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

Structural Failure

For small and medium-sized UAVs, ground handling damage to the airframe is as likely to occur as structural failure in flight. For larger aircraft ground damage becomes less of an issue since such aircraft are basically too large for manual lifting. In either case, a series of take-off, flight, and landing loads must be considered when designing the structure. Perhaps the most obvious structural calculation needed is to size the main spar or equivalent monocoque wing structure used to lift the UAV while airborne. Since the aerial environment is by its very nature a random one, designers typically design against “gust and maneuver loadings” expressed in the form of multiples of gravitational acceleration. Thus one might design a wing spar to deal with 5g loadings, that is, a simple weight load equal to 5 times that of the aircraft, assumed to be lifted by the pressure differential on the wings. Euler-Bernoulli beam theory will then allow one to assess likely the stresses and deflections in the wing. If the deflections are too great or the stresses too close to failure stresses, the design can be iterated to strengthen it. Similar estimates can be made for tail and fin loads linked to likely lift coefficients or inertia loads. Since carbon fiber tubing is such a structurally efficient and low-cost material, it is rarely a problem to provide sufficient strength in a design, even using simple booms and spars. More difficult is dealing with the way such elements are integrated into the rest of the aircraft structure - all highly stressed junctions will need careful detailing later on in the design process. One advantage of monocoque approaches is that local stress raisers are much less likely, though, conversely, it is then no longer simple to assess likely stresses at the outset of the design process.

After considering the main flight loads, the next most important loading that occurs during operation will be landing loads on the undercarriage. Because UAVs are typically smaller than manned aircraft and commonly use less well prepared runways, these loadings are typically much larger compared to flight loads than for manned aircraft. It is because of this that is usual to try and have rather low landing speeds for UAVs. In our experience, one of the main design choices made for any new small or medium UAV is the landing speed. If this can be held below 15m/s, or even 12m/s, then landing loads will be very much reduced. Even so, we have measured acceleration of as much as 200 m/s2 on landing gear on a 25 kg aircraft. Thus the landing gear on such an aircraft may have to tolerate loads of half a ton, if only very briefly. These loads can be such a problem during heavy landings that on some designs we have included mechanical fuses that are deliberately sacrificial and protect the rest of the airframe structure by absorbing energy during the impact and which can then be easily replaced for subsequent flights. In any case, a key design aim will be to structurally connect the main landing gear to the wings and the heavier elements in the fuselage so as to effectively transmit deceleration loads. Again, much of this will be considered during detail design, but due allowance must be made to include sufficient space and weight budget in the initial concept design. Large wheels, suspension systems, and dampers all add mass and cost to a design, as well as possibly impacting on aerodynamic performance if not retracted.

The aircraft structure may also need to cope with fuel and oil spills, exhaust products, rain, and possibly salty environments. For example, we use foam to construct many aerodynamic surfaces but these are often not resistant to gasoline or exhaust products from internal combustion (IC) engines. We thus have to ensure that any exposed areas are clad in suitable protective layers of film of fiber-reinforced plastic. Aluminum parts will be vulnerable to salt damage if not washed off after flights over the sea; steel parts will be subject to rusting unless suitable stainless grades are used. Many aerospace materials are also vulnerable to fatigue failure stemming from vibrations or repeated loadings: engine mounts and wheel axles are two areas rather prone to such problems (particularly if of welded construction). Suitable care must be taken to guard against all these forms of structural failure.

Once the operational issues have been dealt with, consideration must be given to ground handling. While large UAVs will need treating just like manned aircraft and may need tow trucks and hangars, small and medium UAVs are typically packed up into shipping or storage crates after use. This may well involve operators undoing fixings and withdrawing wings, tail parts, and perhaps other components. Given that any structural junction will try and avoid play and slop, this may require noticeable effort, during which time a firm hold of the airframe may be necessary. To avoid the possibility of damage, suitable hard points will need designing into the UAV. Furthermore, any storage crates must be designed to hold delicate structures during transit, again by supporting at known hard points and/or by the liberal use of foam packaging. We typically have large flight cases custom-made for our more valuable or delicate aircraft. It is surprising how much damage can be accrued by trying to split down a UAV and transport it unprotected in an everyday automobile. Finally, it is worth noting that when unpacked and assembled and before flight, aircraft should be placed (and tethered) in locations where people will not accidentally trip over them or stumble onto them, or where they can be caught by unexpected gusts of wind.

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