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Re: Lift formulas and fan design parameters

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Ian Brooks:
Hi

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.

Cheers
Ian

John Robertson:
Hi,
 
Take a look at this area - 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.

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