combatshadow
Topic Author
Posts: 163
Joined: Mon Dec 10, 2007 7:11 am

737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 7:04 am

I have always thought that there has to be a slight disadvantage to the 737 having it's main gear exposed while stored. This has to cause some sort of aerodynamic loss, right? The A320 has to have an advantage here.

I'm not an Aero Eng, but I would think that covering the gear might also help increase efficiency for the type. It may be small, but in the long run, wouldn't it benefit just like adding winglets?

I understand that it would not be an easy fix, and would probably be a nightmare. I'm not asking about that, so much as how much of a difference does it make?

Anyone with some subject-matter experience?
Bob
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 3:13 pm

The drag situation is not as bad as you think, the hubcap in the outside wheel and gap seals make the 737 belly fairly aerodynamic when the wheels are tucked away. It also makes the 737 gear system simpler and saves weight.   
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 3:20 pm

Quoting combatshadow (Thread starter):
I have always thought that there has to be a slight disadvantage to the 737 having it's main gear exposed while stored

If there were, it would have gear doors. There's no design constraint that prevents gear doors, they made a conscious decision not to put them there. The aircraft "on either side" in the development process (727 and 747) both have covered main gear.

Quoting combatshadow (Thread starter):
This has to cause some sort of aerodynamic loss, right?

Sort of. Taken by itself, the drag coefficient of the uncovered gear is higher than the covered gear. But the point of an airplane is to have the best overall performance, not a gloriously streamlined landing gear at the cost of other things. As KELPkid said, the drag isn't as bad as you might think. More importantly, the extra induced drag to carry the weight of the gear doors plus related actuation mechanisms plus the cost of the extra parts and maintenance of the extra parts is bigger then the small drag penalty of uncovered gear. The overall payoff is net positive.

Quoting combatshadow (Thread starter):
I'm not an Aero Eng, but I would think that covering the gear might also help increase efficiency for the type.

Nope. If it did, you'd have seen gear doors show up at least at the 737NG or 737MAX redesigns.

Quoting combatshadow (Thread starter):
It may be small, but in the long run, wouldn't it benefit just like adding winglets?

It is possible that you could get a small net payoff in some sub-factor, like drag, but the overall improvement isn't big enough (if it's even positive) to justify the increased costs.

Tom.
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 4:01 pm

Quoting combatshadow (Thread starter):

I'm not an Aero Eng, but I would think that covering the gear might also help increase efficiency for the type. It may be small, but in the long run, wouldn't it benefit just like adding winglets?

Like Tdscanuck said, it would actually be detrimental to fuel burn and overall efficiency. The reduction in drag is not enough to overcome the weight of heavier doors and actuation. The 737 gets by with slave linkage drive doors and does not have separate hydraulic actuators. The existing two part door is mechanically driven on the gear. Increasing the size of it to cover the wheel and tire would require a more complex design since the doors would not have adequate ground and tire clearance if they were mounted outboard of the gear like they are now.

Airplanes are all about different trade studies. The 737/DC9/A320 is the size point when full size doors start to be beneficial. Many smaller planes have exposed tires. The weight trades on the smaller planes makes it challenging to design doors that improve efficiency. The weight and complex designs required since there is so little ground clearance under the center fuselage means that a drag penalty might be worth it.

By now you can trust the Boeing engineers. If it would improve efficiency, they would have already done it.
If you have never designed an airplane part before, let the real designers do the work!
 
ferpe
Posts: 2651
Joined: Tue Nov 30, 2010 7:44 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 5:06 pm

It might also be said that this discontinuity of the surface is on the underside where you have relatively seen a higher static pressure area. What that means is that any disturbance from the wheel and seals does not escalate into major boundary layer separation and vortices downstream of the area (= major drag) like if would have done if the disturbance was on the side or over-side of the fuselage (all IMO and experience, you care less about the frames underside then overside for this reason).

This disturbance has to be well ahead of the up-sweep of the aft body however because there your boundary layer is getting twitchy.

