JAAlbert
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Question Re: Rear Horizontal Stabilizers

Fri Mar 10, 2006 2:50 pm

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?
 
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zeke
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RE: Question Re: Rear Horizontal Stabilizers

Fri Mar 10, 2006 2:57 pm

Generally do provide lift, however in the opposite direction to the main wing.
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JAAlbert
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RE: Question Re: Rear Horizontal Stabilizers

Fri Mar 10, 2006 3:08 pm

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?)
 
Kukkudrill
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RE: Question Re: Rear Horizontal Stabilizers

Fri Mar 10, 2006 3:56 pm

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.
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RE: Question Re: Rear Horizontal Stabilizers

Fri Mar 10, 2006 6:01 pm

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?
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    wukka
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    RE: Question Re: Rear Horizontal Stabilizers

    Fri Mar 10, 2006 6:58 pm

    Quoting Symphonik (Reply 5):
    What the hell?

    What a complete douchebag.

    We'll see. I wasn't even addressing JAAlbert.
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    zeke
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    RE: Question Re: Rear Horizontal Stabilizers

    Fri Mar 10, 2006 7:08 pm

    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.
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    RE: Question Re: Rear Horizontal Stabilizers

    Fri Mar 10, 2006 7:28 pm

    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.
     
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    RE: Question Re: Rear Horizontal Stabilizers

    Fri Mar 10, 2006 8:49 pm

    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?
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    HiFi
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 3:27 am

    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
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 8:00 am

    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?
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 8:27 am

    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
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 8:29 am

    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.
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 8:30 am

    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
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    airA380
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 8:55 am

    some aircraft have engine at the rear such as MD90/80....is to stabilize or for other reasons?
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 9:06 am

    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).
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    airA380
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 9:31 am

    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
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 9:46 am

    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.
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    zeke
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 12:53 pm

    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
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    RE: Question Re: Rear Horizontal Stabilizers

    Sat Mar 11, 2006 10:33 pm

    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?
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    airA380
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 12:36 am

    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
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    Kukkudrill
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 2:33 am

    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.
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    474218
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 3:20 am

    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.
     
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 6:04 am

    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.
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    airA380
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 6:55 am

    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
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 7:01 am

    Quoting AirA380 (Reply 24):
    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

    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 wings farther back than wing mounteds. Also note how the stretch MD-8x->MD-90 was all in front of the wing due to heavier engines.
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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 6:28 pm

    Quoting AirA380 (Reply 24):
    is centre of gravity always on middle of the plane

    The CoG is typically at about 25-30% of the mean aerodynamic cord of the wing.

    Quoting Kukkudrill (Reply 21):
    That's what I thought too, but the basic problem seems to be that both can shift around quite a bit in flight.

    Which is a good reason to have trim.

    Think of an aircraft as a dart. It has most of the lifting surfaces aft of the center of gravity, in order to point itself into the airflow. If it didn't, it would tumble and reverse to fly backwards.

    Now, an aircraft does something a dart does not. It generates lift to counter its own weight. To do this, it travels with an angle of attack. Think of it as a dart travelling through the air on a level trajectory, with the tip pointed slightly upwards.

    The only way to achieve this with a dart designed to align itself with the airflow is to push down at the rear end of the dart, or to push up at the tip. The former corresponds to the configuration of most aircraft, while the latter is akin to canard configurations.

    Greatly simplified, but that's the gist of it.

    The stabilizers are lifting surfaces and do generate lift. Otherwise, they wouldn't stabilize the aircraft at all. They do not, typically, generate lift in the direction opposed to the effect of gravity though.

    You essentially use trim to set the aircraft up for one angle of attack. It will then seek this angle of attack if disturbed. As the lift will only equal the weight at one airspeed at a given angle of attack (simplifying by assuming relatively small pitch angles), this essentially means that you trim for one airspeed which the aircraft will then seek. If this means climbing, descending or level flight depends on if the thrust exceeds the drag or not.

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    RE: Question Re: Rear Horizontal Stabilizers

    Sun Mar 12, 2006 8:31 pm

    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 provide lift or reduce drag. All lifting surfaces create one or more forms of drag.

    A jet airliner gets its maximum range when it flies at it's best lift/drag ratio. If the wing is at too high or too low of an angle of attack, excess drag is created with resultant less airspeed.

