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Question Re: Rear Horizontal Stabilizers  
User currently offlineJAAlbert From United States of America, joined Jan 2006, 1572 posts, RR: 1
Posted (8 years 5 months 2 weeks 1 day 5 hours ago) and read 4030 times:

Alright, I admit it, I'm green behind the ears and I have a desk job that has nothing to do with aircraft. Pardon my extreme ignorance in advance.

(Now that the disclosure is out of the way) so here is my question:

All airliners have rear horizontal stabilizers (isn't that what the wing structures are called that sit horizontally on either side of the tail fin -- unless of course sitting on top of the tail fin?) Anyways, those things back there is what I'm talking about. I've read that on larger widebodies, those stabilizers can approach the length of the wings of a small airliner, like a 737. My questions are: do these horizontal stabilizers also provide lift to the aircraft? Or are they solely for manuevering the plane while in flight? Would there be any benefit to an aircraft if the rear stabilizers did provide lift, or would that cause problems (having two sets of wings essentially on different parts of the plane)?

Any engineers or experts (okay so everyone else can comment too) available to shed a bit of light on these interesting features?

34 replies: All unread, showing first 25:
 
User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
Reply 1, posted (8 years 5 months 2 weeks 1 day 4 hours ago) and read 4026 times:

Generally do provide lift, however in the opposite direction to the main wing.


We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineJAAlbert From United States of America, joined Jan 2006, 1572 posts, RR: 1
Reply 2, posted (8 years 5 months 2 weeks 1 day 4 hours ago) and read 4017 times:

Zeke! Thanks so much. Okay, so my next question is: what direction do the main wings provide lift (other than "up" I suppose)? And why do the stabilizers provide lift in the opposite direction (to stabilize the aircraft?)

User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
Reply 3, posted (8 years 5 months 2 weeks 1 day 3 hours ago) and read 3997 times:

My understanding is (but more knowledgeable people can confirm or correct) that the horizontal stabilisers are not airfoil-shaped and do not provide lift. They are there to provide pitch (nose up-or-down) control. In some airliners they also contain extra fuel.


Make the most of the available light ... a lesson of photography that applies to life
User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
Reply 4, posted (8 years 5 months 2 weeks 1 day 1 hour ago) and read 3929 times:

OK, so let's see if I've got this right now:

  • At different angles of attack the centre of pressure or centre of lift of the main wings moves aft of the aircraft's centre of gravity. This produces a downward pitching movement.


  • The horizontal stabiliser produces downforce at the tail which also varies according to the aircraft's angle of attack to counteract this pitching effect.


  • This does not mean that the horizontal stabiliser "balances out" the lift of the wings (this statement struck me as odd and got me searching). If it did, presumably the aircraft would plough a furrow in the runway with its rear fuselage and it would not get off the ground.

    Am I correct?



    Make the most of the available light ... a lesson of photography that applies to life
    User currently offlineWukka From United States of America, joined Jan 2005, 1017 posts, RR: 16
    Reply 5, posted (8 years 5 months 2 weeks 1 day ago) and read 3908 times:

    Quoting Symphonik (Reply 5):
    What the hell?

    What a complete douchebag.

    We'll see. I wasn't even addressing JAAlbert.



    We can agree to disagree.
    User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
    Reply 6, posted (8 years 5 months 2 weeks 1 day ago) and read 3903 times:

    Quoting Kukkudrill (Reply 6):
    OK, so let's see if I've got this right now:


    At different angles of attack the centre of pressure or centre of lift of the main wings moves aft of the aircraft's centre of gravity. This produces a downward pitching movement.


    The horizontal stabiliser produces downforce at the tail which also varies according to the aircraft's angle of attack to counteract this pitching effect.

    Thats part of the situation, the other part if the thrust/drag "couple".

    Talking about a conventional aircraft ...

    The centre of drag, and the thrust line from the power plants also are offset vertically from the centre of gravity (CG), having forces in opposite directions offset is called a "couple". Resolving the four main forces around the CG, Lift, Weight, Thrust, Drag, we need some "downforce" on the tailplane so the aircraft can remain in equilibrium (the sum of forces and moments about the CG is zero).

    The downforce can be provided by "lift" being generated over the horizontal stabilizer. On some aircraft the majority of this downforce can be provided by placing a fuel tank in the horizontal stabilizer, this reduces how hard the main wing needs to produce lift, and therefore reduces induced drag in the cruise. The 744(pax)/330/340/380 all can have fuel tanks in the horizontal stabilizers.

