How to determine the right fin size

Much have been said here about the
correct relation between fin size and dihedral, as
most of us know they are interrelated, for too
small fins and we have an aircraft prone to
Dutch Roll, and too big a fin and we have
an airplane that easily enters a spin.

I have wondered about how test pilots
find out the exact correct size and if the method
can be applied to models, and the answer
is an emphatic yes, and no, often our models
are too crowded. For power models it's probably easier,
as their fuselages usually are roomier!

What they do, is to trim the aircraft for glide
and then apply a tiny bit of, say left
aileron, to produce a little bank.
To counter the turn pilot pushes a little
right rudder so that the aircraft
continues forward in a straight line, if
a bit crabbed (yawed).

Now comes the tricky bit for us modellers -
they let go of the stick and see what happens.
This is by no means equal to us letting go
of our tx's sticks, as that means the servos
neutralizes the ailerons.

Our valiant test pilot then repeats the procedure
with the rudder flowing free (feets off the pedals).

If the plane lowers the low wing with the hands off the
stick the fin is too big, or with the ailerons fixed and the
try to turn into the lower wing - entering a spiral dive
the fin is too big, if the wings try to reduce the bank
all is well :-)!

If the ailerons are locked and the rudder let free it should
not try to enter a spiral dive either!

(all these facts gleaned from Darrol Stinton's Flying Qualities
and Flight testing of the Aeroplane, publ. Blackwell Science)

Well, how can we apply this to our one-off designs, modifications,
et cetera? Or if I am a producer of commecial kits, what then?

What we need is a possibilty to "unlock" the ailerons and rudder,
and there is a possibility, and a rather neat one I think :-)!

Copyright, me, of course :-)! :

What you do is reinvent the centre aileron servo system (one servo for
both ailerons, by the means of Bowdens, push-pull lines, push-pull rods,
whatever), but this time it is just a servo without innards, but the output
arm in place , so there is little or no resistance. This servo is then
connected to the aileron servo by pull-pull lines. and the aileron servo
sit on an old classic servo tray, or you use two aileron servos and
an electronic mixer, onboard or in your tx, one servo pulling
the dummy servo one way and one the other.

For the rudder things are much simpler, as a common system is pull-pull.

Here you simply mount the rudder servo on a mixer slider (as used on many
early flying wing), or if the pull-pull system includes a slider already (to
tension the lines correctly a slider a spring is often incorporated), you just
replace the spring with a servo.

As you see, some sort of pull-pull system is needed (could be mainly pushrods
except for the last bit, if you like), as pull-pull is the only way to make
the rudders trail free, that I see workable.

Anyone wanting to use this test system has to send me a picture of the
plane before, and after, modification!

Muchos gracias! Thank you! Tackar, tackar! Danke, Merci!

Tord,
Now taking off his thinking cap to repair his ElectriCub (don't ask
why I need to repair it)!
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to soaring-request@airage.com. Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.

>as most of us know they are interrelated,
>for too small fins and we have an aircraft prone
>to Dutch Roll, and too big a fin and we have
>an airplane that easily enters a spin.

Don't over-generalize here. The effect of
the vertical tail on spiral stability
critically depends on the sign of spiral
stability already present (i.e. is it
already stable or unstable). The latter
can be estimated by Blaine Rawdon's parameter,
which I call "B":

B = EDA * (tail_arm/span) / CL

Case 1: B<5, spiral stability is negative (unstable).
Enlarging the vertical tail will make the spiral grow faster.

Case 2: B>5, spiral stability is positive (stable).
Enlarging the vertical tail will make the spiral die off faster.

The crucial point is that B itself does not depend
on the size of the vertical tail.

In practical terms, enlarging the vertical tail on
a spirally-stable poly glider (Case 2) will actually
IMPROVE its spiral stability, contrary to popular lore.
This is observed in poly DLGs, which have huge
vertical tails but still have good spiral stability.
Ditto for aileron gliders with lots of dihedral.

Enlarging the vertical tail on a flat-wing aileron glider
(Case 1) will indeed make the spiral stability worse,
but I don't see this as a major consideration.
Dutch roll damping is much more important,
especially during a spot landing. Has anyone
ever seen a glider with a vertical tail which was
too big from the viewpoint of controllability?
I doubt it.



RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to soaring-request@airage.com. Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.