1612

Item 1612

DESIGN: AeroVantage ~ PropRotor - Disk - Effective Disk Area

Overview:

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 counterrotating propulsor.

This page contains the essence of the 2x4 PropRotor concept.

The page OTHER: Aircraft - VTOL - AeroVantage elaborates on the application of this concept to an aircraft.

Preamble:

Some Russian aircraft employ counter-rotating propellers, which achieve speeds of up to 570 mph.

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 not 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?).

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.

Outside Information on the subject:

Don Stackhouse talks about Contra-Rotating props Have hard copy.

Misc. Related to Convergence of the Two Streamtubes:

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]

Counterrotating Coaxial Propellers:

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 = ½ 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

Initially displayed: January 30, 2008 ~ Last Revised: September 19, 2008

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.