    The fuel in the horizontal stabilizer tank provides a mass over a moment arm (CG to the horizontal stabilizer) that would be equal to, or close to the amount of "lift" the tailplane would need to generate. The horizontal stabilizer angle of incidence is then modified to reduce to amount of "lift" being produced so the aircraft remains in equilibrium.

    Quoting Kukkudrill (Reply 6):
    This does not mean that the horizontal stabiliser "balances out" the lift of the wings (this statement struck me as odd and got me searching). If it did, presumably the aircraft would plough a furrow in the runway with its rear fuselage and it would not get off the ground.

    Am I correct?

    No it does not balance out the lift of the wings. On takeoff the pilots set the angle of the horizontal stabilizer that is appropriate for the takeoff mass and CG position. This is part of the performance & weight and balance calculations that are performed prior to flight.

    If the angle of the horizontal stabilizer was set incorrectly, it would be possible for the aircraft to get airborne prematurely, or as you suggested, not get off the ground at all.

    The horizontal stabilizer is there to balance out (you will hear pilots say trim) the forces in all steady phases of flight, theses are for “long term changes”. Its design can be a little different to smaller aircraft, in that it generally is "all moving" on larger aircraft. On the rear of the horizontal stabilizer on a conventional aircraft you will also find the elevators which are used “short term changes” in the amount of downforce being produced by the tailplane, these “short term changes” are for the pilot to pitch the aircraft about the CG.

    It’s the short term change in downforce provided by the elevator which actually will rotate the aircraft on takeoff in response to the pilots rearward pressure on the yoke/stick.



    We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
    User currently offlineWukka From United States of America, joined Jan 2005, 1017 posts, RR: 16
    Reply 7, posted (8 years 5 months 2 weeks 1 day ago) and read 3890 times:

    Quoting Kukkudrill (Reply 3):
    They are there to provide pitch (nose up-or-down) control.

    Sorry, Kukkudrill, it was this statement that completely threw me. Generally, on larger craft, the horizontal stabilizer is designed as an almost "upside down" wing.

    And you are correct in that it doesn't completely balance out and negate the effect of the lift on the main wing. It complements it by creating stability through countering forces. Somewhat like the tail on a kite, for want of a better analogy.



    We can agree to disagree.
    User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
    Reply 8, posted (8 years 5 months 2 weeks 23 hours ago) and read 3857 times:

    Quoting Wukka (Reply 7):
    Sorry, Kukkudrill, it was this statement that completely threw me.

    An assumption based on what I knew, that was invalidated by what I didn't know that I didn't know.  Smile

    Quoting Zeke (Reply 6):
    The centre of drag, and the thrust line from the power plants also are offset vertically from the centre of gravity (CG)

    Zeke, many thanks for the info. Presumably, low-winged airliners with engines slung under the wings experience an upward pitching tendency, which would reduce the tailplane downforce needed?



    Make the most of the available light ... a lesson of photography that applies to life
    User currently offlineHiFi From Brazil, joined Apr 2005, 192 posts, RR: 0
    Reply 9, posted (8 years 5 months 2 weeks 16 hours ago) and read 3787 times:

    Quoting Kukkudrill (Reply 8):
    Zeke, many thanks for the info. Presumably, low-winged airliners with engines slung under the wings experience an upward pitching tendency, which would reduce the tailplane downforce needed?

    Yes, the engines placed under the wing usually have a slight pitch-up effect due to the thrust line being lower than the a/c's CG. But it's rather negligible when compared to lift-generated moment.

    For an a/c to be stable, you need to place your centre of pressure aft of your CG. The centre of pressure is the point where lift is produced (lift is actually produced all over the wing, of course, but to concentrate lift on the centre of pressure is equivalent).
    The relative position between CG and wing's centre of pressure is very important.. it is determined by mass distribution, wing placement along the fuselage, wing shape and so on, and it will determine the flying quality and handling of the a/c. If you place the centre of pressure in front of the CG, you have an unstable design. This can be used in order to reach extreme manoeuverability, if combined with artificial stability algorithms, for example (this is quite common in modern military designs).

    In commercial aviation, you better put centre of pressure aft of CG. Big grin
    What does this mean? You have a force (lift) and an arm (distance between CG and wing's centre of pressure), resulting in a moment/torque. In this case, a pitch-down moment.

