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CowAnon
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A400M / TP400 (down between the) engines questions

Mon Jun 22, 2020 6:58 pm

From ANALYSIS: A400M Grizzly bares all, (FlightGlobal, June 7, 2013):

    DOWN BETWEEN THE ENGINES

    For the A400M Airbus has developed a unique down-between-the-engines (DBE) scheme for propeller rotation direction. Engines 1 and 3 rotate clockwise, while 2 and 4 have an idler gear installed that gives them an anti-clockwise rotation. This is a marked departure from the typical counter-rotation scheme that has all engines on a wing turning in the same direction.

    The DBE configuration produces a more symmetrical flow over the wing, improving lift and handling qualities while allowing for smaller (less drag-inducing) tail surfaces. The symmetric propeller slipstreams also allow for mirror image wing structure and reduced structural weight due to lower wing bending moment loads. Finally, the DBE configuration has a benefit for passengers in the cargo compartment, too. Airbus had planned to install active acoustic dampeners to muffle the sound of the propellers. The DBE configuration has the blade closest to the fuselage travelling upwards, which reduces cargo compartment noise by approximately 1dB.

1.Is the additional lift just caused by the adjacent propellers' tips moving in the same direction, or does the benefit occur specifically because the flow is downward below the wing? If the between-engine tips were simultaneously moving the air upward above the wing instead, wouldn't there be a similar benefit? Intuitively, the downward motion seems to make sense as designed, since the increased pressure under the wing would push the wing upward. But air flow above the wing is supposed to be beneficial (though I have a harder time envisioning why).

2. Why would interior noise be reduced by the nearest blade tips moving upward instead of downward?

3. Airbus is rightly proud of the smaller/lighter/less-draggy horizontal and vertical stabilizers resulting from the DBE configuration, but they went with a T-tail configuration for the A400M, which seems to be the most common setup for high-wing aircraft with engines mounted on the wings. However, the Antonov An-70 has high-mounted wings with engines on them, but it has a conventional, lighter tail. Wouldn't a similar DBE-effected size reduction still be available on a conventional empennage? Why didn't Airbus go with that configuration instead? Or is there something wrong or unsafe with the An-70?

4. Airbus specifically had the aircraft and engines go through civil certification, but has anyone ever expressed commercial interest in the A400M or TP400? How much of a hassle would it be to certify different propellers for the TP400 if a customer wanted the engine to go faster than the Mach 0.72 maximum speed of the Ratier-Figeac FH385/FH386 propellers currently certified for it?
 
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zeke
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Re: A400M / TP400 (down between the) engines questions

Mon Jun 22, 2020 11:00 pm

CowAnon wrote:
1.Is the additional lift just caused by the adjacent propellers' tips moving in the same direction, or does the benefit occur specifically because the flow is downward below the wing? If the between-engine tips were simultaneously moving the air upward above the wing instead, wouldn't there be a similar benefit? Intuitively, the downward motion seems to make sense as designed, since the increased pressure under the wing would push the wing upward. But air flow above the wing is supposed to be beneficial (though I have a harder time envisioning why).


I would suggest it is due to the vortex from the propeller similar to the wingtip vortex, having the two in opposite directions next to each other cancelling each other out.

CowAnon wrote:
2. Why would interior noise be reduced by the nearest blade tips moving upward instead of downward?


It would be P factor, the upwards going blade would have a lower relative angle of attack.

CowAnon wrote:
3. Airbus is rightly proud of the smaller/lighter/less-draggy horizontal and vertical stabilizers resulting from the DBE configuration, but they went with a T-tail configuration for the A400M, which seems to be the most common setup for high-wing aircraft with engines mounted on the wings. However, the Antonov An-70 has high-mounted wings with engines on them, but it has a conventional, lighter tail. Wouldn't a similar DBE-effected size reduction still be available on a conventional empennage? Why didn't Airbus go with that configuration instead? Or is there something wrong or unsafe with the An-70?


It simply would be for the roles they saw for the aircraft, they would have gone with a T tail for air drops, air to air refueling (T tail moving some wake away from the trail), loading equipment in the tail

CowAnon wrote:
4. Airbus specifically had the aircraft and engines go through civil certification, but has anyone ever expressed commercial interest in the A400M or TP400? How much of a hassle would it be to certify different propellers for the TP400 if a customer wanted the engine to go faster than the Mach 0.72 maximum speed of the Ratier-Figeac FH385/FH386 propellers currently certified for it?


It is more common these days for military aircraft t go through civil certification with minor military only changes. Reason being is that military aircraft used to go through a certification by each military agency, with the aircraft being civil certified they accept the civil certification and just do the local military certification on the military equipment. This means each military agency does not need to do their own continuing airworthiness (AD, SB etc) for the aircraft they can just follow the manufacturer/EASA. This means the aircraft will enter service quicker for each nation, and cost less to maintain with less staff as the majority of the continuing airworthiness is being done by Airbus/EASA.
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744SPX
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Re: A400M / TP400 (down between the) engines questions

Wed Jun 24, 2020 3:54 pm

The counter-rotation of the An-70's engines probably has something to do with it. In fact the TU-95/114 and Antonov An-22 all have conventional tails and counter-rotating props...
 
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Starlionblue
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Re: A400M / TP400 (down between the) engines questions

Wed Jun 24, 2020 3:58 pm

744SPX wrote:
The counter-rotation of the An-70's engines probably has something to do with it. In fact the TU-95/114 and Antonov An-22 all have conventional tails and counter-rotating props...


