What would be the best fan duct design to achieve maximum thrust?

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.

• obviously the most efficient choice is no duct but use a prop, especially you when you don't care about size – user3528438 Oct 24 '17 at 19:22
• @user3528438 Not necessarily. Shrouded propellers do deliver higher thrust than unshrouded ones, especially if the clearance between propeller and duct is very low. – Koyovis Oct 24 '17 at 23:46
• 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. – Marc Uberstein Oct 25 '17 at 12:01
• 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 – CrossRoads Apr 10 '18 at 18:50
• "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. – CrossRoads Apr 10 '18 at 18:56

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:

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.

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.

• 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. – Marc Uberstein Oct 25 '17 at 11:56