Parasite Drag Tool

[chuy]

I am currently using the Parasite Drag tool for an estimate of CDo. I noticed that as I increase the free stream velocity I get a lower CDo.  I would expect the opposite, as an increase in flight speed would cause an increase in parasite drag.  Do you have any suggestions?  

I have attached two pictures showing an example of using two Vinf values. 

Thanks.

-Chuy

[Rob McDonald]

You are confusing a few different things.

First, CDo is a drag coefficient.  To get a drag force, you need to multiply by V^2 from the dynamic pressure.  So, a slight drop in CDo will still result in the behavior you expect.

Second, the chart you show is for a trimmed airplane -- lift equal to weight.  So, the induced drag drops off because CL^2 is dropping off as the lift coefficient required for lift equal to weight decreases with increasing velocity.

Finally, the CDo change with velocity is really a Reynolds number effect.  Your Reynolds number goes up, so cf goes down, so CDo goes down.

Rob

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[chuy]

That makes sense Rob. For some reason I was thinking of CDo as a drag force and not a drag coefficient. 

Thanks for the clarification.

-Chuy

[chuy]

Also, when using the Parasite Drag tool - do you have to remove any control sub-surfaces in the model?  I noticed that having the sub-surfaces (ailerons, rudder, elevator) gave a different CDo value (less accurate) than when removing them.

Thanks.