don't lose altitude then you just need to be high enough to avoid hitting your wingtip on the ground, thus the 35%.) In reality you'd have to be completely nuts to try this maneuver starting from an altitude of 20ft at normal takeoff speeds. And as I mentioned before, this has very little bearing on the options available to a typical powered airplane. -- Michael Ash Rogue Amoeba Software |
#165
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Lancair crash at SnF
Michael Ash wrote in
: In rec.aviation.student Bertie the Bunyip wrote: 45 degrees isn't enough unless you have a very nice gliding airplane or are starting from a good height to begin with. A 45 degree bank will give you the minimum turn radius and thus minimum altitude lost for any heading change done with a steady state turn. Are you suggesting that there are more... interesting techniques which can be used to turn around, or just that sometimes you are doomed? Sure, a steeper bank than will get you around more quickly and the altitude loss will be lower, mostly because the displacement form the centerline will be minimal. I'm not theorising here. I used to do it as did most people at the place. This was what worked from the lowest altitudes, but it was dangerous and when one of the more experienced guys actually had an engine failure and crashed badly turning around, we decided it might not be such a good idea after all.. Typically I considered about 300' minimum for things like a T-cart or my Luscombe, and about 400' for a Cherokee or 172 or something. In excess of 60 degrees is neccesary to do that. That, of course, is going to bring you over the stall speed if you allow the wing to load up, so a brisk lowering of the nose is required as you come around. The situation has to be assesed moment to moment as you come around and you will be absolutely on the edge of stalling the whole way. If you screw it up you will hit the ground hard. Same is true of the 45 deg method you're proposing. In my experience that's not going to be enough unless you're high enough to have turned crosswind anyway. Most guys aren;t even proficient at doing steep turns at a constant altitude with the power on. Also, probably few turnbacks are planned before departure, that's why the tiny success rate.. Bertie |
#166
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Lancair crash at SnF
On Apr 27, 11:23*am, Michael Ash wrote:
My personal decision height for the analogous glider launch emergency is *usually* a shade under 200 feet. Conditions, performance, obstacles, and landing opportunities ahead can all modify this value. What's your glide ratio? If it were 40:1 would that equate to 800' in a 10:1 Cessna? Cheers |
#167
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Lancair crash at SnF
In rec.aviation.student WingFlaps wrote:
On Apr 27, 11:23?am, Michael Ash wrote: My personal decision height for the analogous glider launch emergency is *usually* a shade under 200 feet. Conditions, performance, obstacles, and landing opportunities ahead can all modify this value. What's your glide ratio? If it were 40:1 would that equate to 800' in a 10:1 Cessna? The things I fly regularly range from 20:1 on the low end to 37:1 on the high end. The 200ft number remains the standard for all of them. Glide ratio can adjust it but it's more a matter of having a comfortable safety margin. I once talked to an instructor who did a 180 in a 34:1 machine at only 100ft, and he said it was a piece of cake. With that kind of performance the turn costs no altitude at all so the decision height is basically all safety margin. Flying something with reasonable performance out of an airport where the landing sites ahead look deadly then I probably would reduce the decision height to something like 100ft. A direct multiplication isn't going to work here for several reasons. First because of the safety margin. You don't need four times as much of that. Second because of speed differences. The power plane needs to go significantly faster to maintain best glide or min sink, resulting in a larger turn, more altitude lost in that turn, and more corrective action to regain the runway centerline after it. The power plane starts out either around best glide or slower than it, whereas the glider generally has extra speed which means extra energy. (This will of course depend on the performance speeds of the plane in question, but most power planes have faster performance speeds.) Third, reaction time. When you hear a bang and your rope goes away it makes the situation obvious instantly. Engine trouble can take more time to diagnose. (One of the more difficult scenarios for a glider is when this happens to the tow plane. You could end up low and slow before realizing what's going on and 200ft may very well not be enough in that case.) Overall I don't think it makes sense to extrapolate from the glider case. From what the power guys are saying, it sounds like you want at least several hundred feet, with the exact number depending heavily on the skill of the pilot and the performance of the airplane. -- Michael Ash Rogue Amoeba Software |
#168
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Lancair crash at SnF
On Thu, 24 Apr 2008 15:40:55 -0700 (PDT), WingFlaps
wrote: On Apr 25, 10:16*am, Stefan wrote: WingFlaps schrieb: (the stall is now damn close -better hope there's no significant wind) ... Now we add in the energy losses from having to accelerate with the wind and to glide speed. Arrrgh! Not the old "turn into downwind" legend again! Better work out your understanding of physics before publicly reasoning about turns. Try reading the statement again, here it is: "Now we add in the energy losses from having to accelerate with the wind and to glide speed." Now perhaps you would like to revise some physics and try to critcise it for us? I await your stumbling analysis of my words with mild amusement. Cheers ***************************************** Long time ago in the 30's I saw on a couple of occasions a Cub take off in a strong head wind 25+ mph and make a 180 degree turn to down wind. They then started losing altitude and mushed into the ground nose high. Any idea why? Pilot's said they had full throttle and proper RPM showed on engines until impact. Big John Big John |
#169
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Lancair crash at SnF
Big John wrote in
: On Thu, 24 Apr 2008 15:40:55 -0700 (PDT), WingFlaps wrote: On Apr 25, 10:16*am, Stefan wrote: WingFlaps schrieb: (the stall is now damn close -better hope there's no significant wind) ... Now we add in the energy losses from having to accelerate with the wind and to glide speed. Arrrgh! Not the old "turn into downwind" legend again! Better work out your understanding of physics before publicly reasoning about turns. Try reading the statement again, here it is: "Now we add in the energy losses from having to accelerate with the wind and to glide speed." Now perhaps you would like to revise some physics and try to critcise it for us? I await your stumbling analysis of my words with mild amusement. Cheers ***************************************** Long time ago in the 30's I saw on a couple of occasions a Cub take off in a strong head wind 25+ mph and make a 180 degree turn to down wind. They then started losing altitude and mushed into the ground nose high. Any idea why? Pilot's said they had full throttle and proper RPM showed on engines until impact. Wind gradient. As they climbed, the wind would increase and they'd lose some airspeed until they were on the backside of the power curve.same thing would happen if you took off with strong tailwind in the first place. Also, the poorer climb angle even without the wind gradient would cause many to get the nose up a bit too much as well if they were not used to it. Bertie |
#170
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Lancair crash at SnF
"Alric Knebel's Rack" wrote in message ... On Fri, 25 Apr 2008 11:06:42 +0000 (UTC), Bertie the Bunyip wrote: It's not so much the loss of altitude that will get you in this manuever. it's the probable loss of control trying to manuever around back towards the field. Firstly, in any emergency that hasnt been drilled, you will have a moment where you will be sitting there with your mouth open in utter disbelief of what has just happened. in fact, even if you have drilled for it you will still have this moment, but if it's been practiced the moment you begine to do something about it will be sooner coming. While you're sitting there wondering what's going on, the speed will be bleeding off. Not good. Nothing you do is useful. You very rarely even come up with a good idea, and when the blind squirrel principal does kick in (astoundingly, you've seem to even defeat random chance with your incompetence), you manage to cock it up so badly that what might have been a useful thing in a normal person's hands turns to low-grade fertilizer. Those are your two claims to fame. Being a complete flake who can't keep even the simplest of things on track for any significant period of time, and being an absolute moron when it comes to understanding what's useful to the piloting community, and implementing it. No, actually he has proven himself a failure and dozens of things, wannabe troll, pilot, motorcycle mechanic, and others. I think it might be fair to label him a "complete" or "master" failure. |
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