Laminar Flow Control

MIflyer · Nov 29, 2020

Odoaker said:
If I understand you correctly, your plane on the runway has zero AoA and zero lift. And during the flight?

Give it a little back pressure and up she goes, but the elevator trim is limited to reduce the severity of stalls. It will stall but recovers on its own even if you keep the wheel back, after which it will stall and recover again. And it cannot spin.

When I look at the drawing you posted, I have the impression that this FOWLER FLAP is the best, because the wing still has an "even" shape, in a sense, this flap "enlarges" the wing. It looks nice.

A Fowler Flap is indeed nice, if rather complicated as well as expensive and heavy.

Odoaker · Nov 29, 2020

I modified my model again and used thin sheet steel wings. It is not visible in the photo, but there is a thin sheet of metal underneath that black fabric. I threw the model out the window, it flew about ten meters straight, lowers the flight about half a meter and ended its flight trapped in the branches of a tree. But it's hard to experiment with such a model. The connection of the sheet metal wings to the cardboard fuselage was extremely unstable. To be precise, there was no joint at all, just a sheet of metal was pressed into the gaps in the cardboard. I will make the next model more solidly.

ThomasP said:
Yes you can make similar models out of thin (very thin) sheet metal, but the problem is that with every thing else being equal (ie same dimensions and geometry) as the density of the material used goes up so does the velocity needed to keep it flying.

That's what worries me. My sheet metal wing model was flying straight but for a short distance. I am afraid that over longer distances, he could slow down and fall.

tyrodtom · Nov 29, 2020

Back in the 50's when I was a kid they had small wood gliders for 5 to 10 cents.
3 pieces of about 1/8 inch balsa wood.
A fuselage with the vertical stabilizer in one piece, two slots, one for the wing to slide through, and one at the rear for the horizontal stabilizer.
You didn't have to add any weight at the front, you adjusted the CG by sliding the wing back and forth in it's slot .
Later I learned you could add dihedral to the wings by putting hot water on the center, and bending gently.
With properly adjusted wings, with a little dihedral in them, and a gentle launch, you could get some quite long flights even on a slightly gusty day.
I learned soon if you wanted a stable, long, flight. Send it off as close to stable flight as you can.

Your home made glider, IMO, is just way too heavy, for it's small wing spread.
Even if you do get your stability issues sorted out, your flights are all going to be pretty short.

They probably still make those little balsa wood gliders, though I'll bet they cost a lot more, look online.
They'll teach you some very small adjustments can make some big differences in flight performance.

Odoaker · Nov 30, 2020

tyrodtom said:
Your home made glider, IMO, is just way too heavy, for it's small wing spread.

But what's heavy about it? Mainly wings. If I make bigger wings, they'll be even heavier.

I can also keep the current wings, but give a smaller, lighter fuselage. Then the wings will be proportionately "larger". But this fuselage is already ultra-light.

I have seen models similar to the one you are talking about. Of course, during my childhood these models were made of some kind of plastic. But for me, the main "challenge" is to make something that glides, not to buy a model in a store. And I would like to make it from "commonly available" materials, and not some rare and unavailable in a normal home. I saw a movie where a guy makes a polystyrene glider model (anyway, terribly over engineered, the guy worked like Mitch Leary). But in my opinion, the polystyrene model is the easiest way. This is how to make a model out of paper. Rather, I mean some elements from old furniture etc.

Odoaker · Nov 30, 2020

MIflyer said:
What people often do not realize is that a flat or symmetrical wing still uses Bernouli because when it is at a suitable angle of attack a wing still creates a situation where the air has to go faster over the top than it does the bottom.

How was it with the P-51? When he was standing on the runway or in the hangar, did he have zero AoA or non-zero AoA?

tyrodtom · Nov 30, 2020

At least look at those models, you might pick up some tips.
You also could look at some videos of aircraft taking off.

A p-51 is tail dragger, so when it's sitting on the ground it has a very nose up AoA. so it accelerates down the runway until the tail provides enough lift to lift the tail to a near neutral AoA, then it accelerates to flying speed , gently dips it's tail to a slightly nose up AoA, and it lifts off.

MIflyer · Nov 30, 2020

Yes, taildraggers have even been described as airplanes with "Take Off Gear" since when sitting on the runway the angle of attack is already positive. Tricycle gear airplanes have "landing gear" because in that three point attitude they tend to want to stay on the ground, although some much more so than others.

