A400M - why the props ?

[atakacs]

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 ?

[Trumpet trousers]

A400M - why the props?
..... 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..

[VinRouge]

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 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.

[dead_pan]

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

[Olympia463]

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.

[spitfirek5054]

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?

[tdracer]

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...

[Herod]

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.

[Big Pistons Forever]

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

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

[Wycombe]

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!)

[atakacs]

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?

[Cat Techie]

Burns half the fuel. That simple.

[tdracer]

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.

[TBM-Legend]

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.

​​​​​​https://youtu.be/8WY2k-uYz5E

[BEagle]

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.

[DCThumb]

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

[Fareastdriver]

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.

[gzornenplatz]

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

[mcdhu]

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

[atakacs]

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 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...

[Haraka]

In the early 80's when the" Future Large Aircraft" (FLA) was being considered , weren't Un Ducted Fans (UDF) also being developed in the USA and Soviet Union?. Might this approach have the attraction to European Industry of also assisting UDF development costs for an anticipated Civilian Market?

[atakacs]

Could be.
I guess an ATR with two of those (detuned) engines might be an interesting proposition...

[Herod]

I always thought the RR Dart was a high-bypass unducted fan ;)

[dead_pan]

Slight aside, but talking of the engine blue note, there is a weird effect if you stand immediately below one on approach. A few seconds after the a/c passes overhead you get this odd sound effect, difficult to describe but like a threshing sound. No silly jokes please, and no, I wasn't imagining it.

[Green Flash]

d_p, I think I've heard this described before and if I remember correctly it is something to do with the interaction of the prop-wash (opposite turning engines)?

[atakacs]

d_p, I think I've heard this described before and if I remember correctly it is something to do with the interaction of the prop-wash (opposite turning engines)?
Might indeed be the case.
How are does birds categorized when it comes to wake turbulence?

[AeroAg92]

Unductec fans have been put aside because they produce a huge amount of noise, not compatible with EU regulations.

[Buster15]

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 ?

It was the nation's who defined the specification.
And we must not loose sight of the fact that the TP400 is actually a very efficient power plant indeed.

[VX275]

Slight aside, but talking of the engine blue note, there is a weird effect if you stand immediately below one on approach. A few seconds after the a/c passes overhead you get this odd sound effect, difficult to describe but like a threshing sound. No silly jokes please, and no, I wasn't imagining it.
Nothing new, as a youngster I used to go with my plane spotting older brother and stand by the approach lights to Ringway. Whenever a BEA Vanguard passed over you got a similar effect, a rushing sound several seconds after the aircraft had passed overhead. Another spotter who was in the ATC, and had been taught about such things, explained it was the 'vortexes off the wings and props'. At the age of seven I didn't have a clue what a vortex was but I knew I liked the noise they made.

[Cat Techie]

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.
Was a simple statement yes, but I am rated on AE2100 and AE3700 powerplants. Cores are very similar. One has a propeller off the LP shaft, the other a Fan. Taking the types, both carry similar pax loads and weights. Yes one can fly 50 knts faster if it wants to and a bit higher. The jet burns a considerable higher amount of fuel that the turboprop on the same 400/500 mile sector and the prop was not much slower. Costs do add up. My last company base had turboprops. Swapped for jets, fuel costs made the base unprofitable, base closed.

[evansb]

Why Turbo-props? The Lockheed Hercules comes to mind. To date, more than 2,500 C-130s have been ordered and/or delivered to 63 nations around the world. Seventy countries operate C-130s, which have been produced in more than 70 different variants.

From the highest of air strips in the Himalayas to landing on aircraft carrier runways in the middle of the ocean, the C-130 exceeds expectations. The Hercules has an ability to tackle any mission, anywhere, at any time. It is even a hurricane hunter.

I've flown on Viscounts in my youth and my teens. Although I've never flown on a Vanguard, I admired its majestic appearance, large windscreen, big port holes, (same as the Viscount's), and a cruise speed of 360 knots. In the late 1980's, a flight on a combi L-188 Electra, (freight and passenger), re-ignited my appreciation of the efficiency turbo-props.

[MarkD]

Could be.
I guess an ATR with two of those (detuned) engines might be an interesting proposition...
if they wanted more power they could just go with PW150s like Bombardier did for the Q400. Instead they seem happy enough pottering along with their PW127s.

Saab 2000s have AE2100s now that I think about it.

[VinRouge]

It was the nation's who defined the specification.
And we must not loose sight of the fact that the TP400 is actually a very efficient power plant indeed.
When it’s not shaking itself apart :E

[tdracer]

if they wanted more power they could just go with PW150s like Bombardier did for the Q400. Instead they seem happy enough pottering along with their PW127s.

Saab 2000s have AE2100s now that I think about it.
The AE2100 was originally developed by Allison (before it was taken over by RR) - and has been looked at for other commercial applications as well.
Given that the AE2100 is already 'detuned' (derated) for it's commercial applications (compared to the C130J), the TP400 would be way too powerful to be useful for a commercial turboprop installation.

[Forrest Black]

A400M - why the props
The simple answer: It replaces aircraft that have props as well - since they were made for tactical missions.

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.
Also compare the A400M's rather modest flap system to the massive blown flaps that an airlifter with turbofans like the C-17 needs for tactical purposes.

Google "AERODYNAMIC DESIGN OF THE A400M HIGH-LIFT SYSTEM" for a paper written by an Airbus engineer (I'm not yet allowed to post links).

So, props have better low speed performance (e.g. takeoff), jets have better high speed performance.
Props also have better low-altitude performance and efficiency. One of the A400M's requirements was extended low-altitude flights. Props save fuel, which leaves more weight for the cargo.

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...
It was the turn of the millennium. How many utopian dreams were linked to that date? There was a high tech stock bubble, everything seemed possible. France was thinking in "Grands Projets", Airbus developed the A400M AND the A380 in parallel ...