Different wing geometry in different flow conditions will drive different choices in airfoil.Īll of these differences are because different aircraft are designed for different kinds of flying. The wingspan and sweep angle will also affect the performance of the wing. The chord length will change the Reynolds number that the airfoil experiences. The details of how the wing is laid out will also affect performance. Jet aircraft and propeller aircraft mostly operate in different speed regimes as well. A Cessna 172 and a DeHavilland Dash 8 are both operating at subsonic speeds, but in very different speed and density regimes. This only scratched the surface of the issue, but I hope I could get the general idea across: Even when optimum L/D is the design goal, the best airfoil shape differs with the other parameters of the aircraft.Īirfoil performance will depend on air speed and density. The root will benefit from a thicker airfoil than the outer wing, where a wide angle of attack range from aileron deflections and roll speeds needs to be tolerated.Īnd then there are designs wich care less about L/D but need to have ideal stalling characteristics and no camber: Aerobatic aircraft have quite different airfoils from all the others, and with good reason. The short and stubby wing will need a different airfoil than the long and sleek one.Īlso, ideally you should vary the airfoil within one wing, depending where along the span you look. The smaller value is for flying fast and high g loads, like in aerobatics, and the higher value is typical for high performance gliders.
Then consider aspect ratio: Depending on the purpose, the optimum aspect ratio of subsonic airplanes is anywhere between 4.5 and 50. Heavier aircraft tend to have higher wing loadings and need to add high-lift devices, which put their own demands on airfoil shape. This translates into a very different range of speeds, so the variation in Reynolds numbers is much wider than the size alone implies. Next, airplanes come with a big variety of wing loadings, from 40 kg/m² in gliders up to the nearly 1200 kg/m² of the Rockwell B-1B. Very fast subsonic designs have to deal with pockets of supersonic flow which put very different demands on the airfoil's shape. So there is a different airfoil for every design Reynolds number, even at the same lift coefficient. A smaller Reynolds number allows for more laminar flow but demands a less steep pressure rise in order to avoid early separation. Smaller aircraft flying at the same speed, air temperature and altitude than a larger aircraft have a smaller Reynolds number which characterises the boundary layer flow. Instead it uses a NACA 0014-1.10 at the root, transitioning to a NACA 0012-1.10 at the tip, meaning the designers had to compromise between two airfoils even within the same wing. A glider airfoil would be terrible because of the need for thickness to accommodate a spar that can carry the engines without breaking under a bad gust. Now take another subsonic aircraft with an emphasis on endurance: the maritime patrol aircraft, the best known example here is probably the P-3 Orion.
#Supersonic airfoil full
This category is populated by gliders, and is indeed full of highly optimized designs ( $C_L / C_d \approx 50$ is not unheard of) with little differences, mostly due to structural considerations. High efficiency is important for aircraft with a mission that emphasizes endurance. Now, to specifically address your Lift-to-Drag requirement: that is simply not a good way to decide "the best airfoil for subsonic flight". Again, this modification is rendered into pointless weight and cost if used outside this particular scenario. A simpler NACA airfoil would be more cost-effective for applications that do not require high-subsonic cruise efficiency as one of their main points.Īdditionally, there are more specific situations, like flying wings, that usually use reflex camber to deal with the lack of horizontal stabilizer. They exhibit good qualities for their typical flight regime, albeit at substantial development and manufacture cost. To follow with this example, have a look at supercritical airfoils. For the same reason why we do not have one single type of aircraft flying all commercial and military missions worldwide: flight has many variables, and there is no single optimum solution.įirst off, you mention transonic flight in your question, which usually involves sonic flow at over at least part of the airfoil, which significantly alters the desired properties of the airfoil.
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