John Kallend's response to Ralph Glasser's letter to Parachutist about lift while tracking

[peek 21]

John Kallend's response to Ralph Glasser's letter to Parachutist about lift while tracking

A question for John and the forum.

Although technically John is correct about the definition of "lift", don't you think it is a bit of a stretch to tell skydivers to de-arch and "make their body the shape of a wing", implying that they will get more lift due to "pressure difference between top and bottom surfaces"?

As aerobatic pilots sometimes say, "With enough airspeed you can make a barn door act like a wing."

But I think that is all that is happening. The improvement in "making your body like a wing" surely cannot be that much.

It's not that it is not true, it's the implication of how much good it will do.

[quade 4]

If you're asking this question to the General Forum, I really think you're going to get a pretty wide range of answers based mostly on "feelings" rather than facts.

My guess is that if you asked some of the top guys over at http://trackingderby.com/ you might get an answer based more on practical application.

quade -
The World's Most Boring Skydiver

[Samurai136 0]

Also discussed in this thread.

edit: Both Parachutist letters are in this thread.

"Buttons aren't toys." - Trillian
Ken

[wwarped 0]

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Although technically John is correct about the definition of "lift", don't you think it is a bit of a stretch to tell skydivers to de-arch and "make their body the shape of a wing", implying that they will get more lift due to "pressure difference between top and bottom surfaces"?

As aerobatic pilots sometimes say, "With enough airspeed you can make a barn door act like a wing."

But I think that is all that is happening. The improvement in "making your body like a wing" surely cannot be that much.

It's not that it is not true, it's the implication of how much good it will do.

well...
I haven't read John's letter to understand the theoretical argument.

I have seen photos of a homemade wingsuit/rig combination worn inside a windtunnel. the design is carefully made to be streamlined and produce lift.

their cutting edge isn't about IF it can be done, but how to MAXIMIZE the lift. amazing stuff.

DON'T PANIC
The lies in learning how to throw yourself at the ground and miss.
sloppy habits -> sloppy jumps -> injury or worse

[Hooknswoop 19]

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But I think that is all that is happening. The improvement in "making your body like a wing" surely cannot be that much.

I can't say for certain why it makes a difference, but it makes a huge difference. On some 105-way dives, no one could keep up with me. on my side of the formation. One skydiver could almost keep even, but would be about 1,000 feet below me at pull time, so I could keep going for another 1,000 feet after he had to hit the brakes and pitch. He ws about a head taller than me and weighed about the same. I could consistantly track at between 70 and 80 mph fall-rate.

Most people dive in their tracks, they think they are tracking great because they are going fast, but it isn't about speed, it is about angle, not speed.

Derek

[peek 21]

I believe you, and I believe all of the respondents who have studied tracking in detail.

I guess what makes me uncomfortable about some people talking about the "body as airfoil" part of it is that I see quite a few instructors telling students about this before the students have even managed a good "delta" position.

It's like it has become one of those urban legends passed around by those who don't really understand it.

Perhaps I should say: For people learning to "track", [The improvement in "making your body like a wing" surely cannot be that much.]

[kallend 2,027]

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I believe you, and I believe all of the respondents who have studied tracking in detail.

I guess what makes me uncomfortable about some people talking about the "body as airfoil" part of it is that I see quite a few instructors telling students about this before the students have even managed a good "delta" position.

It's like it has become one of those urban legends passed around by those who don't really understand it.

Perhaps I should say: For people learning to "track", [The improvement in "making your body like a wing" surely cannot be that much.]

I don't claim to track as well as Hook, but I track better than 90% of the people on the big ways I attend. There comes a point when you get your body shape just right and you can feel the difference. I can get my fall rate down to the low 80s in a regular tight RW suit when I hit the track just right, but the best I can do in a "dead spider" or "hugging the ball" is about 95. Only one possible source for the difference, and that is aerodynamic lift.

Just because you're not the most efficient airfoil doesn't mean you aren't an airfoil at all.

Besides, as you said, I AM technically correct.

...

The only sure way to survive a canopy collision is not to have one.

[Samurai136 0]

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Perhaps I should say: For people learning to "track", [The improvement in "making your body like a wing" surely cannot be that much.]

Over the life of the sport, Ski jumpers have refined their technique and added 100m in distance to their jumps (a football field!). Their techniques looks pretty similar to the flat tracking techniques skydivers use. If they are not generating lift we had better tell them to junk all the aerodynamic lift analysis and training they put into their sport.

