BEG2IAH
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How Does A Plane Take Off?

Sat Apr 24, 2004 8:12 pm

Hi guys.

This is a very simple question. So I expect no complicated answer.  Smile

How does this particular plane take off?


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After the aircraft reaches the take-off speed, I guess a pilot should bring the elevator (on the horizontal stabilizer) up at the very moment of take-off, but in this pic it looks like it's an inch up (deflected) or almost not deployed. Is this small movement of the elevator enough to get this plane airborne? Or is there anything else that lifts a plane off the ground, except for usual lift it gets from the wings and thrust from the engines? Maybe the passengers flapping?  Big grin

Thanks,
Ivan
 
fspilot747
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RE: How Does A Plane Take Off?

Sat Apr 24, 2004 11:36 pm

At higher speeds it doesn't take much deflection to lift the plane's nose up. That, and you also have the suction from the pressure gradient on the wings. Just like when you're driving slow, it takes more turning of the wheel on your part to get the car to respond, but when you're driving fast, just a little bit will send the car flying in another direction.



FSP
 
airplay
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RE: How Does A Plane Take Off?

Sat Apr 24, 2004 11:51 pm

The strange thing about that picture is the obvious lack of flap. Other than that, once the airplane rotates (changes pitch) you don't need to continue large deflection of the elevator. Chances are if the picture was taken seconds sooner, you may have seen a larger deflection.
 
Rick767
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 12:16 am

The elevator deflection does not actually have to be that great to force the tail down at typical takeoff speeds.

Then it's just "a lot of Netwon and a bit of Bernoulli" as our POF teacher used to teach us, to get the plane off the ground.
I used to love the smell of Jet-A in the morning...
 
HaveBlue
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 12:27 am

I agree with Airplay. While Bernoulli creates lift, it doesn't raise the nose to a high angle of attack. The elevator most likely deflected, the nose raised, the elevator was then more or less neutralized about the time the picture was taken.
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Rick767
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 12:47 am

"the elevator was then more or less neutralized about the time the picture was taken"

Unlikely, unless you've accidentally done a really fast rotation and are about to strike the tail, the pitch demand from the flightdeck will not change throughout the rotation (it is one smooth continuous rotation from the deck to 15-20 degrees pitch attitude, with the stick a couple of inches back, not a "two-stage" rotation).
I used to love the smell of Jet-A in the morning...
 
wing
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 6:43 am

Rotation for take off is a continious movement which is approx.3 degrees per second.A sudden or abrupt pull back of the yoke may result a tail strike.
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QantasA332
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 7:11 am

Is this small movement of the elevator enough to get this plane airborne?

I think you're misjudging the use of elevators on takeoff. They simply raise the nose to increase the wing's AoA, thereby increasing the wing's total lift. In other words, deflected elevators aren't really what gets the aircraft into the air, the lift from the wings is. That tiny elevator deflection is enough to raise the nose to an AoA sufficient to produce lift for takeoff. Sure the nose-up attitude in a sense 'points' the aircraft where it wants to go but, as I said, the wing's lift is as always the key to taking flight...

Cheers,
QantasA332
 
rendezvous
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 8:22 am

Another reason for a relatively small elevator movement is the trim. The plane should be trimmed before takeoff so not too much effort is required to get it airborne.
 
CWUPilot
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 8:27 am

Furthermore, sometimes (usually smaller airplanes) you don't even need to touch the yoke in order to get the airplane airborne. Planes are built to fly and will usually jump into the air with very little effort. But yes, the elevator creates a down force on the tail, allowing the nose to rise and the angle of attack on the wings to increase. This increases... Lift!!!  Smile

-CWUPilot
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BEG2IAH
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 8:28 am

Thanks for the replies.

As I understood it, you just need to use the elevators to change the pitch of the plane, and then continue a climb with elevators deactivated. I was just interested in this split of a second action that makes the pitch change. I understand the rest - the lift provided by wings. And, BTW, comparison with slow and fast driving is really good.

Ivan
 
FredT
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RE: How Does A Plane Take Off?

