Squares

General FreeBASIC programming questions.
Richard
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Re: Squares

Postby Richard » May 16, 2017 1:21

Albert wrote:With commercial jetliners the glide ratio is like 1:1 or less. ( They basically just fall from a mile high , at a 45 degree angle.. )
Not true. A Boeing 767-200 with fuel exhaustion has a glide ratio of about 12. That is a slope of less than 5° below the horizontal.
https://en.wikipedia.org/wiki/Gimli_Gli ... g_at_Gimli

Albert wrote:I imagine that bird wings are concave on the bottom of the wing and that , that might make a forward pulling vacume to create the negative drag???
The lower surface of a bird's wing flattens out when the wing and trailing edge feathers carry a load. A concave lower surface is only an advantage when flying at very low speeds with a wing made from a single layer of fabric stretched over a frame.
albert
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Re: Squares

Postby albert » May 16, 2017 2:20

@Richard

But the 767 was traveling 250 MPH when it lost power... that would allow it to glide for a longer time..

The glide ration is calculated as:
You take the plane up to an altitude of 1 mile and release it at speed zero, in still air , how far can it glide.

Commercial jetliners rate at less than 1 mile per mile.
Sail Planes ( gliders) in still air , rate at about 40 to 75 depending on their wingspan..
Hang gliders rate about 25

The airplane with the greatest glide ratio was the ercoupe , it had a stall speed of 19 MPH and could glide like 25 miles from a mile high , at start speed of 19mph. ( you might wikipedia the ercoup or ercoupe can't remember how its spelled i think it was made in 1940's in Germany?? )
Richard
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Re: Squares

Postby Richard » May 16, 2017 3:25

Albert wrote:The glide ration is calculated as:
You take the plane up to an altitude of 1 mile and release it at speed zero, in still air , how far can it glide.
Rubbish. Glide ratio is always specified at the optimum gliding airspeed, not starting at zero. At zero speed an aircraft would not be flying, it's control surfaces would be stalled and yes, it would fall out of the sky like a brick.

The ERCO Ercoupe, your "25 miles from 1 mile high" gives it a glide ratio of only 25.
That may have been pretty good for a powered aircraft during WW2. As an aircraft that was built to be very light-weight, it would be damaged if it flew at greater than 125 knots. There were sailplanes around before then with much better glide ratios.
But now for a L/D of 37, see; https://en.wikipedia.org/wiki/Virgin_At ... lobalFlyer
caseih
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Re: Squares

Postby caseih » May 16, 2017 3:31

If you took an airplane and released it at zero airspeed at a particular altitude, if it's balanced properly it will pitch its nose down as it falls and as it picks up speed the wings will start to create lift which will raise the nose and begin to glide forward, where it will settle in to its glide ratio which is about 12:1 for airliners. In other words if you released the plane at an altitude of 6 miles up, it would fall for a while and then begin to fly and start gliding, and end up on the ground some 60 miles away. That is if the wings didn't snap from the g forces as the planed pulled itself out of the dive.

By the way the record for the longest airliner glide is held by Air Transat flight 236 which had a fuel leak and glided without power for 120km over about 20 minutes. Tells me modern airliners are pretty darn efficient.
caseih
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Re: Squares

Postby caseih » May 16, 2017 3:45

albert wrote:I imagine that bird wings are concave on the bottom of the wing and that , that might make a forward pulling vacume to create the negative drag???

The wing shape you describe (concave on the bottom) is called an "undercambered" wing and this shape creates a lot of lift but also creates a lot of drag. In fact when airliners land they extend slats on the front of the wing and flaps on the back which change the shape of the underside of the wing to increase it's camber (makes it concave in shape). This is pretty important because it allows the airplane to have more lift at lower speeds safe for landing, while at the same time creating drag to slow the airplane down.

Several of my RC airplanes have undercambered wings which can lift a great deal of weight but not at any great speeds because of their drag.

So for jetliners, during take-off they extend slats and flaps slightly to create more lift, but not to the same degree as in landing. And soon after takeoff, once the landing gear are retracted, the slats and flaps are retracted as well to make the underside of the wing clean (that's the actual technical term) and reduce drag. Since the aircraft is already going fast by this point, the lift produced by the clean wing is enough to carry the airplane to full altitude. And of course at altitude the drag is reduced considerably because the air is thinner.

Oh and the reason undercambered wings create more drag is because the downward underside of the trailing edge causes the air mass to curl under and forward a bit. And Newton's third law tells us since the wing is causing the air mass to be pushed forward ever so slightly, this pushes back on the plane. Hence the drag that must be overcome.

If you want to explore quite a deep question, you can ask, how is lift created. And as long as you don't hold to the equal transit idea you'll be fine.
albert
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Re: Squares

Postby albert » May 17, 2017 0:49

I guess i'd have to find a dead sea gull.

and make a mold of it's wings.
and figure out all the joint movements that it could make..
and include in the fact that birds can flex their skin to make the feathers stand up a bit.

