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DIA
Topic Author
Posts: 3053
Joined: Sun Jan 28, 2001 2:24 pm

Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:06 am

Yes, those rear wings. Not much is ever said about these little guys.

I was just thinking. . .aircraft always seem to change their wingspan. . .shortening and lengthening them all the time. Why not the rear horizontal stabilizers? I think the only time I noticed a change, is when the MD-11 made them smaller than the DC-10. Odd. . .for a larger a/c of the same product. I can't seem to recall any other aircrafts that have changed these (shortened or lengthened). . .can you? And why, or why not? I don't need a technical answer. . .just plain, simple english is fine.

Did the DC-9 -10 through -50/MD-80/MD-90 series have different ones throughout their family?
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dl021
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Joined: Fri May 21, 2004 12:04 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:34 am

This question might get a quicker and more accurate answer in the tech ops forum.. Hope you get an answer.
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FinnWings
Posts: 633
Joined: Fri Oct 24, 2003 6:03 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:36 am

Think about it like this way... Wings produce lift for the aircraft. Horizontal stabilizer produce negative lift for the aircraft. So the overall lift of the aircraft is wing lift - horizontal stabilizer lift. So basically it is like a wing, but upside down. Moving the elevators change the amount of lift which is produced from the back... When you pull the stick, elevators go up and horizontal stabilizer has negative angle of attack to relative airflow... now it creates more lift, but because the horizontal stabilizer is "upside down -wing" the lift is produced beneath the stabilizer thus pitching your nose up.

Basically, smaller horizontal stabilizer and vertical stabilizers are always more economical. Same is with wingspan... more lift will mean always more drag as well. In cruise you would like to have as little drag as possible, so bigger wingspan, rudders or stabilizers will only cause more drag. You need bigger span and control surfaces only when operating at low speeds like take-off or landing.

Therefore, MD-11 was very well designed seriously... That aircraft has smaller horizontal stabilizer, because it has fuel tanks at rear. Fuel is used to balance the aircraft during the flight and they will pump fuel from the back to main tanks during flight to get better balance. Such a big stabilizers aren't needed as fuel takes part of their job.

B737-600 is good example of quite a poor aerodynamic design... Short aircraft with huge rudder and quite a big wing: More drag, increased fuel consuption.

I'm not very good to explain these aerodynamic issues, but hope this helps a bit...

Best Regards,
FinnWings
 
DIA
Topic Author
Posts: 3053
Joined: Sun Jan 28, 2001 2:24 pm

Startin' To Get It!

Thu Jul 15, 2004 5:40 am

"Therefore, MD-11 was very well designed seriously... That aircraft has smaller horizontal stabilizer, because it has fuel tanks at rear. Fuel is used to balance the aircraft during the flight and they will pump fuel from the back to main tanks during flight to get better balance. Such a big stabilizers aren't needed as fuel takes part of their job."

Never thought about it this way. Interesting.
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FinnWings
Posts: 633
Joined: Fri Oct 24, 2003 6:03 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:50 am

Yes, DIA....

Even MD-11 has some problems in some areas it was very innovative and more advanced design than more "modern" aircrafts.

I could have done simplier answer to your question, but after been awake 22hrs continously, these aerodynamic issues aren't so easy anymore..  Yawn

Someone with better answer, feel free to add more details or correct me if wrong... I'm going to sleep now!  Sleepy

Best Regards,
FinnWings
 
spacecadet
Posts: 3582
Joined: Thu Sep 20, 2001 3:36 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:52 am

Therefore, MD-11 was very well designed seriously... That aircraft has smaller horizontal stabilizer, because it has fuel tanks at rear. Fuel is used to balance the aircraft during the flight and they will pump fuel from the back to main tanks during flight to get better balance. Such a big stabilizers aren't needed as fuel takes part of their job.

That's highly debatable..

There have been several accidents and several other near-accidents that were attributed to simple loss of control by pilots on takeoff, landing, or even at cruise altitude in MD-11's. Pilots are always blamed in these events, but if you ask the pilots themselves (and one of the major TV networks did a special on MD-11's a while back where they did ask MD-11 pilots), many of them will say the MD-11 is just not an easy plane to control and is not as stable as most other passenger aircraft due to the smaller stabilizer (the DC-10 did not have this problem). Even minute control adjustments can create large pitch oscillations.

Now, it's probably true that under normal circumstances a pilot should still be able to control the plane in any situation. The problem as I see it is in designing a plane that may be within acceptable tolerances but is still not as stable as it could be, for purely economic reasons. Obviously, every plane is a balance between economics and other factors, but safety should always come first.

