B454

OTHER: ~ Rotor Concept - Intermeshing w/ Curved Anhedral (Scimitar Blade)

Overview:

An intermeshing helicopter where;

The obliquity of the masts is much closer to the vertical than on previous intermeshing helicopters.

The rotor disks have large precones.

The blades have a large amount of anhedral.

The construction consists of absolutely rigid rotors.

Advantages for the Intermeshing Helicopter:

The blade tip to ground clearance at all azimuths will be equal to or greater than that of a comparable single-rotor craft.

A significant reduction of the pitch~torque cross-coupling. (The propeller may also help.)

The lift of both rotors is very close to vertical.

The stability of the craft should be enhanced.

Conceptual Sketch & Parameters:

In the above sketch, the obliquity of each mast is 3º.

The precone at the hub is 9º but the actual precone (centerline between hub and 75% of the blade span) is 4.9º.

The center of the blade clears the hub of the other rotor by 2".

The tip of the lower (advancing) blade, when out to the side, is level with the hub.

Anhedral, Aerodynamic Centers and Moments about the Pitch Axis:

The chordline of the blade intersects the pitch centerline of the blade at the pitch bearings and at 75% of disk radius. (0.75R). The chordline of the blade inside the 0.75R is above the pitch centerline and the chord line of the blade outside the 0.75R is below the pitch centerline. A positive change in the pitch of the blade will therefor cause the portion inside the 0.75R to move aft and the portion outside the 0.75R to move forward.

The 0.75R is taken to be the spanwise aerodynamic center of the blade. This 'fixed' position will be different for blades with different characteristics. It is also assumed that the spanwise center of lift and of drag are at the same location; for this discussion.

As the blade rotates about the mast during forward flight, the center of lift and drag will move spanwise. In addition, the movement of the lift and that of the drag may not be identical.

The objective is to;

Minimize any moment about the pitch axis as the center of lift moves spanwise along the blade.
Minimize any moment about the pitch axis as the pitch angle and the angle of attack change.

Two potential means (or combined means) of achieving this are;

Using different airfoils (with different locations of center of pressure) each side of the 0.75R of the blade. I.e. far forward and far aft locations of maximum camber. For example, look at the difference between a NACA 2112 and NACA2912 on NVFoil. Start developing information on OTHER: Aerodynamics - Airfoil - Characteristics

Giving the blade a slight 'S' shape in the plan view. Sine the pitch axis is a straight line the position of this axis, as a percentage of the chord, will vary along the span. The minimum percentage of chord will be at approximately 0.75R/2 on the blade and the maximum percentage of chord will probably be at or near the tip.

Reverse Velocity Region:

This region (which varies) may play havoc with the Curved Anhedral idea on this page, or perhaps it can be used to advantage. LOOK INTO.

Drawing:

The above should/may work if the induced drag holds a reasonable ratio with the induced lift. If this is so then the profile drag will be a constant, in respect to the angle of attack. If this is so, then by putting an appropriate amount of negative twist into the blade (probably negative twist from 37.5% of span to the tip and positive twist from 37.5% of span to the root.), it may be possible to counter the pitching moment of the profile drag.

Working Notes:

The current UniCopter rotor has a precone of 3º and an obliquity of 9º. Therefor the blade has an angle of +12º above the horizontal when it is over the other hub at azimuth-90. This means that if the obliquity of the masts were reduced to 3º the precone angle would have to be 9º. The anhedral over the span of the blade would then remove most of this precone.

The pitch centerline will go through the center of the hub and the chord line at 0.75 R. This means that the inner portion of the blade is above this centerline and outer portion is below this centerline. The actual precone is therefore a reasonable amount less than the 9º root precone.

The above will result in the tips of the blades at 90º azimuth (at the sides) being level with the hub. This is higher then the drooped blades on a comparable single rotor craft..

The apex of the bow in the blade (anhedral) should be at a radius that puts it over the other hub.

