Chrisjdurber
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What Happens During Take Off?

Thu Mar 15, 2007 4:42 am

Okay! please dont shoot me down here!! have got the godkids here and am just doing my duty! So the questio is....


when you are on the runway and you hear the engines roar and start to move what is happening? How does the plane stay on the runway and then all of a sudden rise up?

(okay guys its me now! have got the godkids sat here and have promised them an answer so please dont be too complicated!!!! lol megans main concern is that how does the captain know when to make the plane lift (quote) and how fast does it go) okay there you go. .....

thanks!!!!


p.s. These two are future airliners/net buffs!!!!!
 
PanAm747
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:48 am

Lift is created by speed. Once an aircraft has reached a certain speed, the air rushing over the wings creates a vacuum under the wings that allows the plane to lift off. When airborne, the only thing keeping it moving is its speed.

There are three points involved in take-off. One is the point at which the plane can no longer be stopped safely on the remaining runway. One is the rotation point, where the nose is lifted up and provides the final bit of lift needed to get the plane airborne. The final is the airborne speed.

If the pilot doesn't set the wings properly for take-off, basically you've got a jet-powered hot rod that could drive all the way to Chicago. Only by the proper combination of engine setting, wing settings (flaps and such) and proper mathematical computations involving weight, temperature, and thrust available, can the flight crew properly and safely get their plane off the ground.
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graphic
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:49 am

As the plane speeds up, the wings start to bear the weight more than the wheels. The pilot rotates about the time the wings bear the full weight of the airplane and continues to accelerate across the ground in a slightly nose-high attitude until the wings have enough lift to not only support the full weight of the airplane, but to also create a lifting force of their own, and the plane rises.

Or something like that.
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PanAm747
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:52 am

Quote:
As the plane speeds up, the wings start to bear the weight more than the wheels. The pilot rotates about the time the wings bear the full weight of the airplane and continues to accelerate across the ground in a slightly nose-high attitude until the wings have enough lift to not only support the full weight of the airplane, but to also create a lifting force of their own, and the plane rises.

 checkmark 

Extraordinarily well put!! I wish I could have said it that well.
Pan Am:The World's Most Experienced Airline - P(oor) S(ailor's) A(irline): San Diego's Hometown Airline-Catch Our Smile!
 
kcrwflyer
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:53 am

I thought that the wings were what kept it in the air the whole flight, and not just takeoff. The shape of the wing provides lift as air passing under the wing moves faster than air passing over the wing.
 
dtwclipper
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:55 am

Quoting Chrisjdurber (Thread starter):
How does the plane stay on the runway and then all of a sudden rise up?

Many people here will give you all sorts of scientific answers and talk about lift and drag and co-effiecnts, and quote something called the Bernoulli's principle, but the truth of the matter is, its pure magic!
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Mir
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RE: What Happens During Take Off?

Thu Mar 15, 2007 4:56 am

Lift is created by the air moving around the wing. The engines provide the speed that gets the air moving fast enough to create enough lift to make the plane fly. The plane rises off the runway because when it rotates, the angle that the wing makes with the direction of travel (called angle of attack) increases, which has the effect of creating more lift for any given speed. The increase of lift is enough to get the plane off the ground.

Quoting Chrisjdurber (Thread starter):
megans main concern is that how does the captain know when to make the plane lift (quote) and how fast does it go)

The rotation speed is based upon a number of variables (configuration, weight, etc.). They look it up on a chart (before takeoff), and then when on the roll, they watch the airspeed indicator for that number, and then they start pulling back on the stick. As for how fast it goes, generally anywhere from 120-160kts for an airliner.

I guess that's the simple version.

-Mir
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haggis79
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:08 am

Quoting PanAm747 (Reply 1):
creates a vacuum under the wings



Quoting KcrwFlyer (Reply 4):
The shape of the wing provides lift as air passing under the wing moves faster than air passing over the wing.

most definitely not.... if the air under the wing would be flowing faster than the one over the wing you would have a lower pressure under the wing than over it.... effectively sucking the aircraft downwards not upwards.... certainly something you wanna avoid at all costs  Wink. Same thing holds of course for a vacuum under the wings...

in fact, it's the exact opposite: the air flows faster over the wing than below it creating overpressure below the wing and underpressure above it... both the overpressure below the wing as well as the underpressure above it result in an upward force onto the wing... so the airplane is "carried" by the combined forces of those two effects.
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redngold
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:11 am

Quoting KcrwFlyer (Reply 4):
I thought that the wings were what kept it in the air the whole flight, and not just takeoff. The shape of the wing provides lift as air passing under the wing moves faster than air passing over the wing.

