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This picture of the lift curve slopes for four-digit NACA airfoils and a flat plate is from another question on this topic. It shows that at the same positive AoA, a flat plate generates less lift compared to the NACA airfoils.

enter image description here

An airfoil and a flat plate both creates a low pressure region above; for an airfoil it is suction-induced, and for a flat plate it is due to a separation bubble. Since the (high) pressure distribution below these two are similar, from an arbitrary guess, the lift force generated should at least be the same for both. But why does a flat plate create less lift than an airfoil at the same AoA?

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    $\begingroup$ Any "airfoil" that performs no better than a flat plate wouldn't be useful or interesting. Only those shapes that do better than the simple flat plate get any consideration or use as airfoils. So is your question why such airfoils can exist? As in Bernoulli and all the "what produces lift" discussions? Or is there a different line of inquiry here? $\endgroup$
    – Ralph J
    Aug 21, 2023 at 11:44
  • $\begingroup$ @RalphJ My question is why such airfoils exist. I'm not asking about how they produce lift, as that has been mentioned in my question, but why are those airfoils better, and how they produce MORE lift. $\endgroup$
    – Frank
    Aug 21, 2023 at 12:19
  • $\begingroup$ In the photo in the linked question you can see that the incoming air flows up and over the turbulent area, with an aerofoil without separation at the same angle the air above the wing is directed downwards much more efficiently therefore generating more lift $\endgroup$ Aug 21, 2023 at 12:20

2 Answers 2

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It shows that at the same positive AoA, a flat plate generates less lift compared to the NACA airfoils. From an arbitrary guess, the lift force generated should at least be the same for both

Your guess is right, a flat plate develops exactly the same lift as any other airfoil (at least until stall begins). So, why that difference in the plot?

Convention

The AoA is measured in respect to the geometrical line connecting the leading edge with the trailing edge of the airfoil. By an aerodynamic point of view this line has no special meaning. So why do we use anyway this line to plot the aerodynamic coefficients? Because this line is easy to trace and unmistakable and therefore makes things easy to compare (actually this geometric line does posses an aerodynamic meaning but only for symmetric airfoils since it coincides with the line of zero lift for obvious reason of symmetry).

Now, the NACA airfoils of the 44xy series considered in your picture is a cambered one i.e. an airfoil generating lift already at negative AoA (some -5° in that plot - but remember that this -5° is just a geometrical convention and has no aerodynamic meaning). Anyway a flat plate is not cambered rather symmetrical by definition and therefore its lift coefficient is always 0 at 0° AoA: this comparison is unfair! Indeed, if you shifted the plot of the 4412 to the right so that it also starts by 0 lift at 0°, then you would get an almost perfect overlap with the coefficient of the flat plate.

Or... we can simply make a fair comparison from the beginning and use another symmetrical airfoil like any of the NACA 00xy series, for example the ubiquitous NACA 0012 which has the following coefficients:

NACA 0012 coefficients

Here (blue line) you can see that it develops for example a $C_l$ of 0.5 at 5°, just like the flat plate in your plot. And, being symmetrical, it obviously also goes through the point of 0 lift at 0°, again just like the flat plate.

Ok, but if a flat plate generates the same lift as any other symmetrical airfoil why the horizontal tailplane of a jetliner is built around a NACA 0012 and not a simple flat plate? Because of the stall characteristics: a flat plate begins to stall already at around 5 to 10° while the NACA 0012 at 22° and that's the most important improvement introduced by an airfoil in respect to a simple flat plate.

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  • $\begingroup$ For a symmetrical airfoil, is the pressure force vector always perpendicular to chord, like the flat plate? If so, when we look at this from the perspective of pressure force, how can the symmetrical airfoil delay stall while a flat plate cannot? $\endgroup$
    – Frank
    Aug 22, 2023 at 7:29
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    $\begingroup$ "how can the symmetrical airfoil delay stall while a flat plate cannot?" The answer is: smoother recovery pressure on the rear part of the airfoil with a consequent more stable boundary layer. $\endgroup$
    – sophit
    Aug 22, 2023 at 8:48
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Why different shapes react differently to a fluid flowing past them is ultimately subjective, trying to fit one abstract model to several of these shapes.
How they react is purely a matter of measurement.

Values of x for "because x" include post-stall flow separation, the Coanda effect, Reynolds numbers, (non-)laminar flow, the oversimplifications in children's textbooks, etc, etc.

In this case, where the question has veered to "pre-stall, why does camber increase lift," the intuitive reason is that camber deflects more air downwards.

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    $\begingroup$ Sure, if C_L = C_L. The why-because generalization here is that, all else being equal, more camber means more downwash, which means more lift. $\endgroup$ Aug 21, 2023 at 20:08

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