Why paragliders fly forward?

.. because gliding wings have no drag!


    For many pilots lift opposes weight and drag is the price of moving through the air. The classic aerodynamics explains that lift is a force which is perpendicular to the airspeed (airflow) and drag is a force which is opposite to the airspeed. But lift and drag cannot explain why paragliders, hangliders and sailplanes fly forward. How can they have drag and still fly forward without engine?

    This dead end lift-drag logic results from the perception that lift and drag are independent forces. In fact they are just artificial components of a single aerodynamic force. Every moving through the air object experiences only one aerodynamic force, no matter what its shape, position and speed. This, so called full aerodynamic force R can be divided in whatever components we like, depending on the task we solve.

    For motorized airplanes it seems initially simpler to explain things by dividing the full aerodynamic force in lift and drag components in relative to the direction of movement:
"The thrust is used to overcome drag."
( https://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.html).
"Drag is the aerodynamic force that opposes an aircraft's motion through the air."
( https://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html )
"Lift is the force that directly opposes the weight of an airplane. Lift is a mechanical aerodynamic force produced by the motion of the airplane through the air and is directed perpendicular to the flow direction."
( https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html )

    Better and deeper analysis can be achieved if the full aerodynamic force is examined in relative to the wing surface. Thus it has a perpendicular to the profile normal component Rn and a parallel to the profile tangential component Rt . For simplicity we draw the tangential component along the profile chord and the normal component perpendicular to it.


    The most significant is the tangential component Rt ,which works like an engine. The bigger it is, the bigger is the "thrust force" of the wing. When the tangential component points backward, then it works like a reversed engine thrust force.

    In order to understand the importance of the tangential component of the full aerodynamic force, we have to understand how it is created.
    If we place a symmetrical profile in an airflow, it will experience drag Rx and 2 opposing each other lift components Ry , perpendicular to the airflow direction.
    If we divide this profile and place one half in the airflow, it will experience drag Rx and only one unbalanced lift component Ry, perpendicular to the airflow direction.

inductive ability of a wing profile

    Now it should be easy to understand an important feature of the wing profile. This so called inductive ability is the ability of the wing profile to create tangential force from perpendicular (normal) to its surface airspeed. For simplicity we say tangential force Rt , but we mean tangential component of the full aerodynamic force. When we say that normal airspeed is needed for the creation of tangential force, we don't mean only perpendicular to the wing surface airspeed but we also mean sufficient normal airspeed component .
    For example, if we drop a wing, the airflow from bellow will create a suction around the roundness of the leading edge, which will accelerate the wing forward. The tangential force Rt will add horizontal movement to the vertical fall. Initially 90°, the angle of attack will decrease, but due to the convex shape of the top surface near the leading edge (camber), the tangential force will continue to exist and increase until balance is reached.

steps of inductive ability

    The tangential force Rt depends on:
- wing profile. The ticker profiles with proper upper surface curve (camber) closer to the leading edge are better;
- airspeed. Like lift and drag components of the full aerodynamic force;
- angle of attack; 

    Finally we come to the question, why paragliders fly forward?
Because the weight force G pulls down, which makes an airflow from bellow. The suction from the leading edge and the upper surface camber tilt the aerodynamic force forward (creating tangential component). The wing stops accelerating because of the drag from the lines and the pilot and the whole paraglider reaches balance. 

steps of inductive ability

     Of course there are much more details, but the whole idea about the inductive ability of a wing can help explore:
- paraglider stability and control;
- gliding and soaring technique efficiency;
- paraglider design improvements;
- how birds with flapping wings fly forward;
- etc.;

In physics, induction means an impact by indirect influence (distant by a field or direct by a physical contact).

These ideas are brought by Nikolay Tsarov in 2011 and written by Nikolay Yotov in August, 2014.

All comments and critics are welcome!