by Ollie Brower

Let's say we select a 30% airfoil (NACA 0033), to streamline our axle trees. This is what is used on small aircraft (slow speed), to streamline protruding objects such as axle supports for the wheels, and wing supports (Figure 1).

Fillets on these axle trees help reduce air drag (the Number 1 killer in soap box racing). These fillets reduce parasite drag by making the air flow less turbulent at the body/protruding object transition points. Air flow is always moving to the low pressure areas of your racer, flowing up and down over the car's body. The air flows much smoother when abrupt changes are eliminated or reduced by the use of fillets.

These fillets are calculated for this 30% airfoil, and a 3/4 inch axle. The airfoil is cut into two pieces at the 30% mark, with the axle in between (Figure 2).

The leading edge fillet will have a radius of 15% of the chord. The chord is 3 1/4 inches. This radius will be 1/2 inch (Figures 3 and 4).

The trailing edge fillet will have a radius of 30% of the chord. This radius will be 1 inch (Figures 3 and 4).

With almost all airfoils, the leading edge is much more important in reducing air drag than is the trailing edge. With fillets the trailing edge fillet is every bit as important as the leading edge fillet.

The top and bottom fillets will have a radius of 30% of the airfoil thickness. This calculation is for straight sides. If your sides on top of the axle slope toward the outside, (Figure 5), make this top fillet larger. A radius of 3/8" will be necessary if the slope angle is 45 degrees or larger. This is due to a combination of side angle slope and the thinness of the airfoil (only 3/4").

If your sides are straight, up and down, at the axles, use a radius of 1/4 inch.


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