Drag Polar

[sligo565]

Hi Guys,

Could anyone tell me how to calculate the Drag Polar?

Is it:

1/(pi x A x e)

Where A = Aspect Ratio
e = Oswald Efficiency Factor

[ciurpita]

is this the L/D plot of an aircraft obtained from the total drag of the aircraft (airfoils and body, parasitic and induced) versus lift and hence airspeed, or simply the Cl vs Cd curve for a particular airfoil? doesn't the former quite a lot of information about the aircraft and some experimental/guessimated data, and the later require wind tunnel data?

[mraero]

sligo565,

A drag polar generally refers to a plot of aircraft lift (vertical axis) versus drag (horizontal axis). The equation that you have written is part of the induced drag (drag due to lift), but it is incomplete. The induced drag is your equation, typically called k, times the lift coefficient squared.

To compute the full drag polar, you need need to take into account all components of drag, primarily parasitic drag (form drag plus skin friction and interference) plus induced drag (drag due to lift), as well as have a good understanding of the lift curve of the design, which takes into account the 3d effects of a finite span on the slope of the lift curve, along with other factors.

There are some decent websites available that can help you develop a first order model of your design, which will estimate the items above based on the geometry of your design.

Hope that helps.

Rich

[sligo565]

Hey,

I used the software JAVAFOIL to produce the following graph and numbers..do i just measure off the graph at the area of zero Lift? It's just that its giving me a very high number of 0.07....can i just use the equation:

CD = CDmin + (CL - CLmindrag)^2/pi e A

To find my Drag Coefficient instead of using this such high number of CDo?!

[sligo565]

Here is also a drawing of my conceptual design....its design is much like a sailplane....does anyone know an estimation of CDo of a sailplane that i could use?

[mraero]

Hey,

I used the software JAVAFOIL to produce the following graph and numbers..do i just measure off the graph at the area of zero Lift? It's just that its giving me a very high number of 0.07....can i just use the equation:

CD = CDmin + (CL - CLmindrag)^2/pi e A

To find my Drag Coefficient instead of using this such high number of CDo?!

Yes that is the equation that you want to use. When the lift curve runs through zero lift at zero alpha, the equation reduces to the common parabolic form CD = CD0 + kCL^2. However, when the lift curve does not run through zero lift at zero alpha, which is almost always the case, you will use the full equation as you have written it. One note is that you may find the terms CD_min and CD0 interchanged, with both intended to mean minimum CD on the polar, but the CD_min terminology is the correct one.

Rich