Tag Archives: STUF

This essay is my first “guest blog.” The author, Commander Jim Tritten, US Navy (retired) is a good friend and a Naval aviator of some renown.

Round Engines
Jim Tritten

Remember the first time you sat behind the steering wheel of a new, right-off-the-showroom floor, Prius? Then you turned on the Power switch, depressed the accelerator to the floor, and held on for dear life? Or when you strapped on your first electric golf cart packing hundreds of mousepower of raw energy? Remember how you head was thrown back and you hair streamed in the breeze as you opened the throttle wide? Probably none of you had such an experience or were really impressed with those events – other than how unbelievably boring and quiet they were.
On the other hand, have you ever sat astride a Harley-Davidson chopper with muffler cut-outs venting raw exhaust into the atmosphere – and then done a wheelie? Or, for the less adventurous four wheelers, sitting in your favorite old street drag racer will do: Corvette, Chevy 409, Shelby Mustang etc. The point here is to grasp the contrast. Prius – sensible, quiet and slow start; Harley – boisterous, noisy and nimble.

Most young people today have never seen, let alone flown in, an aircraft powered by anthing other than a jet engine. Even those of you that recognize a propeller in front of an engine generally fail to understand that today it is most likely powered by a jet engine and not an old-fashioned internal combustion engine – a reciprocating engine.

Basic differences in design

Before there were jets, there were thousands of airplane types that lumbered into the skies daily with gasoline-fueled internal combustion engines on the front end of a fuselage or on the wings. These engines ran much like a gasoline-powered automobile engine with obvious differences to support flight, e.g. these engines supported propellers. There was usually, but not always, one propeller per engine. Props generally spun ahead of the engine – but sometimes they were behind the wing or fuselage. The gasoline engine provided the mechanical power that drove the propeller that thrust the airplane through the air.
Flight crew members often tinkered with their automobile engines as adolescents or young adults and had a general knowledge of how these contraptions worked. Internal combustion engines rely on pistons within cylinders to convert pressure into rotating motion. Of the obvious necessary differences with engines for automobiles, none were as obvious as the fact that many airplane engines were round.

The round radial design of multiple cylinders surrounding a crankshaft to drive the propeller provided an expedient way to cool the engine rather than using water jackets found on most automobiles. Round engines had their heyday from the mid-1930s through World War II. They continued providing yeoman service to the military and civilian aviation communities well into the 1980s. Today round airplane engines are mostly found with small flying companies and with hobbyists that own historic aircraft.

The modern jet functions with a turbine in the center of the engine nacelle and a series of blades that spin either before or after a combustion chamber. All of the mechanical rotation takes place internal to the engine with the exception of those having fan blades where air enters. In some cases there is a mechanical connection to a modern propeller as with a turboprop engine.

Another obvious difference in these two power plants is that reciprocating engines leak. Why do we have drip pans underneath our cars as they age? There is an old adage amongst pilots that flew recips. “If it ain’t leaking oil, it is probably empty.” Operating any internal combustion engine without lubrication is a sure-fire prescription for an engine seizure. No engine power in flight means no thrust which means a single-engine airplane becomes a glider. Flight crews did not mind dripping engine oil or the brown film blown back from the engine to the windscreen. Remember those shots of barn-storm era pilots with oil-streaked goggles?
Since these old engines leaked, they also smelled. Not just from oil but also spilled aviation gasoline (AVGAS) and the hydraulic fluid used to move flight control surfaces. Burned oil from an aircraft engine smells very much the same as when it comes from a car. Burned high octane AVGAS has a somewhat different aroma – it is not smoggy. Red hydraulic fluid had a slippery feel and a metallic smell.

Another smell around old airplanes was that of human sweat. Aviators did know how to shower and wash their flight suits, but these old planes and hangers were not air conditioned. Both ground and flight personnel often worked up quite a sweat just getting one of these old birds ready. Everyone carried a rag for drips and those rags got stuffed into pockets and carried around. Flight suits and work overalls might be worn a few times before actually seeing the laundry. “Ripe” was an oft-used phrase to describe soiled clothing. Naval aviation green uniforms for flight crews had the knack of being able to absorb most aircraft fluids and still look unstained.

