Airframe and Powerplant

Tried and True

July 1, 2003

100 years of airplane engine progress

Charlie Taylor built the first engine for the Wright Flyer when the Wright brothers were unable to buy an engine that fit their needs. It took him six weeks and was quite an endeavor. In the book, Kill Devil Hills, Taylor is quoted as saying, "It was up to me. My only experience with a gasoline engine was an attempt to repair one in an automobile in 1901."

The Wright brothers had determined that their airplane would fly with an 8-horsepower engine. When Taylor finished the 1903 Wright engine (improvements were made in 1904 and 1905) it initially produced 12 horsepower from an inline four-cylinder, water-cooled, spark-ignition engine configuration.

When compared to modern general aviation airplane engines, the Wright engine had a poor power to weight ratio (1 hp-to-15 pounds), didn't spin up very fast (maximum rpm was 1,300), had a very low compression ratio (4-to-1), and had to be preheated before starting.

Once the engine was coaxed to life, it ran all out — there was no provision to interrupt the ignition, nor did the engine have a throttle to cut back the fuel-air charge.

The construction details of Taylor's one-of-a-kind engine are fascinating, as is the story of how general aviation engines have evolved since that first flight in North Carolina on December 17, 100 years ago.

The crankshaft

Taylor built the 1903 Wright engine crankshaft using three tools — a metal saw, drill press, and lathe. Since the engine was a 4-cylinder inline configuration, the combustion events occurred every 180 degrees — two pistons were always at the top of their stroke (with one at the top dead center [TDC] on the compression stroke and the other at TDC on the exhaust stroke) while the other two were at the bottom dead center of their intake and power strokes. This means that the connecting rod journals and the main rod bearing journals of the crankshaft were in the same plane.

This allowed Taylor to cut the crankshaft from a piece of machine-grade steel that was 7 inches wide by 1 and five-eighths inches thick by 31 inches long. He used his saw and a drill press to remove all the metal that wasn't crankshaft to create the four cylinder-connecting-rod throws and the five main bearing journals.

Taylor then used a metal-turning lathe and cut the connecting-rod journals and main bearing journals. A 19-pound circular flywheel was attached to the aft end of the shaft to smooth out the power impulses. Modern engines control vibration and dampen power pulses by balancing reciprocating components, and by attaching counterweights to the crankshaft to dampen potentially destructive torque fluctuations. Since almost all modern light-airplane engine propellers are bolted directly to the crankshaft, the rotating mass of the propeller has replaced heavy flywheels.

Crankshafts for modern GA engines are forged out of chrome nickel molybdenum steel that has been poured using a process called vacuum arc remelt (VAR). The VAR process reduces the incidence of impurities and occlusions in the metal. After machining and balancing the forged crankshaft blanks to new dimensions, surface hardening processes, such as nitriding, are applied. These processes create a crankshaft that is extremely resistant to wear.

The aluminum case

The 1903 Wright engine had an aluminum case that was cast by pouring molten aluminum into a sand mold — sandcasting is still used today for engine case halves and cylinder heads.

The Wright case supported the crankshaft with five crankshaft main bearing saddles. Babbitt — a common bearing alloy of tin, copper, and antimony — bearings were poured in place. Modern crankshafts are supported in the case with removable two-piece main bearing inserts. These inserts, also called bearing shells, snap into place and prevent wear to the expensive crankshaft and case halves. Bearing inserts are also used between the piston connecting rods and the crankshaft bearing journals.

The induction system for the fuel-air mixture was also cast into the case. This path doubled back on itself — the longer path heated the incoming fuel-air mixture, thus increasing the rate of fuel vaporization and more evenly mixing the fuel and air.

Cast aluminum cases are still used on modern light-aircraft engines. Today's engines are more modular than the 1903 Wright — two case halves are bolted together around the rotating and reciprocating steel components, such as the crankshaft, camshaft, and accessory drive gears. The Wright brothers chose aluminum for their engine case because it was lightweight, provided adequate strength, and was easy to machine with the tools of the period.

