Wind Tunnel is not the same as Skydiving

[Bill_K 0]

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hello:
I'm a new jumper with 32 jumps and was heading up to Orlando on some work and figured I could use the trip to sharpen my freefall skills in the tunnel there. What I experienced is completely different. Firstly, I was disappointed with my performance. I found it hard to fly stable maintaining heading in the Tunnel Vs in freefall. It seemed more turbulent in the tunnel than it is in freefall. And needless to say, I looked like I'm fighting to remaining stable. My instructor at the tunnel told me that he sees lots of skydivers perform poorly when introduced to tunnel with my experience level. I've not jumped after the tunnel time and would liek to see if it improved my skills.
So I would like to get responses from anyone who can comment on what I saw is normal and any further comments on how I can take what I learnt in the tunnel to improve the freefall skills.
thanks,

I did 30 minutes between my AFF level 6 and 7. I've since gone on to add a few more jumps. The comment that I get just about every jump now is that I fly like somebody with a lot more jumps than the ~20 that I've got currently. I'm now getting invited to fly in larger ways and people are willing to fly with me at the drop of a hat. In my opinion the wind tunnel helped me greatly!!! In fact I called my tunnel coach last week to thank him personally. The next chance that I get I'm going back.

To your point, I struggled the first 2 or 3 minutes in the tunnel. My 10 yr. old kicked kicked his old man's butt for the first couple of minutes. After that, I relaxed and started learning to fly stable and fall straight down with out drifting. Once that got through my thick head I had a ball and learned a lot of good skills. Now I want to go back and grow those.

Just my .02
Bill

[Paulipod 0]

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I can't understand HOW people can "feel" the difference.

The air for sure feels harder/stronger when I fly in a skyventure tunnel - before everyone jumps on my back here and starts bitching - I am not saying one is better than the other.... just different.

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I would think that a tunnel with "X" as an airspeed would feel the same no matter how that airspeed was created.

That would be assuming that the pressure and density are the same. Again back to fluids... is swimming in soup the same as swimming in water? - no matter how fast the flow?

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Question: How many fans do you have?

Just the one.

Bottom line - ask anyone that has flown more than 2mins in each type of tunnel if the air feels the same everywhere.... you'll probably get a few opinions. If for one - if you put me in either tunnel with a blindfold on could tell you where I was flying

Bodyflight Bedford
www.bodyflight.co.uk

[Ron 7]

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The air for sure feels harder/stronger when I fly in a skyventure tunnel

I wonder what the true airspeeds are in each?

Also, which do you feel beter replicates freefall?

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That would be assuming that the pressure and density are the same. Again back to fluids... is swimming in soup the same as swimming in water? - no matter how fast the flow?

Yes water and soup would be different, but we are talking air and air...And that is not so different.

One explanation could be the density of the air. Without a venturi the density *should* be higher in your tunnel. But I go back to my thoughts that just like freefall at 13.5 and 2 being different, I doubt many can tell the difference.

Can you take an altimeter in the cone and let us know what it reads?

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Just the one.

Now....As for having only one fan. I think this may be a big part of the difference in feel. The Bragg tunnel has only the one fan and I can easily tell the difference between a multi fan tunnel and Bragg.

Multi fan tunnels almost seem to loose lift in one or more fans at a time. (Not maybe the best explanation...But it is how it feels). The best technical explanation might involve choking one side of the tunnel and air being a fluid takes the path of least resistance....If I have a large mass blocking air on one side, the air will flow to another side and maybe starve the fan on the obstruction side of air. I "feel" this creates a low spot in the tunnel that seems to loose lift and cause the formation to drift to that side.

That is more a function of multi fan VS one big ass fan....And no matter how much power you have or airspeed it still happens.

Having said that, I am a big fan of the one big fan (pun intended).

I wonder what the cost of each tunnel to run is compared?

"No free man shall ever be debarred the use of arms." -- Thomas Jefferson, Thomas Jefferson Papers, 334

[4wayFly 0]

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Bottom line - ask anyone that has flown more than 2mins in each type of tunnel if the air feels the same everywhere.... you'll probably get a few opinions.

