Three-sixty degree appraisal (hang gliding 1976)
This page continues from Simple versus complex (hang gliding 1975 part 2).
I took this photo of Winklebury hill from the car park at Berwick St John village fête in June, 2013. The aspect reminds me of a trivial event in early 1976, when this was our only north-west site. (Bell Hill had not yet been discovered.) I was at the farm at the bottom, watching one of two flying brothers on approach to the landing field (nearer the hill than where this photo was taken). Flying seated in a Waspair Falcon IV, framed by the hill and some low trees (out of view in the photo) he went right round in a 360-degree turn with little loss in height. He then levelled out and landed normally. That was the first 360-degree turn in a hang glider I had seen.
Doesn’t sound like much, but it was most impressive for those times. However, the pace of advancement was so rapid that, by the end of that summer, 360 degree turns were considered almost routine.
The first prototype of my black V-tail monoplane glider failed to fly, so I reworked it. To take this photo I used a step ladder. Notice my hand holding the fir tree bent over out of the way.
After this photo was taken, I modified the structure (again) so that the root of the sail was attached to the keel tube only at the rear instead of the tube running through the pocket in the sail. That allowed the root to curve upward, greatly increasing the camber of the sail, which caused it to fly very slowly despite its small sail area. On its one and only flight test in that configuration, I flew out from about a third the way up the face of Monk’s Down in north Dorset. Being surprised momentarily by the low airspeed, I pulled the bar in and the glider pitched nose-down in response, as expected. However, it continued pitching nose-down and my airspeed increased to the point where I had the bar pushed out fully to counter the dive. Eventually the glider rotated nose up and slowed. I pulled in to resume level flight, but the glider continued its nose-up rotation and continued to slow, which prompted me to haul myself forward against the control bar again…
I do not recall how many of these pitch oscillations I went through (not many — I was not very high to start with) but luckily it was pulling out at the bottom of a dive when I touched down in the dusty field below the slope, sustaining only a sprained ankle.
Here is what I believe to be the problem:
The high point of the camber shifted longitudinally with changing airspeed. Because of the inertia of the flying machine (most of which is the pilot in hang gliding) airspeed takes some moments to change, resulting in a delay in the movement of the center of lift. I am not sure why the camber should change shape, but it strikes me as likely that the drag of the sail itself (form drag, skin friction, and such like, which increases in proportion to the square of airspeed) would tend to distort the high point of the camber to the rear. That positive feedback mechanism* sent me into a series of pitch oscillations.
Aerodynamicists term that pitch characteristic a phugoid. (I titled an article I wrote about this for my club magazine A Phugoid Men; a pun on the title of the 1992 movie A Few Good Men.)
I have reliable information that the Miles Wings Gulp, an earlier and higher performing monoplane type flex-wing hang glider (see under External links later on this page) suffered from a similar problem, but to a much lesser extent. It too was without curved battens holding its airfoil shape, but its thick sailcloth and cleverly three-dimensional geometry gave it a stable shape. Even so, after the first production batch, Gulps were fitted with chord-wise airfoil curved battens, like modern flex-wings.
Another advanced hang glider, the Eagle III of 1974 (the Scientific American hang glider) seems likely to me to have also suffered from this phenomenon, although I have no solid evidence for that. See Technical accuracy in Painting the Eagle III for a bit about it.
Supersonic fighter-bombers of the Vietnam war also suffered from phugoid if mishandled…
He pushed over violently on the stick. After a control movement such as this, especially at speeds of about 600 knots, the aircraft reacts violently. All his maps, charts and checklists, in fact even the fuel selector knob, which is part of the control panel, flew up into the air and filled the canopy and windscreen. Everything that was not tied down came up. Four’s immediate reaction was to pull back on the stick and he immediately entered a ‘porpoise’. A porpoise is a vertical oscillation where you are just a step behind the aircraft and can’t physically keep up with the machine; each control movement only serves to exaggerate the problem.
— Colonel Jack Broughton USAF (Ret), Thud Ridge, 1969
* Positive feedback mechanism?
“You engineering types are so confusing. Positive feedback is good! We want more positive feedback! It tells use we are doing things right! And why don’t you smile?”
