BritMidDC9
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How Does A Plane 'take Off?'

Thu Jul 26, 2007 8:38 pm

Hey,

Bit of a silly question, but when a plane takes-off, as I understand it, the pilot flying 'pulls back' and the aircraft lifts into the air. What exactly does pulling back on the control colum achieve and how does the aircraft lift into the air. I know speed plays a big part in the take of procedure.

On the other hand, what would happen if during the take off run the pilot did not pull back on the colum? As the speed increases would the aircraft lift into the air of its own accord or would it simply run off then end of the runway?

Thanks for you help

BritMidDC9
 
Spruit
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:06 pm

To save my fingers  

http://en.wikipedia.org/wiki/Take_off

Try clicking on some of the other links too, you'll find lots of interesting information there!

Or try http://www.allstar.fiu.edu/AERO/princ1.htm

Cheers,

Spru!

[Edited 2007-07-26 14:08:40]
E=Mc2
 
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Starlionblue
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:08 pm

Quoting BritMidDC9 (Thread starter):

Bit of a silly question, but when a plane takes-off, as I understand it, the pilot flying 'pulls back' and the aircraft lifts into the air. What exactly does pulling back on the control colum achieve and how does the aircraft lift into the air. I know speed plays a big part in the take of procedure.

Pulling back on the stick/yoke raises the elevators. This changes the lift characteristics of the stabilizer, deflecting air up and pushing the tail down (known as rotation). This in turn changes the angle of attack of the wing, which produces more lift. And the plane lifts off.

As you mention, speed plays a part. Since lift is a function of speed (and flap/slat configuration) if rotation is performed too early (at too low a speed) the wing will not have enough lift to make the aircraft leave the ground.

Quoting BritMidDC9 (Thread starter):

On the other hand, what would happen if during the take off run the pilot did not pull back on the colum? As the speed increases would the aircraft lift into the air of its own accord or would it simply run off then end of the runway?

As I recall the conclusions in a previous thread were "maybe" and "it depends on the aircraft".
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
SilverComet
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:11 pm

As the pilot pulls back, the elevators (at the tail) deflect to create more downforce on the tail on the aircraft. So basically a force is pushing down on the tail, thus lifting the nose of the aircraft. This in turn increases the angle of attack of the wings (looking from the side, that's the angle the wing makes with the incoming air), creating more lift. At that point, the plane has sufficient speed and AoA (and therefore lift) to lift off.

Quoting BritMidDC9 (Thread starter):
On the other hand, what would happen if during the take off run the pilot did not pull back on the colum? As the speed increases would the aircraft lift into the air of its own accord or would it simply run off then end of the runway?

There's no 'one size fits all' answer to that. It depends which aircraft and which runway we are talking about. Given a sufficiently long runway, and provided the aircraft has been properly prepared for takeoff (flaps and pitch trim), even if the pilot did not pull back, some aircraft will lift off of their own accord. This is because although the AoA is still small (nose hasn't lifted off yet), as speed increases, at some point it becomes sufficient to create enough lift for the aircraft to take off.

On the other hand, some aircraft might be designed so that on ground their wings are at the 'zero lift AoA', which means that no matter how much speed you gather, zero lift will be created. That's usually a negative AoA.

In the real world though, with a 3000-5000m runway and a B747-400, if you don't pull back at Vr, chances are you are ending up a big ball of flames at the other end. With a C152 however it's a different story...
 
superstring
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:14 pm

First of all speed is important: This is quite clear because if there is not enough speed the wings won't generate enough lift and the aircraft will not get airborne.

What happens now if the pilot "pulls back" is that the elevators move. The result is that when the aircraft is fast enough the elevators will "roll" the aircraft up -> the angle of attack increases -> lift increases -> and the aircraft gets up into the air.

