Weak links

[Steve Davy]

How do you set the glider at the correct angle on the dolly? Would it be fair to call this "angle of attack" or is there a better term?

[Tad Eareckson]

Think anyone noticed that was sarcastic?

Amongst Davis Show cult members in good standing? I sincerely doubt it. And I'm NOT being sarcastic - much.

Once somebody's been hardwired on this issue it's EXTREMELY difficult to dig himself back out - even if he doesn't have total shit for brains (which pretty much all of those people do).

http://groups.yahoo.com/group/hhpa/message/11400
Question

Zack C - 2010/11/07 05:40:14 UTC

So even the people you want me to listen to you don't agree with 100%. Is there /anyone/ who agrees with you completely? Do you realize how difficult what you're asking me to do is? You're asking me, a not particularly experienced H3, to discard most of what I've been taught about weak links and the convention that has been accepted at pretty much all aerotow flight parks and competitions and trade it for a practice that very few are endorsing. It's very hard for me to believe that the convention is the convention only because everyone following it is an idiot. I'm going to try to get the other side of the story from those who advocate the 130 lb loops. Everyone here seems to use them only because they've been told to (myself included).

http://groups.yahoo.com/group/hhpa/message/11629

Tad Eareckson - 2010/11/11 23:13:24 UTC

I know EXACTLY how difficult this is for you. Your thinking is EXACTLY what mine was over the course of the two years it took me to fully understand these seven words from Tost:

Weak links protect your aircraft against overloading.

http://www.dynamicflight.com.au/WeakLinks.html
http://www.dynamicflight.com.au/Lockouts.html

You need to totally stop thinking of a weak link as anything that'll keep you safe by releasing you at an appropriate time. It's got one mission and that's to protect the glider from excessive positive Gs applied at or near the hang point and if it needs to kill you to accomplish that it'll to it in a heartbeat. It doesn't give a rat's ass what happens to you or the glider after it takes control over from you after you've demonstrated that you're not up to the job - so you've gotta make sure you NEVER let things deteriorate to that point.

Your brain's been hardwired by a bunch of clueless assholes and it's a total bitch to redo it right. It was the most difficult thing I've ever done in hang gliding and it's totally key to understanding towing. And there aren't a whole lot of people who have what it takes.

Zack C - 2010/12/09 04:21:04 UTC

I think you've got me checkmated. No matter what I say you have a response and at this point I've got nothing left...I have no choice but to accept your position. I've seen so much fallacy from the people that push the universal 130 lb loop that I no longer doubt there isn't a good reason we use them.

This goes back thirty years - mostly to Hewett - when he decided to reinvent aviation for hang glider towing - based upon a bunch of mostly clueless assumptions.

I was carrying a lot of that baggage on Ridgely's first weekend of operation early in the 1999 season. They were having a hard time getting gliders up over 125 feet - which is where my first tow, in position in smooth air, abruptly ended.

Me: Why don't we beef them up?

Chad: The way we put them on the bridle the knot is taken out of the equation so the weak link is 260 pounds and the towline tension is therefore 520 pounds and that's already a lot of stress on the glider.

Me: (Ignoring the fact that all these things were blowing with the gliders straight, level, doing fine, under no stress whatsoever...) Oh.

And then half a dozen years later when I get suspicious of what the then late Chad Elchin had told me, test, and find they're half that figure I STILL don't get it. I STILL "think" I'm using the same crappy little piece of string that Karen and Ayesha are for some sane reason.

Now I just wanna kick myself for being that stupid for that long - especially after experiencing the joy of being able to punch through a little turbulence without having my heart in my throat once or twice during what little was left of my hang gliding career.

[Tad Eareckson]

How do you set the glider at the correct angle on the dolly?

The rule is that your (washed out) wingtips should be horizontal.

At Ridgely all the carts were the same design so I knew the settings - keel support and basetube brackets spacing - for my glider but if I was flying something or helping someone else I wasn't at all above getting someone on the Ridgely crew to eyeball things and go with his call on which notch to use.

If you're gonna screw up then screw up nose high. That leaves you a little more vulnerable to crosswind gusts in nasty conditions before you get up to speed but that's rarely a real issue, the setting becomes irrelevant as soon as the keel lifts out of the bracket, you have two hands on the hold-hold downs, and it's a lesser evil than being pinned to the cart as a result of screwing up in the other direction.

Would it be fair to call this "angle of attack"...

No but...

Towing Aloft - 1998/01

With a paraglider, the problem isn't so much maximizing climbout as it is avoiding over-climbing. With a reel-in or payout winch, the operator must be very careful to avoid towing the paraglider too fast as this action will pull the pilot in front of the canopy. The result will be too high an angle of attack, which in turn causes loads on the canopy to rise and increase the risk of lockout or canopy collapse if the weak link breaks.

...this is hang gliding so you can call things whatever you feel like without getting in trouble with anybody - 'cept Mother Nature.

...or is there a better term?

Yes - if you wanna be a freak and do things right.

Of course (unless the wind is blowing) there is no angle of attack when the glider's on a stationary dolly at the back of the launch line - just a PITCH ATTITUDE (relative to the horizon).