This is true - the wings "carry" the plane throughout flight. However, without the thrust of the engines there would not be enough airspeed to create the lift. Therefore it is a balance between thrust and the angle of wings with relation to directional airspeed that keeps the plane flying. Most planes must keep a slight nose-up attitude to achieve this balance, usually around 3 degrees. This is why some people can tell when a plane begins to descend from cruising altitude; the pilot literally levels the nose and this creates more drag, thus beginning descent.
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pilotaydin
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:12 am

in the case of the A340, the earth's curvature enable it to get airborn, but all other aircraft go by newton's third law and the downwash/vacuum theory  Smile
The only time there is too much fuel onboard, is when you're on fire!
 
Pope
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:18 am

Here's a simple way to demonstrate all of this:

Grab a piece of paper. Hold it by its corners against your bottom lip. Blow over the top of the paper and see what happens.

Faster moving air exerts less pressure over the top surface of the paper than the still air below it. Higher (relative) pressure below pushes against the paper and causes the it to rise.

The engines produce the thrust that cause the air to move over the top of the wing. Due to the wing's shape the air moving over the top moves faster than the air moving below. Take that differential in air pressure per sq inch, multiply it by the surface area of the wing and you've got the amount of weight the aircraft can lift.
Hypocrisy. It's the new black for liberals.
 
LAXspotter
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:20 am

Quoting Pilotaydin (Reply 9):
in the case of the A340, the earth's curvature enable it to get airborn

I guess, you wont be flying TK's A340 anytime soon.  Big grin
"Patriotism is the last refuge of the scoundrel" Samuel Johnson
 
LASoctoberB6
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:20 am

Quoting Pilotaydin (Reply 9):
in the case of the A340, the earth's curvature enable it to get airborn,

i dont get it....
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jetjack74
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:24 am

Quoting Chrisjdurber (Thread starter):
(okay guys its me now! have got the godkids sat here and have promised them an answer so please dont be too complicated!!!! lol megans main concern is that how does the captain know when to make the plane lift (quote) and how fast does it go) okay there you go. .....

Check out my videos. I filmed this in a Northwest Airlines DC9 on a ferry flight from IAD-DCA. Flight time was approximately 18 minutes from take-off to touchdown. It's a complete phase of flight:
Taxi and Pre-take-off checklist


The take-off roll.


On the decsent


Final approach into DCA
Made from jets!
 
Pope
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:25 am

Quoting LASOctoberB6 (Reply 12):
i dont get it....

It's a sophomoric swipe at the A340's purported lack of engine thrust.
Hypocrisy. It's the new black for liberals.
 
CYatUK
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:26 am

Quoting Haggis79 (Reply 7):
most definitely not.... if the air under the wing would be flowing faster than the one over the wing you would have a lower pressure under the wing than over it.... effectively sucking the aircraft downwards not upwards.... certainly something you wanna avoid at all costs . Same thing holds of course for a vacuum under the wings...

in fact, it's the exact opposite: the air flows faster over the wing than below it creating overpressure below the wing and underpressure above it... both the overpressure below the wing as well as the underpressure above it result in an upward force onto the wing... so the airplane is "carried" by the combined forces of those two effects.

Spot On. Just to add that the shape of the wing is generally called an "aerofoil".

One of the functions of the winglets is exactly to stop the "interaction" of air between the two sides of the wing that have different pressure when the plane is flying i.e. separate low pressure above the wing and high pressure below.

Quoting PanAm747 (Reply 1):
wing settings (flaps and such) and proper mathematical computations involving weight, temperature, and thrust available, can the flight crew properly and safely get their plane off the ground.

The flaps are there to increase the wing surface area. The more area you have, the more lift you get for the same speed. However, you need to bear in mind that by increasing the wing area, the drag increases as well. That's why flaps are not fully extended for take off.
CY@Uk
 
graphic
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RE: What Happens During Take Off?

Thu Mar 15, 2007 5:33 am

Quoting Haggis79 (Reply 7):
in fact, it's the exact opposite: the air flows faster over the wing than below it creating overpressure below the wing and underpressure above it... both the overpressure below the wing as well as the underpressure above it result in an upward force onto the wing... so the airplane is "carried" by the combined forces of those two effects.

Actually, you're both kinda right. The point of stagnation on a wing surface while varying, is generally about a third up from the bottom of the wing. If I had a diagram this would be much easier, but alas I shall continue anyway... Now, because the splitting of airflow going over the top of the wing and the bottom of the wing doesn't occur right at the bottom of the wing but slightly up from it, the air below the wings is accelerated somewhat (this is why most Pitot masts you see on light aircraft are extended down below the wing surface, so they can tap into the unaccelerated airflow and read an accurate airspeed), creating a slight vacuum below the wings. Because there's more vertical wing area above the stagnation point, the air above the stagnation point is accelerated more, causing a greater drop in pressure than below the wing, thereby offsetting the vacuum below the wing and providing a vacuum of its own to boot.