Except for a turbofan’s outer blades and the exhaust area, flight crews can’t see much inside a jet engine. The crew of a recip had to not only open cowlings and look at the engine but to also inspect the propeller’s blades – especially the tip area. Propellers were often damaged and there were rules of thumb on how much visible damage was allowed. Of course, once the propeller was moving, the true test was in whether it rotated smoothly or vibrated which was a very bad sign. Modern propellers incorporated a variable pitch feature that could change the size of the bite of air taken by the prop or streamline it to the wind in the event of an engine failure.

Getting the airplane airborne

Having completed the external inspection, the flight crew entered the cockpit and went through a series of checks and double checks to verify that all switches and levers were in the correct position for engine start. For a jet, this generally means pulling something or flipping a switch to start the auxiliary power unit – a mini jet engine. Once there is power a single switch is thrown, a temperature gauge is monitored, and the throttle manipulated until the turbine is running smooth and generating a high-pitched whine. There are only a few steps and they are not that difficult which is one of the advantages of a jet. And of course there are no sounds or smells other than the oxygen flowing into your breathing mask.

Starting a recip is more like a seduction. There are three main elements: air, fuel, and spark. The pilot, or flight engineer, opens the throttle “just enough”. The electric boost fuel pumps are turned on and pressure is checked on a gauge. Then the propeller is cranked through a few revolutions to make sure there are no internal hydraulic fluid locks and the magneto switch is turned allowing high-voltage to reach the spark plugs. As the engine is cranked, assisted by occasional shots from the fuel primer, the entire aircraft undulates in rhythm to the slowly revolving propeller.

When a reciprocating radial engine starts, it begins to sputter, make loud noises, and cough like it is trying to heave up its innards. The entire aircraft shakes a bit and the exhaust belches black and white smoke. When it “feels” right, the pilot changes intermittent fuel priming to steady prime. At night, the flight crew is treated to yellow and red flames shooting out from the exhaust stacks. In a single engine airplane those stacks are generally directly ahead of and surrounding the pilot. The aroma of burned oil and AVGAS invades the cockpit as a cloud of smoke obscures forward view.

Not done yet – the pilot needs to let up on the prime button, set the fuel mixture rich and play with the throttles to catch the engine just like when you start an internal combustion engine that still uses a manual choke. When the engine appears to be running smoothly, the flames stop and are replaced by white and cobalt blue exhaust. The battery switch is turned off, external power removed, and then battery power turned back on. At night that meant a brief period of time with no lights on and a massive rotating scythe cutting through the air in front of the engine. It’s not as smooth as a jet start, but infinitely more entertaining.
The unnatural sounds of starting a recip are soon replaced by a comforting purr of a well-oiled machine and the reassuring whoosh of air through the propeller. The sounds of a jet, on the other hand, are either boring or down-right embarrassing. The low frequency sounds of a General Electric TF-34 high-bypass turbofan when advancing the throttles from ground idle can only be compared to those of a household vacuum cleaner. The S-3 aircraft earned the nickname “Hoover” because of the “whooping” sound it made. Hardly manly stuff.

Getting to the runway in a jet involves the addition of thrust and stepping on left, right, or both brakes and perhaps engaging nose wheel steering. Getting to the runway in a tailwheel airplane involves careful planning to ensure that the tail of the airplane actually moves in the opposite direction desired from the turn and that the propeller is not placed between a rising or setting sun and the cockpit. Pilots of tailwheel airplane and helicopters have often been subjected to flicker vertigo at low engine revolutions when ignoring this rule.

Before takeoff, all pilots verify their control surfaces work and that the engine is responding to directions. In a jet this means playing with the throttle and standing on the brakes. In a recip, there is a complicated set of procedures during which the variable pitch propeller is put through its paces. The full power check in a tailwheel airplane involves holding back on the stick to make sure the airplane tail does not lift causing the prop hit the concrete taxiway. Full power checks on large multiengine seaplanes were done where there was plenty of sea room since the airplane would go around in circles as each engine is tested. On most ski-equipped airplanes, the pilot would retract the skis above the wheels and hold the brakes, hopefully in solid snow. Obviously, this is a bit more difficult on solid ice.

