I just completed a Sig Rascal and did not sand the leading edge of the wing.
It is not rounded and somewhat flat. Now that the plane is covered, should I
remove the covering and
sand the wing round or can I still fly?
This is my the first time I have made a plane from a kit, and I was told not
to fly from some people
and others have said it is ok to fly but the plane will have a bit of a
I would appreciate any feedback!
This is almost all book learning, but:
A sharp leading edge will lead to a wing that stalls much more readily
than with a rounded one. Having a nice round leading edge lets the air
go around gracefully when you change angle of attack (i.e. tilt up or
down). It's that air staying attached to the wing that keeps it in a
non-stalled state. In fact, you'll often see small pieces of angle (6
inches long by 1/2 inch) riveted onto the leading edges of the wings of
full-scale aircraft, right at the wing root. Those are there to induce
a stall at the wing root which propagates out to the tips, instead of
having the tips stall and causing a spin.
If the edge is really sharp then the wing will be much more prone to
stalling than it should. This tendency will translate into a desire to
tip stall with little or no warning.
If you're a good flyer you might want to try it out, being very gentle
on the turns -- but don't blame me if it tip stalls and spins in.
TIm took the words right out of my mouth.
Nevertheless, Ed Cregger may be right that it will only look
funny but fly OK.
If I were you, I'd get the best pilot in the club to test fly it for me.
If the thought it flew OK, I'd then try it myself.
I hate covering. The thought of tearing off covering and doing the
job all over again is awful. At the same time, I hate rebuilding
crashed planes. The first six or eight times were OK, but the
older I get the less tolerant I am of my own stupid human tricks. I'd
rather fly than build.
Perfectionism breeds depression. When I'm building and covering,
I use the "six feet and squint" rule. If it looks OK at that distance,
I figure it's gonna fly OK. If anyone gets closer than six feet and
starts to go over the plane with their micrometer eyes, I'm ready
to give them a boot to the behind, so long as I'm confident their
trajectory will take them away from the airframe. I hate
rebuilding, you know.
This would be an interesting thing to test if you could come up with a
way to easily stick on a different leading edge shape and fly. Compared
to what a full-scale full-speed airplane sees air looks like cold
molasses to a Rascal -- this is going to make a difference for things
like the detail of the leading edge.
strips. How about triangle stock taped on the leading edge with carton
sealing tape? You could always shape some stock and tape it on. I may make
some pre-covered stall strips to tape on but this plane is more capable than
my skills even without stall strips. :)
I have done that in the past and it is a good way to test all sorts of neat
things. Asymetric stall strips and other wild sorts of things like flat
LE's. Yes, the sharper the LE the more vicious the break when the wing
stalls. The problem is you have to have a high speed wing to recognize what
you are looking at, OR be very slow at altitude with a good deal of nose
weight. It normally takes about 3 different shapes before you begin to
recognize what you are seeing.
I think the difference in Reynold's numbers makes small-aircraft
design easier--we can do things with the "cold molasses" that
larger-scale aircraft can't. Think of the rubber-band powered
balsa airplanes with flat airfoils.
I seem to misremember one author saying that he had even
strapped on a trainer's wings upside-down and backwards
to win a bet. That seems insane, but I wouldn't be surprised if
the story were true.
One of the columnists who wrote about airplane design said that
the NACA studies for the shape of wing tips offered no guidance
for model aircraft--the efficiences gained in theory are too small
to recognize in practice.
I don't think it would be too hard to design a wing for leading
edge experiments, but I wouldn't expect to learn much from it.
I like the look of rounded leading edges, as a general rule,
and I'm not interested in getting too close to the snap-stall-spin
edge of the envelope (I hate rebuilding!).
There are some pretty serious aerodynamicists (mostly German) working on
model airplane airfoils. It's mostly the sailplane crowd, with some
pylon racers as well. I think for regular sport flying it doesn't make
enough difference to be noticeable unless you really diverge from what's
Ah--I hope I didn't imply that there was no way to reason from one
scale to the other. But there's an art to figuring out what effects will
scale and what won't scale (either up or down). (Ship designers
face the same problem when reasoning from measurements made
on models to what will happen with full-scale ships.)
Scaling up the flat-airfoil rubber-band toy to full-scale would
be foolish. The flat wing flys ok when carrying an ounce or
two, but it wouldn't perform well for a human-carrying plane
(cf. the Rubber Bandit <http://www.rubberbandit.org/ ).
Scaling down full-scale airfoils for models doesn't work well, either,
as a general rule. I think that's why warbirds used to fly so poorly.
Kit designers learned to fit a nice sport wing into the planform of
the prototypes and the world has been a better place ever since.
