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  • Aerodynamic Side loads

    SantoshMore

    SantoshMore

    @santoshmore-FHt1C8
    Updated: Oct 27, 2024
    Views: 1.1K
    Dear All,

    This is santosh more, from mumbai seeking a urgent help from all those how are in aeronautical experties.following are my derivation for side on airbrone antenna which is mounted on the fuselage.I request u all go review the derivation for cross reference.
    1.0 SIDE LOAD:
    1.1.0 CONSIDER DATA :-

    Considered Max Velocity (LCA maneuvering speed), V = 0.9 mach (300 m/s)
    Cross section side Area, A1 = 0.0473015 m2
    Air Density at 60,000 feet, ? = 0.18 kg/m3
    Co-efficient of side load, Cs = 1.28 (For flat surface).

    1.1.1 CALCULATIONS:
    Dynamic pressure = 0.5 x density x velocity square
    = 0.5 x 0.18 x (300)2
    = 8100 kg/m2

    Aerodynamic Load = Dynamic pressure x Area x Cs
    = 8100x 0.0473015x 1.28
    = 490.5 Kg.f

    (will this load act on complete area or a point load acting at center of pressure)

    Please seand the replyies to my mail ID: <removed>
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  • skipper

    Member • Jun 30, 2009

    The aerial extending from the fuselage is equivalent to an elastic rod embedded in a solid or semi-solid material, which will absorb vibrational modes in the aerial (elastic rod).
    You should calculate the aerodynamic loading on the surface area of the aerial and then calculate how much vibrational energy is seen at the point of contact (and "below" it) with the fuselage, or the "major" mass/weight which is responding to all incoming forces/perturbations including the attached aerial.
    So you need to find the difference in inertial "responses" between the aerial and fuselage, under different loads to see if the aerial will stay stable or will absorb more than it should and make the point of contact vibrate beyond a safe limit. Something along those lines.

    I would add that the calculations are not straightforward and several assumptions will be required to simplify things (there are three dimensions to consider, but only one major vibrational mode will correspond to the unwanted 'harmonic' that means mechanical failure under load).

    p.s. sorry I'm not an aeronautics engineer, I just know a little about responses of physical systems; the outline I've given is quite general. You will need to formulate the proper mechanical eqn for the fuselage + aerial (as above, an elastic rod embedded or fixed to another inertial surface) and for the modes, or Lagrange points where the aerial vibrates "safely" to calculate when the limit will be exceeded.
    I would start with the formula for a rod standing upright and responding to being 'energised' or struck, i.e. Fourier analysis of bending modes in an elastic rod, transverse and longitudinal waves, harmonic modes, etc (and, um, etc).

    Your pressure/density values don't include the density of the aerial or fuselage; you should plug these in too, allowing a 'neighbourhood' for the base of the extended aerial, then account for air density against this (as an extended, elastic rod with a density and volume. surface area, and elastic constant, length that determines harmonics, etc). The part of the fuselage that also bends and vibrates in sympathy is the cross-section for absorbance of energy by the fuselage; in flight the fuselage is the 'majority carrier' and the aerial responds mostly at its free end to changes in air pressure/density, while the aerial has a constant density. Pressure/volume and density calculations are straightforward once you have all the coefficients. The latter are not generally that easy to get right, but you can approximate them.
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  • SantoshMore

    Member • Jul 1, 2009

    skipper
    The aerial extending from the fuselage is equivalent to an elastic rod embedded in a solid or semi-solid material, which will absorb vibrational modes in the aerial (elastic rod).
    You should calculate the aerodynamic loading on the surface area of the aerial and then calculate how much vibrational energy is seen at the point of contact (and "below" it) with the fuselage, or the "major" mass/weight which is responding to all incoming forces/perturbations including the attached aerial.
    So you need to find the difference in inertial "responses" between the aerial and fuselage, under different loads to see if the aerial will stay stable or will absorb more than it should and make the point of contact vibrate beyond a safe limit. Something along those lines.

    I would add that the calculations are not straightforward and several assumptions will be required to simplify things (there are three dimensions to consider, but only one major vibrational mode will correspond to the unwanted 'harmonic' that means mechanical failure under load).

    p.s. sorry I'm not an aeronautics engineer, I just know a little about responses of physical systems; the outline I've given is quite general. You will need to formulate the proper mechanical eqn for the fuselage + aerial (as above, an elastic rod embedded or fixed to another inertial surface) and for the modes, or Lagrange points where the aerial vibrates "safely" to calculate when the limit will be exceeded.
    I would start with the formula for a rod standing upright and responding to being 'energised' or struck, i.e. Fourier analysis of bending modes in an elastic rod, transverse and longitudinal waves, harmonic modes, etc (and, um, etc).

