View Full Version : Lift formulas and fan design parameters

27-Aug-09, 12:19 AM

I would appreciate assistance with:

A formula for calculating lift air volume (CFM), perhaps associated with pressure.

A formula for determining lift fan design to generate the appropriate CFM and pressure:
Number of blades
Blade geometry
number of fans required
ducting shape - funnel OK with smaller aperature under craft than intake?
optimizing least noise

My goals are:
(each) fan to be independently powered
least possible noise from fans
low RPMs

Estimated body structure
15 ft x 8.5 ft
1200 lbs unloaded weight + 1200 lbs passengers = 2400 fully loaded
land drive - 4 wheel hydraulic motors
over water drive - 2 water propellers and 2 lift fans - all hydraulic motor drives

Thank you in advance for your assistance and patience with my very basic questions.

Regards, Ron L

John Robertson
27-Aug-09, 08:02 AM

Take a look at this area (http://www.hovercraft.org.uk/forumdisplay.php?f=28) - there are several downloadable documents (with formulae) that may help you.

Based on your design parameters, your biggest issue will be weight. The weight you are proposing will result in a cushion pressure nudging 20lbs/ft2 - that is well above the 10-12lb/ft2 reckoned to be the optimum for a light hovercraft. It may be worthwhile buying a set of plans for a similar sized craft and using that as a basis for your design.

28-Aug-09, 06:42 PM

Thank you very much for your suggestions. Much appreciated.

Yes, weight will be a problem, hopefully overcome with horsepower and adequate fan selection (high compression ability instead of very high flow volume.)

The reason: My design idea is radical; focus on more efficient propulsion (full contact instead of fan thrust) in order to optimize: steering, ATV capability, reduction of noise and vibration, and simplicity of mechanical components by utilizing hydraulic motors.

Thanks again and regards, Ron L

30-Aug-09, 04:37 PM
Can anyone direct me to a book or article which contains graphs of fan output in the range of 30 lbs / sq ft? I suspect the technology would be more of an air compressor, or perhaps a device with mutliple fan blades?

The reason is my hovercraft dry weight will be about 1500 lbs, 2300 lbs fully loaded and about 100 sq ft under the skirt.

Thank you in advance and regards, Ron L

Ian Brooks
30-Aug-09, 07:21 PM

If you're designing from scratch, you might consider getting a copy of Theory and Design of Air Cushion Craft (Yun and Bliault, ISBN 0 470 23621 3). This should help you create a successful design.

I think you may have issues with the projected cushion pressure. Wave making (AKA hump) drag is proportional to the square of cushion pressure, and at 30 lbf/ft^2 you are going well beyond the normal margins for a light hovercraft - quite apart from the excess spray issue, I fear that the craft will be unable to get over hump.

To set it into context, my own craft with a 14' x 6'6" footprint has an all-up weight (inc passengers) under 900lb - this predicts hump drag of 45 lbf, although in practice an acceleration margin of at least 100% is needed, and at 100lbf thrust it was adequate under all but the most adverse conditions (With 200lbf it is has sufficient margin for all conditions).

The acceleration margin is needed because of practical considerations - if there is time for the hump wave system to develop fully, it will interact with the skirt causing draw way higher than predicted. You need to accelerate away before the hump system can develop.

A similar wave drag prediction with your dimensions gives an estimate of 205lbf, suggesting that you will need minimum of 410lbf to provide the acceleration margin, with more required if you wish to use it in adverse conditions. This assumes that the skirt design is ideal - any skirt/wave interactions at hump speed will require further reserves of thrust.