who can explain?

[cjam]

Howdy, if someone can please explain what is happening when a turbo-prop discs I would appreciate it. I always thought I understood props/engines, I understand piston windmilling, What gets me with this is that with the finest pitch set at a high IAS, throttle to idle, it doesn't disc, but when you slow to a certain speed, away she goes. A technical explanation would be good, my desk planner is now completely covered with vector diagrams and it seems to say the opposite should happen, ie when the airspeed decreases the prop aoa gets bigger....aaaarrrgghhhhhh!!!

[Miserlou]

Due to the pressure in the Constant Speed Unit being insufficient, as I understand it, if we are talking descent after dropping skydivers here?

Then I'm guessing there is no flight idle/ground idle gate.

What a/c type?

[safetypee]

Try this; paraphrased from a well know military text.
A ‘discing’ condition is achieved by extending the pitch range of a propeller beyond the normal fine-pitch limit so that a large negative propeller angle of attack is obtained; thus it is possible to make the propeller provide a powerful decelerating force.
This should not be confused with the wind milling action of all propellers when meeting the airflow at a small negative angle of attack.
A braking condition is obtained when the engine is throttled back; the propeller angle of attack is slightly negative and the total force is therefore also negative.
The total force can be split into one component acting rearwards against the direction of flight and a torque force acting in the same direction as the direction of rotation; under the influence of this torque force, the propeller continues to windmill in the normal direction but exerts a braking force on the aircraft.
Although a wind milling propeller provides a strong decelerating force at high speeds, it is not satisfactory at low speeds. If the propeller blade has been rotated further to a large negative angle of attack, the negative thrust remains but the propeller torque is reversed and must be opposed by engine torque if rotation in the correct direction is to be maintained. The magnitude of the braking force will depend on the engine power applied. High negative thrusts are possible and can be used after landing to shorten the run, or to assist in manoeuvring on the ground.
In changing the pitch from normal to a power-on reverse thrust position, the wind milling position has to be traversed. At this point engine torque and propeller torque are acting in the same direction, and it is essential that the pitch change be made at a very high rate if over speeding is to be avoided.
In flight discing / braking has many potential dangers and should not be used. Most commercial engine / airframes have in-flight baulks or restrictions to prevent accidents.

[Miserlou]

Very good explanation, Safetypee.

That's the picture. And if you pull the prop back past the idle gate (into reverse) then the CS unit can't reduce the balde angle due to the aerodynamic force above a certain speed.

[RatherBeFlying]

I can talk about the Viscount. Ground Fine Pitch is automatically actuated on the ground. When landing this reduces the considerable residual thrust at flight idle. GFP failure requires a longer runway.

Same deal for taxi.

Newer turboprops have condition levers; I'll leave that topic to those who have flown with them.

GFP also enables the engine to run at high revs and be able to provide thrust quickly if needed for go-around after touchdown -- something the jets seem to have left behind.

As was seen with the Luxemburg accident, bad things happen if GFP happens in the air.