trav110
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What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 05, 2016 10:41 am

What I'm referring to is the large protruding part in this picture. It looks like a big vortex generator, but why is it so far forward?


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Photo © Yu Cai

 
thepinkmachine
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 05, 2016 10:54 am

I remember reading it is in fact a vortex generator installed to prevent sideways oscillations of the fuselage at high AoA
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zanl188
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 05, 2016 10:56 am

IIRC it provides smooth airflow into the engine at high angles of attack.


I stand corrected...

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040090579.pdf

"The use of strakes on transport aircraft is not
without precedent. For example, the McDonnell
Douglas DC-9 aircraft, when stretched from the Series -30
to the Series -50 configuration, incorporated small
fixed strakes near the nose, reportedly to offset the
change in directional stability caused by the increased
fuselage length ahead of the center of gravity. These
strakes were subsequently also placed on the DC-9-80,
or MD-80, aircraft. However, the flow physics responsible for
the directional stability changes due to the strakes has not been
well studied, nor was the study to optimize strake
size and location well documented."

[Edited 2016-04-05 04:21:52]

[Edited 2016-04-05 04:25:11]
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 05, 2016 2:00 pm

Hi:

Navigation antennas.

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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 05, 2016 2:04 pm

Quoting Apprentice (Reply 3):

Hi:

Navigation antennas.

Rgds

Nope.
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UA444
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Wed Apr 06, 2016 2:09 pm

Notably the 717 removed them.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Wed Apr 06, 2016 11:06 pm

Quoting UA444 (Reply 5):
Notably the 717 removed them.

Because like all DC-9's prior to the -50, it isn't long enough to need strakes.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 08, 2016 8:47 pm

Quoting thepinkmachine (Reply 1):
I remember reading it is in fact a vortex generator installed to prevent sideways oscillations of the fuselage at high AoA

Interesting. Many industrial chimneys have fences wrapped in a spiral fashion around them with the purpose of steadying the vortex shedding in their lee to reduce oscillations.



https://en.wikipedia.org/wiki/Vortex_shedding

http://i.stack.imgur.com/BrjLl.jpg

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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Sat Apr 09, 2016 1:26 am

The more you stretch a design, the less effective the original control surfaces are in controlling the aircraft and the less stable it becomes. Strakes like the ones on the MD-80 family help add surface area and stability in the axis that may be lacking.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Sat Apr 09, 2016 10:12 pm

Quoting jetmech (Reply 7):
Interesting. Many industrial chimneys have fences wrapped in a spiral fashion around them with the purpose of steadying the vortex shedding in their lee to reduce oscillations.

Thank you for that! I never knew that and found it truly interesting!  
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Sun Apr 10, 2016 2:23 pm

Quoting Apprentice (Reply 3):
Hi:

Navigation antennas.

Rgds

Maybe you thought they were talking about something else like the vertical antennas, they are talking about the long horizontal strakes directly below the cockpit windows and below and forward the main front door.

Quoting UA444 (Reply 5):
Notably the 717 removed them

As mentioned, DC-9-30 aircraft never had the strakes. So its not that they removed them, that aircraft never had them. 50 series and longer DC9/MD80 have the strakes.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon Apr 11, 2016 8:12 pm

Interesting thread.

Aside from the less-obvious benefits (directional stability, cleaner airflow to the engines ((?)), etc), I just assumed the strakes were there to add a very small amount of lift to the forward portion of the fuselage during flight… sort of like the ultimate minimalist application of canards.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon Apr 11, 2016 9:18 pm

Quoting jetmech (Reply 7):

Interesting. Many industrial chimneys have fences wrapped in a spiral fashion around them with the purpose of steadying the vortex shedding in their lee to reduce oscillations.

In those cases it's because a cylindrical object in a wind will tend to form alternating vortices in a pattern called a "vortex street." (Google it). Because with a chimney it is impossible to predict the direction from which the wind will come at any given time, the spiral design helps to break up the cross-section from all directions.

In the forward fuselage of the MD-90 we know exactly which direction the wind will come from and so the fences can be made smaller and straight.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon Apr 11, 2016 11:01 pm

Quoting PGNCS (Reply 9):

No worries   .

Quoting DocLightning (Reply 12):
Because with a chimney it is impossible to predict the direction from which the wind will come at any given time, the spiral design helps to break up the cross-section from all directions.

Precisely. It's interesting that chimneys don't have freely pivoting streamlined cowlings to reduce flow separation in the lee of the structure. This would significantly reduce oscillations in addition to the drag force due to wind.