Not the same deal. The An-22 and Tu-95 have counter-rotating props on the same shaft. The A400M has a single prop per engine, rotating in the opposite directions relative to the other engine on the same wing.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
CowAnon
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Re: A400M / TP400 (down between the) engines questions

Thu Jun 25, 2020 8:07 am

zeke wrote:
CowAnon wrote:
1.Is the additional lift just caused by the adjacent propellers' tips moving in the same direction, or does the benefit occur specifically because the flow is downward below the wing? If the between-engine tips were simultaneously moving the air upward above the wing instead, wouldn't there be a similar benefit? Intuitively, the downward motion seems to make sense as designed, since the increased pressure under the wing would push the wing upward. But air flow above the wing is supposed to be beneficial (though I have a harder time envisioning why).


I would suggest it is due to the vortex from the propeller similar to the wingtip vortex, having the two in opposite directions next to each other cancelling each other out.

Right, the propeller tips between the inboard and outboard engines cause less turbulence if they're going in the same direction. My question is, why didn't Airbus design it as "Up Between Engines" instead of "Down Between Engines"? Does it have to do with which direction the wing flexes with UBE vs. DBE?

3. Airbus is rightly proud of the smaller/lighter/less-draggy horizontal and vertical stabilizers resulting from the DBE configuration, but they went with a T-tail configuration for the A400M, which seems to be the most common setup for high-wing aircraft with engines mounted on the wings. However, the Antonov An-70 has high-mounted wings with engines on them, but it has a conventional, lighter tail. Wouldn't a similar DBE-effected size reduction still be available on a conventional empennage? Why didn't Airbus go with that configuration instead? Or is there something wrong or unsafe with the An-70?


It simply would be for the roles they saw for the aircraft, they would have gone with a T tail for air drops, air to air refueling (T tail moving some wake away from the trail), loading equipment in the tail

Okay, that makes sense for for a military aircraft like the A400M. But why would civil turboprops with high wing-mounted engines, like the Dash 8 and the ATR42/72, stick with the T-tail if the conventional tail is viable?

4. Airbus specifically had the aircraft and engines go through civil certification, but has anyone ever expressed commercial interest in the A400M or TP400? How much of a hassle would it be to certify different propellers for the TP400 if a customer wanted the engine to go faster than the Mach 0.72 maximum speed of the Ratier-Figeac FH385/FH386 propellers currently certified for it?


It is more common these days for military aircraft t go through civil certification with minor military only changes. Reason being is that military aircraft used to go through a certification by each military agency, with the aircraft being civil certified they accept the civil certification and just do the local military certification on the military equipment. This means each military agency does not need to do their own continuing airworthiness (AD, SB etc) for the aircraft they can just follow the manufacturer/EASA. This means the aircraft will enter service quicker for each nation, and cost less to maintain with less staff as the majority of the continuing airworthiness is being done by Airbus/EASA.

Interesting, so Airbus/Europrop would've led with civil certification anyway, even if they had zero hope of commercial application. Rolls-Royce did think they might be able to sell the TP400, though. Did any airlines inquire about it?

Rolls-Royce promotes turboprop solution for new civil airliners (Flight International, 1 July 2008)

    "The TP400 engine [for the Airbus A400M military transport] is a very efficient propulsion system," R-R director engineering and technology, Colin Smith, says. "There is a very sound argument to be made for the majority of the 150-seat market, which flies mostly for less than 1.5h [being turboprop-powered]...if somebody does want a high-efficiency turboprop then have we got one for you."
 
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Starlionblue
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Re: A400M / TP400 (down between the) engines questions

Thu Jun 25, 2020 8:54 am

Wing flex isn't a factor when it comes to propeller direction. It has to do with lift and lift distribution. By making each side have the same wing distribution you don't need to fiddle with asymmetric lift. By having DBE instead of UBE you get more lift in the center of the wing instead of the outside.

The T-tail has to do with keeping the tail out of the prop wash. It is always a matter of compromise. The T-tail might be heavier, but evidently the aerodynamic advantages outweighed the structural disadvantages.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
vikkyvik
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Re: A400M / TP400 (down between the) engines questions

Thu Jun 25, 2020 6:17 pm

Starlionblue wrote:
Wing flex isn't a factor when it comes to propeller direction.


I don't know how big an effect there is, but by having counter-rotating props on the same wing, you're certainly reducing the induced bending moment on the wing from the rotating props.

Starlionblue wrote:
By having DBE instead of UBE you get more lift in the center of the wing instead of the outside.


It's interesting, because my first impression would be that the DBE config would create less lift in the center, because the wing is behind the prop's downwash.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
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Starlionblue
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Re: A400M / TP400 (down between the) engines questions

Thu Jun 25, 2020 11:36 pm

vikkyvik wrote:
Starlionblue wrote:
Wing flex isn't a factor when it comes to propeller direction.


I don't know how big an effect there is, but by having counter-rotating props on the same wing, you're certainly reducing the induced bending moment on the wing from the rotating props.

Starlionblue wrote:
By having DBE instead of UBE you get more lift in the center of the wing instead of the outside.


It's interesting, because my first impression would be that the DBE config would create less lift in the center, because the wing is behind the prop's downwash.


I think the increased airflow between the props would lead to more net lift.

Regarding wing flex, I see your point. However, I think the net effect is rather small. Not sure.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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