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Previous techniques first included the Kongsberger technique, developed in Kongsberg, Norway by two ski jumpers, Jacob Tullin Thams and Sigmund Ruud following World War I. This technique had the upper body bent at the hip, a wide forward lean, and arms extended the front with the skis parallel to each other. It would lead to jumping length going from 45 meters to over 100 meters. In the 1950's, Andreas Daescher of Switzerland modified the Kongsberger technique by placing his arms backward toward his hips for a more closer lean. The Daescher technique would be standard for ski jumping from the 1950's until the V-technique was developed in 1985.

"Buttons aren't toys." - Trillian
Ken

[pchapman 279]

All in all I see Gary's point, but I think his short post can be misinterpreted, and can seen to be too hard a criticism of Kallend's letter, which was also pretty short. (... for engineers, anyway.) Kallend's letter in turn doesn't invalidate what Ralph Glasser was trying to get at, even if Glasser didn't understand lift correctly. Kallend says what Glasser got wrong, not what he got right.

Point by point:

-- John Kallend is right about the definition of lift.

-- Yet the concept of Ralph Glasser's criticism of Atmonauti can be valid, given the over-enthusiastic claims of some Atmonauti supporters. Glasser didn't get the concept of lift right, but what he means is that skydivers aren't creating the kind of lift we normally think about from a flying airplane wing -- with lift that is from attached flow, or 'pre-stall' lift. That's to be distinguished from lift that is barn-door in nature, or from unattached flow (on the upper surface), or 'post-stall' lift. I'm not sure what to call it myself, even though it is such a fundamentally important concept in aerodynamics and aviation.

So IF some Atmonauti supporters keep talking about how their bodies are like wings and are truly flying, then Ralph's criticism may be valid. I have seen diagrams published for Atmonauti which also show a misunderstanding of the definitions of lift and drag.

-- Pressure differences between top and bottom have to be the source of lift, whether a wing or object is flying pre-stall or post-stall. If there were no differences, one couldn't create any lift. And de-arching couldn't provide a better track.

-- As for the implied issue of wings vs. barn doors, or deflection vs. pressure differences in Kallend's letter, that's an old debate of Newton vs. Bernoulli for how an airplane flies. The simple answer is that both are true and are part of the process of lifting an object aerodynamically: Pressure differences create the lift force, and when that lift pushes a flying object one way, the air gets deflected the other way.

-- Dearching has been shown to be very useful in providing a good track, so it does need to be taught.
One interpretation of Gary Peek's words indicates that he objects to that, but I think what he means is that he objects to suggesting that it helps due to providing extra attached-flow lift.

I'm not an expert on tracking, but it seems that the lift we skydivers make is basically all post-stall lift. As Glasser says, the body isn't very aerodynamic. Also, consider the angles: Even if a superb tracker were able to get a 1:1 glide, or nearly that good, a common "good tracking position" is only slightly head low vs. the horizon. So we might have a down 45 to 55 degree flight path combined with a body oriented down 15 degrees. This gives an angle of attack of 30 to 40 degrees.

Definitely post-stall lift at that high angle! (Even if one were talking about a wing, not a lumpy, ultra-low aspect ratio skydiver.)

As to whether one should be, say, 10 degrees dearched, vs. 5 degrees (or even 0 degrees dearch), that I'm not sure about. Certainly for inexperienced trackers, one has to emphasize dearching, otherwise they may retain some arch withought realizing it.

In the end do I agree with Gary Peek about his concerns about telling people to "make their body the shape of a wing"? I agree that people shouldn't learn that one is truly flying like a wing normally does (with attached flow lift). I'm strongly against that misconception. But the idea is still a useful basic analogy and training tip, even if it shouldn't be taken too far and used as an explanation of the physics involved.

[kallend 2,027]

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All in all I see Gary's point, but I think his short post can be misinterpreted, and can seen to be too hard a criticism of Kallend's letter, which was also pretty short. (... for engineers, anyway.) Kallend's letter in turn doesn't invalidate what Ralph Glasser was trying to get at, even if Glasser didn't understand lift correctly. Kallend says what Glasser got wrong, not what he got right.

Point by point:

-- John Kallend is right about the definition of lift.

-- Yet the concept of Ralph Glasser's criticism of Atmonauti can be valid, given the over-enthusiastic claims of some Atmonauti supporters. Glasser didn't get the concept of lift right, but what he means is that skydivers aren't creating the kind of lift we normally think about from a flying airplane wing -- with lift that is from attached flow, or 'pre-stall' lift. That's to be distinguished from lift that is barn-door in nature, or from unattached flow (on the upper surface), or 'post-stall' lift. I'm not sure what to call it myself, even though it is such a fundamentally important concept in aerodynamics and aviation.

So IF some Atmonauti supporters keep talking about how their bodies are like wings and are truly flying, then Ralph's criticism may be valid. I have seen diagrams published for Atmonauti which also show a misunderstanding of the definitions of lift and drag.

