A400M - why the props ?
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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 ?
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 ?
A400M - why the props?
Can of worms: check. Opener: check.
I’m off to stock up on popcorn now..
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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 ?
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 ?
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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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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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.
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...
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...
"Mildly" Eccentric Stardriver
VinRouge:
Very subtle!! Not the wording as I know it, but you get the meaning across.
Plus, any decent Air Mobility pilot knows that lots of Torques are always better than EPRs.
Jets are for hot tubs, props are for proper aeroplanes....
Hat, Coat, Briefcase, Roller bag, Emergency slide.......
Hat, Coat, Briefcase, Roller bag, Emergency slide.......
1. Because they sound cool
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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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?
I understand the low speed advantage. Is there a way to quantify it?
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.
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.
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.
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.
Jet engines just hoover it up.
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