how does fin thickness effect stability

how does fin thickness effect stability

Post by Blai » Sun, 16 Nov 1997 04:00:00



I used Stines' formula to calculate the measurements for the clipped
delta fin that I am putting on a bt-60 diameter rocket.  The maximum
thickness for the fin is 10% of the chord root which makes it 8.32mm.
That's about a 1/4 of an inch.  How important is the thickness of the
fin to stability?
Blaine

 
 
 

how does fin thickness effect stability

Post by The Silent Observe » Sun, 16 Nov 1997 04:00:00


Quote:

> I used Stines' formula to calculate the measurements for the clipped
> delta fin that I am putting on a bt-60 diameter rocket.  The maximum
> thickness for the fin is 10% of the chord root which makes it 8.32mm.
> That's about a 1/4 of an inch.  How important is the thickness of the
> fin to stability?
> Blaine

It's my understanding that as long as the fin is thick enough for
strength and stiffness, there's no effect on stability.  On a BT-60
rocket, you'll probably find a 1/8" thickness perfectly adequate, and if
you're mounting with TTMT tabs, you might get away with hard 3/32" balsa
or basswood.

What Stine seems to have been saying in that design sketch is not to
exceed 10% of the chord, to limit drag, rather than that the fin should
always have a thickness of 10% of the chord.

--
There are times when even the Weaver of Skeins can make an awful tangle
of a perfectly simple tapestry.  -- M. A. R. Barker, _The Man of Gold_

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
Rocket Pages             http://members.aol.com/silntobsvr/launches.htm

Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.

 
 
 

how does fin thickness effect stability

Post by Bill Nels » Sun, 16 Nov 1997 04:00:00


: I used Stines' formula to calculate the measurements for the clipped
: delta fin that I am putting on a bt-60 diameter rocket.  The maximum
: thickness for the fin is 10% of the chord root which makes it 8.32mm.
: That's about a 1/4 of an inch.  How important is the thickness of the
: fin to stability?

Thickness of the fin does not matter much.  However, putting an
airfoil on the fin does reduce stability - as it allows the fin
to fly at a higher angle of attack without stalling.

--

 
 
 

how does fin thickness effect stability

Post by Andy Schecte » Mon, 24 Nov 1997 04:00:00


Quote:

> With a flat plate, it basically stalls as soon as there is an angle
> of attack. The corrective force is fairly large at even small angles
> of attack. With an airfoil, you can get various amounts of angle of
> attack - with only slight corrective forces.  The extreme example is
> a cylinder - where there is no angle of attack that will produce a
> change in corrective forces.

You are confused, sir. Stalling is what you DON'T want; it drastically
reduces fin effectiveness. Stalling is what causes airplanes to fall out
of the sky. For the same reason (loss of lift), fin stalling will cause
major instability in model rockets.

What you DO want are fins with a moderately thick airfoil (6-10% of
chord) with a decent airfoil on them. They will provide good LIFT (which
is the corrective force that you DO want) at various angles of attack,
with relatively low drag penalty.

You are correct that a cylinder will make a lousy fin. Its thickness is,
of course, 100% of its chord, and that is way too thick for a fin.

Andy Schecter

 
 
 

how does fin thickness effect stability

Post by Bill Nels » Wed, 26 Nov 1997 04:00:00


: > With a flat plate, it basically stalls as soon as there is an angle
: > of attack. The corrective force is fairly large at even small angles
: > of attack. With an airfoil, you can get various amounts of angle of
: > attack - with only slight corrective forces.  The extreme example is
: > a cylinder - where there is no angle of attack that will produce a
: > change in corrective forces.
:
: You are confused, sir. Stalling is what you DON'T want; it drastically
: reduces fin effectiveness. Stalling is what causes airplanes to fall out
: of the sky. For the same reason (loss of lift), fin stalling will cause
: major instability in model rockets.

No, I am not in the least confused.  Stalling does two things - it
decreases lift and it increases drag.

A rocket does not need lift on its fins for directional stability. If it
did, then plate fins (as are used even on military rockets) would not work
as well as they do.

