climbing/skydiving - sailing/flying

[Tad Eareckson]

Once upon a time the hang glider folk decided that - at all costs - they were gonna stay attached to whatever was left of the glider regardless of the degree to which it had been destroyed by excessive G loading and whatever the rate of parachute eating spin it had developed.

So they looked to the people who really knew how to handle massive shock loads - rock climbers and skydivers - and came away with doubled six thousand pound nylon webbing and six ton carabiners. (Think that'll do it? Maybe not - Let's back up (some of) the webbing just in case.)

BIG FREAKING MISTAKES.

As has already been discussed ad nauseam the carabiner was not designed to connect a wing to a cockpit and we've killed and mangled a lot of people because we have a hard time remembering whether or not we've effected the operation. Hopefully bolt-on harness configurations will make a dent in those statistics 'cause it's pretty damn sure that we're not gonna do much better with attempts at behavior modification.

And we have absolutely zero need to absorb massive shock loads with the suspension or anything else. We're flying stiff but adequately flexible gliders at slow speeds in an extremely elastic medium - the atmosphere. Climbers are anchoring themselves to granite and trying to avoid destroying their pitons at the ends of falls and skydivers are trying to safely decelerate from a hundred and twenty miles per hour without blowing their canopies apart.

These are not the sorts of things with which we are concerned and that's not what our systems should be designed for. We're not trying to slow down. We're trying to go up as fast as possible. We are sailboats that operate in the air in three dimensions.

How To Make A Hang Glider...

Take two sailboats, saw off the hulls and throw them away along with both of the headsails and one of the booms.

Bolt the masts together at their bases, attach the foot of the boomless mainsail to the remaining boom.

Configure a couple of spinnaker poles to use as cross spars (which we've corrupted to "crossbars").

Throw on a control frame and a few wires.

Cannibalize the line, block and tackle, and cleat of the boom vang to reconfigure it for the same purpose and call it a variable geometry system.

The pilot in his harness (sea anchor properly stowed) now substitutes for the hull and the suspension is roughly analogous to the sheet.

YOU DON'T USE NYLON FOR THE SHEET.

Nylon stretches and contracts. It dissipates the energy we're trying to use to go up by converting it to heat. It's a brake.

The only things you use nylon for on a boat are anchor and dock lines, i.e., devices to keep the boat from moving - the opposite of the purpose for which it was designed. EVERYTHING else, everything used to secure and trim the sails and translate wind power into headway, is as low stretch as the owner can afford.

Last winter I stitched together a glider suspension assembly to make up the length difference between the carabiner I had been and speed link I would be using. Had an odd remnant of one inch tubular webbing about the right length that I decided to use up. It had been in my little webbing bag for maybe a decade and a half and I couldn't properly identify it or recall its origin.

A short time afterwards I was in a sail materials shop and - bingo - there was a roll of identical material. Not the nylon I had assumed but polyester. Yeah, that's what I shoulda been using anyway - low stretch and so UV resistant that you don't have the slightest worry about the sun.

About a week and a half ago I got motivated enough to replace the harness suspension. It went in the air on Sunday. Boy could I feel the difference and I spent considerable time looking down at a couple of much better gliders.

Yeah, that's pretty subjective and anecdotal but I absolutely guarantee you that the people smart enough to go to low stretch suspension are gonna start mopping the floor with nylon suspension gliders. I wonder how many decades it's gonna take for the collective light bulb to start glowing.

The bad news is that yesterday I became aware of the availability of one inch Spectra webbing - for 5.2 times the cost of the Dacron (polyester) - and will probably be unable to resist the urge to max out the potential of the prototype.

[theflyingdude]

About a week and a half ago I got motivated enough to replace the harness suspension. It went in the air on Sunday. Boy could I feel the difference and I spent considerable time looking down at a couple of much better gliders.

Yeah, that's pretty subjective and anecdotal but I absolutely guarantee you that the people smart enough to go to low stretch suspension are gonna start mopping the floor with nylon suspension gliders. I wonder how many decades it's gonna take for the collective light bulb to start glowing.

Tad, you really are a piece of work. You should submit your resume to Will Wing or Moyes as I'm sure they'd welcome your engineering expertise and efforts to light the collective light bulb.

