Lift and weight in a climb!

[Comanche250]

Evening, right, I could do with a little clarification. The following was written by my sisters science teacher with regard to a homework entitled 'Forces acting on an aircraft during climb, cruise and descent'. (What follows is word for word what the teacher has written)

"You should be aware that this assessment is used nationally and is assessed aginst accurate science"

"Whilst the aircraft is moving along the runway the weight must be greater than the lift or the aircraft would rise. At the point of takeoff however, if lift does not become greater than weight (as a result of changing airflow over the wings) then the aircraft could not leave the ground. This is a fundamental principle of physics which, if not obeyed by an aircraft at takeoff would make it impossible for it to fly."

"Interestingly a little further in her response (x) correctly states that during descent/landing weight is greater than lift. Clearly both her claims cannot be correct."

Basically the argument arose from her saying that in a climb, weight will be greater than lift.

Anyway, could someone please confirm, (if indeed this statement is correct) that in a climb, as of the moment an aircraft lifts off the ground and starts a climb, WEIGHT will theoretically be GREATER than LIFT?

Many thanks
C250

[BackPacker]

Comanche, it really is very simple. Newtonian laws basically state that if all the forces acting on an object are in balance, then the object is moving along, unaccellerated. If the forces are not in balance, the object is accellerating in some direction. Aviation is no exception.

In the following, I'm going to leave out turns, just assume constant heading.

During a constant-speed taxi, during a constant-vsi constant-speed climb or descent, or cruise, there is no accelleration. All forces are equal. More precisely:
Taxi: The asphalt exerts an upwards force on the wheels equal to the weight of the aircraft, and the thrust of the propellor is equal to the drag of the landing gear and air friction.
Climb: The vertical component of lift plus the vertical component of thrust equals weight plus the vertical component of drag, and the horizontal component of thrust equals the horizontal component of lift plus the horizontal component of drag. (Boy, I wish we could do pictures here...)
Descent: Same thing.
Cruise: Thrust equals drag, lift equals weight.

Now you do not go from taxi to climb, from climb to cruise, from cruise to descent and from descent to taxi immediately. Between these stable flight states, you need to accellerate (or, as you wish, decellerate) the aircraft into its new flightpath. During this accelleration or decelleration, the forces are NOT balanced. And in some cases you have to "overdo" something to actually force the aircraft into the new flightpath. As a passenger you can feel this accelleration or decelleration: when rotating, you're pressed into your seat. When leveling off, you are momentarily lifted out of your seat, and that happens when the descent is initiated as well (although that's not as pronounced). And when you flare and land, you're pressed into your seat again.

As an outside observer, you can see and hear the same thing. Look at an aircraft on a stand. When it starts to taxi, you'll see and hear the RPMs increase. Aircraft accellerates with these increased RPMs (thrust greater than drag) to the intended taxi speed. Then the pilot retards the throttles to a lower RPM. Because as soon as taxi speed is reached, there is no need anymore for an accellerating force/thrust from the engine, only enough thrust to balace out the friction of the tires.

Some of the manoeuvres are actually even more complicated. Here's what happens when you level off at your cruise altitude:
You're climbing. VSI and airspeed is constant, all forces are balanced. About 50-100 feet below your intended cruise altitude (depending on vsi) you push the stick/yoke over so the aircraft noses over into a level attitude. During this pushover you will momentarily feel a bit lighter because the aircraft decellerates its vsi from something to zero (so at that point in time weight is temporarily greater than lift), but at the same time, the power is still 100%, so the aircraft is now accellerating forward. Airspeed increases, but altitude now stays the same. Weight equals lift, but thrust is greater than drag, hence the accelleration. You retard the throttle only once you've reached your cruise speed. At that point the aircrafts forces are in balance again: weigth equals lift, thrust equals drag.

So, in a stable flight condition, be it taxi, climb, cruise or descend, all forces are balanced out. But in the transition state between these conditions (rotate, level off, initiate descend, landing) they are not, hence an accelleration/decelleration.

Now there's one more caveat and that's how the aviation industry defines all these forces.
Weight is the force of gravity. It is by definition working straight down.
Lift is the force generated by the wings. It is by definition working at 90 degree perpendicular to the fuselage axis. This is NON intuitive for people outside the industry!
Drag is by definition acting on the aircraft in the longitudinal axis
Thrust is by definition acting on the aircraft in the longitudinal axis as well

Imagine an airplane which goes straight up (not many aircraft can do that though). Lift is all of a sudden acting in a horizontal plane instead of having a vertical component. So lift does nothing to counteract weight. The only force counteracting weight is thrust. So the only aircraft which can fly straight up (some fighters, the Space Shuttle) are aircraft that have more thrust available than their own weight is. That's one situation where, as you were requesting, weight is more than lift. But that's lift in the aeronautical definition.

