Leezyjet
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Joined: Sun Oct 14, 2001 7:26 am

Why Is The 757 So Tall?

Fri Jul 25, 2003 1:23 am

I was looking at a 757 being loaded the other day and was wondering why Boeing made this a/c stand up so high off the ground. The 737 is bulk loaded and is able to be loaded and unloaded without the use of baggage belts and as the 757 is also bulk loaded I don't understand why they had to make it stand so high off the ground.

I was looking at the engines they do not seem any bigger than those on the 733/4//5/6/7/8 so didn't seem like they required the extra height for ground clearance, after all could they not have just flattened the bottom like they did on the 733 onwards if that was an issue and made the a/c lower to the ground ?.

I then thought about the extra height being required to give the tail more ground clearence on rotation due to the extra length of the a/c, but ruled this out due to the fact that the B739 is just about the same length but is still lower to the ground.

So my question, why did Boeing make the 757 so tall ?.

 Smile
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
 
Rick767
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RE: Why Is The 757 So Tall?

Fri Jul 25, 2003 1:36 am

It is actually for adequate tail clearance on rotation in an engine failure scenario when tail clearance will be minimum (12-24 inches or thereabout, 12" is for the 763 I think, 24" for the 752).

The 764 gear height had to be raised for this reason (I believe it now stands the same height off the ground as the 777, with the same gear oleos?).

And I think the engines are quite a bit bigger than say a 737... they look it to me anyway.
I used to love the smell of Jet-A in the morning...
 
Guest

RE: Why Is The 757 So Tall?

Fri Jul 25, 2003 3:32 am

One of the considerations by Boeing, was to design the 757 with a thought of a fuselage stretch... as a matter of fact, this made the 757-300 possible...
xxx
Boeing had a bad experience in the late years of the production of the 707... Douglas had been able to stretch the DC8 into the 61 and 63 series, and it was impossible to Boeing to match the stretch of the Douglas fuselage, because of tail clearance for rotation being limited with the 707...
xxx
Happy contrails...
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cedarjet
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RE: Why Is The 757 So Tall?

Fri Jul 25, 2003 5:37 am

As ever, the Skipper knows the score. Boeing would have done much better with the 707 if the gear had been taller and allowed a stretch. Shame they left it so late with the 757 - if the -300 had happened a decade ago the 757 might still be in full production now.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
Leezyjet
Topic Author
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RE: Why Is The 757 So Tall?

Fri Jul 25, 2003 9:09 am

Thanks for the answers guys, so basically just planning ahead for a stretch model and tail clearance.

They did leave it very late though to introduce the 753 and 764 though. They should have brought those out years ago. Thing might have been much differant in the A vs B war then.

I've dispatched 757's hundreds of time in the past (maybe even you once Rick  Big grin ) but never thought anything about it until the other day when I was looking at one.

Skipper,

With the B707 wouldn't it have been possible for them to redesign the gear too whilst trying to design the stretch model ?, but I guess by that time they had something much bigger planned !!.

 Smile
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
 
dw747400
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RE: Why Is The 757 So Tall?

Fri Jul 25, 2003 11:26 am

Leezy... Boeing determined the cost of a full gear re design, likely with new wings, engine, etc... would be to great to justify the program. The lack of stretch capability on the 707 accelerated Boeing's efforts to build a larger airplane, leading to the 747. Of course, something bigger would of probably come along anyway, but this made the matter of building a big plane more urgent.
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