This is really where my Venturi thing comes from (yes Ronnie dear I have flown a rig in the tunnel, quite a bit actually) - having flown most of the SV tunnels and Bedford...I feel more 'pressure' in the Venturi SV ones... F...knows why really...Assume the Venturi thing. In Bedford I wear the same lead as freefall, in Orlando/ Perris/ Airkix...with the same team..I double it. Go figure. Makes no sense to me but I am not a fluid dynamicist.

[Paulipod 0]

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but we are talking air and air...And that is not so different

yes - I was just trying to show that flow speed can be the same with other properties different.

When air is compressed I think it acts alot differently to when its not... ie when the air gets to a door this is why it spills out and you get drop-offs.

Wonder if there are any studies on objects falling through compressed air to non-compressed

Bodyflight Bedford
www.bodyflight.co.uk

[Ron 7]

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In Bedford I wear the same lead as freefall, in Orlando/ Perris/ Airkix...with the same team..I double it.

This seams to be saying that all the other tunnels are pushing faster air. Since that is the only reason you would need to wear more weight.

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F...knows why really...Assume the Venturi thing

I'd rather really know why and not just assume something without any basis in physics....Thats just me.

"No free man shall ever be debarred the use of arms." -- Thomas Jefferson, Thomas Jefferson Papers, 334

[rehmwa 2]

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When air is compressed I think it acts alot differently to when its not...

That's exactly right. But air is only treated compressible near and above the speed of sound. Those wind tunnels are usually very small and I don't recommend trying to fly in them.

It upsets the scientists.

(I know you 'mean' -flow directed in a bottleneck passage- when you say compressed . That's an entirely different thing. In a venturi, it's more correct to say accelerated rather than compressed as the effect is a tradeoff in static pressure for dynamic pressure - i.e., velocity, not density.)

edit: or was it above 10% the speed of sound???? I don't recall. But in anycase, a freefall simulator will likely have the flight chamber calcs done at a constant density assumption. The flows near the turbine blade tips? that's a different story. But I don't recommend we fly there either.

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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

[Stumpy 256]

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That's exactly right. But air is only treated compressible near and above the speed of sound. Those wind tunnels are usually very small and I don't recommend trying to fly in them.

he he - especially not the horizontal ones

Images of peashooters....

Never try to eat more than you can lift

[diablopilot 2]

The air is not compressed. It is accelerated instead. There is no pressure spilling the air out the doors in SV tunnel, that's a factor of a body deflecting it out the doors.

THe chamber entry and exit doors don't have to be held closed against a pressure (otherwise they would probably be designed to open inward as that would produce a stronger door). They are imposible to open when the tunnel is running due to a pressure DROP in the anti chamber from the venturi.

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You're not as good as you think you are. Seriously.

[bch7773 0]

for sure the tunnel is not the same as skydiving IMHO. For one thing, there is no wall and door in skydiving that causes burbles. We were turning peices in the tunnel and when we got close to the walls or the doors, we would feel the difference.

Especially sitflying without a rig on. People's rigs tend to catch a lot of air. So people have to learn to fly a different position in the air then in the tunnel.

Is the tunnel a great tool? Yes. Is it the same as skydiving? No

MB 3528, RB 1182

[Paulipod 0]

Yes - the pressure drop inside the tunnel creates acceleration in the airflow (as Mr Bernoulli stated) hence you have an underpressure which sucks on the doors etc... but this pressure difference does make a difference to the antechamber etc.

Flying in a different atmospheric pressure produces a different effects - this must be true otherwise there would be no point in pressurising some scientific tunnels to increase reynolds number etc...

Maybe this is where the difference lies from a technical point...

As pressure decreases so does the reynolds number of a flow (hence this is why they used to pressurise our tunnel for a better reynolds number) - so in a venturi where you lower pressure in the flight chamber you have a different flow type to that in an atmospheric condition

http://www.answers.com/reynolds%20number <- for those that want to know what a Reynolds Number is

This is a very complex field which I have relied alot on our experts for.... but I know for certain based on many experiences in both types of tunnel that you can tell the difference!

