Thrust Drag Lift and Weight?

Question

If the four forces Thrust,Drag,Lift and Weight are equal and opossite(thrust to drag,lift to weight),why would an aircraft move forward?Imagine a block of wood being pulled from all four sides opposite to each other.

Answer 1

Lift equals drag in a constant rate climb or descent, too.  The plane need not be flying  level.

A block of wood being pulled from all four sides opposite to each other can be either motionless, or in motion at a constant velocity.  Push a car by hand at a constant speed:  your thrust is balanced by the forces of drag.  It need not be motionless for the forces to be in balance.

CORRECTION:  where I said "lift equals drag," I meant "lift equals weight."  

Coffeebucks answer is on the money, as usual.

Answer 2

In the scenario mentioned by you, there is effectively no external force acting on the aircraft. So, as per Newton's First Law, the aircraft continues doing what it was doing. That is, if it was at rest, it will remain in rest, if it was in motion, it would remain in motion.

The key point here is that it will NOT accelerate, positive or negative.

Answer 3

It may help if you think of drag by another common term - wind resistance.  Aerodynamic drag is dependant on speed. In general, the faster you are going going the more drag is generated. Think of walking into the wind - if there is no wind, you do not even notice the drag your body creates. On a windy day it is harder to walk into the wind, and during a hurricane you may not be able to move against the wind at all.   An airplane standing still has no drag, so when thrust is supplied by the engine(s) it moves forward.  As it gains speed, drag increases.  When  the amount of drag equals the thrust, the plane will no longer accelerate, but will stay at that speed. If you reduce thrust, drag will slow the plane. As it slows, drag is reduced.  When it slows to a point where drag equals thrust, the plane stops decelerating, and again will stay at this new, somewhat slower speed.  Drag does not always equal thrust, but if there difference beween the two the plane will speed up or slow down until they are equal.

There have been a couple of answers posted that claim an airplane climbs because lift of the wings is greater than the weight of the plane.  That is not correct.  When in a climb, the nose of the plane actually higher than the actual direction of flight, so some of the thrust is  pushing the plane upward.

Answer 4

Newtons first law of motion:

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Therefore, an aircraft in flight will continue at the same velocity if

   1: thrust = drag

   2: weight = lift

because equal forces in opposite directions cancel each other out.

Answer 5

If thrust is equal to drag, the airplane is not ACCELERATING, but it's not necessarily not moving.  Thrust must be greater than drag for an airplane to accelerate.  The plane will continue to accelerate until drag equals thrust (drag increases with increased speed).  At that point, the airplane will resume steady-state unaccelerated flight where thrust does indeed equal drag.

Same with lift and weight - lift equals weight if the airplane is not climbing or descending.  To make an airplane climb, lift must exceed weight, which is accomplished by increasing the angle of attack (assuming a constant airspeed to keep it simple).  To level off, decrease the angle of attack.  If the plane is level, weight equals lift.

If any of the forces change to the point opposing forces are not equal, the plane will climb or descend, accelerate or decelerate.

EDIT:  Mark (below) correctly called out a mistake in my answer regarding lift and weight.  Lift and weight are unequal if the airplane is experiencing vertical acceleration, i.e. a change of vertical speed.  If an airplane is climbing at a constant 500 feet per minute, lift and weight are equal.  To level off requires a change in vertical speed back to zero.  During that transition from a 500fpm climb to level, lift is reduced and weight is the greater force.  After leveling off, the airplane is again in steady-state UNACCELERATED flight (positive or negative), meaning that the vertical and horizontal speeds are unchanging.  In this state, the opposing forces are equal.

Answer 6

If those forces are in balance the aircraft will move forward not because of thrust but because of something known as Newton's First Law.  A body in motion tends to stay in motion unless acted on by an outside force.

Once the aircraft is set into motion, it would require an acceleration force in opposition to its motion to slow it down.  This is generally produced by drag and overcome by thrust.  If thrust and drag are in equilibrium, the aircraft maintains a constant speed and does not speed up or slow down.

Look at your textbook again, it very specifically says that those four forces are in balance when the aircraft is in straight and level UNACCELERATED flight.  If the aircraft is accelerating (or decelerating) then those four forces are not in balance.

Answer 7

If all forces are equal, then the aircraft will not move.  In order for an airplane to fly, it needs to have more thrust than drag, and more lift than weight

Answer 8

This is because of Newton's first law of motion.

"A physical body will remain at rest, or continue to move at a constant velocity, unless an outside net force acts upon it. "

Remember that drag, resulting from traveling through the air, is an external force.

Think about the space shuttle in orbit. It's not using thrust, but it's still moving relative to the earth.

When it says lift and weight are equal, it means that the aircraft will have no relative up/down motion.

When it says thrust and drag are equal, it means that the aircraft will neither decelerate or accelerate.

When you decrease thrust to slow down, you're actually decelerating and if you do it fast enough, you can be flung forwards.

This is because all matter has inertia, or a tendency to resist changes in velocity. This is what Newton's first law is based on.

Answer 9

http://www.allstar.fiu.edu/aero/fltmidfl...

read this a better explanation than i can give.. you need to know this like the back of your hand if you are going to fly ok also study weight and ballance and usefull load ..ok

Answer 10

The aircraft will not move. The thrust must exceed drag and lift exceed weight in order to fly.