For an airplane, the reduced lift/drag ratio is a detriment, as it causes
a rapid decrease in altitude.  Note that it is VERY hard to completely
stall an airplane wing - although it can happen in slow flight in wind
shear.  

For a rocket, there is no lift to start with. For an airfoil fin, lift is
generated when the fin assumes an angle of attack to the airflow. For
small angles of attack - drag only increases slightly, so the restorative
force is mostly lift - and the force is not very strong.  With a plate
fin, the drag increases rapidly - and fairly strong lift also occurs.
The two are additive, and produce a strong restorative force.

: What you DO want are fins with a moderately thick airfoil (6-10% of
: chord) with a decent airfoil on them. They will provide good LIFT (which
: is the corrective force that you DO want) at various angles of attack,
: with relatively low drag penalty.

Keep in mind that I am only talking about small angles of attack here,
not large changes.  

A plate fin will provide even better correction than an airfoil, as there
is a large increase in drag - which provides more restorative force than
can be produced as lift from an airfoil.

--

 
 
 

how does fin thickness effect stability

Post by Andy Schecte » Thu, 27 Nov 1997 04:00:00


Quote:

> ....A rocket does not need lift on its fins for directional stability.
> If it did, then plate fins (as are used even on military rockets) would not work
> as well as they do.

True, rockets can be stabilized by drag alone. I recently flew a rocket
called "Drag Queen" (http://128.230.82.205/rocketpics/may97/andy.html)
that had no fins at all. The idea was to expend maximum newton-seconds
and achieve minimum altitude. :-)  It flies, if you have a taste for
that sort of thing.  :-)

Rockets employing what you call "plate fins" do indeed rely on LIFT for
stability. At low angles of attack, "plate fins" provide perfectly good
lift. G. Harry Stine's famous "Flat Cat" boost glider relies on LIFT
provided solely by flat airfoils, and it glides just fine.

Quote:
> ....For an airfoil fin, lift is
> generated when the fin assumes an angle of attack to the airflow. For
> small angles of attack - drag only increases slightly, so the restorative
> force is mostly lift - and the force is not very strong.  With a plate
> fin, the drag increases rapidly - and fairly strong lift also occurs.
> The two are additive, and produce a strong restorative force.

This is incorrect on several counts. First of all, at small AOA, the
difference in behavior between flat fins and airfoiled fins is quite
small. Second, it is incorrect that the lift force of an airfoil at
small AOA is "not very strong." Do you have numbers to back up this
statement? Third, while it is true that lift and drag are additive, they
are additive only as vector forces. Lift force is acting in the "proper"
vector direction--since it is perpendicular to the long axis of the
rocket, it is very effective in rotating the rocket around its CG back
to zero AOA. Drag force, on the other hand, acts in the "wrong"
direction--nearly parallel to the long axis. To calculate the net drag
force available to rotate the rocket back to zero AOA, multiply the
total drag force by the TANGENT of the AOA. For a five degree AOA, this
is less than 9%, which means that 81% of the drag force is wasted as a
restoring force.

Quote:
> A plate fin will provide even better correction than an airfoil, as there
> is a large increase in drag - which provides more restorative force than
> can be produced as lift from an airfoil.

Again, incorrect. Ask an aeronautical engineer to run the numbers for
you. The lift restorative forces are HUGE compared to the drag
restorative forces, stalled or not stalled.

The biggest difference between the flat and airfoiled fin is the ability
of the airfoiled fin to provide restorative force at a greater AOA.
Ordinarily it isn't important, which is why flat-finned rockets fly just
fine. A slow-accelerating rocket leaving the launcher in a crosswind,
however, can experience such a high AOA that airfoiled fins (that keep
"working" longer) could make the difference between a successful flight
and a cartwheel.

-Andy

 
 
 

how does fin thickness effect stability

Post by James R. Cunningha » Fri, 28 Nov 1997 04:00:00


Quote:


> > ....For an airfoil fin, lift is
> > generated when the fin assumes an angle of attack to the airflow. For
> > small angles of attack - drag only increases slightly, so the restorative
> > force is mostly lift - and the force is not very strong.  With a plate
> > fin, the drag increases rapidly - and fairly strong lift also occurs.
> > The two are additive, and produce a strong restorative force.

