A question for the thinkers...

[The111 0]

Ok, some of you might wonder what I hope to gain from learning the answer to this question... it's a long story, but it came up in conversation the other day between myself (an aerospace engineer who thinks way too hard, and an INexperienced flyer) and a VERY top-notch freeflier.

Is stable arched (belly-down) position "self-orienting"? I made this term up... what I mean is this. Lie on the floor and assume your best RW position. Then get Medusa to cast her spell and turn you to stone (stone with a density distribution equal to your normal human body... basically what I'm getting at here is imagine you are perfectly strong and can hold that position under any conditions). Now, if someone was to toss you (now made of stone) out of a plane in ANY orientation, with any amount of instability or flips/rotations to begin with... would you naturally gain stability? I think yes... but the very experienced FFer I was talking to disagreed. What do you think?

I mean, I watched all sorts of things fall from the plane in Good Stuff, and most of them got nowhere near gaining stability, but they were in very un-aerodynamic shapes. A frozen skydiver, though no longer able to dynamically respond to the airflow, would still be in a very aerodynamic shape, and I think he would stabilize on his own in a steady airflow.

www.WingsuitPhotos.com

[AggieDave 6]

Ever see anyone play badmitton? (I'm not going to ask if you play, since that's embarressing).

The shuttlecock, the thing they hit around. Think of that as a very extreme arched body. The majority of the mass is in the center and it has an "arched" shape.

Hit it up, it tumbled and tumbles, then rights its self and falls straight down.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[Hooknswoop 19]

Yes, it would stabalize into a belly-to-earth orientation. Think early NASA re-entry vehicles.

Derek

[happythoughts 0]

Badmitton birdies do it. They have a shape that deflects the airflow to orient themselves to the ground without any change in their shape.

[The111 0]

Ok, both of you guys are thinking along the same lines as me. I really had next to no doubts that this was the case, but was surprised when the FFer I mentioned disagreed with me.

On to the next question. Do you think this is true for any stable position? I.e. a stable headdown or a stable sit? I think it is still true... these positions are harder for us to maintain as dynamic fliers, because we have to make much smaller more precise corrections with our muscles. But a sturdy body in that orientation with the same mass distribution should fly the same. When we fly sit or HD we are constantly making minor corrections, but that is because we are unable to hold perfectly still in the perfectly neutral (speaking for myself here).

www.WingsuitPhotos.com

[dragon2 0]

I believe that that frozen skydiver would fall perfectly stable.

When I was doing SL jumps and later on doing short delays and even later, practising hop'npops, I wasn't reacting to any airflow at all, I was just punching an arch real hard. I was stable enough

ciel bleu,
Saskia

[dterrick 0]

Given the assumptions of symetricality, etc, the best description I was given as a student is that of a badminton shuttlecock.

Aerodynamic it is, and with the C of G below the centreline of the object. The Unstable Exit drill required for SSE in Canad demonstrates that, even in a short unstable delay, punching an ULTRA hard arch will right you, eventually.

It's all the flailing and screaming that introduced the variables.

Someplace out there I remember reading a fairly detailed aerodynamic description about centre of pressure etc. I'm gonnat bet that Prof. Kallend had something to do with it. Either wait or go search, but the answer is YES, the classic "X man" arch is stable and self- stabilizing, all else being constant.

Dave

Life is very short and there's no time for fussing and fighting my friend (Lennon/McCartney)

[tunaplanet 0]

I'm too sober to answer this question at this moment in time.

Forty-two

[hookitt 0]

Belly flyer, Hmm... student arch.. yes. Mantis.. Probably.

I believe the frozen sitflyer would not gain stability unless spinning and tumbling is considered stable, or it would fly back to earth.

I believe a frozen head down position would gain stablity (depending on how that person flies, but may or may not go true head down. Try belly flying then assume what would be your best head down with out flinging yourself to a head down first. IF I remember I'll try it this weekend but holding frozen enough for an accurate experiment is unlikely.

My grammar sometimes resembles that of magnetic refrigerator poetry... Ghetto

[sammer 0]

An engineer would call the belly to earth arched position "statically stable", but not necessarily "dynamically stable". Hence the potato chipping that is common with students.

Basically, as long as your center of pressure is higher than your center of mass you will tend to fall in that position. You may oscillate wildly though. I think there is a clip in Good Stuff where he shows a car that they attempted to stabilize with a heavy weight attached by four cables. It did fall belly to earth but it was scary looking as the weight swung wildly around in circles.

I think that you would have the same problem with a stiff freeflyer. I have only tried it once...but it felt really damn unstable to me

[kallend 1,623]

Make some cardboard cut-outs and do the experiment.

If aerodynamics were an exact predictive science, Boeing wouldn't need any wind tunnels.

