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peterdsharpe / AeroSandbox

Aircraft design optimization made fast through computational graph transformations (e.g., automatic differentiation). Composable analysis tools for aerodynamics, propulsion, structures, trajectory design, and much more.
https://peterdsharpe.github.io/AeroSandbox/
MIT License
741 stars 119 forks source link

Question about Moments vs Force and which to use for machines learning #6

Closed ghost closed 5 years ago

ghost commented 5 years ago

Here is an image of the output.

https://i.redd.it/6gcmbth06re31.png

My first question what does CY mean? Also, what is the significance of CL/CDi? Finally what is the difference between Moments and Forces(could you please link an article describing the difference; I understand Forces are used for calculating total lift) and are the moments important enough to include in the input of machine learning?

peterdsharpe commented 5 years ago

Hi there! Thank you so much for your interest in the software - please consider starring/watching the repository for future updates!

To answer your questions sequentially:

  1. CY is the coefficient of sideforce (analogous to CL, the lift coefficient; or CD, the drag coefficient). Sideforce is the component of force orthogonal to both the lift and drag components (i.e. to the left/right). By convention, force to the right is positive. It is nondimensionalized in the same way as CL and CD, so you can find the dimensionalized value of the sideforce by multiplying CY by 0.5rhov^2*S (where rho is the air density, v is the freestream velocity, and S is the reference area).

  2. CL/CDi denotes the lift coefficient divided by the induced drag coefficient. (It is important to note that CDi only includes the induced component of drag and neglects the viscous component of drag. This is being implemented currently, but it's a much more difficult problem to do this both correctly and quickly). CL/CDi is a proxy for CL/CD, which is known as the aerodynamic efficiency of an airplane or wing. CL/CD is a key design metric that shows up in many aircraft performance equations (for example, the Breguet range equation or the glide ratio of an unpowered aircraft). Typically, you want to operate in a flight regime near "CL/CD_max", or the conditions where CL/CD is maximized (typically about 3 to 7 degrees angle of attack).

  3. Moments are "forces acting as a distance", also known as a torque (technically there is a distinction between the two, but I use them interchangeably as it effectively makes no difference here). Here's an article that talks about what a moment is: http://web.mit.edu/4.441/1_lectures/1_lecture5/1_lecture5.html

  4. Moments are important if you care about the stability of the airplane (i.e. the ability to keep the airplane pointed in the direction you want). This is a whole rabbit hole you can go down, and I'd recommend skipping thinking about stability if this is your first foray into aircraft design (stability and control is very important, but performance metrics such as lift and drag are even more important to understand first). If you're interested in learning more about stability and control, here's a great lecture series from MIT OpenCourseWare: https://ocw-origin.odl.mit.edu/courses/aeronautics-and-astronautics/16-333-aircraft-stability-and-control-fall-2004/lecture-notes/

Please let me know if you have any other questions!

ghost commented 5 years ago

Thank you for your response! I will definitely star this repo.

codykarcher commented 5 years ago

Issue seems addressed. Closing.