0733.html

0733

OTHER: Aerodynamics - Blade Profile - VR-7 & VR-7b

Overview:

Centers, forces, moments, vectors, etc. related to the VR7 & VR-7b airfoil.

For information related to the specific blade [VR-7b] for the SynchroLite see: DESIGN: SynchroLite ~ Rotor - Blade - Composite - VR-7b - Centers, Radii & Moments

For information related to the rotor disk for the SynchroLite see: DESIGN: SynchroLite ~ Rotor - Disk - Centers, Radii & Moments

Drawing:

Characteristics:

Thickness	t/c	= 0.12
Leading Edge Circle	r/c	= 0.0113
Center at	x/c	= 0.01055
Center at	y/c	= 0.004
Trailing Edge Tab	from x/c	= 0.96
Trailing Edge Tab	to x/c	= 1.01
T.E. Tab Thickness	t/c	= 0.005

Charts:

Coefficients of Lift, Drag, and Moment:

~ from Chuck Beaty

Additional Information:

VR-7 TE=0.tif ~ from Chuck Beaty

For zero pitching moment, trailing edge reflex is +3 degrees, roughly a continuation of lower surface. ~ from Chuck Beaty

Airfoils with Different Tab Settings and Angles of Attack:

Airfoil integral coefficients: Cl (lift), Cd (drag) and Cm (aerodynamic moment in respect to 1/4 of the chord). It's important to note that Cd is theoretically null in a 2-D non-viscous subsonic flow; hence this number is a measure of the numeric errors in the code and in the choice of the points.

The Cm is considered positive when pulling up.

The following calculations are from the freeware program NVFoil.

ά		[alpha] The angle of attack is the angle between the airfoil chord and the velocity vector.
c_l		Coefficient of lift; two-dimensional
c_d		Coefficient of drag; two-dimensional
c_m0		Coefficient of moment; two-dimensional. (pitching-moment coefficient) @ 25% of chord
x_cp		Location on X-axis of center of pressure as a percentage of chord from leading edge.
Airfoil & Tab	ά		c_l	c_d	c_l^3/2/c_d	c_m0	x_cp
VR-7	0		0.299	0.0001		-0.047	41
	1		0.415	-0.001		-0.049	37
	2		0.54	-0.002		-0.05	34
	4		0.782	-0.002		-0.053	32
	6		1.024	0		-0.056	31
	8		1.264	0.005		-0.059	30
	10		1.504	0.013		-0.062	29.5
	12		1.741	0.023		-0.064	29.3
VR-7b - 0	0		0.176	0.002		-0.02	37
	1		0.228	0		-0.021	32
	2		0.402	-0.002		-0.02	30
	4		0.628	-0.003		-0.019	28
	6		0.854	-0.001		-0.019	27
	8		1.079	0.002		-0.019	27
	10		1.303	0.009		-0.019	26.5
	12		1.526	0.017		-0.019	26.5
VR-7b - 2	0		0.111	0.003		-0.007	32
	1		0.225	0		-0.007	28
	2		0.339	-0.002		-0.006	27
	4		0.565	-0.003		-0.006	26
	6		0.791	-0.002		-0.006	25.5
	8		1.017	0.001		-0.006	25.5
	10		1.242	0.007		-0.006	26
	12		1.465	0.015		-0.006	26
VR-7b - 3	0		0.082	0.002		0	25
	1		0.193	0		0	25
	2		0.307	-0.002		0.001	24
	4		0.533	-0.003		-0.001	24
	6		0.759	-0.003		0.001	24
	8		0.985	0		0.001	24.5
	10		1.21	0.006		0.001	25
	12		1.433	0.014		0.001	25.5

I have a plotting of the coefficients from an actual 2-dimentional test, courtesy of CA Beaty. File [VR-7 TE=0.tif] (437KB).

