# Can I have two speeds on one motor? [closed]

I am designing an aircraft and it is my first that will use a combustion engine rather than electric, so I am curious, if I had a single engine in the fuselage and gear shafts to the wings that drove propellers, how would I achieve different speeds on the propellers? The 'plane' itself is more similar to a VTOL so the different speeds are needed. I would be fine with a solution that used smaller electric motors in it's mechanism.

Or maybe I'm approaching this in a terribly inefficient way and there's a much better method to what I'm trying to achieve, I just feel stuck after battling it for a few days.

The aircraft itself will look similar to this:

• Welcome to Aviation.SE. Unfortunately I do not see how your problem could fit in this site, my feeling is that you could be better off at Engineering – Federico Mar 8 '16 at 20:47
• I think this question belongs more on electronics.stackexchange.com – SMS von der Tann Mar 8 '16 at 20:47
• Yes, a differential (wikipedia link) is the key . – Porcupine911 Mar 8 '16 at 22:31
• It seems you also have a constraint to not use variable pitch on the propeller blades, right? – mins Mar 8 '16 at 23:47
• You could add variable pitch to each prop to adjust the thrust each achieves? However, changing the pitch of either prop would change the load on the engine and therefore the speeds of both of them... – Steve Mar 9 '16 at 16:10

The short answer is you probably wouldn't do it this way: Designing a drive system which allows you to split the work force between two output shafts at an arbitrary ratio (effectively an active differential) is pretty complex, and failure-prone. The failure of the differential would be "problematic" (both rotors stop), and the failure of a single drive link would likely be catastrophic (one rotor stops in the VTOL configuration you have an extreme wing-over from the imbalanced thrust).

The generally preferred design for this type of aircraft has two full engines mounted on the wings (for example the V-22 Osprey).
Similar results can be achieved by vectoring thrust from a jet engine (as in the Harrier "Jump Jet"), which can be done with a single engine.

• I figured this would be the case, thank you for the answer. – Alexander Mar 8 '16 at 21:13
• Although the Osprey has a pretty intricate system linking the rotors that allows either motor to drive both rotors should one motor fail. – Jörg W Mittag Mar 8 '16 at 21:51
• @JörgWMittag Yup, that's to avoid the wing-over asymmetric thrust issue in case you lose an engine (which is a distinct possibility if, ya know, people are shooting at you). I think it also uses that linkage for starting them off the APU -- the Osprey is a pretty complicated beast to use as an example, but I couldn't think of another tilt-rotor V/STOL off the top of my head. – voretaq7 Mar 8 '16 at 22:10
• I don't think the fact that you failed to come up with an example of a non-complicated V/STOL tiltrotor is a problem with the top of your head ;-) Rather, I believe it is trying to tell us (and the OP) something about the inherent complexity of this particular niche in the aircraft design space. – Jörg W Mittag Mar 9 '16 at 1:04