Cylinders

The Wright 1903 cylinders, made of cast iron, were screwed into the aluminum case and topped with cast-iron combustion chambers that screwed onto the top end of the cylinders. Today, sandcast (and Superior's investment cast) aluminum heads are heated to more than 600 degrees Fahrenheit before being screwed down onto forged chrome nickel molybdenum steel cylinders to form one-piece cylinder assemblies. The flanged cylinder base is then attached to the engine case with common fasteners. Cylinder parts that wear, such as valve guides, valves, valve seats, pistons, rings, and rocker arm bushings, are easily replaced. This modular design eases cylinder maintenance — individual cylinders can be removed without disturbing adjacent cylinders. Advances in metallurgy also pay dividends.

Teledyne Continental Motors (TCM) and Lycoming harden their cylinder walls to a depth of 12-to 25-thousandths of an inch (0.012 to 0.025 inches) by a process called nitriding, while Superior Air Parts through-hardens its cylinder walls. These processes are proven to drastically reduce cylinder wall wear.

Within the past five years, TCM has started tapering the cylinder barrel cooling fins — the fins are very short near the mounting flange. Tapering reduces weight and improves access for cylinder removal and installation.

Both Lycoming and TCM have produced cylinders with cross-flow head configurations, where the intake system is attached to one side of the head and the exhaust system is attached to the opposite side. This configuration ups engine efficiency by increasing airflow rates into and out of the combustion chamber, and by lessening the contamination of the incoming fuel-air mixture with residual exhaust gases that are backed up awaiting exit out past the exhaust valve.

While intake and exhaust valve material has changed (the Wright engine used cast-iron valves and valve seats), both the first and the latest engines still use poppet-style valves.

Pistons have changed a lot in the past 100 years. The 1903 Wright pistons were cast iron and weighed four pounds apiece. Each piston had three rings (also cast iron) and each ring was five-sixteenths-inch (0.3125 inches) wide. All aircraft pistons today are either forged or cast aluminum, with a four-ring (two compression rings, an oil control ring, and an oil wiper ring) configuration being the most common.

TCM started casting a steel insert into some of its pistons in the 1980s to add dimensional stability to the upper (compression) ring groove. Improvements also have been made in ring technology. Compression rings are now designed so that a portion of the combustion pressure shoots down into the gap between the ring and the piston. This ingenious design uses the gas pressure to push the ring outward toward the cylinder wall, aiding the ring-to-cylinder wall seal.

What oil system? What water pump?

The 1903 Wright engine never ran again after a strong gust of wind sent the Flyer tumbling across the Kill Devil Hills sand dunes following the last of the four flights it made on December 17, 1903. But it's a pretty good bet the time between overhauls would have been only a few hours.

The Wright engine had no oil pump. Oil was thrown around (this type of oil system is called a splash system) within the engine by the action of the reciprocating parts. The friction created by the wide cast-iron rings in close contact with the cast-iron cylinder walls must have been tremendous. Tests show that the power output of the 1903 Wright engine was anything but static — power output dropped off almost immediately because of marginal lubrication and poor cooling.

There was no water pump to circulate the cooling liquid — only a tank that replenished the water that rapidly evaporated as the engine ran. The 1903 engine featured marginal systems, but the engine was good enough to provide reliable power for four flights on December 17, with the longest lasting 59 seconds and covering 852 feet.

With the exception of two liquid-cooled engines produced by TCM and the engines of the Rotax line, all of today's light-airplane engines are air-cooled. Cowl flaps installed on some airplanes of the GA fleet give the pilot some control over the cylinder head temperatures (CHTs).

Since oil (more than one expert cites clean oil as the most important factor in dependable engine performance) plays such a critical role in the health of today's lightplane engines, every modern engine has some provision for controlling oil temperatures.

Oil coolers have replaced rudimentary oil-cooling methods such as directing ram air over the engine oil sump with automatically controlled thermostats. Efficient oil filters catch contaminants and keep engines clean. Multi-viscosity oils and non-congealing oil coolers permit safe operations in cold weather.

