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I'm coppying this question from another web site. I'd like to get a wider range of oppionions. I'm going to put this here as I think it might have shown it self in a base inviroment more then when people are skydiveing feel free to move it if you think fit.

Wing suit CG question?
« on: June 21, 2007, 11:11:38 AM » Quote Modify Remove

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First off,not that I exspect any one to have noticed much less care, I'd like to state that I am back from my trip with all my fingers and all my toes.
I've had a lot of time to sit around in a tent and dwell on the accedent up there this year. I'd like to toss out some questions and see what other peoples opionions are. I'm not saying that any of this was the root cause of the accedent but it was what got me really thinking about it.
Every one up there was of course jumping down with at least some gear. they were also wearing diffrent cloathing and boots then they were used to. By my standards every one includeing my self was jumping very light. With so many people around I felt no nead to carry as much shit as I normally have with me. I was really kicking my self for not bringing a second smaller rig. As light as these people were they were really noticeing a diffrence in their track. All of these people were jumping tracking pants and wing suits. Being "necked" or worse all the time it wasn't quite as noticeable for me. I need to get a bit more hi tec with this. With gear stuffed into any avalobaly pocket they were noticeing a real diffrence in tracking and body position includeing built in turns. People also commented on there Koflocks and seemed to have trouble tracking with them.
Things I noticed. In skydiveing with my javelin I tend to track with my knees just slightly bent. I just can't seem to cup my upper body enough to avoid diveing. With my Kofocks on I can fully extend my legs. Same thing with my wing suit. An old original classic. Really punched out with the rear leg spread I feel like I'm diveing. I find that I again need to bend my knees just a bit. With the boots it ballances much nicer. It's like the upper wing is a bit small. What got me thinking was Jimmys accedent. Aparently he never got flying. I was wondering if any of this contributed to his diffaculty. The weight of his boots and the ballance of the wings on his suit. Now as it happens that he was NOT wearing double boot but that's what got me thinking about this. I was talking to one of the frogs and they've seen issues like this. He felt Jimmy was just trying too hard got too flat and mushed in a stall. He sold me on that idea but I'm still interested in your thoughts on wing ballance between diffrent designes and cg issues with boots and gear.

Lee


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Lee
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nicknitro71
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Re: Wing suit CG question?
« Reply #1 on: June 21, 2007, 12:11:19 PM » Quote

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First, it’s really hard if not impossible to identify the CG of a wingsuit coupled with a pilot because it varies with body position, i.e. every small movement of any body part moves the CG. When designing a wingsuit rather than the CG, the balance between leg and arm wings has to be found. A suit with large arm wings and small leg wing will be inheritably “floaty” and possibly unstable with little forward speed, vice versa for a suit with big leg wing and small arm wings. However we fly a flexible frame so certain characteristics of a suit can be masked by changing our body position. Also what works for my body type might not work for somebody else’s body type. Even if two pilots have the same height and the same weight chances are their legs, torso, and arms won’t measure the same. Also their mass distribution won’t be the same.

As for the boots instance…I do not believe the weight of the boots has nothing to do with this incident. Many people found that flying many wingsuit with boots actually improve the performance. While this is true for some suits, it has nothing to do with shifting the CG aft! On most suits the leg wing can be tensioned better while wearing boots so better leg wing tension equals better performance. If you do not believe it then find out the weight difference between your boots and your sneakers and wear ankle weights to compensate for this difference while wearing your sneakers. Chances are the performance is going to be better with the boots on, not with the sneakers with ankle weights.

Being aware of this problem when I designed the leg wigs of our suits I made the booties in such matter that in order to correctly tension the leg wing forces must be applied towards the outside rather than down. On all our wingsuits the toes and knees should be pointing outwards (sideways). I know many people use wingsuits with sneakers in a skydiving contest and heavy boots in a BASE contest hence the reason for such design including the adjustable booties.

In contrast on my old Skyflier 3 performance was better with boots on and the booties acted like “gas pedals” “press down for speed”. This does not apply to our wingsuits; you must push out your toes and knees for speed.


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krisflyz
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Re: Wing suit CG question?
« Reply #2 on: June 23, 2007, 10:40:04 AM » Quote

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Agree with what Nick said and would like to add the following...

The toes cannot be pointed when wearing boots and if the suit legwing goes all the way to the floor as seen in the attached picture, it creates loads of legwing(trailing edge) camber.

WS BASE Exits
===========

Johnny Utah was at the exit point when I did my first WS BASE jump and he said this...

"You will go a bit headlow, don't fight it". I was shitting myself so didn't hear a thing but its on the video.

From the second second, A little bit headlow is lot better than headhigh or flat. Trying to remain flat too long after exit causes burning up too much altiitude to get the flight started. I have seen this on WS BASE exits...forward movement in a big wingsuit taking 4-5 secs to start.

Kris.

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BootieV2.JPG (42.45 KB, 648x486 - viewed 3 times.)