After you:

- are hooked up and start accelerating the wing is initially at a high (stalled - won't fly) angle of attack.

- have developed some speed the keel lifts out of the bracket as the wing trims to the airflow and the angle of attack decreases. At this point the wing is flying but you're not.

As you continue to accelerate you eventually develop enough airspeed for it to be safe to nose up a little (increase angle of attack) and kiss the cart bye-bye.

At this point the tug's still on the ground 'cause it needs more airspeed than you do so you have to pull back in and stay low for a little while to make things easy for your driver until he lifts off.

When you're towing you're generally pulling in a bit or have the keel attachment far enough fore to trim for tug speed and you tend to think you have your nose pretty low.

But when you're free flying and look sideways at a Dragonfly pulling another glider up it's astonishing to see how high those wings are pitched up.

And, of course, to most of the idiots in this sport (including, to some extent, the ones who wrote Towing Aloft), this means the glider's flying at a high angle of attack. It ain't. It's flying at a high pitch attitude but a low angle of attack.

If, however, the 130 pound Greenspot blows or the tug driver suddenly makes a good decision in the interest of your safety...

http://groups.yahoo.com/group/TTTFlymail/message/11545
Cart stuck incidents

Keith Skiles - 2011/06/02 19:50:13 UTC

I witnessed the one at Lookout. It was pretty ugly. Low angle of attack, too much speed and flew off the cart like a rocket until the weak link broke, she stalled and it turned back towards the ground.

...you have a high pitch attitude AND a high angle of attack.

A lot of people believe the nose "pops" (up) when the weak link blows. It doesn't. It IMMEDIATELY starts trimming down in response to the new force vector situation - but it sure feels like it does.

And one would think that after thirty years of stalls and broken downtubes people would start figuring out that random abrupt increases in angles of attack are bad things but - hey - this is hang gliding and those are our primary means of keeping ourselves safe.

[Zack C]

While I avoid the usage myself, I've never really had an issue with people referring to the glider's angle on the dolly (or platform) as its 'angle of attack' since it is, after all, the glider's angle of attack at the time the tow commences.

While we're on this subject...assuming a constant angle of attack (with constant glider and wind velocity), how is a glider's pitch attitude affected by vertical air movement? It seems like a glider in lift would have to have a lower pitch attitude to maintain the same angle of attack as in still air, but I find this counter-intuitive considering that normally (under its own power or on tow) a plane has a higher pitch while climbing.

Zack

[Tad Eareckson]

...since it is, after all, the glider's angle of attack at the time the tow commences.

Unless the wind is tailing.

I avoid using "angle of attack" like the plague unless I'm actually talking about angle of attack 'cause the misuse drives me crazy and does contribute to the crash rate - 'specially to the 130 pound Greenspot and tug drivers making good decisions in the interest of your safety related crash rates.

While we're on this subject...assuming a constant angle of attack (with constant glider and wind velocity)...

- Constant "wind" - as I'm sure you know - is only relevant in terms of ground track - and the higher you are the harder it is to figure out what the hell the wind's doing.

- Constant angle of attack and constant glider velocity is redundant. If you know your angle of attack you know (or can figure out) your airspeed - and vice versa.

- If there's a CHANGE in the wind - like we commonly experience sinking down or towing up through gradients - the glider's angle of attack (airspeed) will change until it re-trims or we do something with the basetube. If we stall upwind and are dealing with a gradient we're probably screwed. If we stall downwind the gradient's gonna work to our advantage and give us more airspeed with decreasing altitude - but we better have a good pair of wheels or be ready to run by the time we reach the surface.

...how is a glider's pitch attitude affected by vertical air movement?

- Lift is wind. Unless you're better tuned into things than I am it can get pretty tough to tell if you're going up or down when you're high - especially over the flatlands. That's the main reason Flytec makes the money it does.

- Let's make the lift / vertical air movement wide, smooth ridge lift. You'll climb in the upward flowing wind with exactly the same angle of attack, airspeed, pitch attitude, bar position as you would if you were sledding to the LZ in dead conditions.

It seems like a glider in lift would have to have a lower pitch attitude to maintain the same angle of attack as in still air...

- Let's say you're flying at trim in whatever steady or no wind you like at Columbus - constant angle of attack, airspeed, pitch attitude, bar position. You suddenly get slammed by a big wide column of upwardly moving wind from below (sometimes referred to as a "thermal"). You're gonna shove the bar back 'cause your angle of attack will have just gone through the ceiling and you don't wanna stall. (You've done this.) The glider will ACCELERATE upwards, things will stabilize, you'll ease the bar back out. And, if the thermal's smooth and fat enough and you don't turn, you'll resume flying at the same angle of attack, airspeed, pitch attitude, bar position as before - until you fall out the other side.

- So during the acceleration - yes. After stabilization - no.

...but I find this counter-intuitive considering that normally (under its own power or on tow) a plane has a higher pitch while climbing.

