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By: - 31st May 2014 at 18:25 Permalink - Edited 1st January 1970 at 01:00
Hi everyone,It seems fixed wing is far better in term of efficiency, carrying more payload for a longer range with less fuel consumed while helicopter is only used for it's hover and VTOL ability. I want to know the reason behind that, could anyone explain it for me?
For example, AFAIK, heli is better than like F35 in vertical take off because moving a large volume of air at low speed (heli) is better than moving a low volume of air at hi speed (jet). I just want to know by simple reason like that.
Regards
Rotary winged aircraft put the majority of their engine power into lift rather than forward flight, fixed wing aircraft use wings for lift and the engines to overcome drag and generate thrust in the direction of travel.
Turboprop planes are more efficient than turbojets in slow flight but jets are obviously more efficient in fast flight. Props moving large volumes of air at low speeds are generally better for efficiency if you can get the thrust you need to fly at that speed. Helicopters simply don't point the thrust in a direction that's efficient for axial movement.
By: - 2nd June 2014 at 03:33 Permalink - Edited 1st January 1970 at 01:00
It is still not clear to me, but many thanks
By: - 2nd June 2014 at 04:23 Permalink - Edited 1st January 1970 at 01:00
A helicopter "flies the disc", where the "disc" is the circle described by the rotating main rotor blades.
In hover the disc is horizontal so the lift vector created by the disc is exactly opposite the force of gravity.
In forward flight, the disc is angled so the lift vector angles slightly forward. The angled lift vector has a vertical component to keep the helicopter aloft and a horizontal component to give the helicopter forward velocity.
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The lift vector in a helicopter is created by the engine converting fuel into lift by spinning the rotor. This is very fuel inefficient since the lift vector has to be oriented to keep the helicopter airborne, with the vertical lift vector component counteracting gravity.
A fixed wing airplane's propulsion system converts fuel into thrust almost totally in the horizontal, creating forward velocity. Fixed wing lift to counter gravity is the result of differential pressure created by the wing's camber, wing area and speed of the air mass flowing over the wing.
Then there is the V-22, which rotates its "discs" from horizontal to vertical and back again.
By: - 2nd June 2014 at 14:27 Permalink - Edited 1st January 1970 at 01:00
This has nothing to do with "having the rotor make the aircraft takeoff".
People usually think that, while the helicopter engine alone make the helicopter take off, the airplane engine doesnt have to do that, lift comming from something else.
Actually, the airplane engine IS the source of energy for aircraft climb, but an indirectly one, else we would have free energy, something that goes against the laws of nature.
Forward motion is transformed into lift via aerodinamic effects in the wing, and the source of energy for this is the aircraft engine, physics says so (think about that : how much lift a parked aircraft generates ?).
The difference in fuel efficiency comes from the way helicopters achieve forward motion, but this is due to a simple and crucial effect : DRAG.
Drag PERIOD, is the root cause of efficiency difference between a low flying aircraft and a chopper.
Due to VTOL nature, Helicopters fly forward in a attitude that generates a lot more drag than the equivalent aircraft.
This is the reason why Cheyenne had a pusher propeller (http://en.wikipedia.org/wiki/Lockheed_AH-56_Cheyenne).
By avoiding the need to turn the chopper forward to get forward thrust, Cheyenne allowed the designers to build a more streamlined aircraft, with less drag, which would provide near-aircraft like performance, but they hit another helicopter design problem : Retreating rotor blade stall and advancing rotor blade hitting supersonic speeds.
When an chopper is pushed through the air, the blades that composes the rotor fly in two different directions, advancing, where the blade movement is in the same direction as the helicopter movement, and retreating, where the blade is receding relative to the chopper forward motion. In the advancing blade, the sum of helicopter plus blade motion, reaches supersonic speeds at low helicopter speeds, meaning that the blade loses efficiency and can get destroyed. In the retreating blade, the difference of forward speed motion between the helicopter and the blade can mean that the blade achieves low or zero speed relative to the air. This means that the receding blade generates zero lift, spinning the chopper downwards and usually resulting in a crash.
So basically, theres an inherent problem with the usual helicopter form-factor that is inherently less efficient than the fixed wing aircraft form factor.
You can think about this the following way :
What would happen if the chopper gets totally inclined forward, with the rotor cube pointing completly forward ?
(Ignoring the lack of vertical component to neutralize gravity meaning that the chopper would go down)
The form factor of an helicopter at that position is not very aerodynamic, is it ?
Then you have the drag generated by the rotor disk itself, that is way bigger than the equivalent propeller disk from an aircraft.
Etc.
Posts: 12
By: lemd - 31st May 2014 at 16:41
Hi everyone,
It seems fixed wing is far better in term of efficiency, carrying more payload for a longer range with less fuel consumed while helicopter is only used for it's hover and VTOL ability. I want to know the reason behind that, could anyone explain it for me?
For example, AFAIK, heli is better than like F35 in vertical take off because moving a large volume of air at low speed (heli) is better than moving a low volume of air at hi speed (jet). I just want to know by simple reason like that.
Regards