Brett Snellgrove Development note: More linking. Linked commentary. More illustrations.

Paragliders

 c. 2004
Recently, with the introduction of small rigid sections in paragliders like the Gin Boomerang and Apco Kera, I have noticed there has been a great deal of interest from pilots wondering why larger rigid sections, and entirely rigid wings, cannot be used in paragliders.

There appears to be an ongoing fascination with a vehicle that combines the advantages of both hang gliding and paragliding. Unfortunately there are some unique difficulties specific to paragliders that may make rigidifying our soft wings very difficult if not impossible. I have investigated and experimented with the concept for many years, and think I have enough information to venture an informed opinion.

Positioning the major portion of an aircraft's weight far below the wing is an excellent way to stabilize a wing, and renders traditional methods like sweep back, tip washout and reflex in flying wings, or canards and tails as used in more traditional aircraft, unnecessary.

A very low payload however, when associated with connection to the wing by lines rigid in tension but flaccid in compression, introduces a significant problem with pitch, surge oscillation. This problem is kept to a minimum in paragliders by using a very light, low inertia wing, and in using a soft wing that can collapse at low angles of attack. As such, when the wing surges following gust induced pitch back or stall, or encounters airflow at a negative angle of attack to the wing, a portion of the wing collapses, creating drag. The drag assists in bringing the wing back overhead. Add weight and rigidity to the wing and you add considerable inertia to the pitch, surge oscillation and remove the primary means of dampening that oscillation. In rough air there's nothing to stop the wing surging right under the pilot.

This is the major problem that leads to the failure of virtually every attempt to add rigid sections of any significant size to paraglider wings. The Forth Dimension, with its inflatable transverse centre spar, is a classic example. The LD was said to be 11:1, so if a successful rigid paraglider can be achieved the result would be clearly worthwhile.

In an effort to address these problems, I experimented with one-third scale models, adding rigid reflex to a paraglider wing in the form of rigid batons (soft reflex in a paraglider wing can simply blow down at low angles of attack, this is why hang gliders use luff lines to hold the reflex in the trailing edge). Such airfoil sections are auto stable and subject to far less pitch surge oscillation, so they seem ideal for such a concept. However, on testing the models, an immediate problem became apparent: the batons would very easily tangle in the lines. The solution was simple - add a transverse spar as well. This immediately eliminated the tangling problem, but not the pitch surge issue.

A typical, non-reflexed airfoil is not stable on its own, and without additional stabilizing devices, will tumble or rotate about the balance point (the balance point, or centre of lift, is the place on the airfoil where the lift can be considered to act about a single point and is typically 1/3 to 1/4 the distance back from the leading edge of the total chord or width of the wing). Paragliders prevent this tendency to rotate by running lines to the front leading edge and rear trailing edge of the wing down to comparatively heavy payload to hold the wing in a fairly rigid relationship to the airflow and pilot. As such, wing pitch oscillations (or tendency to rotate) are directly limited by the action of the pilot's weight pulling the wing back to a level position relative to the airflow. A hang glider, and indeed virtually all other aircraft, utilise different methods to stabilise the wing. In the case of a hang glider, a single pilot attachment is set somewhat ahead of the centre of lift or balance point of the airfoil.

If you position the centre of gravity (in this case, the pilot) ahead of the balance point at the centre of lift, you create a nose down tendency. Sweep back and washout of the wing tips, and inboard reflex, create a countering force to balance the nose down tendency and stabilise the wing. Think of it like a see saw - as the passenger slides forward of the balance point, the tendency to tilt down is countered by a downward force at the opposite end of the see saw by air pressure pushing down on the upward deflected surfaces providing balance. The further forward you slide on the see saw away from the balance point, the more downward countering force is needed at the opposite end. (The distance forward of the centre of gravity from the balance point is called the static margin, and is a good measure of wing stability. The greatest static margin is achieved by placing the downward force at the end of a long lever - a typical tail.)

With such an arrangement, the wing is somewhat freer to float independently of the influence of the pilot's weight - since aerodynamic forces now serve the same purpose in returning the wing to neutral pitch after most excursions. For example, if the wing is tilted down it flies faster, additional air pressure on the reflexed, washout tips, or tail surfaces push down on the rear of the wing tilting it back to neutral. As such, the pilot can push the bar out or pull it in, or push the stick fore and aft, and let go and the wing will return to neutral on its own - this is termed pitch stability.

Because a typical paraglider airfoil is not auto-stable, and utilises the pilot's weight to stabilise the wing, it is less free to adapt to the oncoming airflow independently to the pilot weight. If a typical paraglider wing is pitched up for example, since the pilot is in such a rigid relationship to the wing, the wing will surge back as the pilot swings forward; pitched down and the pilot will swing back. While resistance to swing (or inertia) stabilises the wing, once the pilot is swinging the resultant momentum can exacerbate, rather than dampen, the pitch and surge oscillation.

