Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

59 Cobra Aero A33i | Dossier Exploiting the forward motion of the vehicle (with no fan assist), air is ducted into the shroud surrounding the cylinder’s cooling fins, the provision for this being tailored according to the specific airframe. Exhaust The exhaust forms a ‘U’ shape to wrap around the base of the crankcase, the standard route taking the gas to the far end and then reversing back to exit near the exhaust port, where there is also a controllable bypass of the silencing segment. The key to the operation of the silencer is summarised by Hilbert as “big volume, small holes”. The main route passes the gas through multiple small holes that “divide big wave fronts into multiple smaller ones that cancel each other out”. This approach does not call for the use of any packing material, which is beneficial in terms of time between overhauls. “We have found that systems that use packing need to be cleaned at frequent intervals,” Hilbert explains. The design of the silencing system involved computer modelling analysis and many iterations, and according to Hilbert, “Nothing in its class comes close from the perspective of the trade- off between performance and noise suppression. You could make more performance but only by being very loud. We are confident that we have more performance than any other engine given our very low noise level.” The exhaust system has passed FAR 33 testing. Back-to-back tests by a customer against an electrically powered version of his UAV found the two-stroke version to be quieter, mainly because of a difference in the propeller requirement for comparable performance. Active exhaust The A33i has a valve that has the effect of lowering the height (and with that the area) of the exhaust port. Since the exhaust port is controlled by the motion of the piston, lowering its height alters equally when it opens on the downstroke and closes again on the upstroke. Clearly, reducing the height of the exhaust port traps the content of the cylinder earlier on the upstroke. According to operating conditions this may or may not improve the purity of the charge (which inevitably contains a percentage of unburnt mixture) but it does increase the effective compression ratio, improving thermal efficiency. On the downstroke, a lower exhaust port opening implies more work is done on the piston, likewise improving thermal efficiency. At the same time, combustion pressure is less when the port opens, reducing noise. The drawback of the lower port is that it is restrictive at higher rpm, reducing peak power. Peak power is also reduced by the noise-suppressing exhaust required for a UAV. For that reason, when the exhaust port valve is opened to increase power, a flap valve is opened that allows the exhaust gas to bypass the silencer. In the A33i, whenever the exhaust port valve is closed, so is the exhaust bypass flap valve, and vice versa. Since port valve and bypass flap valve are controlled together, in Cobra terminology they are either ‘closed-closed’ or ‘open-open’. Running at WOT, on the dyno the closed-closed situation provides more power than open-open up to 6000 rpm, beyond which the open-open state releases far more power. Normally the UAV will cruise or loiter at less than 6000 rpm, at part-load. In that case the closed- closed state, for the power necessary Unmanned Systems Technology | February/March 2017 Operation of the exhaust port valve (top) and the exhaust flap