Web Sites: VR-7: Curves of hysteresis Cl, Cd, Cm

Of Interest to the UniCopter:

Results of a wind tunnel test on advanced rotor configurations present an assessment of the benefits of advanced airfoils, tip shape, blade chord, and blade number. A CH-47D model rotor with VR-7/8 airfoils was tested as a reference rotor. The advanced rotors, incorporating recently-developed VR-12/15 high speed airfoils, were tested to 231 knots in the wind tunnel and demonstrated an improvement of 6.0% in figure of merit and 25% in cruise lift-to-effective-drag ratio over a rotor with VR-7/8 airfoils. The advanced airfoils show significant improvements in stall inception limits, flying qualities boundaries and rotor noise. Improved blade tip shapes provide additional benefits in rotor noise and do not reduce the stall inception boundary.

General comments related to the VR-7 blade:

The VR-7 is aft cambered. Aft cambered-airfoils generally have a worse Mach tuck problem than do forward-cambered airfoils. ~ [Source ~ RWP1 p.422]

It has been estimated that moving the aerodynamic center back 2% of the chord can result in a 10% saving in total blade weight. ~ [Source ~ RWP1 p.421]

VR-7b vs. 0012 by:Self November 8, 1999

It appears that to get the center of pressure of the VR-7b airfoil to not vary too much along the chord of the blade with changes in the angle of attack, the trailing edge tab must be set to -3 degrees. At this setting the Cl seems to be the same as the 0012 and the Cd is worse.

To use the VR-7 or VR-7b with tab of 0 degrees, a stiff blade and ridge pitch links will be required. This will transmit forces back to the pilot.

Pitching Moment: [Source ~ RWP1 p.419] October 4, 2000

Vertol has found that for their rotors the value of c_m0 - the pitching moment coefficient at zero angle of attack and low Mach number - should be slightly positive (about 0.01) to maintain a satisfactory control system oscillatory load level.

~ also ~

At the time of this writing, the accepted limit for c_m0 for main rotors is; c_m0 < 0.02

___________

by Self; The above does not say where the cmo is located (percentage of chord). The pitching moment coefficients in the above table exceed the +0.01. These pitching moments are about the 25% of chord location. I think that all is OK since at 27% of chord the pitching moment coefficients should be acceptable. Check out the validity of this paragraph.

__________

Temporary Random Ravings:

While on this subject; the VR-7 airfoil presents an interesting situation. It appears that this airfoil has a pitching moment coefficient of -0.049; at zero angle of attack, 25% of chord and low Mach number. Vertol has found that the pitching moment coefficient at zero angle of attack and low Mach number should be slightly positive to maintain a satisfactory control system oscillatory load level. It would therefor appear that the aerodynamic center and pitch hinge axis should be located near 27% of chord. It also appears that by moving this aerodynamic center back 2% the blade's weight will be reduce by 10%.

Is it correct to say: As the angle of attack changes the length of the arm (aerodynamic center to center of lift) changes and also the amount of the lift changes. The moment about the aerodynamic center is the product of these two variables and it does not change.

Where does the feathering hinge go (x & z locations) . At the mid-point of the 2 extremes of the center of pressure - I doubt. Could the blade be comprised of 2 different airfoils, one with a positive cm and the other with a negative cm, so that they neutralize each other at all angles of attack and mach numbers? - look into VR-12 & VR-15

_______________

Date: October 08, 2000 12:06 PM
Author: Alain Semet (laserir@aol.com)
Subject: 254 vibrations

I always heard that they picked this profile because of its enhanced efficiency combined with a very limited center of pressure excursions ("practically the same as OO12" were the word used by the ASI principals). Also those moments are pretty small and the stick forces are small even compared to the weight of the cyclic stick so that it would be pretty hard to tell to what the oscillatory forces would be due.

In any case, at this stage still without trim tabs, the vibrations can easily be brought down to a good R-22 feel by compromising between hover and forward flight (I keep it below 65 mph indicated) and can be optimized for either with tracking adjustments.

My thoughts:- It's hard to tell from X-sectional pictures of the Ultrasport airfoil, but perhaps ASI uses the VR-7b with a small amount of trim and compromises between a high lift/drag ratio and a low CofP movement.

Related Information at This Site:

SynchroLite ~ Rotor - Blade - VR-7b - Composite

Last Revised: Sunday, February 11, 2009