Airfoils for Tailless Airplanes: Design and Selection

Airplane Types and Moment Coefficient
Moment Coefficient and Airfoil Shape
Reflex and Moment Coefficient
Reflex and Lift & Drag
Location of Camber and Moment Coefficient
Dangers everywhere
Velocity Distribution and Boundary Layer
Airfoil Design for Light Tailless Airplanes

Velocity Distribution and Boundary Layer

To shed more light on the boundary layer problems, we will have a look at velocity distributions of airfoils with reflexed camber lines.

How camber and reflex change the velocity distribution.

The image above presents velocity distributions of four different airfoils with different combinations of reflex and camber. The moment coefficient of all airfoils is similar.

In general, the velocity distributions of the upper surfaces show high velocities in the first third of the chord length, steadily decreasing as the flow reaches the trailing edge. Depending on the Reynolds number, the steepness of the velocity drop (which, according to Bernoulli's equation, represents a pressure rise) is limited. When the pressure rise is too strong, the flow separates, causing loss of lift and increasing drag.

The velocity distribution on the lower surface pose less problems, with a danger for boundary layer separation near the trailing edge. This will have a destabilizing effect and will cause an increase in drag.

A unique feature of airfoils with a reflexed camber line is the crossover of the velocity distributions in the second half of the chord length. Increasing the amount of reflex, speeds up the flow on the lower surface, while it slows down the flow on the upper side. The enclosed area of negative lift near the trailing edge increases, driving the moment coefficient towards positive (more stable) values.

Increasing the camber will increase the velocity on the upper surface and decrease the speed on the lower surface. The enclosed area of positive lift in the front half of the airfoil also increases, contributing to the lift. When the camber is increased too much, the maximum lift may decrease, because the camber has to be compensated by a larger amount of reflex, putting more stress on the boundary layer. Typically, the maximum lift can be increased to a certain amount, by increasing camber and reflex, but at the cost of a harder stall, which might be dangerous during takeoff and landing.

Dangers everywhere
Velocity Distribution and Boundary Layer
Airfoil Design for Light Tailless Airplanes

Last modification of this page: 21.05.18

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