Piston Engines - Propellers — Page 186, Lesson 243

Let’s pick this up right where the torque meter left off, because that’s the heart of what we’re looking at now. I want to walk you through the torque meter system on a propeller, because it’s a clever piece of engineering that tells the pilot how much torque the engine is actually producing. Here’s how it works. Inside the reduction gear assembly, there’s a stationary gear. Now, “stationary” is in quotes, because it’s not bolted rigidly in place. It’s allowed to float, and its movement is opposed by oil pressure. That oil pressure is generated by something called the torque meter pump. So the torque meter pump builds up oil pressure, and that pressure pushes against the stationary gear, trying to hold it still. Now, within cylinders that are exposed to the torque meter pump output, there are two pistons. These pistons are operated by lever arms that are attached to the stationary gear. So when the stationary gear tries to move, it moves those lever arms, which move the pistons. One of those pistons partially covers a bleed port. That bleed port is a small opening that lets oil escape, and the piston’s position controls how much of that port is open. Here’s the sequence of events. Under low power conditions, the bleed orifice is at maximum area. That means the port is fully open, so oil can flow out freely. In that state, the torque meter oil pressure is just balancing the thrust on the stationary gear. Everything is in equilibrium. Now, when you increase power, the propeller torque increases, and that tries to rotate the stationary gear. As the gear tries to rotate, it forces the pistons further into the cylinders. That does two things. First, it reduces the bleed orifice area, so less oil can escape. Second, it physically pressurizes the oil, because the piston is pushing into the cylinder. The net effect is that the oil pressure rises as a function of propeller torque. So the higher the torque, the higher the oil pressure, and that increased pressure balances the thrust on the stationary gear. That’s the whole principle — the oil pressure becomes a direct measure of torque. Now, let’s move on to the checks you carry out on a propeller after engine start. I want to be clear about something first. The checks and the methods used will vary from aircraft type to aircraft type, and from propeller type to propeller type. So what I’m about to describe is specific to one aircraft, but the principle of thoroughness applies everywhere. Also, remember that there are many other checks carried out on propellers, most of them maintenance orientated. But the pilot is responsible for a thorough pre-flight visual inspection of the propeller before engine start-up. That’s your job, and it happens before you even start the engine. Let’s look at a specific example — the Single Acting Propeller on the PA34-200T Seneca aircraft. After start-up, the engine oil must be warmed up to the level prescribed in the operating manual before any checks are commenced. So you don’t just start checking immediately; you wait for the oil temperature to come up to the specified value. These checks form part of the normal “after start” and “before take-off” checks. The first check is part of the “Power Check.” You set the throttle to 1900 rpm, and you set the propeller rpm lever to 1900 rpm as well. Then you exercise the propeller. The check is that the rpm drops when you select minimum rpm, and the rpm returns to 1900 when you select maximum rpm. Then you repeat that. So you’re verifying that the propeller governor can actually change the rpm as commanded. Next, you reduce the throttle to 1500 rpm, and you check the propeller feathering. Feathering is when the propeller blades are rotated to a position that minimizes drag, typically used when an engine fails. So you check that the feathering system works. Then you close the throttle, or set it to 1200 rpm. That’s the end of the power check portion. Then, in the “Before Take-off” checks, you set the propellers to MAX rpm, and you set the propeller de-icing as required. If icing conditions are expected during or immediately after take-off, you select the de-icing ON, and you check the propeller de-icing ammeter, and also both alternator ammeters. So you’re verifying that the electrical system is supplying the de-icing system properly. Let me show you the feathering installation, because it helps to see the actual hardware. That’s the typical feathering installation for a double acting propeller. And here’s the synchronization system for a light twin. And this is the master engine arrangement for a transport aircraft. Now, let me summarize the key points. The torque meter uses oil pressure to measure propeller torque, with the stationary gear floating and its movement opposed by oil pressure from the torque meter pump. The two pistons, operated by lever arms, control a bleed port, and increased power reduces the bleed area and raises oil pressure as a function of torque. For the checks, you warm the oil first, then do the power check at 1900 rpm, exercise the propeller to verify rpm control, check feathering at 1500 rpm, close the throttle to 1200 rpm, and in the before take-off checks, set max rpm and manage de-icing as required, checking the ammeters if icing is expected. That’s the complete picture of both the torque meter principle and the post-start propeller checks.