We are now going to explore some details of the flow around an airfoil. We will be using the input sliders and boxes located at the lower left and will be watching the Airfoil View window and the Plotter View window. We will also be using the probe on the Plotter Control Panel to look at the flowfield. To start, let's set some initial input conditions (you can also click the Reset button):
Airspeed = 100 mph
Altitude = 0 feet
Angle = 0 degrees
Thickness = 0.5
Camber = 0
Area = 1 sq ft
This gives a symmetrical airfoil pointing into the wind and generating no lift.
In the Airfoil View Panel, the flow goes from left to right. The airfoil shape is colored white. Over the top of the airfoil, the streamlines are colored green, while under the bottom they are colored yellow. (There is no significance to the length of these lines--they are "chopped" just to let you know which way the flow is moving, and they move a little faster at higher speeds for a visual cue.) The white line that hits the nose of the airfoil and leaves the back is called the "stagnation" streamline because this comes to rest at the leading and trailing edges. While the stagnation line appears to flow through the airfoil, the front stagnation line actually splits at the airfoil leading edge, flows around the surface top and bottom, and rejoins at the trailing edge. Streamlines have several special properties. The velocity of the flow is always along a streamline, which means that no flow can cross a streamline. Between any two streamlines, the amount of flow is a constant since no flow can cross the line. Along a streamline we can use Bernoulli's equation to relate pressure and velocity. To refresh your memory:
Now, let's set up the plotter window. Let's first push the "Speed at Surface" button on the Plotter Control Panel. All of the additional buttons which we used before will disappear and the plot in the view window will be a curved line. On the lower axis you will note the words "Front" and "Rear" which gives the location on the airfoil. On the vertical axis will be the value of the Speed.
The green line which appears here corresponds to the "green" upper part of the flowfield in the view window, while the yellow line corresponds to the "yellow" lower part. For this symmetric airfoil, the velocity is the same over the top and bottom and the plots of velocity fall along the same line. There are some interesting things to note in this plot.
To verify this result, let's turn on the probe by clicking the "Activate Probe" button on the Probe Control Panel. On the View panel, you will see a red dot connected by a white line to the top of the panel. This gives the location, relative to the airfoil, of our probe. You can move this probe through the flow field by using the sliders on the Probe panel--up/down and left/right. As you move the slider, the probe location will change, and the speed readout will change on the probe panel.
This gives the value of the selected flow variable at the location of the probe. By default the selection is the speed, but you can change it to measure the pressure by clicking once anywhere on readout. Make sure, for now, that the readout says " Speed." Move the horizontal slider as far to the left as possible. This gives the speed far upstream.
WHAT IS THE UPSTREAM AIRSPEED?
Now move the probe horizontal slider near the center. Do you see the probe in the Airfoil View window?
DID THE SPEED INCREASE OR DECREASE?
Slide the probe from the left part of the view window to the right using the horizontal slider. Notice what happens to the Probe Speed readout.
WHAT HAPPENS TO SPEED?
You should see the speed initially decrease, then increase, as you move over the thickest part of the airfoil, then slowly decrease as you head to the back of the airfoil. If the slider is pushed all the way to the right, the speed should return to the freestream value of 100 mph. This same variation in speed is seen on the plotter window. From the leading edge, the speed increases from zero to a maximum near the thickest part of the airfoil, then decreases to near the trailing edge. Now, let's change the angle of attack to 10 degrees, generating a little lift.
WHAT HAPPENS IN THE PLOTTER WINDOW?
The green and yellow lines now split. The speed over the top of the airfoil is far different from that under the bottom. The zero speed point is no longer at the very front of the airfoil, but moves downstream a short distance. The same feature can be seen in the view window as the front stagnation line hits farther back on the airfoil. The plotter window shows us that the speed over the top (green) is higher than that under the bottom. The maximum speed now approaches 200 mph over the top, which you can verify using the probe. On the upper surface, the speed accelerates from the leading edge to the maximum, then slowly decreases as you move to the rear. On the bottom (yellow) surface, the flow stagnates near the front, then gradually speeds up toward the rear. If we vary the airspeed on the Input Panel, we see the levels of speed change in the plotter; but the general features remain the same. Reset the airspeed to 100 mph, then click the "Surface Pressure" button on the Plotter Control Panel.
The plotter now displays the pressure over the airfoil surfaces, green for the top and yellow for the bottom.
You should see two distinct lines.
Pressure is a force per area. Since the area of the top and bottom surfaces are the same, the forces on the top and bottom depend on the pressure.
WHICH SURFACE HAS THE HIGHER FORCE?
Since the forces are not equal, there is a net difference in force pushing up from the bottom. This net force is the Lift of the airfoil.
If you set the Angle back to 0 on the input panel, you will notice that the green and yellow pressure lines fall exactly on each other.
--> No difference in pressure --> No Lift.
If you put in a negative value for Angle of attack, you will see the green upper surface pressure is greater than the yellow lower surface pressure. We then have negative lift--a net force pushing down from the upper surface.
In general, the greater the difference between the lines of the surface pressure plot, the greater the magnitude of the lift force. And the force will be in the direction of the yellow line. (If it's on the top, we have lift--if it's on the bottom we have negative lift). Once you become familiar with the surface pressure plots, you can explain most of the effects on aircraft lift which we previously encountered! For example, set:
Airspeed = 100 mph
Altitude = 0 feet
Angle = 10 degrees
Thickness = 0.5
Camber = 0
Area =
1 sq ft
Now change Airspeed to 250.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
Now change the Altitude to 30,000 feet.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
Now increase the Area to 100 square feet.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
Kind of a trick question!!
The amount of lift equals the difference in pressure times the
area. While the
difference in pressure has stayed the same--the
area increased, so the lift force
increases.
Now set the Thickness to 0.25.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
Finally, set:
Airspeed = 250mph
Altitude = 0 feet
Angle = 0 degrees
Thickness = 0.5
Camber = 0.2
Area = 10 sq ft
WHAT HAPPENS TO THE PRESSURE PLOT?
Change the Thickness to 0.8.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
Now for some real fun! Set the Angle of attack = -2.
WHAT HAPPENS TO THE PRESSURE PLOT AND THE LIFT?
So what have we learned?
You now know enough to be dangerous!
You can use this program to design and study many different airfoils.
Go
on to the next lesson: The Lift
Coefficient
Return to the FoilSim
Lessons Page