We start this week a new section within Avialogs: How it works. First topic to explore, contra rotating propellers.
1 - What are contra rotating propellers ?
It's two propellers arranged one behind the other, in contra rotation. Several aircraft using this feature are Supermarine Spitfire, Avro Schackleton, Tupolev Tu-95 Bear ... A short list not exhaustive:
Supermarine Seafang Mk 32
Martin Baker MB 5
Tupolev TU-95 Bear
2 - Why contra rotating propellers ?
First, we will look how as single propeller works:
Each blade of an airplane propeller is essentially a rotating wing. As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the airplane through the air.
The high-speed rotation of an airplane propeller gives a corkscrew or spiraling rotation to the slipstream.
At high propeller speeds and low forward speed (as in the takeoffs and approaches to power on stalls), this spiraling rotation is very compact and exerts a strong sideward force on the airplane’s vertical tail surface.
When this spiraling slipstream strikes the vertical fin on the left, it causes a left turning moment about the airplane’s vertical axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective.
The corkscrew flow of the slipstream also causes a rolling moment around the longitudinal axis.
Note that this rolling moment caused by the corkscrew flow of the slipstream is to the right, while the rolling moment caused by torque reaction is to the left—in effect one may be counteracting the other. However, these forces vary greatly and it is up to the pilot to apply proper correction action by use of the flight controls at all times. These forces must be counteracted regardless of which is the most prominent at the time.
All the energy generated in this rotational air flow is lost and the handling of the aircraft can become extremely difficult at low speed and huge power (Think about a 2000hp Rolls Royce Griffon full power during take-off ...).
One solution is adding a second propeller behind the first, contra rotating and cancelling the rotating moment. It will allow to use the engine at his maximum power and get an higher efficiency from the power train.
So if we have a system more efficient, allowing more easy handling, why it is not used on all aircraft ? Main problems with contra rotating propellers are the complexity of the engine required to power such airplane, extra noise and weight.
3 - How does it works ?
It's quite simple: extra gears power the second propeller and one shaft rotate within the external shaft. We will look at it in details with Rolls Royce Griffon 57 (You can read technical documents about the Griffon 57 in Avialogs library):
Spitfire with Griffon Engine