Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

58 at the front of the crankshaft, just inboard of the hub driving the propeller. There is quite a long span between the centre bearing and the nose bearing. “We like a long span for the stability of the shaft,” remarks Johnson. “There isn’t much load at the rear, so the crank journal diameter is reduced in that rear section, and it has only a small and hence light bearing. Nothing runs at the rear of the shaft aside from the bearing, so the tail of the crankshaft is a small stub; it is primarily centring the shaft.” The front and rear webs are shaped for counterbalance, whereas the centre web simply links the two crankpins. Johnson notes that the centre web is known by Desert Aircraft as ‘the peanut’. “That is because in previous years, to reduce weight, we took material from that link, leaving it peanut-shaped [in 2D when seen from the front of the crankshaft], whereas now we leave it simpler in shape in view of stress considerations. “Strength is very important, and any reduction in the area of that link is a stress point.” The two counterbalance webs are of the classic ‘pork chop’ pattern, one up, one down. All the counterbalancing weight is formed integrally with the crankshaft; there is no add-on. The balance factor is 52%. We asked Johnson, is that in view of external or internal vibration? “These are in such frail structures that we have to be very wary of external vibration,” he says. At the front of the crankshaft, the propeller hub slips onto a taper section. Then the retaining nut screws into a thread formed in the nose of the crankshaft, and it has an external taper form that marries with a corresponding taper on the hub. The hub therefore has a forward-facing taper that engages with the nut, and a rear-facing taper that engages with the shaft taper. “That ensures the critical centring of the propeller hub,” Johnson explains. There are two options for generator power take-off. One is to run an extended length of the crankshaft out from the rear, which requires modifying the case with an extra rear bearing. The other option is to run the take-off at the front, whereby the drive is taken from a modified prop hub. “I favour the rear option, letting the prop drive simply do the job it was intended for,” Johnson says. Is there any implication for balance? “Neither option affects how the engine runs; neither affects vibration – the generator just sucks up power.” The forged steel con rods are I-section. “Almost like a flat plate with a little trough each side,” Johnson remarks. The rod is shot-peened. There is a copper finish, the masking for a carburising process that hardens the bearing running surfaces at the top and bottom end. “We run open needle roller bearings top and bottom [directly in the rod],” he notes. The con rods are piston guided, although Johnson reckons that crank guiding could work equally well. The piston pins and the crankpins run directly in the needle roller bearings. Each pin runs directly in its piston – it has “a tight slip fit” and is retained by steel wire C-clips or G-clips. “G-clips are easier, August/September 2017 | Unmanned Systems Technology The DA150 seen from the rear, where it is attached to the airframe by the four mount points visible Nothing runs at the rear of the shaft aside from the bearing, so the tail of the crankshaft is a small stub; it is primarily there for centring the shaft