Starship From South Africa, joined Nov 1999, 1098 posts, RR: 13
Reply 1, posted (15 years 4 months 2 weeks 2 days 11 hours ago) and read 5485 times:
In the case of the Tu-95 Bear.....
it was determined that a propeller capable of efficiently utilizing the enormous power output of the Kuznetsov turboprop engines would have to be 7m (23ft) in diameter. This was unacceptable for a number of reasons, not least of which was the simple difficulty of building and producing such an enormous propeller and in manufacturing landing gear tall enough to accommodate it. It was decided that a contra-rotating propeller configuration would be best suited. Though mechanically complicated and aerodynamically challenging, a design utilising the contra-rotating option could be conveniently accomodated by the propeller manufacturers and the airframe landing gear could be readily designed to meet the more sensible height-above-the-ramp requirement.
Panman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 2, posted (15 years 4 months 2 weeks 2 days 9 hours ago) and read 5471 times:
Okay I've got an exam on propellers (propellors depending on which country you live in) this Friday so I better not be talking crap here.
Propellers have to be capable of absorbing engine power and converting it into thrust. The greater the propeller solidity the greater the engine power that can be absorbed.
Propeller Solidity is the ratio of total blade area relative to the swept disc area. For simplicity sakes it is the sum of the chord widths at a given radius divided by the circumference of a circle at that radius. The radius that is chosen is the propeller master station location which is usually 2/3 to 3/4 along the length of the blade from the hub.
There are two ways in which prop blade solidity can be increased.
(1) By increasing the chord length of the blade (make the blade wider). This has the effect of lowering the aspect ratio of the prop blade. Low aspect ratio wings provide a lot of lift at low speeds but at high speeds the amount of lift they create is offset by the tremendous induced drag. The same can be said for prop blades. There is a point where it is no longer beneficial to make the chord of the prop blade longer.
(2) By increasing the number of blades on the propeller. The side effect of this is that it increases the weight of the propeller assembly tremendously.
Now if the manufacturers in the aircraft shown had decided to use a conventional prop assembly then they would need to attach a propeller with a large number of blades on it in order to convert the engine power into thrust. The problem then would not be with the number of blades on the propeller but with the airflow that is left behind the propellor.
The basic thrust equation is Thrust = Mass x Acceleration. Props are more efficient that turbojets/fans in that they impart a small acceleration on a large mass of air while jets/fans impart a large acceleration on a small mass of air. Not only do the props impart a small acceleration on the airflow but due to the rotation of the prop blades the assembly also imparts a swirling motion onto the air. Some of the engine power is therefore lost in rotating the slipstream. This swirling air mass now moves rearwards and acts upon the horizontal stabilizer. Conventional aerofoils are at their optimum angle of attack somewhere between 3-4 degrees. The swirling mass of air from the prop hits the horizontal tailplane so that the AOA is no longer at its optimum of 3-4 degrees. The air mass also would act at an angle on the vertical stabilizer causing the aircraft to have a tendency to yaw. So the greater the properller solidity the more it would impart a swirl into the airflow and the more this would affect the rear aerofoils of the aircraft.
Now I finally answer your question. To overcome this problem contra-rotating propellers are employed. Contra-rotating propellers consist of two prop assemblies that rotate in opposite directions. The swirling airflow from the first prop assembly is straightened out byt the second prop assembly and the airflow behind the engine is now straight instead of swirling. This now allows for more prop blades to be added, thereby increasing prop solidity and therefore increasing the amount of engine power that is converted into thrust. It is now possible with contra-rotating props to convert upwards of 90% of engine power into thrust.
I hope this helps even though it is long winded.
P.S. Don't confuse contra-rotating props with co-axial props (two separately driven props mounted on the concentric shafts, these also rotate in opposite directions to each other). Even still a counter-rotating prop assembly which is the engine on the port side rotating opposite to the engine on the starboard side.
Mriya225 From French Polynesia, joined Nov 2011, 0 posts, RR: 0
Reply 3, posted (15 years 4 months 2 weeks 2 days 7 hours ago) and read 5467 times:
Starship & Panman,
Thank you both for your answers! Conceptualizing the dynamics at work with prop powered aircraft has never been my strong suit - but I think I'm beginning to catch on. Thanks for your help!
Yes, I was refering to two sets of (counter-spinning) rotors located on the same drive train. You don't need it - but Good luck on your test!