-- Pressure differences between top and bottom have to be the source of lift, whether a wing or object is flying pre-stall or post-stall. If there were no differences, one couldn't create any lift. And de-arching couldn't provide a better track.

-- As for the implied issue of wings vs. barn doors, or deflection vs. pressure differences in Kallend's letter, that's an old debate of Newton vs. Bernoulli for how an airplane flies. The simple answer is that both are true and are part of the process of lifting an object aerodynamically: Pressure differences create the lift force, and when that lift pushes a flying object one way, the air gets deflected the other way.

-- Dearching has been shown to be very useful in providing a good track, so it does need to be taught.
One interpretation of Gary Peek's words indicates that he objects to that, but I think what he means is that he objects to suggesting that it helps due to providing extra attached-flow lift.

I'm not an expert on tracking, but it seems that the lift we skydivers make is basically all post-stall lift. As Glasser says, the body isn't very aerodynamic. Also, consider the angles: Even if a superb tracker were able to get a 1:1 glide, or nearly that good, a common "good tracking position" is only slightly head low vs. the horizon. So we might have a down 45 to 55 degree flight path combined with a body oriented down 15 degrees. This gives an angle of attack of 30 to 40 degrees.

Definitely post-stall lift at that high angle! (Even if one were talking about a wing, not a lumpy, ultra-low aspect ratio skydiver.)

As to whether one should be, say, 10 degrees dearched, vs. 5 degrees (or even 0 degrees dearch), that I'm not sure about. Certainly for inexperienced trackers, one has to emphasize dearching, otherwise they may retain some arch withought realizing it.

In the end do I agree with Gary Peek about his concerns about telling people to "make their body the shape of a wing"? I agree that people shouldn't learn that one is truly flying like a wing normally does (with attached flow lift). I'm strongly against that misconception. But the idea is still a useful basic analogy and training tip, even if it shouldn't be taken too far and used as an explanation of the physics involved.

Well, NASA (and others) have tested "lifting bodies" which do not in general exhibit typical stall characteristis as expected of a wing, and have very high AOA. So lift does not have to be "pre-stall" in order to (a) exist, and (b) be effective.

...

The only sure way to survive a canopy collision is not to have one.

[mdrejhon 8]

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My guess is that if you asked some of the top guys over at http://trackingderby.com/ you might get an answer based more on practical application.

Excellent -- just what I need. Methinks I'll get one of these wristmount GPS units as I often practice my tracking during my solos.

Didn't know I could get skydiving-compatible wristmount GPS logging for about $100!

Many (even if not all) trackers listen to changes to the wind to find out how they are doing in a track, and me, I have to rely entirely on feel and sometimes it's hard to compare my tracking dives, whenever I'm on a solo and not on a tracking dive, and I know my track is sometimes real shitty on some jumps and reasonably good on others.

[pchapman 279]

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Well, NASA (and others) have tested "lifting bodies" which do not in general exhibit typical stall characteristis as expected of a wing, and have very high AOA. So lift does not have to be "pre-stall" in order to (a) exist, and (b) be effective.

Oh yeah, neat aerodynamics. Although the aerodynamics are different, they remind me strongly of modern high wing loading parachutes: When one has a crappy glide ratio, diving for extra speed gives one the energy to create an effective flare and plane out long enough to gently ease down to a landing.

So the issue still remains, how to describe the type of aerodynamics that are "pre-stall" ... in terms of conventional aircraft wings, not lifting bodies or vortex flow low aspect ratio deltas or whatever.

The best I could come up with was 'attached flow aerodynamics'. Can you think of a good way to describe it to those not trained in aerodynamics, other than just saying 'like a normal wing when it is flying'?

[quade 4]

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Oh yeah, neat aerodynamics. Although the aerodynamics are different, they remind me strongly of modern high wing loading parachutes: When one has a crappy glide ratio, diving for extra speed gives one the energy to create an effective flare and plane out long enough to gently ease down to a landing.

You are going to make my head 'splode.

quade -
The World's Most Boring Skydiver

[kallend 2,027]

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Well, NASA (and others) have tested "lifting bodies" which do not in general exhibit typical stall characteristis as expected of a wing, and have very high AOA. So lift does not have to be "pre-stall" in order to (a) exist, and (b) be effective.

Oh yeah, neat aerodynamics. Although the aerodynamics are different, they remind me strongly of modern high wing loading parachutes: When one has a crappy glide ratio, diving for extra speed gives one the energy to create an effective flare and plane out long enough to gently ease down to a landing.