Takeoff

To takeoff in a jet, just push the throttles forward, watch the gauges, release the brakes, and point the nose where you want to go. The most surprising parts of a takeoff in a jet is the lack of noise in the cockpit. The second most surprising is the acrid smell of burned kerosene which is defeated only by wearing an oxygen mask. This is especially bothersome aboard aircraft carriers. If you are the second aircraft behind a jet sitting on a catapult ready for a launch at sea, the smell and stinging to the eyes is awful.

A takeoff in a recip can be something where a pilot really earns his flight pay. Consider a single-engine prop airplane with a tailwheel. After maneuvering onto the runway, the pilot would need to trim in right rudder and extend his right foot pretty well all the way forward to counteract engine and propeller torque. Some pilots would hook their left boot under the left rudder pedal to help with the leverage. The pilot would lock the tailwheel, slowly advance the throttle, and release the brakes.

Stand by for some serious manly noises and vibration – greater than that generated by a Harley Davidson with cut-outs. Simply put, the feeling, sounds, sights, smells and perhaps taste of recips at take-off power are beautiful. The pilot has the sensation of sitting astride raw power that he can directly control. The airplane shudders and shakes and when the pilot releases the brakes, it starts rolling forward and the tail flies up off the ground. The pilot needs to be careful to not allow the nose to pitch forward and knick the prop in the pavement. The pilot manipulates rudder pressure to keep the bird pointed down the runway centerline and allows the airplane to fly itself off the deck. Then he must take out right rudder and trim as needed, suck up the landing gear, raise the flaps slowly, and reduce power a little.

With multi-engine airplane and/or tricycle landing gear, the takeoff evolution is similar and just as noisy and bone-shattering. Imagine four loud recips driving four large propellers that thrust a very large heavy bomber down the runway. It might be equated to the sounds of an entire Harley Davidson motorcycle club at full throttle on an interstate. Imagine one of those thousand plane raids in World War II with four thousand engines running at military power as they all climb en route to drop their load. Incredibly loud.

In flight

Once in the air, jets simply go higher and faster. Props often fly lower and slower although advanced reciprocating engines had superchargers and could support high altitude flight. Props were always slower, even if pointed straight down in a steep dive. Flying lower and without air conditioning meant that flight crews often resorted to their own devices to keep cool. For example, opening the torpedo bay doors and overhead hatches kept the air moving in an S-2. Flying lower often invited flying very close to the ground. Picture a single-engine prop like the A-1 Skyraider skimming the surface of a frozen lake, the 16′ propeller only a few feet from the ice, and ice fishermen scattering.

In flight, there is not too much to monitor with a jet engine. It keeps spinning and sucking up fuel and large volumes of air. A jet is quiet. A recip, on the other hand, needs fuel management and manipulation of the variable pitch propeller. Fuel management is generally done by reducing the fuel mixture control from the full rich setting used for takeoff until the engine starts to quit and then putting a little back on. This keeps the engine running on the least amount of AVGAS. The props then need to be reduced in RPM from their position used during takeoff and landing to a more sedate setting used for cruise. Then just to make sure that everything will still work later, every hour the engine is briefly run at a higher RPM and mixture setting to clear the spark plugs and prevent fouling. The flight crew hears a multitude of sounds inflight. The hourly burnout was often a wakeup call for a dozing copilot whose turn it was to fly next.

Jet engines, however, can support aerial missions only dreamed of by the prop community. They became the power plant of choice both in commercial and military aviation. Jets flight means longer legs at much higher altitudes and the ability to look down upon bad weather. Props are extremely range limited and generally have to fly under or through thunderstorms. Jets need to be exited quickly in the event of a catastrophic emergency and are thus equipped with ejection seats. Props come equipped with parachutes and bailout procedures. Jet flight crews wear oxygen masks and torso harnesses. Prop flight crews carry oxygen just in case. An ejection seat lacks any serious cushion while prop crews have padded seats and the crews might be able to get up and walk around.

