Reversing propellers made their appearance on the King Air A90 that ushered in the 1966 model year. The Straight 90 model of 1964 and 1965 utilized non-reversing propellers similar to those that were installed on the Lycoming-powered Queen Airs being concurrently produced. As one would expect, reversing propellers were a big hit and although they were an optional piece of equipment I don’t think there was ever an A90 that was not built with this option. It has continued as standard equipment on all models that came after it, right up to and including the 260 and 360 models of today.
All pilots who have undergone initial King Air training have been taught about the propeller system in detail. It is one of the more difficult systems to learn and to understand in-depth. The intent of this article is to discuss the method that allows the propeller to reverse but to explain it in a very nonscientific, nonmechanical, non-engineering manner. Other important pro-peller-related systems – e.g., the Overspeed and Fuel Topping Governors – will not be reviewed in any depth, although they, too, are important systems for which the competent King Air pilot must have understanding.
To begin, we must understand what makes a “Constant Speed Propeller.” Two independent variables determine the speed at which a particular propeller will rotate. The variables are (1) the factors that make the propeller want to rotate; and (2) how much the propeller resists that rotation.
Power and airspeed are the factors causing rotation. From our very first flying lesson, it became immediately obvious that pushing the throttle forward – increasing engine power – made the engine/propeller combination increase its rotational speed. This lesson was almost always conducted in a simple, single-engine airplane with a fixed-pitch propeller. In this simple training airplane we also learned that the propeller speed would vary even when the throttle position was not changed: Pull up into a climb and the prop slows down; nose over into a dive and the prop speeds up. Technically, this is because the angle-of-attack of the fixed pitch propeller blades is increasing as airspeed decreases and vice versa. Non-technically, it’s the same effect as when the toy pinwheel held out of Dad’s car window rotates faster the faster the car goes. “Windmilling effect” is the name assigned to this phenomenon. The higher the airspeed, the more windmilling effect the propeller experiences.
When adjustable-pitch or variable-pitch propellers made their appearance they were not initially constant speed propellers. Yes, the pilot now had the ability to change the propeller blade angle and hence the propeller’s angle of attack but no governor was installed that did this automatically. As power and airspeed increased, it was necessary for the pilot to move the propeller control (sometimes a lever, often times an electrical switch) to make the propeller’s angle-of-attack (“bite of air” in layman’s terms) increase, providing more resistance to rotation and hence keeping the RPM at the desired amount.
The addition of a Propeller Governor – the device that could change the propeller’s bite of air automatically – converted the variable-pitch propeller into the constant speed propeller. You realize this name is a lie, right?! The propeller can only keep the selected speed constant when sufficient power and airspeed exist to raise the speed of the propeller up to the desired RPM. Reduce power and slow the airspeed down and the propeller will eventually slow down also. Vice versa – go into a full-power, redline airspeed dive and there is the possibility the propeller will speed past the selected speed. Why? Because the propeller designers pick low pitch and high pitch limits of blade angle travel. The low pitch limit is selected with the goal of preventing excessive propeller drag. The airplane would “fall out of the sky” in the flare for landing if the low pitch blade angle limit were set too close to flat pitch. Vice versa, why have the high pitch limit set above the value needed to get enough rotational resistance to prevent exceeding maximum propeller speed in a reasonably high-powered, high airspeed dive?
When variable pitch propellers began being installed on multi-engine airplanes it became immediately obvious that it was desirable to move the high pitch stop to near 90 degrees: the feathered position that provided the least propeller drag when the engine is shutdown. Thus, with very few exceptions, the propellers on multi-engine airplanes are “constant-speed, full-feathering” propellers in which the maximum blade angle possible is indeed near the 90-degree mark.
How do we convert a “constant-speed, full-feathering” propeller into a “constant-speed, full-feathering, reversing” propeller as installed on most all King Airs? Simple: We move the low pitch limit of blade angle travel from the “don’t fall out of the sky” position where it existed for so many years and now position it to the “back side” of flat pitch.
