Why are large military transport`s fitted with Props?
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Why are large military transport`s fitted with Props?
Having just seen the news report on the Airbus A400 it got me thinking why do Air Force planners specify the turbine propeller on such large military transport aircraft?
What is the advantage of the Prop and why don`t we seem then fitted to medium / large passenger airliners?
Smala01
What is the advantage of the Prop and why don`t we seem then fitted to medium / large passenger airliners?
Smala01
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Probably to do with paying passengers not accepting huge noise levels in cabin and militarily reverse thrust with four huge props much better than four jets for short takeoff and landing requirements and slow flight necessary to launch paratroopers. Easier using props.
To return to the OP's question, the main requirement of a tactical transport is to get heavy loads in and out of unprepared runways.
Maximum acceleration & deceleration are needed. The bigger the area of air that a fan or propellor works on, the more effective it is, so propellers are better than jets for this. Propellers will always be more effective than jets for reverse thrust, as they don't have to suck air in the front and turn it around.
Propellers also send accelerated air over both the top and bottom surface of the wing, as well as over any high-lift devices such as slots & flaps. The lift thus generated is far greater than would be achieved by the wing passing through the air at the aircraft's airspeed alone.
Maximum acceleration & deceleration are needed. The bigger the area of air that a fan or propellor works on, the more effective it is, so propellers are better than jets for this. Propellers will always be more effective than jets for reverse thrust, as they don't have to suck air in the front and turn it around.
Propellers also send accelerated air over both the top and bottom surface of the wing, as well as over any high-lift devices such as slots & flaps. The lift thus generated is far greater than would be achieved by the wing passing through the air at the aircraft's airspeed alone.
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Thanks Mechta - this was exactly the info i was looking for.
On the flip side, what is the disadvantage of the props. Why do we not see them on modern large passenger transports.
Economy? Maintenance Cost?
Many Thanks
Smala01
On the flip side, what is the disadvantage of the props. Why do we not see them on modern large passenger transports.
Economy? Maintenance Cost?
Many Thanks
Smala01
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Take-off
Avman and IB,
The IL76 manages to get airbourne due to the curvature of the earth and lots of luck when I see them blast off...
Always a wonder how a brick flies so well!
TW
The IL76 manages to get airbourne due to the curvature of the earth and lots of luck when I see them blast off...
Always a wonder how a brick flies so well!
TW
Smala01, I'm glad I was able to help.
The disadvantages of propellers include:
In the late 1980s there was a lot of interest in the 'unducted fan' or prop fan which had blade with a very complex swept shape, running outside the engine cowl (look up propfan on wikipedia). As far as I recall, they didn't progress due to issues (perceived or real) with blade containment, noise and vibration.
The other issue for civil aircraft is the public's perception that a 'jet' is a modern aircraft and anything with propellers or propfans is 'a bit second rate'.
The disadvantages of propellers include:
- They are very noisy for people on the ground when the tips reach supersonic speeds. The North American Harvard (Texan) is the often quoted example of this.
- Cabin noise is higher.
- Propellers are less efficient at high speed. See the relatively marginal increase in top speeds at the Reno air races over the years, even with new build aircraft.
- A gearbox is needed to reduce the turbine speed down to optimum propeller speed.
- Turbofans are designed to contain blades if they come off. Its really bad news if a propeller blade comes off and goes through the fuselage.
- Ground clearance may require long undercarriage or a high wing.
- There is a more likelihood of passengers inadvertently walking into a propeller blade, even if it is static, and hurting themselves.
In the late 1980s there was a lot of interest in the 'unducted fan' or prop fan which had blade with a very complex swept shape, running outside the engine cowl (look up propfan on wikipedia). As far as I recall, they didn't progress due to issues (perceived or real) with blade containment, noise and vibration.
The other issue for civil aircraft is the public's perception that a 'jet' is a modern aircraft and anything with propellers or propfans is 'a bit second rate'.
