Pistons vs Turboprops Gregory Bowe II
Goals of This Presentation
- Introduce Turboprops Engine
- Understand how both engines operate
- Why Factors
Piston Engines is an engine that uses one or more pistons in order to convert pressure into rotational motion. They use the reciprocating (up-and-down) motion of the pistons to translate this energy.
How does a Piston Aircraft work?
The Four-Stroke Cycle
When fuel is introduced and ignited within a cylinder, the hot gases produced expand with incredible force, driving the corresponding piston forward within the cylinder, which in turn moves the connecting rod forward and causes the crankshaft to turn. The full rotation of the crankshaft pushes the piston back into the cylinder and the cycle begins again. It is the crankshaft that converts the reciprocating, linear motion of the piston into the rotating motion that drives the propeller
Most piston engines used in aircraft have either a carburetor or fuel injection system to deliver fuel and air to the cylinders. The carburetor mixes fuel and air before it enters the cylinders. Carburetors are common on smaller engines because they're relatively inexpensive.
Piston engines rated at more than 200 horsepower often use a fuel-injection system rather than a carburetor.
A fuel-injection system squirts fuel directly into the cylinders or just ahead of the intake valve.
Magnetos generate electricity when they rotate. So, to start the engine, the pilot must engage a battery-powered starter that rotates the crankshaft. After the magnetos begin rotating, they supply the spark to each cylinder to ignite the air/fuel mixture and the starter system is disengaged. The battery no longer has any part in the operation of the engine.
Advantages of Dual Ignition
- Aircraft have dual ignition systems for safety and efficiency.
- If one magneto system fails, the engine can operate on the other system until you can make a safe landing.
- Two spark plugs improve burning and combustion of the mixture, delivering improved performance.
- A throttle, the control that has the most direct effect on power.
- Propeller control (if the aircraft is equipped with a constant-speed propeller) to adjust the propeller's rotational speed, measured in revolutions per minute (rpm).
- Mixture control to adjust the air/fuel mixture as the airplane climbs and descend
A constant-speed propeller has a governor that adjusts the angle of the blades to maintain the rpm you select. This type of propeller makes much more efficient use of the engine's power. At low speed when maximum power is required (as during takeoff), you select maximum rpm or "full increase" with the propeller control, and the propeller blades meet the air at a small angle. During cruise, you adjust the rpm to a lower setting, and the blades take a bigger bite of the air while turning a lower speed.
Example of Piston Multi-Engine Aircraft
Piper Seminole—PA-44: The Seminole was born at a time when several companies were offering new, four-seat, twin trainers. In the late 70s, Beech introduced the Duchess, Grumman American premiered the Cougar and Piper unveiled the Seminole.
Out of these three aircraft the Seminole is the only aircraft still surviving today.
For a twin, the Seminole is a relative lightweight at 3,800 pounds gross weight, and its counter-rotating propellers help tame the Vmc problem. Single-engine climb at gross is an relatively 212 fpm. NOT good but better than most competitors at the time.
110 gallon fuel supply allows a range of 700 nm
Why a Twin Piston Aircraft?
- First, piston engines are more efficient than turboprops, so their operational cost are lower.
- Low Cost of Entry
- Affordable and Practical for Training Purposes
- Endless Affordable Parts
- Simple Design
What is a turboprop aircraft?
A turboprop engine is a jet engine attached to a propeller. The high-speed turbines generate an enormous amount of power that is transmitted to the propeller through a gear-reduction system. The propeller is really a large fan turned by the turbine.
Interesting Fact - Turboprop engines are much more efficient than pure jet engines at speeds in the 250mph to 350 mph (400km/h to 560 km/h) range. At higher speeds, propellers lose their efficiency, and pure jet engines are a better choice.
How Does a Turboprop Aircraft Work
Complex- The PT6 engine has a three stage axial, single stage centrifugal compressor, driven by a single stage reaction turbine. The power turbine, counter-rotating with the compressor turbine, drives the output shaft. Both the compressor turbine and the power turbine are located in the approximate center of the engine with their shafts extending in opposite directions. Being a reverse flow engine, the ram air supply enters the lower portion of the nacelle and is drawn in through the aft protective screens. The air is then routed into the compressor. After it is compressed, it is forced into the combustion chamber, and mixed with fuel that is sprayed in through nozzles mounted around the gas generator case. Two spark igniter plugs are used to start combustion. After combustion, the exhaust passes through the compressor turbine and two stages of power turbine and is routed through two exhaust ports near the front of the engine. A fuel control system schedules fuel flow to maintain the power set by the gas generator power lever. Propeller speed within the governing range remains constant at any selected propeller control lever position through the action of a propeller governor,
The reduction gear is necessary to convert the high-speed shaft rotation into slower, functional propeller speed.
Throttle Lever controls the amount of power. Its range goes from full reverse through ground idle (GI) to flight idle (FI) and then on up to full power.
Propeller Lever controls the RPM of the engine.
A Condition Lever is a pilot actuated control which is located within the throttle quadrant of a turboprop engine equipped aircraft. It is utilized to control some functions of both the propeller and the engine. These functions vary from installation to installation.
Functions of the condition lever can include any of the following:
- Fuel cut-off.
- Propeller Feathering
- Propeller Un-Feathering
- Low/High Idle Selection
- Propeller Speed Control