xerj wrote:

Thrust from a propeller drops as speed increases to a theoretical zero. When
this happens, what is providing the thrust to overcome drag? The forward
momentum of the aircraft?

If so, is it a case of the speed very slightly and imperceptibly dipping
below the zero thrust speed, getting a little thrust to bring it back to the
zero-thrust condition and doing this over and over again?

Thanks in advance.

Its hard to separate the thrust forces created by the prop from the
torque / hp of the motor exerting the shaft turning force to spin the
prop. Its also necessary to consider that as the speed of the plane
increases, so does its drag (parasitic drag from the plane itself).
At slow speeds, the angle of attack can also create higher drag against
which the prop must work. See:
http://www.petester.com/html/bachap04.html for more information. That
said, I think there are four states we can look at:

The first state is when the plane is accelerating. In this state,
thrust is greater than drag, the prop is creating more forward force
than the drag of our airplane. This is at lower speeds when the drag of
the plane is lower. At low speeds / higher angles of attack as in a
climb, equilibrium is reached at a lower speed due to higher induced drag.

As the speed of our plane increases, so does the drag (so does the
lift). At some point drag will equal thrust. If we throttle to 75%
power, the speed will settle in at a certain point when drag = thrust,
we are at 75% power cruise speed. If we throttle back to 55% power,
then the thrust will decrease (less power to spin the prop) and the
speed will decrease (and the plane's drag, as a factor of speed, will
decrease along with it) until the that equalibrium (drag = trhrust) is
reached. The resulting speed is the cruise at 55% power. This
"equilibrium" is the second state (cruise).

At higher angles of attack as in a climb or low speed flight,
equilibrium is reached at a lower speed due to higher induced drag.
When we throw out some flaps, we are increasing parasitic drag (not very
streamlined) and induced drag (higher angle of attack / aerodynamic
drag). Consider the thrust required to fly at minimum controllable
airspeed without losing altitude.

The third state is when the the prop creates less thrust than the plane.
Consider throttling back and accelerating in a dive. The speed
(pressure) of the air rushing past the prop is causing the prop to spin
at a higher rate than the speed it would turn under the same power at
equilibrium. So thrust is less than drag.

The forth state is when the aircraft is parked and the engine off.
There is 0 thrust and 0 drag. It is a kind of equilibrium, but one that
doesn't really do much for us.

For a given power setting, as the plane flies faster through the air,
the prop takes less "bite" and the drag against the prop decreases. But
it doesn't reach 0 (or 0) drag unless its in the third state. That's
when we want to throttle back so that we don't over-rev the engine. A
constant speed prop simply changes the angle of attack of the prop to
provide higher thrust at higher speeds as the prop "bite" decreases with
speed.



There may be some fine details that don't quite fit the above, but I
think this is generally the prinicpal of thrust (prop + power) vs. drag.