Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Most of the UAVs we build have conventional wheeled undercarriages either with nose- or rear-wheel steering. Obviously, the size and strength of the undercarriage is set depending on the maximum take-off weight (MTOW) of the aircraft. Wheel diameters, however, must also reflect the likely runway conditions - even very light aircraft must have sufficiently large wheels so that they do not have problems rolling over any unevenness in the runway. This is particularly true when dealing with grass strip runways.
The design loads used in sizing the undercarriage and its mounting points stem from two cases: first, one must allow for impact loads on landing. Although the pilot or automated control system will, of course, attempt a smooth landing, significant forces will still arise and care must also be taken to be able to cope with less than perfect approaches. Second, allowance must be made for faults in the runway surface. Even on well-maintained grass strips, it is not uncommon to encounter small mounds or divots that can generate significant and sudden impact loads while taxiing at speed or during landing and takeoff. Suitable suspension systems can help mitigate these loads, but nonetheless the structural mounting points for undercarriages usually need to be among the strongest points in the airframe. When working with SLS nylon structures, we sometimes find it necessary to use metallic load spreaders to cope with the loads being generated. Alternatively, rather large and well-reinforced areas of nylon will be required. Figure 4.18 shows a nose wheel mounting with metallic reinforcing, while Figure 4.19 shows a plain nylon attachment point using two nylon bearings to permit steering.
Figure 4.18 Metal-reinforced nose wheel attachment with steering and retract hinge in aluminum frame attached to SLS nylon fuselage. Note the nose wheel leg sized to protect the antenna.
Figure 4.19 Nylon nose wheel attachment. Note the significant reinforcement around the lower and upper strut bearings.
Figure 4.20 Tails attached directly to the fuselage. The right-hand aircraft is a heavily modified commercial kit used for piggy-back launches of gliders.
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