    Your pressure/density values don't include the density of the aerial or fuselage; you should plug these in too, allowing a 'neighbourhood' for the base of the extended aerial, then account for air density against this (as an extended, elastic rod with a density and volume. surface area, and elastic constant, length that determines harmonics, etc). The part of the fuselage that also bends and vibrates in sympathy is the cross-section for absorbance of energy by the fuselage; in flight the fuselage is the 'majority carrier' and the aerial responds mostly at its free end to changes in air pressure/density, while the aerial has a constant density. Pressure/volume and density calculations are straightforward once you have all the coefficients. The latter are not generally that easy to get right, but you can approximate them.
    Hey hi thr,
    this is santosh more seeking a help frm u,
    can you tell me how to derive a aerodynamic side load on a vertical stabilizer. i hv arrived using dynamic pressure acting upon Vertical stbzr surface area. but in now sure whtr my concept is correct or not ,plz if u can help me out. do reply on <edited> . PLz
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  • skipper

    Member • Jul 1, 2009

    Well, buddy, it isn't a simple thing; there are several ways to analyze the problem and if you can access the real-world model that puts you in a better position.
    You need to know about the methods available (mathematical physics) and the limitations. You have what looks like a relatively "simple" problem to solve, you might not need to go as far as the dynamics and get a good approximation using a "fixed structure" approach. That's where you use standard engineering formulas for beam loading and so on.

    Here's an excerpt from Wikipedia:
    To perform an accurate analysis a structural engineer must determine such information as <a href="https://en.wikipedia.org/wiki/Structural_load" target="_blank" rel="nofollow noopener noreferrer">Structural Load</a>, <a href="https://en.wikipedia.org/wiki/List_of_structural_elements" target="_blank" rel="nofollow noopener noreferrer">List Of Structural Elements</a>, support conditions, and materials properties. The results of such an analysis typically include support reactions, <a href="https://en.wikipedia.org/wiki/Stress_%28physics%29" target="_blank" rel="nofollow noopener noreferrer">Stress %28Physics%29</a> and <a href="https://en.wikipedia.org/wiki/Displacement_%28vector%29" target="_blank" rel="nofollow noopener noreferrer">Displacement %28Vector%29</a>. This information is then compared to criteria that indicate the conditions of failure. Advanced structural analysis may examine #-Link-Snipped-#, <a href="https://en.wikipedia.org/wiki/Buckling" target="_blank" rel="nofollow noopener noreferrer">Buckling</a> and <a href="https://en.wikipedia.org/wiki/Non-linear" target="_blank" rel="nofollow noopener noreferrer">Non Linear</a> behavior.
    There are three approaches to the analysis: the <a href="https://en.wikipedia.org/wiki/Strength_of_materials" target="_blank" rel="nofollow noopener noreferrer">Strength Of Materials</a> approach (also known as strength of materials), the <a href="https://en.wikipedia.org/wiki/3-D_elasticity" target="_blank" rel="nofollow noopener noreferrer">3 D Elasticity</a> approach (which is actually a special case of the more general field of <a href="https://en.wikipedia.org/wiki/Continuum_mechanics" target="_blank" rel="nofollow noopener noreferrer">Continuum Mechanics</a>), and the <a href="https://en.wikipedia.org/wiki/Finite_element" target="_blank" rel="nofollow noopener noreferrer">Finite Element</a> approach. The first two make use of analytical formulations which apply mostly to simple linear elastic models, lead to closed-form solutions, and can often be solved by hand. The finite element approach is actually a numerical method for solving differential equations generated by theories of mechanics such as elasticity theory and strength of materials. However, the finite-element method depends heavily on the processing power of computers and is more applicable to structures of arbitrary size and complexity.
    Regardless of approach, the formulation is based on the same three fundamental relations: <a href="https://en.wikipedia.org/wiki/Mechanical_equilibrium" target="_blank" rel="nofollow noopener noreferrer">Mechanical Equilibrium</a>, <a href="https://en.wikipedia.org/wiki/Constitutive_equation" target="_blank" rel="nofollow noopener noreferrer">Constitutive Equation</a>, and <a href="https://en.wikipedia.org/wiki/Compatibility" target="_blank" rel="nofollow noopener noreferrer">Compatibility</a>. The solutions are approximate when any of these relations are only approximately satisfied, or only an approximation of reality.
    If you aren't familiar with some of this, try seeing what the links have to say. I think that, although Wikipedia isn't authorative, the chances are good that it won't be deliberately misleading. Have you worked on any kind of loading problems, or are you something of a beginner? Your aerodynamic problem unfortunately isn't beginner-level...
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  • nany_mech

    Member • Jul 14, 2009

    Hi Santosh, I am sudheer doing mechanical final year, though I am not an aeronautic student I know a little bit of it.

    so first of all you said that the velocity with which you are dealing is mach 0.9. According to that much velocity, we enter into compressible zone where density changes should be considered.

    Your calculations are satisfactory for basic level and i don't think that we can get aerodynamic load that much easily by multiplying pressure and area to which it is exposed.

    since the altitude is high the temperature changes also should be considered and more over the material properties may vary at that altitude (60,000 ft).

    To simplify your work you can have a little help from any analysis softwares like ANSYS or so which can solve the problem virtually and you will see the aftereffects from its solutions. if you are not familiar with analysis software packages try going for Finite Element Analysis (FEM) which will be very use full in solving problems involving loading at abnormal conditions.

    As i mentioned earlier, though i am not an aerodynamic fellow, I have lot of interest in that field and your question is an excellent one. It is equivalent to major project in design field. I will look forward for it.

    All the best!.....
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