Quoting Redngold (Reply 8):
However, without the thrust of the engines there would not be enough airspeed to create the lift.

Not at all true. Every aircraft has a speed, called L/Dmax, also known as the best glide airspeed, at the very bottom of the airplane's induced/parasite drag curves, that is, the airspeed where the least amount of drag acts on the airplane. To understand L/Dmax, you must first understand drag. Drag comes in 2 basic types: Parasitic, caused by all the shit hanging off the airplane, and Induced: The drag that is the result of the production of lift. Again the diagrams would be handy...anyway... As airspeed increases, parasitic drag increases, because the air behind things like antannaes, rivets, even paint flakes, becomes more turbulent, and the assosciated vacuum becomes much stronger, whereas at low speeds, the air behind these objects isn't as turbulent, so the vacuum behind them is less powerful. Now the exact opposite is true with induced drag. As airspeed increases, induced drag decreases, simply because at slower airspeeds, the wing needs to work harder to produce the lift needed to keep the airplane in level flight. The result is an increased angle of attack. Because the AOA increases, the airflow behind the wing becomes more turbulent, creating a drag force. At higher airspeeds, the wing is pretty much just there, it doesn't need a high AOA to keep the airplane aloft, so the airflow is much more smooth and the drag force is negligable. Now back to L/Dmax. L/Dmax is the airspeed at which the parasitic drag force and induced drag force are exactly the same. Called best glide airspeed because, simply put, that's the speed a pilot shoots for if he loses all power and needs to land safely. You won't fly level at L/Dmax, but the wings will still produce lift and you won't "fall out of the sky."
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redngold
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RE: What Happens During Take Off?

Thu Mar 15, 2007 6:15 am

Quoting Graphic (Reply 16):
Every aircraft has a speed, called L/Dmax, also known as the best glide airspeed, at the very bottom of the airplane's induced/parasite drag curves, that is, the airspeed where the least amount of drag acts on the airplane. To understand L/Dmax, you must first understand drag.

Yadda yadda...

Okay, I may have oversimplified. All airliners can glide for some distance without engine thrust. Keeping at altitude, however, is a different story (unless your aircraft is built to glide and light enough to lift on air currents alone.)
Up, up and away!
 
AAR90
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RE: What Happens During Take Off?

Thu Mar 15, 2007 7:09 am

Quoting Chrisjdurber (Thread starter):
when you are on the runway and you hear the engines roar and start to move what is happening? How does the plane stay on the runway and then all of a sudden rise up?

(okay guys its me now! have got the godkids sat here and have promised them an answer so please dont be too complicated!!!! lol megans main concern is that how does the captain know when to make the plane lift (quote) and how fast does it go) okay there you go. .....

What is happening? The engines are producing THUST which is pushing/pulling the airplane forward. Go fast enough and the wings produce LIFT which lifts the airplane in the air.
How does the plane stay on the runway? The pilots steer it... similar to steering your family automobile (a bit more complex than that, but the kids will understand that analogy).
How does the captain know when to make the plane lift? Tell Megan the pilots have airspeed indicators (similar to your automobile's speedometer) and that is how we know "when to make the plane lift."  Wink
How fast does it go? During takeoffs, anywhere between 100-200 miles per hour --depending upon type of airplane, weight, airport altitude, wing configuration, engine thrust used, and a whole lot of other things. Just one of the reasons pilots have lots of schooling before becoming pilots and lots of schooling after becoming pilots.... we're always going back to school.  Wink

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ChinaClipper40
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RE: What Happens During Take Off?

Thu Mar 15, 2007 8:03 am

Quoting Haggis79 (Reply 7):
...air flows faster over the wing than below it creating overpressure below the wing and underpressure above it... both the overpressure below the wing as well as the underpressure above it result in an upward force onto the wing... so the airplane is "carried" by the combined forces of those two effects.



Quoting Graphic (Reply 16):
...because the splitting of airflow going over the top of the wing and the bottom of the wing doesn't occur right at the bottom of the wing but slightly up from it, the air below the wings is accelerated somewhat..., creating a slight vacuum below the wings. Because there's more vertical wing area above the stagnation point, the air above the stagnation point is accelerated more, causing a greater drop in pressure than below the wing, thereby offsetting the vacuum below the wing and providing a vacuum of its own to boot.

Okay, I THINK I understand this. And this is the reason that wings are basically curved on their top surfaces and basically flat on their bottom surfaces, right? To create more surface area and faster airflow on the top of the wing as compared to the bottom of the wing - and therefore to create lift, right? Okay, then how is it that airplanes can fly upside down? How does an upside down airfoil keep an aircraft in the air? I know it's possible from my years of service in the U.S. Air Force, including rides in F-104s during which the plane was inverted in flight for prolonged time periods (yes, that dates me, doesn't it?). But I never understood the physics of inverted flight - and why it was so easy for the pilots I flew with to simply flip the plane over and continue in level flight at the same altitude. Please don't flame me. I'm a medical school professor, not a pilot or engineer or physicist. My USAF flying was done as a medical officer putting in required flight time. Is it a matter of angle of attack? If so, how does it work? How much difference in angle of attack is required to keep a plane airborne in level flight upside-down versus rightside-up? Can anyone give me a quick and simple explanation of inverted flight for dummies? Thanks.

