Lift your legs?

[RichM 0]

A quick question. Lifting your legs under canopy reduces your surface area presented to the relative and so allows you to achieve a higher speed through the air, allowing an increased chance of getting back from long spots.
Is this technique used in swoop meets for distance?
Rich M

[SkydiveMonkey 0]

I'm not a big swooper (only 71 jumps) but in photos I've got, they seem to lean forward and lot, and some people bring their knees up.
boobies - the cause of, and solution to, all of lifes problems

[skycat 0]

I'm sure hooknswoop will hop in here and explain it better, but I know after his snap hook when he is on the double fronts he is curled up in a little ball until it is time for the canopy to plane out. Then while his legs are still sort of up he taps the toggles to bring it around the corner. At least that is what it looks like from my point of view.

[kallend 1,647]

If you do the math (!!) you will find that the overwhelming majority of the drag comes from the canopy, not your body. Also, your forward speed depends more on the area of the canopy and its lift coefficient than on the drag coefficient.
But I suppose there's a psychological value to it.

[flyhi 24]

One reason to lift your legs is to allow you to plane out a little lower and then slowly lower your legs to transfer your weight from your canopy to your legs.
With your legs extended you have to let the canopy settle in and then convert speed to lift just as your feet touch down to transfer your weight. Timing is critical. If you misjudge your flair, the lift your speed will generate, or your altitude, it can get right exciting.
flyhi

[weid14 0]

depends on many varaiables -- correct? the smaller the canopy and the bigger the person, the more effect the person has. I do notice a difference in getting back from a long spot holding legs up an out of the airstream, so every little bit helps.

[cobaltdan 0]

actually next to the inlet drag, on swoop machines your body is the next highest source of drag on the canopy.
sincerely,
dan

[Tonto 1]

Look at speed skaters, downhill skiers, cyclists etc. Every little bit helps. If you have a colapsable pilot chute and microline, why not tuck up to stop the drag from decaying your speed?
t

[skreamer 1]

Quote

If you do the math (!!)

Yes prof, you have been in the states a little too long good buddy...
"Look before you jump, don't die until you're dead"

[billvon 2,434]

>A quick question. Lifting your legs under canopy reduces your surface area
> presented to the relative and so allows you to achieve a higher speed through
> the air, allowing an increased chance of getting back from long spots.
Not really. It improves your glide; it does not change the parachute's trim speed. If you are backing up in a big way, it will actually hurt you since you will be in the air longer, and thus back up farther. (one of those not-so-intuitive things about aerodynamics.)
>Is this technique used in swoop meets for distance?
I have seen it used. However, be careful using this technique when swooping - your legs can take more impact than your spine, and if you're going to break something, legs are a much better choice than your back.
-bill von

[airdrew20012001 0]

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If you do the math (!!)

Feh! Math bad, Drew no like math. Beeeerr goooooood!
Drewfus McDoofus

[ramon 0]

particularly hookn'swoops comments.
He goes into a hard tuck from about 800 ' for his hook and achieves like 90 mph.
drag does not increase linearly (more like exponentially, unsure...Quade?) with airspeed so at higher speeds every little thing does count.
ramon
"wee girls on a skydive road trip on big bikes...... yikes, dykes and bikes kinda thing...... ", David McKelvie

[flyhi 24]

Quote

>A quick question. Lifting your legs under canopy reduces your surface area
> presented to the relative and so allows you to achieve a higher speed through
> the air, allowing an increased chance of getting back from long spots.
Not really. It improves your glide; it does not change the parachute's trim speed.

Actually, when you tuck up, don't you reduce your frontal area (the projected 2-D image of the 3-D object)?
That would directly impact Bernoulli's equation for dynamic pressure
(P sub d = 1/2 * rho * V^2)
which, if you work out the units comes out to
[(mass/length^3) * (length/time)^2] or [mass/(length*time^2)].
The units on Force = ma are [mass * length/(time^2)].
So P sub d is really Force divided by area (length^2). By reducing the frontal area while in forward flight, we reduce the area affected by the air which reduces the drag. This translates into greater velocity.
I'm pretty sure the math is correct on this, so are we saying the affect is so small it is insignificant and suffers the fate of higher order terms, i.e. neglected?
flyhi

[nws01 0]

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your legs can take more impact than your spine, and if you're going to break something, legs are a much better choice than your back.

