Forums: Skydiving Disciplines: Wing Suit Flying:
Hysteresis, or Why sometimes you barely move forward

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yuri_base

Nov 15, 2006, 11:03 PM
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Yo!

Ever get a jump when it feels all wrong... you max out the suit, but barely move forward and get crappy glide ratio and can't figure out what the hell is wrong?

Well, now you can blame basic aerodynamics instead of blaming yourself! Here's a little masturbation demonstrating why sometimes it's just not your day.

If lift and drag coefficients Cl and Cd as functions of the angle of attack are known, the glide ratio L/D as a function of AoA is simply

L/D = Cl/Cd

the pitch angle (angle of your body to horizon) is

Pitch = AoA - arctan(Cd/Cl)

and horizontal Vx and vertical Vy speeds are

Vx = K*Cl/(Cl^2+Cd^2)^(3/4)
Vy = K*Cd/(Cl^2+Cd^2)^(3/4)

where K is some coefficient proportional to the square root of your wingloading. You can estimate K by matching the polar curve Vy(Vx) with your horizontal & vertical speeds in maxed out position, as measured by GPS.

The attached spreadsheet contains some model Cl and Cd curves for some wingsuit and the resultant L/D, pitch, speed, and polar curves. (For the discussion of lift/drag coefficients at high angles of attack, see this article.) You can drag the points on Cl and Cd curves and see the effects. Also, adjust the coefficient K in cell A2 to realistically match your speeds.

Take a look at the "L/D vs. Pitch" curve. This curve tells us that at a pitch angle of -10 degrees (headlow 10 degrees below the horizon) your glide ratio can be 1.0, 2.1, or 2.7. 2.1 occurs at the stall, at AoA=16, and thus is an unstable situation. 1.0 (AoA=35 degrees, post-stall) and 2.7 (AoA=10 degrees, pre-stall) are potentially stable, if the total momentum of all forces is zero. (For now, let's assume it is.)

So, you can "max out" and your body is at "usual maxed out" pitch of 10 degrees below the horizon, and yet in one case you fly like a bird at L/D=2.7, while in the other case - same body position, same pitch - you fall like a brick at L/D=1.0. In the first case, your fallrate is 39mph, in the second 53mph. Sounds familiar? Horizontal speeds are 106 and 53mph, respectively.

Conclusion: the stall introduces a potential barrier which separates the efficient flying from inefficient. You can assume your maxed out body position and yet be at the slow end of the stall barrier and get poor glide (depending on previous history of flight, hence the "hysteresis" title). To "hop" over the barrier, you need first to increase your forward speed dramatically (by, for example, collapsing the arm wings and thrusting at full throttle with the leg wing). No more wondering, "what's wrong?!"

Something to try this weekend.

Yuri

(This post was edited by yuri_base on Nov 15, 2006, 11:42 PM)
Attachments: Hysteresis1.gif (50.1 KB)
Hysteresis2.gif (18.1 KB)
Hysteresis.xls (36.5 KB)

Tonto  (D 515)
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Nov 15, 2006, 11:57 PM
Post #2 of 55 (11942 views)
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Birds don't know all this shit. That's why they fly well.

All the numbers and spreadsheets in the world don't make you fly.

t

saly  (D 497)

Nov 16, 2006, 12:18 AM
Post #3 of 55 (11932 views)
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Did you skip the maths class as well as Afrikaans?

Theory is good, that is for the academics

Tonto  (D 515)
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Nov 16, 2006, 12:25 AM
Post #4 of 55 (11927 views)
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When I see this stuff - I think "It's winter in the Northern Hemisphere."

Do a search for threads like this one. They SELDOM happen in the summer.

t

(This post was edited by Tonto on Nov 16, 2006, 1:18 AM)

saly  (D 497)

Nov 16, 2006, 12:34 AM
Post #5 of 55 (11921 views)
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Idle minds when you cant fly............

Must he The PhD math class that was skipped

phoenixlpr  (D 3049)

Nov 16, 2006, 1:10 AM
Post #6 of 55 (11908 views)
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Just search for Yuri_base, he had these essays even in may-july.

Nov 16, 2006, 1:16 AM
Post #7 of 55 (11903 views)
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Ever get a jump when it feels all wrong... you max out the suit, but barely move forward and get crappy glide ratio and can't figure out what the hell is wrong?
Yuri

Yea, I've had those, next thing I did was check the wind report and alter my flightplan accordingly.

Tonto  (D 515)
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Nov 16, 2006, 1:25 AM
Post #8 of 55 (11896 views)
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Edited my post.

My point is that anything based on a GPS is bullshit. Aircraft designers knew before 1915 that ground speed and airspeed were not the same thing. GPS tells you squat. See attached pic of my airspeed.

t
Attachments: fast.jpg (38.6 KB)

piisfish

Nov 16, 2006, 1:37 AM
Post #9 of 55 (11892 views)
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All the numbers and spreadsheets in the world don't make you fly.
maybe you can add the spreadsheet surface to the wingsuit for increased performance

redfox

Nov 16, 2006, 1:44 AM
Post #10 of 55 (11890 views)
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GPS may help a lot, but it can display some very interesting errors by itself.

Look at my speed record
(In reality it was a GPS error!!!)