If you were to define lift as the combination of forces that act vertically upwards on the plane (generated by both the wings and the propellor) then the only time that that lift would be temporary less than the weight would be when leveling off from climb to cruise, or when initiating the descent. And by the same token, that lift would temporarily be higher than weight when rotating or flaring to land.

[Field In Sight]

Have a look here and search for "Forces in a climb"

http://www.alphatrainer.com/handouts/ac61-23c.pdf

Once you understand the definition of lift and the vertical components of climb it makes more sense.

Hope that helps.

FIS

[pilotbear]

Climbing is due to excess thrust not lift.

[FlyingForFun]

All the answers so far are correct, but don't directly address the reason for the confusion between you, your sister, and your sister's teacher.

- In a steady climb, lift will be less than weight. The vertical component of lift, plus the vertical component of thrust, minus the vertical component of drag will be equal to weight.

But this only applies in a steady climb.
could someone please confirm, (if indeed this statement is correct) that in a climb, as of the moment an aircraft lifts off the ground and starts a climb, WEIGHT will theoretically be GREATER than LIFT?
This is not true, because it is not a steady climb. At the moment of rotation, lift will indeed be greater than weight, as your sister's teacher says.

What is actually happening is that, as the aircraft is rotated, the angle of attack is increased, and this gives a corresponding increase of lift, until eventually lift is greater than weight and the aircraft becomes airborne. At this point, the aircraft's path through the air changes - it is no longer just horizontal, but it has some vertical element, too, because it is going up. This changes the direction of the relative airflow the wings experience, reducing the angle of attack, so that once a steady climb has been achieved, lift is back to being less than weight.

FFF
----------------

[Tinstaafl]

Weight is the force of gravity. It is by definition working straight down.
Lift... (redacted) ... is by definition working at 90 degree perpendicular to the fuselage axis. (redacted)
Drag is by definition acting on the aircraft in the longitudinal axis
Thrust is by definition acting on the aircraft in the longitudinal axis as well my bolding

Not quite correct. Those forces are defined w.r.t. the flight path, not one of the aircraft's axes. Most of the time the aircraft's longitudinal axis is not parallel to the flight path and may change without a change in the flight path. Consider accelerating in S&L. Aircraft attitude changes but L remains constant.

[BackPacker]

Agreed. But unless you do some seriously aggressive manoeuvring, or you're at the stage where you're holding off for landing, the difference between the longitudinal axis and the flight path will be no more than 10-15 degrees. And in cruise, at the speed the fuselage form and angle of incidence of the wings optimised for, it is close to zero. So I kinda hoped that that difference would be lost on Comanches sister.

[Comanche250]

Cheers FFF, that is exactly what I needed to know, I thought I understood the rest of it seeing as how I passed the bleedin' ATPL performance exam :} its just the transition from t/o to climb that wasnt 100% clear!

[Uncle Ginsters]

Our school-teacher friend was clearly a little mis-placed.....as mentioned in earlier posts. Our good friends at NASA have the answer.....
Ask NASA (http://www.grc.nasa.gov/WWW/K-12/airplane/climb.html)

[FlyingForFun]

Uncle Ginsters,

No, the school teacher is absolutely right. The slide you have linked to on NASA's website relates to a steady climb - not the initiation of a climb such as on rotation.

FFF
---------------

[Superpilot]

Thanks Comanche for asking this one. It was making my balls ache too. And my Perf exam is in a couple of weeks!

No, the school teacher is absolutely right. The slide you have linked to on NASA's website relates to a steady climb - not the initiation of a climb such as on rotation.

Surprisingly, not one of my PoF or Perf teachers made this distinction.

[FlyingForFun]

not one of my PoF or Perf teachers made this distinction
As pilots, it is useful for us to understand the rudimentary ideas of the forces on an aircraft during flight. All the textbooks, etc, I've ever read which are aimed at pilots have only ever explored the forces in a steady phase of flight - never the change from one steady phase to another. I would guess that the reason for this is that changing from one phase to another is quite a bit more complex than a steady state.

On the other hand, a physics student at school has to understand quite a different set of principles. She would, for example, need to learn Newton's laws. Newton's first law states that "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it", and Newton's second law quantifies this some more by looking at the relationship between force, mass and acceleration. What this means for an aircraft about to take off is that, if no upwards force is applied to the aircraft, the aircraft will not take off. It will only take off if there is an upwards force applied, and this upwards force comes from the lift from the wings. This is a very basic principle of physics, and one which should be understood by all science students.

