A400M Propellors

[vulcanised]

Gazing at photos the thought struck me that while almost all large aircraft now have enclosed jet engines, military such as the A400M have exposed propellors.

Is there a reason for this, and why do they have the scimitar-like blades more commonly associated with pushing submarines?

[charliegolf]

The A400M is a turbo prop, ie a propellor aircraft which uses a jet engine to turn the prop. The prop on the outside is not an external version of the spinning compressor blades on the inside of the jet engine. They are still there inside the casing and under the cowlings. The blade shape is the result of advances in aerodynamic science and the manufacturing advances that have happened over the years.

Why not jet engines? Someone who knows more than me will be along soon to explain, I'm sure!

CG

[Future Hunter]

Use of props gives a short field capability and immediate power. Most jets have a spool up time before any amount of power is available from idle. A Turboprop will generally have a constant engine setting (for the jet component) to drive the big fan at the front, and throttle movements will correspond to a change in prop pitch and therefore speed/dragging power (and is referred to as either torque or 'alpha').

Military aircraft require this short takeoff/stop facility (nothing like disc effect if you're coming in hot!) and for tactical situations like low flying, whereas civilian operators merely want to go from A to B. Jets will do this better as they are more efficient at higher levels (where you get a better TAS/IAS ratio), using less fuel, taking less time and losing less money!
So that's why we have the prop-powered Herc and in future A400M for tactical stuff, and big things like Tristar and VC-10 for strat capability.

[Future Hunter]

I heard somewhere that the new curved blades give very little extra aerodynamic advantage compared to traditional ones, yet the cost of manufacture is considerably more!

Of course the main advantage would be how much sound you get from it - on an aeroplane it's not really an issue - they're loud anyway and wil make a noise. Submarines are really careful about how much noise they make as it's how we find them! If they can quietly sneak along with the engine running, they're moving and hard to find- an advantage if someone is trying to throw a torpedo at them.

[vulcanised]

Thanks for those helpful explanations! I did wonder if there might be a STOL factor in there somewhere.

Scimitar blades not for tall Taliban cropping then........

[Jig Peter]

Since WW2 swept back wings have enabled aircraft to fly faster. Sweeping back the blades of the A400M's turboprop engines could be because the speed of rotation would otherwise bring them into the high-drag region - straight props would possibly need a bigger "step-down" from the gearbox to stay well subsonic, which would have a weight penalty.
My 2 pennyworth anyway ...

[WillDAQ]

One of the key factors that swung it was the ability to create sufficient reverse thrust to actually back the aircraft up on the ground. Such a reversal system on a conventional turbo fan aircraft would have been very susceptible to FOD.

[Mudfoot]

Two reasons for the shape: it effectively increases the prop's chord at a given arc moment (not unlike a swept wing's chord), thus reducing the length hence increasing ground clearance while maintaining efficiency. In unimproved landing field conditions, we don't want to cut the grass, do we? ;)

[Fareastdriver]

Normal straight propellors will have the tips approaching M.Cdr one with corresponding drop in efficiency. Swept back tips, it's not a new idea, reduce this effect in the same way as swept back wings. Another large turboprop, the TU 95 Bear, gets away with it by having very slow, very coarse pitch propellors.

[tmmorris]

I must say I was impressed with the quietness of the A400M at RIAT at the weekend - so perhaps quietness is another benefit of the swept tips. It was certainly more popular with my 2-year-old than the F15 or Gripen...

Tim

[Dan Gerous]

Once upon a time, in the long distant past, the original A400/FLTA was going to have jet engines.

[GreenKnight121]

WillDAQ made a passing mention of FOD.
This stands for either Foreign Objects and Debris (the cause) or Foreign Object Damage (the result). During my time in USMC aviation I have seen both definitions used.

This stands as one of the two prime reasons why tactical transport aircraft (C-103, A400M, G.222, C-27J, CN-235, C-295, An-24/26/32, AN-70, etc) have either turboprop (3-5 blades) or prop-fan (6+ blades) engines. Both are turbojets with a shaft drive that turns a large propeller.

Large turbojet or turbofan engines tend to suck in a lot of air, and along with this air tend to be sticks, rocks, people, or whatever is in their area of "suck". On dirt/grass airfields, or damaged paved/concrete ones, there tends to be a lot of such debris.

Their blades are not very sturdy (to hold down weight, and provide maximum thrust to weight), so these objects tend to seriously damage or destroy the engines.

Turboprop/propfan engines, on the other hand have the intakes for their power sections well above the ground (one reason for the high-mounted wings), so the only thing that gets hit by debris are the much larger & stronger propeller blades... and these tend to keep working even if they are somewhat damaged... at least long enough to get the aircraft to a location where they can be repaired or replaced.



The other reason is the previously-mentioned faster response to throttle changes. On a turboprop/propfan engine, the power section tends to be run at or near maximum power during take-off, landing, and low-altitude maneuvers. This means the propellers are also turning at or near maximum designed speed.

Changes in thrust are made by changing the pitch on the blades, from flatter (less "bite into the air, thus less thrust) to steeper (more "bite", thus more thrust). These changes are made in 1-2 seconds, rather than the 10-30 seconds or more it would take for a large turbojet/turbofan to speed up or slow down.

The pitch of the blades can even be changed so that the airflow is from rear to front, thus providing a LOT of braking power for short landings, and for complete maneuvering on the ground without needing tugs (as the blades on each engine are controlled separately).