    When we load our 757's or 767's, we endeavour to have our cg as far aft as possible. The reason for this is buried in some of the discussion above but in general can be summed up this way: The entire stab is a lifting surface. It can generate lift up or down. When it generates lift downwards as during a takeoff manoeuvre, the wing must assume nearly all of the lifting task. However, once in flight, if the wing continues to exert most of the upward lifting force and a negative lifting force must be exercised by the stab, then the wing will be creating more total lift and hence more drag. To obtain a given airspeed, more juice will be required to offset this drag.

    If on the other hand, once in the air the stab exercises either minimal negative lift or even some positive lift (as with an aft cg) then the total lift the wing needs to provide will be reduced and so will the drag. For the same desired airspeed, less juice is now required.

    If the stab produces lift with an aft cg, it too will generate drag as a result but not nearly so much as what the wing would produce at the same all up weight and a forward cg, due to the stab being much smaller as a rule than the wings.

    All this of course applies to a conventionally designed jet airliner. T-tails, anhedral design, delta shapes all have their own characteristics.

    Hope this adds to the discussion.
     
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    RE: Question Re: Rear Horizontal Stabilizers

    Tue Mar 14, 2006 2:20 am

    Quoting Kukkudrill (Reply 10):
    with a right-side-up airfoil on the stabiliser to produce upforce rather than downforce?

    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.

    Quoting Kukkudrill (Reply 21):
    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.

    The shifting is not the only issue.

    Theoretically, yes, you could put CG and centre of lift at the same place. The aircraft would still be stable although oscilation would be virtually undamped, meaning that once disturbed (a gust for example), the aircraft would never go back to equilibrium by itself, remaining in an eternal low-frequency pitch oscilation (for those who are into control engineering, that's placing a pair of poles right on the imaginary axis).

    Now the real world. Undamped oscilation does not exist. You can get close to it, but you cannot reach it (and in this case you do not want to reach it), so:
    - CG aft of centre of lift: unstable design (pair of poles on the right side of the imaginary axis). To ease comprehension, in this case the oscilation mentioned in the paragraph above is not only undamped, it's amplified. Very high manoeuverability since you have virtually no equilibrium. You need artificial stability and control. Highly undesirable in commercial aviation.
    - CG in front of centre of lift, both close together: high manoeuverability with stable aircraft. When disturbed, the a/c is too slow to recover and might even cause nausea to occupants due to oscilation.
    - CG in front of centre of lift, both far apart: extreme stability. So stable that the crew is barely capable of controling the a/c.

    --> Conclusion: the CG must be in front of the centre of lift and the distance between them must be adjusted in order to get satisfactory stability and manoeuverability (in other words, good flying qualities), considering the migration of these points with changes of weight (fuel consumption) and speed.

    Horizontal stabilization can be performed by traditional stabilizer at the rear of with canard at the front. Both have advantages and disadvantages, as usual.

    Quoting 474218 (Reply 22):
    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.

    I agree. They are there to stabilize the aircraft...... by producing lift (or some kind of force generated by the interaction of a solid surface and high-speed air). They are usually simmetrical profiles, producing (positive, negative, longitudinal or lateral) lift only when there is an angle of incidence.

    Quoting AirA380 (Reply 24):
    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

    There is a relation between the placement of any part/equipment and Centre of Gravity... Engines, landing gear, wing, avionic boxes, galleys etc. You will usually come to a point in your project where you will restrict how much weight you can distribute in each part of the plane.

    If you're asking if there's a relation between the position of the engines along the wing and the CG, no there isn't, as long as they are equidistant from the a/c's longitudinal centre line. The position of the engines is usually determined by the influence on aerodynamics and by the loads produced on the wing. There will be an influence on a/c's lateral/directional control and stability, as moments of inercia are dependent on mass distribution along the lateral axis.


    Edits: damn this english language!  Silly

    [Edited 2006-03-13 18:31:20]

    [Edited 2006-03-13 18:41:22]
    no commercial potential
     
    JAAlbert
    Topic Author
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    RE: Question Re: Rear Horizontal Stabilizers

    Tue Mar 14, 2006 11:22 am

    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 -- pardon the joke).

    Still, it is all facsinating!
     
    Kukkudrill
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    RE: Question Re: Rear Horizontal Stabilizers

    Tue Mar 14, 2006 10:26 pm

    Quoting Wukka (Reply 7):
    Generally, on larger craft, the horizontal stabilizer is designed as an almost "upside down" wing.



