Small engines like this - there is an identical creature in the Spruce Goose Museum at McMinnville, OR
- used in aircraft were referred to as Auxiliary Power Units. Similar units used on the ground for a wider range of uses - in WW1, ABC twins were used to drive an airborne spotlight or (for instance) trench pumps: these were referred to as GPU's, variously "Ground" or "General" depending upon whom you believe!
They served various purposes, but almost every example was used to drive a generator in order to keep the on-board batteries charged. Some were very specialised and the ABC MkII APU fitted to Sunderland flying boats could be electrically started from the cockpit and then generate 300W at 24 volts. Alternatively, Bowden cable controls could engage a 2,500 gallon pump to empty both bilge and floats, a similar pump to raise petrol from a bowser to fill the fuel tanks. This done, further controls could engage air compressors that could charge air starting bottles to 400psi inside ten minutes. All this was achieved by a complicated air cooled OHV flat twin buried in the starboard wing root.
B17's had one too. A Homelite TS engine drove a generator & it swayed about noisily on four bed springs. The ground-based unit was inverted but I don't have a clue why - mine is painted white. The green Homelite flat twin is an example of a GPU, ex Canadian Army.
Various other examples exist & yes, they are my specialty, how did you guess???