|Nov 07, 2011, 02:26 AM|
Joined Nov 2011
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, 08:13 AM|
There is a good graphic in this disk loading link showing the relationship between disk loading and lift efficiency
|Nov 07, 2011, 04:04 PM|
LF, thanks for that. I wonder if what I said even relates.
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. 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.
|Nov 08, 2011, 12:04 AM|
For "back of the envelope" design purposes, this equation derived from momentum theory is handy:
Thrust = 13*(Power*Efficiency*Diameter)^(2/3)
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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