I came across online an airplane with two engines driving one prop quite some time ago. For the life of me, I can’t think of what it was. IIRC one of the ‘Big Three’ (Cessna, Piper, Beechcraft) put two engines in the nose of a single-engine airplane, and connected them to a gearbox to drive the single propeller. I want to say that it was a high-wing airplane, which makes me think it was a Cessna; but it might not have been. It might not have been one of the Big Three manufacturers either. I just think it might have been. I think they were piston engines and not turbines.
The cowl was a bit bulbous, where normally it would taper at the bottom. The experiment was deemed unsuccessful, probably because of weight and complexity.
I think that might be problematic. You’d have to ensure that the props are putting out the same amount of thrust. Otherwise one will be ‘dragging’ the other, while the other is ‘pushing’ the one. Seems like a recipe for inducing undue stress on the shaft.
OTOH, helicopter engines routinely drive two rotors. They have flex couplings (one on an R22 shown in the link), but those are for fore-and-aft flexing. I haven’t read the page from which that image was taken. I don’t know if it mentions differential torque. It wasn’t mentioned in training, and I haven’t heard of it elsewhere. So either it’s dealt with somewhere, like in the gearbox, or it isn’t actually an issue and I’m overthinking it.
I’m more familiar with engines for surface vehicles, but occasionally ran into aircraft standards.
Those are some good examples of how and why aircraft certification requirements are very different from automotive design standards. While you MIGHT get into a crash if a car’s engine fails, you’re seriously hosed if an aircraft engine conks out. Consequently, aircraft standards tend to be done to levels that would be considered paranoid if you built a car that way.
(Experimental aircraft are an exception. You can pretty much get away with anything there.)
The dual spark plugs are a good example of this. There’s not just two spark plugs on each cylinder. There’s two completely separate ignition systems, each triggered off a different set of sensors and almost completely isolated. They don’t even have a common power source; aircraft use self-powering magneto ignitions, so the two ignitions will both keep working even in the case of the total failure of the rest of the plane’s electrical system.
Trying to get the direct injected system on the Ecoboost to meet those standards wouldn’t be easy. You’d need two separate sets of direct injectors, each fed off separate high pressure pumps, and some sort of reliable means (or more likely, several overlapping and different means) of detecting if one injector failed so it could switch to the other. It wouldn’t surprise me if a hypothetical aircraft certified Ecoboost engine also would need things like lower compression, a stronger rotating assembly, and more.
