Weak links

[Tad Eareckson]

Alright, I'll give it to you - but a little grudgingly.

Gave your link just a quick skim 'cause through hang gliding I've known too many airline, military, and other flavors of professional pilots to be very impressed with their understandings of high school physics and I've been pretty sick for most of this week and my headache was bad enough to begin with.

So...

http://en.wikipedia.org/wiki/Polar_curve_(aviation)

Effect of wind, lift and sink on best glide speed

The effect of wind, lift and sink on best glide speed is to move the curve within the plot by the amount of each component...

But I don't really like it 'cause I:
- have always used "best glide speed" interchangeably with "max L:D" - and max L:D doesn't care about wind
- would argue that you're not measuring "glide" when you throw air movement into the equations

If you take a chopper up to ten grand, use a GPS receiver to hover over a fixed point, and drop a bowling ball - whose max L:D is zero to whatever - then head, cross, tail winds and lift and sink will have effects on where it comes down.

So let's say there's a big hole in a barn roof a quarter mile east of the drop point.

Did the bowling ball glide there? Or was it pushed there?

Change it to "best range" and I'm happy.

[deltaman]

Hi Tad,

I'm back to understand this.. May you help ?

A glider (or any plane) banked sixty degrees in a coordinated turn is pulling two Gs.

Does that mean that the towline will feel 2Gs in this configuration (coordinated turn) ?

Thanks.

[Tad Eareckson]

No, you can't predict what the tension's going to be based upon what the glider's doing without knowing what's going at the other end of the towline.

I'm assuming you mean in the course of an aerotow.

(And, of course, during a truck tow (payout winch) you can do whatever you want and the tension will stay pretty much the same.)

So are you turning away from or back towards the tug?

Is the tug trying to maintain heading or reacting to what you're doing?

If you're turning away from the tug and everything else is normal/stable then the tension WILL go up but the tug WILL be slowed so the rate of tension increase will be considerably dampened.

http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

Jim Rooney - 2011/08/26 02:44:10 UTC

The forces of an aerotow can get high enough to tear the wings off the glider.
This is no exaggeration... it can be done.

Which is one of several reasons why statements like this from assholes like this are bullshit. If your weak link were two thousand pounds you'd still have to get pretty creative and try pretty hard to break the glider - although you COULD raise hell for tug control if he were stupid enough to leave you on for very long.

As far as I know, no certified glider has EVER been broken on ANY kind of tow under positive loading - and that includes the era when nobody was using weak links.

And, of course, you're using a weak link. So the only way the towline can be feeling two Gs is if you're using a weak link at the top of the US legal range.

Furthermore... It's a little hard to imagine a good reason for a glider to be pulling a sixty degree coordinated turn away from a tug (except maybe Rooney or Ryan trying to actuate one of their instant hands free releases).

But I DO like to plan/design for worst case scenarios and the possibility of a glider being loaded to two Gs by what's going on with a turn plus whatever's coming through the towline is what makes me most happy with a one and a half G weak link.

Planes are generally designed to handle six Gs or more but not expected to be operated at or experience over three.

Three and a half Gs - a turn plus the weak link - I'm not too worried about. Above that I'm starting to think more about wires and downtubes which may be a little worse for wear than they were when they left the factory.

[deltaman]

clear as usual.

but when we say "G" for wl resistance, it's compared to the maximum total load certified
and when we say "G" on the tow line or during a turn, it is compared to the actual load of the glider
That's right ? It's not exactly the same values..

[Tad Eareckson]

Right.

When a sailplane manufacturer specifies a weak link for a particular model, the worst case scenario - glider loaded to its max flying weight - is being assumed. If you knew that the glider would never be loaded to more than a hundred pounds below its certified rating you could specify a weak link a hundred pounds heavier and get the same stress protection.

Similarly, in free flight it will be a lot harder for someone at the bottom of the hook-in weight range to break the glider in aerobatics than someone at the top.

To a hang gliding person a weak link is a lockout protector so he's always talking about the ACTUAL flying weight REGARDLESS of the glider model and its rated capacity.

So when you read something like this:

The United States Hang Gliding and Paragliding Association, Inc.
Standard Operating Procedure
12. Rating System
02. Pilot Proficiency System
12. Hang Gliding Aerotow Ratings
-C. Aero Vehicle Requirements

4. The weak link at the glider end must have a breaking strength that will break before the towline tension exceeds twice the weight of the hang glider pilot and glider combination.

you know that it was written by an idiot.

[Tad Eareckson]

http://www.kitestrings.org/post517.html#p517

Damn.

