I am relatively new to aerodynamics and am curious to know what difference it makes on an aircraft - lift, drag, weight etc - which of the two airfoils is used on the wing.

  • $\begingroup$ i think this will get dupe'd but since someone asked, i'll answer $\endgroup$ – Abdullah Jun 22 at 8:45
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    $\begingroup$ related (if not a dupe): How do paper airplanes create lift if their wings are flat?. Both questions have different approaches (and I think both should be kept on this website) but leads to the same topic: the difference between a flat airfoil and a more usual shape. $\endgroup$ – Manu H Jun 22 at 16:58
  • $\begingroup$ Re "What is the difference between a flat plate shaped airfoil and a droplet shaped airfoil?" -- one difference is that one is shaped like a flat plate and the other is shaped like a droplet. If you are interested in asking about other differences (like the lift coefficient acheived at a given angle-of-attack?) you might consider editing the title to reflect that. Also please include your question in the body of the post, not in the title only. $\endgroup$ – quiet flyer Jun 23 at 15:20
  • $\begingroup$ I get the impression he's asking "why do we not use flat plate wings" $\endgroup$ – Abdullah Jun 23 at 15:37
  • $\begingroup$ What was the downvote for? $\endgroup$ – Abdullah Jun 23 at 16:14

the lift generation mechanisms of these two wing designs are quite different.

Insects use completely flat wings to great advantage. They generate both thrust and lift by shedding vortices in a flight regime in which the wing is operated right at the threshold of flow separation. Dragonflies operate there, and can accelerate from zero to 10 meters/second in one second or less, fly inverted, and execute unbanked turns at full speed.

Cambered wings are used by birds and airplanes and generate lift using (your choice) models based on vorticity and rotational flow, the bernoulli effect, or van Karman vortex shedding.

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    $\begingroup$ Interesting aspect: You compare both shapes in the respective domain where they are the best choice. Flat plates for viscosity-dominated flow and blunt airfoils for inertial-dominated flow. $\endgroup$ – Peter Kämpf Jun 23 at 4:58
  • $\begingroup$ @PeterKämpf yes yes, thanks for that insight! Most valiant! $\endgroup$ – niels nielsen Jun 23 at 5:04

The droplet shaped airfoil has multiple advantages.

  • It provides space for storing stiffening structures. The flat plate foil is very thin, so would need to be quite heavy to maintain stiffness, as stiffness is proportional to the square of the thickness.
  • It provides space for storing fuel. This is important to reduce weight on modern cantilever aircraft. Distributing weight along the wings reduces the bending caused by the heavy fuselage during flight. On the ground, the extra weight (and bending) from the fuel is little, since the fuel is not as heavy as the fuselage!
  • It provides good performance across a range of angles of attack. (Angle of attack is the difference between the direction of motion and the direction you're facing.) The round leading edge allows air to smoothly flow over it when this angle is high, for example during climb or slow flight.
  • It gives gentle stalls. (A stall is when increasing the angle of attack does not increase lift force anymore, and instead decreases it.) enter image description here

Flat plate wings will stall suddenly and violently as soon as the angle of attack needed to stall is reached, as the flow separates across the whole wing at once. (Flow separation is when a region of air moving in reverse develops on the wing, causing the airflow on the wing to flow around this region, which creates less lift and more drag.)enter image description here

But the greater curvature on the top of the teardrop foil causes flow separation to happen at the rear of the wing first, as angle of attack continues to approach the value needed for stall, warning the pilot that any further increase will stall the plane.

But for supersonic applications, the teardrop foil does not work very well. At these speeds, the extra drag from the bluntness begins to outweigh any other benefit. So, an thin, sharp foil, not quite a flat plate, but pretty close, is used.

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    $\begingroup$ actually, stiffness is proportional to the square of thickness. Depth is akin to chord and a flat plate will still be floppy if chord is increased. $\endgroup$ – Peter Kämpf Jun 23 at 4:54
  • $\begingroup$ @PeterKämpf Thanks. Didn't want to get too mathematical so amended the wording to stress that I was talking about the thickness of the wing, not chord. $\endgroup$ – Abdullah Jun 23 at 7:53
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    $\begingroup$ This is Stack Exchange, so we should assume that readers know things like time complexity and big O notation. And we know that O(n) is quite different from O(n²). In other words, it is not too mathematical for the audience here. $\endgroup$ – Peter Kämpf Jun 23 at 8:05
  • $\begingroup$ @PeterKämpf Kay $\endgroup$ – Abdullah Jun 23 at 9:06
  • $\begingroup$ Quite true, @PeterKämpf, but there are a lot of people on Aviation asking very basic questions with little to no knowledge of aviation. (For example, me, when I started here, and I still struggle with wing thickness & chord, though I haven't studied...) $\endgroup$ – FreeMan Jun 23 at 14:33

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