ChinaClipper40
 
LASoctoberB6
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RE: What Happens During Take Off?

Thu Mar 15, 2007 8:13 am

Quoting Pope (Reply 14):

It's a sophomoric swipe at the A340's purported lack of engine thrust.

why would you underpower a massive plane such as the A346? and why wont they fix that?
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kcrwflyer
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RE: What Happens During Take Off?

Thu Mar 15, 2007 8:54 am

Quoting LASOctoberB6 (Reply 20):
why would you underpower a massive plane such as the A346? and why wont they fix that?

It isnt really "underpowered". Its just less overpowered than say.. a 777.

Is it true that all aircraft are actually overpowered in relation to the amount of thrust theyd need to operate safely?
 
DL787932ER
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RE: What Happens During Take Off?

Thu Mar 15, 2007 9:06 am

[quote=KcrwFlyer,reply=21]Is it true that all aircraft are actually overpowered in relation to the amount of thrust theyd need to operate safely?



No.  Silly

But my guess is that you mean multiengine commercial transport airplanes. In which case, kinda. The certification requirement is that the airplane be able to continue a takeoff safely after an engine failure at V1 (below which speed you can stop safely on the remaining runway). So if (x-1) engines produce enough power to continue a takeoff safely, (x) engines produce "more than enough" power to do so. That's a reason why twins like 777s can be considered "more overpowered" than quads like 340s - on a 340, losing one engine still leaves you three, so if you need (x) thrust to take off, three engines produce at least (x) thrust, and four produce (4/3*x) thrust. On a 777, losing one engine leaves you one, so that one engine has to produce (x) for takeoff thrust, and both engines operating can give you (2x) thrust.
F L Y D E L T A J E T S
 
NAV20
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RE: What Happens During Take Off?

Thu Mar 15, 2007 9:47 am

Quoting ChinaClipper40 (Reply 19):
Is it a matter of angle of attack? If so, how does it work? How much difference in angle of attack is required to keep a plane airborne in level flight upside-down versus rightside-up? Can anyone give me a quick and simple explanation of inverted flight for dummies? Thanks.

ChinaClipper40, faster aircraft often have a symmetrical aerofoil section - same curvature on the top and bottom of the wing - because, at the speeds they fly, the turbulence generated by the angle of attack is sufficient to generate enough lift. The angle of attack (incidence angle) only needs to be a degree or two. Flying inverted, all you do is keep an eye on the horizon and raise or lower the nose (tricky because the elevators work in reverse, but it soon becomes automatic) until the aeroplane stays level. Effectively, in inverted flight the aeroplane is flying a little 'nose-up' (i.e. 'nose-down but upside down,' if you see what I mean!  ) but you don't notice that because the angles are small. Some good diagrams on here that may help the kids understand:-

http://www.aeromech.usyd.edu.au/aero/aerointro/what_is_lift.shtml

Quoting LASOctoberB6 (Reply 20):
why would you underpower a massive plane such as the A346? and why wont they fix that?

Bigger engines require more fuel, and fuel is heavy (over six pounds per US gallon). Every airliner design involves a tradeoff between engine power , range, and weight. If they put bigger engines on the A340 it wouldn't 'go the distance.'

[Edited 2007-03-15 02:56:56]
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci
 
707atDc8
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RE: What Happens During Take Off?

Thu Mar 15, 2007 9:54 am

Well kids it's like this, after the driver (Pilot) moves the accelerator (Throttles) forward, he or she !! use's the steering wheel (Tiller) to keep the AeroPlane right in the middle of the road (Runway) , the AeroPlane must reach Superman speed going down the road (Runway) about 180kts (Real Fast) this then creates a big "Marshmallow" like amount of air-pressure under the AeroPlanes arms (Wings) and almost no Marshmallows (air-pressure) over / on top of the arms (Wings) and it's this invisible pressure differential (big fluffy Marshmallows under and no Marshmallows over) that allows the AeroPlane to sit or float through the Air, all the while being pushed very fast by it's Aerosol cans (Jet Engines) under the arms (Wings) .... The End ....
 
NAV20
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:03 am

Quoting 707atDc8 (Reply 24):
he or she !! use's the steering wheel (Tiller) to keep the AeroPlane right in the middle of the road (Runway)

Just to clarify, directional control at speed is also aerodynamic (by using the rudder) and that is worked by the rudder pedals, not the yoke ('steering wheel'). You can use differential braking to control direction at low speeds, but not once the aircraft is accelerating into its takeoff run.

