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#1
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Low approaches in ground effect
First a disclaimer: I understand the security issues involved in the following and would not encourage anyone to try this at home, but I am interested in the theoretical side of it. Imagine you get things wrong and are caught out low on final, still a fair distance out, and it looks marginal whether you are going to reach the runway or not. One technique I have sometimes heard described is to dive for the deck and complete the remaining distance in ground effect. For the sake of the argument we can assume fairly flat ground, free of obstacles, though not necessarily landable. The advantages claimed are usually better glide performance in ground effect and less headwind and absence of downdrafts close to the ground. On the other hand you'll be travelling at higher than optimal airspeed for most of the distance. I am wondering how much truth there actually is to this technique. Would it significantly increase your range and improve your chances of reaching the field or not ? Would it perhaps work better against a strong wind gradient (as I suspect it might), and maybe not help a lot in calm conditions ? I'd be interested in any hard data/analysis or otherwise enlightening comments on this. Please note though, that I am not talking about high-speed competition finishes, rounded off with a beatup and a sharp pullup and all the dangers and other issues involved in that. Cheers CV |
#2
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Don't know whether this answers your question!
A Russian airforce pilot perfected this technique for getting home on minimum fuel using ground effect to prolong his flights. His ideas were developed into ground effect vehicles which the Russians built in a variety of forms. The largest being the size of 747's and were named the Caspian Sea Monsters. The intended development was for military use. the end of the cold war stopped production of the big ones. Some smaller craft have been produced. Briefly the idea is that you force air under the craft with huge forward mounted jet engines, which provide both the air cushion and thrust. Not quite the same as just using ground effect to prolong an approach! Ground effect does work with a low wing glider, other than using the effect to float along way across flat airfields I have never used it to prolong a final glide over rough ground. But practice suggest that you have to be very low to get the best effect and is only short lived. A few hundred yards rather than miles, but then I have never tried to go miles. For Kranoplan information see See http://aeroweb.lucia.it/~agretch/RAFAQ/WIG.html First a disclaimer: I understand the security issues involved in the following and would not encourage anyone to try this at home, but I am interested in the theoretical side of it. Imagine you get things wrong and are caught out low on final, still a fair distance out, and it looks marginal whether you are going to reach the runway or not. One technique I have sometimes heard described is to dive for the deck and complete the remaining distance in ground effect. For the sake of the argument we can assume fairly flat ground, free of obstacles, though not necessarily landable. The advantages claimed are usually better glide performance in ground effect and less headwind and absence of downdrafts close to the ground. On the other hand you'll be travelling at higher than optimal airspeed for most of the distance. I am wondering how much truth there actually is to this technique. Would it significantly increase your range and improve your chances of reaching the field or not ? Would it perhaps work better against a strong wind gradient (as I suspect it might), and maybe not help a lot in calm conditions ? I'd be interested in any hard data/analysis or otherwise enlightening comments on this. Please note though, that I am not talking about high-speed competition finishes, rounded off with a beatup and a sharp pullup and all the dangers and other issues involved in that. Cheers CV |
#3
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Dave Martin wrote: Don't know whether this answers your question! No bearing on the question, really, though it is slightly related, and interesting as well. Ground effect does work with a low wing glider, other I don't know of any low-wing gliders. Most are mid-wing and some older types are high-wing, but that difference is so small I wouldn't expect it to matter. In the situation I asked about we'd want a safety margin of perhaps 5 feet or so off the ground in any case. AFAIK ground effect starts being significant from approx. half a wingspan off the ground, the effect being to increase performance, as if you had greater span. lived. A few hundred yards rather than miles, but then I have never tried to go miles. A few hundred yards would be sufficient for what I had in mind, provided that the other factors balanced out and resulted in a net gain in how far you can glide. Cheers CV |
#4
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"CV" wrote in message news Dave Martin wrote: Don't know whether this answers your question! No bearing on the question, really, though it is slightly related, and interesting as well. Ground effect does work with a low wing glider, other I don't know of any low-wing gliders. Most are mid-wing and some older types are high-wing, but that difference is so small I wouldn't expect it to matter. In the situation I asked about we'd want a safety margin of perhaps 5 feet or so off the ground in any case. AFAIK ground effect starts being significant from approx. half a wingspan off the ground, the effect being to increase performance, as if you had greater span. lived. A few hundred yards rather than miles, but then I have never tried to go miles. A few hundred yards would be sufficient for what I had in mind, provided that the other factors balanced out and resulted in a net gain in how far you can glide. Cheers CV The rule I learned was that ground effect became measurable at one wingspan above the ground and near the ground could double the L/D. Running in ground effect is a lot of fun but you'd better be very smooth on the elevator since the pitch control gets 'twitchy'. (I strongly suspect that unanticipated elevator sensitivity in ground effect is a secondary cause of some of the G103 "PIO" accidents.) The best glide stretching technique is to approach the ground at slightly better than best L/D speed leveling off a couple of feet above the ground. Diving to the ground is dangerous and wastes energy that could better be spent at best glide. Maintaining the usual approach speed seems to work best. Of course, all this assumes that the approach and runway under-run are completely free of obstacles like wires or fences. Bill Daniels |
