1656

Item 1656

DESIGN: UniCopter ~ Pusher Prop - General - Angle of Incidence - Rotor & Propeller

Would this page and Tractor v.s. Pusher be better under; OTHER: Aerodynamics - General - xxxxx?

Objective:

To consider the pros and cons of having the rotors contribute to the forward propulsion of the craft during forward flight.

To improve the efficiency during cruise. To evaluate the optimum angle (in side view ~ X-Z plane) between the rotors, which have a forward thrust component (negative angle of incidence), and the horizontal thrust of a tractor or a pusher propeller.

Sketch:

UniCopter - UAV ~ Tractor Configuration

Using the rotors to contribute to the forward propulsion of the craft during high-speed flight.

From pages 66 and 67 of CICHE WIROPŁATY (Quiet Rotorcraft) (1999)

Rotor Propulsive Efficiency: (η_propR)

If the propulsive efficiency of the propulsor(s) is greater than 0.85 than autorotating rotors could probably be aerodynamically slightly more advantageous than their shaft-driven counterparts.

Shaft-driven rotors usually serve not only as lifting but also as propelling devices, overcoming some of the drag (D). When performing that function they do it at a power increment higher than the DV product (Drag * Speed of flight). Consequently, the rotor propulsive efficiency may be defined as

η_propR= (1.69 D V) / (RP_D - RP_D=0) (49)

Where V is speed of flight, in knots, and RP is rotor power?, in ft-lb/sec.

Eq. (49) rewritten in the coefficient form (notations as in Ref.16) becomes

η_propR= (μ(C_X / 6)) / ((C_P / 5) - (C_P /6)_x=0) (50)

To get an idea regarding the propulsive efficiencies of the rotors three forward inclinations of 3º, 6º, and 9º, of the thrust vector, T assumed to be equal to the lift (T = L) were examined. Data for rectangular tips in Ref.16 were used, resulting in a relationship shown in Fig. 41.

Although the relationship presented in Fig. 41 was determined in a rather cursory manner, it still may be assumed that it reflects a correct trend.

Vertical is 'Rotor Propulsive Efficiency' [percentage]

Horizontal is 'Rotor Thrust Tilt' (mast's forward inclination) [degrees]

Mu [μ] is Tip speed ratio (advance ratio) [non-dimensional]

Ref 16: is Johnson W.: Performance and loads data from a wind tunnel test of a full-scale rotor with four blade tip planforms. NASA TM 81229

Fig.41: Cursory estimate of trend in rotor propulsive efficiency, based on Ref. 16.

A glance at Fig. 41 would indicate that at the rotor tilt of 9º (when it still may be assumed that T = L, as cos(9º) =0.988 and the propulsive force F_prop = 0.156 L), rotor propulsive efficiency would be η_propR≈ 0.85 i.e. similar to those of the propellers. At lower rotor tilts it should be somewhat higher.

Information from Technical Documents on the Sikorsky ~ XH-59A - Coaxial-ABC;

Forward Flight Performance of a Coaxial Rigid Rotor ~ May 1971 ~ (#15):

[This is wind tunnel testing in the early stages of the project. It is only considering the forward propulsion from the rotors, not from the turbojets.]

"For the purpose of this theoretical geometrical study presented herein, the theoretical system considered is one which is required to lift 6,800 pounds and perform in the propulsive mode at a velocity of 150 knots. Shaft angle is held at a constant minus 12º throughout to insure propulsive force generation."
Advancing Blade Concept (ABC) Development ~ May 1976 ~ (#1):

"A design compromise in the selection of a zero degree shaft tilt. This setting was selected to provide for both helicopter and auxiliary propulsion mode testing on a common airframe. Performance improvements would be realized in both configurations by providing a forward shaft tilt in the order of four degrees [-4º] for the helicopter and an aft shaft tilt of approximately two degrees [+2º] for the auxiliary propulsion configuration."