So the issue still remains, how to describe the type of aerodynamics that are "pre-stall" ... in terms of conventional aircraft wings, not lifting bodies or vortex flow low aspect ratio deltas or whatever.

The best I could come up with was 'attached flow aerodynamics'. Can you think of a good way to describe it to those not trained in aerodynamics, other than just saying 'like a normal wing when it is flying'?

Why do the fluid dynamics have to be defined before we can call it "lift". It is lift - end of story.

I bet this generated vortex lift in spades.

...

The only sure way to survive a canopy collision is not to have one.

[JohnMitchell 16]

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I bet this generated vortex lift in spades.

The tapered leading edge of the F/A-18's wing is designed to do the exact same thing at high AOA's. The lift to drag ratio is not as good with vortex lift, but large amounts of lift are produced nonetheless.

[kallend 2,027]

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I bet this generated vortex lift in spades.

The tapered leading edge of the F/A-18's wing is designed to do the exact same thing at high AOA's. The lift to drag ratio is not as good with vortex lift, but large amounts of lift are produced nonetheless.

Lift may be generated in the most amazing ways and we don't have to understand the process to know that it's lift.

...

The only sure way to survive a canopy collision is not to have one.

[wmw999 2,447]

I can't play the video, but if it's the remote-controlled aircraft flying lawnmower, there was one at Ballunar last year, and it was about the coolest thing I saw there.

Wendy W.

There is nothing more dangerous than breaking a basic safety rule and getting away with it. It removes fear of the consequences and builds false confidence. (tbrown)

[LouDiamond 1]

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Lift may be generated in the most amazing ways and we don't have to understand the process to know that it's lift.

That is absolutely brilliant. Do you use that video at all when you teach?

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[kallend 2,027]

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I can't play the video, but if it's the remote-controlled aircraft flying lawnmower, there was one at Ballunar last year, and it was about the coolest thing I saw there.

Wendy W.

Can't play the video? - hey, it's not ROCKET SCIENCE

...

The only sure way to survive a canopy collision is not to have one.

[kallend 2,027]

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Lift may be generated in the most amazing ways and we don't have to understand the process to know that it's lift.

That is absolutely brilliant. Do you use that video at all when you teach?

I use every trick I know to get students to learn stuff.

...

The only sure way to survive a canopy collision is not to have one.

[DZJ 0]

I now know what I want for Christmas!

[ltdiver 3]

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Lift may be generated in the most amazing ways and we don't have to understand the process to know that it's lift.

Thanks for making me grin this morning, John! That was great!

ltdiver

Don't tell me the sky's the limit when there are footprints on the moon

[Joellercoaster 6]

No, but it does appear to depend on some dodgy Internet Explorer feature.

YouTube clicky goodness for the rest of us.

--
"I'll tell you how all skydivers are judged, . They are judged by the laws of physics." - kkeenan

"You jump out, pull the string and either live or die. What's there to be good at?

[riggerrob 643]

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I bet this generated vortex lift in spades.

The tapered leading edge of the F/A-18's wing is designed to do the exact same thing at high AOA's. The lift to drag ratio is not as good with vortex lift, but large amounts of lift are produced nonetheless.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

If you want to go into more detail, an F-18 really has two different sets of wings and two different types of lift.
At shallow angles of attack, the (straight) wings provide most of the lift, but when you fly an F-18 at steep angles of attack, the straight wings stall (loose smooth airflow over the top) and loose most of their lift.
At high angles of attack, the Leading Edge Extensions alongside the cockpit kick in. The sharp leading edges on the LEX kick in to produce massive amounts of vortex lift, similar to delta wings.

LEXs are used on many other jet fighters to improve manuverability at high angles of attack. The SR-71 was one of the first airplanes with LEXes. Technically, tehy are referred to as fuselage chines, but they bend the air the same way as massive LEXes.
LEXs were also installed on F-5 fighter jets and just got bigger on the F-20 Tigershark variant. Lexes were then retrofitted to earlier Harriers and became standard on F-16 and F-18. The F-22 and Joint Strike Fighter may not have LEXes per say, but they have sharp corners on the forward fuselages that generate similar vortexes at high angles of attack.
Another way to look at is that delta wings are not terribly efficient at low angles of attack because of their low aspect ratios, but at high angles of attack, the wing roots of delta wings are stalled, but huge vortexs are generated by the leading edges.
... sort of like hanging on wing tip vortexes ...

[GTAVercetti 0]

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I now know what I want for Christmas!

http://www.redrockethobbies.com/ProductDetails.asp?ProductCode=FTZ3001A&Click=6136

You have to supply the controller and engine and other assorted plane building items.

Why yes, my license number is a palindrome. Thank you for noticing.