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738: "things" At The Bottom Of Vertical Stabilizer  
User currently offlineF.pier From Italy, joined Aug 2000, 1523 posts, RR: 9
Posted (6 years 9 months 2 weeks 4 days 12 hours ago) and read 6385 times:

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Photo © Jiahao su



What are those 4 "objects" you can see at the botton right (4 at the bottom left) of the vertical stabilizer? Are them antennas?

Do they enter in the fuselage inflight of they always stay in that position?
Thank you.

20 replies: All unread, jump to last
 
User currently offlineZeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 1, posted (6 years 9 months 2 weeks 4 days 12 hours ago) and read 6378 times:

Quoting F.pier (Thread starter):
What are those 4 "objects" you can see at the botton right (4 at the bottom left) of the vertical stabilizer?

Vortex generators to help with airflow over the tailplane.

Quoting F.pier (Thread starter):
Do they enter in the fuselage inflight of they always stay in that position?

Stay in that position.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineMender From United Kingdom, joined Feb 2004, 240 posts, RR: 0
Reply 2, posted (6 years 9 months 2 weeks 4 days 7 hours ago) and read 6258 times:

You'll see them on every 737.

User currently offlineAAR90 From United States of America, joined Jan 2000, 3466 posts, RR: 47
Reply 3, posted (6 years 9 months 2 weeks 4 days 1 hour ago) and read 6165 times:

Aft Body Vortex Generators. So important they are not even mentioned in any of my OpMans. MEL says "none" required for flight.  Wink


*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 4, posted (6 years 9 months 2 weeks 4 days 1 hour ago) and read 6159 times:
AIRLINERS.NET CREW
DATABASE EDITOR



There are four vortex generators on each side of the rear fuselage above the horizontal stabiliser. These are to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life. It is also probable that they energise the airflow at the stagnation point at the tailcone thereby reducing drag and giving a slight performance advantage.

Only aircraft after line number 2277 (May 1992 onwards) have vortex generators fitted at production but there are many later 737-200's with and 737-300's without (see photos). The CDL says that if they are not fitted or any are missing "occasional vertical motions may be felt which appear to be light turbulence and is not to be confused with mach buffet."



Source: b737.org.uk

2H4




Intentionally Left Blank
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 5, posted (6 years 9 months 2 weeks 3 days 18 hours ago) and read 6083 times:

Quoting F.pier (Thread starter):
What are those 4 "objects" you can see at the botton right (4 at the bottom left) of the vertical stabilizer? Are them antennas

Aft body Vortex Generators

Quoting F.pier (Thread starter):
Do they enter in the fuselage inflight of they always stay in that position

Fixed.

Anyone having a history behind these,or were they installed on the 1st B731s produced.

regds
MEL



Think of the brighter side!
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 6, posted (6 years 9 months 2 weeks 3 days 14 hours ago) and read 6042 times:

Quoting HAWK21M (Reply 5):
Anyone having a history behind these,or were they installed on the 1st B731s produced.

Looks like 731 line number 2 had them...

http://www.airliners.net/open.file/1166308/L/


Looks like 732 line number 63 had them...

http://www.airliners.net/open.file/0042341/L/

One could conclude that they've been there from day-1....  Wink


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (6 years 9 months 2 weeks 3 days 2 hours ago) and read 5894 times:

Quoting HAWK21M (Reply 5):
Anyone having a history behind these,or were they installed on the 1st B731s produced.

It's really unlikely that they were designed in (purist aerodynamicists hate things like that). They look an awful lot like a fix found in flight test. Vortex generators are very easy to install, have almost no weight and drag penalty, and can fix many kinds of annoying problems.

Tom.


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 8, posted (6 years 9 months 2 weeks 2 days 18 hours ago) and read 5852 times:

Quoting OPNLguy (Reply 6):
One could conclude that they've been there from day-1....



Quoting Tdscanuck (Reply 7):
It's really unlikely that they were designed in (purist aerodynamicists hate things like that). They look an awful lot like a fix found in flight test.

