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Ducted fan efficiency if engine is taken out of the equation (pure electric case)

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karoliina.t.salminen

Well-Known Member
Joined
Oct 7, 2008
Messages
407
Location
Finland
Hi,

There has been some back and forth discussion about ducted fans in general in this forum and elsewhere. Usually the limitations come from the engine that turns the fan. In general it has been concluded that the efficiency of a ducted fan is lower than a prop, except I have seen Orion mentioning some sea plane he has been working with and that analysis concluded that the fans work good up to 200 kts, but I can't find this post anywhere anymore. Well there could be exceptions like the mentioned, but at least in RC aircraft world, the fans tend to produce relatively low thrust for the power they require. If someone would have link to the Orions post, could you please paste it here.

However, if we consider ducted fan from scratch (not placing a existing prop in a duct nor turning that with a combustion engine), would it be possible to design a fan that produces rivaling results to a prop and if so how it should be done? And also why then the EDFs on RC planes tend to have very low efficiency (not only is static thrust lower than on a prop, but it tends to be that also the dynamic thrust seems lower - I do not have accurate measured data, but I have flown one RC plane with both prop and fan and everything else equal, by eyeballing on what I see, I can tell it is going faster, longer and stronger with the prop hands down compared to the fan which was run with same battery and roughly similar amount of current and power)?

The fans interest me especially because it would be interesting to place these inside the boundary layer (partially buried engines near the trailing edge) of a wing and instead of a big prop, propel the aircraft with distributed propulsion and then solving the problem of how to implement suction for active boundary layer control at the same time. I am not aware though how the performance of electric ducted fan would be affected by non-clean air ingested from inside the boundary layer and what that could cause for the efficiency. Luckily EDF is not a jet engine and all the problems associated with jets in boundary layer may not apply. Also it would be interesting to know if it would be possible to combat the potential loss of efficiency by ingesting the boundary layer and thus reducing the drag somewhat this way. Also this kind of distributed trailing edge propulsion could allow both yaw and roll control of the aircraft with adjusting the thrust of the fans through the wing, a bit similar automated control could be developed than the quadrocopters have and this kind of configuration could allow also rather interesting maneuvers for the aircraft potentially.

I have been trying to find information about fan design, but haven't found anything conclusive because either these are experiments by homebuilders without so much science behind (fan not optimal) or they are sort of replicas of jet engines (cold jets) (and again fan not optimal for low speed flight).

I was thinking about a quantity of ballbark 10-20 fans on trailing edge (not talking about single or twin or such relatively conventional configuration), this makes the individual fan side relatively small. Would someone have some facts and opinions regarding this kind of crazy idea. I was thinking this kind of fan configuration would especially fit on a flying wing as these fans could cover the whole trailing edge without interruption with the fuselage as there would be none. How the fans should be designed to meet these goals (to be efficient (not lose too much energy for nothing useful), be optimized for low speed (not transsonic like most studies I can find, rather the GA aircraft speed range <=200 kts and would produce meaningful static thrust on takeoff allowing the plane to get off the ground too).

And there is one more thing: I have seen some papers about using ducted fan to reduce noise. THen why are the ducted fans on EDF jets very extremely noisy then? Like super loud hairdryers in the air. What makes the EDF of a EDF jet both inefficient and loud whereas there are claims that it could be possible to have noise reductions done by using a fan? What could be more cool than a flying wing? A quiet flying wing that makes almost no noise at all.

Best Regards,
Karoliina
 
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