Cordially,

Herwig
Attachments: speed_360.jpg (88.3 KB)

piisfish

Nov 16, 2006, 2:07 AM
Post #11 of 55 (11881 views)
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how is your GPS setup ?
what is the frequency of measure ?

phoenixlpr  (D 3049)

Nov 16, 2006, 2:12 AM
Post #12 of 55 (11879 views)
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So? I have logs that show cruising with a speed of sound.
I went downtown with my car....

phoenixlpr  (D 3049)

Nov 16, 2006, 2:46 AM
Post #13 of 55 (11864 views)
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As I told before, we have to install pilot tube on our helmets to receive proper air speed readings.

cpoxon  (D 11665)
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Nov 16, 2006, 2:53 AM
Post #14 of 55 (11860 views)
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As I told before, we have to install pilot tube on our helmets to receive proper air speed readings.

pitot

phoenixlpr  (D 3049)

Nov 16, 2006, 3:05 AM
Post #15 of 55 (11849 views)
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pitot

Aye!

So were should it be installed? On the helmet? Does turning head has effect on measuring right values?

GeordieSkydiver  (D 105023)

Nov 16, 2006, 3:09 AM
Post #16 of 55 (11848 views)
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A pitot reading will only give you a reading when pointing into the airflow. It will only tell you how fast you are going through the air, not true airspeed (unless you have a static reading too) to be a little more accurate, you need mulitple pitot probes and a flight management computer.

How about just jump and adopt a position that feels right?

Too much brains, not enough 'gut instinct'.

phoenixlpr  (D 3049)

Nov 16, 2006, 3:12 AM
Post #17 of 55 (11844 views)
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How about just jump and adopt a position that feels right?

Too much brains, not enough 'gut instinct'.Smile
It was not how to fly by instruments. Our problem domain is how to collect valid information about our flights.

birdynamnam  (D 179)

Nov 16, 2006, 3:16 AM
Post #18 of 55 (11843 views)
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 Re: [redfox] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post
no, I think that you fly that fast. Oh wait, maybe the speed difference was due to me pulling right in front of you..

redfox

Nov 16, 2006, 6:18 AM
Post #19 of 55 (11778 views)
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 Re: [piisfish] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post
This data was measured with a Garmin Forerunner 205.

Getting a polar curve from your flights is pretty hard from my point of view, weather conditions may affect your flight as much as slightly different body positions, but I guess every wingsuit flyer know this...

Maybe we should learn more about the theory of "intelligent falling"?

http://en.wikipedia.org/.../Intelligent_falling
http://www.theonion.com/content/node/39512

and please look at:
http://www.idrewthis.org/d/20050516.html

Let us work on this! Defying Gravity!

Cordially,

Herwig

piisfish

Nov 16, 2006, 6:21 AM
Post #20 of 55 (11774 views)
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was your GPS set to measure every second ?

VectorBoy  (F 321)

Nov 16, 2006, 6:32 AM
Post #21 of 55 (11759 views)
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 Re: [Tonto] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post
Birds don't know all this shit. That's why they fly well.

All the numbers and spreadsheets in the world don't make you fly.

t

Just wait until Professor Kallend puts you in his Vulcan mind meld, he will cure your lack of faith Tonto.

Tonto  (D 515)
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Nov 16, 2006, 6:52 AM
Post #22 of 55 (11746 views)
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he will cure your lack of faith Tonto.

I have faith! I leap through the door, arms outstretched, powered by childhood dreams of flight formed before I could hold a pencil.

What I lack in theoretical knowledge I make up for with love and desire. My flights inspire me. The images etched in my minds eye cannot be described or reproduced on a spreadsheet, or any other medium other than a child like imagination.

There is no value, no price, no data worthy of the personal feelings my flights generate.

t

GeordieSkydiver  (D 105023)

Nov 16, 2006, 6:53 AM
Post #23 of 55 (11744 views)
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There is no value, no price, no data worthy of the personal feelings my flights generate.

Well said brother.

Nov 16, 2006, 7:22 AM
Post #24 of 55 (11729 views)
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 Re: [yuri_base] Hysteresis, or Why sometimes you barely move forward [In reply to] Can't Post
Hi Yuri,
Very good point - nice observation. I didn't go thru all the math or check all the curves, but what you have pointed out is I think relevant and related to what student pilots (powered aircraft) are taught about the flight regime called the "back side of the power curve" or the "region of reverse command". In this flight regime, you need to add power to fly slower.

An example of this is if you want to slow the plane down to the point just above stall in level flight. At first you keep reducing power and pulling up the nose. This works good until you get to the point where where the drag begins to increase because of the nose high attitude and you need to add more power to compensate for the increased drag. To go slower, you increase pitch even more and now you have to add even more power to maintain level flight. Of course, this continues until the point where the wings begin to stall.

To get out from behind the power curve, you have to add more power (for acceleration) and reduce the pitch angle to get the plane flying faster. Well, in a glider (wingsuit), your only option is to nose down and turn your altitude into airspeed to get back to the desired flight regime.

So, I think I understand what you were pointing out. I'll give this more thought and a try myself this weekend.

I find GPSs work great. Their data is about as accurate as can be in no wind (or light and variable) conditions - not uncommon here in Florida. Yes, there may be a few data points in error that give rise to wild max speed figures etc., but like flying in turbulence and taking readings from an airspeed indicator or altimeter, there will be errors - it's the nature of instrumentation, it's never perfect.

The111  (D 29246)

Nov 16, 2006, 7:49 AM
Post #25 of 55 (11715 views)
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My point is that anything based on a GPS is bullshit. Aircraft designers knew before 1915 that ground speed and airspeed were not the same thing. GPS tells you squat. See attached pic of my airspeed.

Don't you mean groundspeed? Sorry, after your rant it just seemed particularly ironic...

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