    Quoting HiFi (Reply 28):
    They are usually simmetrical profiles, producing (positive, negative, longitudinal or lateral) lift only when there is an angle of incidence.

    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.)
    Make the most of the available light ... a lesson of photography that applies to life
     
    HiFi
    Posts: 189
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    RE: Question Re: Rear Horizontal Stabilizers

    Tue Mar 14, 2006 11:37 pm

    Quoting Kukkudrill (Reply 30):
    Also, what is longitudinal or lateral lift, and why would they be needed? (I assume positive or negative means up or down.)

    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 that stabilizers do produce lift. The traditional conception is that lift is an upward force... OK. But it can also act downward or to the sides or in any direction based on the exact same principle (high-speed air flowing around an aerodynamic shape). It will also confront you with the same limitations, such as induced drag. So lift can be "Up or down" (longitudinal) for the horizontal stabilizer and "left or right" (lateral) for the fin. Positive or negative in any direction.


    As to "who's right?", I'd say I am!     (of course, I may not be the right person to answer this..)
    The horizontal stabilizer does sort of work like an inverted wing, since it produces downward lift (during most of the flight) but it is not designed for the same purpose as the wing and therefore is not an upside-down wing in terms of shape and construction.

    Using a non-symmetrical profile would not be practical in my opinion, for a traditional commercial aircraft, but I don't have the knowledge to tell you if it's commonly used.

    Here's what you have to consider:
    - stability and control
    - structural resistance to loads
    - fitting of equipments (such as elevator actuators)
    - aerodynamic efficiency (basically: drag)

    You end up with a big enough surface for stability, thick enough for equipment fitting and structural resistance and still relatively small and thin for less friction drag. You also design your cruise configuration so that the angle of incidence of the stabilizer is small, minimizing drag during the longest flight phase.
    A symmetrical profile attends to all of these needs and is simpler to design and to manufacture. It is also less vulnerable to pressure drag (since it will be exposed to both negative and positive angles of incidence during a flight) and to eventual wave drag.

    [Edited 2006-03-14 15:41:46]
    no commercial potential
     
    Kukkudrill
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    RE: Question Re: Rear Horizontal Stabilizers

    Wed Mar 15, 2006 5:41 am

    OK, thanks!

    The message you were about to post is too short blah blah blah ...
    Make the most of the available light ... a lesson of photography that applies to life
     
    mknies
    Posts: 11
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    RE: Question Re: Rear Horizontal Stabilizers

    Wed Mar 15, 2006 6:15 am

    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 is the fulcrum, and the HStab is the other side. In a level, cruise configuration the horizontal stabilizer is producing a downforce (which is also a form a lift) that is equaling out the sides. As was previuosly mentioned, the HSTAB is usually an "upside down wing" or a neutal-lift design. The needed lift for trim on a large airliner is achieved by triming the entire stabilizer up or down according to the lift need. I.E. when fuel is consumed or load is shifted or aerodynamic conditions change. As nose up pitch is needed to keep a level flight atitude the HSTAB is angled down which creates more downward lift. As nose downd pitch is needed the HSTAB is angled up which eliminates lift. This is in cruise flight. In all other maneuers, the elevators are used to achieve the desired atitudes.
    As an answer to the argument regarding the CP/CG being on top of each other; As i have been taught, this would cause an enormous instability. Not to mention that it would be nearly impossible to maintain on an airliner.
    Hope i helped with the analogy!!!
    ~Knies
     
    411A
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    RE: Question Re: Rear Horizontal Stabilizers

    Fri Mar 17, 2006 7:10 am

    Many so-called experts here, most without any real aerodynamic detailed knowledge.

    The wing.
    In EVERY case, the airfoil shape used (civil aircraft) is dependant on forward motion thru the air to provide lift.
    In doing so, it has a 'turning moment', I.E. it will naturally tend to pitch DOWN (especially with increasing true airspeed) unless a counteracting force is obtained to preclude this pitch down moment.
    This downward force is provided by the horizontal stabilizer.
    It is, simply, an up-side-down airfoil shape, to provide this required downward force.
    In most aircraft, the center of pressure moves markedly aft as the speed is increased, therefore less 'downforce' is required, to maintain level flight.
    Other designs store fuel in the tail section to help eliminate the required downforce produced by the horizontal tail surfaces, thus reducing drag.

    Known for many many years and is the basis of every subsonic design, to date.

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