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Thread: Elevator trim tabs versus adjustable tailplane

  1. #1
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    Default Elevator trim tabs versus adjustable tailplane

    Hello,

    Yes they appear to do the same thing (adjust the attitude of the aircraft to allow for small changes to COG) but why do some aircraft have trim tabs on the tailplane and others (eg ERJ170) have an adjustable tailplane?

    Also I could never understand how a trim tab (on the tail plane) isn't negated by the fact that if the yoke id not held the air pressure against the trim tab wouldn't result in the elevator being 'pushed' in the opposite direction. Maybe someone could educate me on this.

    Hope I have explained properly.

    Thanks.
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  2. #2
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    to allow for small changes to COG)
    Speed and other factors, even bank angle, affect how much pressure the pilot needs to hold on the elevator. CG changes in flight will generally be because of either fuel burn (but probably not from wing tanks) or someone (or thing) moving fore or aft. While trim will allow the pilot to compensate for the control pressures those changes cause, the speed changes, attitude changes and power changes (other things, too) will also affect how much elevator pressure is required.

    So elevator trim is for one thing: to allow the pilot to reduce or eliminated the amount of pressure held in the pitch axis.

    As to the difference between methods of trimming, this is an aircraft design issue (what did the designer choose? -- they all work), and the results from any of the mechanisms are generally equivalent, though there may be aerodynamic reasons for a specific choice. The tab, of course, is usually the simplest and cheapest (or maybe bungee cords are cheapest).

    Also I could never understand how a trim tab (on the tail plane) isn't negated by the fact that if the yoke id not held the air pressure against the trim tab wouldn't result in the elevator being 'pushed' in the opposite direction.
    I don't quite understand what you are saying. The trim tab (or anti-servo tab or movable horizontal stabilizer or movable tail section) are all to accomplish the relief of pilot-held pressure. The trim tab deflects air, and in the process is itself deflected the opposite direction, taking the elevator with it. So if the tab goes down, it deflects air downward but reacts (Newton's law) by going up, taking the elevator with it. This occurs whether the pilot holds any elevator pressure, though the pilot can fight it by adding opposite pressure on the stick (yoke).

    You might look at this Wiki article and this one or perhaps at other articles that Google will show using this search term (without the quotes): "wikipedia elevator trim" -- there's lots of information, even some helpful pictures and diagrams.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  3. #3
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    Thanks Larry,

    I thought that it would be just the aircraft designers preference (to use trim tabs or adjustable tailplane) but wanted to be sure.

    Regarding trim tabs and what I was asking. I was thinking that if the trim tab was set so that it moved in an upwards direction then the air flowing over it would tend to make the elevator move in the opposite direction (if the yoke was not held) but not as far as the trim tab as it has a larger surface than the trim tab. I just thought that these movements would cancel each other out.
    It seems from your reply that it doesn't.
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  4. #4

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    Funny. That same question has been at the back of my mind since I first started learning about the principles of flight back in the Sixties. Here's what Wikipedia has to say, and it makes sense to me:

    "Because the center of pressure of the trim tab is farther away from the axis of rotation of the control surface than the center of pressure of the control surface, the moment generated by the tab can match the moment generated by the control surface. The position of the control surface on its axis will change until the torques from the control surface and the trim surface balance each other."

    Hope that helps.

    (And thanks for motivating me to actually find out!)
    Last edited by ianhr; 12-05-2017 at 04:05 AM.

  5. #5
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    Thanks Ian

    That makes a lot of sense.
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  6. #6
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    I was thinking that if the trim tab was set so that it moved in an upwards direction then the air flowing over it would tend to make the elevator move in the opposite direction (if the yoke was not held) but not as far as the trim tab as it has a larger surface than the trim tab. I just thought that these movements would cancel each other out.
    Basically that's so but, as Ian points out, you forgot to take the longer lever arm into account. Lots of things in aviation aren't "intuitive" and this is just one of many things that take some "larnin'."

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

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    A simple trim tab or "servo tab" is no more than a type of flap - it changes the camber of the elevator (which is in itself an airfoil), increasing or decreasing its lift coeffiecient, causing it to "float" to a certain position. The horizontal stab, including the elevator, exerts a DOWNWARD force on the tail in most circumstances in a conventional design with the cg forward of the aero center. These features provide POSITIVE LONGITUDINAL STABILITY WITH AIRSPEED - not some stable attitude or altitude setting as many might think.

    Positive longitudinal speed stability means if the airplane's speed is changed from its trimmed speed state it will TEND to return to that speed, and consequently the pilot will sense that tendency in control force. The trimmed state also changes with power setting, cg position and configuration changes (gear, flaps, speed brakes, etc).

    Trimmable horizontal stabs and all-flying tailplanes provide more effective trim capability and less induced drag or "trim drag" but are a little more complex. Flying tailplanes are crucial in providing adequate trim capability in the transonic/supersonic regime.
    Last edited by mikeandpatty; 12-05-2017 at 02:44 PM.

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    Got it. Thanks
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  9. #9
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    OK, I wasn't going to get into it that far, but to add a technicality: "These features provide POSITIVE LONGITUDINAL STABILITY WITH AIRSPEED" -- actually, it's with airflow over the control surface such that, while it does change with airspeed, it also changes with power changes when you have prop blast over the surface (pretty much all tractor singles, some twins), which is why adding or reducing power (in a single) seems to change your trimmed airspeed, and counter-intuitively at that, meaning that a power reduction reduces that air flow, thus causes trimmed airspeed to increase a bit to get the air flow back, with the converse for adding power.

    Don't forget that all the controls interact with each other, so that changing one may mean changing one or more others, too, such as banking for a turn needing elevator and rudder too.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  10. Default

    That is why I said power effects trim point - but not necessarily just propwash - the location of the center of thrust in relation to the cg (above or below it). In a jet, no propwash - still get stability with speed changes in an acceptable design.

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