cancidas
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A400 Prop Design

Wed Aug 08, 2007 4:55 am

in this picture:



the props look as if they are spinning in opposite directions. while the #1 spins CCW, the #2 is spinning CW. on the other wing, the opposite holds true, the #3 is spinning CW while the #4 is CCW. i've never flown a large prop a/c like that, is this normal for that type of a/c or is that just a strange deisng feature of the airbus? i could understand the reasoning to spin the props in opposite directions to balance out the torque on the airframe, but is it really a factor on something that large? if it is, they why wouldn't the #1 and #2 spin CW while the #3 and #4 spin CCW?
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Klaus
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RE: A400 Prop Design

Wed Aug 08, 2007 5:07 am

If the image actually represents the real design in that regard, I could imagine the interaction of the prop wakes and shockwave effects may offer advantages due to lower relative speeds of directly opposing prop blades...
 
9VSIO
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RE: A400 Prop Design

Wed Aug 08, 2007 7:21 am

It certainly seems to be the actual design. Look at the mock-up http://en.wikipedia.org/wiki/Image:A400M_propellers_DBE.jpg
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saintsman
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RE: A400 Prop Design

Wed Aug 08, 2007 3:44 pm

From an Airbus booklet:

"The powerplant uses Ratier-Figeac FH386 propellors that operate at a power rating of over 10000 shaft horsepower and allows the aircraft to fly up to a cruise speed of Mach 0.72. The propellor sense of rotation is down between engines. Such a configuration improves the high-lift performance and reduces the interior noise level of the A400M."


I seem to recall an article that the C130J suffered with propellor problems so it looks like Airbus has learnt from that.
 
aeroweanie
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RE: A400 Prop Design

Thu Aug 09, 2007 12:19 pm

To quote Flug Revue (http://www.flug-revue.rotor.com/FRHeft/FRHeft05/FRH0508/FR0508d.htm):

Quote:
The first time use of counter-rotating propellers on the two sides has had a huge impact. According to Airbus Military, this not only reduces any yawing in the event of an engine failure, but it also results in better airflow conditions over the wing. The horizontal and vertical tail units have been scaled down by eight and 17 percent respectively, while the flap system has also been simplified thanks to better lift values. All of these features have had a beneficial effect on the empty weight which, according to the manufacturer, is “under control”.
 
redflyer
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RE: A400 Prop Design

Fri Aug 10, 2007 4:40 am

Since we're on the topic of the A400 and its powerplant, anyone know why they decided to go with turboprops and not pure jets? The A400 is a pretty big bird as it is and going with turboprops has required an all-new engine be developed, which in turn has led to one of the major reasons for its delayed EIS. Turboprop engines of the size being developed for the A400 seem to bring an order of complexity to the airplane that's not needed and the requirement to counter-rotate each engine seems to add complexity in itself.
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tdscanuck
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RE: A400 Prop Design

Fri Aug 10, 2007 4:58 am

Quoting RedFlyer (Reply 5):
Since we're on the topic of the A400 and its powerplant, anyone know why they decided to go with turboprops and not pure jets?

Turboprops have better low-speed performance and, at that size, fuel burn than the equivalent jet. Good for range, good for short-field performance. It's also easier to implement self-reverse on a turboprop...the C-17 has some nifty but complicated thrust reversers to achieve the same thing.

Quoting RedFlyer (Reply 5):
the requirement to counter-rotate each engine seems to add complexity in itself.

I would assume that the counter-rotation is implemented in the gearbox and not in the powerplant.

Tom.
 
redflyer
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RE: A400 Prop Design

Fri Aug 10, 2007 7:06 am

Quoting Tdscanuck (Reply 6):
Turboprops have better low-speed performance and

I would think the low-speed performance is a function of the wing and not the engine. Turbo-props do tend to fly slower than jets so the wing of a turboprop is usually optimized for slower flight. However, the advertised speeds of the A400 are up there close to jet speeds so I'm curious to know why EADS didn't just go with a regular turbofan instead of an entirely new and complex powerplant.

