[Rick Masters]

...positive pitch recovery characteristics

Please explain why this is important and what you did, in detail.

[Frank Colver]

Aerodynamic engineers please forgive any terminology or minor detail errors here.

As the angle of attack of an airfoil is decreased, in the case of a hang glider by the pilot moving forward, the center of lift moves back. If it moves the CL behind the center of gravity it is now forcing the wing into an ever decreasing angle of attack. This now becomes negative pitch stability and if some other force doesn't counteract it, the glider will continue to increase its dive and can even go inverted. With the old "standard" Rogollo this effect was even accentuated by there not being any forced camber in the airfoil (ribs, curved battens, etc.) so the airfoil of the wing would flatten or even invert which would further increase the negative pitch forces.

SO, an opposing force is needed to counteract the tendency for the wing to continue to pitch down once it has passed a certain lowering angle of attack point. Many aircraft do this by providing a horizontal stabilizer "tail plane" usually behind the wing. This is positioned at an angle of attack that provides a downward force trying to increase the wing's positive angle of attack. As the pilot tries to decrease the wings angle of attack, to fly faster or to dive, the tail plane is working against that lowering of AT. The more the angle of the wing is decreased the more the tail plane forces try to counteract it. This is commonly called positive pitch stability.

Most hang gliders are "flying wings" which don't have a tail with a horizontal stabilizer so this negative pitch opposing force has to be developed in another way but the idea is the same. A reflexed airfoil may be used in which the positive curve behind the leading edge is followed by a negative curve ahead of the trailing edge. Another method is to twist the tips of the wing upward toward the trailing edge. Often a combination of the two with variations is used. Most flex wing hang gliders flying today have sweep back toward the tips and the tips are twisted upward and even have wires to the king post to maintain that twist under all load conditions. Topless gliders have to have it built into the wing structure.

Whatever method is used the idea is to produce a downward rotating force behind the CG to counteract the upward rotating force behind the CG the CG because of the moving center of lift. Try to think of it as a teeter totter with one person as opposed to a teeter totter with a person at each end. The stabilizing force will also try to keep the wing from rotating upward beyond a certain angle of attack. So now we get to "trim". When all of the pitch rotating forces including CG are in equilibrium the wing will want to stay at a fixed angle of attack and the glider is referred to as flying in trim.

On my Skysail (pictured in my earlier post) I used a combination of sweep back with tips twisted upward and an inverted airfoil at the tip to further increase the downward forces. The lift of this tip airfoil is in a downward direction which decreases the amount of twist needed to stabilize the wing.

NOTE that all passive methods to stabilize pitch in an aircraft reduce performance so in the early days of modern (and maybe old) hang gliding, some pilots and builders reduced the amount of twist and/or reflex in their wings and got noticeable improvement in glide performance until one day they allowed the wing to move below that critical angle of attack. Not having provided enough built in dynamic forces to try to force the LE back in a positive direction, the pilot died.

A personal experience I had with this situation was in the 1970's with a rigid wing, using sweep and tip twist for pitch stability, which was being produced in Southern Calif. A guy was flying one at Escape Country and I had seen it there several times. One day, when he showed up, I saw that the tip twist had been greatly reduced. I was concerned and spoke to him at the launch and asked him if he had reduced the twist and he said the factory had done that. I asked him if the factory had tested the wing with the reduced twist and he said they hadn't, but they told him the glider would fly better this way. I watched him launch and fly down to the LZ. It was one of the scariest flights I had seen. The glider was so sensitive to pitch control that the pilot never did fly it smoothly. He was constantly trying to find the trim position, moving back and forth. Not long after that he died in a non-recoverable dive while flying from Blue Mountain.

Does this long disertation answer your question, Rick?

Frank

[Rick Masters]

I've been keenly interested in how hang glider designers, who were largely self-taught, conquered divergence in the middle 1970s. Most of the guys who built rigid wings seemed to have a pretty good understanding of designed-in reflex long before the rogollo fliers figured it out.
        In a Facebook post to Steve Plonk, Dave Cronk says "I have attached a few images of the wing I did in 73. It was flown extensively by myself and others, and it flew pretty well. Influences were Richard Miller and, of course, Frank's beautiful wing."
https://www.facebook.com/permalink.php?story_fbid=1418390651634012&id=1353742711432140
        I assume Dave is referring to your Skysail. Is this correct? In the page he supplied (above), we can clearly see the pronounced trailing edge reflex in the photo of the Cronksail 1. How did the idea of trailing edge reflex on rigid flying wing fabric hang gliders start? Was it a well-understood aerodynamic engineering concept or a result of your own (and your contemporaries') trial and error? Who showed up with it first on a rigid wing?

