page_title: Hang gliding
section_title: Stability and equilibrium
hierarchical_section_title: Hang gliding / Stability and equilibrium
caption_attribution_description: They went off the top of Hyner State Park Lookout on Labor Day 2006.
context_page_description: Hang gliding is an air sport or recreational activity in which a pilot flies a light, non-motorised foot-launched heavier-than-air aircraft called a hang glider. Most modern hang gliders are made of an aluminium alloy or composite frame covered with synthetic sailcloth to form a wing. Typically the pilot is in a harness suspended from the airframe, and controls the aircraft by shifting body weight in opposition to a control frame.
  Early hang gliders had a low lift-to-drag ratio, so pilots were restricted to gliding down small hills. By the 1980s this ratio significantly improved, and since then pilots can soar for hours, gain thousands of feet of altitude in thermal updrafts, perform aerobatics, and glide cross-country for hundreds of kilometers. The Fédération Aéronautique Internationale and national airspace governing organisations control some regulatory aspects of hang gliding. Obtaining the safety benefits of being instructed is highly recommended and indeed a mandatory requirement in many countries.
context_section_description: Because hang gliders are most often used for recreational flying, a premium is placed on gentle behaviour especially at the stall and natural pitch stability. The wing loading must be very low in order to allow the pilot to run fast enough to get above stall speed. Unlike a traditional aircraft with an extended fuselage and empennage for maintaining stability, hang gliders rely on the natural stability of their flexible wings to return to equilibrium in yaw and pitch. Roll stability is generally set to be near neutral. In calm air, a properly designed wing will maintain balanced trimmed flight with little pilot input. The flex wing pilot is suspended beneath the wing by a strap attached to his harness. The pilot lies prone (sometimes supine) within a large, triangular, metal control frame. Controlled flight is achieved by the pilot pushing and pulling on this control frame thus shifting his weight fore or aft, and right or left in coordinated maneuvers.
  Roll
  Most flexible wings are set up with near neutral roll due to sideslip (anhedral effect). In the roll axis, the pilot shifts his body mass using the wing control bar, applying a rolling moment directly to the wing. The flexible wing is built to flex differentially across the span in response to the pilot applied roll moment. For example, if the pilot shifts his weight to the right, the right wing trailing edge flexes up more than the left, allowing the right wing to drop and slow down.
  Yaw
  The yaw axis is stabilized through the sweep back of the wings. The swept planform, when yawed out of the relative wind, creates more lift on the advancing wing and also more drag, stabilizing the wing in yaw. If one wing advances ahead of the other, it presents more area to the wind and causes more drag on that side. This causes the advancing wing to go slower and to fall back. The wing is at equilibrium when the aircraft is traveling straight and both wings present the same amount of area to the wind.
  Pitch
  The pitch control response is direct and very efficient. It is partially stabilized by the sweep of the wings. The wing centre of gravity is close to the hang point and, at the trim speed, the wing will fly "hands off" and return to trim after being disturbed. The weight-shift control system only works when the wing is positively loaded (right side up). Positive pitching devices such as reflex lines or washout rods are employed to maintain a minimum safe amount of washout when the wing is unloaded or even negatively loaded (upside down). Flying faster than trim speed is accomplished by moving the pilot's weight forward in the control frame; flying slower by shifting the pilot's weight aft (pushing out).
  Furthermore, the fact that the wing is designed to bend and flex, provides favourable dynamics analogous to a spring suspension. This provides a gentler flying experience than a similarly sized rigid-winged hang glider.
caption: High performance flexible wing hang glider. 2006
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