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Re: Control surface configurations (was: Typhoon)
OK, a not so quick discussion about balance. I refer to aerodynamic or
hydrodynamic balance, not weight balance, although weight and bouyancy
balance needs to be looked at also (we don't want either to cause or restrict
movement of the surfaces). Looking at the tail of an x-form craft, each of
the control surfaces will generate lift opposite the direction of deflection
(assuming the water is mainly passing straight back, nose to stern, without
significant swirl, normally a valid assumption). This lift will be a
function of the angle of deflection, approximately a straight line until
nearly stalled (lift drops off suddenly). Each force generated can be broken
down into components (up/down and left right are appropriate for our case.
The upper left surface (points to 10:30 on a clock face) will deflect towards
the upper right or lower left (1:30 or 7:30). If the "X" is 90 degrees
surface to surface, the force (let's call it "F") on the upper left surface
will break down into .707F horizontal as well as vertical. That is, a
deflection twards 1:30 produces a force (on the sub) F towards 7:30 which
breaks down to a force of .707F to the left and .707F down. Draw this on a
piece of paper, it makes it a lot easier to visualize. In a similar manner,
the upper right surface points towards 1:30 and the forces for the same
deflection towards 4:30 produces forces of .707F up and .707F to the left.
Adding these together the up and down forces cancel and the net force is
1.414F to the left. If the control function causes the same deflection of
the surfaces and the resultant force is a straight line function, the forces
will always cancel. However, if we only have the top two control surfaces,
there will be a variable drag (drag is a function of deflection) force (to
the stern) at some waterline level. If this point is above the centerline of
the hull, the force will tend to rotate the hull nose up and the stern down
as it increases. The "V" tailed aircraft use this imbalance to bring the
nose up in a turn, kind of nifty actually. Slow moving subs (non-military)
don't need to "bank" into a turn so it is a detriment to them. You can
either design the tail control surfaces so they are centered about the
vertical axis or add two more on the bottom half to cancel out the drag force
imbalance. Now for the kicker! Little of this matters anyway unless the sub
is going fast enough to produce usable control by way of hydrodynamic
forces. Also the rest of the hull would have to be very smooth so as not to
put the surfaces in a turbulent "shadow" causing unpredictable stalls and
loss of control. Turning requires force at a distance to create a moment.
No force, no moment! That is why slow moving subs use thrusters (force
producers!) for directional control. Just put a thruster out as far as
possible on each side and you have a force at a distance (turning moment!).
Power one forward and the other back and you turn! Add a little more forward
to one and a little less back to the other and you go forward while you
turn. And so on and so forth, etc, etc...
Well, that's enough out of me for now, I am sure some have already gotten
bored and filed this post. Hope this helped.
Dick Morrisson
Michael B Holt wrote:
> On Sun, 11 Jul 1999 15:30:48 -0700 Dick Morrisson writes:
> >Michael B Holt wrote:
> >> Disadvantages are complexity and overcrowding of hardware at the
> >> stern and an interaction when turning which produces a vertical
> >> movement when only horizontal movement is desired.
> >
> >A properly designed and functioning "X" tail should be balanced, and not
> >result in horizontal movement during vertical commands or vice-versa.
>
> I wished I could talk back to them. I wanted to point out that aircraft
> don't seem to have that problem.
>
> I should quote the relevant sentences of the book. Anyone want to see
> all of it?
>
> >Even a half "X" like the Beechcraft Bonanza airplane is balanced! The
> key
> >words are "properly designed". If you need some help with this concept,
> let me
> >know, I can provide more detailed info.
>
> OK, let's do it. Tell us more.
>
> Is it possible that the Bonanza moves properly because it does not have
> the lower component? If the tail of the airplane deflects a bit, that
> might
> make less difference than the same deflection in a much smaller
> submarine.
>
> I can't think of any manned aircraft to use the X-stern design (I'll
> probably remember all of them right after I send this).
>
> Can we really use the shape, as our speeds are so low?
>
> Mike Holt
> --
>
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