> This is incorrect on several counts. First of all, at small AOA, the
> difference in behavior between flat fins and airfoiled fins is quite
> small. Second, it is incorrect that the lift force of an airfoil at
> small AOA is "not very strong." Do you have numbers to back up this
> statement? Third, while it is true that lift and drag are additive, they
> are additive only as vector forces. Lift force is acting in the "proper"
> vector direction--since it is perpendicular to the long axis of the
> rocket, it is very effective in rotating the rocket around its CG back
> to zero AOA. Drag force, on the other hand, acts in the "wrong"
> direction--nearly parallel to the long axis. To calculate the net drag
> force available to rotate the rocket back to zero AOA, multiply the
> total drag force by the TANGENT of the AOA. For a five degree AOA, this
> is less than 9%, which means that 81% of the drag force is wasted as a
> restoring force.

> > A plate fin will provide even better correction than an airfoil, as there
> > is a large increase in drag - which provides more restorative force than
> > can be produced as lift from an airfoil.

> Again, incorrect. Ask an aeronautical engineer to run the numbers for
> you. The lift restorative forces are HUGE compared to the drag
> restorative forces, stalled or not stalled.

Andy, you are right in everything you said, and well said, too.  In the
range of 0 to 5 degrees AOA, both the flat and airfoiled fin will have a
lift slope of roughly 0.116, but the airfoiled fin will hang in there
longer.
The Naval Postgraduate School has online panel code for 4 & 5 digit naca
airfoils available at
http://atemi.aa.nps.navy.mil/panel/panel.html
Happy Thanksgiving to everyone.  JimC

- Show quoted text -

Quote:
> fine. A slow-accelerating rocket leaving the launcher in a crosswind,
> however, can experience such a high AOA that airfoiled fins (that keep
> "working" longer) could make the difference between a successful flight
> and a cartwheel.

> -Andy

 
 
 

how does fin thickness effect stability

Post by Konrad Hambri » Thu, 04 Dec 1997 04:00:00




Quote:


>> > ....For an airfoil fin, lift is
>> > generated when the fin assumes an angle of attack to the airflow. For
>> > small angles of attack - drag only increases slightly, so the restorative
>> > force is mostly lift - and the force is not very strong.  With a plate
>> > fin, the drag increases rapidly - and fairly strong lift also occurs.
>> > The two are additive, and produce a strong restorative force.

>> This is incorrect on several counts. First of all, at small AOA, the
>> difference in behavior between flat fins and airfoiled fins is quite
>> small. Second, it is incorrect that the lift force of an airfoil at
>> small AOA is "not very strong." Do you have numbers to back up this
>> statement? Third, while it is true that lift and drag are additive, they
>> are additive only as vector forces. Lift force is acting in the "proper"
>> vector direction--since it is perpendicular to the long axis of the
>> rocket, it is very effective in rotating the rocket around its CG back
>> to zero AOA. Drag force, on the other hand, acts in the "wrong"
>> direction--nearly parallel to the long axis. To calculate the net drag
>> force available to rotate the rocket back to zero AOA, multiply the
>> total drag force by the TANGENT of the AOA. For a five degree AOA, this
>> is less than 9%, which means that 81% of the drag force is wasted as a
>> restoring force.

>> > A plate fin will provide even better correction than an airfoil, as there
>> > is a large increase in drag - which provides more restorative force than
>> > can be produced as lift from an airfoil.

>> Again, incorrect. Ask an aeronautical engineer to run the numbers for
>> you. The lift restorative forces are HUGE compared to the drag
>> restorative forces, stalled or not stalled.

>Andy, you are right in everything you said, and well said, too.  In the
>range of 0 to 5 degrees AOA, both the flat and airfoiled fin will have a
>lift slope of roughly 0.116, but the airfoiled fin will hang in there
>longer.
>The Naval Postgraduate School has online panel code for 4 & 5 digit naca
>airfoils available at
>http://atemi.aa.nps.navy.mil/panel/panel.html
>Happy Thanksgiving to everyone.  JimC

>> fine. A slow-accelerating rocket leaving the launcher in a crosswind,
>> however, can experience such a high AOA that airfoiled fins (that keep
>> "working" longer) could make the difference between a successful flight
>> and a cartwheel.

