Drag effects of fin fillets

[jmattingly13]

Intuitively, the non-filleted case would have a higher drag coefficient due to the odd interaction of boundary layers on both the fin and airframe. As for your math, I'm not sure if I agree with some of your numbers for drag coefficients, but nevertheless, it doesn't make much sense to claim a drag reduction of 20% when the component of reduced drag makes up only a small portion of the overall drag. (I also disagree with the concept of linearly summing the components of drag, but that's a different conversation.) The overall reduction in the drag coefficient (and overall drag) of the rocket would be negligible if your math holds, and thus the claim could be made that while drag is reduced, it is not reduced in any meaningful way.

 

[enderw88]

Intuitively, the non-filleted case would have a higher drag coefficient due to the odd interaction of boundary layers on both the fin and airframe. As for your math, I'm not sure if I agree with some of your numbers for drag coefficients, but nevertheless, it doesn't make much sense to claim a drag reduction of 20% when the component of reduced drag makes up only a small portion of the overall drag. (I also disagree with the concept of linearly summing the components of drag, but that's a different conversation.) The overall reduction in the drag coefficient (and overall drag) of the rocket would be negligible if your math holds, and thus the claim could be made that while drag is reduced, it is not reduced in any meaningful way.

I claimed a 20% reduction of the INTERFERENCE drag, not the drag overall. From an induced drag coefficient of 0.005 to 0.004 (that is where the 0.001 came from). You are correct tht the components of drag do not add linearly. That is where the CDTotal = CD1 + CD2 +CDInterference comes in.

I also agree that for the rockets we are talking about the effect of the fillets on drag would be very difficult to measure. Using them as structural reinforcement is required and they will not noticeably affect drag unless they are extremely thick. (a dispute of that statement in another thread started this one.)

 

[1tree]

Now I will readily admit to not having studied the subject, and having no desire to dig into the math. But in chapter 10 of Handbook of Model Rocketry, Stine states that a filet will reduce drag and gives optimum radius of said filet to be between 4 and 8 percent of the length of the fin root. (Don't have the page because I have it on Kindle.)

Just on what I have read, the idea of a filet makes sense. Having said that, such is not true science. And to find the right answer, one would really need to work with a wind tunnel.


Kirk