“Blade angle” is defined as the angle between the chord line of the propeller’s airfoil shape and the plane of propeller rotation … the disk the prop makes as it rotates. Because propellers have a twist in them, the angle depends on how far out from the propeller’s center (the chord line for measurement) is selected. A bigger angle exists 1 foot from the center than the one that exists at the two-, three- and four-foot locations. The “30-inch station” (30 inches out from the propeller’s center) is the most common choice for blade angle measurement on “smaller” propellers. As the propeller diameter gets larger, it is common to use a point one-foot further out at the 42-inch station.
Neglecting the twist in the blades for a moment, the highest blade angle capable of being attained – where metal hits metal and the blade cannot move further unless something drastically breaks – the feathered position, is a 90-degree blade angle. The lowest angle where metal hits metal is now a negative angle, reflecting the fact that the blade bite is now pushing air forward instead of backward. Typically, this limit of travel angle is near -10 degrees.
Allowing the blade angle to go to -10 degrees is surely desirable after landing when a short stop is desired yet without excessive brake usage. Allowing the blade angle to reach -10 degrees while still flying? Not so good!
Therefore, what reversing propellers contain is a variable, movable, Low-Pitch Stop (LPS). The “how” this movement is achieved will be left out of this simplified discussion. But suffice it to say that when the pilot of a King Air lifts up on a power lever and then pulls it aft behind the location of Idle, he or she is indeed repositioning the LPS to lesser and lesser angles, ending up in the most negative blade angle position when the power lever reaches its full-aft, most-rearward position. Vice versa, pushing the power lever forward causes the LPS to come forward.
When a propeller is exhibiting actual constant speed – the RPM is remaining constant even while airspeed and power changes are being made – it implies that the propeller blade angle is variable –– able to be changed. Increased power and/or airspeed are balanced by more resistance to rotation as the governor makes blade angle increase. Likewise, reductions in power and/or airspeed are balanced by the governor decreasing blade angle to achieve less resistance to rotation.
When the propeller slows down below the selected governor speed it is because the LPS has been reached. The governor is incapable of reducing rotational resistance because the blade is as flat as it can now go so the constant speed propeller has – at least for now – reverted to a simple, fixed-pitch prop with the blade angle being at the LPS setting.
In the simplest of terms, reversing is nothing more nor less than moving the LPS. I’ve said it before and I’ll say it again: A pilot cannot force the propeller to reverse; he can only allow it to do so.
He allows it to do so by moving the LPS. Thus, when the propeller is underspeeding due to a low power setting combined with a low airspeed, the governor flattens the blade angle until it finds the LPS … which may now be in the negative range since the pilot has moved the power lever aft behind the Idle stop. It makes sense that the Pilot’s Operating Handbook (POH) warns the pilot that moving the power levers aft of Idle is only permitting when the airplane is on the ground.
As all of you know, Beta and Reverse are selected by lifting up on the power levers when they hit the Idle stop in the power quadrant and then pulling them further back. That is how we reposition the LPS. Since the power levers must be at Idle before they can be lifted, it means that engine power will be low at this time … High Idle (70% Ng) at most. Thus, one of the two factors that lead to an underspeed condition – low power – is automatically achieved. The second factor needed to reach an underspeed condition – low airspeed – is one of the remaining factors within our control. The other remaining factor is the governor’s propeller speed setting.
Back to the simple fixed-pitch prop on the trainer: With idle power, the speed of the propeller follows the speed of the airplane. Go into a dive, the prop goes faster; slow into a climb, it goes slower. Sit on the ramp with no wind at idle and it may only turn 600 RPM or so.
As for King Airs, have you noticed the minimum speed allowed for a windmilling airstart? It’s 140 KIAS. Where did this number originate? That is the speed at which the propeller can windmill at takeoff RPM with no engine power whatsoever! The force of the air spinning the propeller at its LPS – alone, with no exhaust gas driving the power turbine – spins the prop right up to redline RPM.
Follow me through on this. Let’s use a PT6A-21-powered C90 as an example. At Low Idle, there are indeed some exhaust gases helping to drive the propeller. It will therefore take less windmilling force to achieve the same RPM. Instead of 140 knots required to reach redline propeller speed, now it may happen at 110 knots. At any speed of 110 KIAS or more, we can reach 2,200 RPM at Low Idle. With the propeller levers fully forward, setting the propeller governor at 2,200, we will be seeing 2,200 RPM at any airspeed of 110 or higher. If we were to hold altitude with Low Idle power and allow the speed to decrease, the RPM would start dropping as the speed goes below about 110. At 105 knots the RPM may be down to 2,100; 100 knots might yield 2,000 RPM; 95 knots might yield 1,900 RPM.
