1. #11
    AndyJWest's Avatar Senior Member
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    ... all of which has b****r all to do with ground effect, either in real life or in the sim.
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  2. #12
    JtD's Avatar Senior Member
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    Originally posted by WTE_Galway:
    The apparent division by zero just means mathematically it needs to be approached as a limit case.
    No, it doesn't. Not necessarily.

    thrust hp = engine hp * prop efficiency

    For all situations where neither engine hp or prop efficiency (so basically all situations where engine is on and prop is mounted) thrust hp > 0.

    So,
    T = P/v with P > 0 and v = 0, so
    T = n.d. or, if you want to use a limit case,
    T -> infinite

    That's basically all there is to it, but I'll be going on ranting a bit.

    First note that:

    T =P/v

    is a simplified case assuming a stable system where no acceleration takes place.
    No, acceleration may or may not take place. That's not part of the equation.

    now, rearranging and solving for P:

    P = Tv

    If v=0, then the propulsive POWER of the propeller/engine as a system is ZERO.
    Yes, and no matter how large the thrust is, your P from that equation will always be 0 (i.e. T from P/v = n.d.).

    Getting back to T =P/v.
    Given P = 0 it can be easily shown that for
    the limit case as v->0 then T=0
    (in non mathematical terms - regardless of how small v gets P = 0 and hence P/v is zero and THRUST remains ZERO)
    So, basically you're calculating:

    T = P/v, P = Tv so T = T*v/v so T = T. That's all you've done, and it leads nowhere.

    HOWEVER even from commonsense, if its a stable system where v is constant at zero their can be no EXCESS thrust and the total EXCESS thrust vector must be zero.
    Excess thrust was not the point. Total thrust was.

    I'm used to seeing physics getting raped on this board, but now not even math is spared any longer. That hurts.
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  3. #13
    JtD's Avatar Senior Member
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    Originally posted by AndyJWest:
    ... all of which has b****r all to do with ground effect, either in real life or in the sim.
    Nothing. Sorry. The point where x/0 = 0 should really be a point where I leave a topic because it is getting too stupid. I'll do so now.
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  4. #14
    M_Gunz's Avatar Banned
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    If the v in T=P*n/v is the velocity of air moving through the prop disc and not the plane itself...
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  5. #15
    AndyJWest's Avatar Senior Member
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    Originally posted by M_Gunz:
    If the v in T=P*n/v is the velocity of air moving through the prop disc and not the plane itself...
    ... it still has b****r all to do with ground effect.
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  6. #16
    M_Gunz's Avatar Banned
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    Originally posted by AndyJWest:
    ... all of which has b****r all to do with ground effect, either in real life or in the sim.
    And the discussion had not weaved around to the use and validity of mathematical models, which has to do with?
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  7. #17
    thrust hp = engine hp * prop efficiency
    WTE_Galway got it right.

    Power by definition is force x velocity. If our velocity is zero then our power is zero.

    Its just basic physics.

    http://www.ajdesigner.com/phpp...n_velocity_force.php

    The rest is a clown fest.

    And the proper form of it is actually:

    T = P*n/V
    For example, Thrust Horsepower by definition is:

    Shaft Horsepower x n and the 325 is a conversion factor for using Knots in the BGS system. It is not what some were taught to parrot and without understanding the definition of Thrust Horsepower it does not fit their mold.

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  8. #18
    AndyJWest's Avatar Senior Member
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    Actually, I started the thread after a question was asked about whether, and how accurately, IL-2 modelled ground effect. This involved attempting to answer two questions: (a) how IL-2 models GE, and (b) what are the effects of GE in real life. Without measurement, mathematical models tell us precisely nothing - they can only be a statement about how reality is expected to behave if the maths and the underlying assumptions are correct. It is noticeable that this thread contains (a) some data gathered from attempts at measuring IL-2 GE, and NO verifiable data about real-world GE. On this basis, almost anything IL-2 does might be right, wrong, or just plain irrelevant. In order to simulate something, you need to know what it is that is being simulated. On the evidence presented so far, I could argue that real-world ground effect was caused by proximity to earthworms...
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  9. #19
    NO verifiable data about real-world GE.

    That is not true.

    I gave you a simple method to estimate the effect that agrees with real world results very well in the first few posting on the subject.
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  10. #20
    AndyJWest's Avatar Senior Member
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    Originally posted by Kettenhunde:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">NO verifiable data about real-world GE.

    That is not true.

    I gave you a simple method to estimate the effect that agrees with real world results very well in the first few posting on the subject. </div></BLOCKQUOTE>

    A 'method to estimate the effect' isn't the same thing as 'verifiable data' though, is it? Given the contradictory explanations of GE I've seen, I see little reason to attach any particular credence to estimates.

    I think that the IL-2 GE model is wrong. I've still not got much idea about what it would look like if it was right.
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