All --

Very cool thread.  Like Andy and James said, a flat plate
does generate lift at Non-Zero AoA.  At very small AoA, the
slope is even linear, exactly like an airfoiled fin.

As a matter of fact, the Borrowman Equations are actually
based on the assumption that the fins are thin, flat plates !

At small AoA, the theoretical coeff of lift of a flat plate is:

   CL = 2 * Pi * sin ( Alpha )   #  Alpha == AoA in Radians.

When an angle is very small, the sine of the angle is the same
as the angle itself, so:

   CL = 2 * Pi * Alpha

This means that the slope of the Lift vs AoA curve at small AoA
is simply 2 * Pi, or in Aero terms, dCL/dA = 2*Pi.  When AoA is
measured in degrees, the slope of the lift curve is:

   dCL/dA = ( 2 * Pi ) * ( Pi / 180 )

          = Pi^2 / 90

          = 0.110

Which is pretty close to James' number above.

The original question was 'How does fin thickness affect
stability?'.  That is pretty a complicated question but,
ignoring drag, and assuming you put a good airfoil on the
fins, you can 'get away with' a thickness ratio as much
as 25 % of the chord length.  

OTOH, Hoerner shows in _Fluid_Dynamic_Lift_ ( pp 2-10,
figure 17 ) that a NACA 0018 airfoil ( max thickness =
18% of chord length ) modified with a 'large trailing edge
angle' actually develops NEGATIVE lift out to more than
5 Deg AoA before it finally starts 'working as expected' !

You do need to be careful not to get 'too fat' -- it increases
your drag and reduces your stall angle ( works for people too ;-)

Here is a summary of some airfoil data from _Theory_of_
_Wing_Sections_ ( see below )


   Desig   Thickness    CL   Angle   slope  Max CL
   =====   =========  ====  ======  ======  ======









   Plate      n/a     0.82  10 deg   0.110           From Hoerner
   DblWedge   4.3%    0.83  10 deg   0.113           From Hoerner

   ( note that I measured all these numbers from graphs in
     the books mentioned -- YMMV -- but I did double check ;-)

G.H.Stine suggested 10% -- a pretty good trade-off between
lift coefficient, stall angle and drag.  Looks good to me ;-)

If you are really interested in the subject, I suggest the
following book:

   Theory of Wing Sections
   Ira H Abbott
   Albert E. von Doenhoff
   Dover Publications
   ISBN 486-60586-8
   $14.95

This book is a bargain at $15 -- it includes a summary of NACA
airfoil data and also the answers to your question ;-)

If you are _really_really_ interested, call Mrs Hoerner and
order _Fluid_Dynamic_Drag_ and _Fluid_Dynamic_Lift_:

   Hoerner Fluid Dynamics
   PO Box 65283
   Vancouver, WA 98665
   ph/FAX 360-567-3997

   FDD:  $75 + 6 shipping.
   FDL:  $70 + 6 shipping.

Have fun !  

-- kjh

p.s.  Kites and ( Rockets & Planes ) are completely different
      animals.  Kites fly at large AoA, ( Rockets and Planes )
      are supposed to fly near zero AoA ;-)
--
------------------------------------------------------------

1111 Seacoast Dr.  Unit 41   |  home:   (619) 423-4451     |
Imperial Beach, CA   91932   |                             |

 
 
 

how does fin thickness effect stability

Post by the mighty kr » Thu, 04 Dec 1997 04:00:00


Ahhhh--- this provides some insight into my rolleron design I posted a
couple weeks ago- I have also read Fluid's and done a program to calculate
drag- the above should have been intuitve.

THANKS KONRAD
--
Thanks, cya, mark  8-|

Member:

PHITS of N. Co. NAR Section#565;         NAR# 71034SR565
www.pageplus.com/~cwbrown/phits/phits.htm

Tripoli Rocky Mountain's,  Prefecture  #72;    TRA#5125
www.dohram.com/trm/



Quote:




> >> > ....For an airfoil fin, lift is
> >> > generated when the fin assumes an angle of attack to the airflow.
For
> >> > small angles of attack - drag only increases slightly, so the
restorative
> >> > force is mostly lift - and the force is not very strong.  With a
plate
> >> > fin, the drag increases rapidly - and fairly strong lift also
occurs.
> >> > The two are additive, and produce a strong restorative force.