Quoting Tdscanuck (Reply 6):
at that size, fuel burn than the equivalent jet.

Military organizations are rarely concerned with fuel burn

Quoting Tdscanuck (Reply 6):
I would assume that the counter-rotation is implemented in the gearbox and not in the powerplant.

Your assumption is correct. And that is what adds the complexity -- gearboxes are notoriously complex and maintenance hogs.
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WingedMigrator
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RE: A400 Prop Design

Fri Aug 10, 2007 12:39 pm

RedFlyer, here's some good reading material on the A400M

http://www.leeham.net/filelib/EADS_A400M.pdf
http://www.leeham.net/filelib/EADS_A400M_2.pdf

They mention hot & high field performance, rapid descent and parachute drops as design drivers that swung the trade to turboprops.
 
redflyer
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RE: A400 Prop Design

Fri Aug 10, 2007 1:20 pm

Quoting WingedMigrator (Reply 8):
RedFlyer, here's some good reading material on the A400M

http://www.leeham.net/filelib/EADS_A400M.pdf
http://www.leeham.net/filelib/EADS_A400M_2.pdf

They mention hot & high field performance, rapid descent and parachute drops as design drivers that swung the trade to turboprops.

Excellent links, which provide some answers. Thank you!  Smile
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tdscanuck
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RE: A400 Prop Design

Fri Aug 10, 2007 10:44 pm

Quoting RedFlyer (Reply 7):
Quoting Tdscanuck (Reply 6):
at that size, fuel burn than the equivalent jet.

Military organizations are rarely concerned with fuel burn

Not for economic reasons, no, but they care about range (especially for an airlifter) and fuel burn goes directly to range.

Quoting RedFlyer (Reply 7):
Quoting Tdscanuck (Reply 6):
I would assume that the counter-rotation is implemented in the gearbox and not in the powerplant.

Your assumption is correct. And that is what adds the complexity -- gearboxes are notoriously complex and maintenance hogs.

There's no question that the A400M gearbox is going to have to be an impressive piece of engineering. However, the Russians have had bigger gearboxes than this for decades and have proven you can do it with excellent reliability if you do it right.

Tom.
 
redflyer
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RE: A400 Prop Design

Fri Aug 10, 2007 11:49 pm

Quoting Tdscanuck (Reply 10):
However, the Russians have had bigger gearboxes than this for decades and have proven you can do it with excellent reliability if you do it right.

I have yet to see or read anything that would indicate the gearboxes on the TU-95 (a classic Russian aircraft incorporating complex gearboxes) were anything less than maintenance hogs. In fact, I'd be willing to bet there was an exhorbitant amount of time spent by the airplane in the maintenance hangar after each sortie. As for reliability, the TU-95 was ubiquitious in the media as the classic Soviet threat, but does anyone have hard numbers regarding how "reliable" it was when it came to combat readiness?

But your point is well taken. I do not mean to imply turbo-props with gearboxes (and they all have gearboxes to one extent or another) are not reliable or cost-effective or not mission effective. My question has been why did EADS choose to go with a turboprop equipped airplane since the performance of the A400M is very similar to a comparable jet airplane, and especially since the A400M was going to require an all-new and complex turboprop engine? WingedMigrator's links provide some of the answers to those questions, but I'm not sure the trade-off will have been worth it; especially if the engines continue to prove to be troublesome. Of course, hindsight is always 20/20!
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Starlionblue
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RE: A400 Prop Design

Sat Aug 11, 2007 12:04 am

Quoting RedFlyer (Reply 5):
Turboprop engines of the size being developed for the A400 seem to bring an order of complexity to the airplane that's not needed.

I don't really see how turboprops of themselves are much more complex than turbofans.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: A400 Prop Design

Sat Aug 11, 2007 12:33 am

Quoting Starlionblue (Reply 12):
I don't really see how turboprops of themselves are much more complex than turbofans.

They're both turbine engines at heart, so most of the stuff on the powerplant is of the same order of complexity. However, a turboprop has two major components that a turbofan doesn't: a primary gearbox and a variable pitch prop.

