Aerodynamics mini lesson questions

[lions48]

For a High School level engineering course I was asked to create a very simple guide for solving lift and drag of an airfoil. We recently got a wind tunnel to test airfoils. Prior to this I didn't have any experience with lift and drag and would appreciate any input on the subject. I would appreciate it if you could give me feedback if anything is glaringly wrong and/or could be said better. Thank you!

 

[rcgldr]

From the PDF file: "There is a lower pressure of air on the top of the airfoil because the air takes longer to get to the same point."

This is called "equal transit theory", and it's not true. The air disturbed by a wing is displaced. Molecules that were once adjacent to each other, and then separated into flow above and below a wing remain separated.

The math part of the PDF document is correct, except that coefficient of lift and coefficient of drag are not constants, but instead somewhat dependent on the other parameters in the equation, such as wing chord length and relative velocity between wing and air. The velocity times wing chord length (times a constant) equals something called Reynolds number, which is often used as a basis for plotting the cofficient of lift and/or drag versus angle of attack (such graphs are sometmes called polars).

Although an explanation of lift and drag aren't needed for your specific assignment with the wind tunnel, here is a link to one of many articles that gives a simplified explantion of lift:

http://www.avweb.com/news/airman/183261-1.html

The more complex analysis invovles something called Navier Stokes equations, which normally can't be directly solved, but there are programs that use Navier Stokes as a basis to produce those charts called "polars" given airfoil shape, size, and velocity of the air, where the charts plot coefficient of lift and/or drag versus angle of attack (and/or lift versus drag ...).

You didn't mention if you're allowed to try several different airfoil shapes in the wind tunnel to see which gives the best results.

 

[lions48]

Wow! I was unaware of how the equal transit theory wasn't true! That changes a lot of things.

In regards to explaining lift and how it is generated I think I will put it in the lesson just for the people who are curious about it.

I plan to give them free reign when it comes to the shape of the airfoil to see what they can come up with. I hope to make it a competition of sorts, to see who can get the most lift and drag off of their airfoil.

Thank you rcgldr for your input, it has helped a tremendous amount!

 

[rcgldr]

Wow! I was unaware of how the equal transit theory wasn't true!

One example that somewhat disproves this is the unusually shaped M2-F2 and M2-F3 lifting bodies used as re-rentry prototypes. Part of the reason for the unusual shape is that as re-rentry vehicles, they need to operate at hyper-sonic speeds (or at least survive such speeds), but they also glide reasonably well.

wiki articles:

http://en.wikipedia.org/wiki/Northrop_M2-F2

http://en.wikipedia.org/wiki/Northrop_M2-F3

m2-f2 (and F104) on landing approach:

 

[asteriatic]

First of all, I would like to commend you for taking on the challenge of creating a guide for solving lift and drag of an airfoil. This is a complex topic and it requires a lot of knowledge and understanding of aerodynamics principles. It is great that you are starting to learn about it at a high school level.

To begin with, let's define what lift and drag are. Lift is the force that acts in an upward direction on an object, such as an airfoil, when it is in motion. Drag, on the other hand, is the force that acts in the opposite direction of motion and it is caused by the resistance of the air.

When it comes to airfoils, the shape and design of the airfoil greatly affect the amount of lift and drag it produces. To determine the lift and drag of an airfoil, we use a combination of theoretical calculations and experimental testing. This is where the wind tunnel comes in. The wind tunnel allows us to simulate the conditions of air flow around an airfoil and measure the corresponding lift and drag forces.

To calculate the lift and drag of an airfoil, we use two important parameters: the angle of attack and the airfoil's coefficient of lift and drag. The angle of attack is the angle between the airfoil and the direction of the incoming airflow. The coefficient of lift and drag is a dimensionless value that represents the lift and drag forces of the airfoil.

To measure these parameters, we use specialized tools such as a force balance and a pressure tap. The force balance measures the lift and drag forces acting on the airfoil, while the pressure tap measures the air pressure at different points on the airfoil's surface.

Once we have collected the necessary data, we can plot it on a graph to see the relationship between the angle of attack and the coefficient of lift and drag. This graph is known as the lift and drag polar and it is a crucial tool in designing and optimizing airfoils for different applications.

In conclusion, solving lift and drag of an airfoil requires a combination of theoretical knowledge and experimental testing. With the help of a wind tunnel and specialized tools, we can accurately determine the lift and drag forces of an airfoil. I hope this mini lesson has been helpful and I wish you all the best in your engineering course.