1538

Item 1538

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

Would this page and its subject be better under; OTHER: Aerodynamics - General - Tractor vs. Pusher

Objective:

To compare the aerodynamic efficiency of a tractor propulsor vs. a pusher propulsor on a helicopter.

The question only relates to aerodynamic thrust and overall efficiency of the two actuator disks, in respect to their relationship with each other. The fuselage and aesthetics are not a consideration. The helicopter does not have wings.

Sketch:

Overview:

A rotor provides lift because the pressure differential is greater below the rotor disk then it is above it.

A propulsor provides forward motion because the pressure differential is greater behind the propulsor then it is in front of it.

Position:

A tractor propeller + rotor is more efficient than a pusher propeller + rotor.

Argument 1:

A helicopter has a greater efficiency when in ground effect then when outside of ground effect. This greater efficiency is due to the increase in the pressure below the rotor disk due to the airflow redirection caused by the ground. The ground can be considered as being a mirror image of the helicopter and its streamtube, therefore it is as if two airflows are interacting at 180º to each other. For reference; Bernoulli's Equation

A tractor propeller is forcing air into the same area as is the rotor. This should increase the efficiency of both the rotor and the propeller due to the airflow redirection resulting from the interaction of the two slipstreams. Of course this efficiency will not be as great as that provided by ground effect, because in this instance the two airflows are interacting at only 90º to each other.

Argument 2:

The efficiency of two rotor disks is greater when the two disks are remote from each other then when the are in close proximity. The following sketch shows that a pair of side-by-side disks and pair of extremely remote coaxial disks require approximately 65% of the power that a coaxial disk with very closely spaced rotors will require. In other words, when the two actuator disks are aerodynamically linked in parallel they are more efficient then when they are aerodynamically linked in series.
The implication of this is that the pusher configuration must be less efficient then the tractor. This is because the rotor and pusher are aerodynamically linked in series whereas the rotor and tractor are aerodynamically linked in parallel.

Argument 3:

Both the propeller and the rotor are operating in clean air.

Eng-Tips ~ Aerodynamic engineering Forum ~ Subject: Pusher Vs Tractor configuration

DenisNA (Aerospace) ~ 22 Jan 04 16:16

Overall efficiency of a pusher prop is always lower than that of a tractor one. The additional drag of slipstream is often overestimated. Instead, the slipstream can (and in many cases do) improve the L/D ratio of the plane!

This results from two physical mechanisms: 1. The high-speed slipstream blows over the central wing section and fuselage of a single-engine plane and corrects the spanwise lift distribution, which is always disturbed at this area. 2. The deflected slipstream adds to the lift and requires lower Cl values at all wing sections. This positive effect is most pronounced at relatively low airspeeds where the induced drag is the most important component of the total drag.
The example No1 of the most effective utilization of this effect is Antonov 2 biplane. The positive effect of slipstream in this graceful flying machine increases the L/D ratio by 14% during climb, and reduces the stall speed to completely unbelievable 25 knots (the gross weight is 5500kg). Antonov is in the class of her own.

My note: The above appears to be almost totally related to the wings, and they do not exist on a helicopter.

Rotary Wing Forum ~ Subject: Pusher Vs Tractor configuration

There is a lengthy discussion on this topic on the Rotary Wing Forum. See; Aerodynamic Efficiency: Puller Propeller verses Pusher Propeller? http://www.rotaryforum.com/forum/showthread.php?t=10085

One of the interesting remarks is by Ga6riel, where he says; "notice that in late WW2, most bombers began to shift to clean upper surfaces and underslung engines, while there are other reasons to support why they did this, it was noticed on development Avro Lancasters, that there was an increase in lift performance when the engines went underslung"

Optimal Efficiency:

Both of the sketches below represent identical criteria, except that the left sketch has the actuator disk as a tractor, and the right sketch has the actuator disk as a pusher.

The body has a 3D 'teardrop' shape and there are no appendages.

The tractor configuration will place the body in higher velocity air, however, it probably delays the reduction of the streamtube aft of the actuator disk.

The question is; which of the two basic arraignments will result in the greatest efficiency?

Thoughts:

This concern may not be quite so relevant to the Intermeshing and the Interleaving configurations as it will be to the coaxial. This is because;

The Intermeshing (UniCopter) has the thrust from its two rotors further out to sides then that of the coaxial. In addition the UniCopter's propeller is further forward, under the disk.
The Interleaving has its rotors even further out to the sides.

A pusher propeller on the center of the craft's for/aft axis will be drawing air from under the retreating portion of both rotors where the rotor thrust is relatively insignificant.
Turbofans on a large transport craft will be located under each rotor hub. They will draw air from under the front of the rotor disks and add air to the aft portion of the rotor disks

Picture of Farfadet; for interest;

Related Information:

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

Last Revised ~ July 22, 2008