Fin airfoil question
andrew woodard
What is the best airfoil for rocket fins? Should the LE be round, and
the TE be a sharp angle? Should both the LE and the TE be rounded?
Should both the LE and the TE be at a sharp angle? Which is best for
estes model rockets, and why?
Ay ees,
Andrew Woodard
"Who was that shape in the shadows, who is that face in the mask?"...
"Listen surely, I've exceded expections, tried for three years, seems
like thirty, could you ask as much from any other man???"
Steve Bloom
tapered trailing edge. After that comes a laminar flow. Sharp LE tapered
all the way back to the center line and then tapered to a point at the
trailing edge. Actually I think the laminar flow is more efficient but it
is harder to achieve. With both you run the risk of chipping a fin on
landings. The tear drop mainly the TE and the laminar both.
Steve Bloom
Seattle Area Rocketeer
Skunkworks West
http://www.everett.net/users/bloomer/index.htm
For despaming: Remove the pluralizing letter from bloomers
andrew woodard wrote in message
<13244-37...@newsd-198.iap.bryant.webtv.net>...
Stefan Jones
> Hello,
> What is the best airfoil for rocket fins? Should the LE be round, and
> the TE be a sharp angle? Should both the LE and the TE be rounded?
> Should both the LE and the TE be at a sharp angle? Which is best for
> estes model rockets, and why?
Here's what I've heard through the years.
Rounded leading edge.
Tapered trailing edge. Sharp but gradual.
FLAT outer surface. Or sharply pointed. Never rounded.
A gentle straight taper from root to tip is a good thing, but practically
hard to do really well.
A gentle, continuous airfoil -- the classic teardrop shape stretched out
thin -- is best.
Big factor is practicality and intent. It's hard to make a really good
airfoil. It means lots of careful sanding, and makes the fin more fragile.
(I have in the past put knife-sharp trailing edges on plywood fins for
sport HPR models . . . but after seeing them splinter during normal use
I've cut back, so to speak.)
I'd say: If you're going to use the model for competition, or want
something for altitude record breaking, put on a classic airfoil and to
heck with durability.
If you want a nice sport model that's going to last, put on an airfoil --
they look better! -- but don't overdue it.
Stefan
Paul Diming
What is the best airfoil for rocket fins? Should the LE be round, and
the TE be a sharp angle? Should both the LE and the TE be rounded?
Should both the LE and the TE be at a sharp angle? Which is best for
estes model rockets, and why?<<
As others have said, the rounded leading edge is best with a tapered
trailing edge. I also taper the fin tip. Now be careful because if the
brunt of the landing is on the trailing edge of the fins and they are
tapered too thin, you can end up with frequent chipping of the fins. I
think it is fairly easy to achieve a basic airfoil of this type. I use a
sanding block and lay it paper side up on my bench. I then sand the edges
off of the leading edge just enough so the leading edge is rounded. Doesn't
take too much effort for Balsa. Then hold the fin at an angle to the block
and sand a tapered edge on each side of the trailing edge. It's option to
do the wing tip, but it looks pretty good to either taper or round the wing
tip.
When it comes down to scale models, I make the choice of an airfoil or
straight edges based on appearance and authenticity. For sport and sport
scale models, I almost always use an airfoil.
Why is an airfoil important? Airfoils reduce drag substantially. This is
probably important if you are going for altitude or duration. Straight
edges add drag. IMHO, airfoils are also more attractive.
Good luck!
Paul.
Fly Baby Fly !
Robert Synoski
program to generate profiles.
Stefan Jones wrote in message <370E4132...@SPAMNOus.oracle.com>...
>andrew woodard wrote:
>
>> Hello,
>> What is the best airfoil for rocket fins? Should the LE be round, and
>> the TE be a sharp angle? Should both the LE and the TE be rounded?
>> Should both the LE and the TE be at a sharp angle? Which is best for
>> estes model rockets, and why?
>
Robert Synoski
http://ourworld.compuserve.com/homepages/Harold_Ginsberg/naca.htm
Robert Synoski wrote in message <7eljh5$9qs$1...@news-2.news.gte.net>...
rick oshea
anything ) will work but if youre after altitude, round the l.e. and
sharpen the t.e. , maybe get two smaller rockets and try rounding the
edges on 1 and make sharp l.e. on the other, you never know.
ricko...@webtv.net
Stefan Jones
> Or If you really want to get precise check out this page on NACA foils and a
> program to generate profiles.
The link was missing, Robert. Could you repost?
There's a GREAT illustration at the beginning of one of Estes' technical notes.
Maybe the altitude prediction charts. It shows four shapes, each of which have
the same amount of drag. There's a cube the relative size of a sugar cube, a
sphere the size of an apple, and a 3D teardrop shape roughly the size of a
grapefruit (stretched out). VERY cool way to illustrate the point: Streamlining
is important!