Bodyflight Bedford
www.bodyflight.co.uk

[pilotdave 0]

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Flying in a different atmospheric pressure produces a different effects - this must be true otherwise there would be no point in pressurising some scientific tunnels to increase reynolds number etc...

Actually that's so that the air flows around scale models the same way it flows around full size aircraft. They usually do it by refrigerating the tunnel to increase the air density without changing the pressure.

The reynolds number of the flow in a tunnel is nearly the same as it is in freefall. Reynolds number is not dependent on pressure, but rather density, viscosity, and velocity.

I think the difference in the feel of one tunnel to another to freefall is more based on velocity and turbulence of the flow. In the tunnel, you adjust your body to match the tunnel speed. In freefall, the speed adjusts to match your body position. Throughout freefall, pressure and density and airspeed are changing. Can you tell the difference in feel between the top end of a skydive and the bottom end (starting at terminal of course)?

Dave

[Paulipod 0]

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Can you tell the difference in feel between the top end of a skydive and the bottom end (starting at terminal of course)?

Nope - but thats not what I am saying.... I can tell the difference between flying in our tunnel and air at others... and not becuase the wind is quicker or slower. (which is a little off topic I know)

In our facility (which I have alot of historical test results etc) they increased the pressure to up to 4 atmospheres to specifically increase the reynolds number and decrease turbulance / scale results etc.

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Reynolds number is not dependent on pressure, but rather density

not sure that makes sense - air density changes with pressure... surely changing the pressure would change the reynolds number?

Bodyflight Bedford
www.bodyflight.co.uk

[mattjw916 2]

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I can tell the difference between flying in our tunnel and air at others...

And what would that difference be?

NSCR-2376, SCR-15080

[rehmwa 2]

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I can tell the difference between flying in our tunnel and air at others...

And what would that difference be?

He knows everybody working there.

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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

[Paulipod 0]

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I can tell the difference between flying in our tunnel and air at others...

And what would that difference be?

He knows everybody working there.

yes.. thats the only thing that helps me recognise where I am flying!

Bodyflight Bedford
www.bodyflight.co.uk

[rehmwa 2]

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yes.. thats the only thing that helps me recognise where I am flying!

I can't wait to visit your tunnel. That'll be 3 different flavors. Once we see Dawn, that'll be 4.

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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

[Bodyflight.Net 0]

awww... I'm on your list
different flavors are yummy

[4wayFly 0]

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In Reply To
In Bedford I wear the same lead as freefall, in Orlando/ Perris/ Airkix...with the same team..I double it.

This seams to be saying that all the other tunnels are pushing faster air. Since that is the only reason you would need to wear more weight

Ok let me explain some more then, although I hasten to add I cannot tell you why...In Bedford my team and I all wear the same lead we wear in freefall i.e 2 people wear none at all. In the other SV tunnels I almost double my lead. The others on the team do not. I am the lightest on the team i.e at one end of the spectrum of fallrate. I genuinely feel I cannot fly in my normal way without adding more lead in SV - the difference between our individual fallrates as a team seem exaggerated in the SV tunnels. Bedford feels softer and more like freefall regardless of actual airspeed.

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I'd rather really know why and not just assume something without any basis in physics....Thats just me.

Since the V effect (or lack of) seems to be the primary difference between how the Sv tunnels and Bedford work...I 'assumed' this was why. I assume it is ok to mention my personal experiences on here, Ron, since this is a discussion forum and not a court of law?

[Ron 7]

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Since the V effect (or lack of) seems to be the primary difference between how the Sv tunnels and Bedford work...I 'assumed' this was why. I assume it is ok to mention my personal experiences on here, Ron, since this is a discussion forum and not a court of law?

Well I am no judge anyway, and this is not a court. I just didn't understand how a venturi could make any difference....Being the type of guy that is actually interested in this kind of stuff....Check my teams name at the last US Nationals...Phugoid. I did't understand how a venturi could have a "feel"

To be honest....I still don't. But I'll keep looking into it since I like this kinda stuff. Yes, I have a GF. So while I may be a geek I can't be THAT big of one

I think the major difference between the Bedford an SV tunnels would be the one big fan VS a bunch of fans. I found the Bragg tunnel to be very much different than an SV tunnel.

