The Mechanics Corner 4
I am a few days late with this Mechanics Corner, Halloween was fun and now we are into the beginning of November. Time is flying by. This week I want to talk briefly on the operation of radial engines, specifically why we start and stop the engine with the propeller in course pitch. Some quick background for those that are less familiar with constant speed aircraft propellers.
Most propeller driven aircraft that are beyond the most basic of trainers have what is called a constant speed propeller. In the simplest of terms this means that the pitch of the propeller is controllable from inside the cockpit and once an engine RPM is selected by adjusting the pitch of the propeller, the governor which controls the propeller pitch will endeavor to keep that selected engine RPM constant. This is where the term constant speed comes from. Typically an aircraft will take-off and land with the propeller in the finest pitch, or in other terms, high RPM. This enables the engine to achieve the highest RPM possible, thus allowing maximum HP to be provided for take-off. We land in high pitch because if a go around becomes necessary we will again have the highest HP output available to us.
Most radial engine Warbirds have a propeller hub that exposes a chrome shaft when the propeller is in high pitch and concealed when in low pitch. In order to protect this chrome shaft from rust and corrosion while the aircraft is parked, we shut the engine down with the propeller in low pitch. Since we landed with the propeller in high pitch this results in a specific shut down procedure that needs to be followed. This involves revving the engine up to around 1200 RPM and then bringing the propeller pitch control lever back to full course pitch. It then takes around 15-20 seconds for the propeller to work its way back to course pitch, roughly down to about 800 RPM. Once the RPM has stabilized at its lowest point for a few moments we then shut down the engine like normal by pulling the mixture to idle-cutoff. So, next time you see a radial engine warbird shutting down and you see this process happening you will know why we do it.
In the case of Annie, the propeller was in fine pitch/High RPM during the incident and there was no way for us to change that while she sat out on the ramp. This resulted in the hub rusting, proof positive why we follow this procedure. You can see the rust below in the yellow circle. You can also see what it looks like with the hub in course pitch and the shaft concealed in the red circle.