One thing no jet pilot ever performed is a torque roll. This was generally done at least once to impress fledgling birdmen on the dangers of adding too much engine power during landing or when not watching the impact of too much power during takeoff. A single engine airplane with a large radial engine was taken to a high altitude and the wheels and flaps lowered to replicate the configuration used for takeoff or landing. Then power was rapidly added without any attempt to control the aircraft with the control surfaces. This would result in the plane rapidly rotating around its axis and it might even stall. The lesson to be learned was that if you add power when low, slow, and with the wheels and flaps down, you had to compensate with rudder and aileron to prevent a violent catastrophic roll. Most new pilots got the message the first time.
Landing

To land a jet, just lower the landing gear and flaps, reduce the throttle, aim at where you want to touch down, maintain a fixed attitude and airspeed, and let the bird plant itself on the runway. Once on the deck, deploy a drag chutes or clamshells, throttle back, get on the brakes, and turn off when slow. Hell, a computer can land a jet – even on an aircraft carrier.
Landing a prop airplane is where pilots really earned their flight pay. Again let’s assume a single engine tailwheel airplane. The pilot flies an approach to the touchdown zone and then does a combination of slowly raising the nose and tweaking the throttle to coax the airplane into a three-point attitude. This is all done visually with the forward view from the cockpit obstructed by the engine nacelle. Then, as the aircraft touches down on all three wheels simultaneously, power is reduced, the pilot uses the rudder to get the airplane rolling in the desired direction, and he pulls the stick or yoke into his lap. All this while simultaneously stepping on the brakes without doing it so hard that the propeller nicks the runway.

At the other extreme for props would be landing a large heavy multiengine plane equipped with nose wheel steering. With conventional landing gear, the pilot attempts to land on all wheels simultaneously. For tricycle landing gear it would be the main landing gear and then slowly lowering the nose to contact the runway. Power is reduced and then two things have to occur at about the same time. The propellers need to be cycled through the position where they generate forward thrust into a position that they are actually blowing air ahead of the aircraft. Then power is added and the reverse thrust of the propeller is used to slow the beast. The pilot simultaneously lets go of the yoke and handles a small joystick that directs the nosewheel steering. Quite an unnatural evolution.

Landing a jet on an aircraft carrier is similar to ashore. The pilot sets the attitude with constant airspeed and essentially lets the aircraft plant itself in the landing area. For a prop, the pilot gets a “cut” signal that means cut all power to the engine, lowers the nose down just a bit to spill lift from the wings, and then pulls the stick or yoke back into his lap to re-assume the landing attitude. For tailwheel airplanes, that means a three-point touchdown on a rolling deck. For tricycle gear the optimal method is on the main landing gear. This is somewhat more exciting at night with little reference to a horizon – so it is essentially done by remembering what worked in the daytime. The pilot learns that it has all worked when he strains forward in the straps as the hook catches a wire and the airplane slows to a stop.

When a jet touches down on a carrier flight deck, the pilot adds full power and hopes to engage one of the wires that stop the aircraft. When a prop touches down, the pilot keeps all power off. In either case, if a wire is not caught by the tailhook, called a “bolter”, both aircraft are positioned to takeoff. Since jet engines require more spool up time to go from low to high power, the pilot jams the throttles forward to full power when he feels the wheels touch the deck.

On the other hand, when a prop touches down on a carrier, no power is added. If the aircraft does not engage the cable, the pilot would then jam the throttles forward as the bird approaches the end of the angled landing area. Reciprocating engines can generate power and the propellers provide thrust quickly to get the plane airborne again. Of course the pilot has to remember to combat the torque roll from adding full power. That can ruin your day. If he is late on the power, he can keep the aircraft flying at abnormally low altitude and airspeed by taking advantage of ground effect, but serious pilot skills are required.
Conclusions

It should be clear that jet engines offer many advantages over their predecessors. Jets are the power plant of choice and few commercial companies still manufacture round radial engines for airplanes. The military eliminated enlisted ratings for reciprocating engine mechanics and you can’t find AVGAS today aboard any US aircraft carrier. A similar comparison would be between the old diesel boat submarines and nuclear power.

Yet many old pilots with experience with both round engines and jets have a nostalgic fondness for the old recips. Yes props are harder to operate and cannot support aerial missions in the same ways that a jet can. But when you fly these old birds, the feeling is more like sitting astride a Harley Davidson than driving a Prius. When you fly a round engine with props, the pilot has a job that takes involvement, skill, and finesse. In the words of many who delayed the transition from round engines, “jets are for kids!”