#5
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On Mon, 18 Oct 2004 16:13:41 +0200, CV wrote:
First a disclaimer: I understand the security issues involved in the following and would not encourage anyone to try this at home, but I am interested in the theoretical side of it. Imagine you get things wrong and are caught out low on final, still a fair distance out, and it looks marginal whether you are going to reach the runway or not. One technique I have sometimes heard described is to dive for the deck and complete the remaining distance in ground effect. For the sake of the argument we can assume fairly flat ground, free of obstacles, though not necessarily landable. The advantages claimed are usually better glide performance in ground effect and less headwind and absence of downdrafts close to the ground. On the other hand you'll be travelling at higher than optimal airspeed for most of the distance. I am wondering how much truth there actually is to this technique. Would it significantly increase your range and improve your chances of reaching the field or not ? Would it perhaps work better against a strong wind gradient (as I suspect it might), and maybe not help a lot in calm conditions ? I'd be interested in any hard data/analysis or otherwise enlightening comments on this. Please note though, that I am not talking about high-speed competition finishes, rounded off with a beatup and a sharp pullup and all the dangers and other issues involved in that. Cheers CV I demonstrate to my students ground effect by approaching a 4200 foot runway in a Blanik L-13 at about 55-60 knots. I would be at the threshold a foot above ground. I could float over 3500 feet before I would settle down and use the wheel brake to avoid running off the end of the field. It is very effective. (There was little or no wind when I did this.) I once had to use this technique when I allowed myself to be distracted on base leg. When I turned final I realized I was too low to make the field but had plenty of speed. I dove toward the ground, and put the glider in ground effect and made the field. (There were no obstructions.) A fellow instructor referred to ground effect as the last refuge of the scoundrel glider pilot. |
#6
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Bill Daniels wrote:
The rule I learned was that ground effect became measurable at one wingspan above the ground and near the ground could double the L/D. Running in ground effect is a lot of fun but you'd better be very smooth on the elevator since the pitch control gets 'twitchy'. (I strongly suspect that unanticipated elevator sensitivity in ground effect is a secondary cause of some of the G103 "PIO" accidents.) Interesting observation. Flying in ground effect places the center of pressure of the wing at about mid-chord, while out of ground effect the center of pressure is about 1/4 chord. So in short, ground effect usually has a stabilizing effect by essentially shifting the CG forward with respect to the center of lift. |
#7
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"nafod40" wrote in message ... Bill Daniels wrote: The rule I learned was that ground effect became measurable at one wingspan above the ground and near the ground could double the L/D. Running in ground effect is a lot of fun but you'd better be very smooth on the elevator since the pitch control gets 'twitchy'. (I strongly suspect that unanticipated elevator sensitivity in ground effect is a secondary cause of some of the G103 "PIO" accidents.) Interesting observation. Flying in ground effect places the center of pressure of the wing at about mid-chord, while out of ground effect the center of pressure is about 1/4 chord. So in short, ground effect usually has a stabilizing effect by essentially shifting the CG forward with respect to the center of lift. Al I can say is put a G103 in ground effect and see for yourself. Obviously, flying qualities in ground effect deals with the whole glider and not just the wing. No doubt the flow over the tail is involved as well. Bill Daniels |
#8
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Bill Daniels wrote:
"nafod40" wrote in message Interesting observation. Flying in ground effect places the center of pressure of the wing at about mid-chord, while out of ground effect the center of pressure is about 1/4 chord. So in short, ground effect usually has a stabilizing effect by essentially shifting the CG forward with respect to the center of lift. Al I can say is put a G103 in ground effect and see for yourself. Obviously, flying qualities in ground effect deals with the whole glider and not just the wing. No doubt the flow over the tail is involved as well. I have, and i don't remember any decrease in longitudinal stab. Having the ground that close certainly tends to "up the gain" in the soft mushy grey matter autpilot though. |
#9
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Imagine you get things wrong and are caught out low on final,
still a fair distance out, and it looks marginal whether you are going to reach the runway or not. One technique I have sometimes heard described is to dive for the deck and complete the remaining distance in ground effect. For the sake of the argument we can assume fairly flat ground, free of obstacles, though not necessarily landable. A few years ago Soaring Magazine reported experiments on this undertaken (I believe) by the airforce glider program. They used Blaniks and a very long runway. The conclusion was that it was better to stay at max glide and not dive for the ground. I forget though how much wind they had -- there is some headwind (50 mph) and gradient (to zero at ground level) where it has to be better to dive. And Blaniks are pretty bad at high speed -- though will have more induced drag at any speed in ground effect too. John Cochrane BB |
#10
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One technique I have sometimes heard described is to dive for
the deck and complete the remaining distance in ground effect. For the sake of the argument we can assume fairly flat ground, free of obstacles, though not necessarily landable. Would it perhaps work better against a strong wind gradient (as I suspect it might), and maybe not help a lot in calm conditions ? I'd be interested in any hard data/analysis or otherwise enlightening comments on this. USAF Test Pilot School Candidates did this as a student project 10 yrs +/- ago. Used a G103 and found that one was definitely better off flying speed-to-fly u\into ground effect and then leveling off. Probably diving down would be better in a strong wind gradient but determining the gradient and what speed-to-fly through it would be operationally very difficult. You could probably get a copy of the report from USAF TPS at Edwards, CA. |
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