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#1
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Wing in Ground Effect?
Hi all...
I know this is somewhat off topic perhaps...but maybe somebody here has the tidbit of info I need.... As Im sure you all know....the Lift to drag ratio of a wing goes way up once it gets close to ground.....and roughly speaking the effect becomes pronounced when the wing has an altitude on the same order as the span of the wing or less..... What I was looking for was: 1..... a graph that shows the L/D ratio as the wing gets closer and closer to the ground......anything rough will do...I found such a rough graph once a year or 2 ago on the internet....but I cant find it again...... 2.....question.....as you approach the ground VERY closely (lets assume your landing gear are up AND you somehow maintain a constant forward speed AND you maintain the same angle of attack) does the drag GO down or does the lift go up? 3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3 feet.....flying say 6 inches to a foot about the ground at 40-60mph....(yeah, Im gonna be building the worlds fastest flying lawnmower .....what kind of lifting force could I reasonably expect? .....if you dont have the values for the ground effect itself......I could still use the numbers for "high" altitude values and fudge them from there to get back of the envelope ground effect numbers.... take care all Blll |
#2
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on # 1 look at JavaFoil on the WWW enter your airfoil and if you check the
box called "Include Ground Eff" it will include it on the CL/CD graph "BllFs6" wrote in message ... Hi all... I know this is somewhat off topic perhaps...but maybe somebody here has the tidbit of info I need.... As Im sure you all know....the Lift to drag ratio of a wing goes way up once it gets close to ground.....and roughly speaking the effect becomes pronounced when the wing has an altitude on the same order as the span of the wing or less..... What I was looking for was: 1..... a graph that shows the L/D ratio as the wing gets closer and closer to the ground......anything rough will do...I found such a rough graph once a year or 2 ago on the internet....but I cant find it again...... 2.....question.....as you approach the ground VERY closely (lets assume your landing gear are up AND you somehow maintain a constant forward speed AND you maintain the same angle of attack) does the drag GO down or does the lift go up? 3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3 feet.....flying say 6 inches to a foot about the ground at 40-60mph....(yeah, Im gonna be building the worlds fastest flying lawnmower .....what kind of lifting force could I reasonably expect? .....if you dont have the values for the ground effect itself......I could still use the numbers for "high" altitude values and fudge them from there to get back of the envelope ground effect numbers.... take care all Blll |
#3
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Check the archives from Soaring Magazine (the SSA Publication). Maybe 10 years
ago??, the Air Force test pilots school did a study of ground effect using a glider (I think I remember it being a Blanik). It involved flying at best L/D compared to diving for the ground and riding out the ground effect. It was interesting data. Bob |
#4
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In article ,
BllFs6 wrote: [[.. munch ..]] 2.....question.....as you approach the ground VERY closely (lets assume your landing gear are up AND you somehow maintain a constant forward speed AND you maintain the same angle of attack) does the drag GO down or does the lift go up? Both, "more-or-less". You get a 'cushion' of higher-pressure air that is 'trapped' under the wing. Lift is due to the pressure difference between the top and bottom of the wing. _Either_ a decrease in upper-surface air pressure, or an increase in lower-surface pressure will result in increased lift. The trapped air-cushion also acts like a nearly 'frictionless bearing', reducing drag. I've seen a Cherokee 6 use up more than 12,000 ft of runway, due to ground- effect. Full flaps, the stall warning sounding, and aimed right at the 'numbers', get to about 2' above touch-down, and the PIC *cuts* the engine. Not to idle, but _off_. We follow the stationary prop for more than two miles *without* touching down -- then he kicks the engine, throttles up, and goes around for another attempt. He -had- been cleared by the tower for touch-and-go practice. This one was logged as a "missed touch-and-go". It was amazing how -little- speed we lost during that two mile 'float' down the runawy. Only shed about 5 kts. |
#5
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I've been under the impression that the biggest gain in ground effect is
cutting off the tip vortices thus significantly reducing drag. Bob |
#6
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BllFs6 wrote:
Hi all... I know this is somewhat off topic perhaps...but maybe somebody here has the tidbit of info I need.... As Im sure you all know....the Lift to drag ratio of a wing goes way up once it gets close to ground.....and roughly speaking the effect becomes pronounced when the wing has an altitude on the same order as the span of the wing or less..... What I was looking for was: 1..... a graph that shows the L/D ratio as the wing gets closer and closer to the ground......anything rough will do...I found such a rough graph once a year or 2 ago on the internet....but I cant find it again...... I'll give you the equation for drag in terms of altitude above ground. D = drag D0 = zero lift drag (parasitic) DL = drag due to lift, proportional to lift^2, equl to K x L^2, where K is some constant. D = D0 + K x L^2 Now, K gets adjusted as you get closer to the ground by... K' = K x (33 (h/b)^1.5 )/(1+33(h/b)^1.5) Where h is height, b is wingspan. As h/b goes to zero, K' goes to zero too, leaving only parasitic drag D0 |
