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Old Feb 19, 2013, 08:03 AM
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MHA webmaster
United Kingdom, England, Reading
Joined Mar 2011
212 Posts
If the craft is only being designed for a single purpose, i.e. completing a defined course, then your air gap is dependent on the surface in question, which if smooth only needs to be a couple of mm. In terms of the duct, I assume you are talking about for the lift fan. Experience is that with props, if you make a tight fitting circular duct, it helps greatly with keeping the plenum pressure high. An 8 inch prop seems plenty - if anything slightly oversized for the craft size. A four bladed 6 inch prop inside a duct works well for the Griffon so I don't think you'd need anything bigger for what you are describing. Motor wise, you don't need too much power - there is no point in overrating it as it will lead to wasted power and reduced run times. My first Griffon used a Speed 400 Motor! I later upgraded to a 540 size because the 400 would get hot, but found that my run times reduced for no real gain in air flow or lift. An equivalent to a 480 would probably be ideal with a reasonably high kV but low pitch prop.
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Last edited by jpcwebb; Feb 20, 2013 at 01:16 AM.
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