Frontiercpt
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Why Is The MD-11 Engine Tipped Up?

Tue Jun 15, 2004 3:31 am

Hi! Why is the middle engine(#2?), tipped up the way it is?

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LY744
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RE: Why Is The MD-11 Engine Tipped Up?

Tue Jun 15, 2004 4:36 am

Could have something to do with the way air flows at that particular point (look at the shape of the top of the fuselage), and/or the attitude at which the airplane cruises.


LY744.
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Starlionblue
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RE: Why Is The MD-11 Engine Tipped Up?

Tue Jun 15, 2004 4:38 am

Airflow. MD-80/90 engines are tipped up and out. Underwing engines are pointed inwards.

Basically (and very simplistically) the air moves in an hourglass shape around the fuse, so when the airflow gets to the tail, it's "coming back in" towards the centerline.

There are several threads about this in tech_ops
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Frontiercpt
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RE: Why Is The MD-11 Engine Tipped Up?

Tue Jun 15, 2004 4:50 am

Thanks guys for the information!
 
CX Flyboy
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RE: Why Is The MD-11 Engine Tipped Up?

Tue Jun 15, 2004 8:45 pm

As a matter of interest, Cathay was the first airline many years ago to take delivery of a 747-200 that had it's engines angled downwards 2 degrees lower than usual in order to save on fuel...which it did. I believe all subsequent 747 deliveries had these new angled engines.
 
QantasA332
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RE: Why Is The MD-11 Engine Tipped Up?

Wed Jun 16, 2004 6:40 am

More specifically, it's mainly downwash that tipping the #2 engine up accounts for...

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QantasA332
 
darkblue
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RE: Why Is The MD-11 Engine Tipped Up?

Wed Jun 16, 2004 10:45 pm

Also note that it is generally the inlet of the engine that is adjusted for downwash or upwash. It's difficult to know for sure, but in this case engine #2 does appear to be slightly "tipped up", but not anywhere near the amount the inlet is.

DB
 
delta-flyer
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RE: Why Is The MD-11 Engine Tipped Up?

Thu Jun 17, 2004 12:45 pm

I don't know the reason, but I am skeptical that it is for inlet air flow. I suspect that the aircraft's aerodynamic balance is such that a downward pitching moment is required for optimum flight configuration. The upward pointing engine provides such a moment.

Pete
"In God we trust, everyone else bring data"
 
QantasA332
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RE: Why Is The MD-11 Engine Tipped Up?

Fri Jun 18, 2004 6:42 am

...I am skeptical that it is for inlet air flow. I suspect that the aircraft's aerodynamic balance is such that a downward pitching moment is required for optimum flight configuration. The upward pointing engine provides such a moment.

As Darkblue pointed out, even if the engine is tipped up itself (i.e. if it's not only the inlet that's tipped up), it's only by a tiny bit - not nearly as much as the inlet is. Therefore, any upward component of thrust is quite small. Besides, horizontal stabilizers exist to provide a nose-up moment, so they'd be pointless if creating a nose-down moment (like that provided by upward-tilted rear engines) would improve an aircraft's stability, like you suggested. Aligning with flow is indeed pretty much the only reason for engine tilt; rear-mounted-engines/engine-inlets are tilted up to account for downwash and likewise, many wing-mounted engines are often tilted slightly downward (the A340 illustrates this quite well) to account for upwash.

Cheers,
QantasA332
 
sevenheavy
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RE: Why Is The MD-11 Engine Tipped Up?

Fri Jun 18, 2004 5:12 pm

Hi,

This is slightly off topic but still relevant. I believe that MD-11's had modifications made to and around the #2 intake to improve airflow to the engine and fuel economy. Not sure exactly when but they had been in service a couple of years, I guess mid '90's.

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SevenHeavy
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MD-90
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RE: Why Is The MD-11 Engine Tipped Up?

Fri Jun 18, 2004 9:17 pm

Think of a picture of the 777-300ER prototype and remember how the red and blue lines represented airflow. The think about the no. 2 engine of the MD-11 and see if you can't make a connection.
 
delta-flyer
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RE: Why Is The MD-11 Engine Tipped Up?

Sun Jun 20, 2004 6:40 am

Quantas ... I agree that airflow is an important consideration - but do you know, for fact, that this was the primary design factor for the engine tilt, or are you speculating?

Here are a few observations .....
1) The 727 and L1011 do not appear to have angled inlets. The 727, however, has upward sloping #1 and #3 engines, which increase downward moment.
2) Using the horizontal stab trim adds drag - an aircraft is designed to be inherently balanced for cruise configuration with the stab in an optimum (low-drag) position.
3) the downward moment is the vertical component (which I agree is relatively small) multiplied by the radius arm from the cg - which is large. This may be just enough moment to enable the horizontal stab to be optimized for cruise.

Pete  Smile
"In God we trust, everyone else bring data"
 
QantasA332
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RE: Why Is The MD-11 Engine Tipped Up?

