Airfoil Question

bc2209

bc2209

Well-Known Member
I'm looking for help understanding different wing designs.

I'm trying to understand how aerobatic airplanes, or symmetrical airfoils, create lift.

I understand to a certain degree that there is lower pressure on top and higher on the bottom of a common everyday GA airplane wing, creating some lift.

Is this the same for symmetrical airfoil's? Or does it have more to do with for every action there is a reaction (even more thrust to compensate for lack of lower pressure/higher pressures)?

Confused
 
Symmetric airfoils at zero angle of attack have equal pressure distribution on top as on bottom of the wing. Its not until you go to a non zero angle of attack that the distributions become different.

On most airfoils at zero AoA they still create lift due to the camber of the airfoil.
 
Supercritical airfoils are just shaped in a way to delay the onset of wave drag caused by shockwaves formed on the upper side of the wing.
 
Maximillian_Jenius said:
Besides DC-9 wings and all subsequent variants. What are some other examples of commercial super critical wings?
Click to expand...
Most aircraft that operate above (about) M0.70 require some sort of super critical airfoil.

Citation X and the G650 are probably the most commonly referenced.

Just a nomenclature thing, the airfoil is actually what is supercritical.
 
I assume your studying for the CFI. From the studying I have done, Asymmetrical wings produce good lift but are not ideal for high speed cruise. They have good stall characteristics. The Symmetrical wing has pretty nasty stall as it stalls all together rather rather than from root. It is also better
for high speed cruise. The camber on the upper surface of the airfoil is just one way that lift is created. The main reason in my opinion, is the relative wind created by the forward motion of the airplane. The Wings are just giant Air deflectors.(newtons third law). If it's not in the FAA books I wouldn't go crazy about it.
 
To the OP--I'll try... :)

A symmetrical airfoil creates lift in the same way that a non-symmetrical airfoil does. It's (usually) less efficient than a non-symmetrical airfoil, but with the obvious trade-off of being much less *in*-efficient than a non-symmetrical airfoil when generating negative lift (inverted, etc.).

Pressure distribution is a correct way to describe lift, but the old "particle A traveling a greater distance over the upper surface to meet particle B at the trailing edge and thus lowering pressure" is not really accurate or helpful.

A better way to think of it is that a wing creates *circulation* as it moves through the air. That is, the airflow around a wing can be described as the sum of two flows: A translational flow from fore to aft, and a circulating flow moving aft above the wing and forward below the wing (makes sense if you think of upwash in front of the wing, higher velocity above, lower velocity below, and downwash behind the wing, right?). The circulation results in the pressure differential and therefore the lift. It's not necessary even to have an "airfoil" to create circulation--it can also be done with rotation--that's why golf balls slice and baseballs curve. (If you want to see another cool demo for this, take a plain Bic pen, take the cap off the end and pull the ink stick out of it so you have a straight, hollow tube and scuff it lightly with sandpaper. Set it on a smooth table or countertop, and press down hard with a couple of fingers from each hand, then roll your fingers back so that the pen shoots forward with as much forward velocity and backspin as you can get. Watch the pen fly).

Airfoils are carefully designed to achieve this circulation by their shape and angle of attack (rather than by rotation) while creating as little drag as possible. They are most efficient at the speed and angle of attack for which they were designed, but most of them can create some lift even when quite far from their design point--even when inverted. Symmetrical airfoils are just one case of a not-the-most-efficient-possible airfoil--they still create circulation by virtue of angle of attack and the viscous nature of air, the same as a non-symmetrical airfoil.

Does that help at all?

jspeed87, I would respectfully disagree somewhat with your description of stall characteristics. Some symmetrical airfoils *can* have ugly stall characteristics, as you suggested, but other symmetrical airfoils can actually be quite benign in the stall. Much more stall behavior depends on the wing planform and twist, and airfoil thickness and "bluntness" than whether the airfoil is symmetrical or not. Also, a non-symmetrical airfoil will be most efficient for cruise, as it will be able to create the required lift at the lowest drag.

I recommend this description (starting at 4:20). :)
 
bc2209 said:
I'm looking for help understanding different wing designs.

I'm trying to understand how aerobatic airplanes, or symmetrical airfoils, create lift.

I understand to a certain degree that there is lower pressure on top and higher on the bottom of a common everyday GA airplane wing, creating some lift.

Is this the same for symmetrical airfoil's? Or does it have more to do with for every action there is a reaction (even more thrust to compensate for lack of lower pressure/higher pressures)?

Confused
Click to expand...

It's not a either/or situation between Bernoulli and Newton. I'm not sure where that idea started, but all wings that are producing lift push air down. In level steady-state flight they push down as much force as they are supporting (every action has an equal and opposite reaction). If wings didn't push down on the air, planes wouldn't fly. It's not as if aircraft are one special category of things not beholden to the laws of physics.

HOW they push down on the air is where Bernoulli and fluid dynamics come into play.
 
High_Alpha said:
To the OP--I'll try... :)


A better way to think of it is that a wing creates *circulation* as it moves through the air. That is, the airflow around a wing can be described as the sum of two flows: A translational flow from fore to aft, and a circulating flow moving aft above the wing and forward below the wing (makes sense if you think of upwash in front of the wing, higher velocity above, lower velocity below, and downwash behind the wing, right?). The circulation results in the pressure differential and therefore the lift. It's not necessary even to have an "airfoil" to create circulation--it can also be done with rotation--that's why golf balls slice and baseballs curve. (If you want to see another cool demo for this, take a plain Bic pen, take the cap off the end and pull the ink stick out of it so you have a straight, hollow tube and scuff it lightly with sandpaper. Set it on a smooth table or countertop, and press down hard with a couple of fingers from each hand, then roll your fingers back so that the pen shoots forward with as much forward velocity and backspin as you can get. Watch the pen fly).

Airfoils are carefully designed to achieve this circulation by their shape and angle of attack (rather than by rotation) while creating as little drag as possible. They are most efficient at the speed and angle of attack for which they were designed, but most of them can create some lift even when quite far from their design point--even when inverted. Symmetrical airfoils are just one case of a not-the-most-efficient-possible airfoil--they still create circulation by virtue of angle of attack and the viscous nature of air, the same as a non-symmetrical airfoil.
Click to expand...

This is a plain jane description of the Kutta-Joukowski Theorem. My Aero Eng (nerd) side got a little too excited when I read this.
 
CRJ series have supercritical wings as well. The simplistic way one of our instructors described it to us always kind of stuck with me. You create lift on a "normal" airfoil by accelerating the air over the top of the wing which causes a low pressure area with regards to the underside of the wing.....As you get above about mach .70 this can cause some supersonic flow over portions of the wing (critical mach number) and all of it's associated drag/tuck issues. A supercritical airfoil creates the same pressure differential only it doesn't do it by accelerating air over the top of the wing camber (supercritical wings are relatively flat on top). Instead it does it by decelerating the airflow across the underside of the wing due to its concave shape.This causes lower velocity over the top of the wing (less drag) and smaller shock wave which occurs further aft.

Now I'm sure this is a very "grade school" explanation and somebody will surely quote something from Naval Aviators but it helped me wrap my head around the concept as a new hire a bit.
 
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