What effect will this have? Fin Q

What effect will this have? Fin Q

Post by Des Bromilo » Fri, 08 Sep 2000 06:36:35



G'Day,

a question regarding fins.....
I can understand the relationship of CP with standard fins. but I was
thinking about a slightly different scenario which may be of use...

Imagine a "fin" which was actually two very thin fins placed parallel,
and reasonably close together. The roots of the two fins would be joined
via the body tube connection, and the tips, or other points could be
joined by the use of small tubes (viz 1/8" Launch lug materials) or
small flat braces to reduce the fins flexing, and to make up a sort of
"H Beam".
The theory being that the airstream would flow around the outside of the
two fins, but also (probably a lesser amount) "through" the fin, adding
extra corrective effect.

In it's most bizzare format, you could picture this being modelled as a
series of tubefins glued between two planar fins.

Benefits of this idea (presuming it has aeodynamic benefit)....

Instead of upscaling fins on scale models, you could replace one
oversized fin with two thin fins, of the correct scale size, and get
increased, or the same, corrective force.

OR... Better yet. How about a scale delta fin, with bevels. Instead of
meticulously sanding the bevels, you could make the "fin" out of three
fins. One large one in the center, with two smaller ones (one each side)
with the edges were the bevels would have been. Optically this would
show a crisp edge where the bevel should be, but maybe simpler to make,
and with better CP.

Obviously the aerodynamic merits of this idea would need to be looked
at, but I figure there's plently of "smart cookies" on this ng who could
theorise, or test the concept and state the benefits.

I know that there will be reduced airflow effects "within" the fin, due
to  the nature of the constriction, and double frictional drag.. but
perhaps this is an acceptable tradeoff for true scale size fins, or
reduced nose weight in scale models.

Ideas?, theories?,....

Des

--
Des Bromilow
Queensland Rocketry Association
Brisbane
Australia
 "Inner Child" be darned... I'm having WAY too much fun rediscovering
 my "Inner ***ager"!!!   dyb 11/2/99


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What effect will this have? Fin Q

Post by Peter Cl » Fri, 08 Sep 2000 14:58:24


Quote:

>G'Day,
>Imagine a "fin" which was actually two very thin fins placed parallel,
>and reasonably close together.

I tried this once in the early 1970's. I built two models, one with plain
single ply fins and one with two-layer fins of 1/32" (0,7mm) balsa wood placed
maybe 2mm apart.  They started out a bit oversize. I gave each a string test,
then hacked away a little bit of fin at a time with the X-acto and tested
again, until both were unstable.  My conclusion was that both the single and
the double fin were about the same size when they quit working, so the second
layer didn't do much for good or ill.
Peter W. Clay NAR 18619 SR L1
"What is a tree, but a nut that wouldn't listen to the pessimists?"

 
 
 

What effect will this have? Fin Q

Post by Leonard Fehske » Fri, 08 Sep 2000 23:29:57



Quote:
>My conclusion was that both the single and
>the double fin were about the same size when they quit working, so the second
>layer didn't do much for good or ill.

This doesn't surprise me, for two reasons; first, airflow through a
constricted channel is different from airflow in a large volume (cf.
Healy-Shaw (sp?) effect), and second, the surface area of most of one
fin is in the aerodynamic "shadow" of the other, rendering it ineffective.

len.