    In order to maintain equilibrium, you need "sum of forces = 0" and "sum of moments = 0". Assuming straight and levelled flight, on the vertical axis, you have lift + weight = 0. Now you need to work on the moments.. you have a pitch-down moment due to lift.. you counter it with a pitch-up moment generated by the horizontal stabilizer. The arm in this case is much bigger (distance between CG and the stabilizer's centre of pressure) so you don't need such a big surface as the wing.

    Now by adjusting the angle of incidence of the stabilizer, you change the force produced by it, then changing the moment, and you're able to reach new positions of equilibrium. You're trimming the aircraft.

    A lot of other variables are involved... thrust, drag (the counter-moment produced by the stabilizer is in fact a compensation drag), CG and centres of pressure do not keep their position during a flight etc. But it all works out based on the same principles..  Wink



    no commercial potential
    User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
    Reply 10, posted (8 years 5 months 2 weeks 11 hours ago) and read 3742 times:

    Quoting HiFi (Reply 9):
    For an a/c to be stable, you need to place your centre of pressure aft of your CG.

    This is the key, because my next question was going to be: why not put the centre of pressure bang on the CG? Or, if that's not possible because the two keep moving all over the place with different angles of attack, fuel load and what not, can't you put the centre of pressure forward of the CG and counteract the pitch-up with a right-side-up airfoil on the stabiliser to produce upforce rather than downforce? That way, presumably, you still have stability and the tailplane would work with rather than against the wings, offering an efficiency gain in that you have a net addition to rather than deduction from lift.

    Now aerodynamicists are pretty clever people, so I'm sure they've thought of this already, which means I'm missing something. The question is ... what?



    Make the most of the available light ... a lesson of photography that applies to life
    User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
    Reply 11, posted (8 years 5 months 2 weeks 11 hours ago) and read 3720 times:

    Quoting Kukkudrill (Reply 10):
    This is the key, because my next question was going to be: why not put the centre of pressure bang on the CG? Or, if that's not possible because the two keep moving all over the place with different angles of attack, fuel load and what not, can't you put the centre of pressure forward of the CG and counteract the pitch-up with a right-side-up airfoil on the stabiliser to produce upforce rather than downforce?

    I think the problem is that the plane would be too stable. Eventually you want to pitch  Wink



    "There are no stupid questions, but there are a lot of inquisitive idiots."
    User currently offline777236ER From , joined Dec 1969, posts, RR:
    Reply 12, posted (8 years 5 months 2 weeks 11 hours ago) and read 3720 times:

    Quoting Kukkudrill (Reply 10):
    Now aerodynamicists are pretty clever people, so I'm sure they've thought of this already, which means I'm missing something. The question is ... what?

    Imagine a gust hitting the aircraft and the aircraft being instantly pitched up. If the horizontal stabiliser produces lift, both the wing and horizontal stabiliser will generate more lift, due to the increased AoA. If the aircraft was trimmed before the gust, the increases AoA will mean the wing will produce proportionally more lift than the tail. Given that the centre of pressure of the wing is ahead of the centre of gravity, the aircraft will tend to pitch up further, ie. negative stability.

    Now imagine a gust hitting an aircraft where the centre of gravity is ahead of the centre of pressure. The aircraft pitches up instantaneously, the wing produces more lift. But as the centre of pressure of the wing is behind the centre of gravity, the aircraft restores itself to level flight. ie. positive stability.


    User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
    Reply 13, posted (8 years 5 months 2 weeks 11 hours ago) and read 3716 times:

    Quoting 777236ER (Reply 12):
    Quoting Kukkudrill (Reply 10):
    Now aerodynamicists are pretty clever people, so I'm sure they've thought of this already, which means I'm missing something. The question is ... what?

    Imagine a gust hitting the aircraft and the aircraft being instantly pitched up. If the horizontal stabiliser produces lift, both the wing and horizontal stabiliser will generate more lift, due to the increased AoA. If the aircraft was trimmed before the gust, the increases AoA will mean the wing will produce proportionally more lift than the tail. Given that the centre of pressure of the wing is ahead of the centre of gravity, the aircraft will tend to pitch up further, ie. negative stability.