Based on my attempts at sit flying, I'd guess that a stable sit is not truly stable, but back flying is

...

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

[pilotdave 0]

Stability comes in many shades of gray. It's not just a yes or a no. Belly to earth is more stable than head down or sitflying. So, you may be able to get a head down statue to fall perfectly stable, but once disturbed a little bit, it'll probably go unstable (like I would do if I tried to go head down). The belly to earth statue would be able to withstand a much larger disturbance.

In other words, if you dropped both statues out of a plane in any random position, the belly to earth one would probably stay belly to earth, but the head down one probably wouldn't stay head down. On the other hand if you were in a perfect world with no turbulence or anything and you could throw the head down person out in exactly the right orientation, it might be able to stay stable.

Of course if you could get enough drag on the legs, it would act like a shuttlecock. But I don't care what you freeflyers say, head down isn't so stable!

I'm guessing the belly to earth one would probably be chipping like a tense student. We all know ya gotta relax!

Dave

[The111 0]

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Try belly flying then assume what would be your best head down with out flinging yourself to a head down first. IF I remember I'll try it this weekend but holding frozen enough for an accurate experiment is unlikely.

Note that I don't know how to fly HD as it is, but I don't think that pertains to this next comment.

I don't think we as humans possess the strength and finesse to hold a perfectly straight, stiff HD position while belly to earth. I could be wrong though.

You know, now my perspective is changing. I was thinking that any position would be self stabilizing... my logic was that all the small movements we make must be due to our own mistakes, i.e. not holding perfectly still. But if I made the same analogy for riding a bicycle, or hell, even standing up (something infants can't do) I would be wrong. I could say the only reason riding a bike or walking requires us to constantly compensate (balance) to prevent ourselves from falling over is because we're not holding perfectly still... but if I put a perfectly still dummy on a bike and pushed him down the road, or even tried to make him stand on his own, he would most likely fall, though it complete depends on the shape of his "feet" and his weight distribution whether or not he would be able to stand. So I guess balance, by definition, is using our muscles and brains to make a naturally unstable position more stable. And obviously freefly requires much more balance than RW. Maybe a really fatheaded guy with REALLY baggy pants and a skin tight shirt would naturally fall headdown if he was stiff. But it seems there's not a yes or no answer to account for all situations...

www.WingsuitPhotos.com

[The111 0]

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If aerodynamics were an exact predictive science, Boeing wouldn't need any wind tunnels.

Very good point, I was just thinking the same thing.

www.WingsuitPhotos.com

[The111 0]

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In other words, if you dropped both statues out of a plane in any random position, the belly to earth one would probably stay belly to earth, but the head down one probably wouldn't stay head down. On the other hand if you were in a perfect world with no turbulence or anything and you could throw the head down person out in exactly the right orientation, it might be able to stay stable.

Exactly... that whole "perfect world" point is a good one.

If you read the post I just made about a dummy not being able to stand up or ride a bike... in a "perfect world" with perfectly smooth surfaces and a true continuum of air, he would be able to. But in our world he can't.

I remember learning in solid mechanics about how rods can buckle under compression... I asked my prof what would happen if the compressive force was perfectly balanced, he first challenged me to present a way to do that, and then asked what would happen when someone across the street sneezed. :)

www.WingsuitPhotos.com

[rehmwa 2]

I think sammer is closest, but I don't think he's using 'dynamic stability' correctly. Potato chipping occurs precisely because of dynamic stability (i.e., a perturbation is responded to with an opposite direction correction).

But he is using the concept of center of pressure and center of mass correctly. So that 'stable' body position will seek the stable orientation.

now - will it keep it? yes, for the same reason.

But, will the body eventually become still? no - The relaxed body helps us to damp out small perturbations in the air thus keeping the oscillations to a minimum. The stone body doesn't flex, so it will always have some oscillating mode. (remember your classes on the phugoid and short term oscillation responses to a single 'bump' - they assume perfectly still and uniform air. There is no such thing as perfectly still and uniform air). If you could assume perfectly uniform/still air, then eventually the body would become still - entropy still rules.

(That means that any body position would achieve some kind of minimum energy position eventually - which might include a spin or not - regardless of the body position. Unless the COM and COP are aligned right on top of each other regardless of the body's orientation - COP is dependant on orientation....)

I have a question here though - assuming the parenthetical is true, would the form be restricted to only one axis of spin? or does a minimum energy condition allow more than one spin axis. Think a weighted helicopter blade and then extrapolate....

Now, who wants a drink?

Another good one - if you are inside a hollow sphere of uniform mass (in the shell). How much would you weigh? (hint - gravity is inversely proportional to the square of the distance). Near the shell or in the center, what's the difference?