Sun Apr 25, 2004 6:27 pm

The elevators are in many ways more understandable if you think about them as angle of attack selectors rather than as pitch attitude controls, mainly as the former is really what they do.  Smile

At rotation, things change as the force opposing the weight is moved forward from the main gear to the wings.

Cheers,
Fred
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Bruce
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 2:11 pm

I dont think anything at flight speed requires a lot of deflection. Take ailerons for example, you turn the plane but viewing from the passenger window the aileron panel is barely down but a couple inches. Having said that, the application of flaps full at landing approach would make a huge difference with THAT much deflection. But elevators & ailerons and even inflight spoilers and rudders dont move all that much and as I asked in another thread, it does take much more effort on the stick to make that little bit happen.
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BEG2IAH
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 2:14 pm

Hello Fred.

I didn't really know what the angle of attack was, so I found this great web-site, which is a perfect place to start learning.

http://www.grc.nasa.gov/WWW/K-12/airplane/airplane.html

Now I got the idea what you meant, after reading the following page:

http://www.grc.nasa.gov/WWW/K-12/airplane/incline.html

Thanks,
Ivan
 
DC-10Tech
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 2:39 pm

Rendevous makes a good point. Prior to pushback, the stabilizer trim is set for takeoff (in the green band), which no doubt has a great effect on takeoff.
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QantasA332
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 6:02 pm

Sorry BEG2IAH, I didn't know that you didn't know what angle of attack was when I posted my original reply! I should've explained! Sorry! Embarrassment

Cheers,
QantasA332

 
BEG2IAH
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 7:34 pm

Hello QantasA332.

Well, you motivated me to learn it.  Smile
I have one more question though. Horizontal stabilizers are movable, I mean the whole thing. They can be somewhat rotated. Do pilots change their position during cruise, prior to take-off, or simply put - when do they adjust them? They would be permanently fixed if they didn't have any function, right?

Thanks,
Ivan
 
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Starlionblue
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RE: How Does A Plane Take Off?

Mon Apr 26, 2004 10:48 pm

BEG2IAH: As I understand it horizontal trim is adjusted in two places:
- Next to the throttle quadrant there is an actual handle or wheel.
- As a thumb control on the yoke.

Also, the plane will autotrim. The pilots will select a trim setting (AoA in simple terms) but then that darned cart will roll down the aisle, or pax move about. Autotrim will continually move the stabilizer to retain the angle of attack regardless of shifts in center of gravity induced by people moving about or fuel being guzzled in to the engines or turbulence.

Here is a pic showing the trim wheels on a 737NG. They're the black things on the outside of the throttle quadrant:


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BEG2IAH
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RE: How Does A Plane Take Off?

Tue Apr 27, 2004 5:55 am

Starlionblue,

Thanks for this reply, it makes perfect sense. Clever birds, those planes...  Smile

Ivan
 
DC-10Tech
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RE: How Does A Plane Take Off?

Tue Apr 27, 2004 2:00 pm

Starlion, the autotrim only works when the autopilot is operating. If the autopilot is holding a continuous elevator command (like in a climb/or pitch change) the auto pitch trim will move the stabilizer to retrim the aircraft so a nuetral elevator input is achieved - much the same as a pilot would do. For turbulence penetration autopitch trim is not used, many aircraft have a turbulence button that will prevent the APT from operating.
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Starlionblue
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RE: How Does A Plane Take Off?

Tue Apr 27, 2004 7:07 pm

DC-10Tech. Thx for your clarification. I knew about the autotrim only working with the autopilot on (makes perfect sense of course). I did not know about the turbulence thing. Very interesting.


Trivia: On the MD-8x, stabilizer trim will often be out of kilter. I have flown on lots of SAS MD-8x, and many times they will porpoise (go slowly up and down) during cruise. Asking the pilots about this, they have told me that after a while they give up trying to trim it and this will be adjusted on the ground. The aircraft will slowly pitch up, then the autotrim will overcompensate down. The cycle is about 4-8 seconds and rocks you to sleep.
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DC-10Tech
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RE: How Does A Plane Take Off?

Thu Apr 29, 2004 2:38 pm

That is usually a problem with the autopitch trim LVDT's being out of adjustment. The APT computer uses these transducers to determine the position of the stab. This happens on the DC-10's as well and requires the LVDT's to be nulled. These can be touchy little buggers and sometimes you don't get the adjustments right the first time!
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HAL
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RE: How Does A Plane Take Off?

