Aircraft technical Basics: Aircraft Propellers - Navy Training Courses Edition of 1945: Chapter 3: Constant-Speed Propellers

EFFICIENCY EXPERT

The CONSTANT-SPEED propeller is a first cousin to the two-position controllable propeller. In fact, any two-position propeller can be turned into a constant-speed propeller by the addition of an AUTOMATIC CONSTANT-SPEED CONTROL-a device which automatically maintains the engine rpm constant at any speed you select. It simply changes the blade angles automatically to meet new conditions of altitude, airplane attitude and throttle setting. But it doesn't interfere with the independent setting of engine power at any time.

Both the constant-speed and the two-position types of propellers depend on counterweights and hydraulic oil pressure to do the blade-shifting job. The blades of the constant-speed, however, will be at any angle of pitch, from full low to full high. The two-position, of course, cannot be set at intermediate angles between high and low. What this greater range of pitch means in terms of performance is illustrated in figure 18.

One of the advantages of the constant-speed, as you see in the diagram, is faster climbing ability. Notice that an airplane equipped with a constant-speed propeller climbs to a given altitude in 8 minutes whereas an airplane with a twoposition propeller requires 10 minutes to climb to the same height. As for an airplane with a fixed propeller, it's in the "also-ran" class when compared with the constant-speed.

Another advantage of the constant-speed propeller over the other two types is that it permits power descent without overspeeding the engine.

Figure 18-Comparative airplane performance.

As the air speed increases, the constant-speed control increases the blade angles to higher pitch, holding the engine rpm constantly at the desired speed.

If you set the constant-speed propeller control to give, say, 1,900 rpm, the engine will be kept at that speed within certain limits. The same will be true at any rpm setting selected. If the throttle is opened, the propeller blade pitch is automatically increased. Thus, although the enigine is producing more power and dragging the airplane through the air faster, the rpm remains the same. On the other hand, if the throttle opening is decreased, the propeller pitch will automatically decrease, and the engine will still turn at the same rpm as before.

The improved performance made possible by the use of constant-speed propellers is most noticeable in the case of high-powered airplanes, especially when they are equipped with supercharged engines. On super-power engines, higher pitch settings are needed at all times (except at take-off). This is particularly true when reduction gearing is used between the engine and the propeller. This range between low and high pitch, therefore, is too great for a two-position propeller control to handle.

Figure 19-Thrust horsepower.

Constant-speed propellers not only have the ability to hit each degree of pitch on the scale, but generally have a 10° wider range than the two-speed between full low and full high. Thus, if there are 10° difference between low and high pitch on a two-position, the corresponding constant-speed will have 20° difference between extreme settings. And that wider range means better control of available horsepower.

In figure 19 you have a diagram showing the difference between thrust horsepower when using each of the three types of propellers - fixed, twoposition, and constant-speed - on the same airplane and engine. Incidentally, THRUST HORSEPOWER is the measure used to indicate the pulling power of a propeller, and is not the same thing as engine horsepower.

Take a look at the lowest curve on the chart, starting in the lower left corner. This curve shows the thrust hp available with a fixed-pitch propeller set to give cruising engine rpm at 17,100 feet altitude, with full throttle, in level flight.

Notice that at sea level (0 altitude) this propeller develops only 350 thrust hp. At 10,000 feet, it has developed 425 thrust hp. It isn't until  the airplane has reached 17,000 feet altitude that this propeller develops 475 thrust hp.

Now look at the middle curve which shows the thrust horsepower you can get from a two-position propeller adjusted to give cruising rpm at 10,000 feet altitude under similar conditions. Note that 400 thrust hp is available at sea level, 475 at 10,000 feet, and then, as the ship climbs over 10,000 feet, the thrust horsepower falls off sharply. At 12,000 feet, the output has dropped to 450, and at 14,000 feet, it's delivering only 425. When the airplane has reached an altitude of 17,000 feet, its delivery of thrust horsepower has fallen back almost to the output at sea level.

But the third curve, marked "Constant- Speed," is horizontal, showing that smooth 475 thrust hp is available from sea level up to 17,000 feet altitude.

The basic parts of the constant-speed propeller are the same as those on the two-position controllable pitch propeller, with the addition of a constant-speed control unit.

The constant-speed control, seen in figure 20, is a type of GOVERNOR UNIT, mounted on the nose section of the crankshaft. The unit is coupled to the engine by a suitable gearing to insure that its operating range coincides with the engine rpm range The unit has a simple GEAR PUMP to boost the oil pressure, which in turn shifts the pitch of the blades.