JR

[Dan T]

Tad,

I tried to follow your point and couldn't. You talked about the harness suspension. Is that the two hang straps, one from the harness and one from the glider, with the carbiner in between? If so are you suggesting that a more rigid version of that apparatus would reduce the sink rate? Correct me if I am wrong but I think "climb rate" in a hang glider is really a function of sink rate except that it's in rising air and may apply at bank angles other than flat. I ask this because sink rate is something that could be fairly easily tested, and those glide angle contests that we occassionally hear about would seem to be good candidates. If the performance is as good as you are suggesting I would think it would show up under those conditions.

If I missed what harness suspension is, please restate it. Then please explain the physics/aerodynamics in simple terms for me. Most of your references to sailboats simply confused me.

My guess is you got above those other higher performing gliders because you were coreing the thermals better than the other pilots on that day.

Thanks,
Dan

[brianvh]

Tad is referring to losses due to elastic energy dissipation. If you mounted yourself on a spring, you'd lose energy by bobbing all over every time you hit a thermal. This would be dissipated by the glider being pulled through the air towards the pilot, dampening the oscillation.

I just don't buy that the amount of maybe (at most) half inch stretch in the hang strap will make that much difference. I side with Dan: you thought you would fly better, so you did. Placebo effect. Or more to the point, homeopathic medicine.

[Tad Eareckson]

JR,

Fetal alcohol syndrome, right?

Dan,

Thanks for making the effort.

Definition of terms...

suspension - everything between the glider and the harness
glider suspension - the portion of the suspension above the carabiner
harness suspension - the portion below

Note: With bolt-on systems there is no carabiner and all the webbing packs up with the harness.

Yeah, pretty much what Brian said.

No, a glide ratio contest won't tell you anything 'cause you're flying straight in smooth air - a nice steady 1 G. The suspension elasticity only becomes a factor when the accelerometer needle starts moving around - which it does every time there is a change in the vertical speed of the air in which you're flying and every time you adjust your bank angle, i.e., constantly while you're thermalling - unless it's one of those strong fat smooth things I often hear about but have never actually experienced.

Forget the sailboats for the moment and think of a glider only as a device which serves to pull you up. A construction crane could fulfill the same function over a limited range. Those things use steel cables to transmit the force to lift the concrete blocks and I-beams. They could use bungee cord but they'd burn a lot of extra gas taking up the stretch.

Fill a five gallon bucket with water, tie a loop of bungee cord around the handle, grasp the loop, and lift the bucket a foot off the ground fifty times. Substitute Dacron or Spectra for the bungee and repeat the experiment.

Try propelling a canoe with a piece o' crap plastic paddle that bends thirty degrees with each stroke.

Brian,

Yeah, my position relative to other gliders on Sunday doesn't count for much with respect to relevant data.

I've been busy with the needle and thread lately stitching some test samples and trying to dredge up some high school physics from the dusty corners of my mind in an effort to quantify things. I'm not optimistic. But...

This much I do know.

-

Any glider absolutely WILL climb faster using low stretch material for suspension than it will using nylon.

-

Right now I've got nuthin but gut feeling to go on but if you put a gun to my head I'd guess that a low stretch glider would be a couple of hundred feet over a nylon one after a three thousand foot climb.

But let's say it's only ten feet. We agree that it'll be higher.

It takes about eight feet of webbing installed between my back and the base of my kingpost to give me proper clearance from the basetube. The Dacron in place now is also of comparable massive overkill strength, lighter and thinner, immune to UV, and sets me back a buck twenty more than that length of nylon.

So what's the argument for using nylon?

[RedBaron]

Tad, come on man, you're kidding, right?
If not, let's assume the amount of stretch in Nylon webbing does make a difference. You're then making the mistake treating a hang glider as a fixed-wing aircraft. Stretchy webbing negatively affects your climb rate but would offset the sudden washout due to the quick buildup of positive G's. A static connection would washout the glider more in sudden lift and negate all climb benefits.
You also forget that your harness is not a rigid structure and will give to a quick buildup of positive G's. It will give more if the connection is a 100% static and again offset theoretical climb benefits.
Using your examples, you're not trying to lift a bucket filled with water, you're lifting a balloon filled with water. Likewise, you're not steering a canoe with flexing paddles, you're steering a python with flexing paddles. Replacing the paddles with stiff carbon fiber ones will just make the python flex more.
You're wasting your time buddy. If you want to make your HPAT climb and go better you better start modifying the glider. Move the keel anchor point for the tension wires back an inch for increased span, increased VG tension, less sweep and less washout. Remove the top half of the kingpost and fair the remainder in. Add shims and a longer #1 batten to remove washout further, flatten the battens and lose 20 lbs to bring the stall speed down. Strip all wires. Now that's a project I would be excited to hear about. The design of the HPAT-RACE.