The question of what happens if the runway would be indefinitely long has to do with the aircraft type. If the airfoil has a symmetric shape (like on many aerobatics planes) you will not get up. With big passenger aircraft it ist a little more difficult as the wings generate lift even when there is no angle of attack. However I would guess that it is not possible as the aircraft cannot become fast enought (langing gears, friction...).
 
superstring
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:15 pm

Wow, and I thought that I made a quick response...  Smile
 
eelonghorn
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:43 pm

Could someone with tail wheel steering experience comment on take off? I imagine you cannot start rotation by pulling back on the stick.


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tdscanuck
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:48 pm

Pulling back on the control column moves the elevators up, which pushes the tail down. This rotates the airplane so that the angle of attack of the wings increases and the wings start to generate lift. This allows the airplane to get airborne. You can see this effect pretty clearly if you watch the wingtips during takeoff.

If the pilot did not pull back, the plane would usually just run off the end of the runway. Depending on the exact angle of attack with all wheels on the ground, you might get off the ground without rotating if the wings were at a positive angle of attack and you built up enough speed, but for most planes that would require more runway than you actually have.

Tom.
 
tdscanuck
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:52 pm

Quoting EELonghorn (Reply 6):
Could someone with tail wheel steering experience comment on take off? I imagine you cannot start rotation by pulling back on the stick.

With a tail-dragger, there's two ways to do it. You can just hit the throttle and gain speed until the wing generates enough lift to take off (since it's already at a fairly significant angle of attack). Or, you can gain a little speed until the horizontal stabilizer starts to have some authority, then raise the tail so that the wing is near zero angle of attack (no lift) and gain speed running along on the main gear only, then rotate and takeoff when you hit the right speed.

Tom.
 
Spruit
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 9:52 pm

Quoting EELonghorn (Reply 6):
Could someone with tail wheel steering experience comment on take off? I imagine you cannot start rotation by pulling back on the stick.

I don't have tail wheel experience but how else would you start rotation? You need to deflect the wing to increase lift and you do so by changing the angle of the wing using the elevators, only difference in a talk dragger is the elevators "start" flying first!

Hopefully that makes sense!

Spru!
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oly720man
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RE: How Does A Plane 'take Off?'

Thu Jul 26, 2007 10:00 pm

Quoting BritMidDC9 (Thread starter):
On the other hand, what would happen if during the take off run the pilot did not pull back on the colum? As the speed increases would the aircraft lift into the air of its own accord or would it simply run off then end of the runway?

I think the B52 is a good example of a plane that doesn't rotate in the conventional sense, because of the arrangement of the main undercarriage.
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KFLLCFII
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 1:21 am

By the way, here's an almost identical thread with lots of great information on the topic from a few months ago:
Rotation (by Relic Apr 27 2007 in Tech Ops)
"About the only way to look at it, just a pity you are not POTUS KFLLCFII, seems as if we would all be better off."
 
Fly2HMO
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 2:21 am

Quoting BritMidDC9 (Thread starter):
As the speed increases would the aircraft lift into the air of its own accord or would it simply run off then end of the runway?

Most light GA planes, specially a C172, will take off on their own eventually. Sometimes the slightest aft pull in the yoke will be more than enough for a C172.

A PA44, on the other hand, takes a lot of effort to lift off compared to a C172.
 
FredT
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 3:46 am

Quoting Spruit (Reply 9):
I don't have tail wheel experience but how else would you start rotation? You need to deflect the wing to increase lift and you do so by changing the angle of the wing using the elevators, only difference in a talk dragger is the elevators "start" flying first!

A conventional gear (i e taildragger) aircraft is often configured in such a way that when the aircraft is in the three-point attitude and not climbing, the wing is above the stall angle of attack. This is to facilitate stalled three-point landings, and also to make sure that the aircraft is stalled when on the ground and not about to start flying again. In other words, to unstall the wing and go flying you have to get the tail wheel/skid off the ground. This will eventually happen by itself as the aircraft picks up speed, as it is not trimmed for flying at a stalled angle of attack (even though mains far forward can change this) but can be aided by a slight forward push on the stick... How much prop clearance did you have again, and wasn't the field a tad bumpy?

Either way, before the aircraft goes up the nose goes down. With some exceptions, such as the big taildraggers (Flying Fortress a well known example) which were designed for three-point takeoffs.