Odoaker · Nov 30, 2020

tyrodtom said:
At least look at those models, you might pick up some tips.
You also could look at some videos of aircraft taking off.

I saw, but still do not know, whether a flat wing with a slight non-zero AoA will work. Of course, I believe that in a military jet it can work because a powerful engine will give a lift with a small AoA (without stalling). And, perhaps, in some absurdly light model at low speed, this will also be the case. The model can weigh so little that a minimal lift force will lift it. But I am thinking of a model made of something like a plastic ruler.

tyrodtom said:
so it accelerates down the runway until the tail provides enough lift to lift the tail to a near neutral AoA, then it accelerates to flying speed...

And during this acceleration, what is the AoA? If the pilot didn't do anything (besides just keeping speed), the plane would be traveling on the runway "forever"?

MIflyer · Nov 30, 2020

Most taildraggers will fly off the runway when it gets going fast enough. Tri-gear airplanes might or might not.

Reluctant Poster · Nov 30, 2020

Incorrect Lift Theory

Wings do not simply rely on Newton's Third Law.
Lift from Flow Turning

tyrodtom · Nov 30, 2020

Odoaker said:
I saw, but still do not know, whether a flat wing with a slight non-zero AoA will work. Of course, I believe that in a military jet it can work because a powerful engine will give a lift with a small AoA (without stalling). And, perhaps, in some absurdly light model at low speed, this will also be the case. The model can weigh so little that a minimal lift force will lift it. But I am thinking of a model made of something like a plastic ruler.

And during this acceleration, what is the AoA? If the pilot didn't do anything (besides just keeping speed), the plane would be traveling on the runway "forever"?

Near neutral, about 0 AoA. Less drag .
The pilot is not just keeping speed, he's accelerating.

You really need to watch some videos.

Odoaker · Nov 30, 2020

tyrodtom said:
The pilot is not just keeping speed, he's accelerating. .

Of course, but I was talking about the theoretical situation: the pilot draws maximum power from the engine, the plane accelerates to the maximum, but the pilot does not lift his nose up.

tyrodtom · Nov 30, 2020

Odoaker said:
Of course, but I was talking about the theoretical situation: the pilot draws maximum power from the engine, the plane accelerates to the maximum, but the pilot does not lift his nose u

I don't understand what you're asking.
The pilot either lifts the nose to a slightly positive AoA, or he runs off the runway into the bushes.
He'd have to do it deliberately, because the faster the aircraft goes, the less he'd have to lift the nose to climb.

gumbyk · Nov 30, 2020

I think one of the problems here is that you are wanting a 'one size fits all' answer, when there isn't one. I've flown tricycle aircraft that would run off the end of the runway if you didn't do anything, and I've flown them that would happily get airborne by themselves. If in the landing configuration there is enough of an angle of attack that there is some lift then it will fly, if there isn't, then it won't.
Likewise, there are tail-wheel aircraft that won't fly unless you lift the nose, either due to lack of power to accelerate to flying speed, of the fact that the wing is stalled in the three-point configuration. The Tiger moth will barely get airborne.

As for flat plates - take a look at the tailplane of a cub ,or Tiger - no aerofoils there

Odoaker · Nov 30, 2020

tyrodtom said:
I or he runs off the runway into the bushes.

This is the scenario I asked about. You see, earlyer I thought it was impossible at all, because every plane has a Clark Y airfoil and if it starts going fast on the runway then it MUST be a lift (even if the pilot is dead). And now I know that I was wrong because:

gumbyk said:
I. I've flown tricycle aircraft that would run off the end of the runway if you didn't do anything

The second thing:

tyrodtom said:
As for flat plates - take a look at the tailplane of a cub ,or Tiger - no aerofoils there

Honestly, I wasn't expecting a convex on the tail at all. I thought the tail of the plane is kind of medieval war dart (with the difference that, of course, the rudder and elevator are movable). But I wonder if it is possible a flat "main wing" - as in this picture:

http://www.ericbrasseur.org/glider_physics_28.gif

The author of this drawing seems to think it might work. BUT the weight and speed of this "airframe" are not given. When I made such a model with a flat plastic ruler as a wing - I did not notice any lift.

Odoaker · Nov 30, 2020

tyrodtom said:
Your home made glider, IMO, is just way too heavy, for it's small wing spread.

Thanks for the advice. This time I made such a model and the effect was quite good. The wing spread is proportionally larger here. So a flat ruler can, to some extent, act as a wing. I have the impression that AoA was greater here than in the Internet project. But despite this, the model flew very correctly without a stall. Perhaps during the flight the tail rose higher and the nose dropped lower, and therefore the AoA was smaller. It should also be noted that this ruler is not perfectly flat. It has a light convex, although of course a lot less than the Clark Y.

