Tip path plane, forward flight

[heliflyknow]

What I am saying is when the disc is tilted down in the front, at the 090 and 270 (3o'clock/9o'clock) position the blades are parallel. The advancing blade's leading edge is pointed down, and the retreating side's is pointing up at an equal angle, I think this is the plane that the HFH uses to show flapping that compensates for dissymetry of lift and what leads to the confusion.

[Eric Hunt]

That picture in the HFH is only valid for a hover with a gust of wind, no cyclic input, equal pitch angles on all blades.

 

It is NOT valid as soon as the cyclic is moved, because the pitch angle is now changing all the time, to make the advancing blade reduce pitch to "throw away" enough lift to balance the retreating blade's struggle with increasing pitch to generate lift with the wind coming from behind.

 

In stable forward flight, there is no dissymmetry of lift. Both blades have exactly the same lift. Otherwise, the aircraft would have an unbalanced force, and would roll/pitch until the force went away.

[iChris]

What I am saying is when the disc is tilted down in the front, at the 090 and 270 (3o'clock/9o'clock) position the blades are parallel.

 

The advancing blade's leading edge is pointed down, and the retreating side's is pointing up at an equal angle, I think this is the plane that the HFH uses to show flapping that compensates for dissymetry of lift and what leads to the confusion.

 

There shouldn’t be any confusion since the FAA-H-8083-21A, Helicopter Flying Handbook, gives an explanation in text and diagram. If the helicopter is hovering and the pilot wishes to move into forward flight, he pushes the stick forward, which tilts the swashplate down in front. The pitch of the blade at Ψ = 90º (advancing blade) is decreased and that at Ψ = 270º (retreating blade) is increased.

 

The resultant imbalance accelerates the right-hand blade down and the left-hand blade up. The rotor flaps down over the nose and up over the tail, tilting the rotor thrust vector forward to produce a nose down pitching moment about the center of gravity.

 

FAA-H-8083-21A, Helicopter Flying Handbook, pages 2-19 & 2-20

 

Figure 2-36 illustrates the changes in pitch angle as the cyclic is moved forward at increased airspeeds. At a hover, the cyclic is centered and the pitch angle on the advancing and retreating blades is the same. At low forward speeds, moving the cyclic forward reduces pitch angle on the advancing blade and increases pitch angle on the retreating blade. This causes a slight rotor tilt. At higher forward speeds, the pilot must continue to move the cyclic forward. This further reduces pitch angle on the advancing blade and further increases pitch angle on the retreating blade. As a result, there is even more tilt to the rotor than at lower speeds.

 

A horizontal lift component (thrust) generates higher helicopter airspeed. The higher airspeed induces blade flapping to maintain symmetry of lift. The combination of flapping and cyclic feathering maintains symmetry of lift and desired attitude on the rotor system and helicopter.

 

 

Remember, whether being used for trim or for control, the cyclic pitch is equivalent to flapping in that the changes in rotor conditions due to one degree of cyclic pitch are the same as those due to a one-degree change in flapping. The advancing blade flapping up and the retreating blade flapping down is not the only way to maintain symmetry of lift. That’s just one of ways of increasing and decreasing AOA, We can do the same with cyclic feathering. Lift being directly proportional to the change in cyclic pitch angle.

 

Edited October 20, 2014 by iChris