Mason
Topic Author
Posts: 636
Joined: Sat Jun 12, 1999 12:01 am

Why Are 737s So Low To The Ground?

Thu Jan 06, 2000 6:37 am

I have noticed that the B737 sits VERY low to the ground. After seeing several takeoffs from SEA (including some AA 737-800s), it seems like they will scrape. The A320 (similar size) sits much higher; it looks better, too. So why do 737s have such short landing gear supports and are scrapes common?
 
OPNLguy
Posts: 11191
Joined: Tue Jun 15, 1999 11:29 am

RE: Why Are 737s So Low To The Ground?

Thu Jan 06, 2000 7:45 am

As usual, the general answer is, "it depends"

The first generation 737s (-100, -200) are the same height off the ground. The "Classics" (-300, -400,
-500) are a little higher. The "NextGen" versions
(-600, -700, -800, -900) are higher (slightly) still. Ok, that cover's the differences in height above the ground.

With respect to "tailstrikes" (what we call them in the biz), a major factor is fuselage length. Tailstrikes on the -100 and -200 have always been fairly rare. Out of the remaining versions, the ones with the longest fuselage are the -400/-800 (pretty much the same length) and the -900 (which is even longer. I know the -400 has a small tailskid/strikeplate, and I presume the -800 does and -900 will as well. The -300/-700 (also pretty much the same length) are as not long as the -400/-800/-900, and not as prone to tailstrikes as they are, although they do happen on occasion.

The main factor behind tailstrikes isn't so much fuselage height above ground but (1) amount of fuselage aft of the main gear, combined with (2) excessive rates of rotation during takeoff. Although more rare, tailstrikes can also occur upon landing.

Hope this makes sense...
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
timz
Posts: 6084
Joined: Fri Sep 17, 1999 7:43 am

RE: Why Are 737s So Low To The Ground?

Sat Jan 08, 2000 5:11 am

So why is it low in the first place? Well, presumably Boeing wanted the 737 (and 727?) to be able to operate with a minimum of ground equipment. A person standing on the ground can load baggage and do some (how much?) engine repair, and the built-in airstair is shorter. And of course a shorter landing gear has advantages in itself. Presumably this applies to the DC-9 as well.
 
D L X
Posts: 11638
Joined: Thu May 27, 1999 3:30 am

RE: Why Are 737s So Low To The Ground?

Sat Jan 08, 2000 5:36 am

I think one of the major reasons why the 737 is so low is that the original 737s had low bypass engines under the wing (the cigar like ones). These were not very space-consuming, so the landing gears did not have to be so long. To save weight, you make the gear as short as you can.

Then something weird happened.

The 733-735 series introduced high-bypass engines, but there wasn't enough ground clearance. So, the engineers at Boeing figured out how to sling the engines in front of the wings instead of purely under (like the 747s) to save some vertical space. The engineers at GE helped out some more by relocating engine parts that usually go on the bottom to the sides. Now, the nacelles exhibit that flattened shape on the bottom, but they are also a little fatter on the sides. This gave Boeing so much clearance, that the plane actually *lost* 5 inches in height.

But, the non-circular nacelle takes a small performance hit I believe, so when Boeing made the new 737NGs, they went back to a more circular nacelle, and extended the landing gear. The plane is now 5 feet taller than the 733. Now, part of this is from a larger tail fin to accomodate the greater thrust from the new engines, but the plane does actually sit higher now.

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