intro

Many students transitioning from fixed-pitch Cessna 172s are able to sketch and explain constant-speed props at a basic level when they get to the Seminole, but don’t have a strong understanding of why they have so many benefits and how to operate them efficiently and effectively in different circumstances. The Wikipedia article is a pretty good primer, so I won’t reinvent the propeller. Instead, I want to focus on operational takeaways and help you master the understanding of blade aerodynamics, torque, RPM, and engine thermodynamics to help you operate CS props more effectively. This in turn will make difficult maneuvers in the Seminole easier to execute, thereby saving you time and money in your multiengine training.

Torque, RPM, and power

Reciprocating Otto-cycle engines produce power by combusting an air/fuel charge and using the resultant pressure to push a piston down the cylinder. The force of each stroke is dependent on the density of that charge, which is regulated by the throttle. The linear force of the piston is transmitted to the crankshaft via the connecting rods, which turn it into a rotational force known as torque. Thus, the throttles pretty directly control the torque that the engine produces, which we proxy with the manifold pressure gauge (MP), which measures the pressure immediately downstream of the throttle plate. For this reason, I like to refer to the throttles as “torque levers” on CS-propped airplanes.

RPM is controlled by the prop governor. Prettyflyforacfi has a great video sketching out how it works. In a nutshell, you have two games of tug-of-war looking for balance in the engine:

1. The prop flyweights, nitrogen can, and spring in the hub are all trying to feather the blade, while the oil pressure is pushing the cam rollers in the opposite direction. When the two forces balance, the prop speed stabilizes.
2. The speeder spring and governor flyweights are locked in a second game, where spring pressure and centrifugal force on the weights governs the pilot valve that sends oil pressure to the hub.

One important thing to note is the failure scenario: the oil pressure is more likely to drop than the other hub components, so the rest of the deck is stacked to feather the prop if the oil pressure drops. This is known as “fail-feathered” design that prevents an oil pressure issue from leaving you with a windmilling engine. Single-engine airplanes, by contrast, usually do exactly the opposite, namely feather at a fine-pitch. This is to protect go-around power with a failed governor. A memory aid for the Seminole is “high high low low,” meaning high oil pressure->high RPM, and low pressure->low RPM.

Power is the product of torque and RPM, P=T*RPM. If you increase either input, the power will go up as well. This is why, on takeoff and OEI situations, we go full mix, full prop, full torque. The order of that is important as well: full mix keeps us out of the red box, full prop allows the frequency of combustions to increase, and full torque gives us the maximum force at that max rate for maximum power.

To help the interplay make sense, consider riding a bike with shifting gears: if you start in the tallest gear (big chainring and small sprocket), you can stand on the pedals and the bike will only accelerate gradually. This is because while you are applying lots of torque to the cranks, the rate at which you’re applying it is slow, so the consequent power output is slow (high torque times almost zero RPM gives almost zero power). If you want to get the bike to accelerate quickly, you need to start in a low gear that allows to you put down more torque at a rapid pace.

One final thing to note is that you don’t neatly change any variable in isolation with CS props. A great example of this is during your prop check in the runup: as you pull your prop lever back, you get an MP spike. This happens because the rate at which you’re turning the engine over decreases, which, at partial throttle, means the engine vacates fewer iterations of each cylinder’s volume in a given time frame and doesn’t create as much underpressure in the intake tract. A good read for more is the excellent Manifold Pressure Sucks article by John Deakin.

Efficiency

For more efficient cruise operations in a piston engine, you typically want to pull the prop way back. There are several interrelated reasons to do this:

1. Optimal blade α: each prop will have an optimal angle at which it most efficiently converts power to thrust. As the blade spins faster, the tips start to form shockwave drag and noise, which is inefficient.
2. Reducing pumping losses: if you have to pick a cruise setting between 21MP/23(00) RPM, 22/22, or 23/21, which would you pick? Multiplying all three torque and RPM numbers together gets you about the same answer, but 23/21 is more efficient than the other two because the throttle is not blocking the inflow of air as much. Thus, the reduction in pumping losses gains you efficiency. In the Seminole, you can go up to 4 points “oversquare” per the POH. If anybody tells you to pick square numbers (e.g. 21/21, 22/22, etc.), ask them where in the POH it suggests that, and kindly point them to page 5-26 (right).
3. Rotational friction: the faster the engine internals spin, the more power is wasted with bearing drag, piston ring friction, etc. Minimizing this with lower RPM recovers some of that otherwise lost energy.
4. Combustion efficiency: if you’ve leaned the engine out properly, the flame front will propagate more slowly through the charge during the combustion stroke. Thus you will get higher pressure pushing the piston down the cylinder and convert more of the thermal energy to useful work if you have a lower RPM and allow that cycle to complete itself. Lower RPM for the same output will show lower EGT, meaning the engine is doing more work and wasting less energy out the exhaust.
   

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Windmill Drag

A useful employment of pumping losses is windmill drag: if you need to add drag (e.g. emergency descent), closing the throttles and advancing the props will maximize the number of engine cycles with high vacuum and use energy from the freestream air to turn over the engine. This can be used to get you down quickly. This is like taking an offramp in a standard-transmission car and downshifting into second or third, then taking your foot off the throttle to engine brake to side-road speeds without touching the wheel brakes. In the case of an engine failure, you obviously want to eliminate the drag as quickly as possible.

Maneuvering considerations

A thorough knowledge of CS props will help you make your maneuvers easier when flying the Seminole and other types. Here are some examples:

1. Steep turns: I like to set up at 120 KIAS and 19/25, then find the throttle for what +2 MP feels like, grab two swipes of nose-up trim, then roll in. Going through 30-degrees bank, I add the pre-felt +2 MP and make small adjustments. You can follow this same procedure with 22/23, but 19/25 gives you more granular control. At the low-torque/high RPM combination, your blade α is smaller, so a small reduction in torque will lead to a drop in α and a quick drop in thrust. If you get fast in your steep turn, the ability to remove torque will help you engine brake back to your target speed more quickly. An analogy for this is driving a standard transmission car around a turn: staying in a lower gear midway up the rev band (3000-4500) allows you to modulate speed just with your right foot, thus maintaining the car’s balance through the turn and allowing you to accelerate better past the apex.
2. OEI approach setup: the OEI approach is probably the most hated maneuver on the CMEL and MEI checkride. Thorough knowledge of the CS system will make it easier. I like to set up at 110 KIAS, usually 20/21 (min cruise RPM). This allows more time to brief the chart and plan my “no troubleshoot” cutoff. The second advantage of the coarser blade pitch is less windmill drag: when the DPE/instructor pulls back your torque lever on the “failed” engine, the governor will only try to hold 21, not 23, 25, or whatever else you selected, and you'll suffer the drag amount from 21 at 110 KIAS, not a faster speed or RPM. Third, the “good” engine is not putting out as much power, so the pull to the “dead” side will be more gradual and you’ll have more reaction time. Work smarter, not harder….
   

summary

Understanding not just how to sketch CS props, but how they work and how to harness their characteristics effectively, will help you operate aircraft more efficiently as well as make passing checkrides easier.