Ducted fan with bellmouth inlet

[RandomGuy88]

I am working on a project where we are using ducted fans and just recently we were testing to effect of some bellmouth inlets we made for the ducts. At full throttle the thrust for the fans with no bellmouth inlet was approximately 8.8 lbs and with the inlet the thrust was approximately 10.5 lbs. This is the kind of result we expected but for each test we went to full throttle and the power consumed went down when we added the inlets even though we were at the same throttle setting. I would have expected that by going to the same throttle setting we would supply the same amount of power each time and just get more thrust for an equal amount of power. But we saw thrust increase and power consumption decrease for the same throttle setting. Does anyone know why this is?

 

[jack action]

Power = thrust X velocity

If the thrust increases and power goes down, it only means that velocity decreased more than thrust went up.

One would think it is normal for the velocity to go down with a bellmouth since the better efficiency helps the flow by "increasing" the effective area.

 

[Cyrus]

I am working on a project where we are using ducted fans and just recently we were testing to effect of some bellmouth inlets we made for the ducts. At full throttle the thrust for the fans with no bellmouth inlet was approximately 8.8 lbs and with the inlet the thrust was approximately 10.5 lbs. This is the kind of result we expected but for each test we went to full throttle and the power consumed went down when we added the inlets even though we were at the same throttle setting. I would have expected that by going to the same throttle setting we would supply the same amount of power each time and just get more thrust for an equal amount of power. But we saw thrust increase and power consumption decrease for the same throttle setting. Does anyone know why this is?

First and foremost, my concern is how you are measuring power. My guess is P=VI, which you cannot, and should not do. The correct way to measure power is shaft torque and RPM, as the problem with P=VI is that you are measuring the losses and inefficiencies of the battery and motor as well. These will depend on the voltage of the battery, and the load on the motor.

The potential pitfall about your hunch is that going to max power setting in both cases may not actually be the same power setting! I think your experiment has proven this.