power to weight ratio of vtol - RC Groups
Thread Tools
Nov 07, 2011, 03:26 AM
Registered User
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 ?
Sign up now
to remove ads between posts
Nov 07, 2011, 08:07 AM
Micro Boat Forum Founder
boredom.is.me's Avatar
That's a design thing and power transmission, not power to weight.
Nov 07, 2011, 09:13 AM
Winging it >
leadfeather's Avatar
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.

There is a good graphic in this disk loading link showing the relationship between disk loading and lift efficiency
Nov 07, 2011, 04:52 PM
Registered User

disk loading


can any one explain disk loading
Nov 07, 2011, 05:04 PM
Micro Boat Forum Founder
boredom.is.me's Avatar
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]
Nov 08, 2011, 01:04 AM
Registered User
Illini_Rob's Avatar
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


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Increasing the thrust to weight ratio .. please help khaled_abobakr Power Systems 22 Aug 29, 2012 07:20 AM
Question minimum power to weight ratio for FW sudovod Flying Wings 5 Nov 02, 2011 03:14 PM
Discussion is there a maximum thrust to weight ratio? TheSelf Electric Plane Talk 30 Jun 26, 2011 03:53 PM
Discussion thrust/weight ratio for gliders? RC_Bob Electric Plane Talk 75 Mar 28, 2011 04:01 PM