1468

Item 1468

OTHER: Helicopter - Inside - Interleaving - Downwash in Hover

Overview:

A determanation of the downwash on the fuselage during hover.

Criteria:

The craft used for this evaluation is OTHER: Helicopter - Inside - Interleaving - w/ Enhanced Dihedral

The craft has Independent Root and Tip Control. (Active Blade Twist)

The root pitch control is 2P. Higher Harmonic Control

The tip pitch is 1P.

Sketch and Graph:

Values Taken from the Above Sketch:

The disk radii are 1.0 R

At azimuth 270º the distance from the center of the rotors to the center of the fuselage is 0.58 R.

The azimuth where the blades start overlapping the fuselage is 212º.

The azimuth where the blades stop overlapping the fuselage is 328º.

The Collective value below is that of the root. It is assumed currently that there is no Collective value at the tip.

Values Taken from the Above Graph:

Azimuth:	Radius:	Collective at Root: ⁽¹⁾	Cyclic at Root: ⁽²⁾	Collective at Tip: ⁽³⁾	Cyclic at Tip: ⁽⁴⁾	At Radius: ⁽⁵⁾
180º	∞	12.0º	4.0º	2.0º	0.0º
212º	1.00 R	12.0º	2.1º	2.0º	-2.4º	-0.4º
225º	0.82 R	12.0º	0.0º	2.0º	-3.0º	2.7º
270º	0.58 R	12.0º	-4.0º	2.0º	-4.0º	3.5º
315º	0.82 R	12.0º	0.0º	2.0º	-3.0º	2.7º
328º	1.00 R	12.0º	2.1º	2.0º	-2.4º	-0.4º
0º (360º)	∞	12.0º	4.0º	2.0º	0.0º

______________

Upper horizontal black line.
Upper blue 2P sine wave.
Lower horizontal black line.
Lower red 1P sine wave.
Curved black line. This shows the pitch of the blade over the longitudinal centerline of the fuselage.

Conclusion:

The maximum pitch over the centerline of the fuselage is +3.5º, when the blade is at 270º azimuth.

The pitch over the fuselage when the blade starts over and when it leaves the fuselage is -0.4º.

This looks Not too bad.

It might even be improved upon by implementing a 3P root pitch With a phase shift (of 60º?) which will increase the thrust slightly over the tail boom and decrease the thrust over the front end of the fuselage.

It might even be improved upon by implementing a 4P root pitch

Additional Notes:

See page 1525 for information on the aerodynamic interaction of interleaving rotors that do not have Independent Root & Tip Control.

In the two areas where the tips cross there will be a reduced thrust per blade. This is because of the fact that 4 blades are passing through this area and each blade will be passing through the remnants of an induced velocity, which was produced by the proceeding blade of the other rotor.
Just as per the above and that of a 4-blade single rotor there will probably be another two areas of reduced lift. This is at the inside root of each rotor. The passage of an opposite rotor blade's tip will leave a remnant of the induced velocity. Then the root of the rotor's blade will pass through this area and experience a negative lift. Very high twist may help. Having the widest portion of the chord out toward the tip somewhat may help.

Related Web Pages:

Sometime in the future develop "Optimization of Disk Loading Distribution, during; Hover, Cruise, Autorotation, Transition and Maneuvering'.

Last revised; June 10, 2008