> >> This is incorrect on several counts. First of all, at small AOA, the
> >> difference in behavior between flat fins and airfoiled fins is quite
> >> small. Second, it is incorrect that the lift force of an airfoil at
> >> small AOA is "not very strong." Do you have numbers to back up this
> >> statement? Third, while it is true that lift and drag are additive,
they
> >> are additive only as vector forces. Lift force is acting in the
"proper"
> >> vector direction--since it is perpendicular to the long axis of the
> >> rocket, it is very effective in rotating the rocket around its CG back
> >> to zero AOA. Drag force, on the other hand, acts in the "wrong"
> >> direction--nearly parallel to the long axis. To calculate the net drag
> >> force available to rotate the rocket back to zero AOA, multiply the
> >> total drag force by the TANGENT of the AOA. For a five degree AOA,
this
> >> is less than 9%, which means that 81% of the drag force is wasted as a
> >> restoring force.

> >> > A plate fin will provide even better correction than an airfoil, as
there
> >> > is a large increase in drag - which provides more restorative force
than
> >> > can be produced as lift from an airfoil.

> >> Again, incorrect. Ask an aeronautical engineer to run the numbers for
> >> you. The lift restorative forces are HUGE compared to the drag
> >> restorative forces, stalled or not stalled.

> >Andy, you are right in everything you said, and well said, too.  In the
> >range of 0 to 5 degrees AOA, both the flat and airfoiled fin will have a
> >lift slope of roughly 0.116, but the airfoiled fin will hang in there
> >longer.
> >The Naval Postgraduate School has online panel code for 4 & 5 digit naca
> >airfoils available at
> >http://atemi.aa.nps.navy.mil/panel/panel.html
> >Happy Thanksgiving to everyone.  JimC

> >> fine. A slow-accelerating rocket leaving the launcher in a crosswind,
> >> however, can experience such a high AOA that airfoiled fins (that keep
> >> "working" longer) could make the difference between a successful
flight
> >> and a cartwheel.

> All --

> Very cool thread.  Like Andy and James said, a flat plate
> does generate lift at Non-Zero AoA.  At very small AoA, the
> slope is even linear, exactly like an airfoiled fin.

> As a matter of fact, the Borrowman Equations are actually
> based on the assumption that the fins are thin, flat plates !

> At small AoA, the theoretical coeff of lift of a flat plate is:

>    CL = 2 * Pi * sin ( Alpha )   #  Alpha == AoA in Radians.

> When an angle is very small, the sine of the angle is the same
> as the angle itself, so:

>    CL = 2 * Pi * Alpha

> This means that the slope of the Lift vs AoA curve at small AoA
> is simply 2 * Pi, or in Aero terms, dCL/dA = 2*Pi.  When AoA is
> measured in degrees, the slope of the lift curve is:

>    dCL/dA = ( 2 * Pi ) * ( Pi / 180 )

>           = Pi^2 / 90

>           = 0.110

> Which is pretty close to James' number above.

> The original question was 'How does fin thickness affect
> stability?'.  That is pretty a complicated question but,
> ignoring drag, and assuming you put a good airfoil on the
> fins, you can 'get away with' a thickness ratio as much
> as 25 % of the chord length.  

> OTOH, Hoerner shows in _Fluid_Dynamic_Lift_ ( pp 2-10,
> figure 17 ) that a NACA 0018 airfoil ( max thickness =
> 18% of chord length ) modified with a 'large trailing edge
> angle' actually develops NEGATIVE lift out to more than
> 5 Deg AoA before it finally starts 'working as expected' !

