Piston Engines - Propellers — Page 191, Lesson 247

Right, we’re moving into the propeller section of the piston engine syllabus. I want to walk you through two specific topics here: the double acting propeller and then the diesel engine as it relates to propellers. Let’s start with the double acting propeller. The key word is double acting. In a single acting system, oil pressure moves the blades one way and springs or counterweights move them back. In a double acting propeller, hydraulic pressure does the work in both directions — oil pushes the blades toward fine pitch and also toward coarse or feather. That means there’s roughly twice the amount of actuating oil in circulation compared to a single acting unit. Now, the checks you carry out on a double acting propeller are much the same as on any constant speed propeller. There will be detail differences — basic rpm settings will differ from one installation to another — but the object is the same: to ensure rapid response to rpm control lever signals. That’s the whole point of the ground checks. You want the propeller to react quickly and correctly when the pilot moves the rpm lever. Here’s the sequence. Once the lubricating oil in the main engine has warmed sufficiently, you exercise the pitch change mechanism. Why? Because the oil in the pitch change cylinder is cold and sluggish. By moving the mechanism, you evacuate that cold, sluggish oil from the pitch change cylinder and purge it from the CSU and the oil passages. The CSU is the Constant Speed Unit — that’s the governor that senses engine rpm and moves the pitch change mechanism to keep rpm constant. So you’re flushing the cold oil out of the cylinder, out of the CSU, and out of the oil passages, replacing it with warm, thin oil so the system responds properly. Then, as with the Seneca — that’s the Piper Seneca, a light twin — once the oil has warmed, there’s an engine test procedure. That procedure involves causing the pitch change piston to traverse from the fine-pitch stop to the feathering stop more than once. So the piston travels the full range of blade travel, from fine pitch — where the blades are at low angle for high rpm — all the way to the feathering stop, where the blades are turned edge-on to the airflow to stop the windmilling of a failed engine. And you do that more than once to prove the mechanism works through its full travel. Now, here’s where the double acting propeller adds work for you. Because there’s not only double the amount of actuating oil in circulation, there’s also an extra system to check. You may have to ascertain the correct functioning of the feathering pump — that’s the pump that supplies the high pressure oil to drive the blades into feather. And you also have to check the functioning of the pressure operated cut-out switch. That switch cuts the feathering pump out once feathering pressure has been achieved, so the pump doesn’t keep running against a dead-head. So on a double acting propeller, your checks cover the normal pitch change system, plus the feathering pump, plus that pressure operated cut-out switch. Let me show you the layout of a typical light twin powerplant controls arrangement, because it ties the levers together. Now let’s switch to diesel engines. The diesel engine generally runs at a lower rpm and higher torque than a conventional gasoline engine. Torque is the twisting force the engine produces; diesels produce a lot of it at low rpm. That good torque output translates into greater static-thrust values — static thrust being the thrust produced when the aircraft is stationary — and that gives the aircraft greater take-off performance levels. So the diesel’s torque advantage shows up right at the start of the take-off roll. These features — the low rpm and high torque — also allow the use of Constant Speed Propellers with typically more blades than a conventional gasoline powered unit. More blades let you absorb that high torque at the lower rpm without needing an enormous propeller diameter. Now, because the diesel runs at low rpm, gearboxes may be used to ‘step-down’ the engine’s output rpm to match engine and propeller performances. So the gearbox reduces the already-low engine rpm even further to a speed the propeller is happy with. Here’s the modern part. Propeller control in the modern diesel is co-ordinated with the fuel delivery by means of a ‘single-lever’ concept, similar in principle to the turbo-prop. So instead of separate levers for throttle and propeller pitch, one lever commands both — the system co-ordinates fuel flow and blade pitch together. And finally, fuel scheduling, propeller pitch, torque-monitoring and other parameters are controlled electronically by the FADEC unit. FADEC stands for Full Authority Digital Engine Control. It’s the electronic brain that schedules the fuel, sets the propeller pitch, monitors torque, and manages the other parameters automatically. So to tie it together: the double acting propeller uses hydraulic pressure both ways, and its checks include exercising the pitch change mechanism through its full travel, checking the feathering pump, and checking the pressure operated cut-out switch. The diesel engine, with its low rpm and high torque, drives a constant speed propeller with more blades, uses a gearbox to step down rpm, co-ordinates propeller and fuel with a single lever, and hands all the control to the FADEC.