If It Flies - 2011/04

April 2011 71
Proper setup of skis and floats
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
The ski must be free to allow the model to rotate nose-up for takeoff as well as maintain perfect alignment in flight. The spring/cable
attachment point must be directly above the ski pivot, or binding will result.
The top of the floats should be parallel to the wing chordline, and the step should be
slightly behind the CG. Note the waterline in the illustration.
HI, GANG. In the December column I
dropped an overly simple statement about
how rich running helps produce the CL
Aerobatics (Stunt) run. Although I fully
intended to make that oversimplification,
to be able to make that month’s material fit
the allotted space, it still amounts to a long
dangling thread—begging to be pulled.
Yes, it has been bugging me.
No, that is not what I am going to cover
this month. But I want to take a few
sentences to draw a box around the
subject, with the promise that we will
return to it, someday.
The “four-cycling” sound really is the
consequence of the engine’s doing a
repetitive strong-fire/weak-fire cycle.
That’s why the engine has a slowersounding
note. It’s not the only note you
would hear, and a frequency analysis of
the sound could easily recover the note
that corresponds to the engine rpm.
Yes, I know someone who actually did
this, and I’ll have to bug him for some of
his data. Describing how the strongfire/
weak-fire cycle happens will
eventually take up most of a month’s
column worth of space, but it is simply
amazing how many things happen in the
course of one revolution of a two-stroke
engine and how they set up the initial
conditions for the next revolution.
Enough of that. Let’s move on.
Back at the computer, after shoveling
snow yet again. This puts me in mind of all
the enjoyable frozen-finger flying I have
done from snow, using either skis or floats.
Yes, floats. They are handy when water is
available too.
As a youngster I flew at the
Hackensack Valley Club flying field in the
swamp within sight of the Manhattan, New
York, skyline. That field flooded each
spring. The ankle-to-knee-deep water
persisted for only two or three weekends a
year, and the decomposing carp left behind
as the water receded helped the grass to
grow lush and green.
It also meant that many of us in the
club had a set of floats handy in the
workshop, and maybe even an airplane that
was specially set up for water-flying.
I want to write about the proper setup
of skis and floats for flying from both
kinds of water: fluid and frozen. I’ve made
an observation about something I learned
as a young teenager, which surprised me at
the time. First I’ll cover the part that isn’t
surprising.
When you compare the effect that
aligned floats and aligned skis have on the
flying characteristics of an airplane, the
skis have less of an impact. No doubt that
it is because they are smaller.
On the other hand, even a tiny
misalignment of skis has a dramatic effect
on how the airplane flies, while even
substantial misalignment of floats has a
comparatively modest effect on the
airplane’s state of trim.
How can that be? The answer is in the
different shapes: flat plate vs. torpedolike.
Let’s look at a properly set-up
floatplane. This is where we stop talking
aerodynamics for a while and discuss
something of practical use!
While airplanes and their airfoils vary
all over the place, as a general rule, floats
should be set up parallel to each other,
both when viewed from the side and from
above. And their nominally flat top edge
should be set up parallel to the wing
chordline, and with the “step” in the
bottom of the float placed slightly aft or
directly under the CG. I’ll tell you why in
a moment.
The floats must be mounted firmly,
because water is much tougher than you’d
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think. Even a decent water landing
involves substantial impact loads in every
direction you can imagine.
At speed, water seems to grab things
with the intent to rip them apart. If you fly
from, or even near, water, you might have
seen the results of a water crash. The
pieces are smaller and more numerous than
after a crash on land. The boat racers (both
model and full scale) know this all too
well!
Mount those floats like you mean it;
after all, any care you take in aligning
them will be for naught if the mountings
distort and the floats misalign as a result of
“normal” takeoff and landing loads.
How big should the floats be? There are
three ways I know to answer this question,
and the first is simple. Those who
manufacture and sell floats or float-making
kits rate their products by the range of
airplane weights for which they are suited.
This doesn’t take into account the
difference between a large, light airplane
and a smaller aircraft at the same weight,
though. When in doubt, go up in size for
water and down in size if you plan to use
floats for flying from snow.
The second way to answer that question
is that floats are typically approximately
80% of the overall length of the airplane
from the propeller to the rudder’s end. If
you look at photos of full-scale
floatplanes, you’ll see that this four-fifths
ratio is typical.
The third method is probably going to
be the most useful for those of you who are
thinking about rolling your own. In
general, the front end of the floats should
be maybe a half wing chord in front of the
propeller(s).
As you read earlier, the “step” in the
bottom of the float needs to be a bit aft of
the CG. And since the aft part of the float
is normally slightly longer than the front
half, that determines the minimum overall
length.
With the right-size float, and with the
step located approximately 10% of the
average wing chord behind the CG, the
airplane should float with the back end of
the float in the water but not fully awash.
(Seaplanes require the use of nautical
terminology! “Awash” means that the
water almost covers the end.)
If the aft end of the float is under water,
move the floats back and recheck. If the
stern (another nautical term!) is out of the
water, move the floats forward.
Positioned as I have described, the
floats are set up in what should be the best
compromise between nose-over prevention
and ease of takeoff. I hardly think we need
to discuss nose-overs, except that you
should try to avoid them, but water spray
is another matter.