> You do need to be careful not to get 'too fat' -- it increases
> your drag and reduces your stall angle ( works for people too ;-)

> Here is a summary of some airfoil data from _Theory_of_
> _Wing_Sections_ ( see below )


>    Desig   Thickness    CL   Angle   slope  Max CL
>    =====   =========  ====  ======  ======  ======









>    Plate      n/a     0.82  10 deg   0.110           From Hoerner
>    DblWedge   4.3%    0.83  10 deg   0.113           From Hoerner

>    ( note that I measured all these numbers from graphs in
>      the books mentioned -- YMMV -- but I did double check ;-)

> G.H.Stine suggested 10% -- a pretty good trade-off between
> lift coefficient, stall angle and drag.  Looks good to me ;-)

> If you are really interested in the subject, I suggest the
> following book:

>    Theory of Wing Sections
>    Ira H Abbott
>    Albert E. von Doenhoff
>    Dover Publications
>    ISBN 486-60586-8
>    $14.95

> This book is a bargain at $15 -- it includes a summary of NACA
> airfoil data and also the answers to your question ;-)

> If you are _really_really_ interested, call Mrs Hoerner and
> order _Fluid_Dynamic_Drag_ and _Fluid_Dynamic_Lift_:

>    Hoerner Fluid Dynamics
>    PO Box 65283
>    Vancouver, WA 98665
>    ph/FAX 360-567-3997

>    FDD:  $75 + 6 shipping.
>    FDL:  $70 + 6 shipping.

> Have fun !  

> -- kjh

> p.s.  Kites and ( Rockets & Planes ) are completely different
>       animals.  Kites fly at large AoA, ( Rockets and Planes )
>       are supposed to fly near zero AoA ;-)
> --
> ------------------------------------------------------------

> 1111 Seacoast Dr.  Unit 41   |  home:   (619) 423-4451     |
> Imperial Beach, CA   91932   |                             |

 
 
 

how does fin thickness effect stability

Post by Konrad Hambri » Fri, 05 Dec 1997 04:00:00




Quote:
>Ahhhh--- this provides some insight into my rolleron design I posted a
>couple weeks ago- I have also read Fluid's and done a program to calculate
>drag- the above should have been intuitve.

Awww shucks, I missed that thread ( been trying to get
back up to speed on a new-old job ;-)

How will you spin up the rollerons ?

-- kjh
--
------------------------------------------------------------

1111 Seacoast Dr.  Unit 41   |  home:   (619) 423-4451     |
Imperial Beach, CA   91932   |                             |

 
 
 

how does fin thickness effect stability

Post by Kallen » Fri, 05 Dec 1997 04:00:00


I find this hard to believe. The restoring moment is equal to the force
x offset distance (moment arm) from the CG. For small angles of attack
even a flat plate airfoil produces many times more lift than drag. For a
few degrees of yaw the offset from the CG of the drag vector may be just
a few mm, whereas the offset of the lift vector is the distance of the
CG from the fin's CP, which may be many cm. (depending on the CG
location).

Quote:

> No, I am not in the least confused.  Stalling does two things - it
> decreases lift and it increases drag.

> A rocket does not need lift on its fins for directional stability. If it
> did, then plate fins (as are used even on military rockets) would not work
> as well as they do.
>........
> --


--
-------------------------------------------------------------------------------
John Kallend                            | Phone 312 567 3163
Dean, Undergraduate College             | Fax  312 567 3135
IIT                                     | Web http://www.iit.edu/~kallend
 
 
 

how does fin thickness effect stability

Post by the mighty kr » Sat, 06 Dec 1997 04:00:00


So far it's looking like I'll just build a special pad that plumbs
compressed air from a tank into the rolleron fins thru a hole in the base
as in the HPR article awhile back. Done some physics and I will only get 7

I'm still looking for info on ways to implement the hydraulically damped
hinges though.  This was going to be my level 2 cert bird withe a 35mm
pentax payload. I just got back some pictures and seen on a H180 the need
to make the airframe symmetrical as well as add something to null out the
roll.  



Quote:


> >Ahhhh--- this provides some insight into my rolleron design I posted a
> >couple weeks ago- I have also read Fluid's and done a program to
calculate
> >drag- the above should have been intuitve.

> Awww shucks, I missed that thread ( been trying to get
> back up to speed on a new-old job ;-)

> How will you spin up the rollerons ?

> -- kjh
> --
> ------------------------------------------------------------

> 1111 Seacoast Dr.  Unit 41   |  home:   (619) 423-4451     |
> Imperial Beach, CA   91932   |                             |