I learned that valuable lesson early. I broke three vertebrea in my lower back and bruised the hell out of my tailbone. Now when I know I can not run it out I slide in on my knees. I cringe at the thought of landing on my butt again.
Blue Skies,
Nathan

[billvon 2,434]

>Actually, when you tuck up, don't you reduce your frontal area (the projected
> 2-D image of the 3-D object)?
Yes; you also reduce your drag.
>we reduce the area affected by the air which reduces the drag.
Correct.
>This translates into greater velocity.
Incorrect. Try the following on an aircraft:
Trim for a 200fps descent at 100kts. Now, without touching the yoke, add a little power. (Note that, since thrust and drag oppose each other, this is like reducing drag.) The plane will initially accelerate. Its nose will come up, and after a short time, will stabilize again at 100kts, but will now be descending more slowly, say at 100fps. This is a characteristic of pitch stability - the aircraft wants to return to its "trim" speed.
Parachutes are the same way. If you increase your speed, the parachute will "nose up" to get back to its trim speed. If you decrease it, it will nose down to recover its airspeed. Note that if this didn't happen, your airspeed would be all over the place - you'd either keep speeding up unless you added toggle, or you'd slow down until you stalled, depending on minor influences of wind, turbulence etc.
Now, while it's true that your airspeed is the same in the long term, it is also true that you can change your airspeed for a _short_ time, for example during a high performance landing. The parachute will be trying to recover to its trim speed the entire time, which is why you can get long swoops out of a parachute without too much toggle input initially - it's trying to "nose up" and get back to trim speed.
There are, however, some things you can do to increase your airspeed. Adding a little front riser increases the trim angle, and increases the speed at which you fly. Letting the risers spread _may_ increase your airspeed depending on your canopy and how it's trimmed (or, more importantly, how far it's out of trim.)
-bill von

[flyhi 24]

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This is a characteristic of pitch stability - the aircraft wants to return to its "trim" speed.
Parachutes are the same way.

I'd never considered stability with respect to a parachute, but now that I think about it, it makes sense. A parachute, by design, is statically stable. If it were not stable, it wouldn't plane out at the bottom of the approach without input, and we'd being digging out every approach. This planing out is its attempt to seek out its equilibrium condition after being affected by a control input; the carving riser turn. Good insight. Mahalo.
flyhi

[SBS 0]

I think most of the reason for people lifting their legs in a distance event is to not touch the ground until the last possible instant. In theory, during a swoop, the less surface area you have, the faster you will go. Thus is why someone loading a canopy at 2.0 who is 5'7" will go faster than someone loading a canopy at 2.0 who is 6'3". Thus is why cyclists and Jim Slaton wear spandex. Seems to me, though, that in cycling, you are talking about a huge amount of drag over a long period of time in a race...the amount of actual energy that is gained by wearing spandex or lifting your legs on a swoop is minimal in comparison, but still exists. In the intermediate speed event a couple of weeks ago, the difference between 5th and 10th place was about half a second. I would have to say that had the 10th place person been wearing something with slightly less drag or lifted his legs a little bit more, the final standings may have been influenced.
Steve

[crazy 0]

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Parachutes are the same way. If you increase your speed, the parachute will "nose up" to get back to its trim speed. If you decrease it, it will nose down to recover its airspeed.

May be i'm wrong, but i think that if you reduce your drag ("increase you speed"), in addition to nose up, the canopy will fly slower. The reason is that the canopy needs less speed to compensate the resultant (weight+drag of the pilot). The opposite (flying large flags under a "small" canopy) seems to increase significantly the airspeed.
Come

[billvon 2,434]

>May be i'm wrong, but i think that if you reduce your drag ("increase you speed"),
> in addition to nose up, the canopy will fly slower. The reason is that the canopy
> needs less speed to compensate the resultant (weight+drag of the pilot).
I don't think so - drag and "thrust" still balance. Drag goes down, and "thrust" (i.e. anti-drag vector of weight) goes down as well since the angle flattens out. Lift stays the same since your weight doesn't change, and the anti-lift vector is close to 90 degrees so that vector quantity doesn't change much.
>The opposite (flying large flags under a "small" canopy) seems to increase
> significantly the airspeed.
I didn't notice that when I jumped our massive flag. The glide angle went way down but the canopy didn't seem to fly any faster.
-bill von

[crazy 0]

Once more you seem to be absolutely right. I should have done the math before posting.
Come