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[rehmwa 2]

OK - real life? The air is not still, so your statue will never stop responding to the constant little bumps and jumps...

Whether you call that 'stable' or not, I don't know. We are pretty free with that term for students and newbies.

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[kallend 1,623]

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Another good one - if you are inside a hollow sphere of uniform mass (in the shell). How much would you weigh? (hint - gravity is inversely proportional to the square of the distance). Near the shell or in the center, what's the difference?

The answer you are looking for is ZERO at all points inside the shell- Isaac Newton proved that around 1665. The proof is simple and needs nothing more than geometry - no calculus required.

However, you omitted part of the question - the hollow shell is the only mass around. You cannot make an anti-gravity machine from a uniform hollow shell and float away from the Earth.

...

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

[SkymonkeyONE 3]

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If aerodynamics were an exact predictive science, Boeing wouldn't need any wind tunnels.

Very good point, I was just thinking the same thing.

Matt, since you are doing all this "thinking", did you think to pack away your freefly pants before you left Palatka? There was a set hanging on a gear rack that looked a lot like yours.

To give my answers to your question:
yes, a person in a student-type belly to earth position will self-right himself. I also believe that a person in a head-down position with a perfectly straight back, exhibiting strong power-legs, while grabbing the bottom of their pack-tray will fall straight head down eventually. I don't think that would happen to a frozen person in a "typical" head-down.

Chuck

[happythoughts 0]

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...but was surprised when the FFer I mentioned disagreed with me.

What? You are discussing stability with a FFer? Isn't that like discussing sex with a Trekkie? (They've seen the technical specs, but never actually tried it.) I spend all my time trying to get them to understand it. Seems hopeless.

[The111 0]

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Matt, since you are doing all this "thinking", did you think to pack away your freefly pants before you left Palatka? There was a set hanging on a gear rack that looked a lot like yours.

Nope, I don't have pants, just a full Michigan suit, which is definitely hanging in my closet right now. Thanks for thinking of me though.

www.WingsuitPhotos.com

[sducoach 0]

If you are an aerospace engineer and had to ask this question, I have one for you.

What are you working on during the day? If it's an airplane please tell me what type, please.

Blues,

J.E.

James 4:8

[kallend 1,623]

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An engineer would call the belly to earth arched position "statically stable", but not necessarily "dynamically stable". Hence the potato chipping that is common with students.

Basically, as long as your center of pressure is higher than your center of mass you will tend to fall in that position. You may oscillate wildly though. I think there is a clip in Good Stuff where he shows a car that they attempted to stabilize with a heavy weight attached by four cables. It did fall belly to earth but it was scary looking as the weight swung wildly around in circles.

I think that you would have the same problem with a stiff freeflyer. I have only tried it once...but it felt really damn unstable to me

I'm not sure that the wobbling or potato chipping is due to instability of the object itself. I suspect irregular vortex shedding has a lot to do with it, which is a property of the air. This is the reason that flags and tree branches wave even in a steady wind, and wires "hum" in the wind. Cylinders with their axis perpendicular to the airflow are susceptible to vortex shedding, and a skydiver in a belly to Earth position can be approximated by a bunch of cylinders perpendicular to the wind. Irregular vortex shedding can be strong enough to cause chimneys to fall down in moderate winds, which is why many tall chimneys have vortex breakers on them.

...

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

[The111 0]

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If you are an aerospace engineer and had to ask this question, I have one for you.

What are you working on during the day? If it's an airplane please tell me what type, please.

Blues,

J.E.

I already said I was pretty sure without a doubt that my view was correct, it had nothing to do with the 4 years of college courses I took, but just with intuition. The reason I posted it here was because it surprised me to hear a very world-class freeflier disagree. I wanted to hear what others would say. And as Kallend pointed out, aerodynamics is a very largely experimental science. Even here among people who are both experienced skydivers and great minds there is still a lot of uncertainty as to *exactly* how a "rock-solid" RW or FF body would react.

FWIW, I don't even work as an aerospace engineer, I'm current working on a shoulder mounted missile system for a major defense contractor. If you think you might be using this system when it hits the field, let me know, and I'll warn you so you can stay away from it and protect yourself.

www.WingsuitPhotos.com

[pilotdave 0]

Vortex shedding may not technically be related to stability, but it is related to "skydiver stability" in a general sense. I mean if it causes a belly flyer to potato chip, it might cause a freeflyer to flip out of control. It's just a cause of a disturbance. So the point is that the flexibility of a real person allows us to counteract those wobbles, where a solid belly flyer shape wouldnt.

But if chipping is only caused by vortex shedding, how are we controlling it by relaxing? I mean, am I really subconsciously countering each wobble, or are the vortices not being shed as unevenly, or what?

Dave