[Edited 2007-03-15 03:07:09]
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci
 
707atDc8
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:10 am

Hey !!! NAV20, it's just a joke for KID'S !!!, you know how kids are don't you ??, remember ???, you got to get their attention with the Funny stuff first, then drop the Real deal ... Remember Ha,ha first, then the Homework....
 
graphic
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:14 am

Quoting ChinaClipper40 (Reply 19):
Okay, then how is it that airplanes can fly upside down?

Dammit now I need a diagram  Wink

Ok, the coefficient of lift equation.

CL = L / (1/2pV2 A)

CL is the Coefficient of Lift, L is the lift produced, p is the density of air, V is the freestream (not affected by the wing surfaces) velocity of the air, and A is the surface area of the lifting surface. Simply stated, assuming free-stream velocity and air density remain constant, then in order to achieve a large coefficient of lift (and thus a large lifting force), the larger the lifting surface area, the faster the airflow over it, and therefore a greater lifting force.

http://www.airliners.net/uf/536921701/1173924636Q2FdlC.jpg

As you can see, in inverted flight, the airplane will have to maintain a nose-high attitude, and when it does this, the point of stagnation moves down far enough on the wing (actually it makes a cushion that basically fills in the gap) such that the lifting area, A, is greater than the bottom of the wing, A1, which allows an inverted airfoil to produce lift.
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NAV20
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:30 am

Quoting 707atDc8 (Reply 26):
Hey !!! NAV20, it's just a joke for KID'S !!!,

Sorry, 707, bridled a bit at the 'steering-wheel' bit!   We may as well get it right as far as Chrisjdurbar is concerned, it's up to him how much he explains to the kids.

[Edited 2007-03-15 03:45:43]
"Once you have flown, you will walk the earth with your eyes turned skywards.." - Leonardo da Vinci
 
KevinSmith
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:34 am

Quoting Dtwclipper (Reply 5):
quote something called the Bernoulli's principle

Oh please!!!! That Bernoullii guy was a quack. Streamtubes and A1V1=A2V2=A3V3, geez what a nut job.
Learning to fly, but I ain't got wings.
 
fridgmus
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:34 am

Jetjack74,

Thanks for the great videos Jack, especially for those of us outside the Aviation Industry!  bigthumbsup 

Marc
The Lockheed Super Constellation, the REAL Queen of the Skies!
 
KevinSmith
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:41 am

Quoting ChinaClipper40 (Reply 19):
Okay, then how is it that airplanes can fly upside down?

Well they really aren't if you are flying at a 0 degree inverted pitch attitude.. You change the camber of the wing when up side down as well as the AOA, hencforth extreme lift reduction is the result. To maintain lift in an inverted attitude you need to push the stick down so the now negatively cambered wing gets to an angle of attack that can produce lift. A CL max AOA chart helps to see this greatly.
Learning to fly, but I ain't got wings.
 
hmmmm...
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RE: What Happens During Take Off?

Thu Mar 15, 2007 11:20 am

Don't fill your Godkids with all that Bernoulli nonsense.  Wink Bernoulli spoke of the compressibility of fluids, that have very little to do with this concept of making airplanes fly with vacuums from different air pressures. It is commonly claimed that Bernoulli's principle means that a difference in velocity creates a difference in pressure. Bernoulli never made any such claim.

As some have noted, an airplane can generate lift whether or not the wing has any difference in camber. This Bernoulli dynamic, as commonly misinterpreted and repeated, accounts for a very small portion of lift, if any at all. Lifting force is accomplished more by Newton tham Bernoulli. When all that air hits the wing at an angle, it is deflected downwards, causing the wing to deflect upwards. Newton's Third Law. You can test this with your hand out the car window. The camber of your hand will not matter. That would be the best way to teach your Godkids how an airplane takes off.
An optimist robs himself of the joy of being pleasantly surprised
 
graphic
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RE: What Happens During Take Off?

Thu Mar 15, 2007 12:16 pm

Quoting Hmmmm... (Reply 32):

The problem with Newton's laws and Bernoulli's principle being used in this argument is that they are both valid arguments. Fluid flowing faster than other fluid does create a pressure differential. You can test this by rolling down your car window, holding a small light object (small paper ball, or gum wrapper rolled into a ball) just inside your car and you'll notice it gets sucked out when you let go. Its also known that bernoulli's principle holds true because wings produce vortices. At the wingtip, the air is not going to follow the chordline of the wing. Nature abhors a vacuum (thank you Foltz1), the air flowing faster atop the wing than on the bottom creates a vacuum, so therefore nature is going to do whatever is necessary to fill that vacuum, hence the airflow, rather than following the chordline, will rush to the wintip, up and beside it, and down into the airstream on top of the wing, creating a vortice. The difference in pressures, however, is really a biproduct of Bernoulli's principle. Bernoulli's principle simply put, is that air velocity increases as volume decreases. Going back to my diagrams, the air in front of the wings (A + A1) is a given volume of air. When this given volume of air hits the wings, the amount of air stays the same, but the volume is drastically reduced, hence the increase in air velocity over the airfoil, and the accompanying partial vacuum.