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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 12, 2016 2:59 am

Quoting jetmech (Reply 13):
Precisely. It's interesting that chimneys don't have freely pivoting streamlined cowlings to reduce flow separation in the lee of the structure. This would significantly reduce oscillations in addition to the drag force due to wind.

I've never seen one of those on an industrial chimney, but I have seen what you describe on a home's chimney. It has a large vane on top and, if you squint and use your imagination a bit, it resembles a knight's helmet.
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DocLightning
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 12, 2016 3:36 am

Quoting jetmech (Reply 13):

Precisely. It's interesting that chimneys don't have freely pivoting streamlined cowlings to reduce flow separation in the lee of the structure. This would significantly reduce oscillations in addition to the drag force due to wind.

Too mechanically complex and possibly self-defeating You'd have to keep the swiveling mechanism lubricated and it might start to oscillate with the vortices. A fixed structure is much simpler.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 12, 2016 5:30 am

Quoting pilotpip (Reply 8):
The more you stretch a design, the less effective the original control surfaces are in controlling the aircraft and the less stable it becomes. Strakes like the ones on the MD-80 family help add surface area and stability in the axis that may be lacking.

Are you sure ? What about the much larger tail on the 747SP versus the 747-100/200s ? You are right in that these strakes helped with aerodynamic stability.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Tue Apr 12, 2016 6:30 pm

Quoting TSS (Reply 14):
has a large vane on top and, if you squint and use your imagination a bit, it resembles a knight's helmet.

One of these things?

http://www.ventfansxpress.com/images/products/sku722.jpg

In this case, it appears the device would most probably produce a vortex pair to enhance the draft of the chimney.

Quoting DocLightning (Reply 15):
You'd have to keep the swiveling mechanism lubricated and it might start to oscillate with the vortices

The presence of a streamlined cowling in the lee of the chimney would act to reduce the intensity of vortex shedding. That being said, you'd probably still need some sort of damping system.

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TSS
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Thu Apr 14, 2016 9:12 pm

Quoting jetmech (Reply 17):

Quoting TSS (Reply 14):
has a large vane on top and, if you squint and use your imagination a bit, it resembles a knight's helmet.

One of these things?

http://www.ventfansxpress.com/images/products/sku722.jpg

In this case, it appears the device would most probably produce a vortex pair to enhance the draft of the chimney.

That looks very much like it from what I can tell, yes. The one I've seen is mounted on a chimney that exits at the apex of a fairly steep (12/12 or thereabouts pitch) roof atop an old farm house, so I've never gotten a really close look at it. It was already on the house when it last changed owners in 1971, and it vented a furnace that burned coal.

Quoting DocLightning (Reply 15):
Quoting jetmech (Reply 13):

Precisely. It's interesting that chimneys don't have freely pivoting streamlined cowlings to reduce flow separation in the lee of the structure. This would significantly reduce oscillations in addition to the drag force due to wind.

Too mechanically complex and possibly self-defeating You'd have to keep the swiveling mechanism lubricated and it might start to oscillate with the vortices. A fixed structure is much simpler.

Doc does bring up an interesting point- As far as I know the swiveling mechanism on the one I'm familiar with has not been lubricated or serviced in any way in 45 years (Who'd want to scale a metal roof with a 45 degree pitch carrying a can of 3-in-1 oil? Any volunteers?), yet it still moves freely in the wind.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 5:47 am

Quoting trav110 (Thread starter):



Well, so sorry but imho and out of memory, nothing to do w/ vortice generator or aerodynamic. Just a Radio-Navigation DIRECTIONAL ANTENNA.

Rgds
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 6:06 am

Quoting Apprentice (Reply 19):
Well, so sorry but imho and out of memory, nothing to do w/ vortice generator or aerodynamic. Just a Radio-Navigation DIRECTIONAL ANTENNA.

No one else seems to think so. And if so, why do only the long ones have it, and why don't other airliners have similar ones?
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 6:17 am

Stantionblue:

I'm not thinking. As I wrote, it was OOMemory. I have the Mx rating for this a/c and used to certify it back to service. Still, I may be wrong, of course, since I have no Mx Manuals with me to check.
And yes, other a/c do have Radio Navigation- Directional Antennas. In other parts (regulary in botton middle fuselage) and other forms.