Sun Jun 20, 2004 9:03 am

All I can say is that only airflow issues are mentioned in what I have read, and it definitely makes sense. After all, horizontal stabilizers exist and are sized according to the need to produce a nose-up moment (to counteract the natural nose-down moment present in most flight regimes). Accordingly, it's a nose-up moment that's necessary to maintain balanced and stable flight in most cases. Why, then, would one want to tilt engines up, thus producing an unecessary nose-down moment? That'd just be counterproductive. Keep in mind that as a result of the tilting there is certainly that aforementioned moment (it's more than compensated for by the horizontal stabilizers), but the actual reason behind the tilting isn't for balance, as I said - airflow considerations are really the only driving factor.

Sorry, it's a bit hard to explain here, but hopefully you see what I mean...

Cheers,
QantasA332
 
vikkyvik
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I Accidentally Deleted The Topic Title.

Sun Jun 20, 2004 2:07 pm

If the engine inlet were not aligned with the local airflow, that would also add drag, in much the same way as the stab trim, or whatever.

I will concur with QantasA332 in saying that it seems unreasonable for the engine to be tilted up to counteract the stabilizer's nose-up moment. Why not just reduce the stab size, instead of tilting the engine upwards; reducing the stab size would reduce drag, and reduce the nose-up moment. So the argument that the engine is tilted to produce a nose-down moment seems unreasonable.

Alright, hopefully this is sensible. Good night.

~Vik
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WomBat151
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RE: Why Is The MD-11 Engine Tipped Up?

Thu Aug 05, 2004 7:58 am

I don't think it's because of the moment, because this force would be very small. I also don't think that its the airflow.
I think that the back of the engine is pointed down because the back of the airplane is also pointed down. The piece that connects the engine with the fuselage has the same shape over the complete length. If we had a perfectly horizontal engine, it would hang higher above the fuselage at the back, resulting in a much more fragile connection between engine and fuselage.

It would be possible, but i think it would be less easier to do and this seems to be a great sollution.
Ian @ EHAM (AMS), 3,1NM of SPY VOR radial 205
 
MD11Engineer
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RE: Why Is The MD-11 Engine Tipped Up?

Fri Aug 06, 2004 1:11 am

The No. 2 engine on a MD-11 (and a DC-10) is hanging from a pylon aft of the vertical stab. It is loosely conncted to the inlet tube by a bell,outh attached to the fan case, which permits tjhe engine to move independently of the fuselage, restricted by a diagonal rod at the bottom of the engine.
The primary structure of the stabiliser consists of 4 huge "banjo fittings", huge forged pieces looking like -O- (just turned 90°) They are attached to strenghtened frames in the fuselage and extend upwards into the four spars of the stab. The engine pylon is attached to the rearmost banjo fitting.
The inlet tube itself doesn´t carry any engine load.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
MakeMinesLAX
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RE: Why Is The MD-11 Engine Tipped Up?

Fri Aug 06, 2004 8:16 am

QantasA332 wrote:

...horizontal stabilizers exist and are sized according to the need to produce a nose-up moment (to counteract the natural nose-down moment present in most flight regimes). Accordingly, it's a nose-up moment that's necessary to maintain balanced and stable flight in most cases. Why, then, would one want to tilt engines up, thus producing an unecessary [sic] nose-down moment?

Sorry, it's a bit hard to explain here...

My intention is not to pick on QantasA332, but I'd like to clarify something in case some people (i.e. those who are not versed in aeronautics) are drawing the wrong conclusion based on this explanation.

In short, nose-down moment is not "a bad thing"

A controllable aircraft is one in which the center of mass is forward of the center of lift. In other words, such an aircraft should naturally tend toward nosing-over. Obviously, the stabilizer counteracts this moment, and is providing "negative lift".

Furthermore, an increase in velocity yields an increase in lift (from the main wing), and thus a larger nose-down moment. It indeed does not make sense that the engine angling is for the purpose of adding to this moment, since the velocity and thrust increase in tandem.

BTW, the velocity/attitude coupling explains the need for shifting fuel to the rear in aircraft such as the Concorde and B-1. I was amazed to find out that the F-14 doesn't shift fuel - the stabilizer lies below the plane of the main wing, so the downwash from the latter when swept "pushes down" on the stabilizer!
 
MD-90
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RE: Why Is The MD-11 Engine Tipped Up?

Sat Aug 07, 2004 12:00 pm

That's just because Grumman engineers are smart folks Big grin


Why tilt the engines up to produce a downward vector component of thrust when the horizontal stab is already producing downforce itself to hold the nose up (which makes the aircraft stable)? It would be working against itself.

Now, there are exceptions to this, notablly the Piaggio P-180, which has three lifting surfaces that actually all have positive lift (the foreplane, which is not a canard, main wing, and a tail that actually produces lift). One reason why a turboprop with 11,550 max gross rating has the same main wing area as a 172.
 
QantasA332
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RE: Why Is The MD-11 Engine Tipped Up?

Sat Aug 07, 2004 5:08 pm

Why tilt the engines up to produce a downward vector component of thrust when the horizontal stab is already producing downforce itself to hold the nose up (which makes the aircraft stable)? It would be working against itself.

Exactly what I was trying to say (in many fewer words). Thank you, MD-90, for that reiteration!

Cheers,
QantasA332
 
AmericanAirFan
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RE: Why Is The MD-11 Engine Tipped Up?

Wed Aug 11, 2004 5:10 am

Porbably helps with attitude pitch at cruising altitude and better thrust performance maybe no one has ever noticed but MD-80's have engines tilted down as well.

-AmericanAirFan
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