Paraglider Flight Dynamics
A complete understanding of the physics of flight and how a wing works is an entire science unto itself. This section will give you a basic understanding of flight – and in particular – the forces and critical angles that a glider requires to fly.
Forces
Paragliders, like all gliders, are subjected to three forces: lift, drag, and weight/gravity. All these forces have a magnitude and a direction. The weight acts through the center of gravity and is always directed toward the center of the earth. Lift and drag are aerodynamic forces and act through the center of pressure. The lift is directed perpendicular to the flight path, and drag is directed opposite the flight path. The magnitude and direction of the lift and drag forces depend on the size and shape of the glider with payload, air density, and airspeed.
Lift is generated by “turning” or redirecting the air around a wing. The turning of the airflow results from the air molecules staying in contact with the surface of the wing due to relatively fast moving airflow which generates low pressure regions at the top surface. Think of the wing as “yanking” down on the air molecules above it. We know from Newton that Force = Mass * (Change in velocity / Time). Velocity has a speed and a direction. By changing either the speed or direction of an airflow, you generate a force, in this case, lift.
A change in velocity causes a force, and a force will cause a change in velocity. Because the velocity and direction of the air around the wing vary, so do the amounts of lift generated at various points on the wing. The average of those forces is the lift vector. Simply put, the force of a wing pulling air molecules downward into the low pressure at the top surface, generates an opposite force that pulls the wing upward.
The motion of a glider through the air generates drag. In a powered aircraft, thrust from the engine (kinetic energy) opposes drag. A glider must trade altitude for speed, or trade the potential energy of its altitude for kinetic energy or speed. Thus, gliders must always descend relative to the air in which they are flying.
Gliders are able to stay aloft for hours at a time because they are relatively efficient and descend slowly. If the glider is subjected to air that is rising faster than its descent rate through the air, the glider will gain altitude and increase its potential energy.
Angle of Attack
The angle of attack is the angle created by the difference between the chord line of an airfoil and the relative wind. The chord line of a wing is an imaginary line drawn from the center of the leading edge to the trailing edge. Relative wind is what the wing feels as it travels through the air. Most gliders are designed with the optimum angle of attack achieved at trim or with no extra input on the glider. The angle of attack can be changed by the pilot through use of the brake toggles (brakes) or the speed system. As a non-powered aircraft our deviation from the mean angle of attack is relatively narrow. You should be careful and thoughtful about your glider’s angle of attack for both safety and efficiency.
The chord line and angle of attack change with brake input, or when the wing suddenly encounters a vertically moving column of air. As the angle of attack increases, the amount of lift also increases, up to a point (minimum sink). If the angle of attack is increased beyond “minimum sink” the amount of lift decreases while drag increases until the airflow over the wing can no longer remain attached to the top surface. The lift force is instantly zeroed and the drag vector, in opposition to the flight path, becomes perpendicular to the horizon.
Beyond the stall angle, the glider is no longer generating lift and the only force slowing your descent is drag. This situation is usually caused by a pilot who is using too much brake input to control their glider and can lead to spins and stalls, especially when flying in thermals.
Wingtip Vortices
A byproduct of lift is a rotating spiral of air that is attached to the wing and travels with the airfoil. At the wing-tips, the vortex, sometimes called wake turbulence, is shed and produces a net descent relative to the air around it. The slower or heavier an aircraft, the more air it must redirect, and the larger the circulation in the vortex. The vortex, like a thermal, is subject to the air around it and can drift with the air-mass for a significant distance. Care should be taken when flying a paraglider in the vicinity of other aircraft as vortices are equivalent to turbulence and can cause deflations. Vortices from powered aircraft and even tandem paragliders with relatively high wing loading can create strong wake turbulence.
Glide Ratio
The glide ratio is the ratio between the horizontal distance covered, and the vertical distance covered. Another common term you may hear used is the lift to drag (L/D) ratio. which is an indication of overall aerodynamic efficiency. A glider that produces a lot of lift and very little drag has a high L/D ratio and will travel farther or carry more weight. A glider that produces relatively little lift and a lot of drag will travel a shorter distance. The L/D ratio of a paraglider remains relatively constant while it’s glide ratio over the ground may change dramatically due to rising or sinking air or wind. Most beginner paragliders have a glide ratio of 8:1 in still air, while a top competition glider achieves close to 10:1.
Maximum Glide is the ratio at which the glider will fly the farthest distance possible. The speed of maximum glide is usually considerably more than minimum sink.
Minimum Sink is a term that it used when you are flying your glider with enough brake pressure to slow the descent of the paraglider to the lowest possible rate prior to stalling the wing.
Polar Curves
Every wing has a polar curve that describes the speed of the wing through the air and the amount of lift it generates. Most paragliders, with no brake input, fly at about 23 MPH through the air. At that speed they sink at roughly 250 Ft/min. If the angle of attack is increased and the speed decreased, the glider will generate more lift, but fly slower.
Minimum sink, or the wings greatest lift factor, is roughly 200 ft/min at 17 MPH. If the angle of attack is increased beyond minimum sink to just above the stall point the sink rate increases again to 300 Ft/Min and the airspeed drops to 12 MPH. If the speed system is used and the angle of attack is decreased beyond trim the airspeed will increase to 35 MPH and the sink rate will increase to 500 Ft/Min or more. The more efficient the paraglider, the flatter its polar curve and the more useful its full range of speed becomes.