Then make a wind tunnel , and try to figure out how they get negative drag , to suck into a headwind.

Back in the mid 90's i built many airplane wings using 3 ft wingspans..
The traditional wing shape , flat on the bottom and domed on top. nosedives and flips upside down when the wind hits it.
You have to slope the underside to counter act the pushdown effect of the top half arch of the wing.
So if you split the wing in half top to bottom, the bottom portion needs to be arched deeper than the top half. to make it fly level.

You calculate the downward push of the top half of the arch, then the bottom half arch should be 1/2 the top arch and 50% more steep.

I was going to make a light aircraft out of polycarbonite sheeting. which weighs 50% of aluminum.
And was working on designing wings to get the best "glide ratio".

I didn't know or realize that some birds can position their wings to allow them to get sucked into a head wind..
So i guess god made me go in a big round about way to understand flight.

Now maybe i can design the negative drag wing.
caseih
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Re: Squares

Postby caseih » May 17, 2017 4:58

Did you make whole airplanes or just wings? Your description of what happened to them is puzzling. I have several model airplanes with different airfoil shapes including flat bottom, undercambered and semi-symetrical (same shape top and bottom). They all fly well when "wind hits" them without flipping over and they can fly straight and level when trimmed.

Negative drag is by definition thrust! :)
albert
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Re: Squares

Postby albert » May 17, 2017 23:21

@Caseih

I just made wings , and secured them to a fence post with rubber strands from out of a sock top, elastic threads.
And waited for the wind to blow, at my dad's house the wind always blows about 45 MPH from the SW all afternoon till night.

The flat bottom wing , with no bottom leading edge curvature, just nosedived.
Several others jittered up & down.

I tried all different types of wings , even an elongated "S" shaped
I had a book about the Wrights wing experiments and a book called "Janes: fighting aircraft of WWI"
I went through all the experiments...(several dozen wings).

The only ones that lifted , were ones that had a bottom , leading edge curvature, sharper than the top leading edge curvature.
if the top and bottom leading edge curvatures are the same , it will hover , but not lift.
caseih
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Re: Squares

Postby caseih » May 19, 2017 3:36

Oddly enough I have several RC model airplanes with various airfoil shapes (including no shape at all, just a flat plate) and they all definitely lift and fly. Some are more efficient at gliding than others. The undercambered wings don't glide particularly well, but they do lift well. the flat-bottomed wings glide quite well, though. My favorite plane has a semi-symmetrical wing so it can fly upside down as well as right side up.

I imagine 45 MPH is too fast of a wind for the wing size and thickness of your wings. A thinner wing will do better in higher winds. Most of my planes with wingspans between 20 and 40 inches fly between 20 and 30 mph. But also wings don't really fly well without the rest of the airplane to balance them.
dodicat
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Re: Squares

Postby dodicat » May 19, 2017 11:51

45 mph is a full gale.

Regarding Bernoulli and the equal transit idea, it is not fair (IMO) to compare a paper plane (flat wings) to a shaped wing.
A paper plane merely (IMO) falls to the ground slowly at a small angle, as a parachute falls to the ground slowly at a large angle.

I think flying upside down is forcing the wings aspect to use Newton's idea of thrusting upwards under big throttle.

I made gliders and little propeller planes when I was young.
Radio control was expensive in those days.
We just had to follow the whims of power flight on foot.
caseih
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Re: Squares

Postby caseih » May 19, 2017 19:06

What is flying if not falling and missing the ground? :)

There's a company that makes a little tiny battery/motor/prop/rc receiver unit that's designed to attach to the back of paper airplanes. Kind of a gimmick to be fair, but it does actually fly. A paper airplane wing very much generates lift. And it turns out a flat plate does generate lift also. Many RC airplanes are nothing but flat foam plates. Generates lift, but a flat plat wing also has a fair bit of drag. They will glide without thrust, just not as well as a smooth airfoil shape. They might have less drag than an undercambered wing, but not nearly as much lift of course. A flat plate can only generate lift due to the angle of incidence. And the underside of the wing causes a fair amount of drag, similar to why an under-cambered wing has more drag. Whereas an airfoil has a built-in incidence, and a smooth air path.

Turns out that whether you use Bernoulli's equation or Newtons laws of motion the results are the same. Wings generate lift by pushing air down.

The equal transit idea is a fallacious way to trying to explain Bernoulli's equation I think. The fact that wings can fly upside down pours cold water on equal transit.

It's still true, though, that as air is forced up and over the air foil (or flat plate) it does lower the air pressure, so the increased air pressure under the wing creates lift. But the two air masses don't join up again. Wind tunnel tests can clearly show that. Or you can think of the wing as directing air down as it flows over (and under the wing) and the specific impulse of that reaction creates lift. Anyway it's really interesting and quite a deep subject.