Rigid Coaxial (ABC) Rotor System Stability and Control Characteristics ~ May 1976 ~ (#4):

[Rotor is providing all lift and propulsion] "Aircraft pitch attitude is nearly level up to 60 knots airspeed. The nose then goes down with airspeed, as increased propulsive force is required. A nose down attitude of about 12 degrees is required at 156 knots airspeed."

Advancing Blade Concept (ABC) High Speed Development ~ May 1980 ~ (#3):

[The following 3 lines imply that the rotors provide little to no propulsive thrust during cruise]

"In the auxiliary propulsion mode, the ABC rotor power requirements are reduced with increasing airspeed with maximum rotor L/D being achieved at or near autorotation. The power required for the auxiliary propulsion devices is, of course, highest at high speed."

"Conversion to the auxiliary propulsion configuration ........ installation of two P3A turbojet engines ........ and change of the horizontal tail incidence from +10 to -5 degrees."

"However, with the rotor flying at close to autorotation at high speed, differential collective pitch has little impact on yawing moment ....."

Assumptions on the Sikorsky ~ X2 - Coaxial-ABC; (Today)

From the pictures it looks like the X2 has an angle of incidence of zero degrees. However, a drawing of a larger craft appears to show an angle of incidence of approximately -4 degrees.

I think that it was mentioned that the rotors would operate in autorotative mode during cruise.

Does this mean that they are to operate in full autorotative mode with the rotor disks tilted up at the front? [Like the CarterCopter?]

Or, does it mean that the rotors will provide all the lift and no propulsion, thereby having a relatively horizontal rotor disks.

This section will be enlarged if and when more technical information is made available.

Thoughts:

Re the Sikorsky ~ XH-59A - Coaxial-ABC:

This craft uses two P3A turbojet engines mounted midway along the fuselage.
If this helicopter was to have taken advantage of the relatively free forward component of rotor thrust (up to 9º angle of incidence, as described by Stepniewski), would mean that the angle between the rotors' thrust line and the turbojets' thrust line must have been made less than 90-degrees.

Re the Sikorsky ~ X2 - Coaxial-ABC:

This craft uses a pusher propeller.
As with the XH-59A, If this helicopter is to take advantage of the relatively free forward component of rotor thrust (up to 9º angle of incidence) it means that the angle between the rotors' thrust line and the propeller's thrust line must be less than 90-degrees.
However the angle between the rotors' thrust line and the propeller's thrust line appear to be 90-degrees.

Questions Related to the ABC Concept, Irrespective of Rotor Configuration:

During cruise, will not the vertical location (Z-axis) of the center of drag be very dependent on whether the rotor is contributing to forward thrust versus acting in the autorotative mode? (i.e. the change in the induced drag.)

Is the propeller to be non-rotating during hover to medium cruise or is it to rotate but deliver no thrust during hover to medium cruise?

Do the rotors only contribute to forward propulsion during slow cruise, or do the rotors contribute to forward propulsion during all cruise speeds?

Does the pitch of the propeller or the rotational speed of the propeller increase with the increase of speed of the craft?

Does the propeller have a single speed, a two-speed (on/off) or a variable speed?

Notes:

My Thinking:

The rotors' thrust line and the propeller's thrust line must be less than 90-degreess say 85-degrees. In other words; the rotors will have an angle of incidence of ≈ -5º.
The rotors and the propeller will contribute to both lift and forward propulsion. The rotors providing >95% of the lift and the propeller providing ≈ 85% of the forward propulsion.
Moving the cyclic stick forward will cause the craft and rotors to tilt forward in a manner similar to that of a conventional helicopter, however, the angle of tilt will be substantially less..
This small angle of incidence of rotor (≈ -5) will theoretically provide an improvement in the efficiency of the craft during cruise. It is assumed that a 5º nose high angle of the fuselage during hover will not be detrimental.

Related Information:

DESIGN: UniCopter ~ Pusher Prop - General - Tractor vs. Pusher Propulsor

This patent may have some relevance; CarterCopter ~ 'Tilting mast in a rotorcraft'

Last Revised: July 24, 2008