Cheers

[Tad Eareckson]

Janni,

Point by point...

Nope, totally serious.

A hang glider IS a fixed wing aircraft. Wills - not Sikorsky.

I don't know where you're getting the "sudden washout...". Maybe you know something I don't but anyway - we're not talking about quick buildup of lotsa Gs (much as I wish we were). We're mostly talking about relatively small changes in vertical air movement and adjustments in bank angle. If we're feeling much more than about 1.5 Gs it's not often enough to have any bearing on this discussion.

No, I hadn't forgotten about the flexible pod harness with the squishy human inside. The water balloon thing is valid but a bit extreme. Still, you can pick up a water balloon on the end of a Dacron string easier than with one of nylon (my bungee analogy was a bit extreme too).

Although the principles are the same I'm not talking about steering the canoe - I'm talking about powering it. In this analogy the paddle blade is the wing, its shaft is the suspension, and the clown in the boat substitutes for the clown in the harness.

I don't buy the python at all. Let's make it an anaconda - that's a more appropriate snake. I don't buy the anaconda at all. We've got nothing - including the pilot - that flexes like an anaconda can. And, anyway, if the anaconda wants to go in the same direction you do, and thus cooperates, the both of you are gonna get there a lot faster and easier with the carbon fiber paddle.

As I've told others before you - these little tweaks are not about getting my HPAT to move better. My old glider is just a test platform I use to help figure out how to move ALL gliders better.

I'm not wrong about this. If what you're saying were valid then it would apply to sailboats as well and they'd be well advised to employ nylon for their main sheets and find other uses for the high tech, low stretch, four bucks a foot stuff they're using now. Hiking straps are also pretty analogous to glider suspension and that webbing is low stretch.

And - unlike the situation in hang gliding - the performance differences between boats employing low and high stretch rigging is easily observable.

Sailing has got infinitely more history, racing experience, technology, money, participants, and brains than hang gliding will ever be able to dream about and our best stuff is mostly hand-me-downs from them. The more we pay attention to what they're doing the better we'll have our shit together.

You're wrong but I really appreciate your participation in and contribution to the discussion.

[Batman]

Ahh hell ... here we go again.


/goes to get popcorn

[jimrooney]

You'll need a drink with that... I'll go get some soda.

[Batman]

/passes Jim the popcorn ...

[Tad Eareckson]

Janni,

P.S.

I do want to remind you that a low stretch glider has already been up for an extended thermal flight which involved a twenty-five hundred foot gain and the alleged loss of performance due to increased twist did not appear to be a problem.

The Dacron glider and a nylon glider were pimping off of each other at various times.

The Dacron glider was a kingposted HPAT with the coating peeling off the leading edges. It was state of the art nineteen years ago.

The nylon glider was a sleek topless Litespeed. It's state of the art now.

The Dacron pilot was a not particularly current 55 year old.

The nylon pilot was a 33 year old rabid air junkie.

After an extended climb in which the HPAT maxed out for the day at 4036' the Litespeed which had entered the thermal below it remained below it some hundred feet or so.

This observation sure isn't proof that the low stretch suspension is imparting a significant performance advantage but it's pretty damn good evidence that it ain't doing no harm.

General rule...

You wanna make something - bicycle, automobile, sailboat, hang glider, sailplane, fighter jet - perform better, you get rid of the slop.

[Batman]

Helicopters are FULL of slop. That's why the ground repels us!

[theflyingdude]

Helicopters are FULL of slop. That's why the ground repels us!

I think paragliders resemble that remark.

JR

[RedBaron]

Glad I made your day, Tad. Don't be too modest, you're a pretty good thermal pilot. Skill beats equipment every time.

[toto's_ride]

/knocks over the soda and ruins the remaining popcorn.
Damn low performance soda.

[Tad Eareckson]

For the first time this year obtaining first hand evidence that we have not yet poisoned the second and last breeding species of falcon out of existence in the eastern US in the course of my lifetime was what made that day for me. Who topped out a few feet above whom or whether I or anyone else flew at all didn't matter in the least by comparison.

But as long as we're burning all these fossil hydrocarbons to get to flying sites and up to launch/release altitudes it would be nice if we maxed out the efficiency of how we take care of business upon arrival.

That day the lift would be fairly abruptly shut down by overdevelopment about 15:15. With only one tug running - pretty much par for the course - the line was moving at it's usual glacial pace.