Not saying the MD-11 is an unsafe plane or that I wouldn't fly in one (I have, and would again). Just saying that I don't think this particular aspect of its design was well-thought out overall; it was a purely economic decision that negatively affected the plane's safety in however small a way, and has been a contributing factor in several accidents.
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!
 
planemaker
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Joined: Mon Aug 25, 2003 12:53 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 5:59 am

One thing that FinnWings perhaps didn't clearly point out is that typically the longer the moment arm (the further the control surface is from the aircraft centre of gravity) the smaller the control surface will need to be. Think of it like a lever, the longer the lever, the less force that needs to be applied. The MD-11 was a stretch on the DC-10 so in addition to the benefits of the fuel trim tanks, the H-Stab was further distance from the centre of gravity. He gave the 736's V-Stab as a similar example.
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henpol747
Posts: 563
Joined: Mon Jun 25, 2001 5:53 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 6:13 am

I see this one is going straight to Tech/ops!!
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avioniker
Posts: 1100
Joined: Tue Jan 01, 2002 5:38 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Thu Jul 15, 2004 6:34 am

Now that it's here in Tech/Ops
Take a look at the 737 Jurassic/Classic/NG
The Horiz got bigger with each.
As for the drag on the 600, you might want to check the data. One of the reasons for no deice system in the rear of the 737's is that the leading edges are so slick it isn't required.

In short, the horizontal stabilizer is looked at and redesigned with every aircraft factory modification of every type. At least here in the States. Hard to tell what the EADS people are doing...
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
md80fanatic
Posts: 2365
Joined: Tue Apr 13, 2004 11:29 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Fri Jul 16, 2004 3:24 am

The MD-11 was designed with "relaxed stability" in mind. That is no defect...it is by design. The DC-9/MD-80 lines had the same relaxed stability. So the pitch axis is a little more sensitive, this is not always a bad thing. The MD-11 put the pilots MORE into the aircraft they are flying, not disconnecting the pilot from the action. There are as many pilots who like the handling as those who dislike it.
 
N766UA
Posts: 8386
Joined: Thu Jul 29, 1999 3:50 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Fri Jul 16, 2004 11:53 am

Boeing redesigned the stabilizer with the NG series 737s.
 
AJ
Posts: 2304
Joined: Wed Nov 03, 1999 3:54 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Fri Jul 16, 2004 7:00 pm

Another change was to the Boeing 747 when the Special Performance model was produced. 6' extensions were added to the vertical and horizontal stabilisers due to the shorter moment arm on the 'SP'.
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Fri Jul 16, 2004 8:05 pm

The MD-11's HS is smaller as its arm is shorter. The negative lift required is less than on the DC-10 (which has a huge HS). Safety is not the issue as the airplane is built to and certified under Part 25 of the FAR's.

As for the DC-9/MD-80 question... The Stab is the same, the elevators are not. On the MD-80's, Douglas added another tab (Anti-Float tab, P/N 5930497-501,-502...) to it.
Molon Labe - Proud member of SMASH
 
9V-SPJ
Posts: 671
Joined: Thu Dec 21, 2000 1:51 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Sun Jul 18, 2004 12:19 pm

Most of it has to do with the torque produced by the horizontal stabilizers whose main components are the elevators. The longer the moment arm from the elevators to the aircraft, the less force you need to actually make the aircraft go up and down, as it is compensated by the longer moment arm. Its like a door, if you push on the edge furthest from the hinge, you require more force if you push it from, say the middle of the door. Same goes for aircraft, the md-11 is longer than the DC10, so the you don't need such a big force exerted by the elevators to make the aircraft change pitch.
This is what i think the size of the elevators has to do - in physics!

9V-SPJ
 
XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Sun Jul 18, 2004 12:41 pm

Not to be nitpicky..but Miamiair- dont you mean the MD-11's arm is longer?
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Boeing747_600
Posts: 612
Joined: Tue Oct 19, 1999 4:01 am

Question About Stabilizer Trim Settings In Large J

Thu Aug 05, 2004 4:37 am

As a hobby, I'm trying to develop a generic mathematical model of stable flight configurations using the longitudinal stability and control power derivatives in Jan Roskam's book.

For the Boeing 747 for example, my computations suggest that for the following parameters:

Gross Cruise Weight 300,000 Kg
Center of Gravity 25% of MAC
Pressure Altitude 30,000 ft

I actually need positive (upward) tail lift (negative stabilizer trim) to maintain level flight at Mach numbers greater than 0.72

Regardless of the numbers I compute, my question is whether it is even possible to actually create positive tail lift with any stab trim setting in a Boeing 747-400 or any large jetliner for that matter.

The reason I bring this up is because the Stab trim Display on the Center Console in the 747-400 only goes from 0 to 15 units (0 being the "APL Node Down") limit.

I'm not sure as to whether 0 corresponds to a limiting Stab Trim setting of 0 (Zero Tail Lift) or whether there's some sort of calibration involved that actually allows for stab trim settings that produce upward tail lift.

-Sharat

P.S: I'll be glad to share details of my rudimentary work with anyone that's interested.
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: Rear Horizontal Stabilizers: I Don't Get It.

Wed Aug 18, 2004 10:07 pm

XFSUgimpLB41X:

Actually it is shorter. When the DC-10 was stretched to the MD-11 fuselage, the CG moved aft. Shorter distance=shorter arm. Also the MD-11 has LSAS longitudinal Stability Augmentation Sys...Like a yaw damper in the pitch axis.
Molon Labe - Proud member of SMASH
 
L-188
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

RE: Rear Horizontal Stabilizers: I Don't Get It.

Fri Aug 20, 2004 6:37 pm

Yeah but the tail moved aft also.

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