[Edited 2011-11-30 09:07:18]
Non French in France
 
redflyer
Posts: 3881
Joined: Thu Feb 24, 2005 3:30 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 7:08 pm

Quoting tdscanuck (Reply 2):
If it did, you'd have seen gear doors show up at least at the 737NG or 737MAX redesigns.

Interesting comment regarding the MAX. I know Boeing is extending the nose gear by 6", and they are changing the stern to render it more aerodynamic. But did they tweak the MLG set-up at all to accommodate the new engines? I thought to myself if the 737 will ever get gear doors this would be it, since I'm assuming the wing-body fairing will be modified to some extent.
My other home is a Piper Cherokee 180C
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 7:13 pm

Just out of curiosity: are hubcaps ever "lost" in operation? Is the 737 dispatchable without a hubcap? If so, what is the fuel burn penalty?
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
cmf
Posts: 3120
Joined: Sun Jun 12, 2011 11:22 pm

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 7:35 pm

Quoting tdscanuck (Reply 2):
There's no design constraint that prevents gear doors, they made a conscious decision not to put them there.

The decision was done a long time ago so many of the parameters considered at that time has changed, flight length being a major. It is possible that if they made the decision based on today's parameters they would have done it differently.

What is clear is that the difference hasn't been enough to justify the change.
Don’t repeat earlier generations mistakes. Learn history for a better future.
 
Goldenshield
Posts: 5005
Joined: Sun Jan 14, 2001 3:45 pm

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 7:38 pm

Quoting KELPkid (Reply 6):
Just out of curiosity: are hubcaps ever "lost" in operation? Is the 737 dispatchable without a hubcap? If so, what is the fuel burn penalty?

More than likely there's a CDL for it, and it's probably be as bad as gear door seal on the CRJ (relatively, of course.)
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 
yeelep
Posts: 724
Joined: Mon Apr 04, 2011 7:53 pm

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 9:37 pm

Quoting KELPkid (Reply 6):
Just out of curiosity: are hubcaps ever "lost" in operation?

Never heard of it happening, they are held on by three bolts that are safety wired.

Quoting KELPkid (Reply 6):
Is the 737 dispatchable without a hubcap?

Don't have access to manuals right now, but I'm going to say no. The hubcaps drive the wheel speed transducers for the antiskid, so I doubt they are able to be CDL'd.
 
Goldenshield
Posts: 5005
Joined: Sun Jan 14, 2001 3:45 pm

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 10:05 pm

Quoting yeelep (Reply 9):
The hubcaps drive the wheel speed transducers for the antiskid, so I doubt they are able to be CDL'd.

If this is the case, then Anti-skid can be deferred, and thus, it'd be an MEL, not a CDL, although there could very well be a drag/fuel penalty applied.
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 
yeelep
Posts: 724
Joined: Mon Apr 04, 2011 7:53 pm

RE: 737 Main Gear Storage Aerodynamics

Wed Nov 30, 2011 10:50 pm

Well I decided to take a look at the 737NG CDL/MEL. I was wrong, the outboard hubcap can be CDL'd by 32-41-2B, provided MEL 32-2B is applied to the outboard antiskid channel.

The performance limitations per hubcap:
Enroute climb: 115lbs.
ETOPS enroute climb: 630lbs.
Fuel burn increase: 0.288%

[Edited 2011-11-30 14:50:57]
 
737tdi
Posts: 1116
Joined: Sun Sep 02, 2007 10:05 am

RE: 737 Main Gear Storage Aerodynamics

Thu Dec 01, 2011 3:00 am

Quoting yeelep (Reply 11):
Well I decided to take a look at the 737NG CDL/MEL. I was wrong, the outboard hubcap can be CDL'd by 32-41-2B, provided MEL 32-2B is applied to the outboard antiskid channel.



On a different but similar note. Any of the three doors on the strut/wing can also be CDL'd.

737tdi

Who is online

Users browsing this forum: No registered users and 15 guests