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Groundschool — Theory of Flight

Weight-shift control and parawings

Revision 29 — page content was last changed 12 August 2015

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The aerodynamic principles expounded in the preceding modules apply to all heavier-than-air aircraft. However those single-place or two-place sport and recreational aeroplanes — whose legitimate flight operations are enabled by Civil Aviation Order 95.32 or by Civil Aviation Order 95.8 — are not 3-axis control aircraft so the stability and control features differ.

Such aircraft are either powered ram-air parachutes or aeroplanes where flight control is attained primarily by the pilot's ability to move the aircraft's centre of mass fore-and-aft and/or side-to-side relative to a framed wing, i.e. weight-shift control. The following notes are just a short introduction to such aeroplanes. For comprehensive manuals — produced by the Flight Standards Service of the United States FAA — read the 'Weight-Shift Control Aircraft Flying Handbook' (the download is 57 megabytes) or the 'Powered Parachute Flying Handbook' (the download is 15 megabytes).


10.1 Hang glider and 'trike' wings and carriages

Hang gliding started in 1891 with Otto Lilienthal's first flights; see the history of Australian powered recreational aviation. In 1948 Francis Rogallo, an American aeronautical engineer, experimented in delta-shape flexible wings, which culminated in a project to evaluate his Rogallo parawing concept for suitability as a recovery vehicle for the Gemini spacecraft. That project was finally dropped in favour of parachute recovery, but the technology acquired helped kick-start the modern hang-glider industry.

The flexible, swept-wing design provides high lift, reasonable L/D, smaller pitching moments and subdued stall characteristics. The wing is aerodynamically balanced in pitch, because in flight a download is applied at the rear of the flexible wing by a slightly reflexed aerofoil and/or the outer wing sections are washed-out. Longitudinal stability is derived from the reversed centre of pressure (cp) movement — as angle of attack (aoa) increases from the cruising aoa the cp moves backward, which pitches the nose down. The swept-back leading edge provides good lateral stability, although the directional and lateral stability of such wings is also dependent on aoa, being most stable at low speeds.

The weight-shift control aeroplane
The hang glider technology has developed to encompass heavier and faster, primarily weight-shift control, two-place, powered aircraft with metal-framed, double fabric surface wings; generally called microlights or trikes. Smaller versions may be known as 'nanolights'. See the Airborne Australia website.

The powered aircraft carriage (or lightweight cart) — consisting of the pilot/passenger seating, instrument binnacle, pusher engine and propeller mounting and a steerable tricycle undercarriage (from which came the term 'trike') — is primarily suspended, via a streamline-section metal mast, from pitch-and-roll hang point hardware attached to the tubular metal keel of the framed wing structure. The fore-and-aft position of the hang point hardware is ground-adjustable to allow an increase/decrease in the aircraft's trim speed. The pilot's control frame and bar is a fixed part of the wing structure; if the wing is strutted, the inboard end of each strut will be terminated at the control bar. The control bar's neutral position is the aircraft's trimmed level flight position at cruising speed so the aircraft could be flown 'hands-off' the control bar. This arrangement provides direct pilot contact with the wing and the feel for how it is flying. There are no ailerons, flaps or spoilers.

Carriages may be an open frame metal structure or a partly or fully enclosed composite pod. Seating for pilot and passenger is usually a very close tandem arrangement. The concept of the carriages and the light-weight carts are similar for trikes, nanolights, gyroplanes and some powered hang gliders.

The wing primary load structure is aluminium tubing plus a lot of hardware fittings forming triangulated structures that are supported by secondary triangulated structures of aluminium tube plus stainless steel rigging cables.

The components of the rather complex sail structure are generally cut from polyester materials and sewn together. Shaped battens contained in chord-wise sail pockets provide the aerofoil shape. The sail is only tightly attached to the aluminium frame along its leading edges and wing tips, leaving the trailing edge and much of the rear section of the sail free to flex and twist under load, altering the aerodynamic forces generated by the left and right halves of the wing.

In flight the aircraft's centre of gravity (cg) is normally located vertically below the carriage hang point and horizontally near the propeller's extended line of thrust. There is no tailplane and there are no control surfaces like rudders or elevators. Aircraft speed is controlled by rotating the wing, in the pitching plane, about the pitch-and-roll joint thereby altering the wing angle of incidence. To increase speed – for the same power setting – back pressure is held on the control bar (i.e. seemingly pulling the suspended load toward the bar) to reduce wing incidence and thus the angle of attack (aoa). To reduce speed – for the same power setting – forward pressure is applied to the control bar (i.e. seemingly pushing the suspended load away from the bar) to increase incidence and thus aoa. These control bar movements shift the cg fore or aft in relation to the vertical line of the centre of pressure — hence the 'weight-shift' term. The throttle controls climb and descent.

In cruising flight the wing centre of pressure is vertically coincident with the aircraft centre of gravity. At slower speeds (higher aoa) the wing cp is vertically aft of the aircraft cg creating a nose-down pitching moment. At higher speeds (lower aoa) the wing cp is vertically forward of the aircraft cg creating a nose-up pitching moment.