In engineering terms, as distinct from corporate HR arts graduate terms, positive feedback is a runaway response that causes instability; an out-of-control situation. Negative feedback is where the mechanism tends to damp out excursions from the required control setting. The Watt governor of the first successful steam engines is an example I learned in CSE engineering at secondary school.
“The what governor?”
Oh, never mind…
I contracted out the construction of my sails to Roly, a local hang glider sailmaker (who, at age 17, had built his own Skyhook IIIA from plans) but I built the airframes myself. My hang gliders all flew — eventually. However, I was always at least two steps behind the state of the art.
Notice the absence of an emergency parachute at this time. This version had a larger control frame to accommodate flying prone. I also did away with the double side flying wires of the first version.
On a later version of this glider, I created the deflexor cables as a single wire running through a pulley (or equivalent device) at the nose so that the tension on each side would always be the same. It just seemed like a good idea at the time. On a flight test, the glider flew normally and I was about to land it, avoiding wet ground, when the horizon whipped round and, still in the prone position, I was dumped onto the ground — actually into a couple of inches of water — with the glider and me facing the opposite direction of my approach.
Some time later, when I was away from home, my mother took a call from Steve Hunt of Hiway Hang Gliders (then based in Brighton on the Sussex coast) who heard about the incident. When I next saw Steve, a year or so later, I had forgotten all about it. I will never know what insights he might have provided.
Incidentally, this hillside is inside Bournemouth Airport airspace. When they found out about my hang gliding there, they had no objection to it, but they wanted me to telephone their air traffic control before I flew. They said also that I should refrain from flying higher than the collision light towers on the hill. (I had done that only once, in a winter gale. Scary!) It is hard to imagine in today’s extremist ‘democracy’ populism in which the slightest whim of the masses trumps the welfare of any individual, but I suggested the following: I was then flight testing experimental hang gliders, which clearly incurred risk of control problems. Could they shut down the airport while I carried out this important work?
Oh, to be a 20-year-old hang glider designer/test pilot in the 1970s!
Anyways, the guy on the other end of the telephone (he sounded young and enthusiastic, definitely on my side) said he would look into it (shutting down the airport on my say-so whenever I felt like it) and he (or someone) would get back to me. This was in 1976. You gotta understand that we were looked up at then. (For the wrong reasons, I feel now.) I don’t recall whether it was the same guy who got back to me, but whoever it was said they had no mechanism (or process) for shutting own the airport except for an actual emergency and, hopefully, the process we had agreed would prevent any such thing arising.
To see what became of this airframe, see under Simulator in Aviation computer-based training.
Mean Machine 1 was the first of three swallowtails built by Martin O. It is flown here by Chris S. Note the scallop — removal of sailcloth from the trailing edge in a curve — and reduced billow compared to a standard Rogallo. Martin flies paragliders nowadays (2019). I have photos of Chris flying a powered ultralight in 2019 on my page Defying Newton, powered ultralight operations at Newton Peveril in Dorset, England.
As well as stretching the Swallowtail concept, the Firebird, made by Birdman of Marlborough, Wiltshire, England, was said to be so quick to rig it opened like an umbrella. At that time Birdman contracted out their sail-making and, according to one expert insider, the Firebird suffered from a ‘rattly’ sail. (The one I saw flying at Monk’s Down looked impressive, but the sail certainly fluttered more than did that of the Swallowtail, for instance.)
During the long hot summer of 1975, Birdman made Roly the sail-maker an offer he could not refuse, so in about July 1976 he left Kestrel Kites of Poole, Dorset, to join Birdman in Wiltshire. Their premises, former chicken sheds, had no sail loft initially. As well as leading the creation of the Birdman sail loft, Roly added a second batten each side to the Firebird, in the style of late Swallowtail types such as the Hiway Cloudbase. The two battens each side largely eliminated the ‘rattle.’
Ron’s green glider (in the photo) also seems to be a Firebird with the extra battens. Roly informs me that several Firebird owners brought their gliders back to the factory to have the sail modified with the new batten layout.
So what if it’s freezing cold in the north wind on a bare hillside in the middle of nowhere with a swamp for a landing field — or is that slush and snow? We’re going flying anyway!
See my related topics page Birdman of Wiltshire, England.
In 1976 I was a hang gliding instructor, first teaching beginners informally at my local site, then working for a hang gliding school in south Wales.
