Item 1612

DESIGN: AeroVantage ~ PropRotor - Disk - Aerodynamically Variable Disk Area	▬	▬▬
Aerodynamic Rotor Effective Area [AREA] ~ Effective Disk Area ~ Variable Effective Disk Area [VEDA] Related page: OTHER: Aerodynamics ~ General - Optimum PropRotor	Coaxial for Speed	Tandem for Lift

Overview:

• A pair of propulsors that are aerodynamically convertible between;
• Two independent rotors in hover, which result in a larger disk area with a lower induced velocity.
• A set of contra-rotating coaxial propellers in cruise, which result in a smaller disk area with a higher induced velocity.
• Side view of transitioning between hover and cruise;

· The effective aerodynamic disk area is 50% smaller during cruise then it is during hover. This is achieved by combining the two rotors into one coaxial counter-rotating propulsor.

Perceived Advantages:

• Cruise: Some Russian aircraft employ coaxial counter-rotating propellers that achieve speeds of up to 570 mph.
• Hover: The streamtubes from the PropRotors do not place a downloading on the wings or fuselage.
• Transition: The streamtubes from the PropRotors do not place a downloading on the wings or fuselage until the forward speed has resulted in the wings assuming some of the lift.

Wake Contraction:

• March 24, 2008: A thought about the streamtube diameter at the aft propeller. The following sketch etc. is based on Leishman' evaluation of coaxial helicopter rotors. However, the freestream air velocity of propellers (during cruse) will be considerably higher then that of a helicopter rotor (during climb). Therefore, I wonder if the rate of the reduction of the streamtube diameter may not vary with changes to the freestream air velocity but the distance aft of the disk may. In other words, perhaps the aft disk should have a radius that is greater than the shown 0.8 of the forward disk.

• "... the wake of the upper rotor contracts quickly (within 0.25R below the rotor) so it can be considered fully contracted by the time it is ingested by the lower rotor. The ideal wake contraction is 0.707 but in practice it is found closer to 0.8." ~ Aerodynamic Optimization of a Coaxial PropRotor page 3. (see next line)
• Leishman's statement above references this technical report. Taylor, M. K., "A Balsa-Dust Technique for Air-Flow Visualization and Its Application to Flow Through Model Helicopter.Rotors in Static Thrust," NACA Technical Note 2220, 1950. Have hard copy of text but did not print out photos. After reading this report and having a cursory scan of the pictures, I wonder if the wake does contract so quickly.
• This implies that the diameter of the rear disks should be 0.8 times the diameter of the forward disks, assuming that the gap between the disks in forward flight is greater than 0.25R of the forward disk.
• In turn, this implies that the effective aerodynamic disk area is √((πR² + 0.8 πR²)/2) = √((3.143 + 2.011)/2) = √2.57/π = √0.82 = 0.91R.
• Wake Contraction appears to be related to the rotor thrust coefficient; the higher the C_T the greater the contraction. Rotary-Wing Aerodynamics, Section I, page 192; by W. Z. Stepniewski

Streamtube ~ Some Initial Thoughts:

• The basic objective is to have the forward and the aft PropRotors, on each side, located on a common axis during forward flight.
• The above is a little white lie. The streamtubes on the V-22 appear to move toward each other (and acquire a smaller diameter) as they descend from the rotor disks, therefore the lateral ctr-ctr stagger on the rear PropRotors may be somewhat less than the ctr-ctr stagger on the front PropRotors. See velocity picture below; but note that downwash on the wings may be having an affect .
• The idea is to have the diameter of the aft PropRotor equal the diameter of the front rotor's streamtube at the location of the rear PropRotor. This should make the 'effective aerodynamic disk area' be the average of the actual front and rear disk areas.
• This should mean that the total effective disk area of the PropRotors in forward flight is half the total disk area of the PropRotors in hover.
• This means that the induced velocity of the PropRotors can be much greater in cruise than in hover, and this should result in a greater lift in hover (greater disk area) and faster speed in cruise (greater induced velocity).
• The exact vertical location (Z-axis) of the PropRotor's longitudinal axii (X-axis) will be determined by the parasitic drag of the craft.
• The forward PropRotors and their motors maybe approximately 1.0/0.8 larger than the aft PropRotors and their motors. Or, perhaps all motors will be the same size (but turn at different RPMs?). Or, perhaps have a different blade count and/or chord, etc.
• The longitudinal locations of the vertical axii of the front and aft PropRotors will be determined in conjunction with the desired CG of the craft, the moments generated by the PropRotors and the wings etc.

