how does fin thickness effect stability

how does fin thickness effect stability

Post by The Silent Observe » Sun, 23 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. <<

> Huh?

> When a fin (or wing) stalls it goes from producing lift to just drag. The lift
> that a fin can generate as a stability corrective force is more than it can
> produce as mere drag for any given angle of attack.

Wait a minute, George.  It isn't that an airfoil "stops" producing lift
when it stalls -- it's just that there's a very sharp drop in the
lift/drag ratio.  There have been a number of experiments with "stalled
flight" and the military has proposed utilizing stalled flight as a
standard maneuvering enhancement in the next generation of fighters.

A stall is generally avoided in normal flight, though, because the
turbulence over the wing produces so much extra drag, combined with a
distinct reduction in lift, that the result is usually a sharp drop in
altitude -- a very hazardous thing, given that it's most likely to occur
close to the ground during landing or takeoff.

Bill's right, though, in that a flat plate generates all it's lift in a
stalled condition.  Likewise, a simple kite, such as the ordinary
diamond kits most kids fly at one time or another, normally flies in a
stalled condition, even though its airfoil is somewhat more complex than
a flat plate -- the drag of the stalled sail helps keep the kite line
taut, even as the lift holds the kite and line off the ground.

It was my understanding at one time that an airfoil, even one with zero
camber (i.e. "symmetrical"), would generate more lift at low angles of
attach than a flat plate; I've been corrected on this, and now
understand that the lift is almost the same in unstalled flight, but the
streamlining delays the stall, while reducing induced drag, making it
more desirable in most cases.

Quote:
> So you do not want a fin to stall easily. Therefore an airfoiled fin produces
> better stability than a flate plate fin.

I'll second this part of the statement -- I've seen any number of
flights of Long Shots where the upper stage fails to ignite, with the
rocket often ejecting the D booster motor; in most of these cases,
instead of the lawn dart one might expect, the rocket will tailslide,
and fall into a flat horizontal descent, with all fins stalled -- and
thus avoid serious damage.  That's good for the specific case, but
stalled fins in normal flight are generally bad -- they slow the rocket,
but apply their restoring force mostly opposite the direction of travel,
instead of always at right angles to the fin surface; that means (in
most cases) that less restoration is available because only a fraction
of the drag acts to restore the rocket to pointing in the direction of
flight.

--
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 » Mon, 24 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. <<
:
: Huh?
:
: When a fin (or wing) stalls it goes from producing lift to just drag. The lift
: that a fin can generate as a stability corrective force is more than it can
: produce as mere drag for any given angle of attack.  

Are you talking about an airfoil or a flat plate (such as most rocket fins)?

An asymmetrical airfoil produces lift. Neither a flat plate or a symmetrical
airfoil produce lift when aligned with the airflow. To produce "lift", they
have to be kept at an angle to the airstream.

Airfoils generally do not stall completely - without large changes of angle of
attack.  With small changes, both drag and lift increase.  One of the primary
purposes of an airfoil is to make it less sensitive to changes in angle of
attack.

With a symmetrical airfoil, there is a small increase in lift at small angles
of attack, and also an increase in drag.

With a flat plate, there is no lift. Any angle of attack produces a fairly
large increase in drag - more than for a symmetrical airfoil. There is also
a force vector due to the airflow striking the "exposed" surface at an angle.

: So you do not want a fin to stall easily. Therefore an airfoiled fin produces
: better stability than a flate plate fin.

It is not that simple. Your statement is certainly true for an airplane, which
relies on lift to stay in the air.

--


 
 
 

how does fin thickness effect stability

Post by James R. Cunningha » Mon, 24 Nov 1997 04:00:00


Quote:




        some stuff snipped out

Quote:
> It was my understanding at one time that an airfoil, even one with zero
> camber (i.e. "symmetrical"), would generate more lift at low angles of
> attach than a flat plate; I've been corrected on this, and now
> understand that the lift is almost the same in unstalled flight, but the
> streamlining delays the stall, while reducing induced drag, making it
> more desirable in most cases.

Hi, I'm Jim Cunningham, and this is my first posting here.  I don't know
much about model rockets (wish I did), but I'd like to comment on
induced drag anyway.  The coefficient of induced drag equals the coeff.
of lift squared, divided by (pi times the aspect ratio).  The
coefficient of lift equals lift divided by (half the density times the
velocity squared times the planform area).  Induced drag is a function
of circulation, is unrelated to streamlining, and is not affected by
it.  In fact, when no lift is being produced, there is no induced drag
no matter what shape the airfoil has.  As a specific example, an
asymetric airfoil when at the appropriate negative angle of attack
produces no lift and consequently no induced drag. Of course, other
components of total drag are quite strongly affected by streamlining.

Quote:
> > So you do not want a fin to stall easily. Therefore an airfoiled fin produces
> > better stability than a flate plate fin.

> I'll second this part of the statement

I'll third it.  JimC

        more stuff snipped out

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

Hey, me too.  JimC
 
 
 

how does fin thickness effect stability

Post by Geoff Gran » Tue, 25 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. <<
>:
>: Huh?
>With a flat plate, there is no lift. Any angle of attack produces a fairly
>large increase in drag - more than for a symmetrical airfoil. There is also
>a force vector due to the airflow striking the "exposed" surface at an angle.

I believe that you are wrong in saying that flat plates do not create
lift.  They will certainly produce less than  an airfoiled surface,
but the distance (air has to travel) over the "top" of a flat plate is
still greater than that of the distance over the "bottom" in a +AoA.
Still, I'll check my source and post back if my memory failed me.
Perhaps the effect is just not taken into account usually?

Quote:
>: So you do not want a fin to stall easily. Therefore an airfoiled fin produces
>: better stability than a flate plate fin.

An airfoiled fin also produces more skin friction drag, and probably
interference drag as well. Keep in mind that the fins only need to be
as big as they are because of low starting velocity.  The size of fin
needed at 600mph is miniscule compared to fin size needed at the end
of the rod.  In other words, you're _usually_ only using the fins
until T+ 3 or so, after that most rockets are going fast enough to
correct their flight profile with a fin around 1/10 the sized used.
 
 
 

how does fin thickness effect stability

Post by HogDenv » Tue, 25 Nov 1997 04:00:00


Fins are a hassle to attach.  Don't know why you'd want to bother.
hogdenver

 
 
 

how does fin thickness effect stability

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


:
: >With a flat plate, there is no lift. Any angle of attack produces a fairly
: >large increase in drag - more than for a symmetrical airfoil. There is also
: >a force vector due to the airflow striking the "exposed" surface at an angle.
:
: I believe that you are wrong in saying that flat plates do not create
: lift.  They will certainly produce less than  an airfoiled surface,
: but the distance (air has to travel) over the "top" of a flat plate is
: still greater than that of the distance over the "bottom" in a +AoA.
: Still, I'll check my source and post back if my memory failed me.
: Perhaps the effect is just not taken into account usually?

I am assuming that the thickness of the plate is negligible.  For small
angles of attack, the restorative force is primarily drag - there is
little lift generated as the V(delta) is very small.

For larger angles of attack, there is a lot of turbulance behind the fin,
so there is a fair amount of lift.  You can see this in some of the flat
kites that are flown - such as one I have.

--