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RiggerLee
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Re: Wing suit CG question?
« Reply #3 on: June 23, 2007, 01:42:34 PM » Quote Modify Remove

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On the boot question. I was just unpacking and I weighed my boots. my shoes are 1 lb 12 5/8 oz. My Koflacks are 6 lb 5 1/4 oz. It's a pretty big diffrence. Now Jimmy was wearing a light weight single boot and I think that was really just a case of some one trying too hard to fly fast.

Lee

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krisflyz
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Re: Wing suit CG question?
« Reply #4 on: Today at 06:50:51 AM » Quote

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Quote from: RiggerLee on June 23, 2007, 01:42:34 PM
On the boot question. I was just unpacking and I weighed my boots. my shoes are 1 lb 12 5/8 oz. My Koflacks are 6 lb 5 1/4 oz. It's a pretty big diffrence.

Lee


That is a substantial difference in weight. It will move the CG a little bit. I did some approximations and calculated that the difference in CG for me(44 inch legs(approximated as a cone) and 30.5 inch rest(approximated as a rectangular block of same dimensions as the waist) the CG will approximately move 1 inch back with 5 lb weights added to the ankles.

Will moving the CG back help L/D? Or is that related to where the center of lift is located as well?

Kris.

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RiggerLee
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Re: Wing suit CG question?
« Reply #5 on: Today at 11:19:41 AM » Quote Modify Remove

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Think of it as more of a moment issue. the boots are probbable 30 in behind the cg. that's a pretty good lever arm. Put a 5 lb weight on the end of a 3 ft broom stick and try to hold it horizontal by the other end. If your in ballance before then thats the amount of moment that you have to over come to remain in ballance. I find it to be noticeable. With my old original bird sute I can really extend and streatch out my legs. I find that it flyes much better for me.
As another example. Back when we first got the early bird suits around here one of the best and fastest flyers was Howey. This kind of surprised us all as he was kind of a tubby guy. I'm thinking the better proformance that he was showing might have been from the fact that his cg was a bit lower. Women tend to have wider hips and less upper body muscular development. There cg tends to be lower. I wonder if that might be why some of them fly so well? I'm curious what your thoughts might be on how this might relate to diffrent wing suite designs that have differing sizes of wing and how they seem to ballance.

Lee


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Lee

I've experimented with shifting CG quite a bit over the past year. (see, for example, "It's not the pilot, it's the shoes!" and experiments with V-2)

No doubt, CG has major effect on wingsuit flight. Just try ankle weights for yourself and feel the difference. While everybody's balance is different, I found that wearing Hanwags with Phantom leads to a stall (although flights with <3000ft of usable altitude can actually benefit from slight tail-heavy misbalance through flatter, longer planeout) - so I wear lighter hiking shoes on base flights specifically for that reason - while Hanwags+5lbs (total) on ankles help achieve better glide on V-2.

Put enough weight on your feet and you'll fly feet first.

Lee, can I buy another "Turkey Boogie 2005" shirt? I spilled coffee on girl's breasts. Please PM.

I am not so sure the CG shift accounts for all of the performance increase. Can you post some links to material that discuss CG of gliders and the relation to L/D performance?

The hanwags are 2 inches longer than my usual shoes and about 0.25-0.5 inches taller. Wearing hanwags makes the suit fit a bit tighter. That has an effect on performance. Suit fit has about 20% impact on performance(read below). On a suit that has the legwing tailored like the V2 where the trailing edge of the wing attaches near the front of the feet, creates lots of camber.

My Phantom was made for me when I weighed nearly 90kg. July last year(weighing 78-80 kg) the L/D in the suit was 2.1-2.2 on average skydives ( 2.4 on the best ones) and 2.4+ on BASE jumps of over 35 secs delay. Towards the end of the year(72 kg) the L/D was less than 1.8. About 20% decrease than average performance.


In your experiments with weights, how much improvement is due to weights alone? Any suit that fits poorly enough to allow you to wear hanwag shoes is not optimal to begin with. How can you conclude that there is a major benefit is from CG shift alone?

Kris.

Dont you think the added year/experience also adds a lot performance/skillwise?.

The performance decreased 20% over the year. I cannot see the point you are trying to make .

Kris.

Sorry...read that wrong

Maybe you didnt jump enough that year and just started sucking performancewise

I got it tattooed. now i can check it before every jump. I got sick of moment arm calculations.

sadly, my 'Roll' Axis tat is X rated.

You actually got a tat on(or in the vicinity of) your bollocks? .

Kris.

no of course not!!!!, but thats where it would go.

I jumped V-2 in sneakers and heavier weights instead of Hanwags and lighter weights and CG effect was about the same. My V-2 is loose even in Hanwags. I agree with your point about the importance of tight fit.

Cool, I am unable to find any material (online) about the importance of the location of CG for gliders. Please post links if you know of any.

Kris.

I am not sure if standard glider control theory applies. you probably need flying wing theory.