Thu Apr 29, 2004 4:15 pm

Let me see if I can give you a brief idea about trim and how it works:

If you were to let go of the control wheel at any point during a flight (assuming autopilot is off), most planes would assume some particular airspeed depending on the configuration of flaps/slats and elevator trim. This is true for almost any airplane except inherently unstable ones like the stealth fighter (F-117).

Whatever this 'trimmed' airspeed is, it does not depend on the power setting. Remember, I am talking about airspeed only, not climb or descent rate. For the private pilots out there, I've found from my instructing days that many don't really understand it, even after they take their checkride. But here is a demonstration that may help. Next time you're flying start out well above the ground (maybe at least 3000ft agl, just so you don't have to worry about the ground) with the airplane in a trimmed cruise configuration. Let go of the controls, and if you've trimmed it properly it should stay somewhere near level flight. You may go ahead and use the ailerons gently to keep the wings level. Note your airspeed. Then reduce power a little bit - maybe a few hundred RPM and let the plane stabilize again. (It will take a couple minor oscillations but should damp out quickly). What will happen is that you'll keep the same airspeed, but start descending. Now add some extra power, say a few hundred RPM more than what you started with. You should keep the same airspeed, but now start climbing. Now using this example of the extra power, if the airplane were to stay level it would accelerate. But that acceleration causes more airflow over the wing and tail, and since the tail provides a downward force (trust me, it comes in advanced aerodynamics classes) more airflow causes more downward force, the nose goes up, and your speed is restored to the 'trimmed' airspeed.

The whole point of this - even for the non pilots here - is that the trim of an aircraft allows it to fly without control pressure at a given airspeed. What you don't want to have to do as a pilot is have a mis-set trim which would require you to be Arnold Schwarzenegger hauling back on the control wheel just to get yourself airborne, or to have the nose suddenly spring skyward well before you've reached the appropriate flying speed. So when the pilot of any aircraft from small Cessna to Boeing/Airbus jets sets the trim for takeoff, they know what speed they want to use for their initial takeoff and climb, and set the trim accordingly. Of course in the big jets it is also affected some by weight and balance issues since those planes have a much bigger envelope in which to load the plane. But the end result is the same. With the trim set for takeoff, and all that power added, what happens is a climb away from the airport at the speed best suited for that phase of flight.

And finally as for it not looking like there is much elevator deflection at rotation, there isn't. Much of the force is provided by the trimmed position of the forward portion of the stabilizer on the 737 in the picture. The stabilizer trim is set (in large jets like these) for the initial climb speed. The actual speed of rotation and takeoff is a little bit less, so yes the elevator is used to raise the nose. But it's just a small change since by that point you're already close to your desired climb speed. The stabilizer itself is what's providing most of the downward force, not the elevator. You don't want the elevator deflected much because that would cause much more drag. And designing aircraft is first and foremost all about reducing drag.

Sometimes we joke that planes actually fly because of all the inflated egos in the cockpit. In reality though, it's the laws of aerodynamics, and the brilliance of the design engineers that make it so.

HAL

[Edited 2004-04-29 09:19:56]

[Edited 2004-04-29 09:24:31]
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FredT
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RE: How Does A Plane Take Off?

Thu Apr 29, 2004 6:41 pm

HAL,
good post, but not 100% technically correct regarding the reason for the nose going up as the speed increases. Technically correct (hopefully!) explanation follows - those who read on have been warned!  Smile

You really trim for a given angle of attack. This implies a given airspeed, as there is only one airspeed which for a fixed angle of attack (AoA) creates the same amount of lift as the weight of the aircraft. The aircraft, when left alone, will always return to this angle of attack. That is static stability, to speak aerodynamicese.

Now picture this: You’re flying level at a slower airspeed than trimmed. To achieve this, you must apply back pressure to the stick Holding back pressure means you will be flying at a higher AoA than trimmed. As you let go of the stick, the aircraft will go back to the trimmed AoA. This will mean lowering the nose a few degrees at the most. However, the lower AoA will decrease the lift generated. The lift is no longer equal to the weight of the aircraft. The aircraft accelerates downwards.