    Then again, I was totally off base and 777236ER got it  Wink



    "There are no stupid questions, but there are a lot of inquisitive idiots."
    User currently offlineAirA380 From Bangladesh, joined Mar 2006, 179 posts, RR: 0
    Reply 14, posted (8 years 5 months 2 weeks 10 hours ago) and read 3708 times:

    some aircraft have engine at the rear such as MD90/80....is to stabilize or for other reasons?


    I'm flying without wings!!!!!!!!
    User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
    Reply 15, posted (8 years 5 months 2 weeks 10 hours ago) and read 3703 times:

    Quoting AirA380 (Reply 14):
    some aircraft have engine at the rear such as MD90/80....is to stabilize or for other reasons?

    - Cleaner wing design, although this is less of a problem nowadays.
    - Ground clearance. Tail engineds can have shorter gear, easing fueling, maintenance, boarding with shorter airstairs.
    - Less noise in front cabin (more in rear).



    "There are no stupid questions, but there are a lot of inquisitive idiots."
    User currently offlineAirA380 From Bangladesh, joined Mar 2006, 179 posts, RR: 0
    Reply 16, posted (8 years 5 months 2 weeks 10 hours ago) and read 3693 times:

    Quoting Starlionblue (Reply 15):
    Ground clearance. Tail engineds can have shorter gear, easing fueling, maintenance, boarding with shorter airstairs.
    - Less noise in front cabin (more in rear).

    so why dont they have that on A380/747/777



    I'm flying without wings!!!!!!!!
    User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
    Reply 17, posted (8 years 5 months 2 weeks 10 hours ago) and read 3690 times:

    Quoting AirA380 (Reply 16):
    Quoting Starlionblue (Reply 15):
    Ground clearance. Tail engineds can have shorter gear, easing fueling, maintenance, boarding with shorter airstairs.
    - Less noise in front cabin (more in rear).

    so why dont they have that on A380/747/777

    Aaah, but I didn't list all the drawbacks of rear engine mounting  Wink
    - Complex plumbing for the fuel system.
    - Need to strengthen the rear fuselage (= more weight).
    - Wing mounted engines counter wing bending moment and allow a lighter wing.
    - Large turbofans are too large to mount on the tail anyway.
    - On large aircraft, tail mounting makes engine maintenance harder due to distance from the ground.



    "There are no stupid questions, but there are a lot of inquisitive idiots."
    User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
    Reply 18, posted (8 years 5 months 2 weeks 6 hours ago) and read 3663 times:

    Quoting Kukkudrill (Reply 8):
    Zeke, many thanks for the info. Presumably, low-winged airliners with engines slung under the wings experience an upward pitching tendency, which would reduce the tailplane downforce needed?

    If all you did were to add thrust, the aircraft would pitch up and reduce speed, the aircraft would come to rest in a new equilibrium for that thrust setting, and trim condition.

    If you wanted to remain level after applying thrust, you would need forward pressure on the control column/stick to trim for that condition you would effectively be reducing the downforce as you suggested.

    Quoting Kukkudrill (Reply 10):
    Now aerodynamicists are pretty clever people, so I'm sure they've thought of this already, which means I'm missing something. The question is ... what?

    Its part of the A380 FBW system.

    Quoting AirA380 (Reply 16):
    so why dont they have that on A380/747/777

    The main reason is engines on the wing, like fuel in the wing counter the lift being generated resulting in a lighter wing structure.

    Another reason is for mass and balance, on a 747 or 380, it would be very tail heavy.

    And another reason ..... http://www.orizzle.com/htm/v/023.htm



    We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
    User currently offline777236ER From , joined Dec 1969, posts, RR:
    Reply 19, posted (8 years 5 months 1 week 6 days 21 hours ago) and read 3630 times:

    Quoting Zeke (Reply 18):

    Its part of the A380 FBW system.

    Wait, you were replying Kukkudrill who was talking about the centre of lift being ahead of the centre of gravity. Are you trying to say the A380 is statically unstable?


    User currently offlineAirA380 From Bangladesh, joined Mar 2006, 179 posts, RR: 0
    Reply 20, posted (8 years 5 months 1 week 6 days 19 hours ago) and read 3614 times:

    shouldn't centre of gravity and centre of lift be at same point....I'm a civil engineer so don't qoute me on that... Wink


    I'm flying without wings!!!!!!!!
    User currently offlineKukkudrill From Malta, joined Dec 2004, 1123 posts, RR: 4
    Reply 21, posted (8 years 5 months 1 week 6 days 17 hours ago) and read 3596 times:

    Thanks for the info guys!