Rgds
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 6:24 am

My friend GOOGLE:

https://www.google.com/imgres?imgurl=http%3A%2F%2Fwww.navfltsm.addr.com%2Fn56v-n-cropped-borderc.jpg&imgrefurl=http%3A%2F%2Fwww.navfltsm.addr.com%2Fndb-nav-history.htm&docid=xhwBdlv7Xv3DbM&tbnid=t6RxUYaV_PG9kM%3A&w=245&h=319&hl=en-us&client=safari&ved=0ahUKEwjWn9Pz_4_MAhVBTSYKHZj8CBEQMwgeKAIwAg&iact=mrc&uact=8

https://www.google.com/imgres?imgurl=http%3A%2F%2Fflighttraining.aopa.org%2Fimages%2Fstudents%2Fpresolo%2F0208ANTENNA02L.jpg&imgrefurl=http%3A%2F%2Fflighttraining.aopa.org%2Fstudents%2Fpresolo%2Fspecial%2Fantenna.html&docid=EbiDB4xkCcAWGM&tbnid=YHi-2SrUfEOJNM%3A&w=300&h=201&hl=en-us&client=safari&ved=0ahUKEwjWn9Pz_4_MAhVBTSYKHZj8CBEQMwg8KBYwFg&iact=mrc&uact=8
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 7:03 am

Quoting Apprentice (Reply 21):
'm not thinking. As I wrote, it was OOMemory. I have the Mx rating for this a/c and used to certify it back to service. Still, I may be wrong, of course, since I have no Mx Manuals with me to check.
And yes, other a/c do have Radio Navigation- Directional Antennas. In other parts (regulary in botton middle fuselage) and other forms.

The pictures you linked show ADF antennas, one with a fairing and one without. No way those strakes are ADF antennas. I could maybe buy VOR antennas, but their relative angle is typically much wider.

I'd be happy to be proven wrong but those Google pics don't show anything like what is mounted on the MD-80.

[Edited 2016-04-15 00:12:10]
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 8:57 am

Sorry, I give up. This, for me, is not personnel so if needed I agree: Vortex Generatos

Only they are an special type of them:
Vortex Generators, in all type of planes, are a thin (1/8") plate of Al,
MD 80s are around 3/8", they are not metal but made from composite, from radiotranslucent composite
And the more shocking difference is that they have an electrical connector to which a plug from an a/c system is attached during replacement. My best guest is that this plug is for a/c to be able to count how many vortex/sec are been made
  

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Dalmd88
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 9:09 am

Quoting Apprentice (Reply 19):
Well, so sorry but imho and out of memory, nothing to do w/ vortice generator or aerodynamic. Just a Radio-Navigation DIRECTIONAL ANTENNA.

Worked MD88 overhaul for years. I was Autoland certified on the MD88. You are wrong. These are not antennas of any kind. They are aerodynamic strakes.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 9:19 am

Quoting Dalmd88 (Reply 25):

Dalmd, hi:

As I said at the begining, I may be wrong. And since I don't have regular access to md80 amm, I can't probe nothing.

So for me, for now, is finished..

Thanks./ Regards
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 10:00 am

And last, but not least, plese note 2ea "vortex generators" on this Cubana's Ilushin IL-62M, at each side in the midlle / botton of fuselage.
The russian called this generators: radio-navigation antennas:


https://www.google.com/imgres?imgurl=https%3A%2F%2Fimg.planespotters.net%2Fphoto%2F276000%2Foriginal%2Fcu-t1283-cubana-ilyushin-il-62m_PlanespottersNet_276722.jpg&imgrefurl=https%3A%2F%2Fwww.planespotters.net%2Fphoto%2F276722%2Fcu-t1283-cubana-ilyushin-il-62m&docid=TElahetQKwDXnM&tbnid=2131i688q9QrPM%3A&w=1000&h=679&hl=en-us&client=safari&ved=0ahUKEwiY0e_SqZDMAhUGFx4KHVRSDEoQMwgqKA8wDw&iact=mrc&uact=8

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“An4; IL18; IL6; Tu5; D10; MD11; MD83; B32; B34: B37; B744; B748; B752; B763; B772; B773; B77W; A320; A332; A333; A342; A343.
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Fri Apr 15, 2016 3:11 pm

Quoting Dalmd88 (Reply 25):
Quoting Apprentice (Reply 19):Well, so sorry but imho and out of memory, nothing to do w/ vortice generator or aerodynamic. Just a Radio-Navigation DIRECTIONAL ANTENNA.Worked MD88 overhaul for years. I was Autoland certified on the MD88. You are wrong. These are not antennas of any kind. They are aerodynamic strakes.

