1. #21
    If ground effect is working, then I stand corrected. I'll go back to blaming my botched landings on myself.

    Now if I could just get static campaign designers to start throwing in some random wind components...
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  2. #22
    AndyJWest's Avatar Senior Member
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    As JtD suggested, turning off 'wind and turbulence' seems to remove the increase in speed at very low level I'd measured earlier. I double checked my results, and also flew a reciprocal course to confirm it wasn't an effect of wind - I'd set the weather to 'clear'

    One thing I can't understand is how, if the ground effect phenomenon disappears with 'wind and turbulence' off, he is getting the results he showed in his graph, though to be honest, I'm not entirely sure I understand what he is measuring.

    This finding has some interesting consequences: if you select a 'no-wind' weather setting, you will find takeoffs and landings easier with 'wind and turbulence' on!
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  3. #23
    JtD's Avatar Senior Member
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    It's not the ground effect that disappears, it's the speed boost. That's not related. The ground effect gives you more lift at the same angle of attack just above the ground (which is also what I was measuring).

    Viikate said he edited the code to turn it off, so there's no difficulty switch to do so, I guess.
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  4. #24
    AndyJWest's Avatar Senior Member
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    I'd assumed that the speed boost was due to ground effect - reduced induced drag giving excess power to increase speed at a given throttle setting. If the speed boost isn't down to ground effect, then what is causing this?

    Off-topic, I must remember my autopilot wasn't designed to do 180 degree turns at 10m. It's just as well the max bank angle for bombers is set at 30 degrees.

    EDIT ----
    I have now confirmed I get the same results for my Ju-88 test using the standard 'level stabilizer' autopilot function as I do using my AP.
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  5. #25
    K_Freddie's Avatar Senior Member
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    This pic is 'wrong' with regard to bernoulli's principle, very wrong!!!
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  6. #26
    AndyJWest's Avatar Senior Member
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    The pic might well be wrong, but I'm not sure I see the relevance?

    If you are saying 'it is wrong because the airflow over the top must meet up with the same airflow at the bottom when it meets the trailing edge', or something like that, I think this is a misinterpretation anyway.
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  7. #27
    Sorry to break it to you Freddie, but applying bernoulli's principle and equal transit time to describe how an airfoil generates lift is a fallacy.

    A false explanation for lift has been put forward in mainstream books, and even in scientific exhibitions. Known as the "equal transit-time" explanation, it states that the parcels of air which are divided by an airfoil must rejoin again; because of the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom. Therefore, because of its higher speed the pressure of the air above the airfoil must be lower. Despite the fact that this "explanation" is probably the most common of all, it is false.
    Link

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  8. #28
    JtD says:
    That's not related.
    IRL, it is very much related....

    JtD says:
    The ground effect gives you more lift at the same angle of attack just above the ground
    Negative Ghost Rider.

    You do not get "more lift", that is not how it works at all. That is fundamentally not correct. The wing only produces the amount of force required.

    In a climb or descent, the wing will produce the amount of force equal to weight offset by the angle of climb or descent. The forces are in balance.

    In a steady altitude coordinated turn, the wing will only produce the centripetal force required and the weight offset by a component of thrust.

    The effect of an aircraft entering GE is the wing will move to a lower angle of attack at the same velocity. You must reduce power to maintain the same velocity.

    Real pilots who do not understand this can get in trouble and kill themselves on both take offs and landings.

    Ground effect reduces induced drag and the airplane is able to reach a speed where it can stagger off. As altitude is gained, induced drag increases as the effect of the ground effect diminishes. Twenty or thirty feet up, ground effect vanishes, the wing encounters the full effect of induced drag and the struggling airplane which got off the ground on the ragged edge of a stall becomes fully stalled and drops to earth.
    http://www.pilotfriend.com/tra...raining/aft_perf.htm


    Drag is reduced and at a constant power speed will increase.
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  9. #29
    Originally posted by Kettenhunde:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> JtD says:
    The ground effect gives you more lift at the same angle of attack just above the ground
    snip

    The effect of an aircraft entering GE is the wing will move to a lower angle of attack at the same velocity.

    snip </div></BLOCKQUOTE>

    If you disagree with JtD, then why are you saying that the lower AoA is required?
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  10. #30
    AndyJWest's Avatar Senior Member
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    I think we are getting into an unnecessary question of semantics here. For an aircraft to remain at a constant altitude, the upward force - 'lift' has to equal the downward force - 'weight'. Since ground effect gives the same lift at a lower AoA, to maintain this balance, the AoA needs to be reduced. I think my experiments seem to suggest this is occurring, though I haven't measured it directly, and JtD seems to be observing a similar effect. There seems to be a little uncertainty about measurement techniques, but so far I've seen no evidence that either (a) IL-2 isn't modelling the effect, or (b) it is giving implausible results for the scale of the effects. If anyone can come up with quantitative figures for what these effects should be in real life, we will be able to make a comparison. Otherwise, arguing about abstract questions will tell us little. Has anyone got any real data on predicting ground effect?
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