Table of Contents

Honors Project

Introduction

The goal in this honors project was to design an airfoil with a specific pressure distribution in mind. The desired pressure distribution depends upon the application, which is why there exist so many airfoils.

The application in mind was that of a high-speed tunnel boat. Tunnel boats have two hulls that are adjoined by an airfoil. As the velocity of the tunnel boat increases, the bow of the boat generates enough lift to raise itself out of the water. This causes drag to become much smaller as surface area exposed to the water (which is almost one-thousand times denser than the surrounding air) lessens. Tunnel boats such as these are capable of achieving incredible high speeds on water due to the significantly lower drag. Many are even capable of traveling at low aircraft speeds.

forces on tunnel boat
Figure 1. A tunnel boat is shown with all acting forces drawn.


The problem that exits with tunnel boats is that they are statically unstable and require constant "pilot" correction, much like the Wright brother's aircrafts. Specifically, the aerodynamic center of tunnel boats tends to be considerably ahead of the center of mass. While this is normally acceptable, if the boat pitches to too high an angle of attack, the tunnel boat will continue the rotation and flip over. At high speeds, this can be a fatal accident.

A possible solution to this problem would be to use an airfoil that stalls out at a low, specific angle of attack. This would cause the generated lift to suddenly drop, thus significantly reducing the pitching moment. The drag would experience a sudden increase, which would decelerate the tunnel boat until the boat once again reached equilibrium. It is important to note that air drag actually produces a positive pitching moment, however at low angles of attack the moment arm is too small to be a significant contributor to the whole moment.

An airfoil that behaved like this would develop a strong adverse pressure gradient at a specific angle of attack. Fluids will detach from a surface if the adverse pressure gradient is large enough (i.e. dP/dx >> 0), thus causing flow separation. When the flow separates, lift drops dramatically and the body accumulates a large amount of pressure drag. Thus, the pressure distribution sought is one that rapidly develops this adverse pressure gradient at a sought angle of attack.

Next: Experimental Method and Set-up

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