I guess some Aerodynamic problem during tess warranted them.
If only Manufacturers printed a book on how & what caused them to adjust certain features on a type Aircraft in its history.something which the AMM does not tell. Smile
regds
MEL



Think of the brighter side!
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6387 posts, RR: 54
Reply 9, posted (6 years 9 months 2 weeks 2 days 4 hours ago) and read 5769 times:

The fuselage narrows at this point at a quite substantial angle. That increases the local airspeed quite a lot. (many planes have a more pointed aft fuselage than the 737, and therefore don't accelerate the air as much).

When the plane is going fast the local airflow can go supersonic, or just about sonic, or transonic.

The vortex generators creates a vortex in the boundary layer in such a way that at least the boundary layer remains subsonic. That way slight vibrations in the control surfaces are eliminated.

Without those vortex generators, then at least when making a minor sideslip (in turbulent air) some chock-waves could slam a little on the rudder and elevator.

The same vortex generators are seen on many planes on the upper surface of the wing in front of the ailerons where the serve the same purpose. More seldom they are seen on the lower surface on the horizontal tail in front of the elevator hinge.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 10, posted (6 years 9 months 2 weeks 2 days ago) and read 5735 times:

Quoting AAR90 (Reply 3):
MEL says "none" required for flight.

Since the MEL is for functional systems, the vortex generators should not be in the MEL. However, they should be found in the CDL.


User currently offlineAAR90 From United States of America, joined Jan 2000, 3466 posts, RR: 47
Reply 11, posted (6 years 9 months 2 weeks 1 day 21 hours ago) and read 5703 times:

Quoting 474218 (Reply 10):
Since the MEL is for functional systems, the vortex generators should not be in the MEL. However, they should be found in the CDL.

Sorry, since we're going all electronic manuals, the MEL and CDL are in a combined pdf file.  Wink



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (6 years 9 months 2 weeks 1 day 4 hours ago) and read 5578 times:

Quoting Prebennorholm (Reply 9):
The fuselage narrows at this point at a quite substantial angle. That increases the local airspeed quite a lot.

I must be missing something here...diverging duct in subsonic airflow should result in a local airspeed decrease, not increase.

Tom.


User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 13, posted (6 years 9 months 2 weeks 1 day 3 hours ago) and read 5569 times:

Quoting OPNLguy:
Looks like 731 line number 2 had them...

http://www.airliners.net/open.file/1...08/L/

Interestingly, the aircraft in the backgroud of that picture (D-ABEI) doesn't seem to have them;

http://www.airliners.net/open.file/0223671/M/

(Sorry for the improper linking - the proxy here is acting up and won't allow me to use the buttons above. The usual  img {/img} isn't working either.


Also, apropos of nothing, I was interested to find when doing a search for other searches of D-ABEI, pictures of a 733 as well. I had no idea that they recycled registrations that way, (I'm not a spotter), though it's obvious they would have to, now that I think about it.

[Edited 2007-10-17 18:30:49]


Todos mis dominós son totalmente pegajosos
User currently online737tdi From United States of America, joined Sep 2007, 786 posts, RR: 1
Reply 14, posted (6 years 9 months 2 weeks 13 hours ago) and read 5496 times:
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I have never worked on a 737 that didn't have them. I can tell you this, those things will tear your skin off. Funny that this topic is here today. I was on a rudder change last night and those things bit me 20 times if not more. They are incredibly pointy (engineer word) on the edges. Disconnecting the rudder pcu and the lower 3 hinges put your knees, hands, head in direct contact with them and they will draw blood. I'm no engineer but just looking at them, they look like they are designed for postive airflow across the rudder and to reduce turbulence on the same, I could be completely wrong here though.

737tdi


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 15, posted (6 years 9 months 1 week 6 days 16 hours ago) and read 5419 times:

Quoting 737tdi (Reply 14):
I was on a rudder change last night and those things bit me 20 times if not more

True.
Common like the VHF Antenna & the Inboard MLG door  Smile
regds
MEL



Think of the brighter side!
User currently online737tdi From United States of America, joined Sep 2007, 786 posts, RR: 1
Reply 16, posted (6 years 9 months 1 week 6 days 12 hours ago) and read 5390 times:
Support Airliners.net - become a First Class Member!