All turboprops and jets have auxiliary gearboxes and they're of approximately the same power range on either, so that's a bit of a wash. A turbofan puts its power to the fan through a direct drive...simple, easy, and reliabile. A turboprop has to run all that power through a reducing gearbox; this is doable but you need to do it right and it's not easy.

The fan in a turbofan is a fixed geometry critter...there are major materials challenges but no moving parts. The prop on a turboprop has all the same material challenges (they're worse in some ways since the load per blade is higher) and has a variable pitch mechanism burried right at the highest load point to boot.

Tom.
 
Klaus
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RE: A400 Prop Design

Sat Aug 11, 2007 1:27 am

Quoting Tdscanuck (Reply 10):
Not for economic reasons, no, but they care about range (especially for an airlifter) and fuel burn goes directly to range.

Plus it adds to the strain on the overall supply chain when deployed away from established bases; Every tanker is a potential target as well...

Quoting RedFlyer (Reply 11):
(and they all have gearboxes to one extent or another)

Aren't there gearless direct-drive turboprops with low-speed turbine stages? I seem to remember reading about that variant a while ago... Probably not as efficient but simpler and possibly more reliable...
 
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Starlionblue
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RE: A400 Prop Design

Sat Aug 11, 2007 2:17 am

Quoting Tdscanuck (Reply 13):
They're both turbine engines at heart, so most of the stuff on the powerplant is of the same order of complexity. However, a turboprop has two major components that a turbofan doesn't: a primary gearbox and a variable pitch prop.

Ah, good point.
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redflyer
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RE: A400 Prop Design

Sat Aug 11, 2007 2:25 am

Quoting Starlionblue (Reply 12):
I don't really see how turboprops of themselves are much more complex than turbofans.

The propeller has to rotate at a much slower speed than the turbine shaft. That means the power has to go through a conversion process (speed reduction) usually via a gearbox to slow it down.

Quoting Tdscanuck (Reply 13):
A turboprop has to run all that power through a reducing gearbox;

 checkmark 

Quoting Klaus (Reply 14):
Aren't there gearless direct-drive turboprops with low-speed turbine stages? I seem to remember reading about that variant a while ago... Probably not as efficient but simpler and possibly more reliable...

I don't know. I can't imagine it would be too efficient as you say because the turbine would have to spin at a much slower and less efficient speed.
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columba
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RE: A400 Prop Design

Mon Aug 13, 2007 3:48 am

Quoting Tdscanuck (Reply 10):
Quoting Tdscanuck (Reply 6):
at that size, fuel burn than the equivalent jet.

Military organizations are rarely concerned with fuel burn

Not for economic reasons, no, but they care about range (especially for an airlifter) and fuel burn goes directly to range.

I think in todays day and age they also are concerned about fuel burn maybe not as much as airlines but the money safed on fuel can be spend else where. Also a fuel economic aircraft can be better sold to the tax payer.....
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meister808
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RE: A400 Prop Design

Mon Aug 13, 2007 4:17 am

Quoting RedFlyer (Reply 5):
anyone know why they decided to go with turboprops and not pure jets

It all really comes down to propulsive efficiency - thrust production will be most efficient when the speed of the propulsive fluid most closely matches the speed of the ambient fluid around it. In this case, those fluids are air, and there's two different ways of producing the same amount of thrust - either you accelerate a large amount of air a relatively small amount (with a propeller) or you accelerate a small amount of air a relatively large amount(with a jet). As cruise speeds increase, jets become more efficient, but propeller-driven aircraft, especially with modern advances in propeller design, can be very efficient well up into the .7M range, and will always be more efficient at slow airspeeds and low altitudes. When EADS designed this aircraft, they were looking for the best tradeoff between cruise and tactical maneuvering - the turboprop is the clear choice for the tactical stuff, and modern advances in turboprop technology meant that the cruise speed didn't have to suffer all that much, either.

Clear choice? The turboprop.
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