power to weight ratio of vtol - RC Groups
Nov 07, 2011, 03:26 AM
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# power to weight ratio of vtol

Can any one explain to me why the martin jetpack need 200 HP to lift a 535lbs, but the Sikorsky Cypher with a 50 HP wankel rotary engine can lift 340 lb more then half of what the martin jetpack can with just 1/4 the horsepower ?
 Nov 07, 2011, 08:07 AM Micro Boat Forum Founder That's a design thing and power transmission, not power to weight.
Nov 07, 2011, 09:13 AM
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Quote:
 Originally Posted by kingofking1 Can any one explain to me why the martin jetpack need 200 HP to lift a 535lbs, but the Sikorsky Cypher with a 50 HP wankel rotary engine can lift 340 lb more then half of what the martin jetpack can with just 1/4 the horsepower ?
It is about disk loading. A helicopter has a large disk swept out by the rotor blades. Low disk loadings give increased thrust for the same power. The jet pack has a very high disk loading and is inefficient for static thrust, but it would be good for high speed propulsion.

Nov 07, 2011, 04:52 PM
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 Nov 08, 2011, 01:04 AM Registered User For "back of the envelope" design purposes, this equation derived from momentum theory is handy: Thrust = 13*(Power*Efficiency*Diameter)^(2/3) Units: Thrust [lb] Power [shaft HP] Efficiency from 0 to 1 -- typically 0.5 to 0.8 for rotors and props. (This is efficiency of the disc converting power to thrust -- not the same as efficiency during forward flight) Diameter -- of prop or rotor disc in [ft] All you have to do is make a handy dandy spreadsheet with different numbers plugged in for different configurations to see just how important disc loading is to minimizing power required (and fuel flow).
 Nov 08, 2011, 03:50 PM Registered User so rotor blades are more efficient then prop for lift thing but prop are more efficient for high speed

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