Contra Rotating Propellers

Topics: Propeller, Counter-rotating propellers, Contra-rotating propellers Pages: 16 (4553 words) Published: December 10, 2012
Chapter 1: Introduction
Spitfire Mk XIX, Tu-95 Russian strategic bomber, Douglas XB-42 Mixmaster, what makes these Aircrafts different. For most vehicles, the primary power consumer is the propulsion system. Advances in CFD and fabrication have increased the efficiency of the single propeller significantly. However, further increases in efficiency can sometimes be obtained from having two propellers in a contra- rotating configuration. Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimize the effect of torque. Examples include some aircraft propellers, resulting in the maximum power of a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are also common in some marine transmission systems, in particular for large speed boats with planing hulls. Two propellers are arranged one behind the other, and power is transferred from the engine via planetary gear transmission. The configuration can also be used in helicopter designs, where similar issues and principles of torque apply Axially-aligned contra-rotating propellers have long been recognized as an extremely efficient arrangement. The two propellers are designed such that the aft propeller removes the swirl induced by the forward propeller; as a result, a dual contra-rotating propeller will be on the order of 10% more efficient than a single propeller of equivalent power. An additional benefit is that the two propellers are torque balanced, which is advantageous in directional control and reduced heeling moment of single-shaft vessels. Contra-rotating propellers were installed in two U. S. Navy submarines in the late 1950’s to early 1960’s. These propellers were 1

mounted co-axially on a concentric shaft line; each shaft was powered by a separate motor. Significant problems with shaft seals and bearings precluded installation on future vessels. Current employment in vessels of significant size includes, an inboard-powered propeller axially aligned with a podded propeller on ferries, and concentric shafts with gearing from a single motor in heavy tankers. However, there is a significant complexity and cost increase that this increase in efficiency must be balanced against. This configuration has several advantages over a conventional propeller. The most significant is the approximately 10-15% gain in efficiency. Another significant advantage of a counter-rotating propeller design can be that there is no net torque applied to the vessel being propelled if designed properly. Depending on the vehicle configuration, this can be a critical advantage over a single propeller thruster. However, there are significant disadvantages as well. The additional gearing required to generate the contra-rotating motion adds more mechanical loss to the system, as well as another point of failure. Appendix 1 shows the list of aircrafts with contra-rotation propellers.


Chapter 2: Working Principle and Operating Principle
2.1 Operating principle When airspeed is low, the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a singlepropeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow. The tangential air flow also causes handling problems at low speed as the air strikes the vertical stabilizer, causing the aircraft to yaw left or right, depending of the direction of propeller rotation. If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. Fig 1 shows the difference in air flow between conventional propulsion...
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