Coefficient of Parasitic Drag - What is the Preferred Basis of Measurement?

Question

In attempting to determine the coefficient of Parasitic (Zero Lift) Drag - the cD0 - for various aircraft, I have discovered that there appear to be several ways this number is represented: by reference to wing area, frontal area, flat plate area (which may be the same thing) and wetted area.

The standard equation for Parasitic Drag is: Parasitic Drag = cD0 X dynamic pressure X reference (wing area, etc)

If my understanding is correct, the Parasitic Drag is determined, either experimentally or by adding components together, and the person making the computation can choose the basis of measurement for the cD0.

Is there a preferred basis of measurement? (It is easiest to use wing area because that is what you use to compute lift and induced drag.)

Is there a way to determine which basis is being used or referenced? (I have found some equations that use cD0 but do not specify which basis they are using.)

If not, is there a way to "ballpark" the number to guess which one you have? For example, if I plug a cD0 into a set of equations that compute Parasitic Drag using wing area, I would expect that the cD0 should result in a sea level top speed that approximates the published sea level top speed for the aircraft. If the computed top speed is significantly lower, the cD0 I am using may be for frontal area. If the computed top speed is significantly higher, the cD0 I am using may be for wetted area.

I also read somewhere that, at Vy, the Induced Drag is 3X the Parasitic Drag. Is that true?

Answer 1

What is the Preferred Basis of Measurement?

The basis of measurement depends on the method and/or the component whose drag is being evaluated.

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As "parasitic drag" I consider here that part of drag which is not proportional to the square of $C_l$.

Parasitic drag can be simply calculated as the sum of the drag of all the components which make up the aircraft:

$C_{D_{parasitic}}= C_{D_{fuselage}}+C_{D_{wing}}+C_{D_{landing gear}}+C_{D_{nacelle}}+…$

Each term of $C_{D_{parasitic}}$ is normally estimated:

• either using known standard historical values from similar design; these values are normally scaled via the basic dimension of the component; for example Raymer¹ gives a $C_d$ of some 0.25 for a wheel and tyre assembly, which has to be multiplied by its frontal area;
• or via simplified equations which take as input the basic parameters of the component, like length and diameter for fuselage or thickness and sweep angle for wing; for example again Raymer gives for the fuselage a quite simple equation which depends on its length, diameter, external (wet) surface, Reynolds and Mach numbers.

Some term depends also on $C_l$ (landing gear for example).

Since $C_{D_{parasitic}}$ always refers to wing surface, all of its coefficients have to be divided by $S_{wing}$ before being summed up.

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¹ Daniel P. Raymer, Aircraft Design: A Conceptual Approach, AIAA Inc.