Four forces in level flight.

5 Answers

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   Donnie on Dec 22, 2012

   If thrust were greater than drag, the plane would be increasing is speed. If less, the plane would be slowing down. If lift were greater than weight, the plane would be climbing. If less, the plane would be descending.

   In physics or any other branch of science, if there is no acceleration (change in velocity), then there is no force … or any such force is cancelled out.

   Newton’s second law of motion is force = mass * acceleration. The plane certainly has mass. So, no acceleration (i.e. no change in velocity) means no force or at least the force has been cancelled.

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   Koehn on Dec 22, 2012

   The first one.

   Put another way: when bicycling over level ground, you need to keep pedaling or you’ll slow down and eventually stop.

   Newton said that an object in motion tends to remain in motion. If we didn’t need thrust, you could climb to altitude, shut off the engine, and coast the whole way there. But we have drag (parasitic and induced), so we need that engine to keep thrusting the air backward to overcome that drag.

   When in level flight thrust begins to exceed drag, the airplane climbs. When in level flight thrust becomes less than drag, the airplane descends. This is quite easy to verify in the airplane by putting it in level flight and then adjusting the throttle.

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   John D. Collins on Dec 22, 2012

   Any time the four forces are balanced, meaning the sum of the vertical forces and the sum of the longitudinal forces are the same, the aircraft will be in un-accelerated flight. This is true in steady level flight, a steady climb, and a steady descent. At a given airspeed, the airplane will climb anytime the power available exceeds the power required for level flight and will descend anytime the power available is below the power required for level flight. If you start out level at a given airspeed and add power, the airplane will accelerate and climb until the forces are once again in balance. Similarly, if you reduce power, the airplane will accelerate and descend until the forces are once again in balance. If you maintain a constant airspeed, the lift will remain unchanged in a climb or descent, but the direction of the lift vector will have an angle with respect to the vertical. The same is true with drag, the magnitude of the drag will remain the same, but not its direction. Weight will also remain unchanged. The variable that changes when airspeed is held constant is the thrust.

   On the other hand, if you change the angle of attack while keeping the power the same, the lift and the drag will have to change to reach equilibrium.

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   Jim F. on Dec 23, 2012

   Great answers, but I need to correct Donnie on one thing: Excess lift does NOT cause a climb. (Don’t worry, this is a common misconception, for myself included until I took an aerodynamics course.) A climb is actually caused by excess thrust. Here’s an explanation on this here (starts below figure 2): http://www.free-online-private-pilot-ground-school.com/Aerodynamics_in_flight.html

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   Dave on Dec 23, 2012

   Thanks all, what I was completely forgetting a body in motion stays in motion….

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