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Old Nov 07, 2011, 02:26 AM
kingofking1 is offline
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Help!
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 ?
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Old Nov 07, 2011, 07:07 AM
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That's a design thing and power transmission, not power to weight.
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Old Nov 07, 2011, 08:13 AM
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Quote:
Originally Posted by kingofking1 View Post
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.

There is a good graphic in this disk loading link showing the relationship between disk loading and lift efficiency
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Old Nov 07, 2011, 03:52 PM
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disk loading

can any one explain disk loading
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Old Nov 07, 2011, 04:04 PM
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LF, thanks for that. I wonder if what I said even relates.

From Wiki:
In fluid dynamics, disk loading or disc loading is the average pressure change across an actuator disk, such as an airscrew. Airscrews with a relatively low disk loading are typically called rotors, including helicopter main rotors and tail rotors; propellers typically have a higher disk loading.[1] The V-22 Osprey tiltrotor aircraft has a high disk loading relative to a helicopter in the hover mode, but a relatively low disk loading in fixed-wing mode compared to a turboprop aircraft.[2]
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Old Nov 08, 2011, 12:04 AM
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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).
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Old Nov 08, 2011, 02:50 PM
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so rotor blades are more efficient then prop for lift thing but prop are more efficient for high speed
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