The bow (anhedral) in the blade should probably not be normal to the chord of the blade. It should probably be about 8º off normal so that the lead/lag of the apex and the tip is as little as possible when the blade is at its mean operating pitch.

Instead of the anhedral being a large curving arc, perhaps the spar should basically be a straight line to the top of the other rotorhub, then a small radius curvature and then a straight line to the tip.

For a thin flat plate at a low angle of attack the lift coefficient Clo is equal to 2.0 times pi (3.14159) times the angle a expressed in radians (180 degrees equals pi radians):

Clo = 2 * pi * a [Clo = 2 * 3.14159 * (6 * 3.14159/180) = 0.6580]

The drag coefficient Cdo is equal to 1.28 times times the trigonometric sine of the angle a:

Cdo = 1.28 * sin(a) [Cdo = 1.28 * sin(6 * 3.14159/180) = 0.0023] * Why is this different from the value below? Is it only induced drag?

NACA 0012 airfoil from table in Theory of Wing Sections

Angle of Attack:	Coefficient of Lift:	Coefficient of Drag:
0	0.0	0.0060
2	0.22	0.0062
4	0.45	0.0070
6	0.65	0.0076 *
8	0.88	0.0091
10	1.10	0.____
12	1.28	0.0133

If the above drag (in column 3) is induced and profile then assuming that the coefficient of profile drag is 0.0060 the coefficient of induced drag looks like it might have a fairly close relationship with the coefficient of lift, at angles sufficiently below stall.

The objective is to create/find an Aerodynamic Center below the chordline for the blade elements at < 0.75R and above the chordline for blade elements at > 0.75R.

Information on this subject;

[Source ~ PHA ch.7]

[RWP1 p.417]

OTHER: Aerodynamics - Airfoil - Characteristics

Flight-Control for This Rotor:

The root of the blade provides little lift during hover. During fast forward flight, the advancing root provides lift but the retreating root will be providing some negative lift. Due to the arrangement of the intermeshing configuration, it may be advantageous to extend the blade spar so that it is equal to or greater than the stagger. Then provide only a 1P or less independent root control, which will be set by the forward velocity. The preferred method of control is probably ~ OTHER: Rotor Concept - Independent Root & Tip - ARR w/ Floating Root Method

It should be noted that this does not necessarily entail separate root and tip pitch links (i.e. 1P control over the root and the tip). It only requires a means of (slowly) changing the twist of the blade (much less than 1P).

This means that the tip and the root are directly linked. It is not the root linked to the hub and the tip linked to the hub. I.e. the twist change is within the blades. A single pitch horn controls the complete blade.

This may not be the optimal method for very fast forward flight, where the reverse velocity should be utilized.

Concerns:
It may be impossible to effectively overcome the (varying) moments about the blade's pitch axis.

An alternative idea is OTHER: Rotor Concept - Intermeshing w/ Precone/Decone

I.e. Will the fact that the center of lift will vary along the span of the blade at different azimuths and that the chord line of the blade is a different elevations in respect to the pitch centerline cause problems?

Have not yet calculated blade to blade clearance at crossing. For initial reference see; Rotor - Disk - Blade to Blade Clearance - 3-blade rotors 0854

Will the large conning angle at the front of the disk cause problems in fast forward flight, or could forward cyclic overcome any problem?

Will the profile drag of the blade at 0 and 180º azimuths, due to the large curved anhedral, cause a problem?

Will it be possible to incorporate Independent Root & Tip Control with the concept on this page?

Can and should the stagger be reduced?

OTHER: Rotor Concept - Intermeshing w/ Precone/Decone

OTHER: Rotor Concept - One-Piece Hub and Spars

DESIGN: UniCopter ~ Rotor - Disk - Pre-cone

DESIGN: UniCopter ~ Rotor - Disk - Blade to Blade Clearance - 3-blade Rotors

Initially displayed: February 14, 2005 ~ Posted on PPRuNe: February 18, 2005 ~ Last Revised: February 6, 2007

The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent.