Luff in the time of cholera


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Luff in the time of cholera

…he flew off a 700-ft. hill and flew down 600 ft. diving faster and faster the whole way, ending it up in a 100-ft. vertical luff dive, and landed in a large oak tree. The tree sprung so severely that it flung him back out like a giant trampoline. He ended up with a couple of bruises and scrapes…

— Hang glider designer Terry Sweeney interviewed in the USHGA magazine Hang Gliding, August 1977

Flying the Skyhook IIIA standard Rogallo hang glider in early 1975

The piece of fabric stitched between the rear rigging wires by the keel, visible in these two photos, was my half-hearted attempt at a device known as a sail feather. Standard Rogallos were thought by some to become unstable in pitch in a steep dive because, with the air flowing parallel to the airframe, the sail deflated and ceased to provide lift, which is essential for control in a weight-shift controlled aircraft. Several fatalities in the USA had been attributed to luffing dives. The idea was that the fixed sailfeather, small though it was, would act as an up-elevator, preventing such an extreme situation developing.

Incidentally, the piece of webbing across the top of the control frame (it was attached by hose clamps) made carrying the glider less painful on the shoulders. And, although it might look as though the rope goes around my neck, those lines of my cobbled-together prone harness went all the way to the thick plywood seat (worn on the front).

Experimental prone harness in March 1975

Experimental prone harness in the back yard, March 1975

However, in the April 1975 edition of the USHGA magazine Ground Skimmer, top American pilot Chris Price pointed out that, as long as the sail begins luffing from the trailing edge, the glider remains pitch stable, partly because of the drag caused by the flapping sailcloth. Exactly that was demonstrated by Chris Wills in the 1976 movie Sky Riders.

Pitch stability of this kind, where in a dive the sail begins luffing from the trailing edge, was (as far as we could determine) provided by a slight upward (concave) bow in the keel tube, itself fixed by the fore-aft flying wires being slightly long and, importantly in this scenario, the fore-aft upper rigging wires being slightly short. That flex (called reflex) in the 18 ft long tube was only an inch or two. Of course, the tube itself has no direct aerodynamic effect, but the large area of adjacent sailcloth, particularly at the trailing edge ten feet behind the centre of mass (that is, behind the pilot) acted as a crude equivalent of a tailplane rigged at a lower angle of incidence than the wing. Bear in mind that the root of the sail of a standard Rogallo, at the keel, had no camber at all. The large inboard part of the wing provided more pitch stability than lift, as far as we knew.

…in a FULL luff dive the pilot is at zero Gs–he has no weight. The sail has no lift. The sail has lost its reflexed shape. There is generally a slight restoring force in the fact that the center of drag of the system is a few inches above the center of gravity. But this requires airspeed and lots of it! Furthermore, the force is small and minor misalignments can overwhelm it.

— John Lake, inventor of the sailfeather, writing in Ground Skimmer, July 1975

In an opposing view, Chris Price argued that most, if not all, fatal luffing dives were actually side-slips, where a sailfeather would be unlikely to be effective. Ultralight Products jumped in on the same side as their rival manufacturer, Wills Wing:

It took all of Bob’s strength to hold the glider in a dive as its natural tendency is to pull up. This phenomenon is totally in contrast to the self-styled “experts” saying that the glider would continue on into the ground.

— Pete Brock of hang glider manufacturer Ultralight Products writing in Hang Glider magazine, spring 1975. The Bob he refers to is Bob Wills.

Still from 'Sky Riders,' 1976

Chris Wills demonstrates the Wills Wing Swallowtail’s resistance to the luffing dive in the 1976 movie ‘Sky Riders’

Clearly, sufficient pitch stability was essential in preventing entry to an unrecoverable dive. Bob Williams wrote in to Ground Skimmer, August 1975, reporting that, where his fore-aft rigging had become loose, his glider dived 50 feet after a stall. After re-tensioning the top wires to restore the keel reflex, the problem went away.

And, in the November edition, a competition pilot reported the following:

The keel had over two inches of reflex and the sail had flutter even at low speeds. I had changed the reflex to about 1¼ inches two weeks before…

Then, when carrying out a practice whip-stall, this was the result:

The glider immediately whipped 180 degrees to a nose down dive and I was just going along for the ride… I could feel no bar pressure either way, and am convinced that my full out push was useless.

— Dennis Pagen in Ground Skimmer, November 1975

The glider tucked upside-down, breaking the king post then the cross-tube, and Dennis sustained a broken leg among other injuries when he impacted the ground on top of his inverted glider. He recovered fully and went on to win the US championship in 1978 and to become the foremost author of books about hang gliding.

Years after the standard Rogallo was consigned to history, hang gliders could still be rendered ‘pitch divergent’ from rigging mistakes. For an example that affected an upgrade to a Sky Sports Sirocco II, see Chris Gonzales’ contribution at the end of Hang gliding 1974.


Pitch divergence was not a characteristic unique to improperly rigged standard Rogallo hang gliders (that is, without sufficient keel reflex).

…an IP [instructor pilot] called in as he crashed. He said that the ship had tucked in a simulated forced landing and the controls had no effect on the dive. Then he died.

— Robert Mason describing the Hughes TH-55A training helicopter in his Vietnam War memoir Chickenhawk, 1983


Lastly, a gratuitous snippet from another age…

Because of her age, she had been chosen to greet Charles Lindbergh with a bouquet of roses when he came here on his goodwill flight, and she could not understand how a man who was so tall, so blond, so handsome, could go up in a contraption that looked as if it were made of corrugated tin and that two mechanics had to push by the tail to help lift it off the ground.

— from Love in the Time of Cholera by Gabriel García Márquez, 1985

Internal links

A painted history of hang glider design

Paint it black–my review of the 1976 movie Sky Riders