#7
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"B Lacovara" wrote in message
... I've been under the impression that the biggest gain in ground effect is cutting off the tip vortices thus significantly reducing drag. Could be dangerous for me to try to interpret Hoerner (in "Fluid Dynamic Lift"), but the effect on the vortices is indirect, as it's more relevantly the downwash behind the wing which is being altered and therefore induced angle of attack, and hence drag. He thus measures and graphs the height above the ground as the height from the trailing edge. There's really no "cushion" of air involved, as the pressure of the air under the wing can be zero or negative with respect to free-stream air, per normal except at high lift coefficients, and wing can still exhibit ground effect. Fred F. |
#8
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"Robert Bonomi" wrote in message ervers.com... In article , BllFs6 wrote: [[.. munch ..]] 2.....question.....as you approach the ground VERY closely (lets assume your landing gear are up AND you somehow maintain a constant forward speed AND you maintain the same angle of attack) does the drag GO down or does the lift go up? Both, "more-or-less". You get a 'cushion' of higher-pressure air that is 'trapped' under the wing. Lift is due to the pressure difference between the top and bottom of the wing. _Either_ a decrease in upper-surface air pressure, or an increase in lower-surface pressure will result in increased lift. The trapped air-cushion also acts like a nearly 'frictionless bearing', reducing drag. I've seen a Cherokee 6 use up more than 12,000 ft of runway, due to ground- effect. Full flaps, the stall warning sounding, and aimed right at the 'numbers', get to about 2' above touch-down, and the PIC *cuts* the engine. Not to idle, but _off_. We follow the stationary prop for more than two miles *without* touching down -- then he kicks the engine, throttles up, and goes around for another attempt. He -had- been cleared by the tower for touch-and-go practice. This one was logged as a "missed touch-and-go". It was amazing how -little- speed we lost during that two mile 'float' down the runawy. Only shed about 5 kts. Obviously, this Cherokee 6 had the "eternal motion" upgrade. A Cherokee 6 is a fairly draggy airframe. Assuming no odd circumstances ( i.e. moving from a large tailwind to a large headwind or a severely downhill runway), I'd pay $50 to anyone who could float a Cherokee 6 over 1000' down the runway if the excercise was started with the flaps down, the engine stopped, and the aircraft a 2' of altitude and within 10 knots of stall speed. KB |
#9
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"FF" wrote in message ...
"B Lacovara" wrote in message ... I've been under the impression that the biggest gain in ground effect is cutting off the tip vortices thus significantly reducing drag. Could be dangerous for me to try to interpret Hoerner (in "Fluid Dynamic Lift"), but the effect on the vortices is indirect, as it's more relevantly the downwash behind the wing which is being altered and therefore induced angle of attack, and hence drag. He thus measures and graphs the height above the ground as the height from the trailing edge. There's really no "cushion" of air involved, as the pressure of the air under the wing can be zero or negative with respect to free-stream air, per normal except at high lift coefficients, and wing can still exhibit ground effect. Fred F. The helicopter manual I have here says the same thing: the presence of the surface reduces AOA. However, this is for a hovering machine, and the ground prevents the generation of the descending column of air that can occur at altitude, necessitating the higher AOA (more collective) to maintain altitude in the hover. In a fixed-wing airplane, the ground's effect on AOA will be somewhat less than that. Kerschner says that tip vortices (or rather, the ground's interference with them) are definitely a part of the reduced drag, as well as the reduced AOA caused by reduced downwash angle. An airliner in the piston days left Honolulu for the Mainland, and before the halfway mark it had lost two of the four engines. The pilot turned back to Hawaii, prepared the pax for ditching as the airplane settled toward the ocean, and found that once he was within a few feet of the waves it stopped sinking. He made it all the way back to Honolulu that way, forcing it a bit lower to gain a little speed to pull up high enough to get to the runway. Can't remember where I read that, and the details might be off some, but it's true. I think. Anyone else recall it? Dan |
#10
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BllFs6 wrote:
1..... a graph that shows the L/D ratio as the wing gets closer and closer to the ground......anything rough will do...I found such a rough graph once a year or 2 ago on the internet....but I cant find it again...... This site has reams of gouge (Navy term). http://www.se-technology.com/wig/index.php 2.....question.....as you approach the ground VERY closely (lets assume your landing gear are up AND you somehow maintain a constant forward speed AND you maintain the same angle of attack) does the drag GO down or does the lift go up? Both. Drag goes down due to span-related ground effect. Lift goes up due to chord-related ground effect. 3.....lets assume a wingspan on the order of 6 feet and a chord of 2 to 3 feet.....flying say 6 inches to a foot about the ground at 40-60mph....(yeah, Im gonna be building the worlds fastest flying lawnmower .....what kind of lifting force could I reasonably expect? .....if you dont have the values for the ground effect itself......I could still use the numbers for "high" altitude values and fudge them from there to get back of the envelope ground effect numbers.... Check that web site. Might have the answers. |
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