# Why induced drag shows up as reduced pressure downstream of the wing?

Quote from book (Understanding Aerodynamics: Arguing from the Real Physics, Doug Mclean):

"When we look at the force/momentum balance, the induced drag shows up in the flowfield primarily as reduced pressure downstream of the wing.."

Which pressure is reduced downstream of the wing, dynamic pressure (measured by aero-rakes)?

Downward flow around the wing cause induced drag, so higher the speed of that flow, induced drag is higher and more reduction in dynamic pressure behind the wing?

• The quote does not appear to be symbiont within the text I reviewed. On what page does this quote appear? Commented Dec 10, 2023 at 3:34

In a wind tunnel, drag shows up as a reduction of the speed (momentum to be precise) of the airstream behind the body.

The drag that normally appears in any plot of the aerodynamic coefficients, is determined exactly by measuring this speed reduction, which is termed momentum deficiency. This momentum deficiency is what the engine has to supply to keep the aircraft flying.

Which pressure is reduced downstream of the wing, dynamic pressure (measured by aero-rakes)?

As you have correctly guessed, the momentum deficiency is measured by aero-rakes behind the body. Integrating the difference in the total pressure from the aero-rakes with respect to the total pressure measured in front of the body, the aerodynamic drag can be calculated.

It should be clear now that, in a wind tunnel, only the total aerodynamic drag can be measured and not each and every component making it up: parasite drag, induced drag, viscous drag, ... you name it. So, induced drag cannot be really measured, it can only be estimated mathematically by means of its definition.

So higher the speed of that flow, induced drag is higher?

Induced drag is defined as:

$$D_i = \frac{L²}{½ \rho V² \pi e b²}$$

All the rest being the same, a higher speed $$V$$ gives a lower induced drag.