Paper Airfoil Aerodynamics4 - Curvature,
Viscosity, and Lift | |
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I.
A. Next B. Back At low speeds, fluid tends to flow easily across the surface of an object. The viscosity of a fluid plays an enormous role in flight, so much so in fact that the British scientist Lord Raleigh noted that an airfoil would not work in a non-viscous environment.
The viscosity of a fluid is a measurement of that fluid?s resistance to shearing. Fluids behave in such a way that, unlike solids, it is not the amount of shear placed upon the liquid but the rates at which that shear is applied that determines its resistance to flow.
From the perspective of a small amount of fluid, flowing along with
the greater stream of fluid the following behaviors can be deduced. As the fluid particle passes over
the surface, viscous forces cause it to stick to the surface. Meanwhile, the rest of the flow
continues on its way, providing a shearing force to that
particle.
A wing provides lift because the viscosity of air causes an
acceleration in the flow of air as it moves to equalize the pressure
difference of the wake of the wing.
Changing the direction of the air as it flows over the wing brings
about this acceleration, and therefore an increase in velocity, and a
decrease in pressure. Without
this viscous force to change the direction of the flow, it would be
impossible to fly.
More and more particles continue to stick to the surface of the
object, as well as the original particle due to viscosity effects. The further from the object that
this layer of stagnant air reaches, the more susceptible it is to
fluctuations in the flow of air, and therefore the more likely it is to
remain very close to the surface of the object. This layer of air is referred to
as a boundary layer.
In an ordinary airplane wing, this boundary layer is only on
average about the thickness of a sheet of cardboard. A large boundary layer would lead
to a great amount of viscous drag.
Figure 4.1 gives a visual schematic of the boundary layer of fluid
along an airfoil.
Now let?s say the fluid particle is moving with the flow of air
over the surface of the object and that there already is a good quality
boundary layer already attached to the surface of the wing. Let us also say that the surface
of the object is smooth and continuous along the limits of our
experiment. From the
perspective of the fluid particle, the curvature of the object changes
over time with respect to its velocity.
Should this change in curvature along the direction of flow over
time be too great, the fluid particle will no longer be able to stick to
the surface of the wing and will separate from it. This effect is known as the Coanda
effect. Figure 4.2 and 4.3
show this effect in action. This separation of airflow means that it can no longer be accelerated by motion over the wing as it attempts to equalize the pressure difference created by the wing?s wake. This boundary layer separation, as it is called, is commonly referred to as a stall. As a wing is pitched up, the curvature of its forward portion increases with respect to the flow of air. Once this change in curvature becomes to great, the airflow separates from the wing, causing a failure in lift. Figure 4.2 shows a stall as it occurs due to this excessive angle of attack. I.
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Figure 4.1 ? The boundary layer of fluid across an airfoil.
Figure 4.2 ? Smooth airflow (top) and boundary layer separation (bottom).
Figure 4.3 -- The Coanda effect in action. |
