Yes, that is correct. The protuberance input is for adding the extra drag of
the protuberance, the extra drag of the fin brackets.
RASAero II assumes that adding the fin bracket to the fin has no effect on the
fin lift (fin CNalpha and fin CP).
Charles E. (Chuck) RogersRogers Aeroscience
-----Original Message-----
From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
To: arocket@xxxxxxxxxxxxx; Redacted sender crogers168 for DMARC
<dmarc-noreply@xxxxxxxxxxxxx>
Sent: Thu, Oct 15, 2020 6:44 pm
Subject: [AR] Re: Modeling fins and fin brackets
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> 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
-Bob
