1062

Item 1062

DESIGN: Rotor Blade ~ General - Comparison for UniCopter
Should this page be UniCopter????

Moment Coefficient - NACA 0012 to 23012 :

The moment coefficient of the NACA 0012 is 0.0 from below 0º up to stall.

The moment coefficient of the NACA 23012 is between -0.012 and -0.038 from below 0º up to stall.

Prouty says that the Aerodynamic Center for the 23012 should be at 24% of chord. This means a larger leading edge weight. From NVFoil it looks like the aerodynamic center should actually be aft of 25%, like 26%. I've got something wrong.

Comparison - NACA 23012 to VR-7b:

If the VR-7b has 2º of tab then the Cl is very close, the Cd is better for the 23012 and the Cm is close.

VR-7:

It appears that all helicopters that use the VR-7 profile inboard uses the VR-8 or VR-9 outboard. For example;

Bell 209 AH-1E&F Cobra (K747 blades)	Boeing VR-7	Boeing VR-8/-9
Bell 301 XV-15 (Boeing ATB)	V43030-1.58/VR-7	Boeing VR-8
Boeing 107 CH-46E Sea Knight	Boeing VR-7	Boeing VR-8
Boeing 179 YUH-61	Boeing VR-7	Boeing VR-8/-9
Boeing 414 CH-47D Chinook	Boeing VR-7	Boeing VR-8

Notes:

For general airfoil information see: OTHER: Aerodynamic - Airfoil

Idea:

Use one set of tooling (female molds etc.) to produce any of the following;

(Maybe should have 8'-6" R. w/ 8" to 4.75" chord) - BUT want small aspect ratio.

Six rigid blades (3 CW & 3 CCW) for a two-seat Sport Pilot helicopter. ~ UniCopter II (GW = 1100#) (high RRPM) (high disk loading)

Same six blades as above, unmodified, for a high performance single-seat Sport Pilot helicopter. The maximum angle of attack must be greater (& less - for inverted flight?) than standard. ~ UniCopter I (GW = 700# w/ heavy loading) (high RRPM) (high disk loading)

Four blades (2 CW & 2 CCW), unmodified, could be used for a single-seat Sport Pilot helicopter that has a teetering hub but no conning angle. (GW = 600#) (medium RRPM)(medium disk loading)

Four blades (2 CW & 2 CCW), with lighter internal construction, could be used for a single-seat Ultralight helicopter that has a teetering hub and conning angle. ~ SynchroLite (GW = 550#) (low RRPM) (low disk loading) Concern ~ can a blade root be designed that will take the UniCopter's loading and also be 'foldable' for the SynchroLite?

Considerations Regarding the Airfoil:

Assuming that fast forward speed is a desired attribute for the Unicopter, then the VR-7/8 blade will probably not be the optimum. It may be desirable to start with the symmetrical NACA 0XX, but twist will still necessitate two molds.

	Twist	To reduce length of arm to center of lift
	Taper	To reduce length of arm to center of lift
	Thin tip (for advancing blade)	ABC reduces the concern about retreating blade stall
	Thick root	For strength
	Cutout	If the RRPM is 737 and the air speed is 200 mph then the reverse airflow extends 3.75' out the retreating blade at azimuth 270º. The implication is that the airfoil should only exist for approximately the outer 5 feet of the blade. ????
	Pitching	Stronger blades can take torsion but it may put vibration and loading back into the control system.

The following is a copy from UniCopter - Rotor

Taper and twist on the blade will reduce the distance from root to center of thrust (length of moment arm) slightly.

Blades to be all composite (carbon) construction.

A high lift asymmetrical airfoil can be used because of the extreme rigidity of the rotor.

A strong arm (cutout) between the hub and start of the airfoil, which can be the mast-to-mast distance.

The airfoil profile can be thicker toward the root, for extra blade strength.

Any tendency to cone, at average loading, is offset by building a small amount of downward bow in the span of the blade and/or anhedral at the blade tip.

It may be advantageous to use blades with a reduced span and increased chord than conventional blades. The reduced span will shorten the length of the thrust's moment arm and this will result in a reduction in the weight of the blades and the hub. This weight reduction will not offset the required increase in horsepower, but. The faster turning rotors will also be advantageous for fast forward flight. Maybe - maybe not?

Consideration of Airfoils re Sufficient 'Meat' on the Bottom of the Spar, when Converting from NACA 00xx:

The aerodynamic center of the NACA 0012 is 25% of chord.

The aerodynamic center of the NACA 23012 is 24% of chord.

The aerodynamic center of the VR7-b is 27% of chord.

Notes:

With ABC the retreating blade is of less importance therefor;

Retreating blade stall is not so much of a concern. An airfoil with a higher maximum coefficient of lift and a less gentle stall can be selected.
The airfoil can have a thinner profile.

Thinner airfoils have a much higher drag divergence Mach number and will be a natural choice for the blade tip region. ~ [Source ~ PHA p.284]

May want to consider the newer airfoils VR-12 and VR-15. See [Source ~ PHA p.286] Used on Boeing 360.

Additional information on airfoils; OTHER: Aerodynamic - Blade Profiles

Supercritical Airfoil:

It appears that perhaps a supercritical airfoil will offer no advantage since the retreating tip on the UniCopter will not be operating near stall. The VR-7 laminar flow airfoil might be ideal, or perhaps the NACA 230xx airfoil. Look into further.

See Prouty's article 'Airfoil Choices' in Vertiflite Summer 2005

DYNAMIC STALL ON A SUPERCRITICAL AIRFOIL

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Last Revised: August 10, 2005