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For an ultralight, should two engines weighing 10kg each and making 7hp each OR one engine weighing 20kg and making 14hp be used?

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  • $\begingroup$ Does the FAA allow multiple engines in ultralight or LSA categories? $\endgroup$
    – Harper - Reinstate Monica
    May 13, 2018 at 16:24
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    $\begingroup$ @Harper What makes you think OP, whose profile specifies their location as Nigeria, has any interest in what the FAA has to say about anything? $\endgroup$
    – user
    May 13, 2018 at 21:30
  • $\begingroup$ The Colomban Cri-Cri is a twin-engine homebuilt with 2 15-hp engines. It also has a 4 engine electric model that was built as a demonstrator. $\endgroup$
    – Ron Beyer
    May 14, 2018 at 1:34

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Normally you have more power relative to engine mass in the bigger engine. If there is no weight advantage (like in your example), other considerations must be added:

  1. Two engines allow to spread the load out. By placing one on each wing, the fuselage weight can be reduced which in turn reduces the wing root bending moment.
  2. If the engines drive a propeller, asymmetric thrust and gyroscopic effects can be eliminated with two engines if one engine runs clockwise and the other runs counterclockwise.
  3. Two engines need twice the number of controls, fuel lines and instrumentation. This is both a cost and a weight issue.
  4. And the obvious one: If one of two engines fails, you still have the other to keep you flying. However, this is only true if your design is capable of flying with the remaining engine only. Now your rudder authority and minimum thrust requirement must be adequate to keep the aircraft flying with the resulting asymmetric thrust.

Note also that with two engines an engine failure is statistically twice as likely.

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    $\begingroup$ And don't forget 4A. If one of two engines fail, you still have the other to get you to the crash site. $\endgroup$
    – CrossRoads
    May 13, 2018 at 21:38
  • $\begingroup$ I guess there is a propeller efficiency should be taken into account. If propeller disc diameter is (nearly) same in both cases then one-engine design total efficiency would probably be lower. $\endgroup$
    – Eugene
    May 27, 2018 at 23:40
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Since, Peter Kämpf and the OP presumably considered turboengines (or piston engines), I would like for the sake of future completeness talk about electric engines.

Some concepts of electric powered aircrafts (of small dimensions of course), consider using a fully distributed propoulsion system, involving several engines (see this project from NASA and this project from french ONERA.

So in this case, it seems that in addition to what Peter Kampf said, the more you have engines, the better.

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  • $\begingroup$ However, decreasing propeller blade span decreases its efficiency, which is currently crucial for electric-powered aircrafts. $\endgroup$
    – Eugene
    May 27, 2018 at 23:43
  • $\begingroup$ @Eugene Indeed, that is probably why the propellers are fitted inside a casing. Also, this is part of the compromise related to the very integration of the engines on the fuselage. $\endgroup$
    – BambOo
    May 28, 2018 at 5:58
  • $\begingroup$ This is all good when you receive power from burning fuel. Although you will have nasty fuel consumption, it will be reasonable considering craft's flexibility. But modern batteries has too little power density compared to fuels, so inefficient designs shorten flight time dramatically. There is probably a change for on-board generators, but this only contributes to overall inefficiency. Propellers that small (fans, in fact), even ducted, are very inefficient. Although I myself like ducted fans, I admit their inefficiency. $\endgroup$
    – Eugene
    May 30, 2018 at 22:40
  • $\begingroup$ For electric engines, the efficiency chain is for sure more complex to analyse. Between the energy storage, engine efficiency, mass to weight ratios, ... There is much work to improve that ! With so much work on electric drones, I guess we will see some improvements. $\endgroup$
    – BambOo
    May 31, 2018 at 9:21

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