Concern:

• Alignment of Streamtubes:
• I think that; the forward proprotor should be centered slightly below the chordline of the wing so that it causes a little more pressure under the wing, and that; the aft proprotor should be centered slightly above the chordline of the wing so that it causes a little less pressure over the wing. Will this result in the streamtubes of the fore and aft rotors not being concentric with each other?

Outside Information on the subject:

• Airbus : Extensive Testing A30X Configurations
• "For othet techies / nerds: you can recognize an old / fake open rotor picture by counting the blades. Not having the front and aft rotors have the same number was a lesson learned in the late eighties; resonance & a lot of irritating noise. On the newest open rotor studies I've seen the front and aft rotor blades even do not have the same length / shapes."
• Don Stackhouse talks about Contra-Rotating props Have hard copy.
• AERODYNAMIC OPTIMIZATION OF A COAXIAL PROPROTOR ~ J. Gordon Leishman Have copy on E-drive.
• US patent 5,381,985 ~ Wingtip mounted, counter-rotating proprotor for tiltwing aircraft
• WIND-TUNNEL TESTS 0F SINGLE- AND DUAL-ROTATING PUSHER PROPELLERS HAVING FROM THREE TO EIGHT BLADES ~ Have hard copy.
• The single-rotating pusher propellers were from 0 to 7 percent less efficient then the corresponding tractor propellers, and from 0 to 10 percent less efficient than the tractor propeller tested with the wing. The dual propellers provide about the same efficiency, within a few percent, irrespective of whether they were tested as tractors or as pushers and whether or not the wing was present.
• Wiki: Contra-rotating propellers http://wapedia.mobi/en/Contra-rotating_propellers
• US 7,584,923 ~ Tilt-rotor aircraft ~ September 8, 2009 ~ Burrage; Robert Graham ~ Have hard copy. & EP1704089 & GB2409845
• AN-70: Wide-Body Transport Aircraft with Short Take-Off and Landing
• Bell x-22A:
• Why are the fore and the aft tiltrotors not axially aligned?

Convergence of the Two Streamtubes:

• Velocity magnitude of the V-22 in hover about five aircraft heights over the aircraft carrier deck
• My thoughts;
• It appears that the streamtubes converge at an angle of approximately 3º from the axis of the respective rotor disk.
• The blue area of negligible airflow under the inner portion of both disks must due the obstruction of the wings and fuselage. These obstructions will not exist with the AeroVantage.
• Could the 3º of convergence be partially or totally due to these obstructions?

• Velocity magnitude of the V-22 in hover about five aircraft heights over the aircraft carrier deck

A one-fifth-scale wind tunnel model has undergone testing in the Transonic Dynamics Tunnel (a unique transonic wind tunnel) at NASA's Langley Research Center during summer 2006. The "semi-span" model (representing the right half of the aircraft) measured 213 inches in length, and had powered 91-inch rotors, operational nacelles, "dynamically representative" wings.[5]

The primary test objective was to study the aeroelastic effects on the aft wing of the forward wing's rotors and establish a baseline aircraft configuration.[1] Alan Ewing, Bell's QTR program manager, reported that "Testing showed those loads from that vortex on the rear rotor [are the] same as the loads we see on the front [rotors]," and "Aeroelastic stability of the wing looks exactly the same as the conventional tiltrotor". These tests used a model with a three-bladed rotor, future tests will explore the effects of using a four-bladed system.[4]

Contra-rotating Coaxial Propellers:

• Contra-rotating propellers ~ Wikipedia
• Tu-95 Bear Strategic Bomber
• Post: Historically there have been several attempts at counter-rotating props, with results from marginal to disastrous. Something about shock-waves between the two props, as I recall, breaking or shattering the props. Don't think I want to risk the possibility of putting prop pieces through the rotor. Maybe if someone did some static tests over several hundred hours I could be convinced otherwise. But until then. ~ from Rotary Wing Forum - RockyMeLad
• Reply: The Russian Tu-95, with for counter-rotating props, is still in service, since (I believe) 1952. That's 55 years of first-rate service. Some observers say the Russians intend to keep them in service for at least 20 or 25 more years. Throughout this time it has been the fastest prop-driven aircraft in active service. ~ from Rotary Wing Forum - Bruno
• Reply: Momentum Theory: The fundamental laws of motion say it all:
  
  Force = mass x acceleration
  
  Kinetic energy = 1/2 mass x velocity squared
  
  From these two relationships, we can extrapolate:
  
  Static thrust, for a given power, is proportional to the 2/3 power of propeller diameter. Double propeller diameter and thrust increases by a factor of 1.59.
  
