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Static & Dynamic Stability ID 811225

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  • Static & Dynamic Stability ID 811225

    Hello,

    would like some feedback/explanation as to why that is, concerning the question :

    With increasing altitude and constant IAS the static lateral stability (1) and the dynamic lateral/directional stability (2) of an aeroplane with swept back wing will:

    (1) increase (2) decrease- correct answer.

    My textbook only dealt with dynamic stability when considering the longtitudinal axis (pitch) and then only expanded on aerodynamic damping as an influencing factor. This question also lacked an explanation below it in the BGS database, so I am at a loss of information for the moment.

    Thank you

  • #2
    Re: Static & Dynamic Stability ID 811225

    Hi Skymaster15L

    during a roll with a swept wing aircraft the down going wing will see a side slip airflow that affectively makes the wing a straight wing, this increases its CL for a given change in alpha and lateral static stability i.

    As you increase altitude, mach number increases and CL increases for a given alpha, this will increase the static lateral stability more. Dynamic stability reduces for all axis with altitude due to reduced aerodynamic damping.

    Hope this helps

    John H
    Last edited by John H; 25-03-2013, 10:34.

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    • #3
      Re: Static & Dynamic Stability ID 811225

      Cleared it up. Thank you

      Regards

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      • #4
        Hi.

        For this question, I am struggling to understand that aerodynamic damping decreases (and therefore dynamic stability). I thought aerodynamic damping was dependent on dynamic pressure, which is constant (constant IAS) so the increasing altitude effects on density are nullified by increased TAS. Please help!

        Thank you.
        Last edited by TouchTheFaceOfGod; 14-04-2018, 20:11. Reason: Added the TAS bit

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        • #5
          Consider and aircraft at sea level. A disturbance has caused the aircraft to oscillate around its lateral axis, pitching at a rate of, say, 3 degrees per second.

          Due to the downward (or upward) movement of the tail, the direction of the relative airflow over the horizontal stabiliser will be changed, which changes its angle of attack. This will change the forces it creates, and the result, hopefully, will be the aerodynamic damping that gives us the dynamic stability that returns us to steady flight.

          Now let us maintain IAS but take the aircraft to 40000ft. In undisturbed flight, the forces created by the stabiliser will be the same, but what happens if a disturbance gives us the same rate of pitch?
          TAS will have approximately doubled. In one second, the aircraft will have travelled twice as far horizontally for the same vertical movement of the tail. Sketch it out, and you'll see that the change in stabiliser alpha due to the pitching movement will be less. The change in the forces created by the stabiliser will be less, so stability and aerodynamic damping are reduced.

          I'll improve the explanation we give to this question.

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          • #6
            Thank you Ben.

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