XBe02Drvr · Dec 1, 2020

Odoaker said:
perhaps, in some absurdly light model at low speed, this will also be the case. The model can weigh so little that a minimal lift force will lift it. But I am thinking of a model made of something like a plastic ruler.

If you're trying to make a hand launch glider to fly at hand launch speeds, weight is your enemy. "Absurdly light" is a basic requirement. I spent most of my teen years building flying models, including many gliders, ranging from 20 CM wingspan hand launch featherweights to 2 and 3 M tow launched and slingshot launched big gliders. These were all built out of balsa wood, most framed and covered with tissue paper, but some (including up to 2 M) with solid balsa wings formed by gluing flat pieces together at an angle and sanding them to a cambered airfoil shape. These solid wing gliders did not perform as well as the balsa frame and tissue paper ones, because they glided faster and at a steeper down angle than the built-up ones, but they were more durable in the inevitable collisions with trees, buildings, and the earth itself. I got a lot of exercise chasing after some of the larger balsa and tissue "floaters", as they would sometimes catch thermals and gain altitude, extending the glide while the breeze carried them away. One of them landed on the hood of a school bus that was stopped to let a few kids off, startling the driver.
What I learned:
WEIGHT IS YOUR ENEMY!
WING CAMBER IS NECESSARY.
CoG LOCATION IS CRITICAL.
IT'S ALL ABOUT LIFT/DRAG.
THE HEAVIER IT IS, THE FASTER IT GLIDES.

Odoaker · Dec 1, 2020

XBe02Drvr said:
If you're trying to make a hand launch glider to fly at hand launch speeds, weight is your enemy. "Absurdly light" is a basic requirement. I spent most of my teen years building flying models, including many gliders, ranging from 20 CM wingspan hand launch featherweights to 2 and 3 M tow launched and slingshot launched big gliders. These were all built out of balsa wood, most framed and covered with tissue paper, but some (including up to 2 M) with solid balsa wings formed by gluing flat pieces together at an angle and sanding them to a cambered airfoil shape. These solid wing gliders did not perform as well as the balsa frame and tissue paper ones, because they glided faster and at a steeper down angle than the built-up ones, but they were more durable in the inevitable collisions with trees, buildings, and the earth itself. I got a lot of exercise chasing after some of the larger balsa and tissue "floaters", as they would sometimes catch thermals and gain altitude, extending the glide while the breeze carried them away. One of them landed on the hood of a school bus that was stopped to let a few kids off, startling the driver.
What I learned:
WEIGHT IS YOUR ENEMY!
WING CAMBER IS NECESSARY.
CoG LOCATION IS CRITICAL.
IT'S ALL ABOUT LIFT/DRAG.
THE HEAVIER IT IS, THE FASTER IT GLIDES.

You're right about everything you write, but I'm still trying to get the ruler and other items found in my desk to glide I know there will be no "best" flying structures possible. The last model thrown (rather gently, not at high speed) from a height of 10 meters flew some 30 meters ahead and landed quite gently. I estimate that the pilot had a chance to survive the landing without any broken legs, although there were of course 4 nails in the role of the "pilot".

BTW BV40 is just an interesting "glider"! The pilot is surrounded by steel armor. Landing speed: 125 km / h. You think something like this could land in a field or a meadow?

Odoaker · Dec 1, 2020

MIflyer said:
Give it a little back pressure and up she goes, but the elevator trim is limited to reduce the severity of stalls. It will stall but recovers on its own even if you keep the wheel back, after which it will stall and recover again. And it cannot spin.

And how do pilots solve the problem of changing the center of gravity? I mean, when some extra passenger takes a seat behind your back.

MIflyer · Dec 1, 2020

Odoaker said:
And how do pilots solve the problem of changing the center of gravity? I mean, when some extra passenger takes a seat behind your back

You use the trim for level flight adjustments. And if you do the CG calculations and the passengers put you outside the allowed envelope, you do not let them get in the airplane.

Actually, Aft CG is better for efficiency. The tailplane provides Negative Lift to counterbalance the Positive Lift provided by the wing. Moving more weight aft decreases the amount of Negative Lift required to keep the airplane balanced. The bad news is that Aft CG is much more likely to cause the airplane to stall and spin and be unrecoverable.

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