# Stability and Control of an aircraft [duplicate]

Why is the pitching moment coefficient of an airplane said to be constant about the aerodynamic centre of an aerofoil

• You should read how it flies? and search in this website for other questions about pitch stability. – Manu H Sep 22 '19 at 13:14
• You should edit the title so that it is more specific (less generic) so that it is easier to browse questions about a specific subject (e.g. pitch stability) only reading the titles. – Manu H Sep 22 '19 at 13:16
• Alright thank you so much – caleb olojo Sep 22 '19 at 13:18
• I don't understand what you are after. On one hand you ask about the "moment coefficeint an airplane", but on the other hand you talk about "an aerofoil". An aircraft is a 3D structure which has typically several lifting surfaces, and aerofoil is a 2D cross section of wing. Both have very different aerodynamic behaviour and their aerodynamic centres can't be compared. Please rephase your question to be more clear. – DeltaLima Sep 23 '19 at 10:34

Why is the pitching moment coefficient of an airplane said to be constant about the aerodynamic centre of an aerofoil?

Because the phrase "aerodynamic center" is defined as meaning "the point about which the pitching moment coefficient is constant."

That's it, that's the entire answer. If I've misunderstood the question, please let me know.

Consider this equation:

$$C_m = C_{m0} + \xi C_L$$

where $$C_{m0}$$ is $$C_m$$ at no lift, and $$C_m$$ is the pitching moment.

At the aerodynamic centre $$\xi = 0$$ and $$C_L = 0$$, $$C_m = C_{m0}$$ which is a constant.

• Hi Shishir Maurya. I edited the equation into the answer, as opposed to having it included as an image. Please check to make sure I didn't introduce any errors while doing so. You can find a Mathjax tutorial over here, and this tool is handy for fancy symbols. – user Sep 22 '19 at 19:34
• Thanks, I am new here. I will surely learn the Mathjax and you edited correctly. – Shishir Maurya Sep 29 '19 at 14:45