What does the pressure distribution over a glider's wing look like?

Question

Can someone explain the design of glider wing airfoils and the subsequent pressure distribution over them?

I hypothesize that:

1. The pressure distribution should form a resultant force in the forward, upward direction, since two components of force are required to maintain equilibrium in the flight of a glider: a) lift - in equal opposition to weight, and b) a forward component of the total reaction generated by the airfoil - in opposition to drag.

2. Thus I conclude that the angle of attack of a glider airfoil relative to flight direction would be negative, in order to achieve a forward component opposing drag; although it would be positive relative to airflow, such as to generate low pressure over the wings. Either that or the camber is designed such that the two mentioned criterias here, (a) and (b), are upheld.

Should there not be any opposition to drag, I would expect to see the glider decelerate in the forward direction, until the airflow stalls over the wings, and the glider loses lift and falls.

I am also curious to know about the various designs of glider airfoils: a) what are the criterias defining them, and b) how do they differ from those of powered aircraft, if there are any differences at all?

Answer 1

The pressure distribution over a glider's wing is no different from that of a well-designed aircraft. A glider can maintain flight by flying slightly downwards, so the direction of motion is not straight but slightly down. Doing this in rising air will result in sustained flight.

Look at the sketch above: The flight path angle $\gamma$ is negative and in still air the flight path vector $\text{x}_k$ is parallel to the direction of the airspeed. Due to the inclination of the flight path, the lift vector is tilted forward such that it's horizontal component is exactly equal to the horizontal component of drag, and the vector sum of lift, drag, and weight is zero. This is shown on the right where I moved the vectors into a closed sequence which demonstrates that all forces balance.

From the viewpoint of the glider, the lift is pointing straight up and drag directly backwards, but the weight force is slightly tilted forward. In a way, it seems that the thrust of a glider is its weight force.

Now let the whole air packet in which the glider flies move up. The glider will still sink down within this air but relative to the ground it will gain altitude if the upward air velocity is high enough.

Answer 2

I think you are approaching this problem wrong. If you think about how a glider flies, there is no forward force component on the airfoil that opposes drag. By definition, drag is the force that opposes the motion of the craft and lift is perpendicular to the drag force.

Suppose we drop the glider from some height in a typical flight position (ie fuselage parallel to the ground). The only way this glider is going to fly forward (assuming no tailwind) is by dropping the nose, creating a negative angle of attack with respect to the ground. The lift generated by airfoil here will have both horizontal and vertical components relative to the ground. However, you are also losing altitude in this orientation. The point here is, without initial speed (forward momentum) the only way to create forward speed is to convert potential energy into kinetic energy by losing altitude, so a negative angle of attack will create forward speed, but will cost you altitude.

As to your other questions in regard to glider airfoil design vs propelled aircraft airfoiled design, glider airfoil are typically low camber, low lift, and low drag. This is because while higher camber airfoil generate more lift, they also generate significantly higher drag. This would cause the glider to lose forward momentum faster. Since lift is a function of velocity, your glider loses lift during this time and will fall out of the sky. With propulsion, these airfoil can be used to carry heavier loads (since more lift is generated and the propulsion directly counteracts drag forces). Gliders are typically light and therefore require less lift to stay aloft. The low drag profiles allow them to maintain forward momentum and speed without losing much altitude.

In general, you always want the pressure below the wing to be higher than the pressure above the wing. As soon as this condition is not met, the plane/glider will begin to fall from the sky. Also note that just because the pressure below is higher than the pressure above does not guarantee the aircraft will stay aloft. The upward force generated by this pressure (F=P x Area) must exceed the downward force caused by gravity (F=Mass x Gravity).

Answer 3

This question focuses on a very interesting mechanistic question about what exactly happens when a (model) aircraft is simply dropped from a height.

How does the aircraft start to glide?

If the aircraft is held horizontally, and let go, gravity will cause acceleration. Aerodynamic drag forces now act on the surfaces of the aircraft. Longitudinal stability will start to point the aircraft in the direction of motion because the center of drag and the center of gravity are off-set, causing rotation, but the wings are still stalled. Not yet a glider.

In drawing diagrams, at the start, when released, only gravity and drag act on the plane. The "wind" is striking the underside of the aircraft. If you are a parachute, those are the vectors drawn.

But even parachutes can move sideways by off-setting center of drag and center of gravity. The "sideways" motion, combined with downward motion, is the new flight path.

Mechanisticly, what is drawn is a rotation as well as drag and gravity forces. The "tilt" caused by rotation gives vertical drag a horizontal component, enabling one to change where the parachute will land.

when rotation from drag forces lowers AoA enough to un-stall the aircraft, the wing becomes more efficient at producing upwards and "sideways" forces than vertical drag

The trim of the plane will have rotating forces in balance at an airspeed and AoA sufficient to produce adequate lift.

The glider now flies away in a straight path with the lift, drag, and weight force vectors summing to zero at a constant (steady state) velocity.

Fairly complicated procedure to throw a paper airplane, but illustrating why there is more than just trim to a tail.