- It's climbing due to THRUST (its or somebody else's). Thrust is like an extra force of gravity with a forward (totally for aerotow, partially for surface) component and hopefully enough in the way of pounds to allow you to climb. The plane is trimming to the resultant of the gravity (down) and thrust (forward) force vectors and that'll be nose higher than normal.

- If you run out of gas or your 130 pound Greenspot decides to make a preemptive strike against a lockout it thinks you might have if it doesn't - the way it did for Jonathan - you'll trim back down to just gravity (assuming you don't hit the runway first).

- In dead air - with or without power - if you're pitched down you're going down.

- If you're in a mass of rising air you can be pitched down and still be going up.

- So it's the difference between being LIFTED up BY the medium and (let's make it its own power or aerotow) being PULLED up by something PUSHING OFF OF the medium.

That work?

[Steve Davy]

Wouldn't the pitch attitude have to be set VERY low in order to become stuck to the cart?
At some point the keel comes up as the glider trims, so the initial pitch attitude would have to be lower than that, correct?
Also wouldn't a driver be able to note the bar position he had used in the past as a reference for his attitude?
If the pitch attitude was set too low would it not be obvious?

[Zack C]

Unless the wind is tailing.

How did I know you were going to say that...

To me it's understood that when they say 'angle of attack' they mean in a headwind (and one that's not so strong the keel's already lifting). It's sort of like when people talk about a glider's 'best glide speed' it's understood they mean in still air. I do agree that misunderstanding the concept of angle of attack could have serious consequences and would correct any such misunderstanding were it apparent.

Constant "wind" - as I'm sure you know - is only relevant in terms of ground track...

Apparently I don't know, or I wouldn't have asked the question. =)

What tripped me up was I started thinking about what would happen in 1000 mile an hour lift, and it seemed like the glider would have to nose down pretty hard to keep a sane angle of attack with that kind of relative wind. But of course (discarding acceleration, as I prescribed) the glider will be moving upward at 1000 miles an hour minus its sink rate so the relative wind will still only be slightly from below. I wasn't thinking.

Thanks for clarifying.

Zack

[Tad Eareckson]

Wouldn't the pitch attitude have to be set VERY low in order to become stuck to the cart?

I was a lot more scared of aerotow dolly launching than I needed to be when Ridgely stared up. Five years prior I had launched in a crosswind for a step tow. I came off too soon, got rolled a bit, and Mike Robertson "helped" me out by gassing it. I didn't really understand what had happened on that one until I reviewed my log five months ago and I was expecting that stuff like that would be a regular feature of Dragonfly towing. It took me several years to realize that stuff like that was NEVER happening.

There were some problems with Ridgely's early brackets design which made them a bit sticky and I was getting a bit stuck. They modified them and I was still getting a bit stuck. After I while I figured the problem was that I was anticipating a problem, becoming anxious, and creating one - don't ask me how. Self fulfilling prophesy thing. After that I started relaxing and feeling things out and from then on always had clean separations. But I guess I stayed a little gun-shy on dolly issues.

And you probably don't wanna push your luck too much on the tail high end for a couple of reasons.

At some point the keel comes up as the glider trims, so the initial pitch attitude would have to be lower than that, correct?

- When Julia Kucherenko got smashed up at one of Davis's little funfests on 2011/05/11 it was found that the bracket had been banging and denting her keel as a consequence of bounce while the cart was rolling on the field.

- You also want a bit of daylight back there so you can push out enough to clear the basetube brackets.

I suppose you could stay on long enough to pull a wheelie - like some folk do - and make that a non issue, but that makes cart recovery more of a hassle and I always tried to come off as soon as I had safe airspeed out of sadly misplaced consideration for the Ridgely crew. (Same way I pinned off as soon as I thought I had something I could hook. No good deed...)

Also wouldn't a driver be able to note the bar position he had used in the past as a reference for his attitude?

Yeah, he should. Bar out in front of his face.

If the pitch attitude was set too low would it not be obvious?

Yeah, and if people used releases they could actuate when they NEEDED TO a high tail rollout could be aborted with less hassle and danger than is encountered when the Rooney mandated 130 pound Greenspot lockout protector kicks in to protect someone - like Jonathan.

[Tad Eareckson]

It's sort of like when people talk about a glider's 'best glide speed' it's understood they mean in still air.

I'd go with "by definition" over "understood". Best glide / max LD is like stall speed - wind is NEVER a consideration.

What tripped me up...

Try talking about balloon drops with some folk.

"Shouldn't you have your nose pointed into the wind when you release?"

"It's a balloon. It's PART of the wind. There is no wind."

"But shouldn't you turn the glider into the wind AFTER you release?"

"You're in the basket at ten thousand feet. The wind is blowing a hundred miles an hour from the west. You light a cigarette. Which way does the smoke go?"

[Zack C]

Best glide / max LD is like stall speed - wind is NEVER a consideration.

I always thought 'best glide speed' was the airspeed that resulted in the maximum distance over ground, which is only going to be max L/D speed in still air. A quick search found an airline pilot forum discussion that seems to confirm this:
http://www.airlinepilotforums.com/archive/index.php/t-48233.html

Zack