Water spray in the propeller will
literally chew up a wooden propeller in a
handful of takeoffs, and it robs an
unbelievable amount of power. Sure, a
nylon composite propeller might be a
better choice, but water spray in the
propeller disc needs to be avoided.
The chines, or spray rails, on a float are
like skinny downturned gutters on the
edges of the bottom of the hull. They
deflect water spray to both sides rather
than letting it go up into the propeller. A
1/4-inch-wide spray rail on the hulls in
front of the propeller might be the only
thing needed to solve a bad water-spray
problem, but good float designs probably
won’t need them.
By the way, that step on the bottom of
the float is there to break the Coanda effect
attachment of the water to the hull, making
takeoff much, much easier than it would be
otherwise. Aviation pioneer Glenn Curtiss
invented it, along with the aileron. (That
ought to start an argument!) The diagram
shows the preferred alignment of floats to
the airframe.
The only thing I haven’t covered is
water rudders. I am merely a part-time
water flier, but I’ve never used them.
Most pilots I know who do use them
create some sort of linkage or springloaded
mechanism to pull them up out of
the water for takeoff, because it makes a
big difference in takeoff performance.
The full-scale fliers normally lift them
out of the water as soon as they have
enough air over the rudder for acceptable
control authority. If I regularly flew from
water, I’d put steerable water rudders on at
least the starboard float.
Skis are another matter altogether. For
one thing, they can be much smaller than
floats, especially on hard-packed snow.
Come to think of it, large tires work on
hard-pack too. If you plan on flying from
72 MODEL AVIATION
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freshly fallen powdery snow, skis need to
be practically as large as floats!
The problem is that skis are flat-plate
flying surfaces, in the air, and small
misalignments have roughly three to five
times more effect than the torpedo-shaped
floats would if both were the same size.
The best method I have seen for setting up
skis for snow is to replace them with small
floats; I mean, to set them up so that they
can rotate in the nose-up direction but are
spring-loaded back to parallel with the
wing.
By doing this, the airplane can sit flat
on the snow, rotate nose-up for takeoff,
and then the skis return to parallel with the
wing chordline for flight. It’s amazing
how much aileron trim a little tweak can
require.
The setup is as shown in the diagram.
A strong but flexible cable attaches the tail
of the ski to an upright that is attached to
the main landing gear leg. This cable’s
length is adjusted so that when it is taut,
the ski is parallel to the wing. The cable is
kept taut by a rubber band or spring
(rubber bands aren’t all that good in the
cold weather) that is stretched between the
upright and the front or tip of the ski.
If the model has a tricycle landing gear,
the nose ski can be treated similarly or
simply locked at the correct angle. The
mains are the ones that need to rotate for
takeoff and the landing flare.
There is one other difference between
skis and floats, and it clearly falls in favor
of skis—for some airplanes, at least. Skis
offer little or no side area, while floats
have plenty of it low on the airframe. Not
only that, but floats have more side area in
front of the CG than they do aft of it,
which reduces the yaw stability of the
aircraft.
In the case of my trusty sport model,
the Carl Goldberg Models Tiger 60, this is
no problem. The long-tailed Tiger has yaw
stability to burn, while shorter-tailed
airplanes might develop the tendency to
sashay slowly from side to side, or even
develop an annoying tendency to drop the
tail into the turns. This can turn an
otherwise enjoyable aircraft into a
complete dog.
The cure is added vertical fin. Fullscale
pilots typically add a pair of
semicircular fins to the underside of the
horizontal stabilizer—one on either side.
I have seen a modeler add a fin to the
top of the stern (more nautical stuff!) of
each float to fix the yaw stability deficit,
so that the airplane reverts to its usual self
as soon as the wheels are replaced. I’ll bet
you could even use thick clear plastic if
you don’t like the thought of how it would
look.
So why, when all is said and done, do
skis create more trim problems than floats
when they are misaligned?
The lift created by a flat-plate flying
surface, even a fairly narrow one such as a
ski, is somewhere between three and five
times that created by a float-shaped or
fuselage-shaped object with the same
projected area and the same angle relative
to the oncoming air. The difference also
partly depends on the size of the object
and airspeed, or the Reynolds number.
Do you remember maybe two years
back, when I wrote about computing the
neutral point for a blended-body/wingtype
airplane, or any airplane for that
matter? I claimed that the area of the nose
and tail of the fuselage (as viewed from
the top) also figures into the calculation. I
have a refinement to that statement.
Because the nose or tail is a skinny
tubelike object, that area counts only close
to one-third as much as the same area
would if it were part of the wing or
stabilizer.
In aerodynamic texts there are tables
and graphs of this area effectiveness
multiplier for a variety of fuselagelike
shapes and for objects such as drop tanks.
They are useful for approximation
purposes, although long ago the
professionals changed to computational
fluid dynamics modeling. That’s right; a
“virtual wind tunnel.”
I’m out of room, but I enjoy the daylights
out of water-flying and hope that you
might too. I’ll be back next time. Until
then, have fun and do take care of
yourself. MA
74 MODEL AVIATION
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 e Academy is looking for the best helicopter
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