# If I put two ductfans behind one another, would I got the sum of the rated thrust or other thrust values?

I am trying to design a VTOL system for a drone and I don't want to have extending motor arms that make it look like a quadcopter. I want to use duct fans and thrust vectoring nozzles. However, the aircraft is 25 kg and the drag caused by vertical takeoff is 2 N. But most of the RC duct fans I see online can only deliver 2 to 3 kg of thrust. So I am thinking whether I can put two ducted fans behind one another, but how much more thrust can I get? will it be the sum of the two motor, or other numbers?

• How did you come up with 2N drag for vertical takeoff? But forget about that how would you lift 25kg even if you had 6kg of thrust?
– Jim
Jan 18, 2022 at 4:01
• I use ansys and put in 1 m/s vertical climbe rate. And there are going to be a lot of duct fan combo Jan 18, 2022 at 4:32
• It should work but the aft fan won't be very efficient. The aft fan blades needs to have a much high pitch in order to be efficient in the high speed flow. Jan 18, 2022 at 7:50
• But if you want a 25kg aircraft to vertically takeoff, and you only have 2-3kg of thrust per fan, then you have to stack up 10 fans? Jan 18, 2022 at 7:54

## 1 Answer

No, two fans stacked in series will not produce the same static lifting force as two fans in parallel. Several factors will be in play.

First is that by default the ducted fan blade angles will be assuming intake air at low speed. If you want to add a second stage it needs to have an a higher angle of attack on the second stage (something like the changing slope of a single fan blade but stretched across two fans). This can be partially achieved by increasing RPM but that moves away from the optimal power point of the motors unless major re-design is done.

The second is that the cross sectional area needs to change. If you have one fan capable of pulling 1 cubic meter if air through it's intake per second and pushing it out at 1 meter per second, it is moving one cubic meter per second. If the following stage is increasing that to 2 meters per second, it is trying to pull 2 cubic meters per second from the first stage! Hence the taper seen in multi stage turbines.

The third is drag through the combined duct. Drag goes up with square of speed, so the details of the surface finish and motor supports starts to matter far more than the original designer intended, so for example helicopter design.

Blades also work best in undisturbed air, and the airflow going into stage two or three of a stack will not be that without carefully designed stators between stages.

• Don't forget the induced power. Drag grows with square of speed, but of the blades, and that initially only slowly when the orbital velocity is much greater than the flow velocity. But the induced power, due to energy given to the stream, grows with the flow velocity (just linearly) from the start. Jan 18, 2022 at 14:48