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I want to design a UAV to fly at faster cruise speeds than usual. Setting aside the powertrain selection, which will probably be a big part of this, I want to know what kind of design choices I can make that would make this aircraft more efficient at faster speeds, vs designing a traditionally efficient aircraft (Fly it wherever the L/D is the highest) and flying it faster.

From what i understand, wherever the max CL/CD is at is the most efficient aoa/speed to fly at. How would we design an aircraft to solve max cl/cd for a certain speed? i.e., if i wanted to design an aircraft to cruise and be its most efficient at, say, 130mph instead of 60mph. Is there design choices that'd make it more efficient at those speeds (Low lift airfoils, picking airfoils for the right RE numbers, etc), or is this as simple as making an aircraft with a high CL/CD and making it fly at faster speeds?

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    $\begingroup$ Are you asking how to generally increase L/D or how to set the wing such that at 130mph it is at its highest L/D? $\endgroup$
    – sophit
    Nov 14, 2022 at 18:41
  • $\begingroup$ @sophit yeah basically how to set the wing such that 130mph is the highest L/D $\endgroup$
    – Tarik Agcayazi
    Nov 15, 2022 at 15:41
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    $\begingroup$ Understand. Anyway I was thinking that the point where L/D is max isn't actually related to the max speed rather it is the condition maximising loiter for a jet and range for a propeller or a glider. $\endgroup$
    – sophit
    Nov 15, 2022 at 21:24

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Look to purpose built F1 racing airplanes for your answer. Basically, smallest possible frontal area, overall cleanliness, and small wings. If you want to go at multiples of best L/D speed, you need to raise the wing loading by making the wings small (and obviously, optimizing airfoil selection and such).

Problem then is high landing speeds. If you can't live with the resulting landing/takeoff speeds with itty bitty wings, the workaround for that is high lift devices that deploy from the little wings to reduce the landing speeds.

So an optimized design would have the smallest cross section possible, careful attention to parasite drag, cooling drag etc, and small wings, offset by, say, double slotted full span flaps and possibly leading edge slats to raise the max L/D of the wing you have in the takeoff/landing configuration, enough to compensate for the lack of wing area.

Of course, the high lift devices add a lot of weight, which might force you to make the wings bigger anyway, which add more weight again, and off on the design merry-go-round you go.

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  • $\begingroup$ I see. I'm trying to have as much range as i could get as well. Would I follow this philosophy and try to squeeze as much L/D out as i could as well, and just spin around the design merry-go-round until i arrive somewhere favourable? $\endgroup$
    – Tarik Agcayazi
    Nov 15, 2022 at 22:20
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    $\begingroup$ There's so many variables. If you create a large fuselage tank for all the fuel, you have to account for spar bending from the fuel load. If you put the fuel in the wings and, say, external tip tanks, you save weight in the wing spar but it costs you speed. If you want extreme range, you will probably be forced to use some form of wing mounted underslung or tip mounted tanks. If you have an engine decided in advance, and you have a range/speed target, you design around those objectives. $\endgroup$
    – John K
    Nov 15, 2022 at 23:00

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