Newton's 3rd law also works: For every action, there is an equal and opposite reaction. Going back to the air coming off the wing, there is a downwash caused by the faster air coming down off the back of the wing. Because there has to be an equal and opposite in order to satisfy Sir Newton, there is an upwash hitting the front of the wing, hence, even in level flight, the wing really wont have a 0 degree AOA, as air is always hitting the wing at a slightly upward angle. When the airplane comes close to the surface, we get what is called "ground effect," caused by the downwash off the back of the wing being drastically reduced due to surface interference. Because the downwash is reduced, the upwash is also drastically reduced, and the drag accompanying it goes away, hence airplanes fly much better within about a half-wingspan (or was it full?) lenth of the surface.

Now here's where Newton and Bernoulli disagree. Bernoulli would tell you that the differing velocities of the airflow over the wing creates a partial vacuum with which the air rises into, however Newton's 1st law states that an object at rest will remain at rest, and an object in motion will remain so in motion, until acted upon by an outside force. This means that an object, or say a parcel of air filling up an area of, lets say area A (up in my diagrams), will remain at a constant velocity until an outside force is acted upon it, therefore it cannot be accelerated, even by a wing, unless there is a vacuum already there. But didn't Bernoulli state that the vacuum was created by the accelerating air? Try to wrap your head around that one for awhile.

1Those of you who have gone to UND will know who I'm talking about
Demand Media fails at life
 
FlyDeltaJets87
Posts: 4479
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RE: What Happens During Take Off?

Thu Mar 15, 2007 12:32 pm

Anyone know where we can find that rejected takeoff video in the AC 767? That guy certainly knew what he was talking about and could help clear things up.  biggrin   wink   stirthepot 

Quoting Chrisjdurber (Thread starter):
lol megans main concern is that how does the captain know when to make the plane lift (quote) and how fast does it go)

Two men dressed in pilots' uniforms walk up the aisle of the aircraft. Both are wearing dark glasses, one is using a guide dog, and the other is tapping his way along the aisle with a cane.

Nervous laughter spreads through the cabin, but the men enter the cockpit the door closes, and the engines start up. The passengers begin glancing nervously around, searching for some kind of a sign that this is just a little practical joke. None is forthcoming.

The plane moves faster and faster down the runway, and the people sitting in the window seats realize they're headed straight for the water at the edge of the airport property. Just as it begins to look as though the plane will plow straight into the water, panicked screams fill the cabin.

At that moment, the plane lifts smoothly into the air. The passengers relax and laugh a little sheepishly, and soon all retreat into their magazines and books, secure in the knowledge that the plane is in good hands.

Meanwhile, in the cockpit, one of the blind pilots turns to the other and
says, 'You know, Bob, one of these days, they're gonna scream too late and we're all gonna die' !!




Anyway, here's a takeoff video from the passenger perspective if they want to see one of those: AirTran 737-700 at ATL;
AirTran 737 takeoff from ATL

ATL">Add to My Profile | More Videos
"Let's Roll"- Todd Beamer, United Airlines Flight 93, Sept. 11, 2001
 
dc1030guy
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RE: What Happens During Take Off?

Thu Mar 15, 2007 1:11 pm

Quoting Hmmmm... (Reply 32):
Don't fill your Godkids with all that Bernoulli nonsense. Bernoulli spoke of the compressibility of fluids, that have very little to do with this concept of making airplanes fly with vacuums from different air pressures. It is commonly claimed that Bernoulli's principle means that a difference in velocity creates a difference in pressure. Bernoulli never made any such claim.

As some have noted, an airplane can generate lift whether or not the wing has any difference in camber. This Bernoulli dynamic, as commonly misinterpreted and repeated, accounts for a very small portion of lift, if any at all. Lifting force is accomplished more by Newton tham Bernoulli. When all that air hits the wing at an angle, it is deflected downwards, causing the wing to deflect upwards. Newton's Third Law. You can test this with your hand out the car window. The camber of your hand will not matter. That would be the best way to teach your Godkids how an airplane takes off.

I disagree with what you are saying -- partly. Yes, I would agree that some lift is created by the air hitting the bottom of the wing. However, this does not explain "spoiler float" and why big airplanes crash when there is a small amount of frost on the top of the wing. Why would an airplane's spoilers float up in the wind when they lose hydraulic power? (They will do this ... very similar to the blowing air over a piece of paper held to your lips). The only answer that I can think of is that as the air rushes over them, it creates a sucking force. And then, with the frost -- if lift was all about the air being forced downward under the wing, then why would something so thin on top of the wing make a difference?

