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hotelbravo
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A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:26 am

I've seen quite a few pics of the A346 now in profile, and it always seems to me that the inboard engine always points down a bit, while the outboard engine seems to point up a bit:
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Photo © Bernardo Andrade



Do anyone know why this is? The only explanation I can come up with is that the configuration compensates for wing flexing in flight, that is, that the inboard and outboard engines are both perfectly level in flight due to wing flexing (but are not level on the ground).

Cheers.
 
LY744
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:28 am

Who said the engines need to be straight and level anyways?

Maybe they're designed to get the most out of the relative airflow, or something fancy like that.


LY744.
 
aogdesk
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:29 am

Not familiar with the A340, but the thread title certainly is an eye-opener!!  Smile
 
Slarty
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:32 am

Interesting observation. This might explain all the fully-loaded 34X PAX shaking/clatter/yaw/pitch and lack of V2 climb???

They remind me of the 707 in the 60s.
 
dogfighter2111
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:34 am

Nah, if you look closely they are level. It is probably the shape of the intake.

There is a United B733 on A.net showing the aircraft heading straight for you but the engine was facing the person standing next to you  Smile

Thanks
Mike
 
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ZSOFN
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:35 am

You might get a better response over in Tech/Ops. I think it's because the airflow at the point where each engine is is at that angle. It's easy to presume the air is just running straight across (i.e. in the direction of flight) but the body vectors the airflow in the direction indicated by the engines. I remember during a fluid dynamics module being shown how air is moved by the aircraft even quite far in front of the aircraft itself - lift is generated a good 20m or so (don't quote me on that - it was a while ago) in front of the aircraft and further behind.
 
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longhauler
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 8:39 am

I would guess it would be something to do with the way the wing looks when loaded, and not when sitting on the ground. If you have ever seen an aircraft wing flex upward during takeoff, you would understand that these wings are designed with flying in mind, not taxiing.

Find a picture of an A340 flying, under normal wing loading, and the position of the engines might look different.
 
kaitak744
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 11:04 am

Well, I have a very accurate 1/200 scale model of the A340-600, and the engines are also pointed at slightly different angles. I always thought this was a model flaw, until I read this post.
 
bohica
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 11:20 am

Take a good look at the 747. The outboard engines point up a few degrees. Maybe someone at Airbus or Boeing can explain why the engines on the 340 and 747 are at different angles.
 
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hotelbravo
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 11:32 am

Maybe the outboard engines need to be pointed up a few degrees to reduce stress on the wing caused by the engines at low speed and on the ground? The larger moment of the outboard engines causes greater stress on the wing, so the outboard engines are pointed further up. The upwards angle may cause a slight penalty at high speeds, but this is more than offset by structural weight savings due to lower stresses.

Does this make sense?
 
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zeke
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 11:36 am

Engines are generally pointed towards the nose of the aircraft as a bow wave comes off the nose, it improved the engine efficiency by a toe-in towards the nose.

Also a slight angle down is common on RR engines as it aligns the airflow into the intake in the cruise condition where the aircraft is at a 2-3 degree nose up attitude, this improved the SFC.

[Edited 2005-09-20 04:37:46]
 
NAV20
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RE: A346 Engines Point In Different Directions?

Tue Sep 20, 2005 11:46 am

In a perfect world of course you'd want all the engines to have parallel thrust lines. But it isn't unheard-of to tilt some (or even all) of the engines upwards if there are some circumstances (usually low-speed situations) where the liift generated by the wings is insufficient to maintain acceptable performance.

It isn't just the A340 - someone's already mentioned the 747. And if you look at pictures of the A380 you'll see that the outboard engines on that are tilted upwards too.
 
jayspilot
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 4:37 am

wing flex in cruise.
 
teamregal
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:07 am

I'm not an aerospace engineer, but I think it's purposely designed that way to compensate for the unbalanced flow of air as well as potential force exerted on the fuselage.


REGAL
 
PyroGX41487
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:23 am

Quoting Slarty (Reply 3):
Interesting observation. This might explain all the fully-loaded 34X PAX shaking/clatter/yaw/pitch and lack of V2 climb???


No. The lack of V2 climb was an A342/A343 problem exclusively. The A340-500 and -600 are almost what can be described as overpowered (not 757 style, but might close  Wink )
 
Geo772
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:33 am

Most engines are mounted with a slight toe in or out. Usually by a degree or two.
 
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jumbojim747
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:43 am

Same thing when your looking at any b707 or dc8 .
I have also noticed it for some time.
Cheers
 
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hotelbravo
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:54 am

If the 707 and DC8 have the same engine deflections, this would implies that it has nothing to do with wing flex. Wing flex in the 707 is negligible compared to that in the 346. So aerodynamic or structural factors seem more likely to be the reason at this point.
 
tockeyhockey
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 5:58 am

a little bit of thrust up and a little bit of thrust down seems like they would cancel out, yet provide a more balanced vector, making the jet feel more stable to fly.
 
LY744
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RE: A346 Engines Point In Different Directions?

Wed Sep 21, 2005 8:37 am

Quoting Hotelbravo (Reply 17):
So aerodynamic or structural factors seem more likely to be the reason at this point.

7 replies up:

Quoting Zeke (Reply 10):
Engines are generally pointed towards the nose of the aircraft as a bow wave comes off the nose, it improved the engine efficiency by a toe-in towards the nose.

LY744.

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