I first saw a Chargus Midas in late 1976 when its pilot, together with his aristocratic-looking girl friend with long brown hair (I remember her clearly to this day!) visited the hang gliding school in south Wales where I was instructing. (How was I to obtain a girl friend like that? I had no idea, but I soon concluded that hang gliding instructing was not the way.) Compared with the primitive Rogallos I flew at the time, the Midas appeared almost extra-terrestrial. As one university student put it the following year (when I too was at college, where surely there were unattached girls…) the Midas was “just a pair of leading edges.”
The Midas, designed by Martin Farnham, was manufactured by Chargus of Buckinghamshire, a few miles north-west of London, England.
The key to the Midas series, apart from the fully battened sails, were the rolled keels. I remember making every one of those using a rolling machine. (Three big rollers on a metal frame.) You slowly increased the pressure on the middle one whilst pushing the tube backwards and forwards to create the curve.
— Robin G. in Can you name this glider? — a topic on the British Hangies Facebook group (link later on this page)
Technical: The first flex-wing hang gliders I know of with permanent camber curves in their keel tubes were the Seagull 4 of 1974, the Wasp Nova of 1975 — both of which also had a large amount of compensating reflex — and the Sun Sail Sun IV of late 1975. (See More developments in Hang gliding 1975 part 2.) The latter had a short keel, although not as short as the Midas, and a small amount of camber. Like the Midas, it relied on ‘truncated’ (fixed) tips for dive recovery.
Chargus was run by former Royal Navy marksman and motor racing engineer Murray Rose. In 1977, he flew a Midas with a Soarmaster-type power unit at the annual event at Mere in Wiltshire. It was noisy and it was under-powered, but it flew. However, the high thrust line of engines mounted above the pilot proved to be dangerous and that layout was soon abandoned in favour of the power trike, with pilot, engine, and propeller mounted on a detachable frame with wheels.
I next encountered Murray in 1980 testing a flex-wing trike powered ultralight with an experimental 3-blade carbon fiber propeller, in which he climbed out from the airfield despite a down-draft and turbulence caused by wind curling over from the forested edge of the runway. By the early 1990s, he was at Solar Wings in Wiltshire using finite element analysis to compute stresses and strains through the compound curves of flexible wings and airframes. (By then the Solar Wings factory was part of Pegasus Aviation, which made powered ultralights.)
As of this writing in 2019, Murray Rose is a successful thriller writer using the name Walter Gunn. (Info source: SkyWings, August 2018.)
See also my related topics page Chargus of Buckinghamshire, England.
A bit about hang glider design philosophy as I see it…
The front fourth of a wing does about half the lifting, so ‘a pair of leading edges’ might seem all you need. (See the Chargus Midas in the preceding section The man with the golden gun.) Just cut away the sail behind the quarter-chord line and extend the span a bit to compensate for the reduced sail!
A couple of problems: First, extending the span normally reduces roll authority and your ability to counter turbulence, particularly in weight-shift controlled flex wings. It also makes it hard to counter washout (wing twist) because the triangulation between the root and the leading edges holds down the roached area near the tips. (In wings like the Midas, the fixed tips do part of that job.) In addition, no matter how much you increase the aspect ratio (spindliness) of your design, it is still true that the front quarter does half the lifting. You need the rear three quarters too.
Having said that, the Wills Wing team in 1974 found a worthwhile improvement in cutting away the trailing edge of the standard Rogallo in a ‘helical curve,’ particularly after they also added tip roach supported by battens. The area removed added little to lift (even before it was removed…) and its rippling fabric added much to drag.
Wings like the Midas and indeed modern flex-wing hang gliders can be considered an extension of that design philosophy.
However, the increments in performance achieved during this period were mostly small. Exceptions included radical innovations such as the Miles Wings Gulp of 1975 and Gryphon of 1976, which embodied drawbacks of their own. (See the related topics menu Miles Wings Gulp and Gryphon.)
An illustrative experience occurred when I modified a balsa and tissue Rogallo I used for testing on summer days in the back garden where I live near the south coast of England. Such tests are less than scientific, partly because on even the most oppressive anticyclonic day of blue haze and withering sunshine, there is always some wind, even in a sheltered garden. The model in question was a simple Rogallo, which I modified by adding tip roach supported by a pair of radial battens (strips of balsa) each side. However, after modifying one side, on a whim, I took it out to see how it flew. The hugely asymmetric glider flew straight and level!