Different audience, different knowledge required. But it would be very useful if a differentiation would be made between the two, so that a student (whether a school science student or a student pilot) who is taught one set of rules understands clearly that these rules only apply in certain circumstances, and when.

FFF
------------------

[Uncle Ginsters]

FFF,
Oops, I mis-read the original post [typing much red-faced] :\

Apologies, i should have read the whole text more closely.

But i agree with the above - this is generally a much mis-understood phase of flight in terms of the aerodynamics going on.

Uncle G :ok:

[rondon9897]

QUOTE Climbing is due to excess thrust not lift.


Wow bad news for glider pilots then--they cant climb!

[rondon9897]

QUOTE Climbing is due to excess thrust not lift.


Wow bad news for glider pilots then--they cant climb!

QUOTE Anyway, could someone please confirm, (if indeed this statement is correct) that in a climb, as of the moment an aircraft lifts off the ground and starts a climb, WEIGHT will theoretically be GREATER than LIFT?

It actually depends on what increment above the stalling speed you lifted off the ground.

Once the thrust vector is inclined away from the horizontal it will provide a degree of lift and is the reason why you need less lift in a climb.Think about a vertical rocket to understand this, its lift is provided by 100% thrust=no lift vector

The closer you lift off to stalling speed the less lift is need from the aerofoil if you find this hard to understand try cutting the engine at the exact stalling speed and lifting off and after you have had hospital treatment try again but this time go 30 knots above the stall speed cut the engine and lift off, you will get airbourne and as there is obviously no thrust the lift is greater than the weight.

Backpacker--if you are an instructor try to keep things simple then you will get more time to go backpacking


ASK CAPTAIN JON

[FlyingForFun]

Anyway, could someone please confirm, (if indeed this statement is correct) that in a climb, as of the moment an aircraft lifts off the ground and starts a climb, WEIGHT will theoretically be GREATER than LIFT?

It actually depends on what increment above the stalling speed you lifted off the ground
I'm not sure it has anything to do with the increment above the stalling speed?

According to Newton (who is fairly well respected in the world of physics!), the vertical acceleration is proportional to the net vertical force.

What that means for our aircraft as it lifts off is this: take the vertical component of lift, and add to that the vertical component of thrust, then subtract the weight and the vertical component of drag. That gives the vertical force. If you are left with a positive amount, then the aircraft is accelerating vertically, in other words the VSI will be increasing. If you are left with zero, the aircraft is not accelerating vertically, and the VSI (once you allow for any lag in the instrument) will show a steady climb.

At the moment the aircraft lifts off the ground, it does not instantly achieve its initial climb rate. It achieves this initial climb rate gradually, because the vertical components of lift and drag are greater than the weight and the vertical component of drag. (The vertical component of both thrust and drag will initially be approximiately zero because those forces act horizontally, but they will both increase as the aircraft is rotated.) This, according to Newton, causes the aircraft to accelerate vertically. As its vertical speed increases, the angle of attack of the wings decreases because of the change in the direction of the relative airflow the wings experience. This reduces the lift, and hence the vertical comonent of lift, and reduces the vertical acceleration. The aircraft is now still accerating vertically, but not as much as before - but again, this vertical acceleration decreases the angle of attack further, and so on, and so on, until eventually the upwards forces and the downwards forces are equal. Now the aircraft is in a steady climb, and the vertical component of the lift is slightly less than the weight (by an amount equal to the vertical component of thrust minus the vertical component of drag).

FFF
-------------

[rondon9897]

I'm not sure it has anything to do with the increment above the stalling speed?

Thats the difference you see--I am sure!

ASK CAPTAIN JON

[BillieBob]

Wow bad news for glider pilots then--they cant climb!In relation to the air surrounding the glider, that is true. A glider will only increase its altitude if it is attached to a winch, an aerotow or if the volume of air immediately surrounding it is also increasing altitude (e.g. in a thermal). In any steady state (climb, level or descent) the forces acting on an aeroplane must be in equilibrium, if they weren't the aeroplane would be accelerating in one plane or another.

In the case of a powered aeroplane and in relation to the vertical plane - in a climb the resultant of thrust and lift is balanced by the resultant of weight and drag; in level flight lift is balanced by weight and the down force of the tailplane; in a glide descent weight is balanced by the resultant of lift and drag. In any of these examples, if the total forces UP are greater than the total forces DOWN, the aircraft will accelerate upwards, and vice versa.