Stefan
Bob Kaplow
> Or If you really want to get precise check out this page on NACA foils and a
> program to generate profiles.
Correct! Two to check out (from memory, may be of) would be the NACA 0006 or
0008, and from SoarTech #8 the SD6080 (is that the symmetric stab airfoil?).
Somewhere I had a Postscript program that wouldnot only plot airfoild from
coordinates, bu t for the 4 digit NACA series would generate the airfoil
directly from the 4 digit number. yes each of the 4 digits mean something
and you plug them into an equation to get the whole NACA series. pretty neat
actually.
Bob Kaplow NAR # 18L TRA # "Ctrl-Alt-Del"
Kaplow Klips: http://members.aol.com/myhprcato/KaplowKlips.html
NIRA: http://www.nira.chicago.il.us NAR: http://www.nar.org
SPAM: u...@ftc.gov postm...@127.0.0.1 otherwise, I'm not on MARS!
Jerry L. Golden
woodard) Wrote:
>What is the best airfoil for rocket fins? Should the LE be round, and
>the TE be a sharp angle? Should both the LE and the TE be rounded?
>Should both the LE and the TE be at a sharp angle? Which is best for
>estes model rockets, and why?
>
Like a elongated teardrop feathering to a knife TE.
That's ideal anyway ; what you'd probably want to achieve is a rounded
LE and a TE that is tapered down to 1/32" or so then rounded. The
tapper should be roughly 8 or 10 to 1 ratio ( about 1/4" long for
1/16" fin stock). This will give the TE some strength - important
against chipping on landings, and it's very difficult to get a
straight edge when it's a knife point - you'll normally end up with a
wavey or jagged edge if you do it by hand.
I use nail files, the cardboard ones, you can get them from 4" to 12"
long with course on one side / fine on the other. Lay the fin on a
table with the edge at the table's edge, clamp it down with one hand
and sand with the other. Try to keep the file at a roughly 5-10 degree
angle and gently & uniformly take down one side at a time. Takes me
about 45 min to an hour to get a set of 3 balsa ones. Plywood takes
2-3 X longer. It can get rather boring, and your hands get stiff.
Try to get the fin fillets about the same shape by smearing the glue
/ epoxy with your fingertip after you run a bead (easier said than
done). Keep both sides of the fins symmetrical.
If the TE of the fins are placed close to the aft end of the
body tube you'll see roughly how well you've done after the first
flight : the delay smoke will "backwash" into spots that are not
streamlined enough and char / discolor the paint. The amount of that
backwash is directly proportional to the drag. In time you'll get a
feel for streamlining just by looking at various items on the rocket.
Due to the compressibility of air the best shape for the LE is
rounded ; anything else just causes more surface for the air to have
to flow over. But because air has weight it can't move fast enough to
fill in behind an abrupt TE transition - so you get a vacuum there.
Air will follow a smooth TE transition of about 5-15 degrees - I
usually aim for about 6-8 degrees - and not cause a vacuum.
This only applies to sub-sonic models in the 200-700 FPS range.
You can get formulas that will give you the ideal angles but
they can get a bit hairy to the uninitiated. Also you can make a set
of max.altitude competition fins and literally spend all day on them,
only to squeeze another 3-4% in altitude and have to repair them
almost every flight. Most of the time it's not worth it.
Good luck - have fun.
JLG
Bob Kaplow
> In article <7eljh5$9qs$1...@news-2.news.gte.net>, "Robert Synoski" <robert....@trcinc.com> writes:
>> Or If you really want to get precise check out this page on NACA foils and a
>> program to generate profiles.
>
> Correct! Two to check out (from memory, may be of) would be the NACA 0006 or
> 0008, and from SoarTech #8 the SD6080 (is that the symmetric stab airfoil?).
The NACA 00XX series are all symmetric airfoils suitable for fins, stabs and
rudders, boat hulls or whatever. The correct Soartech references are SD8020,
J5012, and NACA0009. IIRC there's one more, but I forgot the scrap of paper
with the notes. Supposedly the SD8020 is improved to reduce the dead band
around zero degrees. useful for F3B control surfaces. Maybe important for
fins as well.
The biggest concern we probably have for our models is to have low drag
fins. The best way to accomplish this is to make the fins thinner. The last
2 digits of the NACA number indicate the fin thickness, so a NACA 0009 is 9%
thick (compared to the chord), the NACA 0006 would be 6% thick, and a NACA
0004 would be only 4% thick. They are all generated by a set of equations
and formulas, and at least the symmetric series scale easilly. If you take
the coordinates for a NACA 0009 and multiply the thickness by 2/3 you get
the coordinates for a NACA 0006.
The Selig (S series) and Selig Donovan (SD series) are not generated by
formulas. I can't comment on what a SD8020 might do if thinned by 1/2 or
2/3, and whether it would have any similarity at all to the SD8020.