Hawk21M: You ain't kidding. I got 13 stitches in my noggin a couple of years ago from the gear door. Was repacking a gear and stood up in the wrong place. I have a very bad habit of locating sharp objects with my head. You would think I would learn, but these danged airplanes just keep getting in the way. That door will get you pretty regularly when changing the 1 or 4 tire. Later

737tdi


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 17, posted (6 years 9 months 1 week 6 days 10 hours ago) and read 5369 times:

Quoting 737tdi (Reply 16):
That door will get you pretty regularly when changing the 1 or 4 tire.

I know just what you are talking about.The End tends to scrape a individual well.
regds
MEL



Think of the brighter side!
User currently offlineZuluLima From United States of America, joined Jul 2007, 302 posts, RR: 3
Reply 18, posted (6 years 9 months 1 week 6 days 1 hour ago) and read 5308 times:

You'll see vortex generators in other places on the 737, the largest being the "horn" on the inboard side of the engines. Here you can see them in action (whispy vortex over the wing) along with the 4 on the rear fuselage and one on the vertical stab itself:



Here are 8 short ones on the upper wing that keep control authority for the outbord flaps:



There are also 3 small VGs on the leading edge that I think help the ailerons:




I didn't get a 'Harumph' outta that guy!
User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 19, posted (6 years 9 months 1 week 6 days ago) and read 5296 times:

Quoting Prebennorholm (Reply 9):
The fuselage narrows at this point at a quite substantial angle. That increases the local airspeed quite a lot. (many planes have a more pointed aft fuselage than the 737, and therefore don't accelerate the air as much).

When the plane is going fast the local airflow can go supersonic, or just about sonic, or transonic.

The vortex generators creates a vortex in the boundary layer in such a way that at least the boundary layer remains subsonic. That way slight vibrations in the control surfaces are eliminated.



Quoting Tdscanuck (Reply 12):
I must be missing something here...diverging duct in subsonic airflow should result in a local airspeed decrease, not increase.

I'm gonna have to agree with Tdscanuck on this one. What I suspect happens is that the tapering fuselage does indeed cause the subsonic airflow around the tail cone to slow down. From Bernoulli's law, we know that if the velocity of flowing air decreases, the static pressure increases correspondingly.

The boundary layer of this airflow is thus moving against an adverse pressure gradient, an adverse pressure gradient being one where static pressure increases as we move along a surface. The boundary layer, or layer of air adjacent to the skin which already has reduced velocity and hence energy, can eventually be slowed to a stop and even forced to reverse direction due to this adverse pressure gradient.

At this point, the boundary layer will separate from either side of the aft fuselage surface. Between the separated streamlines on either side of the fuselage, will be a large low pressure region (wake) with a highly unsteady vortex shedding characteristic. The low pressure region causes pressure drag, but of more concern is the vortex shedding and the turbulence in generates.

The vortex generators alleviate this problem by generating stream-wise vortices. These vortices force high velocity and high momentum air outside the boundary layer to mix with the sluggish air inside the boundary layer. This re-energises the boundary layer and allows it to travel much further against the adverse pressure gradient before it separates. If the boundary layer separates off a bluff body (aft fuselage) further along the surface, the size of the low pressure wake region and severity of the vortex shedding will be less.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (6 years 9 months 1 week 5 days 23 hours ago) and read 5293 times:

Quoting ZuluLima (Reply 18):
Here are 8 short ones on the upper wing that keep control authority for the outbord flaps:

I'm not sure that's what they're for...the flaps aren't really control surfaces and they're slotted, so they get most of their boundary layer boost from high-pressure air coming off the bottom of the wing. Someone stated in another thread that those VG's were there to reduce transonic drag effects, although I don't know the physics behind that.

Quoting ZuluLima (Reply 18):
There are also 3 small VGs on the leading edge that I think help the ailerons:

In cruise, they look like they're below the stagnation point so they wouldn't have any significant effect on the ailerons. They may be to help the slats during low-speed/high-angle flight.

Quoting JetMech (Reply 19):
What I suspect happens is that the tapering fuselage does indeed cause the subsonic airflow around the tail cone to slow down.

I think JetMech's explanation is dead on, and consistent with the aero guys I've talked to at Boeing. The 737 has a very strong fuselage taper at the back end (much more than any other Boeing or Airbus) that can lead to separation.

Tom.


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