The other parts of the governor unit include the RELIEF VALVE ASSEMBLY (controlling the maximum governor oil pressure), the PILOT VALVE (designed to equalize the pressure forces of the oil), the CONTROL SHAFT ASSEMBLY, the FLYBALL ASSEMBLY and GASKETS.

Figure 20.-Constant-speed control unit.

You can examine these parts in figures 21 and 22. The model of the Hamilton Standard Constant Speed Control is designated by a series of numbers and letters which refer to the head, body, and base of the governor and indicate the design of each. This designation is always located on the "trademark plate" which is fastened to the head or to the base.

There are six positions in which the numbers and letters can be inserted, three appearing before the dash and three after the dash, as shown by the following:

Figure 21-Front view of constant-speed control governor unit.

Of the three positions appearing before the dash, the first is always a number and it refers to the type head assembly used. The second is always a letter which designates the type body assembly used, and also the range of the governor. The third is always a number which designates the type base assembly used.

Each position appearing after the dash refers to the corresponding position in front of the dash, and indicates any modifications to which the head, body, or base assemblies have been built. For example, in the first position after the dash there appears a letter. It refers to the first position before the dash and indicates the alteration from the standard head assembly to which the control is built. The second and third positions after the dash indicate deviations from the standard body and base assemblies, respectively.

Figure 22-Rear view of constant-speed control governor unit.

Here is a typical illustration:

MODEL 1A1-A5

This governor consists of a type 1 head altered to modification A, and called a 1A head. A type A body altered to modification 5, and called an A5 body. And a type 1 base which has not been altered, and is called a 1 base.

From this it is seen that, if no modifications are made to the head, body, or base, then the second group is entirely omitted. If the body is the only part not modified, the body designation is shown with an  "0" modification. For example, in the model 1SI0-GOA, the head is type 1G, the body is type S, and the base is type 10A.

HOW IT WORKS

You'll recall from earlier discussions that the pilot uses a hand control to set two-position propeller blades to either low or high pitch. With the constant-speed propeller, he also has a hand control. Once the pilot has selected the engine speed he wants, and sets the control accordingly, he can forget about it until he wants some other engine speed. The control unit takes care of changing the blade pitch as needed to keep the engine speed where the pilot wants it.

Suppose, for example, you want the engine to Operate at 1,900 rpm, and set, the control for that many revolutions. If the engine revolutions fall below this speed, the governor unit gets busy. If the revolutions of the engine go above 1,900 per minute the governor unit again goes into action and increases the blade angles until the engine again makes 1,900 rpm.

In figure 23, you see the governor unit with some of its covering removed to show its insides. Notice the oil pump. The job of this pump is to take the engine oil and boost its normal pressure 200 to 300 percent or up to about 180-200 pounds per square inch (psi). Although this is considerably more pressure than is needed to shift the pitch of the blades, the excess pressure does no damage and is there in case it's needed. There's a pressure relief valve just above the gear pump to maintain this higher pressure.

At the top left-hand corner of figure 23 is a detail sketch of the gear pump, showing the two gears which receive the oil from the engine, by way of the INLET, and pass it along to the propeller cylinder via the OUTLET under increased pressure.

Just below the pump there's a pilot valve (not labeled in the diagram). The high-pressure oil from the gear pump fills the space between the necked-down section of the pilot valve and the drive gear shaft. The pressure relief valve is spring-loaded, and serves as a backstop for the oil.

Figure 24-Diagram of onspeed, overspeed, and underspeed.

Now look at figure 24. Whenever the airplane engine is running, the flyweights in the revolving cage near the top of the governor unit whirl around. If the engine and propeller are in balance, the propeller is ONSPEED, and the unit idles, as in the top sketch. The flyweights are in balance, the pressure and drain ports are all closed, and the oil from the gear pump is by-passed through the relief valve back to the inlet side of the pump.

But what goes on when the engine is turning faster than the pilot wants it to turn? Such a condition might result, for instance, when an airplane is nosed down into a dive, or when the throttle is opened rapidly. The unit is then in an OVERSPEED condition, as shown in the center sketch. What then takes place happens in less time than it takes to tell it. At the top of the sketch, below the pulley, you'll see a spring called the RPM SPRING. To the right and left sides of this spring are shown the flyweights, or flyballs, which are driven by the engine. If the engine goes fast, the flyweights speed up. If the engine goes slowly, the flyweights slow down. The flyweights and the rpm spring are just in balance when the engine is onspeed.

But now the engine is not onspeed. It's overspeed. That means the flyweights are swinging around faster than the speed for which the spring tension has been set. So the flyweights tend to fly outward and cause the spring to compress upward.

