# Ducted fan thrust in confined space

I would greatly appreciate any insight into this.

I'd like to place a ducted fan inside of a box with the bottom cut out. The fan is attached to the sides of the box by support rods. The ducted fan pulls in air from the top of the box and accelerates it towards the opening at the bottom, as shown in the attached picture.

My question is, how big should the dimensions of the box be in order to match the thrust of an unenclosed fan? is there a simulation approach I can take to better understand how the box's size and the placement of the fan affect thrust, without going full-blown with CFD?

• Intuitively, I think the thrust from that would be zero, regardless of dimensions. If you're pulling air up from the bottom of the box and blowing it back out of the bottom, then the in and the out exactly cancel, and you're left with just the weight of the contraption and some inefficiency products (heat+noise), with no net lift. Unless I've misunderstood your purpose. Jun 20 '20 at 1:30
• Can you put holes in the sides instead? Jun 20 '20 at 3:10
• I see where both of your points come from. @AaronD I agree if we're just looking at the outlet surface, it doesn't seem like it will produce thrust. However, if the spinning propeller creates low pressure above it, where does that thrust go? Also, what happens if the ducted fan is shorter/longer than the sides of the box? Jonathan that's an interesting question too, is there a way to generalize removing any combination of the 6 sides? Jun 20 '20 at 3:53
• We used a ducted fan held on cables to check the thrust - a set of masses was used to “pull” the fan back to its initial position. Jun 20 '20 at 7:45
• "if the spinning propeller creates low pressure above it, where does that thrust go?" There is a pressure gradient across the air which is accelerating into the fan. That is equal and opposite to the "low pressure above the fan". Jun 20 '20 at 12:55