A400M - why the props ?
 

Having some time for musing I was wondering about the advantages of the turboprop vs. using more conventional turbofans for the A400M.

Designing a completely new engine was obviously a very expensive proposition (and turned out to be even worse than that...) so what are the expected gains ?

..... because the customer (OCCAR) insisted on props, the company preferred turbofans.
Can of worms: check. Opener: check.
I’m off to stock up on popcorn now..

you also get a lot of lift off the prop flow meaning smaller wing area which is useful in austere areas and (may be talking garbage here) the energised flow over the leading edge of the wing means you don’t need leading edge slats, which significantly cuts down on complexity of the design. The P factor was supposedly removed with opposing rotation of the props and also cancel out a lot of the asymmetric flow issues typical of Multi engine props.


Plus, any decent Air Mobility pilot knows that lots of Torques are always better than EPRs.

1. Because they sound cool
2. Because if it had turbofans it would be a C-2 or KC-390

Vin Rouge is right. The propeller can be accelerated more rapidly to produce extra airflow over the wings and lift when flying slowly - an advantage for STOL operations. Noise may also be less with this choice of thrust production. I admit I was surprised to see the A400M had props but on reflection I can see why.

A440M is that a newer variant?

Let's see if I adequately can explain this in non-engineer terms :p
To a first approximation, props are constant power devices, while (pure) jets are constant thrust devices. The relationship is thrust x speed equals power - so the faster you go, the more power a jet produces while the less thrust a prop produces. So, props have better low speed performance (e.g. takeoff), jets have better high speed performance.
Modern technology has blurred that difference. High bypass turbofans pretty much split the difference between props and jets - producing adequate low speed performance and good high speed performance, while the latest high speed props provide improved high speed performance while retaining their excellent lower speed performance. But that basic relationship is the same - props are better slow, jets are better fast.

Now, that 's all engineering theory. For the specific instance of the A400M, some committee apparently made the decision that the low speed advantages of props were more important than high speed advantages of fan jets. I suspect politics played a role...

VinRouge: Very subtle!! Not the wording as I know it, but you get the meaning across.

Jets are for hot tubs, props are for proper aeroplanes....

Hat, Coat, Briefcase, Roller bag, Emergency slide.......

An A400M came over my residence whilst climbing through about FL240 outbound from Brize earlier today.

I didn't need ADS-B Exchange or to go outside to eyeball it to know what it was. The sound is utterly distinctive (and great!)

I certainly agree about the sound being very distinctive but I guess it was probably not high on the list 😏
I understand the low speed advantage. Is there a way to quantify it?

Burns half the fuel. That simple.

That used to be the case, but not any more.
Yes, a prop will typically burn less fuel/mile, but it's nowhere near half.

The Boeing YC-14 solved the jet/STOL problem eons ago...


​​​​​​

The earlier iterations of the 'FSTA' were a 4-jet design. The earliest I saw was 'Solution 10'. Over the following years it changed to a 4 prop design, after about another 4 dozen 'solutions'.

The design was more or less fixed, then the fin and tailplane were redesigned and the prop rotation became 'down between engines'.

Engine and prop development was....protracted in both software and hardware elements. Single lever power controls with such a wide speed range from max thrust at brake release to idle thrust at max IMN presented huge problems - the 8 blade prop design is ingenious, but far from simple. Turbofans would, on balance, probably have been much simpler.

It started with fans. Then changed to props. But they forgot to move the horizontal stab.....

With propellers you can bang it on a dirt airstrip and thump in reverse thrust. Any crap thrown forward and up will be slung away from the engines by the centrifugal forces generated by the props.

Jet engines just hoover it up.

Why props?
 

In the words of an ancient Flight Engineer, "How can you feather a hole?"

Besides, you can't feather a hole!!
Cheers
mcdhu

That's indeed correct (although the YC-14 had apparently very good STOL characteristics 40 years ago, even on dirt strips). But in any case, is that capability worth 10bn€ (low ball guestimate) in R&D and many years delay in the program ? Wasn't the only issue but clearly engineering those from scratch was a very risky proposition. I don't see the reward to be commensurate, so I guess there were other matters considered...