A guy was flying one at Escape Country and I had seen it there several times. One day, when he showed up, I saw that the tip twist had been greatly reduced. I was concerned and spoke to him at the launch and asked him if he had reduced the twist and he said the factory had done that. I asked him if the factory had tested the wing with the reduced twist and he said they hadn't, but they told him the glider would fly better this way. I watched him launch and fly down to the LZ. It was one of the scariest flights I had seen. The glider was so sensitive to pitch control that the pilot never did fly it smoothly. He was constantly trying to find the trim position, moving back and forth. Not long after that he died in a non-recoverable dive while flying from Blue Mountain.

That was Don Fielder, HG fatality # 88. He bought the second production model of the Sundance from Jeff Magnan. He hadn't been flying it for very long when he was killed on March 22, 1975. The accident was apparently an example of classic divergence, with a near vertical dive to impact that was unresponsive to anything the pilot did. Howard Long, the first Mitchell Wing pilot, sent the accident report in to USHGA Accident Chairman Robert V. Wills.

As the angle of attack of an airfoil is decreased, in the case of a hang glider by the pilot moving forward, the center of lift moves back. If it moves the CL behind the center of gravity it is now forcing the wing into an ever decreasing angle of attack. This now becomes negative pitch stability and if some other force doesn't counteract it, the glider will continue to increase its dive and can even go inverted.

[Frank Colver]

Yes, Dave was referring to my Skysail.

I'll add some more info about what the Rogollo flyers did to improve dive recovery of their wings.

One thing done by everyone who lived to fly again was to bend the keel upward toward the aft end. This introduced reflex to the center portion of the wing.

ANOTHER THING THAT WAS VERY IMPORTANT WAS TO MAKE SURE THAT THE HEM ALONG THE TRAILING EDGE DID NOT BEND DOWNWARD. Some "standard Rogollo" pilots discovered that their glider would glide flatter if they turned the TE hem downward. They paid for this better performance with their lives.

The Best improvement to dive recovery by the "standard Rogollo" was John Lake's "Sailfeather". This was a tiny triangular "sail" that mounted between the guy wires running from the control bar to the aft end of the keel. Since the wires went up to the keel this forced the Sailfeather to also angle up and it became a stabilizing tail plane as in a familiar aircraft horizontal stabilizer. The sail tension could be adjusted by a cord so as to adjust the amount of downward force it produced in normal flight. The Sailfeather could be easily mounted on any Rogollo type glider. Once John came up with this design I never flew without it on my Eipper FlexiFloater. It even made pitch control during launch easier. Paul MacCready once told me that John's Sailfeather was the most important improvement ever made to the Rogollo wing.

I tested the Sailfeather concept by building a large scale model "standard Rogollo" and dropping it in various degrees of dive from a second story window. I had a small amount of reflex in the keel. I could drop it in dives to a point and it would pull out. As the launch angle got steeper it would take longer to pull out and at a certain down angle it would impact the ground before pulling out to level flight. Then at a steep enough angle it would just dive straight to the ground.

I then added a scale Sailfeather and not only would the glider pull out from any straight down vertical launch dive I could even launch it inverted and it would pull out before reaching the ground!!!!

I came into hang gliding already aware of these pitch stability and dive recovery requirements from a history of model aircraft building and design, as I was growing up. I especially liked all wing designs so I had built and flown those "tailless" types of models. Before building the full size Skysail I built and flight tested a 5' scale model of my design. I still have that model to this day and I've shown it at anniversary and vintage HG events. It is somewhat beat up but it still flies! I wish I still had my full scale Skysail but it burnt up in the San Diego Aerospace Museum fire many years ago.

BTW - Rick, thanks for posting the Cronk Sail info. I was going to do that but I figured I had already written enough to put the reader to sleep. A couple of years ago I asked Dave if he still had that and/or the Cronk Kite. He replied that he had neither.