Valerie launched ahead of me and was soon back in line ahead of me again commenting that her weak link had popped at a point at which it was under no significant excess load. (Yeah, no shit. That's what happens when you use one that's less than half as strong as it needs to be. A new generation prepares to uphold the sacred tradition.) So between that and another one or two previous useless breaks and relaunches I got screwed out of a quarter or half hour of airtime AND got to subsidize the cost of the fuel, wear and tear on the tug, and pilot and crew time 'cause - collectively - we're still too fucking stupid to know what a weak link is supposed to be doing for us and what it can't.

I have a feeling that if a weak link break were treated as, instead of a random and inevitable act of God, the pilot error that it actually is and thus the cost were borne individually rather than collectively and the perpetrator had to wait at the end of the line until everyone else had had a fair shot that the course of evolution would soon take a sharp turn for the better. This is why communism never worked very well.

So anyway I can now quantify - to some extent - the differences between the nylon and Dacron webbing.

I've got about a meter's worth of distance between me and my suspension tangs so I formed a couple of meter long loops out of 2050 millimeter lengths of webbing of the two materials with 50 millimeter stitched overlaps.

I mounted each in one of my torture devices and measured the differences in lengths when loaded to two and three hundred pounds. The nylon elongated six millimeters, the Dacron a third as much.

The test wasn't fair to the Dacron cause the 6000 pound flat nylon was slightly wider and much thicker than the 3800 pound tubular Dacron - giving the former a cross sectional area about 2.42 times the latter.

I don't know if this extrapolation is valid but I'm guessing that nylon of equal dimensions would be stretching about fourteen millimeters.

I have not tested the Spectra but the web says it stretches about a third what Dacron does so - do the math. It's gonna be a difference very hard to read off of a tape measure.

Anyway - every time we hit a friendly bump we waste energy taking up stretch that we could be using to gain altitude. Sometimes it's only a matter of a few feet per minute to make the difference between slooooowly sinking out and and slooooowly climbing into something that'll make the day worth having gotten out of bed for.

Ever notice that everything else on the glider low stretch?

[brianvh]

Okay Tad, let's do the numbers. Have to make a lot of assumptions here:
* every few seconds (let's say 5) you experience a change in acceleration of 0.5 g...this is being very generous.
* between every change in acceleration, the energy of stretching the strap is completely dissipated.
* according to your measurements, 1 g produces a stretch of 14 mm. 0.5 g would then stretch it 7 mm
* average sink or climb rate is 200 ft/min which is about 1 m/s.
* pilot plus glider mass of 300 lbs = 150 kg

Since gravitational energy change goes as mgh, and g is about 10 m/s^2 we get (150kg)(10 m/s^2)(1 m/s) = 1500 J/s energy change of a glider doing typical flying stuff.

For a spring, energy = 0.5*K*x^2 where K = mg/extension = (150 kg)(10 m/s^2)/(0.0014 m) = 10^5 N/m approximately
so energy per second = 0.5(10^5 N/m)(0.007 m)^2/(5 sec) = 0.49 J/s

From my calculations, the elastic dissipation is about 0.3 % of the energy changes during typical flying. Do with the numbers what you may.

[Tad Eareckson]

Oh boy. I knew I should've waited till AFTER my senior year to start smoking weed.

They were right - that stuff IS a gateway drug to the harder, more dangerous, really addictive stuff like the alcohol upon which I am now hooked (at least I dodged the tobacco bullet). I'm sure I'm gonna hafta kick that habit before I can get a proper grip on the equations.

I'm not yet together enough to know how this is gonna affect my case (seems like it should bolster it but it's in the denominator) but...

Don't you have one too many zeros in 0.0014 m?

At this point I'm perfectly willing to accept the possi-/proba-bility that the difference in feel of the glider I reported was entirely the placebo thing and, yeah, the change in acceleration allowance you gave me is extremely generous.

But from my dumb glider driver (Me Tarzan. Dacron good. Nylon bad.) perspective whatever number you end up with on the bottom line it's gonna be something and it's gonna be entered on the liability side of the balance sheet.

The number may be so small that it's not worth a retrofit. But do we have to keep on building them that way?

For the aforementioned dollar twenty increase I've cut the stretch, lowered my drag profile, and stopped worrying about the UV which Betty reported causing a harness strap to fall apart on her sewing machine.

This is just like the straight/curved pin thing, albeit way less dramatically so. Deliberately start out with a pre-damaged component just 'cause:

the probability of one being in a situation in which reasonable performance is required is low; and
with the right combination of technique, testosterone, adrenalin, and luck one can probably make the thing work anyway?

