wing leading edge question

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.

Don't bother sharpening the leading edge. It truly won't make much difference until you get it too sharp, which is not good. It just looks funny being flat, but it will fly fine.

Ed Cregger

On Tue, 13 Sep 2005 21:53:04 -0700, Tim Wescott wrote 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.

Marty

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.

wrote in

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. :) mk

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 for building.

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

13th flight.

Then this young man discovered Bridi Hobby Enterprises models and lived much happier.

Ed Cregger

On Wed, 14 Sep 2005 08:17:29 -0700, Tim Wescott wrote in :

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!).

Marty

Hey, I like that idea -- that would let you do some testing with any plane that had plastic covering.

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 sensible.

"Martin X. Moleski, SJ" wrote in news: snipped-for-privacy@4ax.com:

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.

On Wed, 14 Sep 2005 09:37:44 -0700, Tim Wescott wrote in :

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 ).

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.

Agreed.

Marty

On Wed, 14 Sep 2005 18:32:06 GMT, Mark Miller wrote in :

Makes a lot of sense to me.

The tradeoffs we face are very different from the tradeoffs that need to be resolved in full scale.

Marty

On Wed, 14 Sep 2005 12:05:58 -0400, "Ed Cregger" wrote in :

There you go--data from an experiment!

Heh heh.

Even after all these years, the "OUCH!" comes through loud and clear.

Bridi is the father of many happy designs.

Marty

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. Oh well.

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...

Ed Cregger

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 same problem.

Dan

On 15 Sep 2005 08:34:48 -0700, Dan_Thomas snipped-for-privacy@yahoo.com wrote in :

As a general rule, you are of course correct.

I doubt very much that the fellow who started this thread is going to make the same mistake twice. :o)

Marty

It will fly fine S. If you are a beginning pilot get an experienced type to preflight the bird and trim it out on the first flight. Then have fun,

On Fri, 16 Sep 2005 17:48:24 -0400, "Ed Forsythe" wrote in :

And let us know how the experiment turns out!

I haven't built the kit, so I've been picturing your leading edge as if it were a rectangle that stood on its small side with the long side butted up against the ribs. If it's that system, you'd have to remove the covering pretty entirely to sand the airfoil shape into it.

If, instead, your piece of balsa is a square piece that is glued into notches in the ribs, so that a pointy edge is your leading edge, you may be able to sand the covering off just the pointy part, along with the unwanted balsa, and then just wrap a

1" strip around the leading edge. If you're working with something like Monokote, it will blend in very nicely with the original covering. Or you could use a contrasting color as if you intended it as part of the color scheme from the very beginning.

Marty

R O F L O L ! ! !

I like that rule...