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: