Ask the Expert: Propeller Speed in Climb, Cruise and Descent

This week I flew with an owner-pilot of a beautiful, late-model King Air 300, doing a bit of in-aircraft recurrent training. He mentioned that when he received his Initial training in this plane a few years ago the instructor emphasized, often and forcefully, that the propeller speed must be reduced back to 1,500 RPM (from the takeoff speed of 1,700 RPM) at 400 feet AGL on every departure. “How important is that?” he asked.

In my opinion, it is not important at all and I cannot understand why the instructor would have taught that it is! The intent of this article is to discuss the pros and cons of our selection of propeller speed in climb and cruise. For many King Air pilots this is neither confusing nor controversial in any way. They merely follow the manufacturer’s checklist procedures. Other pilots consider this a matter of technique and vary the RPM based on conditions. Please read on to gain a deeper understanding of why we choose the propeller speed that we do.

I will begin by reviewing the basic formula for the power that is being delivered to the propeller shaft. Bear with me here as I make sure you understand the exact definition of power.

Power Formula
The dictionary defines torque as “A force that produces a twisting or rotational effect.” And what is a force? Merely a push or a pull, measured in pounds (lbs) in the typical American system of measurement. Work is done, energy is expended, when a force moves through a distance. Let’s say that we want to push a heavy chair from one side of a room to the other. It seems to most of us that work would be done when the force is applied, when we bent our arms and gave the chair a good shove. Although I agree that our muscles would get tired if we kept this pose for a lengthy period of time, technically work is not done until the chair moves. You see, we could go find some heavy board or cabinet, tilt it so that it is lying against the chair, and now the chair is still experiencing the same pounds of force as if we were still pushing, yet we could go take a nap! But when the chair actually moves, then the board or cabinet falls off and if more motion is desired someone has to do the work of repositioning the force-producing object. Better yet, why don’t we just push hard enough to get the chair moving and then walk to the desired new location?

It is obvious that the amount of work we did depended on the force (lbs) we used and the distance we traveled (feet). It could take the exact same expenditure of energy to move a heavy chair a shorter distance or a lighter chair a longer distance. It is the product of force multiplied times distance that determines the amount of work that was done. Thirty pounds moved 10 feet requires 300 ft-lbs of work, the same as 20 pounds moved 15 feet.

Power is the rate of work being done. In human terms, you can think of it as how quickly you get tired. If we meandered across the room and took a minute to move that chair, we would not get as tired as if we sped across the room in ten seconds.
Instead of the linear motion of the chair, visualize a manual ice cream maker being used at a Fourth of July picnic. Before the milk/sugar/flavoring concoction starts to gel, a little torque is required to turn the crank handle. Even Grandma won’t get her heart rate elevated much while she turns the crank rapidly. Lazy Uncle John takes over after the handle gets stiff but he, too, doesn’t start panting since he is only making one complete revolution of the crank every minute or so. Not much power is being applied and the ice cream is slow in forming. Finally, Cousin Sam – a six-foot four-inch football player who can’t wait to show how strong he is at age 16 – steps in. He spins the handle even faster than Granny did, overcoming the heavy resistance at the same time…in other words, applying lots of power!

I hope by now that it seems logical to you that the power applied to anything that rotates – be it the ice cream maker or the King Air’s propeller – is the product of torque (ft-lbs) times speed (RPM – Revolutions Per Minute). In the case of the King Air, maximum power can never be achieved unless torque and Np (propeller speed) are both at their maximum, redline values.

POWER = TORQUE X PROPELLER SPEED

A minor challenge appears when we choose to measure power in the common unit of horsepower (HP) and yet we measure torque in ft-lbs and Np in RPM. Although the formula is not in error, the answer will not be correct until conversion factor, K, is included. One HP is equal to 5,252 ft-lbs-RPM so the constant conversion factor needs to be the reciprocal of that, or 0.0001904. For our purposes, 0.00019 is close enough. Thus, the power at the propeller shaft (SHP – Shaft Horsepower) becomes:

POWER (SHP) = TORQUE (ft-lbs) X PROPELLER SPEED (RPM) X K (0.00019)

By the way, the 300-series are the only domestic King Airs that present torque in units of percent, not ft-lbs. For these, 100% = 3,244 ft-lbs.

Climb RPM
Time for a pop quiz. What, if any, model King Air has a time limit for operation at maximum, redline, propeller speed? Yes, there are limits for when the redline is exceeded – due to Primary Governor failure and proper operation of the Overspeed Governor – but, you’re correct, no other propeller time limit exists. It is completely permissible to operate with the propeller levers full forward all the time. What is the disadvantage of doing so? Only one – noise. It is the desire to reduce both exterior and interior noise that causes the Beechcraft checklist writers for most King Air models to specify a cruise climb RPM that is less than takeoff RPM. The F90-series is the exception here, with the checklist keeping takeoff RPM throughout the climb. However, the F90’s maximum Np is only 1,900 RPM, already 300 less than previous 90-series models.

Years ago, before the B200 made its appearance in 1982, few King Air models were capable of maintaining maximum rated power while climbing. To get that maximum power, remember, both torque and Np had to be at their maximum values. This was impossible, however, because so much power would cause ITT to get uncomfortably toasty. Only at rather low altitudes, on cooler days, could early King Airs enjoy their rated power capability. Even on cooler days, it was rare to be able to maintain full power above about 5,000 feet.

As most of our readers have experienced hundreds of times, torque goes up as RPM goes down when the power levers are not touched, and ITT changes not enough to observe. This means that reducing RPM for improved noise levels in climb rarely actually results in a power decrease, since the rise in torque will compensate for the fall in RPM. Since we are ITT-limited, torque has already fallen below redline, so there is room for it to increase as Np decreases. (Bigger bite of air, slower RPM, more rotational resistance…more torque.)

A simple example: A -20-powered A90 is climbing through 6,000 feet and, due to ITT constraints, torque has fallen to 1,000 ft-lbs, while the propellers are still turning at redline 2,200 RPM. Our power formula shows that 418 SHP is being produced. If the prop levers are now pulled back, bringing RPM down to 2,000, torque will automatically rise to about 1,100 ft-lbs and SHP won’t change. (Actually, it may change a tiny amount. The explanation for that will come later in this article.)
It is my educated guess that the main reason most King Air checklists specify a lower cruise climb RPM than redline is that power changes little if any when this is done – since we have become ITT-limited and torque is not at redline – yet the noise situation improves.

In the last few years, however, both Blackhawk and Beech have offered STCs or introduced new models that enjoy exceedingly flat-rated engines that can develop their full-rated power well up into the high teens or even low flight levels. ITT is never of concern until very thin air at high altitude is reached. The -135A conversions on C90s and E90s, the C90GT-series, the B200GT series and those 200s modified with -52 or -61 engines…all are in this category in which ITT is never a restraint on power until the airplane climbs quite high.

In these models, if we bring the propeller speed to anything less than redline, we give up some power and some climb performance, while we gain less noise inside and outside. Is the loss of performance worth it?
In many cases, the answer is “yes.” Why not give the passenger a slightly quieter cabin at the expense of only a five or 10% reduction in climb performance? On the other hand, maybe those same passengers would prefer climbing above the low altitude turbulence, or icing, or mountainous terrain more rapidly, but with a slightly louder cabin. In that case, leave the prop levers full forward to enjoy the extra power. Or maybe it’s just the crew onboard, wearing noise-cancelling headsets, who desire a little more oomph in the climb; then leave the props at takeoff RPM.

To summarize: When ITT is not a limiting factor, realize that any reduction in propeller speed will yield an identical reduction in power. That is, if we pull props back to 1,900 RPM from 2,000 RPM – a five percent reduction, since 100 is five percent of 2,000 – then our shaft horsepower has also taken a five percent hit. On the other hand, when we are ITT-limited and we have been forced to accept a drop in torque, the noise reduction comes with no significant change in performance since torque has the room to rise as RPM falls.

Cruise RPM
It is the rare King Air – probably a B200GT, 250, or a 200-series member modified with Blackhawk -52 or -61 engines – that is not ITT-limited by the time it levels off at typical cruise altitudes. As the climb reached the higher levels, eventually the power levers could no longer be advanced to compensate for the decreasing torque due to an ITT constraint. From that point in the climb, torque and hence power were decreasing.

For ease of discussion, let’s presume that we are flying a B200GT and by the time we level off at FL260 and accelerate into cruise speed, the torque is down to 1,600 ft-lbs with the propeller still at its maximum value, 2,000 RPM. Since 1,600 ft-lbs at 2,000 RPM multiplies out to the same power as 2,000 ft-lbs of torque at 1,600 RPM, why not just pull the props all the way back to 1,600, watch the torque rise to 2,000 ft-lbs automatically, and enjoy the same speed but in a significantly quieter cabin? In many cases, you can do exactly that! But not in all, especially not in the -135 and -135A-powered airplanes (F90s and modified C90s and E90s) or even in E90s, 100s, or A100s that sport Raisbeck-Hartzell four-blade propellers. Here I need to discuss two things: Power Turbine (PT) efficiency and Propeller Efficiency.

Power Turbine Efficiency and Propeller Efficiency
Most of the readers of this magazine know that it is possible to start a PT6 and operate at Low Idle while someone holds the propeller stationary. Doing this makes the Power Turbine have zero efficiency. It is obvious that the engine is developing power – it’s running, the generator may be supplying electricity, the gas generator may be providing bleed air – yet with the prop speed at zero, there is no horsepower being delivered to the propeller shaft. Any amount of torque, times zero RPM, still yields zero SHP. No power is being extracted from the exhaust gases when they flow across the blades of a stationary Power Turbine.

Although in no way as extreme, a similar action takes place when the Power Turbine turns slower than its designed optimum speed. And what is that optimum speed? It is the one that results when the propeller is turning at redline, takeoff, RPM. For example, for members of the model 200 family, takeoff RPM is 2,000 and the Reduction Gear Box (RGB) between the propeller shaft and the Power Turbine shaft has a 15:1 ratio. When the Power Turbine rotates 15 times, the propeller rotates once. At takeoff RPM, it follows that the PT speed is 30,000 RPM. At a prop speed of 1,600, now the PT rotates at 24,000 RPM. The extraction of exhaust gas energy is a little less efficient at the slower speed, but only marginally so.

On the other hand, when Raisbeck props are put on an E90, for example, the 15:1 RGB is not changed but the propeller governor is adjusted so that maximum RPM is no longer 2,200, but instead is 1,900. The bottom end of the primary governor’s range is “re-clocked” for 1,600 from the old minimum of 1,800. The Power Turbine that was designed to be most efficient at 33,000 RPM (2,200 X 15) becomes significantly inefficient when turning at 24,000 (1,600 X 15). That is more than a 27 percent speed decrease!

Here is how to see this change in PT efficiency to prove it to yourself. On a deadhead leg or a test flight, set a realistic cruise torque with the propeller levers full forward giving redline Np. Set a round number, such as 1,500 or 900 ft-lbs – something realistic for your exact model. Now, without changing altitude or touching the power levers, reduce Np to the minimum value you can comfortably set. Go to the calculator on your smartphone and work a simple math problem: Multiply the original torque value by the ratio of old to new prop speed. Here’s the formula:

New Torque = Old Torque X (Old Np/New Np)

This new torque value presumes engine SHP did not change. Since power lever position and fuel flow and ITT did not change, we would logically believe that SHP would also not change. But it does. It goes down. In some cases, quite dramatically so.
As an example, if we had been using 900 ft-lbs at 1,900 RPM in our Raisbeck-modified E90, then came back to 1,600 RPM, the new torque should be 900 x (1900/1600), or 1,069 ft-lbs. Is it? Look at the torque gauge and see what it reads. It will be less than 1,069, perhaps 980 or so. If that is the number, we have lost over eight percent of our SHP, even though we are burning the same amount of fuel. How can this be? Simply because the slow speed of the PT limits its efficiency, its ability to extract useful power from the exhaust gases flowing across its blades. Is the noise reduction worth that much loss of fuel efficiency? Each pilot/owner needs to decide that for him or herself.

The 200-series is probably the best in this area. Had we set 1,500 ft-lbs at 2,000 RPM in our 200 demonstration, then reduced the props to 1,600, the formula would say that the new torque should be 1,500 X (2,000/1,600) or 1,875 ft-lbs. I bet the actual value will be very close to that, probably 1,850 or even more, so there was insignificant efficiency loss. That is why most operators of 200-series models that have been modified with Raisbeck props usually climb at 1,800 RPM and cruise at 1,600 RPM, instead of the standard checklist-suggested values of 1,900 and 1,700.

On to propeller efficiency…

Just because we may be delivering the same torque to a propeller shaft turning at the same speed as another, there is no guarantee that both propellers are delivering the same thrust. Instead of a carefully-designed and manufactured propeller, what if a wooden club of some shape were bolted to the shaft? We might not get any useful thrust at all, even though we could spin that club at a high RPM with lots of torque.

As you know, the propeller blade’s cross-section is an airfoil and its thrust is derived by generating lift, much as a wing does. Like a wing, if the propeller blade’s angle-of-attack (AOA) with its relative wind becomes too great, the airfoil starts to stall and loses its lifting/thrusting ability. The lower the airspeed, the lower the propeller speed, and the greater the torque, the greater the tendency of the blade is to reach its stall AOA.

