Undercarriages

[KelvinD]

Forgive me if this a daft question but:
I have often wondered what sort of considerations go into the design of an aircraft's landing gear. There seems to be such a wide difference between the various models.
Some examples have the landing gear hanging with a rearward bias, presumably resulting on the rearmost wheels touching down before the forward ones. Boeing 777 are an example of this. On the other hand, the 767 has the landing gear has a forward bias so the forward set of wheels will touch down first.
And then, there is the 747 with one set of wheels on each side hanging almost vertically!
The Boeing 737 puzzles me too in so far as they don't have enclosed wheel bays, just a wheel shaped "hole" in the belly. Doesn't this cause noise and drag, as the belly is not 100% clean?
A spot of enlightenment would be very much appreciated.
Thanks

[Krystal n chips]

There are plenty of design engineers on here who can offer you explanations as to the complexities of u/c design, but this may help in the interim

https://aviation.stackexchange.com/questions/14569/why-do-certain-aircrafts-landing-gear-tilt-up

As for the 737, the outer wheels on the main u/c have a hub cap. When the gear is retracted, around the edge of the wheel well are some very effective and robust flexible rubber seals which act as wheel bay seal. There are also fixed fairing doors attached to the outside of the leg to complete the seal.

You really have to see a 737 jacked up, with the gear retracted, to understand how this simple, but effective, design works. There's no discernible noise for the pax in the cabin either.

If there are performance related issues, again, that's one for the designers to explain.

https://www.youtube.com/watch?v=9uPBgOlhuLc

[KelvinD]

Thanks for that K&C.

[dixi188]

Embraer E170/190 also has no doors to cover the main wheels when retracted.

[galaxy flyer]

Then, there’s this Rube Goldberg contraption.

https://m.youtube.com/watch?v=uJfy-KdvTXE

[H Peacock]

Great video of the C5 gear, but why such a long delay in getting the gear up after getting airborne? Is there much of a delay between moving the gear lever and the retract sequence commencing or was this just a very late selection?

[galaxy flyer]

The retract time 28 seconds; the delay is unusual.

[tdracer]

With regard to the 737 landing gear, you need remember the 737 was originally designed to be a short range jet - basically 1000 miles or less - and relatively slow (by jet standards). Hence a little extra drag due to the exposed landing gear was considered a good trade vs. the extra cost/weight of going fully enclosed.
The 737 range has gone up with the various iterations such that it is now a trans-continental aircraft, and wing changes on the NG increased the cruise Mach a bit (~0.02 faster than the -3/4/500), but going to a fully enclosed landing gear is simply too major a redesign to justify the cost.

[RVF750]

I'd rather they moved the -100's leftover Ailerons from half way along the wing to the end first! At least when they copied the 737, Embraer fixed that bit!

[PEI_3721]

“Form follows Function”. Landing gear design depends on the role of the aircraft and the structural arrangement, e.g. high / low wing: see “BAC R-STOL Studies.pdf”
https://www.dropbox.com/s/z5vcyu4shsovyqx/BAC%20R-STOL%20Studies.pdf?dl=0
Landing gear studies pages 25-30.
The alternative design strategies led to A300 type of aircraft (1) and HS 146 (2), two radically different implementations.

Also:
http://i67.tinypic.com/21vhis.jpg

http://i68.tinypic.com/x2mgee.jpg

http://i63.tinypic.com/fk2o05.jpg

[DaveReidUK]

Landing gear design depends on the role of the aircraft and the structural arrangement, e.g. high / low wing

Though not necessarily.

Dash 8: high-wing, nacelle-mounted gear

ATR: similar mission, also high-wing, fuselage-mounted gear