> Somewhere I had a Postscript program that wouldnot only plot airfoild from
> coordinates, bu t for the 4 digit NACA series would generate the airfoil
> directly from the 4 digit number. yes each of the 4 digits mean something
> and you plug them into an equation to get the whole NACA series. pretty neat
> actually.
I dug out an old tape from my DEC days, and located the AIRFOIL software. My
memory wasn't quite accurate here. Sadly for anyone except Helen and myself,
it's a VMS text library and command file, which builds a postscript file
from the command procedure. Anyone who can make use of it is welcome to a
copy. And anyone fluent in Postscript and more who wnats to try to hack it
into something that can be used elsewhere is welcome to try. The is no
copyright statement, so as long as the derrivative is left to the public
domain...
Bob Kaplow
> The NACA 00XX series are all symmetric airfoils suitable for fins, stabs and
> rudders, boat hulls or whatever. The correct Soartech references are SD8020,
> J5012, and NACA0009. IIRC there's one more, but I forgot the scrap of paper
> with the notes. Supposedly the SD8020 is improved to reduce the dead band
> around zero degrees. useful for F3B control surfaces. Maybe important for
> fins as well.
The other symmetric airfoil in Soartech #8 is the NACA 64A010 at 10% thick.
Andy Schecter
>The biggest concern we probably have for our models is to have low drag
>fins. The best way to accomplish this is to make the fins thinner.
Bob: perhaps for the sake of discussion, I think you're oversimplifying the
situation. Low drag is nice, but thin fins that flutter produce a lot of
drag. And thin fins that fail are, well, a REAL drag.
It certainly depends on the power range your model is operating in. At
higher velocities (like >0.5 Mach), thin fins can be a real mistake. I've
seen M motors strip 1/8" G-10 fins off halfway through the burn.
Torsion stiffness increases more or less as the square of thickness. Bending
stiffness increases pretty much as the cube of stiffness. (For solid fins).
If you get too thin, you can get into trouble very quickly.
Here are a few comments by Bob Parks on the subject:
"The super thin fiberglass fins that have gotten common on competition model
rockets are very prone to flutter, and do NOT have lower drag than a thicker
airfoiled fin."
"For SUBSONIC flight, there is only a slight decrease in drag as you go
thinner, and anything under about 8% probably makes no difference at all.
(in some cases, you can keep laminar flow on a good, thicker airfoil while a
thin one is turbulent.. so thicker could be LESS drag)."
"In all cases, a "real" airfoil will be lower drag than a "flat plate"."
"The ONE problem with thicker airfoils is at very low speeds right at
launch.. at the very low reynolds numbers, the thicker airfoils can "quit"..
This could be one of the reasons that some models will go unstable at launch
occasionally, but fly well at other times."
Of course, it is fairly difficult to put a good airfoil on 1/16 balsa. But
that's OK for small sport models. On some of the larger model rockets,
however, and certainly on HPR stuff, thick fins with a decent airfoil make a
lot more sense than the extremely thin fins you sometimes see.
-Andy
Bob Kaplow
> Bob Kaplow wrote:
>>The biggest concern we probably have for our models is to have low drag
>>fins. The best way to accomplish this is to make the fins thinner.
>
> Bob: perhaps for the sake of discussion, I think you're oversimplifying the
> situation. Low drag is nice, but thin fins that flutter produce a lot of
> drag. And thin fins that fail are, well, a REAL drag.
Whoops. What I was thinking clearly isn't wat I wrote above. I was looking
at the SoarTech publication. The airfoild there, NACA 0009, SD8020, etc. are
all in the 9-10% thickness. That means that a fin with a 10" root chord
would be 1" thick. IMHO, that is way too thick, even for an M powered model.
Rather than a NACA 0009 or an SD8020, I was trying to imply that a thinner
NACA 0004 might be a better choice. It's still plenty thick for the purposes
of avoiding the structural problems you refered to.
Is that any better?
Ryburn Ross
What would be the best thickness to use for a 24mm model about 18 inches
tall that would fly on G200's? (Just something I have floating around in
my mind). What airfoil would be best for a mach 2+ rocket (maybe a two
staged G200). I was considering tackling something like this this summer
using Fiberglass sheet for the fins. I was planning on using 3/16"
stuff, 30 minute epoxied on, with fillets of fiberglass cloth with 2
hour epoxy. Would it hold together??? Would heating become a problem at
these kinds of high velocities? Anyone here done anything like this?
I figure that if I can't buy highpower motors like H's I might as well
try some high performance models. The fastest that I have sent something
so far is mach .6 and that was with a three stager.
--
Ryburn Ross
http://www.geocities.com/capecanaveral/hall/3641/wframes.html
http://pages.rzsoft.com/ross/videos.html
"To live is to suffer, but to survive, well, that's to find meaning in
the suffering."
