The Four Forces
Introduction to the "Fources"
The above diagram illustrates the four forces that affect an airplane while in flight. Lift is the upwards force produced by the wings which counteracts gravity and keeps the airplane from falling. Weight is the downwards force produced by gravity that pulls the mass of the airplane towards the Earth. Thrust is the forwards acting force produced by the engine and propeller, it is essentially lift in a horizontal direction. Drag is the rearwards acting force, or the resistance of the airplane to move forward through the air.
The Forces in Balance
When the airplane is in stabilized flight, then all of the forces are balanced. This is the case whether the airplane is level, climbing, or descending, as long as the airplane is neither accelerating nor decelerating. A lot of people tend to get confused right here, believing that excess lift or excess thrust allows an airplane to climb. This is partly true: as the airplane is transitioning from straight and level flight into a climb, excess lift is needed to accelerate it vertically. Once the airplane is established in a steady rate climb, the forces are once again in balance.
I know, at this point you're wondering, "But isn't the power at a higher setting now that we are in a climb?" Yes, it is, but all the forces are still balanced. The reason that a higher power setting is needed is because as the airplane's nose is tilted upwards, the thrust, lift, and drag forces follow it, but the weight force remains straight down towards the earth.
In the above diagram you can see smaller arrows that are part of the weight force. These smaller arrows are called components, they are there to help us understand how we are affected by the weight force since it is no longer lined up with the rest of the forces. You can see that most of the weight force is acting opposite to lift, but there is a small amount acting in the same direction as the drag force, which is opposite to thrust. In effect, this has increased our drag, so we need to compensate by increasing thrust. The technical explanation is that we are increasing thrust to compensate for the rearward component of weight. Notice what happens in a descent.
There is now a small component of weight acting in the direction of thrust, which means we can reduce power to compensate and keep the airplane at the same speed. Note that if power is not reduced when entering a descent, then the small component of weight acting in the direction of thrust will put the forces out of balance because total thrust will now be greater than total drag. Since thrust is greater, the airplane will accelerate. This is why the speed increases when entering a descent if power is not reduced. The reverse is also true, speed will decrease when entering a climb if power is not added.
On to the next section...Lift
|
