f.pier
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738: "things" At The Bottom Of Vertical Stabilizer

Sun Oct 14, 2007 4:02 pm

Look at this picture


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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.
 
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zeke
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Sun Oct 14, 2007 4:11 pm

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.
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Mender
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Sun Oct 14, 2007 8:55 pm

You'll see them on every 737.
 
AAR90
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Mon Oct 15, 2007 2:54 am

Aft Body Vortex Generators. So important they are not even mentioned in any of my OpMans. MEL says "none" required for flight.  Wink
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2H4
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Mon Oct 15, 2007 3:14 am



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

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Intentionally Left Blank
 
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HAWK21M
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Mon Oct 15, 2007 9:50 am

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
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OPNLguy
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Mon Oct 15, 2007 1:47 pm

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
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tdscanuck
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Tue Oct 16, 2007 2:05 am

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.
 
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HAWK21M
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Tue Oct 16, 2007 9:41 am

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
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prebennorholm
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Wed Oct 17, 2007 12:01 am

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.
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474218
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Wed Oct 17, 2007 3:55 am

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.
 
AAR90
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Wed Oct 17, 2007 6:46 am

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
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tdscanuck
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Thu Oct 18, 2007 12:29 am

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.
 
BAE146QT
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Thu Oct 18, 2007 1:27 am

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]
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737tdi
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Thu Oct 18, 2007 3:16 pm

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
 
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HAWK21M
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Fri Oct 19, 2007 12:29 pm

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
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737tdi
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Fri Oct 19, 2007 4:01 pm

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
 
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HAWK21M
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Fri Oct 19, 2007 5:45 pm

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
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zululima
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Sat Oct 20, 2007 3:09 am

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!
 
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jetmech
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Sat Oct 20, 2007 4:22 am

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
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tdscanuck
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RE: 738: "things" At The Bottom Of Vertical Stabilizer

Sat Oct 20, 2007 4:33 am

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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