The reason today's engines look just like the engines built in 1945 is that the changes that have brought about improved engine usability and safety are improvements in fuel, oil, manufacturing, and metallurgy. Many improvements have been incorporated — but the majority of the changes are not visible to the naked eye.

Carburetor?

The fuel system of the 1903 Wright engine consisted of a metal can (with a capacity of one gallon) that was mounted below the top wing on a wing strut. A tube ran down the strut to the engine. Raw fuel (the fuel that was bought from a nearby marina is estimated to have been between 30 and 50 octane) squirted out of a small hole in the end of the tube into an open-ended can on the top of the engine. Air drawn through the can picked up the fuel and swept it into the aluminum case, carrying it through the serpentine routing on the path to the cylinders.

Present-day fuel delivery systems, while not as simple as the Wright system, have changed little since the 1950s.

The Wright brothers solved the problem of igniting the fuel-air mixture in the cylinders by installing a set of points in each cylinder. A direct-current generator, driven by a leather-covered rub wheel that contacted the spinning flywheel, supplied ignition system energy. When the points opened, an electrical arc was produced that ignited the fuel-air mixture.

During the past 70 years, light-airplane engine ignition systems have drifted away from a dual mechanical-point magneto ignition system from time to time, but not very far. Even the newest GA airplanes still use two separate magnetos for ignition. Since light-airplane magnetos have fixed spark timing, impulse couplings or starting vibrators are required to retard the spark timing and generate a "hot" spark during starting. But improvements are on the horizon (see " Airframe & Powerplant: Bag of Chips," January 2000 Pilot).

Within the past 10 years, Unison Industries has produced two products that improve magneto systems. The SlickStart unit lessens loads on the starter and aircraft battery by producing a cascade of high-energy sparks during starting. The LASAR system replaces the magnetos and improves both starting and operating economy by automatically adjusting ignition timing for best results by monitoring engine CHT, manifold pressure, engine speed, and crankshaft position.

At AOPA Expo 2002, TCM introduced its full authority digital engine control (FADEC), a fully automatic system that replaces both the magnetos and the carburetor or fuel-injection system. FADEC controls not only ignition timing, but also adjusts and controls the fuel-air ratios at each cylinder with microprocessor-controlled timed-pulse fuel-injection nozzles. FADEC does away with the mixture control knob — there's an rpm control function in the works.

Unison Industries is working with Lycoming on its digital fuel and ignition control system. According to a Unison spokesman, the Epic system has been in flight-testing for the past year and certification is hoped for in early 2004.

General Aviation Modifications Inc. (GAMI) is in the process of certifying its PRISM (pressure reactive intelligent spark management) system. Test-cell runs have proven that the company's closed-loop system of manipulating the ignition spark event to control the timing and peak pressure of the combustion gases lessens engine temperatures and stresses, and provides what it says is the best combination of power and economy.

These engine fuel and ignition control systems will permit full power operation of existing engines on unleaded 95-octane avgas. If true, these systems brighten the future for the high-powered piston engines that require 100-octane fuel since it's not clear how long leaded fuel will remain available.

After a flurry of interest a few years ago, activity on the diesel-engine front has narrowed to two companies. Thielert's 135-hp four-cylinder turbocharged four-stroke engine ( www.centurion-engine.com) is being certified and seems to be well on its way to market, with the SMA 230-hp SR 305 engine not far behind. Both companies say they intend to produce 300-hp diesel engines.

Toyota, Bombardier, and Honda are positioning themselves to introduce their versions of light-aircraft reciprocating engines. My guess is that these engines will have automatic mixture controls, be liquid cooled, have variable ignition timing, and have some sort of rpm control system — creating a true single-lever powerplant control.

New reciprocating engines will appear in the next decade. These engines are already in development and advancing toward certification and the market. It's doubtful that they will replace a tried-and-true, dependable, air-cooled, fixed-ignition, carbureted or fuel-injected, opposed four- or six-cylinder engine, at least for GA pilots. Taylor and the Wrights got it right the first time.


E-mail the author at steve.ells@aopa.org.