The Wright brothers' real breakthrough was that they recognized that a wing is inherently unstable, but rather than trying to stabilize it with other large wings as many others were trying to do, the key is to balance this instability with other forces. So they figured out that a heavier nose can be balanced with a canard (or a rear horizontal stabilizer nowadays) countering the nose's desire to go down. So in a modern aircraft the center of gravity is always in front of the center of lift so that the tail has to constantly push down slightly. This brings about inherent stability because if the aircraft is flying too slowly the nose drops and gravity accelerates the craft. This in turn causes the tail to exert more downward force (or the canard to exert upward force on the nose), which raises the nose and slows the aircraft. This repeats until equilibrium is achieved. It's genius! The Wright brothers may not have been the first to actually "fly" but they were the first to recognize how these things can bring about stable flight.

Now of course we have planes like the B-2 which are just wings, and the only thing that makes them flyable is computers constantly adjusting control surfaces and thrust.

Anyway, I'm rambling. Just find it interesting.
Richard
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Re: Squares

Postby Richard » May 19, 2017 23:01

caseih wrote:Turns out that whether you use Bernoulli's equation or Newtons laws of motion the results are the same. Wings generate lift by pushing air down.
Do they really?
That does not explain why the pressure above the wing is reduced by more than the pressure below is increased. Surely the wing lift is generated by the partial vacuum above the wing pulling air down behind the wing. Airfoils suck.
albert
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Re: Squares

Postby albert » May 19, 2017 23:39

The little rubber band powered , balsa wood planes you get in the corner stores , where you buy kites. have a flat wing..
The wing goes into a slot, that's pitched up slightly front to back , several degrees . so it acts as a kite to provide lift.

The flat tail plate keeps the wing from planing out ( flying flat ) , it weights the back end down , to keep the wing angled up , into the wind.
So it always acts as a kite.

The rubber band propeller keeps the plane flying to counteract the push back effect of the angled wing.

But somehow seagulls can drift into a head wind , by adjusting their wings. ( negative drag )
So some part of the seagull wing , creates a vacume in front of the wing to pull the seagull into the wind..

If commercial planes could suck into a headwind, they could save millions a year on fuel costs..
caseih
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Re: Squares

Postby caseih » May 21, 2017 1:39

Richard wrote:
caseih wrote:Turns out that whether you use Bernoulli's equation or Newtons laws of motion the results are the same. Wings generate lift by pushing air down.
Do they really?
That does not explain why the pressure above the wing is reduced by more than the pressure below is increased. Surely the wing lift is generated by the partial vacuum above the wing pulling air down behind the wing. Airfoils suck.

Lift is a complex interplay of lots of different principles and actions including the Coanda effect (which as I understand it is what glues the laminar flow to the wing surface), the Bernoulli effect, and Newton's third law. The idea that lift is generated by suction turns out to be the exact same thing as saying the wing pushes the air down. Both ideas are essentially equivalent and both are right. Pushing the air down creates a partial vacuum or low pressure zone by definition.

Think about a propeller or even a common fan. It's essentially a rotating wing. Whether you say it pushes the air or is pulled by vacuum, the fact is air moves and the specific impulse of that air is thrust (lift). So it's one and the same thing. Air that's pushed down by a wing is by definition higher pressure. If a plane stays in they sky, there must be constant thrust equivalent to the force of gravity. Barring rocket engines, the only way to generate this force is to push air down.

The interesting bit is that a great deal (most) of the lift and downward air movement is caused by the air over the wing, rather than the air under the wing. When a wing stalls the laminar flow of air detaches from the top of the wing and thus you lose the effect of the specific impulse of the moving air mass over and down the trailing edge. Air under a wing can and does generate lift but as I said before under-cambered wings end up with a curling of the air mass forward at the trailing edge, which creates drag (since the air mass is attached to the wing).

The thing that is definitely false is the idea of equal transit being responsible for the pressure change. It's a bit amusing to see so much confusion on lift, even from pilots. For such a simple idea, it's quite complicated. Here's a good summary of what I was trying to say: https://www.grc.nasa.gov/www/K-12/airplane/right2.html . Note the summary line: "Lift is a force generated by turning a moving fluid." The entire site is actually a pretty good exploration and explanation of lift. Best I've seen.
Last edited by caseih on May 21, 2017 2:08, edited 2 times in total.
caseih
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Re: Squares

Postby caseih » May 21, 2017 1:59

albert wrote:But somehow seagulls can drift into a head wind , by adjusting their wings. ( negative drag )
So some part of the seagull wing , creates a vacume in front of the wing to pull the seagull into the wind..

If commercial planes could suck into a headwind, they could save millions a year on fuel costs..

If a seagull is soaring into the wind and not losing altitude that's because the air flow is angled upwards at a rate that, given the bird's glide ratio, equals the force of gravity. I know this can occur because I've flown RC airplanes in such winds. Could just be a thermal, could be upward winds reflecting off some surface feature, or it could be due to wind and pressure gradients. If there are different gradients of wind speed that a bird flies through and as it crosses the boundary between different gradients it can trade speed for altitude.

As for sucking a plane into a headwind, well, that's by definition what an engine does! :)

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