Aircraft movement in the lateral plane (rolling and subsequently turning) is initiated by the pilot applying sideways pressure on the A-frame control bar — which is fixed relative to the wing. As perhaps 80% or more of the total aircraft mass is represented by the carriage and its occupants and that mass suspended below the wing has considerable inertia then, rather than moving the carriage sideways left or right the control bar movement rotates the wing about the hang point. Consequently the aircraft starts to bank while, at the same time, the action effectively shifts the cg in relation to the wing aerodynamic centre (hence 'weight-shift'). The aoa has to be increased at the same time by forward pressure on the control bar, providing the centripetal force for the turn. The only other flight control is the throttle. As there is no control for rotation about the normal axis, weight-shift aircraft are sometimes referred to as 'two-axis' aircraft. A trike is limited in manoeuvrability; pitch angles of 45° and bank angles of 60° are the recommended maximums; otherwise the usual physics apply for turning, climbing and descending.

The hang glider and powered hang glider
The very light powered hang glider (PHG) system is similar; the main difference — apart from fewer, and lighter, hardware fittings — is the lack of a carriage or cart. the hang gliders have weight-shift control (i.e. body shift) by the pilot moving their body fore-and-aft or sideways relative to a simple, fixed, triangular control bar and frame system rigidly attached to the wing. The pilot's harness is directly attached at a hang-point on the tubular metal wing keel structure. In addition to the trikes the Hang Gliding Federation of Australia [HGFA] administers a class of powered hang gliders that have an empty weight under 70 kg plus another class, sometimes classified as nanolights, that have a maximum take-off weight under 300 kg.

PHGs employ a specialised 10–20 hp paramotor, fuel tank and propeller cage rigidly attached to a light frame within a harness suspended from the hang-point. The pilot is harnessed to the frame in a standing/running position for foot-launching of the aircraft and in a sitting position when airborne. Some PHG, particularly two-place aircraft, may use a light three-wheel cart. The cart relieves much of the physical loads on the pilot when launching and simplifies take-off and landing when carrying a passenger.

The non-powered hang glider (HG) system is much the same as the PHG without an engine. Generally the aircraft must be foot-launched. After launching the pilot is either in a seated position or a prone face-down position.

For descriptions of some currently available hang gliders, nanolights and trikes see the AirBorne Australia website.

10.2 Powered parachutes

The ram-air parachute wing
The parachute wings used by sport parachutists, paragliders [PG], powered paragliders [PPG] and powered parachutes [PPC] function quite differently to the traditional circular, umbrella-shaped emergency 'chutes. The latter are 'descending only' parachutes, dimensioned so that the drag of the parachute canopy counters much of the weight of the load, limiting the rate of fall to a terminal velocity around 18–25 km/h (5 to 7 m/sec). During the descent the parachute's path acquires a horizontal component as it drifts with the wind, though an encounter with a vertical gust will increase or decrease the rate of descent. The load might be a person who has 'bailed-out' of an aircraft, or even an aircraft plus the occupants — if it's part of a rocket-deployed emergency recovery system.

Parachute wings or parawings, on the other hand, also generate lift, allowing the person or aircraft to glide with a fairly low rate of sink and thus to ascend with any parcel of rising air that has an ascent rate exceeding the aircraft's sink rate. Typical L/D ratios for unpowered paragliders are around 8:1. As the 'chute and harness weigh less than 20 kg, L/D much depends on the weight of the pilot and the selection of wing size.

Parawings are steerable and provide that high degree of manoeuvrability demonstrated by skydivers and paragliders; and they can be flared for a soft landing. The parawing is generally rectangular in shape; higher aspect ratio elliptical wings provide better performance but are not as stable as a low aspect ratio rectangular wing.
Parachute wing construction
When the open end of a closed tube is aligned with, and exposed to, a continually moving airflow, the flow within the tube is halted and the rather small amount of pressure energy (see 'stagnation pressure') needed to halt the airflow within the tube is additional to the ambient atmospheric air pressure. This is the basis of the 'ram-air' parachute wing used in the sport parachutes, paragliders and powered parachutes. The design of the skydiving parachutes is a little different from the others as the system must cope with high shock-loads generated as it opens to arrest a free-falling body and the aspect ratio is very low, perhaps less than 2:1 to 2.5:1, to facilitate their very close canopy formation descents.

Ram-air wings are formed from a low-porosity material, such as rip-stop nylon, and consist of an upper and a lower fabric surface separated by fabric load-bearing ribs; thus creating a number of individual wing cells open to the airflow at the leading edge and sealed at the trailing edge. The rib fabric is cut in an aerofoil shape (i.e. a parafoil) with interconnecting cross-ports cut into them, so maintaining an equal pressure distribution across a group of cells. In flight, although fabric permeability has a slight effect, the ram-air pressure within the cells is near the stagnation pressure — the highest — and is enough to form the semi-rigid wing shape (a cambered upper surface and a flatter under-surface) that generates lift, providing the gliding/soaring flight ability and the manoeuvrability of parachute wings — as long as the stagnation pressure holds. Once established, the higher stagnation pressure is inside the mouth opening and there is airflow into the cells, then back out over both the upper and lower surfaces. The better designs of parawings have smoother flow.