  With a single propeller, as much as 20% of applied power is wasted by rotating the slipstream, depending upon rpm and power loading of the propeller disc (HP/ft²). In principle, counter-rotating props can eliminate this waste. In practice, interference limits the improvement. ~ from Rotary Wing Forum - C Beaty

Optimal PropRotor Diameter - Fore and Aft

• Belief or Miss-belief?:
• That the is an optimal diameter for propellers; and assuming that that the cruise speed is a given then simplistically the diameter must be large enough to overcome the parasitic drag of the fuselage but not so large that the profile drag of the propeller starts increasing the total drag.
• WORK ON THIS SECTION

Related Information:

• Related page on comparative disk areas; OTHER: ~ Aerodynamics - Rotor Disk - Dual Configurations
• See Quad PropRotor design, which has rear props that are smaller than front props. 1961 Agusta design on DESIGN: AeroVantage ~ PropRotor - Outside Information - Quad Rotor and Related Material
• The Attributes of a Variable-Diameter Rotor System Applied to Civil Tiltrotor Aircraft Have hard copy.
• Aerodynamic Design of VTOL Micro Air Vehicles Have hard copy.
• Old idea that might eventually become viable due to developments in composite construction, electronic, electric drives and Nano technology etc. From Mirek on Rotary Wing Forum.

• Author: Bernard Lidenbaum
• Title: V/STOL Concepts & Developed Aircraft, History Report, Vol I (1940-1986)
• Publisher: Universal Energy Systems, Dayton OH, for Flight Dynamics Laboratory (AFWAL/FIA). ID: AFWAL-TR-86-3071, Vol I
• Link1: http://contrails.iit.edu/DigitalColl...71volume01.pdf (26.25 MB)
• Link2: http://www.dtic.mil/dtic/tr/fulltext/u2/a175379.pdf

Playing Around:

• xxx

	Hover:	Cruise:	Non-dimensional Ratio:
Disk Area:	A + a = Aa	0.91A
Blade Area:	B + b = Bb	Bb
Solidity Ratio:	Bb / Aa	Bb / 0.91A
Induced Velocity:

o Based on balanced torque; not balanced thrust

Comment:

• The advancements in electric power (hybrid or other) are increasing the viability of this configuration.

Rough Notes:

• The three alternative propulsion arraignments for cruise are;

1. Coaxial counterrotating PropRotors rotating at a reduced speed (and lower motor and propeller efficiency)
2. Either Propeller: Stop and feathering the front or the rear PropRotor(s) and use one only for propulsion
3. Front propeller: Stop the rear PropRotor(s) and have its blade fold back against or into, the nacelle and use only the front proprotor for propulsion.

o The Quad V-44 rotor may be able to do 2 and 3.

• This page contains the essence of the Effective Disk Area concept.
• For work-up of rotor aerodynamics see; DESIGN: AeroVantage ~ PropRotor - Disk - Rotor
• For work-up of propeller aerodynamics see; DESIGN: AeroVantage ~ PropRotor - Disk - Propeller
• Pages OTHER: Aircraft - VTOL - AeroVantage - 1x2 Overview and OTHER: Aircraft - VTOL - AeroVantage - 2x4 Overview elaborates on the application of this concept.

Comparison of Rotor diameter to Propeller diameter for comparable rotary-wing and fixed-wing craft;

This is done to see what the change in diameters should be to be optimized for both types.

Types:	Rotary-wing:	Fixed-wing:	Ratio:
Manufacturer:	Robinson	Diamond
Model:	R22 Beta II	DA 20
Gross weight:	1370 lb	1,764 lbs
Empty weight:	855 lb
Passengers & baggage w/ standard fuel:	400 lb	600 lbs	0.67:1
Horsepower:	Lycoming O-360 180 hp	TCM IO-240-B3B 125 hp	1.44:1
Disk/prop diameter:	25 ft 2 in. dia.	5 ft 6.9 in. dia.
Disk/prop area:	498 sq. ft.	24.4	20.4:1
Wing area:		125 sq. ft.

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Initially displayed: January 30, 2008 ~ Last Revised: August 9, 2012

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