Total Drag (man...)
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  1. #1
    Senior Administrator PRB's Avatar
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    Total Drag (man...)

    While doing some calculation on total drag, I may have swerved, uncontrolled, into a truth. I found that total drag equals static (CD0) drag + induced (CDi) drag, and that CDi has to do with dynamic pressure and is also influenced by AOA. So I found that as speed increased, induced drag increased, which made sense to me, since dynamic pressure would be increasing. But I also found that below a certain speed, induced drag started going up again! What was happening? AOA was going up, is what was happening! This implied an “optimum” speed, at which induced drag, and therefore total drag, would be at a minimum. Might this speed be related to that which might produce max range? Did I stumble into something important, or is my math out to lunch (again)?
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  2. #2
    (Found this thread by accident thanks to a phantom subscription in my control panel...)

    I can tell you from first hand experience that there is a sweet spot regarding speed and fuel consumption. On my bike, I get the best gas mileage around 60MPH, but above that, it starts dropping off due to increased drag. Common sense would state that this concept holds true for any powered vehicle. The trick is in finding that "sweet spot" - which generally requires considerable testing. Before computers, all you could do was start with an educated guess and then fly several sorties at varying speeds, then calculate consumtion after each flight. With planes, altitude also plays a huge role. Higher altitude reduces drag and aldo reduces fuel use due to the fact that fuel metering is based on total air volume. But if you get too high you loose thrust and end up going using more fuel trying to go nowhere fast. Come back down and the increased air density increases both drag and fuel use. Of course, both ground-based and in-flight computers make this easier now.
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    There are 4 simple ones:
    Minimum (stall speed),
    Maximum speed, where max thrust = max drag - probably have one at each altitude
    maximum range, also altitude related depending on engine output &
    maximum endurance which will also be altitude related.
    In other words there are a lot of different parameters to be calculated for optimum performance & modern aircraft come with an Operating Manual with loads of graphs.........I've also not mentioned weight.....which of course varies with fuel consumed & stores which might have been dropped...
    Happy hunting the optimums...
    Keith

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    Senior Administrator PRB's Avatar
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    I guess what I was getting was this: Given straight and level flight, will induced drag follow a curve, such that there is an optimum speed above which, and below which, induced drag will increase? Because that's what my calculations suggest. The only way this makes sense to me is that below a certain speed, assuming straight and level flight, AOA will increase, thus presenting more surface area to the wind, which would make induced drag increase. For the FDG2 B-17E, this "minimum drag" speed, at sea level, is around 130 knots. Does this make any sense?
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  5. #5
    That certainly makes sense, but those calculations are above my pay grade! :isadizzy:
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  6. #6
    Paul: It is fairly easy to visualize this if one looks at the curves, such as in a book as Aerodynamics For Naval Aviators. For the effect on performance one has to take into account the power or thrust available, which also changes with speed. Jet power available is approx a straight line, increasing with speed, for prop aircraft it starts out higher at low speed than the jet, but increases at a decreasing rate and actually decreasing past a certain point. The Lift over Drag max is at a higher speed than the minimum power required and would be nominally related to max endurance and max range. Most aircraft use a 99% best range solution, which is about 5% faster than the absolute max range speed. It is quite illuminating to study the differences between prop and jet aircraft and the best speeds. Something I always thought amazing was that the max gliding distance of an aircraft is almost totally independent of it's weight! Cheers: t

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