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If you are learning how to fly a constant speed prop, it can be a pretty intimidating experience at first, but it shouldn’t be…
Imagine you are driving in your car using the first gear. It is excellent at very low speeds and nearly impossible to accelerate. And driving on the interstate with only one gear available is not something you want to do. It is the same principle, just converted to an aircraft. But I want to encourage you that it can be a reasonably easy transmission once you understand what is happening to the aircraft and why.
During the initial climb phase, you will often aim for a manifold pressure of 25 and an RPM of 2500. Always consult the operator's handbook for the most efficient power setting during the cruise. Finally, you should increase the RPM during descent to prepare for a possible go-around.
Since the cockpit layout in a constant speed prop is slightly different from a fixed pitched propeller. To make everything as straightforward as possible, in this article, we will go through a virtual flight and discuss what to consider when flying a constant-speed prop in detail.
Whilst writing this article, I have drawn upon my own experiences as a pilot, those of several pilot friends of mine and consulted the aviation community as a whole to best find how to fly a constant speed prop.
How Does A Constant Speed Prop Work?
As the name suggests, the constant-speed propeller can automatically adjust the blade angle to maintain a continuous RPM independent of the airspeed. It does that by using a governor with a manifold pressure gauge. The governor is constantly measuring the engine load and increasing or decreasing the oil pressure to the pitch-changing mechanism of the propeller.
There are two types of constant speed propellers: mechanical and hydraulic. Mechanical props use a governor to change the pitch, while hydraulic props use oil pressure to change the angle. Most modern aircraft use hydraulic props because they are more reliable and require less maintenance. The Pilot’s Handbook of Aeronautical Knowledge provides great in-depth technical knowledge and explanations if you want to deep-dive into this topic.
As mentioned earlier, the main advantage of a constant speed propeller is that you use the most efficient settings for each flight phase. It is the same as it would be much harder to use a bicycle in just one gear for going uphill and downhill versus a mountain bike with multiple gears.
In other words, you can fly at lower RPM and manifold pressure during the cruise to save fuel while still having the ability to increase the power when needed, like during takeoff or go-arounds.
How To Fly A Constant Speed Prop?
Many student pilots are pretty nervous before flying a constant speed propeller aircraft for the first time. Suddenly, there is an additional control lever in the cockpit, and you might be worried that you will mess up.
You don't have to learn everything from scratch, that is the good news. There are just a few significant differences to remember compared to a fixed-pitch propeller, for example, on the Cessna 172.
Before we start, just an important note: Below are some power settings for each phase of flight as an example just to show you what a typical range is. I am not a pilot, and each aircraft type is different. Therefore, the only thing you should refer to find out if a power setting is within the limits of your aircraft type is the pilot operating handbook and the Engine's Operating Manual.
The takeoff power setting is usually reached by selecting the control lever to full forward throttle. However, remember that you might need to reduce the power slightly during the takeoff roll to avoid wheel spin, especially if the runway is wet. Also, don't forget to increase the RPM to 2600 or 2700 once airborne.
Always keep an eye on the engine instruments at all times and ensure you are not overloading the engine. An overloaded engine will lead to a loss of power and could eventually damage the engine.
When operating during takeoff, a full-power throttle with Manifold pressure around 28-30 inches is the norm. This creates an oversquare condition. Later, we will discuss the benefits and risks of an oversquare operation.
The climb power setting is usually around MP 25 and RPM 2500. As you can see, the difference in the takeoff power setting is not that big. However, you might need to increase the RPM slightly during the initial climb to avoid stalling the engine and maintaining sufficient speed.
When you reduce the power configuration, it is helpful to keep the principle of left-to-right in mind before moving the control lever. This means you generally reduce the MP first and then the RPM. To increase the power setting, on the other hand, you would increase the PRM first, followed by the MP.
The cruise power setting is where you will spend most of your time while flying. And it is also the flight phase, where things get interesting with a constant speed propeller. As I mentioned, you can select any RPM between maximum power and idle and still have good efficiency. But what is the most efficient cruise power setting?
The most efficient cruise power setting has the lowest fuel flow possible while maintaining an excellent true airspeed.
To give you rough guidance, as a rule of thumb, you want a fuel flow of around 10 to 12 gallons per hour. Engine temperature should be in the green, and you want a True Airspeed (TAS) of approximately 140 knots. Engine worn, the noise level in the cabin, and fuel efficiency are other factors that should be considered.
These numbers will vary depending on the aircraft and the current conditions. But as a general guideline, they should give you a good starting point.
The standard cruise power setting is usually around MP 23 and RPM 2300. As you can see, both the manifold pressure and the RPM are lower than during takeoff and climb. This is because we want to save fuel during the cruise and operate the engine as efficiently as possible.
In general, the higher the altitude you are flying at, the lower the power setting you will need. This is because there is less air density at high altitudes, so your engine does not have to work as hard to produce thrust.
Descent And Landing
During descent, you will likely have to pull the throttle back to control airspeed, and you can do so by reducing the During descent. Apart from that, your landing is just like in a fixed-pitch propeller. You can make adjustments on the trust throttle as needed.
The descend power setting is usually around MP 21 and RPM 2100. Again, the manifold pressure and the RPM are lower than during takeoff, climb, and cruise. This is because we want to save fuel during the descent.
Also, during descent, keep in mind that you must slowly increase the RPM to prepare for a possible go-around. If you don't do that and suddenly need full power, the propeller might not be able to provide enough thrust because it is not turning fast enough.
What Is Engine Over/Under Square Operation?
If your manifold pressure is higher than your rpm, you are said to be oversquare. For example, a over square setting would be a power setting of 23" of manifold pressure and 2'100 RPMs.
On the other hand, If your manifold pressure is lower than your rpm, you are undersquare.
Most larger turbine-powered transport category aircraft spend the majority of their time undersquare.
In most situations, operating your engine at or near the "sweet spot" where power and fuel efficiency are maximized is best. However, sometimes, you will find it necessary to operate outside of this range.
For example, you may need to operate above square during takeoff to produce maximum power. And during descent, you may need to operate below square to control your speed.
Knowing how to correctly operate your engine at different power settings is essential to flying a constant-speed propeller aircraft.
What Are The Benefits And Risks Of Oversquare Operation?
Often, flight instructors warn their student pilots to avoid an oversquare situation, as it could cause immediate and severe damage to the engine. Luckily the truth is not so black and white.
While it is true that there can be risks associated with oversquare operation, it only occurs in extreme situations outside all parameters mentioned in the Pilot's Operating Handbook and the Engine's Operators Manual. These manuals are there to provide guidance and protect the engine, so as long as you follow their guidelines, you will be fine.
The main risk would be forcing the propellers to go so slow with a low RPM that it can cause damage to the engine. This could lead to a loss of power and an increased chance of stalling.
To avoid this, always keep an eye on the engine instruments and ensure you are not overspending the engine. I would always recommend sticking to the cautious side and using a higher power setting if you are ever unsure about your power settings.
However, within these limitations, operating oversquare has a few benefits worth considering.
The two most important benefits are probably fuel efficiency and passenger comfort. And as a result of the slower turning engine less wear and tear on the moving parts.
Especially with passengers on board, the single most significant factor for cabin noise is the engine or propeller. So if you can reduce the RPM, that's the only way to reduce engine noise. And it will make your next trip even more comfortable.