CRGsFuture
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Why MD80/DC9 Series Engines Are Tilted?

Thu Aug 03, 2006 1:35 pm

Hey all, yesterday I was at the second level at LGA's CTA terminal. I had to wait to pick my friend up, so I spotted for about 3+ hours before hand. Some people watched and stayed quiet but an interesting man from upstate looked like he knew what he was talking about? Or so I thought. One of the points he brought up was why the MD-80/DC-9 series engines are tilted?

He said a pilot told him

Quote:
Due to the rotation of the Earth.

However that's not true because the airplane flies through atmosphere. So to all MD80/DC9 pilots as well to 717 pilots, is their a better answer to this?

My understanding was due to a plane has to be at a certain pitch to maintain a stable altitude, having the engines tilted would allow the plane to pitch and the engines stay straight, is that the reason?
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MD-90
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RE: Why MD80/DC9 Series Engines Are Tilted?

Thu Aug 03, 2006 1:50 pm

I believe it's to better capture the airflow after it passes over the wings.

Refer to the 773ER prototype, remembering that the curved lines represent airflow:


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speedracer1407
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RE: Why MD80/DC9 Series Engines Are Tilted?

Thu Aug 03, 2006 2:49 pm

Curvature of the earth? That's pretty funny.

Based on other threads on this subject that I've read, It's my understanding that local airflow at the back of the plane dictates that the engine intakes point up a bit to maximize efficiency.

Notice also that when viewed from above, an MD-80's (and all other DC-9 variants) engine nacells are toe'd out for the same reason.

Also, the intake and the nacell might appear tilted, but the engine within, as I understand it, is not tilted to the same degree.

O
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Starlionblue
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RE: Why MD80/DC9 Series Engines Are Tilted?

Thu Aug 03, 2006 7:05 pm

As has been said, any diversion from the longitudinal axis is to better harmonize with local airflow. Underwing engines tend to be a bit toed in for the same basic reason.

The Mad Nitpicker Strikes: It's spelled "nacelle".

Quoting CRGsFuture (Thread starter):
My understanding was due to a plane has to be at a certain pitch to maintain a stable altitude, having the engines tilted would allow the plane to pitch and the engines stay straight, is that the reason?

Airliners are designed to fly with a slight pitch up since it is more efficient to do so. But airflow will still curve and twist around the fuse much more than just to "compensate" for this. Local flow conditions are the reason.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
vikkyvik
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RE: Why MD80/DC9 Series Engines Are Tilted?

Thu Aug 03, 2006 11:08 pm

As has been stated, it's simply a function of aligning with the local airflow. Look at some photos taken from directly in front of a 777. It almost seems like both engine intakes are pointed directly at you, because they are in fact angled slightly inward.

Quoting CRGsFuture (Thread starter):
He said a pilot told him

Quote:
Due to the rotation of the Earth.

Out of curiosity, what was the reasoning behind this answer? What exactly about the rotation of the Earth did he think necessitated angling the intakes?

~Vik
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CRGsFuture
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 12:54 am

Quoting Vik:
Out of curiosity, what was the reasoning behind this answer? What exactly about the rotation of the Earth did he think necessitated angling the intakes?

That due to everytime a plane flew, if the engines were straight the pilot would have to adjust downward to stay on the Earth. However by having tilted engines, it allows the plane to naturally follow the Earth's curve. Look I'm pretty gullible, but that reason was on belief. Thanks for all who answered.
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Arrow
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 2:10 am

Quoting Vikkyvik (Reply 4):
Out of curiosity, what was the reasoning behind this answer? What exactly about the rotation of the Earth did he think necessitated angling the intakes?



Quoting CRGsFuture (Reply 5):
That due to everytime a plane flew, if the engines were straight the pilot would have to adjust downward to stay on the Earth.

 Silly C'mon guys. This is a pilot with a sardonic sense of humour. Sort of like suggesting that the front suspension on a car should keep it .000005 mm higher in front so it will remain level for the passengers as it tracks along the curvature of the earth. I'll ask my mechanic to make that adjustment the next time I take my car in, then I won't feel like I'mfalling into the steering wheel everywhere I go.
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vikkyvik
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 3:02 am

Quoting Arrow (Reply 6):
This is a pilot with a sardonic sense of humour.