"Let me go my way... but walk with me"
Andy Schecter
>What would be the best thickness to use for a 24mm model about 18 inches
>tall that would fly on G200's? (Just something I have floating around in
>my mind). What airfoil would be best for a mach 2+ rocket (maybe a two
>staged G200). I was considering tackling something like this this summer
>using Fiberglass sheet for the fins. I was planning on using 3/16"
>stuff, 30 minute epoxied on, with fillets of fiberglass cloth with 2
>hour epoxy. Would it hold together??? Would heating become a problem at
>these kinds of high velocities? Anyone here done anything like this?
Will it hold together. This kind of question gets asked a lot. It's
impossible to answer (although RSO's have to try and answer it at every
launch). Two people may construct identical designs with equivalent
materials, yet one model may be much stronger than the other. There are a
hundred tips and techniques that experienced modellers use to make their
rockets stronger. I suggest that you find someone in your area who is
skilled in fiberglass construction techniques and watch him/her build a
couple rockets.
Look at sounding rockets to get an idea what the pros use for supersonic
airfoils. The "double wedge" (sharp leading and trailing edges, with a high
point in the center) is a popular design.
Heating would not be a problem with G200s because of the extremely short
period of time your model would be at high velocity.
-Andy
jIM k.
>
> Bob Kaplow wrote:
>
> >The biggest concern we probably have for our models is to have low drag
> >fins. The best way to accomplish this is to make the fins thinner.
>
>< . . . >
> Here are a few comments by Bob Parks on the subject:
>
>
< . . . >
> "For SUBSONIC flight, there is only a slight decrease in drag as you go
> thinner, and anything under about 8% probably makes no difference at all.
> (in some cases, you can keep laminar flow on a good, thicker airfoil while a
> thin one is turbulent.. so thicker could be LESS drag)."
>
Comparing the data in "Abbot and Von Doenhoff" NACA 0006 has a minimum
section drag coefficient of .0045 at a Reynolds number of 3 million and
.005+ Cd at Rn= 6M and 9M. The NACA 0012 data shows a Cd just under .005
at all three Reynolds number.
Now, based on this data, it looks like there would be no significant
difference in drag between 6% and 12% thick fins of the same size (area:
chord * span). Note: the reference area used to convert Cd to drag for
these sections is the planform area not the frontal area used to
reference Cd we usually use for rockets.
As long as I'm flipping though the book...
The bottom of the "drag bucket" for a 66(sub 1)-012 is way down around
.0035 to .004, but it doesn't look like the traditional "teardrop" shape
- the maximum thickness is at 66% of the chord and the back of the
section has a bit of a concave look to it. The 66-006 looks to have a
slightly lower minimum drag, but the width of the "bucket" (the low
drag, laminar flow portion of the drag/lift curve) is a lot narrower and
probably would give worse results than the -012 in real life (at
Reynolds numbers between 3 and 9 million)
--
Jim K. !When Great Britain changed to the Gregorian calendar in
1751,
Ji...@ili.net !the day following Sept. 2 was declared to be Sept. 14.
Thinking
!that somehow they were being cheated out of 11 days,
people
!rioted. Slightly more than 248 years later, people
celebrated
!Jan. 1 2000 as the start of the new millennium.
Dave Cook
mention is that thicker airfoils provide lower drag when flying at angles of attack other
than 0. You get nonzero angle of attack either via flutter or by gross rotation of the
airframe. A very thin airfoil is only better if it doesn't flutter AND the rocket has no
pitch/yaw motion. Flutter is totally awful because you get both turbulent flow and angle
of attack-induced drag.
The low speed problems he mentions are really interesting. Things get worse as the
control surfaces (fins) get smaller too because the viscous air begins to look pretty
soupy to the vehicle.
Dave Cook
On Thu, 15 Apr 1999 06:09:35 -0400, "Andy Schecter" <asch...@compuserve.com> wrote:
>Here are a few comments by Bob Parks on the subject:
>
>"The super thin fiberglass fins that have gotten common on competition model
>rockets are very prone to flutter, and do NOT have lower drag than a thicker
>airfoiled fin."
>
>"For SUBSONIC flight, there is only a slight decrease in drag as you go
>thinner, and anything under about 8% probably makes no difference at all.
>(in some cases, you can keep laminar flow on a good, thicker airfoil while a
>thin one is turbulent.. so thicker could be LESS drag)."
>
Stefan Jones
same?
Does "through the wall" construction reduce flutter at all, or is the flutter caused by the
material itself flexing about?
Stefan
Alan Jones
Yes. TTW fin mounting significantly increases the root bending
stiffness and will increase flutter speeds. Without TTW construction
the fin root bending stiffness is largely determined by body tube wall
flexure. OF course if the fin marial itself is to wimpy, it is
pointless.
Alan Jones