I'm going with Bernoulli on this one.

Pat
 
Shawn Patrick
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RE: What Happens During Take Off?

Thu Mar 15, 2007 7:18 pm

Quoting Hmmmm... (Reply 32):
his Bernoulli dynamic, as commonly misinterpreted and repeated, accounts for a very small portion of lift, if any at all. Lifting force is accomplished more by Newton tham Bernoulli.

Thank you!!

Quoting Dc1030guy (Reply 35):
And then, with the frost -- if lift was all about the air being forced downward under the wing, then why would something so thin on top of the wing make a difference?

You are right, Bernoulli's lift does contribute to the total lift, but it is much less than you think. When an airplane is in level flight, all of the forces are balanced, so any frost will decrease the amount of "Bernoulli lift" and you will need more power to overcome that.
 
YYZSaabGuy
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RE: What Happens During Take Off?

Thu Mar 15, 2007 7:56 pm

Quoting Pilotaydin (Reply 9):
in the case of the A340, the earth's curvature enable it to get airborn, but all other aircraft go by newton's third law and the downwash/vacuum theory



Quoting Pope (Reply 14):
It's a sophomoric swipe at the A340's purported lack of engine thrust.

Sophomoric yes, but still worth a laugh. Unfortunately, I was drinking coffee while reading it......  biggrin 
 
beechnut
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RE: What Happens During Take Off?

Thu Mar 15, 2007 9:00 pm

Hey I enjoyed the videos! Potomac river approach into DCA is a cool, semi-Kai Tak affair.

Brought back many memories, I had the pleasure of a couple of jumpseat rides on AC DC-9s before 9/11 killed such forms of amusement...

Beech
 
Max777geek
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:00 pm

Quoting PanAm747 (Reply 1):
Once an aircraft has reached a certain speed, the air rushing over the wings creates a vacuum under the wings that allows the plane to lift off.

Actually the vacuum is created "over" the wings, while high pressure stays under. The explanation of this is in the shape of the wing itself, which is "more" rounded in the top than in the bottom side, causing air to walk more road up than below, concentrating more air below (high pressure) the wing and "vacuum" (low pressure) over. The air pressure will try to match itself by pushing up the wing, causing the plane to lift, which the wings are hopefully well attached..
 
Max777geek
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:35 pm

Quoting Max777geek (Reply 39):
Actually the vacuum is created "over" the wings

Im sorry I saw too late the concept was already explained in oher posts.

The interesting fact of the wing shape causing lift, in my opinion, and Im talkin to the beginner
of the thread, is that this lift is actually used in airliners also to change the direction of the plane
in its 3 axes.

If you saw the proposed diagram you notice that torsion in the 3 axes of the plane is achieved by
moving a part of the wing causing yes, wind pressure over the rising part, and the principle is the
same, as the rounded area of the opposite side of the moving part, cause air to make more road,
using the same pressure game to move the wing a bit "down". Since the force to apply would be
a lot, a part of this part of the wing can move again itself, like in a servo steering system, helping
with pressure in the opposite direction to move the part of the wing to cause the big wing to move.
This you can notice when you seat at a window where you can see the back of the wing. The same
applies to the vertical stabilizer :

http://www.airliners.net/open.file/0968841/L/
http://www.airliners.net/open.file/0561847/L/

here you can see the "big" and the "little" moving parts.
 
motopolitico
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RE: What Happens During Take Off?

Thu Mar 15, 2007 10:45 pm

Quoting Haggis79 (Reply 7):
most definitely not.... if the air under the wing would be flowing faster than the one over the wing you would have a lower pressure under the wing than over it.... effectively sucking the aircraft downwards not upwards.... certainly something you wanna avoid at all costs Wink. Same thing holds of course for a vacuum under the wings...

WOW it took 7 whole replies before those grossly inaccurate statements were refuted. Those two guys even had RRs higher than zero. Thank you, Haggis. Makes me think twice about the information I get on a.net though. Although the Tech/Ops forum seems to be populated by folks who know what they're talking about.
Garbage stinks; trash don't!
 
Stratofortress
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RE: What Happens During Take Off?

Thu Mar 15, 2007 11:08 pm

Put your godkids in a car, open the windows, and have them stick out their hands.... Then explain to them that airplane has really big hands, and really big engines that work kinda like hair dryers (e.g. A340... Sorry. JK. I couldnt help it)
Forever New Frontiers
 
DeC
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RE: What Happens During Take Off?

Thu Mar 15, 2007 11:48 pm

Quoting Jetjack74 (Reply 13):

Amazing videos my friend, thanks a lot for sharing.
DEC
 
User avatar
jetjack74
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:11 am

Quoting Fridgmus (Reply 30):
Jetjack74,

Thanks for the great videos Jack, especially for those of us outside the Aviation Industry!

