#### DynaSoar

##### Well-Known Member
Obviously, fillets are good for fin/body strength. Less obvious, but often said, is that they reduce drag. No doubt this is true, because it's mentioned in write ups of such vehicles as the Pegasus launcher and X-33. They reduce interference drag. OK, fine.

But how much fillet? How much is enough? Is there such a thing as too much, and how much would that be? What factors go into figuring this out, and how is it to be measured? I seem to recall from somewhere that the measure is in radius of curvature, but I don't know how that relates to the other dimensions or other factors.

Anyone seen any hard numbers generated in figuring this out? Or is the calculation going to be so complex that we're stuck with trial and error and wind tunnels?

#### Stymye

##### Well-Known Member
for aerodynamic purposes the ideal fillet is
4%-8% of the fin root chord..any larger begins to increase friction drag.on small model rockets it's obviously not as critical

got this from stines handbook, I think he got it from the MIT studies tho

#### Stymye

##### Well-Known Member
you would measure it from the fin body joint to the edge of the fillet in each direction (rather than across the face of the fillet)

#### polaris

##### Well-Known Member
Ok, I must be a little bit slow. Could you work out an example?

If I have a fin that is 1/8 thick with a root cord length of 5 ½, what are the dimensions of the fillet?

#### DynaSoar

##### Well-Known Member
Thanks Styme. The book was right here on the table, but propping up the wife's laptop. I shoulda looked.

Originally posted by Polaris
Ok, I must be a little bit slow. Could you work out an example?

If I have a fin that is 1/8 thick with a root cord length of 5 ½, what are the dimensions of the fillet?

Split the difference of 4 to 8 and call it 6%.

5.5" times 0.06 = 0.33". One third of an inch wide from the edge on the body to the edge on the fin. 3/8" (0.325) is a good close measure.