Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Wing-housed Retractable Undercarriage
Although it is common practice in large aircraft to use a main undercarriage the retracts into the wing body, this is rarely seen on small UAVs because of weight, cost, and complexity issues - the most common exceptions being where aero-modelers are building accurate scale copies of large aircraft. It is, instead, more conventional to fix the undercarriage to the main fuselage structure (with tail-wheels on the empennage in some configurations). If, however, the drag from the main undercarriage is a significant issue, then electrical or pneumatic retract
Figure 3.12 Simple FDM-printed wing tip incorporated into the outermost wing rib.
Figure 3.13 UAV with pneumatically retractable undercarriage - the main wheels retract into the wings while the nose wheel tucks up under the fuselage (wing cut-out shown prior to undercarriage installation).
systems can be fitted. Figure 3.13 shows one such aircraft that was fitted with a pneumatic retract system - in flight, the sudden acceleration caused by the reduction in drag on retracting the undercarriage was very noticeable. Suitable pockets in the underside of the wing are required to house the whole system along with structural elements to transfer the landing loads to the main spar and onward to the rest of the airframe. We make such components from SLS nylon and also increase spar diameters to allow for the loads seen - we have routinely measured accelerations of 20g in undercarriage elements during landing and even seen low-cycle fatigue failure in undercarriage axles.
Integral Fuel Tanks
While it is quite normal in full-sized aircraft to place fuel tanks within the main wing structure, this is rarely seen on small UAVs. The issue is that the materials used to construct small UAV wings are rarely compatible with storing fuels. We have, however, built integral fuel tanks into the main lifting surfaces by using SLS nylon. This is only really justifiable where there is some other structural requirement being met that calls for the extra weight and strength of the nylon over our conventional foam wings. On the SPOTTER aircraft, the central wing between the two nacelles is used to support the payload, and thus it has to be able to support considerable masses. Given this requirement, we made the part from SLS nylon, fitted a rear flap to it, and made the entire hollow structure into a large fuel tank, see Figure 3.14 and also Figures 2.3 and 2.6.
Figure 3.14 Integral fuel tank in central wing section for SPOTTER UAV.
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