I saw that section but my question was more related to whether the fin
mounts had any influence on the effectiveness of the fins. I would
think a tall enough fin mount would act as a mini-fin in itself but the
protuberances input seems more related to drag.
-Bob
On 10/15/20 10:35 AM, (Redacted sender crogers168 for DMARC) wrote:
See the Protuberance Inputs section in the RASAero II Users Manual.
Fin Brackets are specifically addressed on Page 28 and Figure 31 of the RASAero II Users Manual. But I'd read all of the Protuberance Inputs section on Pages 24-29.
Note there are other protuberances which can be modeled in RASAero II, as shown on Pages 24-29, including pictures of actual protuberances on rockets/launch vehicles, and descriptions on how to model them.
Charles E. (Chuck) Rogers
Rogers Aeroscience
-----Original Message-----
From: Ben Brockert <wikkit@xxxxxxxxx>
To: arocket@xxxxxxxxxxxxx <arocket@xxxxxxxxxxxxx>
Sent: Thu, Oct 15, 2020 1:11 am
Subject: [AR] Re: Modeling fins and fin brackets
You can get down in the weeds and start arguing about boundary layers, shock effects, etc. but for the purpose of analysis by rocket software, yes the brackets are part of the fins for aerodynamic purposes. And the sheet part is what’s going to flutter first.
Yours looks good to go, but channeling one of Charles Pooley’s favorite ideas for tank construction, I wonder if you could weld two hexagonal sheets of metal together along all their edges and then inflate it a little bit hydraulically, same as how a pita is inflated by steam. Then saw it in a half and you have a stiffer fin than the equivalent thickness in flat sheet, and a vaguely aerodynamic shape.
On Thursday, October 15, 2020, Robert Watzlavick <rocket@xxxxxxxxxxxxxx <mailto:rocket@xxxxxxxxxxxxxx>> wrote:
When modeling fins in software such as RasAero or OpenRocket,
what's the best way to handle the fin bracket? Does it contribute
as much aerodynamically as the fin does? For example, I have fin
brackets that are the same length as the fins but are 1 inch tall.
The fins themselves have a 5 inch span with 1 inch gripped by the
brackets. It seems from a flutter standpoint that the span is only
the 4 inches that are protruding from the bracket but from an aero
standpoint, the fin span is 5 inches. Is this correct? Or is
there some diminished effectiveness of the fin since part of it is
gripped by a much wider bracket? The fins are 0.090 thick and the
bracket width where it grips the fin is about 0.35 inches. Body
OD is 6 inches.
I'm about to trim the fins down to make them smaller but I don't
want to cut off too much. Here is a "before" picture with larger
fins and longer brackets:
http://www.watzlavick.com/robe rt/rocket/rocket1/photos/dsc_
8310m.jpg
<http://www.watzlavick.com/robert/rocket/rocket1/photos/dsc_8310m.jpg>
-Bob