Now what if we took this same propeller/engine/airframe combination but installed a new governor that has a maximum speed setting of 1,900 instead of 2,200 RPM? To achieve an underspeed condition – the prerequisite for allowing the propeller to follow the moving LPS into Beta and Reverse – our IAS must be below 95 knots. Do you see where I am going with this? The F90, debuting in 1978, was the first member of the King Air 90-series that used 1,900 instead of 2,200 RPM as its propeller’s normal speed limit. Beech felt it necessary to add a statement into the POH stating, “CAUTION: Propellers will not reverse at speeds above 95 KIAS.”
I often repeat the adage, “For every good, there’s a bad.” The good thing about slower propeller speed is less noise. The bad thing is that it makes reverse harder to obtain. More than one King Air pilot has found out the hard way that touching down with too much excess speed is a sin that use of reverse cannot cure! Of course the reason why is that the propeller blade angle is still being determined by the governor, not by the position of the Low Pitch Stop. The propeller has not yet reached the state in which it is allowing us to bring it into reverse.
I should add here that this discussion about being impossible to reverse when airspeed is high began by mentioning the F90. I used the phrase “same propeller/engine/airframe combination.” I lied a little, didn’t I? Compared to the C90 of its day, the F90’s wings are shorter, it has the T-Tail, it uses -135 engines instead of -21s, and it has four-blade props instead of the three-blades which were still used on the C90s of 1978 vintage. Yet, the relationship of airspeed to propeller speed at idle are still very similar, as I described.
Rest assured that although the 200-series uses 2,000 as its propellers’ maximum speed and the 300-series uses a mere 1,700 RPM, these models hit their LPSs quite easily due to their particular propeller’s size and shape. With power at Idle, they will be ready to reverse in almost all cases unless the speed at touchdown is much, much, too fast.
The “RVS NOT RDY” annunciator – Reverse Not Ready – illuminates when the landing gear handle goes down. It goes out when we push the propeller levers fully forward. Do you understand the significance of the reminder? Will extinguishing the light by advancing the prop levers guarantee that we can reverse the props? No.
This annunciator simply reminds us that selecting the highest propeller governor speed – which we are doing when the prop levers go fully forward – means that our moving of the LPS will more likely be successful since we are more likely to be in an underspeed condition if the governor is set for its highest governing speed.
You have all seen the placard on the power quadrant that states, “CAUTION: Reverse only with engines running.” At the start of this article I promised that my discussion would be in a “nonscientific, nonmechanical, non-engineering manner.” Let me try to explain why this caution exists and what it really means.
The only time the power levers can move the Low Pitch Stop to obtain Beta and Reverse is when the blade angle is actually sitting on the LPS. We might say that we need the propeller trying to go flatter, trying to restore the onspeed condition, trying to push the LPS flatter, to help us pull the LPS to smaller and smaller blade angles. If we don’t have this help, this push from the propeller, then we feel resistance as we pull back and try to move a LPS that is not yet ready to move. With enough force, we can elongate or stretch the reversing cable. If that occurs, in the least it can set the LPS to an incorrect, higher blade angle. In the worst case, it can cause the propeller to go to and stay in feather!
A sidenote: Most of us have pulled the propeller levers into feather at shutdown while making the mistake of leaving the power levers in Beta: An easy boo-boo to make, right? Well, don’t do it again but relax … you probably did no harm. Unless you had the power levers quite deep into Beta – or at the Ground Fine position for those models that have it – combined with a very tight friction setting, the chance of damage is slim.
If you have followed our discussion this far, then it should be obvious that using Reverse in flight to increase the descent rate during an emergency descent is impossible. Power levers to Idle, prop levers fully forward, flaps to approach, landing gear down and we nose over to about -12 degrees pitch attitude to maintain the landing gear extended speed limit. Even with Idle power and maximum propeller speed (Np) selected, the high airspeed will cause the propellers to be solidly in the prop governing range. If we pull back into Beta now not only are we in violation of the limitation on lifting the power levers in flight but also all we will achieve is the possible stretching of the reversing cable since the blade angle is not even close to the LPS.