The BAYNET.com

Agency Posts Preliminary Report on Glider Wreck

Marty Madden - 2011/07/21

The National Transportation Safety Board (NTSB) has issued a preliminary report on the Friday, July 15 glider crash that resulted in the death of a St. Mary's County man and injuries to a Calvert County man.

The accident occurred at approximately 3:35 p.m. near St. Mary's County Regional Airport in Hollywood.

The aircraft, a Slingsby T-49B glider, sustained substantial damage when it collided with trees while maneuvering for landing. The preliminary report indicates the glider's occupants were attempting to make an emergency landing in the median of Route 235 but missed the mark.

The NTSB report confirms that the survivor of the crash, identified by the Maryland State Police (MSP) as Nicholas John Mirales, 52 of Prince Frederick, was piloting the glider when the accident occurred. The craft's copilot, identified by the MSP as James Michael Dayton, 55 of Mechanicsville, sustained fatal injuries as a result of the crash and was pronounced dead at the scene.

"The glider pilot provided a comprehensive written statement and a brief interview following the accident," the report stated. "According to the glider pilot/owner, the glider was purchased a week prior to the accident and he has accrued about one hour of flight experience in it. He assembled the glider and completed all post assembly checks prior to departing. The pilot and copilot then performed the before-takeoff checks outside the aircraft and confirmed the tow release operation and confirmed 'trim and spoilers closed.'"

The NTSB report stated that during the craft's initial climb, "the glider pilot noticed the glider 'wasn't climbing.' About 100 feet above ground level and over the trees beyond the departure of the runway, the glider pilot observed the tow plane rudder 'waggle' back and forth, and his copilot shouted 'release, release, release!' the glider pilot pulled the release handle, released the glider from the tow and entered a left turn to the north for a forced landing on the north/south divided highway east of the airport. The glider overshot the highway and collided with trees on the east side of the roadway."

The NTSB report stated the pilot of the plane towing the glider provided a similar account of the events leading up to the tragedy. "He observed the glider spoilers were at least partially deployed above and below each wing and rapidly 'wagged the rudder,' to alert the glider pilot to 'check his spoilers.' At that moment, the glider released from the tow, banked to the north and struck trees adjacent to the highway."

Peter Knudson, a spokesman for the NTSB, said a final report on the accident could take as long as a year to complete.

The glider did not crash shortly after takeoff when it "BECAME" disconnected from its towing plane.

[deltaman]

Three and a half Gs - a turn plus the weak link

Does that mean that "Gs" felt in turn are not felt in the towline ?

If not, is that why you don't go further that 1.5G (wl) ..to never exceed 3 or 3.5Gs on the glider, values you think as a maximum to experiment in flight?

[Tad Eareckson]

Does that mean that "Gs" felt in turn are not felt in the towline ?

In that example the towline is feeling (very briefly) - as a consequence of the turn - the maximum tension the weak link will allow.

In theory, the weak link doesn't guarantee that you can't break the glider on tow. You can break the glider in free flight if you try hard enough and if you were really good and really creative you might be able to figure out some way to break it while still connected to a tug through a legal weak link.

And it's also possible to have a two G weak blow when you're standing on your tail, going up like a rocket, and needing two and a quarter Gs to ride out the climb and live. But...
- You:
-- probably have a bigger risk of being hit by an asteroid
-- can use ribbons along the runway to make sure you don't get into a situation like that
- The air that could create a situation like that is going to kill you anyway if you don't hit it exactly right - and nine out of ten times you won't.

Also note that even with virtually the entire world population of solo gliders towing with those stupid PATHETIC loops of 130 pound Greenspot we virtually never see anything more serious than downtube breaking crashes if the tug and glider are using good releases and doing everything else reasonably well.

If not, is that why you don't go further that 1.5G...

In the real world if you fly one and a half Gs and don't do anything extremely stupid (like putting a release lever on a downtube) you will never:
- have a serious problem
- come anywhere NEAR to maxing out your weak link

And if you go much higher than that you will have to start worrying about:
- beefing up the releases and materials and hardware you're using to connect things to your harness and glider
- what's going on with things at the front end.

And on the goddam Dragonfly with that idiot tow mast breakaway that Bobby built in you had to worry about what's going on with things at the front end a long time ago.

...values you think as a maximum to experiment in flight?

There is NOTHING that can be learned by putting tow system components into the air - none of this stuff functions any differently at two thousand feet going 35 miles per hour than it does on the kitchen table at home. EVERYTHING you need to know can and MUST be determined on the ground through load testing, mathematical models, logic, and ten year old kid common sense.