The suspension lines are dimensioned to form the wing into an anhedral arc in flight, thus a PPC usually has a fairly low effective aspect ratio (around 4), but the arc adds to the system's pendular stability because the lift vector at most cell positions will have a lateral component.

Turning is accomplished by increasing drag on one side of the wing — by pushing foot pedals or steering bars or pulling steering toggles — which in turn pull down on the brake lines attached to the wing trailing edge. This is supplemented by weight-shift — the pilot leaning. The deflection acts like fully lowering a flap increasing drag on that side and the aircraft yaws and turns. The greater the deflection, the steeper the turn — and the greater the height loss, unless power is increased. Braking both wings simultaneously and reducing power will flare the aircraft for landing (the increased drag slows the wing, the cart swings forward and up a little before touching down); excessive braking may stall the wing. Sport parachutes need fine relative speed, direction and descent adjustment systems for canopy formation manoeuvring.

Parawings are used in paragliders, powered paragliders and the powered parachutes described next.
The powered parachute aircraft
A powered parachute aircraft [PPC] is a two-part system consisting of a cart for one or two occupants with engine and propeller plus the parawing and suspension lines. About 80–90% of the total system drag is contributed by the wing. PPC with rectangular wings have a low L/D — between 3 to 5, but L/D is greater for elliptically shaped wings. PPCs normally cruise at only one aoa and airspeed — around 30–35 knots, although the aoa of some wings can be trimmed in flight to change aoa a little. The aoa of some wings can be changed by shifting weight fore or aft, and maintaining that pilot/passenger position — much the same as altering the trim state of a three-axis very light aircraft by the pilot leaning forward or back. All parawings are capable of stalling (the cells lose their pressure differential and the upper wing surface collapses) if badly mishandled, or if flown in turbulence greater than 'low'.

The engine, pilot and passenger are usually accommodated (side-by-side or tandem) in a tricycle undercarriage vehicle — similar to the trike — and often with the parachute lines being led into four attachment points — two forward for the leading edge lines and two aft for the trailing edge lines. The cg is low on the cart, the thrust line is above it and the line of drag is very high. Although it is a two-part system, the two parts act as a whole provided the state of trim is maintained. If power is increased above cruise power, the thrust will initially push the cart forward of the wing — increasing pitch — and the PPC will climb at the designed speed. Rate of climb is dependent on throttle opening and all-up weight. Similarly, if power is decreased, the pitch will decrease and the PPC will descend. In normal cruise, climb and descent, the wing automatically adjusts to the aoa.
Pendular stability
For pitch and roll stability, the PPC relies on the natural pendular stability provided by the long vertical separation between the aerodynamic centre of the wing and the cg. As a dry nylon wing and suspension lines etc probably weighs less than 20 kg the cg of the two-part system will be within the cart. The wing acts as the suspension point for the 'weight' (the cart and crew) of the pendulum and the suspension lines act as the pendulum rod.

Any turbulence will tend to move the wing further than the cart, because of the cart's much higher inertia, and the pendular action quickly restores the normal state after the perturbation — although the normal state is probably a slight gentle oscillation of the cart because of its freedom to swing longitudinally and laterally. In smooth air the PPC can generally be flown 'hands-off'. A gust from the front has the effect of moving the wing back, in relation to the cart. This will temporarily increase aoa and thus lift, because V² is maintained, and the aircraft will rise a little until the cart swings back under the wing and aoa is returned to normal. A gust from the rear has the effect of moving the wing forward, and decreasing aoa and thus lift. The aircraft will sink a little, until the cart swings forward and aoa is returned to normal.

Pendular stability is dynamic, so there will be a few oscillations of rising/sinking after such disturbances. Gusts with a vertical component will affect aoa and wing-loading as with three-axis aircraft. In addition to atmospheric disturbances, transient changes in attitude, aoa and airspeed can be induced by over-controlling — fast throttle changes, radical control inputs and fast weight-shifting. The wing will usually — depending on torque at varying rpm settings — turn into the relative airflow and take the cart with it. This can be a problem in the take-off or landing roll if not conducted directly into wind, or if conducted in turbulent conditions.

For more PPC information see Aerochute International.

The next module in this Flight Theory Guide discusses take-off considerations.

Groundschool — Flight Theory Guide modules

| Flight theory contents | 1. Basic forces | 1a. Manoeuvring forces | 2. Airspeed & air properties |

| 3. Altitude & altimeters | 4. Aerofoils & wings | 5. Engine & propeller performance | 6. Tailplane surfaces |

| 7. Stability | 8. Control | 9. Weight & balance | [10. Weight-shift control] | 11. Take-off considerations |

| 12. Circuit & landing | 13. Flight at excessive speed | 14. Safety: control loss in turns |

Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing | 1

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