Presumably the minuscule added weight of the battens exactly countered the minuscule additional lift on that side. While it is unlikely that the model can be considered an accurate simulation of the full size glider, I nevertheless feel that it demonstrated how small were some of the improvements gained from apparently large modifications.
One day teaching beginners on a low slope in Wales in 1976, we found that those who flew one particular glider (the sole example of the type that we used) kept nosing in hard shortly after launching. We determined the correlation with the incidents and the specific glider only gradually. Why did Fred Bloggs nose in just now when he was OK two minutes ago. And why has Joe Soap just nosed in? Eventually one young fellow, a student at the nearby Polytechnic of Wales, was winded so badly in a hard impact he was unable to breathe. He just lay on the ground curled in a ball for a minute. I thought we were witnessing a fatality. However, in a few minutes he was up and about and able to describe his experience. My relief was short lived.
“Everard, take that glider up and find out what’s wrong with it.” Mike, the chief instructor, was a former RAF pilot, glider pilot, and scuba diver.
I launched with plenty of speed (for safety) and was settling into the short period of level flight afforded by the low grass-covered slope when the glider slowly, but unstoppably, pitched nose-down and gathered speed, in much the same way as in stall recovery, but I was nowhere near the stall. I ‘pushed out’ on the control bar for maximum rearward weight shift (equivalent to hauling back on the control stick in a conventional glider or airplane) and it levelled out and flew normally from then on until I landed (for a total flight time of no more than thirty seconds).
“That glider is grounded,” said Mike. No explanation from me was needed; everyone saw what I did and what the glider did.
I do not know why it behaved like that or why the fault had not been discovered earlier, or whether the problem was inherent or the result of incorrect adjustment. (The type had been in use for about a year by then.)
We kept the gliders in a disused cinema at the back of rambling premises in Merthyr Tydfil. ‘Rambling’ isn’t strong enough. It was like another dimension. Looking out one window onto a vertical hole between structures, there was a river flowing underneath… Late in the year, we instructors were all issued with identical red and white winter jackets and we strode up the Merthyr main street, about 10 feet tall. I half expected to bump into a local girl whose song Lost in France was climbing the pop charts. In retrospect, the people in that street likely thought were were a bunch of English fools and I never did get to meet Bonnie Tyler. (One pop star I did meet through hang gliding is Elkie Brooks, but that was in the early 1990s.)
Mike the chief instructor had immigrated from Russia with his mother — who barely spoke English and lived in rooms in the back of the sprawling building (which included a disused cinema/movie theater large enough to accommodate several rigged gliders). Fronting onto the High Street, was a junk shop.
He said that the barrel rolls were sudden and unexpected and that they occurred as one maneuver. He hit his head on part of the kite but had zero Gs and positive Gs intermittently, but no negative G forces. As quickly as the rolls started they then stopped, and Bob was again flying straight and level with no damage…
— George Worthington in Hang Gliding, September 1978, describing top Arizona pilot Bob Thompson being barrel-rolled in a competition in the Owens Valley, California
Mike sometimes had friends round and I overheard them discussing how much money they had made the day before. Then, barely pausing, switching to the subject of hang gliding, one of them described how turbulence had rolled him upside-down a few days earlier. (This was in a Birdman Firebird — see Firebird earlier on this page — and it was before we adapted emergency parachutes for hang gliding.) He was lucky to be alive. These were kick-ass people with a kinda strange background. Mike had flown for the RAF in his younger days.
This topic continues in Hang gliding late 1970s and early 1980s.
I was a kamikaze, my review of Ryuji Nagatsuka’s 1973 book, which possibly influenced my direction in life at this time
Whisky Kilo Two Eight tells all, my review of Trailblazers, Test Pilots in Action by Christopher Hounsfield, 2008
Can you name this glider? topic on British Hangies Facebook group, containing Chargus Midas manufacturing info by Robin G.
Miles Wings Gulp and Gryphon related topics menu in Hang Gliding History
The Fight to Preserve South Wales’ Jewish History in the Jerusalem Post