Edit:
I knew I'd seen a god vid explaining this on the net. Found it! Check the flight ops vids, starting about 13:30 or 14:00 mins into the movie if you are in such a rush that you are prepared to rob yourself of a lot of interesting stuff on flying the mighty Fort.
http://www.zenoswarbirdvideos.com/B17.html

[Edited 2007-07-26 21:01:16]

[Edited 2007-07-26 21:02:39]
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vikkyvik
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 7:32 am

Quoting Superstring (Reply 4):
The question of what happens if the runway would be indefinitely long has to do with the aircraft type. If the airfoil has a symmetric shape (like on many aerobatics planes) you will not get up.

If the symmetric airfoil is mounted at a positive incidence angle, then you'll still be generating lift (depending on the deck angle on the ground as well).

Quoting Tdscanuck (Reply 7):
This rotates the airplane so that the angle of attack of the wings increases and the wings start to generate lift.

The wings are already generating lift. Rotation increases the lift significantly, however.
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speedracer1407
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 2:42 pm

Quoting FredT (Reply 13):
A conventional gear (i e taildragger) aircraft is often configured in such a way that when the aircraft is in the three-point attitude and not climbing, the wing is above the stall angle of attack. This is to facilitate stalled three-point landings

In what cirumstances would a stalled three point landing be desirable or necessary?
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FredT
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 3:47 pm

Quoting Speedracer1407 (Reply 15):
In what cirumstances would a stalled three point landing be desirable or necessary?

When you want to be on the ground with weight on the wheels, i e without significant amounts of the weight of the aircraft being carried by the wings. One example which comes to mind is when you want traction for directional control on the ground or for braking.
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AirWillie6475
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 4:03 pm

Quoting BritMidDC9 (Thread starter):
how does the aircraft lift into the air.

By the collective will of the passengers.
 
aviopic
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 4:57 pm

Quoting EELonghorn (Reply 6):
Could someone with tail wheel steering experience comment on take off? I imagine you cannot start rotation by pulling back on the stick.

You start the take off run with pushing the controls forward as the tail wheel needs to lift first.
When the a/c is level and has obtained enough speed you gently pull backwards so lift off is almost done horizontal with just a slight angle of attack, hitting the runway with the tail wheel during rotation is asking for trouble.

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This is exactly what it looks like at lift of.
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tdscanuck
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 6:33 pm

Quoting Vikkyvik (Reply 14):
Quoting Tdscanuck (Reply 7):
This rotates the airplane so that the angle of attack of the wings increases and the wings start to generate lift.

The wings are already generating lift. Rotation increases the lift significantly, however.

It depends on the ground angle of the aircraft and the mounting angle of the wing.

If you work the math for a 737-800, you find that the cruise Cl is about 0.058, which translates to a wing angle of attack of 0.5 degrees.

Since the fuselage is pitched up about 2 degrees in cruise and that same aircraft is nose-down about 1.5 degrees on the ground, s the wing angle of attack on the ground is about negative 3.5 degrees. This is skewed a little bit during takeoff because the slats are out (neglitable effect) and the flaps are deployed (increases the angle of attack a little), but the short version is that the wing is generating little, if any, lift prior to rotation.

Tom.
 
FredT
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RE: How Does A Plane 'take Off?'

Fri Jul 27, 2007 9:44 pm

Quoting Tdscanuck (Reply 19):
Since the fuselage is pitched up about 2 degrees in cruise and that same aircraft is nose-down about 1.5 degrees on the ground, s the wing angle of attack on the ground is about negative 3.5 degrees. This is skewed a little bit during takeoff because the slats are out (neglitable effect) and the flaps are deployed (increases the angle of attack a little), but the short version is that the wing is generating little, if any, lift prior to rotation.

Which is the case for most larger airliners. A good way to see this is to observe them taking off on a day when there is drifting snow on the ground. You can see when the vortices (tell tale signs of lift generated) start forming, and it is not until rotation.

Rgds,
/Fred
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