To: helmet fire
I don’t know what your definition of cone angle is but here is mine. First of all we have to remove one thing from the equation and that is some helicopters have a left bias built into the mixing unit and when the pilot pulls collective tail rotor propeller effect is automatically compensated for. We will not consider this as it has an effect on cone angle nor, will we consider pilot input to compensate for translation as this too effects the cone angle.
With the helicopter on the ground and the rotor turning at speed the cyclic is in the rigged neutral position the blades are in the pure radial position as the only force acting on them is centrifugal.
At this point there is zero cone angle. If you have a Bell with a pre-coned head and it is in the same condition as described above that is the zero cone angle for that aircraft. It is most apparent on an articulated rotor system so we will deal with that type of rotor as any change in the cone angle effects lead and lag.
This can be best described by viewing the helicopter from the left-hand side. When the blades are in a pure radial position the blades are in line with the rotor head so there is a straight line from tip to tip passing through the rotorhead. At this point in time the driving axis (mast) and the driven axis (a line perpendicular to the rotor disc and coincident with the mast) are in alignment and, there is no cone angle.
When the pilot pulls collective in order to hover the blades will cone up to the point where lift is balanced out by the centrifugal forces and now the disc forms a flat ‘V” with the flat portion being the rotorhead. At this time the driven and driving axes are still coincident with each other. The flat “V” is the basic cone angle and all other movement of the blade disc will be about this point.
When the pilot pushes forward cyclic the blade over the tail will raise and the blade over the nose will fall. If you draw a line from the higher rear blade to the lower forward blade you have described the tip path plane for this condition. Now, draw a line that is normal to the tip path plane and intersect the center of the rotorhead. This line represents the driven axis. Draw a line coincident with the mast. The difference in angular displacement between the two lines can be also construed as the coning angle. It is this difference that results in lead and lag due to Coriolis forces or conservation of angular momentum. The greater the angular difference the greater the lead lag. If the pilot brings the two lines to a point where they are coincident with each other there is no leading or lagging.
Confused?
1. Flapping has nothing to do with cone angle.
Response:
The way I learned it flapping of the blades above the radial position is what prescribes the cone angle. The higher the flap the greater the angle.
2. Precession has nothing to do with cone angle.
Response:
Precession has everything to do with the cone angle. Whether you believe in aerodynamic or gyroscopic precession precession is what causes the tilting of the disc and as a result it creates an angular difference between the driving axis and the driven axis.
3. While the tip path plane changes with the introduction of cyclic, and the disc is tilted left (American helicopter) due to tail rotor roll, neither of these have anything to do with cone angle.
Response:
As I indicated above compensation for tail rotor propeller effect will cause a change in cone angle. That is why I removed it from the equation and referenced the viewing angle for the rotor system.
[ 28 November 2001: Message edited by: Lu Zuckerman ]
[ 28 November 2001: Message edited by: Lu Zuckerman ]