Beveling/airfoiling required for supersonic flight?

[RocketRoll]

Looking to start on a Wildman Blackhawk 29 and aiming for ~900 mph maximum velocity. I've beveled/airfoiled my fins in the past for flights at this speed, but it's a pain to do by hand and it's always tough to get consistent results. Out of curiosity, I simmed different fin cross-sections in OpenRocket and RAS Aero II to see exactly how much more velocity I'm getting out of this extra effort.

Results (note: motor is an H182) are below. Is it really only a ~10-25 mph difference?? I know my max altitude will be affected, but I'm only concerned with velocity. If this is correct, maybe I'll save myself the trouble and leave the LE and TE blunt on the fins.


OpenRocket:

Square Fin Cross Section	Rounded:	Airfoiled Fin Cross Section:
886 mph	891 mph	895 mph

RAS Aero II:

Square:	Rounded:	Double-Wedge:
881 mph	904 mph	905 mph

 

[heada]

I've always been told that surface finish has more impact than cross-sectional shape. What difference do you get between rough, smooth and polished surface?

 

[boatgeek]

How thick are the fins? The thinner the fins, the less the profile matters. At minimum I would round them for an attempt at Mach.

 

[RocketRoll]

I've always been told that surface finish has more impact than cross-sectional shape. What difference do you get between rough, smooth and polished surface?

"Rough" drops it to 872 mph for square cross-section fins. The figures in the table were for polished.

 

[RocketRoll]

How thick are the fins? The thinner the fins, the less the profile matters. At minimum I would round them for an attempt at Mach.

Haven't measured them myself yet, but the Rocksim file has them as 0.062" thickness. They're quite thin.

 

[dhbarr]

At these speeds, no matter what your shape you're blasting air out of the way.

I think you'll find your top speed more affected by mass -- but the fin-fin-ish reeeeally effects the final altitude.

 

[dr wogz]

one past TRF-er went and bought 3D printed NACA airfoiled fins. something like $30 a piece.. neat but.... and these were for some small rocket; 38mm dia body or 54mm.. something like that.. And a precision nosecone too, Von Karmen - something, with blah blah blah in it...

 

[The_Quacken]

I just built a Blackhawk 29 and honestly due to their thinness I found beveling the fins wasn’t much of a chore. That said the Fins are very thin so idk not sure if it would matter

 

[G_T]

Unless you can get a shallow taper on the trailing edges, consider just leaving that part squared off with sharp edges. DO NOT ROUND THE TRAILING EDGE. That will actually have more drag than leaving it squared off. Rounded gives you the highest drag you can get. Most people don't realize that about thin flat plate airfoils. A shallow taper to a sharp trailing edge of course is best. But if you are thinking of just doing a short taper, just go ahead and leave it squared instead.

With the considerable sweep angle of the leading edge, even a 45 degree bevel would be an improvement. Something like 60 degrees would be better. It doesn't have to be a long thin bevel. It doesn't even have to go all the way to a sharp edge though that will reduce drag some of course.

The motor burn is short on these. Max speed is at motor burnout. So there isn't enough time for the drag force to cost much on the maximum achieved speed. Where the drag difference on various profiles for the fins will show up is in max altitude. There is enough time for the drag force difference to make a difference over that duration.

Gerald