Sunrise Aviation

The last column got us through taxi and the start of the takeoff roll with the constant speed prop mechanism.

Differences from fixed pitch props become apparent the minute takeoff power is applied. Thanks to low blade angle, redline RPM is quickly reached and full rated horsepower becomes available, delivering substantially better acceleration than fixed pitch models.

As airspeed increases during the takeoff roll, for reasons discussed in the first column, prop angle of attack begins to decrease. This lessens prop loading, and RPM starts to increase. The governor, designed with redline as an absolute maximum limit, senses this increase and immediately allows more hydraulic pressure to the hub, causing the blades to twist and take bigger and bigger bites of air. The increased bite increases prop workload, and engine speed remains constant.

For the pilot, the only indication in the cockpit that all this governor activity and propeller change are happening during the roll is that the engine accelerates to and not beyond redline on the tach. Only if the governor failed to do its job would there be any visible change: engine overspeed. This gives you an extra chore during takeoff: you must check for adequate power output at the very start, as you would with a fixed pitch prop, and then ensure that redline is not exceeded during takeoff acceleration.

ANGLE OF ATTACK
One note of technical importance: the easily observed fact that engine RPM remains constant during takeoff acceleration can be used to demonstrate that propeller angle of attack also remains constant. The angle of attack determines prop workload and RPM; therefore, if RPM stays constant, so must angle of attack. This observation is important: it is the constant speed mechanism’s ability to establish and maintain prop angles of attack, rather than RPM, that provides the greatest benefit to the pilot. The importance of this will become more clear as cruise performance is discussed in subsequent columns.

MANIFOLD PRESSURE
Shortly after takeoff you will want to make the first power reduction; at this point, it becomes necessary to distinguish between manifold pressure and RPM.

With fixed pitch props, changes in throttle result in changes in RPM, making the tachometer an effective indicator of power settings. With a governor, however, increasing and decreasing power has no effect on RPM. Changes in power merely result in blade angle changes manipulated by the governor to maintain constant tachometer readings (the changes in propeller bite do, of course, produce changes in aircraft speed).

Without the tachometer to inform you about power settings,the manifold pressure gauge is provided to let you know how much power the engine is producing. It does this by reporting the atmospheric pressure in the intake manifold--the tubes carrying air to the engine. This is a particularly accurate way of reporting power: the pressure in these tubes is what pushes air and fuel into the combustion chambers of the engine and actually controls the power produced.

Prior to engine start, the atmospheric pressure inside and outside the engine will be the same--approximately 30" at sea level (29.92" for purists). This will be reported by the MP gauge. After engine start the demand of the pistons inside the cylinders draws outside air through the air filter and intake manifold. As long as there is no restriction to block this flow, outside pressure will fill each cylinder to the maximum as the intake valves open, producing full power and full RPM. As long as the exchange between outside and inside remains freely open, MP should continue to read 30".

As you probably don’t wish full power right after engine start, you naturally begin with the throttle somewhat closed. The restriction provided by the throttle in this position ensures there will be a partial vacuum in the intake system: the pistons are demanding more than is being supplied. At idle, when the throttle restriction is greatest, expect to see about 10" of MP, indicating that only one third of the outside air pressure is being made available to push the fuel/air mixture into the cylinders. With increases in throttle settings, the imbalance between inside and outside pressure reduces, until finally, as in the full throttle example above, MP should theoretically be the same as outside pressure. Realistically, thanks to inefficiencies of the entire system, 29" is probably the best you’ll see at full power, sea level.

Next month, techniques for managing MP and RPM.