It's true that we can get away with a lot, but it's not the Re effects -
it's amazing what absurd full-scale airplanes actually flew. But in
general, we're a lot less fussy than the full-scale builders. Consider a
nice sporty .46-powered trainer; its weight is about 10% fuel at takeoff
and it has enough fuel for only about 15 minutes of flying at about 50
mph. Can you imagine a real plane that operated like that? Even if you
went to airliner-style fuel loading (30% of takeoff weight), you'd get
only 45 minutes of air time, and at a cruise speed of 50 mph, you'd have
a range of less than 40 miles! Hard to run a business that way . . .
Looking at it another way - if Boeing spent $5Billion to develop an
airliner that was 20% draggier than they thought, so it burned 20% more
fuel than it's competitor, they'd be out $5Billion.
When I build a model plane, I don't even know what the drag coefficient
is or should be. And if it's 20% draggier than it should be, then I'll
just have the throttle pushed a little higher in level flight. Over the
life of the airplane, say, 100 flights, I'll probably use an extra $2
worth of fuel. I'm okay with that.
We also grossly overpower our models (oh, yeah!), so we can tolerate some
inefficiencies elsewhere in the design. We put in big washouts, when we
remember to, and we tend to have larger-than-scale tails because we want
the stability and we don't want to control our CG to within a small
fraction on an inch and we don't care so much about a little extra weight
and drag in the tail.
And, if the plane will do something terrible under conditions that occur
once every thousand flights - well, on a model, most of 'em don't live
that long anyway. But if it's a real plane, you'd have a few dozen
crashes PER DAY, and people would be getting killed.
Reynolds number effects certainly have a big impact our models, but it's
generally in a bad way. The only benefit is that, since we won't get the
penefit of a the precise airfoil shape (for example), we don't have to
spend a lot of time trying to GET the precise airfoil.
"If we can hit that bullseye, the rest of the dominoes will fall like a
house of cards...Checkmate."
Once upon a time there was a young man that had built and flown mostly
control line models with pre-shaped leading edges. He found himself building
an Andrews Trainer Master in great haste. Working a full time job and
playing music in a band most evenings, this young man did not have much time
Looking at the Andrews plans, he noted that the leading edge ON THE PLANS
was flat and not curved at all. Puzzled by this, the young man called the
manufacturer and was promptly told to build the model exactly as shown on
the plans. That is what the young man did. The leading edge of the wing had
a 1/2" to 3/4" flat all the way across the wing. Following the
manufacturer's instructions, said model was built with this flat area.
The model was test flown by this young man and flew just like every other
model airplane at the field that day. Other models had rounded leading
edges, his did not, but his flew well.
As you have probably guessed, I was this young man. I took the
manufacturer's instructions literally. I attribute this to lack of sleep.
What the manufacturer did not mention was that if you were using plastic
film covering instead of silk and dope, the fiberglassing effect of the silk
and dope at the wing's center section was missing. The result was a 100'
power dive into the ground when the wing snapped. This resulted in a total
wipe out of the brand new engine and brand new Orbit radio, which was far
from being cheap in those days. I never forgave the son-of-a-bitch that
designed that airplane.
This was new territory we were walking into. Monokote had just came out and
no one told us about the lack of strength of many kit wings without the
benefit of silk and dope. To add insult to injury, the crash occurred on the
Then this young man discovered Bridi Hobby Enterprises models and lived much
That debacle is one that I cannot explain sufficiently to this day. I have
never seen anyone else do anything as stupid as building a model like that.
I had one of OS's then very hot Pylon special engines on that model. It
looked just like the H model that I had owned before, but made an
unbelievable wail when running. I was buzzing past many of the sixty sized
pattern models that inhabited our field. That probably helped in breaking
the wing during a full power split-S.
Ah, the good old days...<G>
Air has trouble flowing smoothly around corners. It prefers
rounded shapes. A flat-faced leading edge probably won't affect the
model's performance in flight, since modelers have to keep speeds
higher than normal to avoid stalling; they don't have the benefit of
being in the cockpit and having an airspeed indicator.
It's in the low-speed regime where that flat leading edge's
behavior will show up. At high AOA the stagnation point is below and
aft of the leading edge; that's the point where the oncoming flow
separates to flow either under or over the airfoil. The air moving over
top has to actually move forward and negotiate two corners; it gets
torn up and stall will happen sooner and be less predictable. When the
airplane is a bit slow and suddenly rolls over and crashes right after
takeoff or just before touchdown, the builder may or may not understand
what happened. If he doesn't, he might build another airplane with the
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.