Thank you Dalmd88, you are correct. They are aerodynamic strakes, they are pneumatically heated when the TAI system is on (wing mode), and are metal (incidentally, they are electrically heated on the MD-90 if anyone is interested). Let me be clear: They are NOT antennas. Period.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Sat Apr 16, 2016 5:26 am

Quoting PGNCS (Reply 28):

PGNCS, Loud and Clear. Thanks for clarification.

To keep alive and old practice that served me well, if I got an ipc, I will take a look.

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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Wed May 04, 2016 5:16 am

Quoting CALTECH (Reply 16):
The more you stretch a design, the less effective the original control surfaces are in controlling the aircraft and the less stable it becomes.
Quoting CALTECH (Reply 16):
Are you sure ? What about the much larger tail on the 747SP versus the 747-100/200s?

It depends on the direction of the stretch. Moving the vertical fin further aft from the CG, will increase lateral stability. Apply a side load to the fin while flying forward, the fin slides into the airstream on the opposite side and gets pushed back towards center. The further the distance from the CG, the larger the moment arm, therefore the smaller fin required to provide the righting moment. Actually more important for an engine-out situation where the asymmetric thrust will yaw the aircraft.

Increasing the length of the fuselage ahead of the CG will reduce lateral stability. When a side load is applied to the forward fuselage, or the aircraft yaws, there is no righting moment. The air load pushing the fuselage to the side increases as the yaw angle increases.

When stretching an aircraft with wing mounted engines, you roughly stretch equivalent length forward and aft of the wing to maintain CG and stability, though you can reduce the fin size as the plane gets longer, it usually not worth the effort. However if you reduce the length of the plane, you will probably have to increase the size of the fin, unless the shorter length was accounted for in the original design. The 747SP was a result of having to inccrease the fin size, while the oversized fin on the 737NG series was due to the needs of the -600 short body (the -800 could have had a 3 ft shorter fin).

With the rear engine DC-9 series, CG concerns required that stretches forward of the wing longer than stretches aft of the wing because the engine weight aft of the CG offset a lot of fuselage tube weight Fwd of the CG. The increased fuselage length forward of the CG, without a significant increase in vertical fin moment arm, decreased the lateral stability, which had to be offset by either increasing the size of the fin or changing the aerodynamics over the fuselage (another option would be a fly-by-wire fight control system, but that would be an extreme fix). The strakes provide that required airflow modification.
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Thu May 05, 2016 1:27 am

Here is the explanation for the strakes, straight from the authoritative source:
AIAA paper 85-4067 "Aerodynamic Bugs: Can CFD Spray Them Away?"
R. S. Shevell, Stanford University
(formerly chief of aero at Douglas Aircraft)

DC-9-50 and MD-80 Fliselage Strakes

"Another example of complex aerodynamic interactions was found in the DC-9-50 and MD-80 directional stability at high angles of attack". Vortices shed from the nose of the extended fuselages of these airplanes in sideslips at high angles of attack degraded directional stability, Figure 17. The problem was caused by interaction of these vortices and the vertical tail. The solution was found to be fuselage strakes, Figure 18, developed and optimized in wind tunnel testing. The strakes, when placed in a low forward position on the fuselage, generate vortices that alter the flow past the vertical tail so that good directional stability is maintained over the required ranges of angles of attack, sideslip, and flap deflection. The original problem involved interaction between the fuselage boundary layer and the high angle of attack flow. These phenomena plus the interaction with the vertical tail could not be handled analytically. The wind tunnel remains essential for such complex flow interactions."
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Sun May 08, 2016 7:25 pm

Quoting CALTECH (Reply 16):
Are you sure ? What about the much larger tail on the 747SP versus the 747-100/200s ? You are right in that these strakes helped with aerodynamic stability.

That was a shrink that needed a bigger fin because it had a shorter moment arm.

Quoting FrmrKSEngr (Reply 30):
With the rear engine DC-9 series, CG concerns required that stretches forward of the wing longer than stretches aft of the wing because the engine weight aft of the CG offset a lot of fuselage tube weight Fwd of the CG. The increased fuselage length forward of the CG, without a significant increase in vertical fin moment arm, decreased the lateral stability, which had to be offset by either increasing the size of the fin or changing the aerodynamics over the fuselage (another option would be a fly-by-wire fight control system, but that would be an extreme fix). The strakes provide that required airflow modification.

Thanks, this is a good summary.

Quoting dlednicer (Reply 31):
Vortices shed from the nose of the extended fuselages of these airplanes in sideslips at high angles of attack degraded directional stability, Figure 17.

