pelmet wrote: ↑Sun Feb 18, 2018 1:50 amI'm still trying to figure out why they designed to propeller on the Twin Otter to not be able to go into reverse unless the prop levers are above 91% (full forward is 96%) unlike the King Air which doesn't seem to have this restriction.
Pelmet:
I can't comment on the King Air, I have no knowledge of that aircraft. But, I can explain why the propeller levers have to be full forward (set to 96%) before you can move the power levers aft of the idle stop (into the ground range area) to use reverse thrust. It's kind of a lengthy explanation, because I have to provide some background information before getting to the point and providing the answer.
A propeller on a Twin Otter can be controlled either by the constant speed portion of the governor, or by the beta-reverse valve. Both of these sub-systems use the same methodology to control the propeller (they supply oil under pressure to move the propeller towards a finer blade angle, thus opposing the action of the internal spring and external counterweights which are always trying to drive the propeller towards feather, feather being a coarser blade angle). Although they use the same methodology, they function to achieve different objectives. The objective of the constant speed portion of the governor (the CSU) is to maintain a propeller RPM selected by the pilot using the propeller lever, and the CSU maintains a set RPM by varying blade angle as required to achieve the set RPM. If RPM is too slow (underspeeding), the CSU admits more oil to the propeller to fine out the blade and enable it to rotate faster (due to less resistance to rotation). If RPM is too fast (overspeeding), the CSU reduces the oil supply, and the propeller moves to a coarser blade angle, which slows it down (due to greater resistance to rotation).
The objective of the beta-reverse valve is to maintain a set blade angle, without any concern at all for whatever the RPM may be. When the propeller system is rigged by the technician, the technician sets the idle blade angle at +11°. As long as the power levers are not moved aft of the idle stop, into the ground operations range, the beta-reverse valve will ensure that the blade angle does not decrease below +11°. When the pilot twists the power levers and pulls them aft, into the ground operations range, a linkage connected to the power lever pulls on a lever attached to the beta-reverse valve and opens the beta-reverse valve, admitting more oil into the propeller. The propeller blade angle decreases. There is a follow-up linkage connected to the a feedback ring on the propeller dome that moves the beta-reverse valve back towards a closed position, thus regaining an equilibrium at the position the pilot has selected when he or she pulled the power levers aft. If the pilot pulls the power levers further aft, the same process repeats: beta-reverse valve opens a bit, oil flows into the prop, prop dome moves outwards, blades move to a finer blade angle, feedback ring moves beta-reverse valve back towards closed, equilibrium achieved once again, this time at a finer blade angle.
Now, before we go further, take careful note that in each of the above scenarios, both the CSU and the beta-reverse valve use the same method of moving the propeller towards finer blade angles: they admit more oil into the propeller dome. And, they use the same methodology to move the propeller towards coarser blade angles: they restrict or even cut off the flow of oil going to the propeller.
From the above, we can extract two key rules necessary to understand propeller behaviour:
1) It takes
A LOT of oil to make the propeller move to a finer blade angle (full reverse being the 'most fine' blade angle), and;
2) If you cut off (or substantially reduce) the oil supply, the propeller will move to a coarser blade angle (feather being the extreme end of a coarse blade angle).
Keep those rules in mind, they will come in handy later on in this explanation.
If the CSU can get the propeller to achieve the pilot's selected RPM (as set with the propeller lever), then the beta-reverse valve stays wide open and does not interfere with or control oil flow to the propeller. But, if the CSU cannot get the propeller up to the set speed, the propeller will keep moving towards a finer and finer blade angle as the CSU continues to supply oil to the propeller. Eventually, the beta-reverse valve will step in and say, in effect,
"that's fine enough, I'm not letting you move that propeller to a finer blade angle than +11°", and the beta-reverse valve will start moving towards the closed position and reduce oil supply to the propeller in order to limit blade angle.
Based on what has been said so far, you can extract two conclusions:
1) The CSU always gets "the first opportunity", so to speak, to control the propeller, and;
2) The propeller has to be in an underspeeding condition (in the opinion of the CSU) before the beta-reverse valve will operate.
The reason that the pilot is asked to push the propeller levers forward at low power settings (by way of the 'RESET PROPS' light illuminating) is
to force the propeller into an underspeed condition, thus handing control of propeller blade angle over to the beta-reverse valve. Remember, as long as the CSU can achieve selected propeller RPM, the beta-reverse valve will not operate... it will remain wide open, allowing the CSU to control blade angle.
Now, getting to your question (I'm paraphrasing it)
"Why is it necessary to move the propeller levers fully forward to release the mechanical interlock before you can twist the grips and move the power levers back into the ground operations range?"
Easy to answer. Just consider the following scenario:
1) Pilot is landing on a very, very, short strip.
2) At the end of that very short strip is the town cesspool, and it is full.
3) Pilot obviously wants to stop before running off the end of the strip.
4) But this particular (imaginary) Twin Otter has no mechanical interlock installed on it, so, it is possible to pull the power levers back into the ground range when the props are set to 75% Np.
The pilot flies a perfect approach, exactly on speed, touches down on the first 6 inches of the strip, and promptly twists the grips and hauls the power levers back into row 3 of the cabin. The propellers move to full reverse, and the engine starts to roar as Ng rises. As Ng rises, the propellers start turning faster and faster. As soon as the propellers reach 75% Np (which is what the prop levers are set at), what is the CSU portion of the propeller governor - which up to this moment, has been in an underspeeding condition - going to do? Guess what, it's going to start to govern Np at 75%. And how does it do that? By reducing the oil supply to the propeller. And what happens when you reduce oil supply to the propeller? See Rule 2 up at the beginning of this explanation.
So, here's our pilot, engines howling, power levers all the way back, coming ever closer to the cesspool at the end of the strip, and suddenly the CSU decides to substantially reduce the oil supply to the propeller (or even dump oil from the propeller) in order to limit RPM to 75%. The prop will respond by rapidly moving to a much coarser blade angle, and as soon as the prop coarsens up above 0°blade angle, it will develop forward thrust, even though the power levers are all the way aft. The pilot will wind up in deep s___, off the far end of the runway.
So, as you have probably figured out by now, the reason for the mechanical interlock is to make sure that the props are always set at the MAX RPM (96% Np) setting before the pilot can move the power levers aft of the idle stop, this to prevent the CSU from suddenly deciding it is going to "govern" the props when the pilot is commanding reverse thrust.
Hope this explains things satisfactorily.
Michael
PS: The 91% Np limitation you mentioned is a different matter. It is a restriction imposed on propeller speed when reverse is selected, by reducing fuel supply to the engine. The reason is the same - we don't ever want the CSU portion of the prop governor to become active when heavy reverse is selected. The selected Np in reverse is 96% (the mechanical interlock ensures this is so). To prevent Np from reaching 96% when reverse is selected, a totally different mechanism in the governor assembly reduces fuel supply to the engine as soon as Np reaches 91% when the power levers are aft of idle.