Imagine, however, if a gust hit the wing and rather than pitching the wing up and oscillating the wing back, swinging the pilot forward, the wing itself simply tilted upward independently to the pilot. Then, due to the inbuilt stability, came straight back down as the gust past with the pilot remaining directly under the wing. This could be achieved by simply suspending the pilot at a single point ahead of the balance point in the same manner as a hang glider. Imagine the case of a surge: rather than the wing swinging forward and tilting downward relative to the airflow (because lines to the leading edge pull it down leading to a negative angle and collapse), if the wing was not rigidly held at the leading edge, the wing would be free to tilt upward (rather than pulled downward) as it surges forward maintaining a neutral angle of attack relative to the airflow. There would be no tuck under and the surge would be immediately dampened. So rather than pitching up and down as the wing swings back and forward it would simply translate forward and backward maintaining the same neutral angle of attack relative to the airflow.

I decided, since I was using an auto stable airfoil, I would abandon the traditional method of stabilising a paraglider with front and rear lines, and attempt to stabilize my rigid paraglider in the same manner as a hang glider. I suspended the payload from a transverse spar (ahead of the balance point but back from the leading edge) from three lines (one from either end of the spar and one from the middle to add support to the spar and prevent upward bend under load). I ran lines from the leading edge and trailing edge of both wing tip batons to the payload to control pitch and support the tips, but rather than rigidly attaching the tip lines to the payload, these lines ran through a pulley to allow the wing to float. There were only seven lines in total. The pitch control system was such, that in the absence of pilot input, the wing was free to float in pitch after the manner of a hang glider.

In the models it worked well. I did some wind tunnel (house fan and cardboard box) tests and found the wing could be tipped as far as 40 degrees negative to the airflow and still pop back up when released, with no tuck under. If a rigid paraglider ever becomes a viable proposition I suspect this is the form it will most likely take. The model did appear to fly well and tolerate turbulence to a great extent.

However, after some consideration I came to realise the inherent problem with such a concept. In extreme turbulence even the most stable hang gliders have been known to tuck and tumble. With the pilot rigidly and intimately associated with the wing, he is free to exert control inputs during the tuck and bring the wing under control - he also tumbles with the wing. In extreme turbulence a rigid paraglider style wing suspended far above the pilot on lines would tuck independently to the pilot, rendering the control lines flaccid and inoperative. The tumbling wing would then tangle in the lines creating an unrecoverable disaster. Whether a current soft paraglider would be any safer in the sort of turbulence that would tumble such a rigid, paraglider style wing, is a matter for debate, but what is certain is the rigid wing would be unrecoverable once tangled. Unfortunately it's the sort of problem that only becomes apparent at the worst possible time, and I have no intention of finding out the hard way as a test pilot.

As such I turned my focus in a different direction - rather than trying to evolve a paraglider toward a hang glider, perhaps a hang glider could be modified to make it more like a paraglider. I began to consider various ideas for making a hang glider lighter and more portable with convenience approaching a paraglider. I investigated various bowspit designs, and even looked into cable leading edge wings like the Whitney porta-wing of the mid 70's, but none seemed to offer sufficient performance advantages over paragliders to make the effort worthwhile. Those concepts that did, were not sufficiently more portable than current hang glider wings. Then one afternoon, while watching windsurfers setting up, I hit upon the idea of eliminating the cross tube by curving and pre-loading the leading edge. I built several working models with a fibreglass rod leading edge and even took it to the radio controlled model stage, until one day when surfing the web, I found an article about someone who had beaten me to it many years earlier. This is the story of the Longbow.

The inventor was Bill Brooks, who is now chief technical officer for the BMAA. At the time, 1992, he was working for Solar Wings. Basically, Bill was also looking for something with the portability and convenience of a paraglider and the performance of a hang glider. He wondered if a curved, pre-loaded bow for the leading edge would eliminate the necessity for a cross tube, kingpost and upper rigging, and allow the leading edge to be removed and broken down for easy transport. Interestingly, like me, he also got the idea from looking at windsurfers. But unlike me he took it a lot further than the model stage and beat me to it.

To tolerate positive and negative loads, the leading edge bow needed a radius of 14.8m and used 2 5/8' diameter aluminium 17swg HT30TF tube with 2 1/2' inner sleeve progressively tapering to 60mm 7075-t6 tube with 1mm wall thickness at the tips. The leading edge broke down to five sections of 2.4m and rolled inside the sail with the batons remaining inside the wing. The fold up package was so small it could fit inside a small car for transport. With a tube this large it was able to tolerate all positive and negative loads without cable support and may have been the first true topless wing?