Good subject! I've attached the 3 view of my Skysail on a grid of 0.20". The glider was built in 1971 but I didn't draw the 3 view until '73, when someone asked for it.
Frank

Skysail 3 view 1-10-73.jpg (348.75 KiB) Viewed 7178 times

[Rick Masters]

Paragliding is a completely different sport and should not be associated with hang gliding.






Form a national hang gliding association.
Leave paragliding behind.

[JoeF]

This seemed to have an affinity for the topic:
http://www.bendigoadvertiser.com.au/story/5153352/life-jacket-fails-safety-testing/

[Rick Masters]

The joy of paramotoring        July, 2017

You can't do this with a recreational hang glider.
Mounting a propeller and engine on your back below your paraglider
- and then tripping on your take off run - can get the job done, though.
While it is possible to do this on a powered hang glider,
usually you need to be careless in an engine test before you strap in,
as the prop is out of reach of the pilot.
At good ridgelift or thermal venues, a non-powered hang glider (or paraglider)
can stay aloft longer than a powered one can before it runs out of gas.

[Rick Masters]

https://player.vimeo.com/video/135561989

[Rick Masters]

VIDEO: http://video.dailymail.co.uk/video/mol/2018/01/22/4866737117952586484/1024x576_MP4_4866737117952586484.mp4

You don't fiddle around with a hang glider when you launch.
You don't struggle or argue or fight.
You embrace her and feel her tremble and sigh, responsive to your touch.
You pay attention to the air ocean you are about to enter.
You don't turn your back to its waves, lest they sweep you away.
You don't make a bunch of ridiculous and distracting choreographic* moves at the critical moment of take off.
You stand firm and focus on the sky that beckons you.
You stand firm and focus on the movement of air coming up the hill towards you.
A gust? A thermal? A lull? The devil?
Your hang glider is completely ready to go, trusting you to guide her.
You stand there, confident, in quiet excitement, feeling her fly on your shoulders
    like an eager and wondrous and powerfully fleet, obedient gryphen about to be released from her cage.
You see the grass ripple.
You watch the birds.
You llsten to the words of the wind.
Sometimes you wait for signs of a thermal teasing the bushes.
You wait for the right moment, when the wind feels perfect,
    with your fabulous wing already flying, inches above you, responsive to your every command.
You trust her.
Then you just take a few steps and fly away into heaven.
You are joyously and instantly and smoothly transformed into an ethereal being.
There is no drama.
Drama is for idiots.



___________________________________________________________
*choreography
NOUN
The sequence of steps and movements in dance or figure skating, especially in a ballet or other staged dance.
- examples from the Oxford English Dictionary -
‘A simple piece with meaningful choreography that your dancers can perform well is better than a flashy number that's beyond their abilities.’
‘The choreography, music, lighting and most of all the dancing all combined to convey this pain.’
‘His choreography surrendered to gravity and dealt in angles and broken lines as well as broken phrases.’
' If anyone finds the Croc I lost, hit me up.'    http://www.dailymail.co.uk/news/article-5296131/Paraglider-whipped-air-slammed-ground.html
                

[Rick Masters]

Why so many more soaring parachutists die compared to hang glider pilots



A member of the search and rescue team regards the corpse of a soaring parachutist who fell vertically to his death following a collapse.

"He fell flat on the ground. We do not know the reason for the fall, but he was using all the necessary safety equipment," said the rescuer.

Of course, all heavier-than-air aircraft can stall, but soaring parachutists often die in vertical falls unrelated to stalling. Hang glider pilots rarely die in vertical falls unrelated to stalling.

Many years ago, I encountered sudden sink at Plowshare and landed (well, slowly crashed) on the side of a mountain much like this.
The limbs of the bushes grabbed at my flying wires, control bar and leading edges.
I came to a stop like a softball into a catcher's mitt.
No damage to the hang glider.
Not a scratch to me.

Hang gliders are more difficult to learn to fly, but its worth it.



Kenyans reach the bodies of Thomas Lednik (PG fatality #1662) and Kimberly Ann Pace (PG fatality #1663), yesterday, two weeks after Gerd Schegler (PG fatality #1661) fell to his death after launching from the same site.
All died in vertical falls following a collapse in turbulence.
I would think intelligent people would have figured out why this is happening long before now.
Paragliders collapse in turbulence and kill their occupants in vertical falls.
Its not complicated.



"If we continue the trend, we are going to get a lot of new fatalities," said a police supervisor. "That is ridiculous."
Paragliding is now banned at the Kiero Escarpment.