Small UAV wheels are commonly made from injection-molded plastic or spun or turned aluminum and fitted with rubber tires (either solid foam or pneumatic). They can be purchased in a range of sizes, though the choice gets rapidly limited above 150 mm diameter. In the larger sizes, roller or ball bearings may be fitted as may wheel brakes. We tend to avoid both unless the aircraft is substantially over 30 kg in weight. Assuming a conventional undercarriage is to

Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.

©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.

Some typical small UAV undercarriages

Figure 7.1 Some typical small UAV undercarriages.

be fitted the main wheel, diameter represents a significant design variable; large wheels give better ground handling, lower rolling resistance (thus aiding takeoff), and a greater ability to deal with less than perfect runways. The penalties of large wheels are twofold: most obviously, they weigh more, and as with all aircraft components, weight is a key factor; perhaps more important, however, is the added drag large wheels generate if they are not retracted out of the airflow. It is a well-known fact of aerodynamics that circular objects have startlingly more drag than those of an airfoil shape but similar frontal area. According to Hoerner [9], typical drag coefficients, based on wheel outer diameter and width, vary from 0.12 to 0.35 as compared to streamlined shapes where values less than 0.01 are normal. Thus the drag of the

An aircraft with spats fitted to its main wheels to reduce drag

Figure 7.2 An aircraft with spats fitted to its main wheels to reduce drag.

whole fuselage is typically less than that of the main wheels when unretracted. This drag can be reduced if the wheels avoid having hollow centers (as most do) and fairings (commonly termed spats) are placed over the upper part of the wheel, see Figure 7.2.

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