I have a few brushless motors and I am trying to get the best thrust out of each motor. I am busy doing some research but can't get a clear answer. What would be the best fan duct design to achieve maximum thrust?

Project: mini hover bike prototype with 2 or 3 motors. Duct size doesn't matter at this point.

  • 1
    $\begingroup$ obviously the most efficient choice is no duct but use a prop, especially you when you don't care about size $\endgroup$ Oct 24, 2017 at 19:22
  • 1
    $\begingroup$ @user3528438 Not necessarily. Shrouded propellers do deliver higher thrust than unshrouded ones, especially if the clearance between propeller and duct is very low. $\endgroup$
    – Koyovis
    Oct 24, 2017 at 23:46
  • $\begingroup$ It all depends on the specific setup for the motor and props. With the correct setup you can increase thrust and reduce power consumption by to 50-60%. (so I have read). I'm going to use @Koyovis research and 3D print some test ducts and props. $\endgroup$ Oct 25, 2017 at 12:01
  • $\begingroup$ You could take a look at the ducting on the Moller flying car also moller.com The M200 had 8 ducted fan motors. moller.com/brochures/Neuera-Brochure.pdf $\endgroup$
    – CrossRoads
    Apr 10, 2018 at 18:50
  • $\begingroup$ "The M200G Neuera Volantor A revolutionary 2-passenger VTOL ground effect vehicle" Wonder if electric motors could substitute for fuel powered motors now? Battery technology has come a long way. $\endgroup$
    – CrossRoads
    Apr 10, 2018 at 18:56

1 Answer 1


It's not easy to find research on small ducted fans. I did come across this interesting masters thesis from 2009, which does show some relevant results - at low propeller speeds.

Duct design

The document describes three duct designs, one with a variable bypass inlet and two simpler ones. The simplest one is made from PVC tuning and a foam inlet lip:

enter image description here enter image description here

Geometric information is given as:

  • Inner diameter = 14.5"
  • Duct Chord = 4"
  • Duct Length (C/D) = 0.28

Maximum Thrust

One of the results of the project is that a ducted propeller delivers more thrust than an un-ducted one. Here is one of the results, listing $C_T$ as a function of tip Mach numbers - which are really low and therefore hard to generalise into practical rotor and fan configurations.

enter image description here

Tip Clearance

Best results were obtained with a propeller with square tips with the smallest gap between propeller and duct that could be achieved. This was a 3-blade propeller, but the research project was severely limited by the small motor used, and therefore low maximum torque that could be exerted on the propellers.

  • $\begingroup$ Thanks, great answer! The information and linked document will definitely push me in the right direction. I guess there is no 1 answer as every motor, prop, number of blades, motor torque etc. differs. $\endgroup$ Oct 25, 2017 at 11:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.