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Old Jan 10th 2019, 08:11 PM   #1
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Why does Cd increase then decrease after M1 but drag keeps increasing?

When I was looking a project that included Mach vs Cd and Mach vs drag Force for a grid fin at different altitudes with different mach numbers, I saw that as Cd approached Mach 1, it increased but after passing Mach 1, it started to decrease but the drag force kept on increasing.

I initially thought that as it approaches trans sonic speed, shockwaves start to form inside the lattice (choked flow), which starts to create a little more drag. As it nears M1, a bow shock will form which would start diverting air away from the lattice and at the same time would create significant drag. A normal shock wave and oblique shockwave should form at this time, further increasing drag as speed increases and the bow shock eventually disappears. Yet as the fins start to go past M1, the oblique angle increases which would be associated with lower temp/pressure drops which would explain why the Cd decreases ? Wouldn't this contradict the Mach vs Force graph where the drag force keeps increasing?

This feels wrong but I did the best I could. I'd love to hear the flaws in this attempt.
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Old Jan 11th 2019, 03:42 AM   #2
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The drag coefficient CD is the Drag Force divided by the dynamic pressure q (and a representative area).
The Dynamic Pressure is proportional to the velocity squared.

At low speeds, while the Force will be (approximately) proportional to the velocity squared, the coefficient will be almost constant.

As the speed approaches the speed of sound, compressibility effects mean that the Force is no longer proportional to the velocity squared, it increases even faster.
This means that the coefficient is no longer constant with respect to speed, it starts rising.
(this is called the drag rise Mach number).

Once you get past Mach 1, the relationship between Force and speed again changes,
such that (although the Force continues to rise, the Drag Coefficient starts to drop.
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