![]() ![]() Once you have this, you can adjust the twist to deal with any predicted tip stall problems, and find the AoA needed for any desired C_L. Schrenk's Approximation is a simple graphical technique that finds the lift distribution on an untwisted wing of any given planform shape. ![]() If you put washout in the wing, that reduces the lift at the tips - so you'll need to increase the incidence angle to have the same overall C_L. You should choose an airfoil which has its minimum drag near that C_L.įor lower aspect ratios, you also need to consider that aspect ratio lowers effective lift curve slope - so you'll need a higher AoA than the section value to get the same C_L. Find the C_L value for your desired flight condition, then set the wing incidence so the fuselage is level when the wing is flying at that C_L. If you select 8° of incidence, it would be optimized for the slowest flight, but unless you are making thermal records, it will penalize you heavily for slightly faster speeds, because the fuse would be actually pointing down by 5° when you fly at Cl 0.8, angle of attack 3°įor FPV, I try to design everything around this sweetpoint of 2-3° angle of attack (if it were Clark Y) (because often I fly in strong winds, so sometimes I need faster speeds than the ideal L/D speed).Ĭhileflora: You have the right idea, but you really should consider three-dimensional effects and not just the airfoil section characteristics. So in theory, for Clark Y one could go from 3 to 8 incidence, and the best one probably would be in the range of 3-4°. optimizing for higher speeds you will have to gain more. When selecting a wing incidence I would go with the lower angle of attack for the range of "best" anlges of attack because you will have much more fuse drag at higher speeds than at lower speeds, i.e. For instance, for Clark Y, reynold number 500.000, the angle of attack for best L/D is between 3 and 8°. if you select Cl for best L/D, that Cl would give you the angle of attack. The "just" depends on the wind you expect to deal with.įor wing incidence, this should be set so that the plane is level for the most of the time, i.e. But unless this is for a competition free flight model or one of those heavy lift models that needs to fly at the max Cl for most of the flight the slight drag isn't a deal breaker.Īs Mathew put, it, but maybe a little bit more explicit:įor designing a plane for FPV use, the best way to look at it:īest Ground Distance made without wind: best L/Dīest Ground distance with tail wind: minimum sinkīest Ground distance with head wind: "just" above the best L/D. Set the wing to that angle.īesides, for the time you do fly very slow to max out the flight time or use the least power even having the fuselage pointing upwards by some angle isn't a deal breaker. So where's this leave you? Decide on how your model will fly for MOST of the flight. The wing at a 0 degree angle (cambered wings develop lots of lift even at negative angles of attack) and the fuselage pointed at the ground due to the strong angle. But imagine this same model flying around at a sporty pace. The intent being that for a 6* angle of attack which models of this size max out at for angle before stalling the fuselage is aligned along the glide path through the air for minimum drag. Notice the high angle of incidence of the wing compared to the center line of the fuselage. Here is a free flight rubber model that uses this idea. The reason for setting the wing angle as you suggest applies where we want to set the fuselage so it offers the least frontal area during flight at the minimum sink speed. The rest of the time gliders fly either at or a touch slower than the best L/D while searching for lift or somewhat faster if pushing back upwind. RC gliders would fly at that setting more than many but even with that style of flying maybe only for half a flight and often only while riding in thermals so we maximize the altitude gain rate. And for some a Cl point or two shy of the stall speed.īut other than free flight duration models and perhaps the heavy load carry design powered RC models there's pretty well no models that spend significant time at this minimum sink or minimum energy flight mode. For most airfoils used on models this occurs at just shy of the stall speed. The flight speed and trim setting that produces the minimum sink speed in a glider is also the same setting that will require the minimum power to fly level for a powered model. On the other hand best L/D is the setting on a glider that covers the most ground for a given amount of altitude used up. First off the speeds and Cl values for minimum sink and best L/D are different.
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