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I've been recently looking for answers to this question on the internet but the jungle of propellers, fans and electric motors didn't quite get me an answer. How would I have to design an electric engine to produce as much thrust as possible on an area as small as possible, so the thrust/area ratio should be high. Also, assume there's enough power available. Would I use propeller and if so, how many blades? Would I use a fan like the turbofan of a jet engine?

With the optimal solution, which thrust do you think a 500 mm prop/fan could produce?

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    $\begingroup$ Thrust-to-area ratio is an odd figure - why do you want to optimise for that? If power is not an issue, one would probably make an extremely long series of increasingly complex compressors to accelerate pretty much indefinitely, so perhaps you want to add some more constraints to your question as well (most likely: single or double stage compressor/fan/propeller) $\endgroup$
    – Sanchises
    Aug 22, 2018 at 11:41
  • $\begingroup$ Why use propeller? Use a compressor, e.g. driving the compressor stages using a electric motor, and jet the high pressure stream. $\endgroup$
    – user3528438
    Aug 22, 2018 at 15:57
  • $\begingroup$ @Simon Henn it would help a lot to specify your target flying speed. Unless it is greater than around 450 mph, find a way to put a prop on there. Multiblade props or fans will drain your battery much faster for a given amount of thrust. Ducts are more efficient than open props only at lower speeds, losing efficiency in comparison at higher speeds due to drag. $\endgroup$
    – Robert DiGiovanni
    Dec 2, 2019 at 22:42

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Generally, if you want to optimize for area/diameter (without the complexity and weight of the many/multi stage option mentioned in comments to the question), you'll want to use a ducted fan (roughly similar to the fan on a turbofan). Not only do you avoid tip losses due to the end plate effect of the duct (if the clearance isn't excessive), you can run the fan at arbitrarily high speed, even with supersonic tips, without the extreme drag and noise production of an open propeller in such a regime.

Look at the design of EDF (Electric Ducted Fan) engines used to simulate jets in small radio control models; some of these can product thrust comparable to fans driven by high performance methanol/nitromethane glow engines (15-25 kW/L displacement, with 10 cc, 1.5 kW engines driving fans 15 cm diameter), with 10-20 minute flight duration on a charge with light enough batteries to fly well. Scaling these up to 500 mm diameter will generally require shortening the blade chord relative to diameter, and you'll need to optimize pitch for the combination of rotation speed and torque available.

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  • $\begingroup$ Noise yes, but the wave drag will still be all there if you run supersonic. It is still never advisable to have supersonic tips. $\endgroup$
    – Zeus
    Oct 22, 2018 at 1:43

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