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I saw this question Is there a rule of thumb for spacing between jet engines?, which referred to How do you choose engine placement for under wings engines?, but I was wondering how the engine spacing is approached for propeller aircraft. There are some examples as shown in this question (How far should the two propellers on a twin-engine plane be apart?), but I'm looking for a more systematic approach.

Compared to jet engines, propeller engines are larger, and have a much larger stream tube going into the engines. This will probably make their performance much more sensitive to the lateral gap between the engines.

I found this paper Aerodynamic interaction between propellers of a distributed-propulsion system in forward flight, which indicates for a three propeller setup:

At the thrust setting corresponding to maximum efficiency, the efficiency of the middle propeller is found to drop by 1.5% due to the interaction with the adjacent propellers, for a tip clearance equal to 4% of the propeller radius.

This is one data point, but I wonder how the influence varies with prop spacing. It makes sense to me that at a far-enough spacing, the engine can be treated as independent. I'm wondering what this far-enough spacing is, and how the influences vary with prop spacing.

There is a wide range of gaps available, some aircraft, such as the X-57, have a spacing that is very small - the tips are touching.

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Image source

So my question is:

What is the aerodynamic influence of prop spacing (to each other and to the engine) in multi-prop engine setups?

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    $\begingroup$ @sophit Because the power of electrical motors scales linearly with their mass. For the same power, no matter if you have a single large electrical motor or many small ones, the mass will be similar. For combustion engines it is not the case: one large engine will be much lighter than two small engines with half the power each. Distributed propulsion is not necessarily better, but it has the potential to do things that would not be feasible with a mono-/twin-engine configuration. $\endgroup$
    – Robe
    Commented Oct 21, 2022 at 10:49
  • $\begingroup$ @Robe I don't know about the first part of your answer but I guess you're right with the second one: blowing on the wing gives more lift also at low speeds. Thanks for your comment $\endgroup$
    – sophit
    Commented Oct 21, 2022 at 11:42
  • $\begingroup$ @sophit Reading these comments made me think about propeller tip speeds and the impact that may have on propulsive efficiency. Are there advantages to a larger number of smaller propeller disks over a smaller number of larger disks? To Robe's point, perhaps the weight cost of additional combustion engines justified compromises in propeller disk size, and once that weight cost is removed, smaller is better? $\endgroup$
    – Max R
    Commented Oct 25, 2022 at 22:53
  • $\begingroup$ @Robe I suspect that in addition to weight, combustion engines benefit from a smaller count for other reasons as well. Overhaul costs do not scale linearly with displacement, so 16 tiny engine overhauls are quite likely more expensive than 2 large engine overhauls. Same would be true of oil changes, spark plugs, magnetos, etc. There's a lot of combustion engine costs that don't necessarily scale smoothly with engine count. And I potentially have to route 16 throttle cables, and fuel lines, and primer lines, and and and... $\endgroup$
    – Max R
    Commented Oct 25, 2022 at 23:01
  • $\begingroup$ @MaxR: very good points. From the very basic momentum theory we know that the bigger the radius, the more efficient the propeller/rotor is. Many slower rotating propeller can reach the same efficiency but each of them introduces anyway inefficiencies that somehow sum up. On the other hand they are small and light and their rotating speed can be varied in a relative simple way and therefore be used to control the aircraft, see lilium, volocopter and so on. As usual it's a matter of compromises and I personally prefer one very good engine than 64 of them that can get broken. $\endgroup$
    – sophit
    Commented Oct 26, 2022 at 4:43

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A major consideration is the single engine performance when one of the engines fail. If the engines and props are farther away from the aircraft centerline, then the adverse yaw experienced with an engine failure is more extreme. This would make is much harder to control the aircraft.

In a typical twin-engine propeller driven airplane, when you shut down one engine, the airplane will yaw and roll agresively towards the dead engine. The pilot has to apply maximum rudder pressure towards the good engine to keep the airplane from completely rolling out of control. This is one reason the rudder and vertical stabilizer of twins are larger than single engine planes.

Another factor is the structural strength needed to support a heavy piston engine far out from centerline. So, in general, designers would want the props as close as possible to centerline while taking into account aerodynamic factors such as the prop slipstream and the increased drag caused by the engine nacelles being too close to the fuselage.

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  • $\begingroup$ Why does drag increase if the engine nacelles are placed close to the fuselage? $\endgroup$ Commented Dec 31, 2022 at 7:31
  • $\begingroup$ I think the question was more about the aerodynamic interference when the rotor are close to each other. $\endgroup$
    – sophit
    Commented Dec 31, 2022 at 11:31

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