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IFlyVeryLittle
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C-17 gear retraction side doors

Thu Aug 19, 2021 2:11 pm

Suddenly, the gear retraction mechanics of C-17s is tragically in the news. Beyond the obvious, I was struck by the vertical fuselage doors that seem to open on the side to allow for the gear mechanism to operate without hitting, then close for the remainder of the gear's movement. Any idea how that kind of engineering solution was arrived at? Seems like something that wouldn't have been a designer/engineer's first choice.
 
GalaxyFlyer
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Re: C-17 gear retraction side doors

Thu Aug 19, 2021 4:39 pm

It is a requirement of mounting the gear in sponsons to allow for a flat floor. Rather typical Douglas design—half-arsed, pragmatic.
 
GalaxyFlyer
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Re: C-17 gear retraction side doors

Thu Aug 19, 2021 6:50 pm

And, us C-5 guys would laugh at the Buddha for things like the actuator doors.
 
IFlyVeryLittle
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 12:41 pm

I just looked again, and the doors have doors.
 
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N14AZ
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 1:31 pm

Thank you for starting this thread. Since I saw this video of the teenagers falling of the aircraft I was wondering: how did they manage to stay that long on the gear doors?!?

Here is a picture of the „fuselage doors“

Image
Source: https://commons.m.wikimedia.org/wiki/Fi ... detail.jpg
 
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Moose135
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 3:49 pm

IFlyVeryLittle wrote:
Any idea how that kind of engineering solution was arrived at? Seems like something that wouldn't have been a designer/engineer's first choice.


Aircraft design is a series of compromises.
 
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Aesma
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 8:32 pm

The landing gear has to be as sturdy as practical, so I guess you don't want to make the mechanism too complicated and failure prone (it is already quite complicated...). So with that mechanism in mind, to avoid the doors you would just make bulges on the side. Either they didn't like the look of it, of the performance penalty was worse than with the doors.
 
Sooner787
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 8:40 pm

The C-17 that took off with all the teens hanging onto it,
was it able to retract it's gear after takeoff?
Heard gruesome reports of finding a body in the main wheel well
as well as various other remains stuck in the gear .... :(
 
GalaxyFlyer
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Re: C-17 gear retraction side doors

Fri Aug 20, 2021 9:02 pm

I pretty much guarantee it went up, 3000 psi hydraulics is difficult to stop with flesh.
 
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LyleLanley
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 4:38 am

Considering the design requirements required of it, I think the C-17 MLG was designed to operate pretty ingeniously. Any more sponson and the acft drag/fuel-burn raises precipitously, but any less and weird mechanical difficulties result. There are a lot of faults to be bestowed onto McD designers/engineers, but I think the dudes in charge of the landing gear did their jobs right. It's easy for C-5 dudes who have all the room in the world to complain, but does it work great, barring some refugees? Yes. Incidentally, the C-141 does it's MLG sequence in a similar manner.

https://youtu.be/sDgM4x66LTM
 
mxaxai
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 11:55 am

Aesma wrote:
So with that mechanism in mind, to avoid the doors you would just make bulges on the side. Either they didn't like the look of it, of the performance penalty was worse than with the doors.

Engineering a door in an unpressurized section is trivial. The increased maintenance can be dealt with later and the weight penalty is small.

Adding bulges, on the other hand, requires simulations and tests by the aero department. This becomes expensive quickly and can require changes in other areas. The door is a far less risky, cheaper and faster solution to problems late in the design process.
 
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bikerthai
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 12:11 pm

mxaxai wrote:
The door is a far less risky, cheaper and faster solution to problems late in the design process.


As an engineer, I would disagree with this statement.

A bulge is always easier to design and the aero drag can be estimated if acceptable. A door is way more expensive than a bulge.

The door was selected probably because the aero drag was not acceptable or there was a second reason the door was preferred, like maintenance access.

by
 
mxaxai
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 1:09 pm

bikerthai wrote:
A bulge is always easier to design and the aero drag can be estimated if acceptable. A door is way more expensive than a bulge.

It's not just drag. The bulge would be right in front of the paratrooper doors. Turbulence created by a bulge in this area could prevent safe drops. The A400M had aero issues that required a redesign of the deflector doors and restricted its paratrooping capacity for some time.

But you're right, there might be secondary reasons to choose a door. And if the engineers had known that the gear would be this large, they almost certainly would've chosen a larger sponson.
 
Noray
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 3:21 pm

mxaxai wrote:
And if the engineers had known that the gear would be this large, they almost certainly would've chosen a larger sponson.

This assumes that the gear requires more space than planned initially. Do we know? Maybe the sponsons were larger in an earlier design stage and these doors were invented as a measure to reduce their size? Seeing it that way, it looks like a clever idea to me.
 
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bikerthai
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Re: C-17 gear retraction side doors

Sat Aug 21, 2021 3:55 pm

Noray wrote:
Maybe the sponsons were larger in an earlier design stage and these doors were invented as a measure to reduce their size?


The aerodynamics is fixed early on. The space allocation would have been pre-determined. Perhaps the requirement for the landing gear increased and they needed more robust actuators, thus clap trap doors had to be designed.

bt
 
Willjet
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Re: C-17 gear retraction side doors

Mon Aug 23, 2021 1:16 am

My all time fav mil transport aircraft!
 
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N14AZ
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Re: C-17 gear retraction side doors

Wed Aug 25, 2021 1:57 pm

Found this video that shows the gear retraction:

https://m.youtube.com/watch?v=oTz21kfPczo
 
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william
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Re: C-17 gear retraction side doors

Mon Aug 30, 2021 1:45 pm

Here is another video of it, does seem like a high maintenance item. Alot of work to keep a flat floor.

https://www.youtube.com/watch?v=sDgM4x66LTM

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