But thats just a guess. One day I may go to Bedford...But I just can't see me making a trip just for a tunnel.

"No free man shall ever be debarred the use of arms." -- Thomas Jefferson, Thomas Jefferson Papers, 334

[dzjnky 0]

I can think of several major differences between the various tunnels, and each of them will have some impact on the "feel" and the response of the airflow to people flying around in it...

1) Walls or not. The lack of walls at AAC or Ft. Bragg will not only influence the shape of the airflow, but it can also influence how the air responds to being deflected by a body in the airflow - air flowing around a body won't get pushed back towards the body when there is no wall.

2) Blow vs. suck vs. recirculating. I think the biggest impact would be on turbulence in the flight chamber, but there is also the issue of being able to partially stall the fan(s) in a sucker. That effect can definitely be felt during flight.

3) single fan vs. multiple fan. Not really sure what the influence would be here - I guess it would be easier to stall a smaller fan, but then you are only stalling one of multiple fans. I don't understand the dynamics of stalls with the larger single fan.

4) Probably the most important influence, one that I didn't see discussed earlier in the thread: Diameter of the tunnel. The cross-section of a 12 ft. tunnel is about 113 sq. ft. The cross section of a 16 ft tunnel is about 201 sq. ft. or about 175% of the area of the smaller tunnel.

I'm going to make some gross assumptions now, to illustrate my point - the absolute numbers are probably way off, but I hope this will help folks to understand what I'm thinking... Let's guess that a skydiver on their belly presents a cross-sectional area of about 3 sq. feet (roughly 1 ft wide, and roughly 3 ft from shoulders to knees? I know, wider in the middle, especially for me! but for a rough estimate, close enough...) That means that a 4-way team will have an area of about 12 sq. ft, which in the 12 ft tunnel represents about a 10% blockage of the tunnel, but only about a 6% blockage of the 16 ft tunnel. If the undisturbed airspeed in both tunnels was 120 MPH, then to get all that air past the tunnel fliers, the air has to pass the fliers in the 12 ft. tunnel at about 132 MPH, but only 127 MPH in the 16 ft tunnel.

Result?? The light guy on the team needs more lead to stay with his teammates. I'm sure the real situation is much more complicated than this, but I suspect this is where a bunch of the difference in "feel" is coming from. This also leads me to guess that a single belly-flier probably won't notice as much difference in the various size tunnels as would 4-way teams.

And while we are speculating... Another possible difference between the various tunnels (and no way am I an expert here...) could be the control systems - I don't know how complex the control systems are, but if one tunnel uses an "open loop" control system - basically set the power, and you get whatever airspeed you get, and another tunnel uses a "closed loop" control system that dynamically changes the power setting to maintain a relatively constant air speed, I think that would introduce a significant difference in the way the tunnels would "feel". Again, I have no knowledge of the tunnel control systems, so this is pure speculation on my part.

Note that I did not discuss the "Venturi effect" that others have mentioned. I tend to agree with those who think that is a "red herring", and if there is such an effect, it is probably so small as to be unnoticable. I think that the presence of walls that restrict the airflow (or rather the deflection of the airflow), and the relative size of the tunnel to the fliers present are much more important than just the fact that the flight chamber might be narrower than other parts of the airflow system.

Sorry for the ramble - this got a bit long - I hope this gives someone "food for thought"

[tunnelfly 0]

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One day I may go to Bedford...But I just can't see me making a trip just for a tunnel.

Well, there's actually two tunnels, just minutes apart.
Not to mention the good food and lovely weather...

No.1 reason NOT to be an astronaut: ...You can't drink beer at zero gravity...

[rehmwa 2]

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awww... I'm on your list
different flavors are yummy

Your's is outside - that is a very attractive feature

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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]

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Yes, I have a GF. So while I may be a geek I can't be THAT big of one

Sure you can, she just has to be very understanding.

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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

[Ron 7]

Now that posts makes great sense!

I never thought about the power needed to go around the blockage...Nice angle.

"No free man shall ever be debarred the use of arms." -- Thomas Jefferson, Thomas Jefferson Papers, 334