Yeah, that makes a lot of sense.

Do you think for a nanosecond that if this idea came out of Wills Wing instead of yours truly that the two millimeter sidewire crowd wouldn't be sprinting for the sewing machines if they thought they could scrape up anything in the ballpark of another third of a percentage point of performance for nothing?

Thanks much for the equations. I'd have never been able to put that together.

[brianvh]

you're right. 3% ratio, but that was a generous estimate of 0.5 g every 5 seconds. Call it 1% on a good day when you've got a white-knuckled deathgrip on the basetube for the entire flight. I'd guess this enhancement about equal to what the dumbasses who strip the plastic off their flying wires get. So yes, people would indeed go ahead and swap out the nylon for dacron. Personally I don't think it's worth the effort, but some people (see above) would.

[dbodner]

I daresay that stripping your wires would probably provide a bigger boost than changing your suspension material. Doing both also raises risks, probably very slightly. Reducing the springiness would reduce comfort (slightly) and increase (slightly) the chance of injury in case of a rapid deceleration.

For me on my Falcon or Sport2, any advantage of a stiffer suspension will be noise amid my frequent flying mistakes. However, I suppose if I were a top pilot flying in comps, a stiffer suspension may just make sense.

[Tad Eareckson]

Brian,

HEY! I resemble those remarks!

David,

Yeah, I was afraid someone would bring up those points.

Pacific Airwaves had a continuous nose wire which ran from a control frame corner, through an elbow sleeve at the nose, to the other control frame corner. That's not very strong but normally all it's gotta handle is the force with which you're pulling in. If all you do is push out you can throw it away.

At Hyner on the morning of 1989/07/03 Dave Collins, in the course of doing those idiot spot landings then required for his Three, was focused on the spot so intently that he forgot about the ground and flew into it. The basetube stopped and the wire failed right were you'd expect it to.

Dave's body was yawed out of alignment and when the still level Mark IV wing slammed flat down to the ground the keel acted as a guillotine and broke his neck. (He recovered well enough to reappear at the training hill once or twice but vanished from the sport anyway.)

I described this accident to Ken Brown and his response was "We don't build them to crash."

I was never much satisfied with that reaction 'cause:

it was rather chintzy engineering to begin with;
somebody was damn near quadded as a result; and
there wasn't much in the way of a penalty for doing it right.

In the case of suspension however - yeah, I'd go with "You don't build them to crash." The engineering is good, nobody's gonna be crippled as a consequence, and it only costs another buck twenty.

If we were really after a lot more no cost safety and comfort we would all fly suprone. But that'll never happen 'cause we'd feel less like Superman.

And if comfort is a primary concern - there's always the couch and tube. On the rare occasions when I've got a shot at going up I'll happily trade a little for a few more feet on the track log.

Stripping the wires...

I stripped the nose wires 'cause I got tired of the old coating sliding down and bunching up and I'm not crashing often enough for it to make a difference. I should strip the side and tail wires 'cause we don't use parachutes and the Type 18 bridle should be able to handle it anyway. And why the hell are we using coated wires for the upper rigging to begin with? (Maybe we're not anymore?)

More on the sailboat analogy...

Sailors don't have any pitch control - waves are about the only things that affect that rotation.

We don't have much in the way of yaw control - a sailors use rudders.

For pitch and roll control we move our bodies around relative to the airfoil while suspended from webbing.

For roll control sailors move their bodies around relative to the airfoil on the deck or - when they get serious - outside of the windward rail while suspended from low stretch webbing.

[brianvh]

Tad;
I know the concept of wire crew has gone completely out of your world view, but we HATE people who strip their wires. The major discomfort to wire crew on a gusty day may be the biggest safety concern. I'd get fat, laminated wires just to make my wire crew like me.

And if using a will's wing harness, the hang strap is maybe a foot on a keel mount. you'd have to replace all the harness suspension to find anything noticeable (and we are talking about a day with lots of high accelerations involved). Not even remotely close to worth it.

[RedBaron]

Okay, Brian, so I'm a dumbass and you hate me. Gotcha.
Know what, I'm not gonna change my wires just because your hands are as soft as a ballerina's, you big girl. Leonie is tougher than you

[brianvh]

Set me to wrestling her and we'll find out who's tougher. I've always been envious of catfights. Just tell me which buttons to push and we'll settle this.

[RedBaron]

Big words from someone who got beaten up by Rhonda.