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Why is it that a plane as big as the C-17 (the A400M is slightly smaller though) use turboprops instead of turbofans? Both aircraft seem to have the same purpose and are both capable of landing at short runways. So why does the A400M have turboprops?
I want to know why two very large planes have different propulsion systems.
This answer needs some background information.
What became the A400M started as the Future Large Aircraft or FLA in 1983. Airbus was worried what to do with their engineers after work on the A320 was completed, and the next projects, soon to be called TA9 and TA11 (TA for Twin Aisle; they would eventually mature into the A330 and A340), were still far, far away. So the idea of a Transall follow-up was dreamed up to keep Airbus busy, and the engines for this plane would help Snecma to fund the development of their next engine after the CFM56. Yes, the FLA started life with four turbofans!
However, the European military did not like to have turbofans on something that should have the shortest field length requirements possible, and especially their desire for the new transport aircraft to backup under its own power made them to demand turboprops. So in the late Eighties the FLA was converted to turboprops. These were to use the core of the M88 engine, so Snecma would be happy with the new configuration.
This project went through many ups and downs in the quarter of a century between its start and the first flight of the A400M, and many of the sometimes ugly details should better never be mentioned …
Both aircraft seem to have the same purpose
Not entirely.
The RAF see the C17 as a strategic airlifter, the A400 is seen as a tactical airlifter replacing the turboprop C130.
Strategic airlifters are more likely to make longer, higher-altitude journeys. Where jets are more fuel efficient. They are more likely to operate between airports with concrete runways where FOD is less of an issue.
Tactical airlifters are more likely to make shorter, lower-altitude journeys of the sort where turboprops are more efficient. They are more likely to operate between smaller airfields where FOD may be more of an issue.
References:
Some random internet website alleges the fuel costs in \$ per Nautical mile are 19.63 for A400 and 88.14 for C17. They don't say for what mission profile. At a minimum, this illustrates that there is some motivation for matching engine types to expected mix of mission types.
The similar sized AN-70 also uses turboprop engines, illustrating that this choice is not unusual for this sized aircraft.
As RedGrittyBrick showed, the aircraft are certainly similar in external dimensions, with the A400M being a bit smaller than the C-17. However, the differences are in what the aircraft are designed for. The C-17 is designed to carry heavy cargo long distances, while the A400M is designed to carry lighter cargo shorter distances.
The C-17 is powered by 4 turbofan engines which can together produce 160,000 lbf of thrust. The A400M is powered by 4 turboprop engines that can together produce 44,000 shp, which will come out to probably less than 30,000 lbf. Though both types of engines lose power with altitude, the turboprop will also lose more power with speed.
The difference in power corresponds to a difference in the weights of the two aircraft. The C-17 MTOW is 585,000 lb, while the A400M MTOW is 310,852 lb. The max payload weight is similar: 170,900 lb for the C-17, and 81,600 lb for the A400M. The C-17 is about twice as heavy and can carry twice the payload as the A400M. The C-17 also offers a larger cargo area, with 1.5 m extra width and 3.7 m extra length.
Due to the similar size, both aircraft can carry a similar amount of troops. The C-17 carries up to 134 and the A400M carries up to 116. However, the C-17 can only carry 102 paratroops, while the A400M can still accommodate 116. The A400M can also accommodate 66 stretchers, while the C-17 can only accommodate 36. This illustrates that the A400M is designed to carry lighter cargo such as personnel, but the C-17 isn't really more capable.
As RedGrittyBrick also mentioned, this leads to a difference in roles. The powerful turbofans on the C-17 can carry a lot of payload, but they need longer distances to make up for the higher fuel consumption. The C-17 can fly 2,420 nmi with max payload, and the A400M can fly 1,780 nmi. Similarly, the C-17 can fly 5,610 nmi with paratroopers, while the A400M can fly 3,450 nmi with a 20-tonne payload.
While the c17 is twice as big as the A400m by payload and range, the primary reason for using turboprops on the latter is for takeoff from short, unimproved airfields rather than aircraft size.
The four 17ft props on the A400m turboprop can move much larger cylinders of air than can the four 6.5ft fans of the c17. This generates much more thrust when static or moving slowly as in takeoff or landing, particularly important on poor fields. The c17's primary role is strategic, or long distance flight. These are conducted at a higher airspeed where the smaller fan is better suited and large props cause excess drag.
Boeing claims the c17 can land on unimproved airfields. The reality is that it is so heavy that one or two passes makes anything but concrete unusable, and there is a significant inspection/repair bill anytime they go agricultural.
Similarly, Airbus claims the A400m has strategic capability, but payload, speed and range make it an inferior choice.
Simple physics means that turboprops, at low speed, give more thrust for a given amount of fuel burnt than due turbofans.
Complex engineering means that building a turboprop aircraft in the 300 ton category, like the C-17, is impractical: it would need about 120,000 shp, total. Building the engines is fairly simple--Pratt, GE, and Rolls-Royce all make gas turbine engines that produce 30,000+ shp -- but neither the needed gearbox nor propellers are within current state of the art. The real answer is not that Airbus was stupid or France corrupt (if you think politics doesn't come into US procurement, I've got some nice oceanfront property in Nebraska), but that the C-17 is too big for turboprops.
Turbo props are better for lower speeds. The blades are that size because it also has to pulled along at the same speed as passenger aircraft. Turbo fans would ingest too much debris on landing on non prepared air fields.
This thing can fly at 110kts to refuel helicopters and as fast as a passenger aircraft. It's civil registered so it can fly along civilian airways too. Carries more, flys slower, flys faster, higher and longer than any competition. It's also European so if you are European too......be very proud of what Airbus has acheieved.
Turbo-prop configuration is blowing air over the wings and developing lift as soon as the blades start turning, this does not happen with turbofans. This means that turbo prop can have much shorter take-off distance than turbo-fan - which has to wait until forward motion of the aircraft develops lift. Turbo prop engines are lighter and more rugged than turbofan and less likely to ingest foreign material. The larger and more numerous the blades the slower they can rotate to produce same thrust and this also makes them quieter.
