CFM for a hovercraft

[IndyITGuy]

I'm not sure if this should be in mathmatics or not so I apologize if I posted this incorrectly.

 

I've seen a few DIY hovercraft designs that use leaf blowers as the means for lift/thrust. Those leaf blowers are usually around 440cfm... A carpet dryer puts out 2400cfm, does this mean I would get the same lift/thrust from a carpet dryer that I would get from 5/6 leaf blowers at 440cfm each?

[swansont]

!

Moderator Note

Similar discussion is here http://www.scienceforums.net/topic/75155-converting-cfm-to-thrust/

[dimreepr]

The old adage of “more equals better” holds little weight, in this instance, as the weight (gross vehicle) determines the volume of air needed, IOW, any increased capacity would be wasted.

[ewmon]

Let's review the categories of equipment you might consider using. Each category uses a different design based on its purpose.

 

A compressor is a machine for raising a gas to a higher level of pressure without necessarily moving it (such as pumping up a bicycle tire).

A blower is a machine for moving volumes of a gas with a moderate increase of pressure (such as a leaf blower).

A fan moves large amounts of gas with a low increase in pressure (such as a carpet dryer or a building ventilator).

 

I suspect that a leaf blower falls into the "blower" category and that the carpet dryer falls into the "fan" category. The leaf blower is designed to generate aerodynamic forces against objects in order to move them. The carpet dryer is designed to blow high volumes of air across a carpet so as to augment the evaporation of water after it has been washed etc.

 

As has been said before, CFM is not the characteristic that determines whether the equipment will lift/move a hovercraft. For all three categories, I suspect there's a pressure threshold over which the equipment will begin to fail (that is, fail to produce the pressure differential). Fans have the lowest threshold, blowers have an intermediate threshold, and compressors have the highest threshold.

 

Let's look at what you want to do, and see what sort of characteristic derives from it. You want to create [a differential] pressure under a hovercraft that will lift the hovercraft off the ground and allow it to ride on a cushion of air. Compute the weight of the hovercraft as used (including rider(s) and fuel etc. Compute the area within the skirt. Divide the weight by the area. This computes the pressure differential between the ambient air and the pressurized air under the hovercraft, and creating such a differential will cause the hovercraft to "lift off".

 

I suspect that this differential would fall within the do-able range for many blowers, but that it would exceed the threshold for most/all fans.

 

I used another kind of "blower" for a homemade hovercraft — a vacuum cleaner. For its intended use, a vacuum cleaner generates a pressure differential in reverse by creating a partial vacuum to suck objects into the vacuum cleaner.

Edited July 4, 2013 by ewmon

[Externet]

It is not only air volume per second. That does not provide lift. That sustains lift.

Lift is provided by pressure of the fan. The fan pressure has inverse relationship with the area of the vehicle; less pressure neded for a larger footprint craft area (size).

But larger footprint craft area usually means more craft weight, which turns the relationship with pressure direct, not inverse.

So there is a sweet spot for pressure.

To make things more complex, pressure drops sustantially with roughness of the riding surface and leaks under the skirt.

 

A fan delivering the optimal pressure for a given hovercraft, should also yield the proper volume. Volume determines the rate of inflation which is important to raise the craft from siiting in the surface in shorter time, and to recover height from bumps quickly.

I suggest you sign up with the 'Hoverclub of America' and access the technical discussions, formulas and construction keys. There is a bunch of great fellows there with knowledge and experience to share.

 

Thrust is another story.

When a single fan has to provide lift and thrust, it is a third story.