Baically, these strakes employ the same concept as nacelle strakes on engines, they create predictable vortices at high-alpha. In the case of the longer DC-9 variants, it helps the vetical tail, with nacelle strakes it helps the overwing flow. Another piece of trivia on the 9s, the MD-87 had the stretched upper fin for OEI directional stability because it was shorter and had the 200 series engines. This was carried over to the MD-90 and 95 (717) as well for the same reason. More info here: http://airlinercafe.com/page.php?id=396
 
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon May 09, 2016 5:33 am

Quoting FrmrKSEngr (Reply 30):
The 747SP was a result of having to inccrease the fin size, while the oversized fin on the 737NG series was due to the needs of the -600 short body (the -800 could have had a 3 ft shorter fin).

  

Quoting CALTECH (Reply 16):
Quoting pilotpip (Reply 8):The more you stretch a design, the less effective the original control surfaces are in controlling the aircraft and the less stable it becomes. Strakes like the ones on the MD-80 family help add surface area and stability in the axis that may be lacking.
Are you sure ? What about the much larger tail on the 747SP versus the 747-100/200s ?
Quoting LH707330 (Reply 32):
That was a shrink that needed a bigger fin because it had a shorter moment arm.

Which is why I questioned his post. If the further away a control surface after a stretch is made, he said it becomes less effective. The example of a 747-100/-200 then would need the bigger fin according to the claim made, versus a 747SP, which is not the case.
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Florianopolis
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon May 09, 2016 1:04 pm

Quoting CALTECH (Reply 33):
Which is why I questioned his post. If the further away a control surface after a stretch is made, he said it becomes less effective.

I read that original post to say that when the fuselage is stretched so much in front of the CG, the control surfaces are relatively less effective at controlling the stretched jet than the un-stretched jet. In other words, because the moment arm of the destabilizing fuselage ahead of the CG grew so much more than the moment arm behind the CG for the vertical tail, the vertical tail was relatively less effective than before the stretch. (This is basically what KSengineer said in #30)

If you read it that way, everyone can be right.

Quoting dlednicer (Reply 31):
The strakes, when placed in a low forward position on the fuselage, generate vortices that alter the flow past the vertical tail so that good directional stability is maintained over the required ranges of angles of attack, sideslip, and flap deflection.

I like this answer much better than the vortex-shedding-chimney answer.

[Edited 2016-05-09 06:10:03]
 
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CALTECH
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Wed May 18, 2016 4:14 pm

Quoting Florianopolis (Reply 34):
I read that original post to say that when the fuselage is stretched so much in front of the CG,
Quoting pilotpip (Reply 8):
The more you stretch a design, the less effective the original control surfaces are in controlling the aircraft and the less stable it becomes.

Not what was posted. Nothing about a stretch in front of the CG.

Quoting Florianopolis (Reply 34):
If you read it that way, everyone can be right.

I would say we were all wrong, including my use of the 747SP vertical stab. That is not a control surface, just a stabilizer. Bad example to use. Primary flight control surfaces are the ailerons, elevators/stabilator and rudder. Secondary flight control surfaces are LE devices, TE flaps, Speedbrakes/Spoilers, Balance, Anti-Balance, Servo and Trim Tabs and the movable Horizontal Stabilizer
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RE: What Purpose Does This Frontal Fin Serve On MD-88?

Mon May 23, 2016 3:43 pm

Quoting CALTECH (Reply 16):
Are you sure ? What about the much larger tail on the 747SP versus the 747-100/200s ? You are right in that these strakes helped with aerodynamic stability.
Quoting FrmrKSEngr (Reply 30):

When stretching an aircraft with wing mounted engines, you roughly stretch equivalent length forward and aft of the wing to maintain CG and stability, though you can reduce the fin size as the plane gets longer, it usually not worth the effort. However if you reduce the length of the plane, you will probably have to increase the size of the fin, unless the shorter length was accounted for in the original design. The 747SP was a result of having to inccrease the fin size, while the oversized fin on the 737NG series was due to the needs of the -600 short body (the -800 could have had a 3 ft shorter fin).

The design condition that requires a taller or larger area Fin is engine out criteria. You need enough fin and rudder authority to keep the airplane on the runway if you lose an engine on take off. The 747SP was so short that the fin had to be increased to meet this requirement because the length of the lever arm was short and the wasn't enough lateral stability at low speed if you lost an outboard engine and needed to complete the takeoff. it also helps with dutch roll tendencies in cruise.

[Edited 2016-05-23 08:44:59]

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