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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
The piece of fabric stitched between the rear rigging wires by the keel, just visible in the colour photo of my Skyhook 3A, was a half-hearted attempt (by the original author of this web site) 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.
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. (See Rogallo wing definitions and diagrams for clarification of the parts of a glider.) 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.
For a video clip taken from the 1976 movie Sky Riders where Chris Wills demonstrates the Wills Wing Swallowtail’s resistance to the luffing dive, see under External links later on this page.
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. (See the Dennis Pagen related topics menu.)
How much keel reflex was enough? As far this author is aware, there was no really scientific way of determining that. Frank Colver (of the Colver audio variometer) carried out some experiments with a flying scale model. With no keel reflex and without a sailfeather, when launched in a steep dive from a second story window the sail luffed and the model dived vertically into the ground. He then attached a sailfeather (a scaled down one) and repeated the experiment…
The glider would not only pull out of a luffing dive, launched straight nose down but I could drop it at a negative angle and it still pulled out.
— Frank Colver (1)
Frank’s experiments likely came as close to scientific as anyone’s. A lot of gliders in those days did not have sufficient reflex (for a variety of reasons). In that respect John Lake’s ‘sailfeather’ might well have saved the day.
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 part 1.
In Britain, here is what happened to an up-and-coming pilot flying a licence-built Electra Flyer Cirrus 2 (the larger version of the popular Cirrus 3):
My first glider did one. I had no idea what it was at the time. Shining Tor, winter of 1982 on my Scott Kites Cirrus 2. From about 200′ and me pulling full speed it pitched straight down without warning. No pitch pressure during the dive. My head was pointing at the ground.
It yanked out miraculously with immense G force at about twenty feet from the hill top.
I only realized later (when I had a safe glider) what had actually happened.
— Justin Needham, who subsequently became a top competition pilot (2)
That photo is not of Justin Needham, but Irv Alward having failed to reach the LZ in his Cirrus 3 at Escape Country in April 1976. Nonetheless, it reveals several pitch stability shortcomings of such designs when compared to more modern wings, including the following:
- Absence of reflex bridles to hold up the trailing edges inboard in a sail ‘blow down’ situation (luffing dive)
- Absence of tip struts to hold up the trailing edges outboard, also in a sail ‘blow down’ situation (luffing dive)
- Leading edge deflexor cables that can be over-tightened, creating a deeper camber that, while increasing the lift of the wing, can also decrease its pitch stability to a dangerous degree.
Pitch divergence was not a characteristic unique to improperly rigged or inadequately designed and/or manufactured hang gliders…
…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
When we lived in modern times
While flex-wing hang gliders of the mid-1970s were, in the main, stable in pitch — that is, when you pulled the control bar back to increase speed, aerodynamic force caused the control bar to resist your pull, a question of dive recovery remained. In all these flexible wing hang gliders pitch stability depended on adequate airspeed to hold the sail’s aerodynamic shape. In severe turbulence, the glider could be pitched nose-down into a dive. If the sail inverted — that is, the air flow bore down on it from above instead of from below — the sail would lose its normal aerodynamic properties and the glider would be likely to ‘tuck’, that is to pitch over upside-down.
Two innovations compensated for that shortcoming: Reflex bridles and tip struts (or ‘blow down’ tubes). Reflex bridles, sometimes referred to as luff lines, run from the king post top to the trailing edge of the sail. In normal flight they are slack, but at low and negative angles of attack, they hold up the trailing edges, conferring an automatic ‘up elevator’ effect, countering the dive.
The New Zealand-manufactured Lancer II of 1977 (see Lancer II in Graeme Bird’s hang gliders) used an early implementation of reflex bridles, based on a Moyes (Australia) design.
Tip struts, firmly attached to the leading edge tubes, do a similar job, but farther outboard, where reflex bridles would tug on the sail at a disadvantageous angle. The simplest tip struts are not attached to the sail and are unmoving. Like reflex bridles, they only have their effect when the sail ‘blows down’ on them.
Many flex-wings made after the mid-1990s lack king posts, so they cannot have reflex bridles. They have their dive struts inside the double surface sail. They are hinged where they meet the leading edge tubes, so they ‘float’ freely upward with the sail’s billow-shift, but are limited in their downward angular movement.
The 1978 Sky Sports Sirocco used a combined tip strut and reflex bridle system.
Lastly, a gratuitous snippet from another time…
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
Related
Dove in Hang gliding 1978 and 1979 part 2 about a problem with a prototype licence-built Electra Flyer Dove
Flex-wings with tails related topics menu
Paint it black–my review of the 1976 movie Sky Riders, which includes film of Chris Wills demonstrating the Swallowtail’s inherent resistance to the luffing dive
Vehicle-based testing in Tom Price’s flying machines
External links
Big Blue Sky, 2008, by Bill Liscomb on YouTube starting at one hour, two minutes, and 29 seconds in, where Taras Kiceniuk Jr. narrates film of a pilot-less standard Rogallo in a luffing dive (then Pete Brock, off camera, talks about the need for safety standards)
James Coburn – Sky Riders (1976) on the modcinema YouTube channel starting at 3 minutes 17 seconds for Chris Wills whip-stalling his Swallowtail into a luffing dive
References
1. Frank Colver message on the HangGliding.org forum
2. Justin Needham comment in this topic on the British Hangies Facebook group
Hang Gliding History 