You can learn more in a couple of minutes with something like:

http://www.flickr.com/photos/psucvollibre/6055817574/


or:

http://www.flickr.com/photos/aerotowrelease/8317889807/


than a couple hundred professional pilot shitheads can in a dozen lifetimes.

And...

http://www.hanggliding.org/viewtopic.php?t=21377
AT several Questions

deltaman - 2011/04/04 07:14:09 UTC

I won't travel in US to learn where to put wl on the V-bridle system.

...you won't have to travel to the US to help get things figured out.

[Tad Eareckson]

The NTSB report stated that during the craft's initial climb, "the glider pilot noticed the glider 'wasn't climbing.' About 100 feet above ground level and over the trees beyond the departure of the runway, the glider pilot observed the tow plane rudder 'waggle' back and forth, and his copilot shouted 'release, release, release!' the glider pilot pulled the release handle, released the glider from the tow and entered a left turn to the north for a forced landing on the north/south divided highway east of the airport. The glider overshot the highway and collided with trees on the east side of the roadway."

The NTSB report stated the pilot of the plane towing the glider provided a similar account of the events leading up to the tragedy. "He observed the glider spoilers were at least partially deployed above and below each wing and rapidly 'wagged the rudder,' to alert the glider pilot to 'check his spoilers.' At that moment, the glider released from the tow, banked to the north and struck trees adjacent to the highway."

FAA Glider Flying Handbook

Figure 7-2. In-flight aerotow visual signals.

Something Is Wrong With Glider - Close Airbrakes!
(Towplane fans rudder.)

Glider: Release Immediately!
(Towplane rocks wings.)

Bill Bryden - 1999/06

Rob Richardson, a dedicated instructor, died in an aerotowing accident at his flight park in Arizona. He was conducting an instructional tandem aerotow flight and was in the process of launching from a ground launch vehicle when the accident occurred.

Rob had started to launch once but a premature towline release terminated this effort after only a few meters into the launch roll-out. It is suspected the cart was rolled backwards a bit and the towline was reattached to begin the launch process again. During the tug's roll-out for the second launch attempt, the tug pilot observed the glider clear the runway dust and then begin a left bank with no immediate correction. At that point he noticed that the launch cart was hanging below the glider and immediately released his end of the 240 ft. towline. The tug never left the ground and tug pilot watched the glider continue a hard bank to the left achieving an altitude of approximately 25 feet. Impact was on the left wing and then the nose of the glider. Rob was killed immediately from severe neck and head trauma.

http://www.chgpa.org/forums/viewtopic.php?f=2&t=1079
$15 pacifiers

Jim Rooney - 2005/09/22 14:05:50 UTC

Sure, being on tow at the wrong time is an extremely bad thing. But don't tell us that being off tow at the wrong time is all sweet and wonderful. Yes, we prepare for it, but that doesn't make it a safe situation. It makes it a manageable situation. There are times where it's better to be on tow than off tow. Ask anyone that's dragged a dolly into the air.

http://www.chgpa.org/forums/viewtopic.php?f=2&t=2467
weak links

Jim Rooney - 2007/08/01

Whatever's going on back there, I can fix it by giving you the rope.

It's more of this crappy argument that being on tow is somehow safer than being off tow.

Peter Knudson, a spokesman for the NTSB, said a final report on the accident could take as long as a year to complete.

Why?

[Tad Eareckson]

To Tad Eareckson:

Steve Kinsley - 2006/11/23 03:38:11 UTC

I have never seen a weak link used towing sailplanes. But I only know aero tow. Maybe if you winch tow you could build up enough force quickly enough that an actual weak link would be useful.

I saw something recently (probably something that you sent me) where a hang glider pilot had a weak link inside of a weaklink like the Tost system to avoid the sudden gust/bump break.

Because I fly with a release that can be activated in milliseconds, I think a weak link is a pain in the butt that adds little or nothing to my safety. Not sure that this is the case for everyone though. It might have helped Holly. I can see it taking 5 seconds to find and activate a shoulder release. That's a long time. So is 3 or 4 seconds. I think there are a lot of cases where weak links go before someone can find and activate a release.

I fly with one because I suspect I wouldn't be permitted to launch w/o one.

Sailplanes are a piece of cake to tow. Much easier than a hang glider. I dunno about having your hand on the release. Maybe ok if it requires a long pull -- most do. But why? Maybe if you are not familiar with the aircraft. The main concern is an inadvertent release. Only had two incidents where I had to release early. One a tow plane blew the engine. Second one I was rear seat and trying to find something on a map when we took off with the brakes/spoilers open and could not even given two brains working on the problem figure out why we were in brick mode.