BUT...
when a main wing produces lift, it generates a forward pitching moment.
putting a lifting surface (canard) out front eliminates the pitch, but creates issues.
most aircraft use a horizontal stabilizer behind the wing. this surface creates a DOWN force to counter the pitching moment.

simply shifting the cg aft decreases the amount of downforce required. less downforce means less induced drag and better performance.

I just have no clue how this applies to wingsuits...

hmm... kinda true. for most, meaning probably about 90% of aircraft out there, that is true. (these being standard configured wing/tail aircraft)

modern sailplanes, the most bad ass, effiecient gliders to exist (some guy is building one with the achievable goal of 100/1 GR) still have the same configuration, but are tuned so that all horizontal lifting surfaces produce their share or more of the positive (up) lift to stay aloft, this makes the glider extremely unstable, and several pilots have died tuneing their ships this way.

this is also true, only more so, in military fighter/bombers. say the F16, its CG is so far behind the CP point (center of pressure) that it requires a computer to actualy fly it. the pilot is only telling the computer what he/she wants to do, then the computer translates it, and sends it to the surfaces.

im no WS expert at all, but from what i see, a wingsuit is what i described, like a glider with all horizontal lifting surfaces producing positive lift. thus, their instability.

this is translated to novice aircraft pilots as 'stability'. the farther forward the CG is, the more stable the aircraft is. (a ball in a bowl) as the CG moves back, induced drag is reduced, but the aircraft becomes less stable.

Thanks for the explanation.
Here are a couple of links if anyone is interested.
http://www.meretrix.com/...g/notes/cg-perf.html
http://www.westwingsinc.com/cgeffect.htm


That is my point exactly.

I have found a few links that have the same explanation you have given above. It seems reasonable to assume that this reasoning would apply for a glider if the implementation of the pitch control is the same as that for an airplane.

Wingsuits do not use an element that produces a downward force to maintain balance. The legwing and the arm wings provide lift on different sides of the CG. Kind of like balancing a seesaw by pulling it up on both sides instead of pushing it down on both sides.

The pilot can balance the suit. I can see how CG and Center of Lift being as close to possible each other would be advantageous.

I have not found a reasonable explanation of why shifting the CG aft works in the wingsuit context.

Kris.

Lee, what was the vertical drop and horizontal distance Jimmy flew before the impact?

well, from a very basic viewpoint...

when static stability exists, an aircraft returns to "normal" after being disturbed.

airfoils tend to pitch forward.

so, the pitching forward gets balanced by some other force. (some applications place a "reflex" on the trailing edge to induce a counter moment... this could be mimiced by bent knees.)

whatever counters this moment would be assisted by an aft shifted cg. thus, the wingsuit would not need to pitch as much nose down. a flatter pitch could lead to a better glide.

humans are very good at adapting. they have also done things not anticipated by established theory. instead of wondering, why not just go experiment?

 
Yuri, I'm going to pass the ball on this. Jimmy was part of Chads groop. I was just kind of a naibor. We were at times hikeing togather and jumping the same cliffs but I don't think I ever made a jump with Jimmy. I never jumped that exit point. I did not personaly see the accedent. I was not involved directly in the body recovery or any investigation.

I don't think you'll find any surprises when you look at it. Still I think Chad and the people that were on the jump with him should have the opertunity to present a formal statement before I cloud the air with any of my speculations. Contact Chad/Dexterbase or one of the other wing suit jumpers that were on that load with him and see if they will write up a description of the exit point and the accedent.

Although jimmy's death is what got me thinking about all this I do not beleave that the boots he was wearing or the gear was the root cause of this accedent. Ask Chad for the details of the suit, rig, boots, expereance with them, etc. But I saw the body and he was not wearing a heavy double boot. He'd made other jumps there with the same configuration.

I was talking to one of the frogs that's a rather experanced wing suit flyer. They've seen things like this where some one does well till they get to a lower cliff or one with a ledge. Then they are stressed, try too hard and mush in a stall. He sold me on this idea and I beleave that is what happened here.

Lee

I considered the very basic airfoils tend to pitch forward theory, if we consider the wingsuit as a whole to be the airfoil. But the lift from the armwings counters the lift from the legwing and we cannot treat the wingsuit as a whole to be an airfoil.

Unless we assume that

a) the moment of the lift from the legwing about the CG is greater than the moment from the arm wing lift about the CG
and
b) The suit and pilot combination must be at a flatter pitch than the suit and body charecteristics are balancing at for best L/D

an aft shift in CG may not be benficial.

I know that my problem when flying a phantom last year was that the suit tended to pitch me headup and put me in a stall(not an actual stall but in a slow flight mode). Adding weights to my leg is hardly the solution to alleviate that problem.

Getting a suit with a relatively bigger legwing is the route I took. We'll see how that works out. I'd be experimenting a lot if the weather permitted.

Kris.

An article on CG and L/D of sailplanes by a sailplane manufacturer.

http://www.dg-flugzeugbau.de/schwerpunkt-e.html

Kris.