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fspro
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Joined: Tue Jan 10, 2017 9:47 pm

Placement of Engines on Wings

Thu Sep 10, 2020 7:09 pm

In considering the lack of reversers on the outboard engines of the A380 and the amount of rudder needed for an IFSD due to asymmetric thrust, I was wondering: what factors dictate how far from the fuselage the engines are placed on the wing? It seems like, if they are closer to the fuselage, the A380 could have reversers on its outboard engines and aircraft would need less rudder in case of an IFSD and a smaller rudder would reduce drag. So I think I must be missing something that requires the engines to be placed farther from the fuselage, but I don't know what it is. Thoughts?
 
GalaxyFlyer
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Re: Placement of Engines on Wings

Thu Sep 10, 2020 7:42 pm

The wing structure can be lighter because engines mounted further out balance the lift bending the wing upwards, so your lighter rudder is offset by heavier wing. A number of factors go into Vmcg/Vmca considerations beyond mere fin size, hydraulic power of actuators, rudder travel, aerodynamics of the fun itself (it’s a wing after all) to start.

The 747 engines are fairly far outboard with reverses, the lack of outboard reversers on the A380 is not dictated by engine position.
 
FGITD
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Re: Placement of Engines on Wings

Fri Sep 11, 2020 5:46 am

Like galaxyflyer said, they act somewhat as a counterweight. Wings want to flex up, engines keep them down.

I believe the 380 didn't get reversers on the outboard for the weight savings, and FOD prevention. They're high up, but also tend to hang over the side of the runway. Also it just didn't need them, so why bother?
 
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Starlionblue
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Re: Placement of Engines on Wings

Fri Sep 11, 2020 5:49 am

Airbus had planned for the A380 to not have reversers at all. A compromise was reached.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tomcat
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Re: Placement of Engines on Wings

Fri Sep 11, 2020 1:47 pm

In the event of an uncontained engine failure you may want to avoid the other engine of the same wing to sit in the way of the debris. Placing the outboard engine far enough from the inboard engine creates a longitudinal separation between both engines (at least on a swept wing). I don't know whether this is the main driver for the engine separation on a 4 engines aircraft. Consideration like aerodynamic integration and structural are also driving the position of the engines along the wing.
 
Max Q
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 5:32 am

I remember when Qantas had their uncontained A380 engine failure they landed with only one operable reverser, none fitted to the outboards of course and lost the reverser on the failed engine


IIRC they had little runway left
The best contribution to safety is a competent Pilot.


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Starlionblue
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 5:59 am

Max Q wrote:
I remember when Qantas had their uncontained A380 engine failure they landed with only one operable reverser, none fitted to the outboards of course and lost the reverser on the failed engine


IIRC they had little runway left


True, but they were also 50 tons over MLW, and thus faster than normal. They blew a bunch of tyres.

The landing performance calculation showed 100 meters remaining, though I'm not sure if that was factored or actual.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
hitower3
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 3:19 pm

Side question: Why are the engines placed forward of the wing?
Is this also an aerodynamics related choice (area rule), or is it for protecting the wing fuel tanks in case of an uncontained engine failure, or...?
 
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Starlionblue
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 3:44 pm

hitower3 wrote:
Side question: Why are the engines placed forward of the wing?
Is this also an aerodynamics related choice (area rule), or is it for protecting the wing fuel tanks in case of an uncontained engine failure, or...?


- Airflow in front of the wing is "clean" and not affected by the wing.
- Hanging the heavy engines up front relieves torsion on the wing spar. The leading edge wants to twist up, and having the engines out there acts as a counterweight.
- Uncontained engine failure is a consideration but probably not a major one.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
LH707330
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 10:55 pm

Starlionblue wrote:
hitower3 wrote:
Side question: Why are the engines placed forward of the wing?
Is this also an aerodynamics related choice (area rule), or is it for protecting the wing fuel tanks in case of an uncontained engine failure, or...?


- Airflow in front of the wing is "clean" and not affected by the wing.
- Hanging the heavy engines up front relieves torsion on the wing spar. The leading edge wants to twist up, and having the engines out there acts as a counterweight.
- Uncontained engine failure is a consideration but probably not a major one.

It probably also saves some structural weight because the engine is pulling forward on the wing. Putting them anywhere else would result in weird behaviors when adding or reducing thrust.
 
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LyleLanley
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Re: Placement of Engines on Wings

Mon Sep 14, 2020 11:38 pm

Starlionblue wrote:
Uncontained engine failure is a consideration but probably not a major one.


FWIW, from Joe Sutter's book, 747, he stated it's a regulatory requirement that the turbines cannot intersect the wing spar, due to unconfined failure and wing integrity.
"I've sold monorails to Brockway, Ogdenville, and North Haverbrook, and, by gum, it put them on the map!"
 
hitower3
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Re: Placement of Engines on Wings

Tue Sep 15, 2020 2:24 pm

Thank you for your valuable information!

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