The leading edge tube was assembled from five sections and posted (slid) into the wing sleeve, then loaded (bent) with a winch that was built into the sail to provide the 180kg of pre-load required - taking about ten turns to fill the sail. It was then held securely in place with a winch pin. The lower lines were then attached to the leading edge boom and it was ready to go. Assembly time was minutes.

The resulting tension in the wing was such that there was little tip washout in flight, so weight-shift only roll control was poor. Furthermore, since low sweep back was combined with minimal washout, reflex was required in the wing batons for pitch stability. In some respects the wing was probably more similar to a plank style flying wing than a hang glider.

Since roll control with weight shift alone was ineffectual, and wing tension precluded a functional billow shift mechanism, D handles were added to the A frame with lines that lifted the tip trailing edge to dump lift and provide excellent roll control. This sounds very similar to the tip lifters currently under investigation for augmenting roll control in stiffer, high performance hang glider wings, but activated by hand controls rather than coupled with weight shift. The advantage of hand controls meant the wing could be more or less flown on the ground making ground handling very easy.

The disadvantage of permanent, built in reflex in a flying wing is pitch stability comes at the cost of efficiency - reflex airfoil sections typically have lower coefficients of lift. Kingposted hang gliders get around this problem with tip washout and reflex that only comes in at lower angles of attack when the sail blows down and the luff lines hold up the trailing edge. This does however create a "hole" on pitch tests as the wing approaches lower angles of attack before the reflex kicks in and becomes effective.

On pitch testing of the Longbow there were no such "holes" at low incidences on the pitch curve. The wing was pitch tested to 400kg positive 150kg negative and proved stable up to 50mph. Internal struts held the trailing edge up at low incidences (the first sprogs?). Furthermore, because of the built in reflex, it was virtually impossible to stall. At sustained high angles of attack the wing would oscillate, alternately throwing of tip vortices rather than stalling the entire wing.

Since the wing was essentially topless, some of the inefficiency of a reflex airfoil section was offset by reduced line and kingpost drag and the wing was estimated to have an LD in the vicinity of 9:1. Comparable to the best single surface hang gliders with far greater portability, significantly reduced weight and set up time, and with greater control efficiency.

At the time of writing the wing had only been truck tested and in short glides with S-turns off bunny hills. There had been no soaring flights as the author was concerned with spiral dive tendency shown in models (low sweep back and washout?).

The designer felt the weight and convenience could be further enhanced with a composite leading edge tubes and Kevlar lines. He also felt the reflex could be reduced and glide improved with a short V-tail. Solar Wings liked the idea, but declined to take it any further as it was felt the flying public would consider the concept too radical.

In later correspondence with an associate of Bill Brooks, I had the opportunity to pass some questions on to Bill. He confirmed it was the tendency of the wing models to spiral dive, and Solar Wings' lack of interest, that put a halt to further development. He also said that Bill felt the spiral dive tendency could be overcome with tip rudders, but this would detract from the simplicity of the wing which was the whole point of the concept. Some inbuilt dihedral in the wing would also solve the problem, but would be difficult to achieve in a curved bow style leading edge.

Unfortunately, this was the last I've heard of it. Clearly, a wing with the portability and convenience of a paraglider, and the performance and handling characteristics of a hang glider (speed, wind penetration and tolerance of turbulence) is an achievable goal. It may be achievable by increasing paraglider rigidity (without reinventing the hang glider) but this direction is fraught with aerodynamic and engineering problems. More likely, the solution will be something along the lines of the Longbow.

Interestingly, Laurent de Kalbermatten designed a wing called the Woopy-fly, which, despite external appearances, is similar in many ways to the Longbow. By eliminating the curved leading edge and opting for a straight, internal tube in what is essentially a elliptical, plank style wing, he was able to add dihedral to the wing and likely eliminated the spiral dive tendency. Rather than internal batons, he opted for an inflated, paraglider style wing that utilises battery operated turbine motors and a closed leading edge to maintain inflation. The wing has built in reflex for pitch stability, as the pilot is positioned close to the wing rather than suspended far below, losing the stabilising effect of the very low centre of gravity in a paraglider. Roll control is achieved by a centre rudder, and coincidentally the L/D is said to be the same as the Longbow at 9:1.

Apparently the wing will be released for sale in the near future. Whether pilots will accept such a radical concept will be interesting to see. My personal feeling is reliance on battery operated motors for wing inflation may deter some pilots. If this proves to be the case however, it should be a simple matter to replace the inflated wing with a battened one with minimal weight or convenience penalty.

Whether the end result is a rigid Woppy-fly or a straighter Longbow, I'll leave to the experts to debate, but either way we might see a wing that meets the requirement of both paraglider and hang glider pilots and all that arguing over a combined magazine will be a thing of the past...

YouTube - Gin Boomerang 5
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