As the spring moves up, the pilot valve also moves up. And as the valve moves up, it uncovers the propeller port, shown at the left near the bottom of the sketch, and allows the oil to run out of the propeller cylinder and down into the sump. As the oil drains out of the propeller cylinder, the cylinder moves in on the piston, and the centrifugal force on the counterweights on the propeller blades increases the blade pitch.

As soon as the pitch has reached the point where the propeller is stabilized and is allowing the engine to turn over at the desired rpm, the ONSPEED condition shown in the top sketch is reached. The spring tension and flyweights balance, the pilot valve closes the port, and the propeller and engine are together again.

Now look at the third sketch. Here the constant-speed control unit is operating to stabilize the propeller of an engine that's running UNDERSPEED.

The greater the tension of the rpm spring, the faster the flyweights have to rotate before they can compress the spring upward. When the flyweights don't rotate fast enough to compress the spring, the spring forces overcome the force of the whirling flyballs and the spring moves down. In so doing, it moves the pilot valve down.

Again the pilot valve uncovers the propeller port, only this time the downward motion closes the drain to the sump. At the same time it opens the port from the booster pump into the pilot valve and allows oil from the booster pump to be forced into the propeller cylinder. The cylinder moves out on the piston, the blade angles are reduced, the propeller becomes stabilized, and there you are.

Incidentally, the same oil is used over and over for the constant-speed mechanism, drawing on the engine lubrication supply only when the quantity of oil in the mechanism isn't sufficient to do the job. This oil economy is the result of the following operation:

When the oil pressure builds up to about 180 psi, the relief valve opens, allowing some of the oil to circulate around the pump and come back into it again on the low-pressure side. Thus, there's always a basic supply of oil within the unit to keep a normal balance.

The following rule applies to all constant-speed propellers and it's worth remembering:

In every controllable propeller adjustment for the initial flight, the LOW PITCH should be set HIGH enough so that the airplane can stay in the air safely even if the propeller should fail to adjust beyond full low pitch after taking off. Similarly, for the initial flight, the HIGH PITCH should be set LOW enough so that the airplane can operate safely even if the propeller should shift to high pitch and remain there.

The safety of the airplane and the lives of the crew may depend on the observance of this practice. So fix it in your memory as one of the things you know for certain about constant-speed propellers. As a matter of fact, this precaution applies to two-position controllable propellers as well as to constant-speed. But it's especially important with the constant-speed because the working pitch range is greater.

What should the range be? That will depend on the type of airplane and the desired performance. A propeller with a 20° pitch range is commonly used for high-performance airplanes with supercharged engines, while a propeller with a 10° pitch range is adequate for general use. It is unnecessary and undesirable to use a pitch range greater than actually needed. You may sometimes hear about the wonderful improvement in cruising efficiency when a propeller has been set beyond the limits specified in service bulletins. Don't be fooled by such scuttlebutt.

Parts for counterweight-type two-position controllable propellers and constant-speed propellers are practically interchangeabl e. On both the twoposition and the constant-speed, by changing the angle at which the counterweights travel and the slope of the counterweight cams, the blade range may be varied. By an interchange of counterweight brackets, ranges of 6, 8, 10, 15, and 20° call be obtained. The weights are adjusted by means of adjusting screws like the one shown ill figure 25.

Figure 25-Adjusting screw assembly.

When a range of 20° is desired, an additional spring assembly must be used to help the counterweights return the blade to high pitch. It is because of the increased angular travel of the counterweights and the slope of the counterweight camss that the spring assembly is needed with 20° range propellers.

Some propellers may be fitted with a slinger ring-type de-icing device like the one shown on the hydromatic propeller in figure 26. A tube which leads the de-icing fluid from the slinger ring onto the blade shank is fitted over the barrel bolt at the edge of each blade. A small feeder tube is mounted on the two bottom studs of the engine thrust plate to lead the de-icing fluid from the de-icing pump to the slinger ring. Once the fluid

Figure 26.-Slinger ring type de-icing device installed an hydromatic propeller.

has been deposited in the slinger ring, it is thrown out by centrifugal force through the bracket and nozzle assemblies to the blade shanks.

You can see the parts of the de-icing device in figure 27. The slinger ring is mounted on the rear barrel half, and secured by screws. A bracket and nozzle assembly is attached to the leading edge barrel bolt at each blade, and connected to the slinger ring by hose couplings. These couplings and the attaching screws are wired in place. The feeder tube is mounted on the two bottom studsof the engine hose thrust plate and connected to the supply line from the de-icer pump.

REMOVAL

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DISASSEMBLY OF GOVERNOR

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INSPECTION AND REPAIR

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ASSEMBLY

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INSTALLATION

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