Haha, I totally missed the fact that it was a pilot who originally claimed the reason. That is funny.

Quoting CRGsFuture (Reply 5):
Look I'm pretty gullible, but that reason was on belief.

No worries - it's much better to ask and get the correct answers than to go on believing hearsay.

That said, don't believe everything you read here either  Wink

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
Dougloid
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 3:36 am

The line of thrust of each engine is a little off center relative to the centerline of the fuselage to promote stability. One can see the same thing on some recips whose engines are a little 'splay footed'. It's more or less the same reason that wings and stabilizers have a certain amount of dihedral or anhedral.
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speedracer1407
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 4:27 pm

Quoting Starlionblue (Reply 3):

The Mad Nitpicker Strikes: It's spelled "nacelle".

You know, as a student of english literature for 8 years, having worked on my masters degree in the same for 2, and as an adjunct professor starting in 3 weeks teaching "college writing," it really is amazing that I can't spell for crap. Spell check is a wonderful thing. I really should use it once in a while.
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Starlionblue
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RE: Why MD80/DC9 Series Engines Are Tilted?

Fri Aug 04, 2006 10:21 pm

Quoting Speedracer1407 (Reply 9):
Quoting Starlionblue (Reply 3):

The Mad Nitpicker Strikes: It's spelled "nacelle".

You know, as a student of english literature for 8 years, having worked on my masters degree in the same for 2, and as an adjunct professor starting in 3 weeks teaching "college writing," it really is amazing that I can't spell for crap. Spell check is a wonderful thing. I really should use it once in a while.

No worries. English is tricky because it's a pretty convoluted mishmash of influences. I used to have a huge problem with the apostrophe for possesive/contractions.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Arrow
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RE: Why MD80/DC9 Series Engines Are Tilted?

Sat Aug 05, 2006 12:49 am

Quoting Speedracer1407 (Reply 9):
You know, as a student of english literature for 8 years, having worked on my masters degree in the same for 2, and as an adjunct professor starting in 3 weeks teaching "college writing," it really is amazing that I can't spell for crap. Spell check is a wonderful thing. I really should use it once in a while.

Don't feel bad. Every newspaper I ever worked for (and that's nearly a dozen over 30 years) spelled my name wrong in my byline at least once, and some several times.
Never let the facts get in the way of a good story.
 
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Starlionblue
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RE: Why MD80/DC9 Series Engines Are Tilted?

Sat Aug 05, 2006 1:14 am

There's always http://www.rosboch.net/various/bobs_guide_to_the_apostrophe.gif

This useful guide handles everything except the "its" possesive exception.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
David L
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RE: Why MD80/DC9 Series Engines Are Tilted?

Sat Aug 05, 2006 1:24 am

Quoting Starlionblue (Reply 12):
everything except the "its" possesive exception

Made a bit easier when you consider that other forms of possessive pronouns don't present the opportunity, e,g, our, your, her, etc.
 
access-air
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RE: Why MD80/DC9 Series Engines Are Tilted?

Sat Aug 05, 2006 2:09 am

Tilted???
Are you referring to the engione nacelle from the front to the back or are you referring to the tilt of the Thrust reverser mechanisms??? If its the thurst reverser mechanisms, be care ful not all DC9s have this tilted feature....
The DC9s that I have seen this on are ANY MD-80 series....the DC9-50 series, some DC9-30s and even DC9-15s. (Air Canada's DC9-30s even tho powered by lower thrust engines than many newer DC9s had the tilt in their buckets.) I think they gave this treatment to their small fleet of DC9-15s as well...
I cannot remember of it is standard on the DC9-40 series or not....
It actually chaged with the DC9-50 as it was thought to make the thurst reversers more effective? I remember reading that some place...Other DC9s could be retrofitted...Most airlines chose not to......
Is this what you are talking about?

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