    Quoting AirA380 (Reply 20):
    shouldn't centre of gravity and centre of lift be at same point

    That's what I thought too, but the basic problem seems to be that both can shift around quite a bit in flight.



    Make the most of the available light ... a lesson of photography that applies to life
    User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
    Reply 22, posted (8 years 5 months 1 week 6 days 16 hours ago) and read 3588 times:

    To answer the original question, the horizontal stabilizer (and the vertical stabilizer) are stabilizers, not lifting surfaces. While they my or may, not provide a small amount of lift, that is not why they are installed. They are there to stabilize the aircraft. The horizontal stabilizer in the pitch mode and the vertical stabilizer in the yaw mode.

    User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
    Reply 23, posted (8 years 5 months 1 week 6 days 13 hours ago) and read 3572 times:

    Quoting AirA380 (Reply 20):
    shouldn't centre of gravity and centre of lift be at same point....I'm a civil engineer so don't qoute me on that...

    If they were statically so, the plane wouldn't be able to pitch, since pitching essentially involves moving the center of lift.

    The CoG also moves around quite a bit. Just think of the trolleys. They don't weight a lot but with a long enoug arm...

    As I see it:

    A "stable" aircraft, that is one which will return to a stable condition without control inputs, has the CoG ahead of the center of lift. This is the case for ALL commercial aircraft. Some, like the 340, the 380, Concorde, MD-11 have a slightly less stability achieved by moving the CoG closer to (but still ahead of) the center of lift. The closer the CoG to the center of lift, the more efficient it is (since you don't have to counteract the moment arm of the CoG with negative lift from the stabilizer). But the closer the CoG to the center of lift, the less stable it is, meaning it can take a lot of oscillation for it to stabilize without inputs from the controls.

    An "unstable" or "relaxed stability" aircraft has the CoG behind the center of lift. This is done to make the aircraft more maneuvrable. Less relaxed aircraft can turn faster since they are moving with the CoG instead of against it. This is only possible with a lot of computer support since it is impossible for a human to react fast enough to the constant spontaneous pitch changes. Without constant and exact computerized control input an unstable aircraft very rapidly (on the order of a couple of seconds) goes out of control and perhaps even breaks up. This kind of system has only been implemented on fourth and fifth generation fighter aircraft such as F-16, Gripen, Rafale, Typhoon, Raptor.



    "There are no stupid questions, but there are a lot of inquisitive idiots."
    User currently offlineAirA380 From Bangladesh, joined Mar 2006, 179 posts, RR: 0
    Reply 24, posted (8 years 5 months 1 week 6 days 12 hours ago) and read 3567 times:

    is centre of gravity always on middle of the plane...is there any relation between placement of engine on wing (ignore rear mounted engines) and centre of gravity...as engine are rather heavy


    I'm flying without wings!!!!!!!!
    25 Starlionblue : As has been explained, CoG tends to be more a bit forward of the wings. Of course engines have an effect. That's why tail mounted designs have the wi
    26 FredT : The CoG is typically at about 25-30% of the mean aerodynamic cord of the wing. Which is a good reason to have trim. Think of an aircraft as a dart. I
    27 Yikes! : I don't post a lot anymore but I will wade into this one a little bit. Nearly every horizontal surface on a modern jet aircraft is designed to either
    28 Post contains images HiFi : Put that in front of the a/c and you got a canard. But the centre of pressure/lift is still aft of the CG. The shifting is not the only issue. Theore
    29 JAAlbert : Gosh guys, I had no idea my question would generate such discussion! Thanks for all the info -- most of which I fear flew over my head (hahahaha. ahem
    30 Kukkudrill : Who's right? Also, what is longitudinal or lateral lift, and why would they be needed? (I assume positive or negative means up or down.)
    31 Post contains images HiFi : I guess I wasn't very clear on this one! "longitudinal" or "lateral" lift is not part of any nomenclature or convention.. I just wanted to emphasyze
    32 Kukkudrill : OK, thanks! The message you were about to post is too short blah blah blah ...
    33 Mknies : The best way to look at the traditional horizontal stabilizer is a teeter-totter. The center of gravity is one side of the TT, the center of pressure
    34 411A : Many so-called experts here, most without any real aerodynamic detailed knowledge. The wing. In EVERY case, the airfoil shape used (civil aircraft) is
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