In other words, the flight path turns downwards.

The aircraft remains at the trimmed angle of attack, so the downwards flight path will result in the pitch attitude lowering further as the aircraft aligns itself with the relative airflow.

Then you pick up speed. The lift increases until it again equals the weight of the aircraft. Your flight path stops turning downwards.

Theoretically, you could already be in a stable descent as this point. Typically though, the dive will be a bit steeper than the power setting and trim dictates at this point. This means the speed will increase past the trimmed airspeed. Then, the lift will be greater than the weight of the aircraft which equals to an upwards acceleration. Again, your flight path turns upwards and with it, the nose comes up as the aircraft remains at the trimmed AoA.

This cycle might repeat a few times with decreasing amplitude of the oscillations in descent rate and airspeed before the aircraft finds the stable pitch angle for a constant rate of descent. If this happens, the aircraft is said to be dynamically stable in addition to statically stable.

Some aircraft can in some configurations be dynamically neutral, meaning they will continue to climb and descend alternately left to their own devices until you manually home them in on the trimmed airspeed in straight (not necessarily level) flight. The amplitude of the oscillations will not become either larger or smaller.

Yet other aircraft can become dynamically unstable. If left to their own devices, the oscillations will keep getting bigger and bigger, until they either reach an amplitude where they are stable or cumulugranitus intervenes.

In fact, most aircraft will oscillate slightly around level flight but the oscillations are typically too small to be noticed.

Cheers,
Fred
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vikkyvik
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RE: How Does A Plane Take Off?

Thu Apr 29, 2004 7:15 pm

Hmmmm....

Sounds like you guys are describing a Phugoid mode. Correct me if I'm wrong. But I do know that the Phugoid mode is characterized by a constant AOA.

Also, Fred, as far as I have learned, static stability means that without any disturbance, the object will remain motionless relative to whatever coordinate system. For instance, a rigid pendulum positioned directly upwards will stay there if it is not disturbed, but will fall to the downwards position if disturbed. If I remember correctly, this is static stability but dynamic instability. A pendulum hanging directly downwards, however, is statically and dynamically stable. Point being, an airplane at a certain AOA that will return to that AOA if disturbed must be statically and dynamically stable.

As I said, please correct me if I'm wrong. But I thought "static" referred to an object being stationary, and "dynamic" referred to an object in some sort of motion.

~Vik
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FredT
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RE: How Does A Plane Take Off?

Thu Apr 29, 2004 9:20 pm

In aerodynamics, static stability is just what I said: The aircraft will return to a given angle of attack if disturbed.

An often used example is a marble and a bowl. If you put the marble in the bowl, it is statically stable. If you disturb it from the position of equilibrium (i e the equivalent of the trimmed angle of attack) it will return there.

If you turn the bowl upside down and balance the marble on top, it is statically unstable (JAS 39 Gripen). If you move it even slightly away from the position of equilibrium, the disturbance will increase over time. The speed at which it doubles is a measure of the degree of static instability.

A ball on a flat plate has neutral static stability. It will remain where you put it. An aircraft with neutral static stability (F-16) will simply maintain any angle of attack you set it at.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
QantasA332
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RE: How Does A Plane Take Off?

Fri Apr 30, 2004 6:16 am

Below is the classic "bowl" visualization of the three types of static stability, just to clarify things...





I won't bother to explain it myself, as it has already been demonstrated well enough. For anything more, check out any introductory book on flight/aerodynamics, as stability and control is always addressed...

Cheers,
QantasA332
 
FredT
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RE: How Does A Plane Take Off?

Fri Apr 30, 2004 6:43 am

Thx QA332. A picture says more than a thousand words!
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
QantasA332
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RE: How Does A Plane Take Off?

Fri Apr 30, 2004 7:18 am

You're very welcome, Fred! Pictures certainly do say more than a thousand words, especially when the only other means of communicating is through typing...

Cheers,
QantasA332
 
vikkyvik
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RE: How Does A Plane Take Off?

Fri Apr 30, 2004 10:22 am

Thanks guys - it's been awhile since I learned this stuff.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".

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