The propeller designer attempts to optimize the propeller’s efficiency for both takeoff and cruise situations. The higher takeoff RPM combined with the relatively low takeoff speeds work well, as does the lower cruise RPM combined with relatively fast speeds. (It is my experience that some propellers seem to excel at this better than others!)

Working only from the power formula, it would seem that whenever torque is less than redline due to an ITT constraint, it would be silly not to reduce RPM, increase torque, keep SHP the same, and enjoy a quieter ride. This sometimes goes by the term “Chasing the Torque.” As torque falls in the climb, chase it with a decrease in RPM to force it back up and…lose nothing? Well, without a change in Power Lever position, the formula would certainly lead one to conclude that there is no downside risk to this operation. We can make it quieter, yet never lose performance. What a win-win situation!

Throw in recognition of Power Turbine and propeller efficiency losses that can occur at lower prop speeds and lower airspeeds, however, and we find that we do in fact lose enough performance that the cost is often not worth the benefit of less noise.

Personal Flight Testing
Okay, the ball is coming back into your court. On your own individual airplane, with all of its modifications and options, with its standard three-blade, standard four-blade, or after-market four or even five-blade propeller, you should do your own investigation and conclude what is optimal for that exact machine.

Cruise performance optimization is easy. At your typical cruise altitude, set power your usual way at your usual RPM during a longer flight in smooth air. Record parameters carefully – especially RPM, Torque, ITT, Fuel Flow, and IAS – then select a different RPM and let everything totally stabilize. Record the new values. Keep doing this throughout the range of propeller speeds you feel comfortable using. While you are waiting for stabilization, it might be a good idea to use the formula here to see what the new torque should be for equal power and note if there is any deviation.

Climb optimization is more subjective, but after you are high enough that torque has decreased, try both higher and lower propeller speeds, letting the torque float down and up as it will, and see if the climb rate changes enough to be noticeable.
I will even offer some typical “answers” of what some of you will find. First, in C90s with Raisbeck props, as you bring RPM down from 2,200 to 1,900, I will wager torque rises exactly as it should to maintain power and that IAS does not decrease at all, but maybe even creeps up a knot or two. But back at 1,600 – wow, do we lose it! The torque rise is not nearly what the same-power formula says – we are losing PT efficiency – and the IAS probably drops off by 10 knots or more. That is why Raisbeck Engineering decided to limit their recommended cruise RPM to 1,750. At that value, you will start to see a minor drop in Power Turbine and propeller efficiency, but you will likely conclude that the two or three knot loss in speed is worth it for the significantly quieter interior.

For you F90 pilots and C90-series/E90-series with Blackhawk -135As, you will probably conclude that cruising at 1,700 instead of 1,900 RPM offers so much better cabin noise level for a very small loss in speed that it’s worth it. On the other hand, the efficiency bugaboo really shows up down at 1,500 RPM.

The 200-series? You probably have it best of all – 1,700 RPM cruise is great, and with Raisbeck props installed, 1,600 probably yields no noticeable performance loss but is a bit quieter. (If you have the passive noise-cancelling “tuning forks” in your interior, you may decide that 1,700 still sounds the best.)

With the 300-series, your governor range is so relatively small – 1,700 down to 1,450 – that both PT and propeller efficiency changes are tiny. Now’s a good time to use max RPM until torque starts to fall, then chase the decreasing torque with RPM reductions until you reach 1,500 and stay there for cruise.

Descent RPM
During the descent, approach, and landing, it is no longer best power and performance we are after. Instead, safety (still) and comfort are probably the main goals. Quite early in the descent, we can make the cabin more quiet – it already got a little noisier due to the extra IAS we picked up as we descended – by bringing the props back to the lowest possible governing speed. Even with noticeable PT and propeller efficiency losses that may show up, the low or medium power settings we will be using on our way to the airport can still easily be reached. Of course, we must remember to select high RPM for a balked landing or missed approach as well as to run the prop levers full forward before using Beta or Reverse.

Do many pilots bring the RPM lower for descents? No. Is the noise difference significant? Not usually. So if you choose to leave props alone for the descent, that is 100 percent acceptable and you are in the vast majority. But if you choose to use a lower RPM, that is also okay, and not hurting a thing.

Thanks for reading this lengthy presentation. Now, if you feel like it, plan to do a little experimentation on your next few flights. Have fun!

If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.