Flow and The Windtunnel

Ludwig Prandtl 1904 - 1,5m water test channel Picture: Estate administration University of Göttingen (Germany)

The word "airflow" or "airstream" (in german "Strömung") has become ingrained in us through repetition. "Airflow" is a legitimate term when it comes to sailing ships, water, pipelines or wind power, for example. Otto Lilienthal did not yet use the term "Airflow", he spoke of "wind". Ludwig Prandtl is said to have introduced it to aviation for the first time. As an engineer he developed suction systems, as a professor a water channel and later a wind tunnel. This may be where the dilemma of terms began.

The wind tunnel is an experimental set-up. The object stands in it, fixed by brackets with sensors. The air mass is moved.

The results of measurements are similar to reality if you put a lot of effort into them. For example, it is about the Reynolds number (Re number). The aim is to achieve a Re number in the wind tunnel using a wide variety of measures, as it occurs in reality. The Re number describes the ratio of the inertia forces to the viscosity forces. The closer you get to the desired Re number, the more realistic the measurement results will be. Smoke threads are rarely blown in for optical reasons, in which case you can see the path of the air particles lined up next to each other and the image of a stationary flow (streamlines) becomes recognisable.

The image of permanent smoke trails (Streamlines) is not the reality of flying. Such smoke trails are not the trajectory of a single air particle (which is deflected vertically) but are a series of successive air particles.

Picture: NASA Langley Research Center Windtunnel

In fluid mechanics, streamlines (trajectories of lined-up particles) are referred to as steady flow. If there is no flow (i.e. the air mass has not been set in motion) but a wing / propeller / rotor blades are set in motion, the equalising movements of the displaced air particles are referred to as unsteady flow. The word flow for the sake of flow.

If you feel a headwind on the ground, then you might believe in a flow, but as soon as the aircraft is flying, the TAS is relevant for safe operation - regardless of how strong the wind is. In the event that entire air masses shift, the entire observation system / inertial system is in motion and in this system the air has no speed but the aircraft does.

Every pilot knows that they are moving with their aircraft and that the air is almost stationary - not the other way around. In real flying in the open air, the kind of airflow suggested by smoke trails in a wind tunnel does not occur, so it is astonishing that this rational insight is immediately lost in the next breath.

It is therefore a question of choosing the right inertial system. Smoke trails in the wind tunnel or drawn streamlines are seen from the inertial system of the measurement object (stationary flow).

In order to understand how the impulses (air pressure) make themselves felt on the wing, it is necessary to choose the air mass as the observation system. Streamlines do not reflect the direction of momentum / impulse. Air pressure acts on surfaces at right angles.

Lift is primarily an air pressure system around the wing cross-section. The movements of the pushed out and reunited air particles are essentially vertical. The aircraft essentially moves horizontally. The term "flow", which for everyone suggests a direction that follows the body surface, is misleading for the real movement of the air particles (unsteady flow - just because you write "flow" on it doesn't mean it's flow).

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