wjvail

My Feedback: (3)

Yup in all cases... in level flight and if velocity is static. If thrust does not equal drag the plane is ether accelerating or decelerating. It's simple statics. The sum of the forces must equal zero if the object is static (i.e. not accelerating.) If you'd like to talk about a situation where thrust doesn't equal drag, that is the area of dynamics and we see that on every takeoff roll. That is not what we are really talking about here. (It gets a little tangled in the usage of terms if you are talking about, say, a plane in a steady torque roll. In this case you have to say lift = weight. In this case we would be using engine thrust plus wing generated lift (0) to equal aircraft weight.)


Thrust, in this case, is the sum of prop provided thrust plus the component force of gravity and MUST equal the drag of the airframe. Airplane drag is the drag coefficient of the airplane times velocity squared.

An example may help. Take the example of a 7 lb Alpha .60 trainer with a Evolution .60 turning a 12x6 at 12,000 rpm. If you remove the engine and prop and replace them with ballast, I think we can all agree the component of gravity contributing to thrust in level flight is zero and thrust provide from the (missing) engine is zero and the total trust is... zero! Now take this same arrangement and point it straight down. Engine provided thrust will still be zero but gravity will be providing 7 lbs of thrust (force). The plane WILL continue to accelerate until airframe drag equals 7 lbs.... and NO faster. I would suggest this speed for an .60 trainer is around 90 mph.

Now 1/2 way into this dive reinstall the engine. A 12x6 may have an actual pitch of around 7" and may unload to 13,000. That computes to a prop speed of around 86 mph. At this point the newly installed engine is now not pulling the airplane. It is increasingly becoming DRAG. At about 90 mph engine trust will be close to zero and gravity will remain... 7 lbs. 0 prop thrust + 7 lbs gravity = 7 lbs thrust = 7 lbs drag.

Bill