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Home arrow Engineering arrow Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

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Undercarriages

Undercarriage design can be something of a Cinderella subject when considering new aircraft. It may, however, dominate much of the inner wing and main fuselage layout, and for some types of UAVs become a significant design driver. Of course, not all UAVs have undercarriages, since for some applications catapult launch and belly or net-based landing can be an advantage. We have certainly built such aircraft, including SULSA (the Southampton University Laser Sintered Aircraft - the world’s first fully SLS airplane), but generally it is more normal to operate from a conventional grass strip runway with a wheeled undercarriage. These are commonly of tricycle layout with soft wheels and some form of suspension. Since there is a large drag penalty associated with leaving wheels in the airflow, it can be desirable to retract the undercarriage in flight, but this brings much added complexity and additional failure modes, weight, and cost. Again, we have used retract systems, but mostly choose not to do so since our flight cruise speeds are rarely above 40 m/s (for vehicles that must operate in adverse weather conditions where significant headwinds are probable, then retract systems are all but inevitable). In addition, it is normal to have some form of steerable system, generally linked to the rudder, so that ground maneuvering is easier. Using a nose wheel arrangement protects the nose of the aircraft (and the propeller if a tractor arrangement is in use), while tail wheels generally require shorter struts and are typically lighter and simpler to design and do not obstruct any forward-looking under-hung sensors (but can restrict rotation angles on takeoff and also need to allow for any overturning moments caused by the combination of propeller thrust and main wheel drag); the final choice is often dependent on the particular preferences of the design team. Figure 7.1 shows a range of typical small UAV undercarriage systems.

 
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