Weight-shift aircraft (xhtml w3c 12/09)

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Groundschool — Theory of Flight

Weight-shift control and powered 'chutes

Revision 25 — page content was last changed December 9, 2008; consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au

Module content

10.1 'Trikes'
10.2 Powered parachutes

10.1 'Trikes'

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. The Paraglider Research Vehicle project was finally dropped in favour of parachute recovery, but the technology acquired helped kick-start the hang-glider industry. The technology has since developed to weight-shift controlled, powered-aircraft, commonly called 'flexwings' or 'microlights' or 'trikes'.

The flexible, swept-wing design provides high lift, a high L/D, a small pitching moment and subdued stall characteristics. The wing is aerodynamically balanced in pitch, because a download is applied at the rear of the wing by a reflexed aerofoil (the trailing edge is bent up — reverse cambered) and/or the outer wing sections are washed-out. Longitudinal stability is derived from the reversed cp movement — as aoa increases, 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 aircraft body — consisting of the pilot/passenger pod, pusher engine mounting and a tricycle undercarriage (from which comes the term 'trike') — is suspended from a pitch-and-roll joint attached to the wing structure. This hang-point is usually forward of 25% MAC. There is no tailplane and there are no control surfaces like ailerons, rudders or elevators. Pitch and roll are controlled entirely by shifting the whole trike body either to the sides or fore and aft, via pilot pressure on the control frame — which is fixed relative to the wing. This action effectively shifts the cg in relation to the wing aerodynamic centre, hence 'weight-shift'. The only other flight control is the throttle. As there is no 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.

For more information on the rather complex aerodynamics of the flexwing, check out the Aerial Pursuits trikes page, particularly the section on how a microlight turns.

10.2 Powered parachutes

A powered parachute wing is a ram-air parachute, and has a cambered upper surface and a flatter under surface, formed from a low-porosity material such as rip-stop nylon. The two surfaces are separated by a series of ribs that create 'cells', typically open to the airflow at the leading edge and with internal cross-ports interconnecting the airflow. The principle employed is that the stagnation pressure — dynamic plus static — within the cells is greater than the external pressure, so the wing forms, and maintains, an aerofoil shape in flight — so 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 have smoother flow.

About 80–90% of the total system drag is contributed by the wing. Powered parachutes [PPC] have a low L/D — around 3 or 4. The wing is designed to form an anhedral arc under load. Thus, a PPC usually has a fairly low effective aspect ratio (around 4), but the arc adds to stability because the lift vector at most cell positions will have a lateral component. They normally operate at only one aoa and airspeed — around 25–30 knots, although 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 light aircraft by the pilot leaning forward or back. All parawings are capable of stalling (the cells lose their pressure and collapse) 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 normally 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 vehicle, 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 vehicle 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.

Turning is accomplished by increasing drag on one side of the wing — by pushing foot pedals or pulling steering toggles — which in turn pull down on brake lines attached to the wing trailing edge. The deflection increases 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 will slow the PPC and increase rate of sink; excessive braking may stall the wing. However, the newer generation of shaped wings are significantly more efficient than the older standard rectangular wings and can be flown using only weight-shift.

For pitch and roll, the PPC relies on a natural pendulum stability provided by the long vertical separation between the aerodynamic centre and the cg; the wing acts as the suspension point for the pendulum. Any turbulence will tend to move the wing further than the vehicle, because of the vehicle's higher inertia, and the pendulum effect quickly restores the normal state after the perturbation — although the normal state is probably a gentle oscillation. A gust from the front has the effect of moving the wing back, in relation to the vehicle. This will temporarily increase aoa and thus lift, because V² is maintained, and the aircraft will rise a little until the vehicle 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 vehicle swings forward and aoa is returned to normal. Pendulum 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 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 vehicle 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.

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

Groundschool — Flight Theory Guide modules

| 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