When the engine developed enough power to lift the helicopter, the main problem was how to counter the main rotor forcing the fuselage to rotate in the opposite direction of the rotor. This effect is known as TORQUE.
The classic solution was a small tail rotor to push the fuselage in the opposite direction of the torque force.
Another popular solution were tandem rotors. The primary advantage of this configuration is the ability to lift heavy loads whose position relative to the helicopter's centre of gravity is less critical than in the single rotor configuration. Because there is no anti-torque rotor, full engine power can be applied to lifting the load. Disadvantages of the tandem rotor system are a complex transmission and more drag due to its shape and excessive weight.
And coaxial rotors, in which the goal is obtain a notably compact design. Being the fuselage independent of the lifting system dynamics permits the design to be strictly functional as related to the helicopter mission.
After the torque effect, the other main problem was the tendency of the helicopter to roll laterally in the direction of the retreating rotor blades as the advancing blades pass through denser air and generated greater lift than the retreating blades that pass through less dense air.
This problem was eventually solved by the introduction of a flapping hinge in the rotor head, which allowed the advancing blade to climb slightly, thereby reducing its angle of attack and the amount of lift generated, while the retreating blade fell slightly, thereby increasing is angle of attack and the amount of lift generated.
List of Rotor Configurations in detail
NOTAR also utilizes Coanda Effect with the rotor downwash across the tailboom and an internal airflow through the tailboom to produce a sideways "lift", or more correctly "thrust" to counter main rotor torque.
The jet thrust from the nozzle at the end of the tailboom is primarily used for directional control, with a very small contribution to anti-torque force.
|User Contributed Notes|
mike jones ( seattle wa usa )
You have it a little backwards. Disymetry of lift is offset by the advancing blade staying relatively flat (flat pitch) while the retreating blade climbs, (increasing angle of attack) thus reducing lift on that side of the rotor disk. forward speed is thus limited to stall speed on retreating blade. Retreating blade stall sets limits on forward speed on all pure helicopters. Mike, 28 year Chinook pilot.
daniel ( miami fl usa )
Actually he was right the first time. If the flapping hinge were not there, the advancing blade would be at the same angle of attack as the retreating blade, but the advancing blade has more incoming airflow therefore produces more lift. The flapping hinge allows the advancing blade to flap upward therefore reducing its angle of attack which reduces its net lift. The retreating blade doesn\'t produce very much lift so it flaps downward which increases its angle of attack therefore increasing its net lift.
dave sullivan ( yeovil somerset united kingdom )
Sorry 28 yr old chinook pilot, the others are right. Look back at your theory of helicopter flight class notes.
james grenfell ( brisbane qld australia )
Guys, I am not able to piece your comments together here. I am new to Heli\'s but believe I have a strong knowledge of fixed wing machines. If an advancing blade flaps up this would increase in AOA? Retreating blade down will decrease AOA? A flapping hinge should maintain lift for the advancing blade and increase AOA and lift for retreating blade to negate roll in the direction of the retreating blade. If someone can clear this up i would be greatfull.
sang\iewa ( nairobi, kenya )
hey J. Grenfell, I think you are having the same problem I had until I realised that flapping up refers to the rear edge of the rotor flapping up thereby reducing the AOA rather than the front edge! If the front edge flaps up then the AOA increases. If the rear edge flaps up then the AOA decreases. By the way Im just a wannabe pilot. I took a grand total of 3 hours of heli pilot lessons in LBC, CA, USA!! So excuse me if Im way off the mark, but its the only way it makes sense to me!
Ranger75 ( Borneo )
Hello chaps, let me try...as the adv blade flaps up,it meets its rate of flapping up airflow which effectively increase the lenght of induced airflow hence reposition the relative aiflow component upwards and reduces the effective AOA;the reverse also happens on the retreating side. As a consequence lift value stays constant(CL&V2 cancellout within the lift famulae)for a single rotor throughout its 360 travel. Another words dissymetry of lift is resolved by itself by allowing them to flaps up and down about their flapping axis. Naturally the area of stall will increase from the root outboard of the retreating portion of the rotor disk, a factor determining Vne; plus of cos due to compressability issue at the tip portion (at 90 deg posn) of the adv area of the rotor disk - so dont go beyond Vne!. Thanks 241040HJune2010
will ( san diego )
gentlemen, there is no easy way to explain how the advancing blade flapping up decreases the angle of attach and the blade flapping down increases it with out drawing it out on a board. the first thing i can say is that if your not a CFI you shouldnt be speculating on this because your just further confusing other readers who are going to regurgitate this to other people and further spread the ignorance. I am not trying to be mean or making fun of anyone, but the first thing your taught when you become a CFI is dont make a claim unless you have something to back it up. always look up the answer. that being said a ASA have lots of books that spell it out. What it comes down to is the plane of rotation of the rotor blades. If the blade flaps up i.e. the new plane of rotation is in an upward direction, this would lower the induced angle of attack and vice versa. draw it on a board. it is measure off the plane of rotation
Micko ( New Zealand )
The advancing blade (into the airflow) experiences a greater airspeed and hence a greater lift than the retreating blade. This causes the blade to swivel upwards about the horizontal hinge at its root. If this hinge is inclined outwards, so that the leading end is further from the hub than the trailing end, then as the blade rises the angle of incidence is reduced. The converse is true for the retreating blade. As lift is reduced the blade drops and the angle of incidence increases. The simplest example of this is the two blade autogyro with its teeter head.
Mick ( New Zealand )
The important thing about the hinging of the blades is that the hinge is NOT normal to (making a right angle with) the blade axis. It is set at an angle, with the leading edge end further from the hub than the trailing edge end. The upshot is that if the blade rises the trailing edge rises further than the leading edge, decreasing AoA, while if it falls the trailing edge falls further and AoA is decreased. The teeter hinge on a basic autogyro, is the simplest example of this concept.
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