Marc



Quoting DeC (Reply 43):
Amazing videos my friend, thanks a lot for sharing.

No problem
Made from jets!
 
Femme
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:19 am

Fascinating thread to read and contains a lot of my often wondered questions answered .....Thanks guys !!

Still marvel at the whole take off thing though, always will....

Claire
Women don't have hot flushes, they have Power Surges....
 
ultrapig
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:22 am

You godless secularists 43 posts and not one mention of God? It is he/she who makes the plane rise and if you are bad he/she makes it go down. Allah/Jesus/Hashem/Buddah (and without meaning to insult any of the other equally politically correct deities) are great!
 
PanAm747
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:24 am

Quote:
Actually the vacuum is created "over" the wings, while high pressure stays under. The explanation of this is in the shape of the wing itself, which is "more" rounded in the top than in the bottom side, causing air to walk more road up than below, concentrating more air below (high pressure) the wing and "vacuum" (low pressure) over. The air pressure will try to match itself by pushing up the wing, causing the plane to lift, which the wings are hopefully well attached..



Quote:
most definitely not.... if the air under the wing would be flowing faster than the one over the wing you would have a lower pressure under the wing than over it.... effectively sucking the aircraft downwards not upwards.... certainly something you wanna avoid at all costs . Same thing holds of course for a vacuum under the wings...

in fact, it's the exact opposite: the air flows faster over the wing than below it creating overpressure below the wing and underpressure above it... both the overpressure below the wing as well as the underpressure above it result in an upward force onto the wing... so the airplane is "carried" by the combined forces of those two effects.

Thank you! I knew no matter which way I remembered it, it would be wrong. Part of me even thought the laws of physics would change JUST to prove me incorrect...  rotfl 

Quote:
The flaps are there to increase the wing surface area. The more area you have, the more lift you get for the same speed. However, you need to bear in mind that by increasing the wing area, the drag increases as well. That's why flaps are not fully extended for take off.

Again, thank you for the clarification. There are more than just flaps that extend out from a wing; I should have been more precise.

The 727 is always what comes to mind with wing design and lift-devices. In the early 1960's, Boeing was searching for a way to satisfy United's desire for a plane that could fly fully loaded from Denver (hot and high) as well as Eastern's desire for a plane that could get off of La Guardia's shortest runway, fully loaded, and fly all the way to Florida non-stop.

Eventually, the wing became the key in the design of the 727 - high lift devices were installed that would increase the total wing area by 25%, enabling a much lower take-off speed. Once airborne, the devices were pulled in, and the 727 actually scared some military installations as it zoomed by faster than any known commercial airplane at the time!

Unfortunately, there were several incidents where the flaps themselves led to a premature descent and stalling of the plane - An AA 727-23 crashed at CVG and a UA 727-22 both crashed short of the runway when the pilots used the highest level flap setting and the drag associated with put the plane down WAY too early. IIRC, Boeing discontinued that level of flap setting in later models.
Pan Am:The World's Most Experienced Airline - P(oor) S(ailor's) A(irline): San Diego's Hometown Airline-Catch Our Smile!
 
Ferroviarius
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:54 am

Good afternoon!
Interestingly enough, phenomena quite similar to those that keep an airplane - or a helicopter, where the lift is created by "rapidly rotating wings" - in the air, are causing the propulsion of a - western style - sailing boat, particularly if it is moving towards the grand main direction of the wind (some explanation for that on en.wikipedia.org, look up "sailing").
And, perhaps more trivial but nonetheless with a considerable impact on the mood of acoustically un-experienced, freshly married housewives and hence with a considerable impact on the mood of the society: What makes an airplane fly, also makes husbands snore!

The complete - if possible at all - description of the phenomena making an airplane fly - at least in the sub-sonic range - can, as far as I know, not be given in verbal terms. One has to use the appropriate partial differential equations.

Also, some airplanes, e.g. the Saab Draken (first so ever Delta Wing Airplane, as far as I know), the Concorde, TU144, etc. pp make use of what is called "Vortex lift".

Finally, another phenomenon to be mentioned here is the "Coanda Effect", occasionally used for airplanes travelling at ultra low flight levels.

Best,
Ferroviarius
 
YULWinterSkies
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RE: What Happens During Take Off?

Fri Mar 16, 2007 12:56 am

Quoting Chrisjdurber (Thread starter):
please dont shoot me down here!!

No, instead thank you for bringing in a topic different from the usual B vs A boredom...

Quoting Pilotaydin (Reply 9):
in the case of the A340, the earth's curvature enable it to get airborn

... although it did not less more than 10 replies to bring that back on the board, even though it's a joke.
When I doubt... go running!