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A400M - why the props ?

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A400M - why the props ?

Old 15th May 2020, 16:52
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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 ?
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Old 15th May 2020, 17:10
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A400M - why the props?
..... because the customer (OCCAR) insisted on props, the company preferred turbofans.
Can of worms: check. Opener: check.
Iím off to stock up on popcorn now..
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Old 15th May 2020, 18:56
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Originally Posted by atakacs View Post
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 ?
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 donít 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.

Last edited by VinRouge; 15th May 2020 at 19:30.
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Old 15th May 2020, 18:57
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1. Because they sound cool
2. Because if it had turbofans it would be a C-2 or KC-390
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Old 15th May 2020, 19:27
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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.

Last edited by Olympia463; 16th May 2020 at 14:51.
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Old 15th May 2020, 20:20
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Smile

Originally Posted by Olympia463 View Post
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 A440M had props but on reflection I can see why.
A440M is that a newer variant?
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Old 15th May 2020, 20:39
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Let's see if I adequately can explain this in non-engineer terms
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...
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Old 15th May 2020, 20:42
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VinRouge:
Plus, any decent Air Mobility pilot knows that lots of Torques are always better than EPRs.
Very subtle!! Not the wording as I know it, but you get the meaning across.
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Old 15th May 2020, 21:03
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Jets are for hot tubs, props are for proper aeroplanes....

Hat, Coat, Briefcase, Roller bag, Emergency slide.......
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Old 15th May 2020, 21:25
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1. Because they sound cool
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!)
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Old 15th May 2020, 22:18
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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?
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Old 16th May 2020, 00:06
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Burns half the fuel. That simple.
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Old 16th May 2020, 00:26
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Originally Posted by Cat Techie View Post
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.
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Old 16th May 2020, 01:03
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The Boeing YC-14 solved the jet/STOL problem eons ago...


The Boeing YC-14 is a twinjet short take-off and landing (STOL) tactical military transport aircraft. It was Boeing's entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition, which aimed to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport.
​​​​​​
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Old 16th May 2020, 07:05
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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.


Last edited by BEagle; 17th May 2020 at 12:54.
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Old 16th May 2020, 07:45
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It started with fans. Then changed to props. But they forgot to move the horizontal stab.....
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Old 16th May 2020, 08:45
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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.
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Old 16th May 2020, 10:37
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Why props?

In the words of an ancient Flight Engineer, "How can you feather a hole?"
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Old 16th May 2020, 10:39
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Besides, you can't feather a hole!!
Cheers
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Old 16th May 2020, 11:09
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Originally Posted by Fareastdriver View Post
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.
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 10bnÄ (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...

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