Development of Air-Cooled Aeroengines

The first succcessful aeroengine was water cooled (Wright Flyer I's Win Fiz, a four cylender inline, lying on its side), like common car practice.  However, the different use of engines in planes vs. cars allowed the development of air-cooled engines that are easily cooled by propellor air.  The lack of a coolant pump, hoses and radiator meant they could both weigh less and were much more reliable (fewer parts to break and fluids to leak).  Since they all had a very large fan bolted directly to them (the propellor) and were exposed to even more airflow once at cruising speed, they were supposed to have little need for the troublesome complexities of water-cooling systems that cars needed, but that often didn't work out when the crude lubrication systems were factored in.  They had the cylenders (always an odd number) radiating out from a common crankshaft.  Most of the WWI planes, especially the early ones, used this design.  The crankshaft was firmly bolted to the airframe while the propellor was bolted to the engine, which was designed to rotate.  This rotation allowed the engine to then act partly as its own flywheel, allowing lower idle speeds and smoother starts, compensating for the very primitive fuel delivery systems of the day.WWI Observation / Bomber As the engines became larger, to increase their output and support larger, more capable aircraft designs, the rotary design's rotation of the entire engine created increasing gyroscopic effects.

A twin-engine bomber from WWI, at the Paul E. Garber Facility in the suburbs of Washington, D.C.

When a Sopwith Camel's flight stick was pulled back, not only would the nose rise, but because of the gyroscopic effects it would also move to the left.  When the stick was pushed forward, the nose went down and to the right.  This effect had to figured into gun aiming as an extra variable, reducing accuracy.

Early Water-Cooled Engines

The highest-performance WWI fighter planes used water-cooled engines.  The SPAD used a Hispano-Suiza automobile V8 unique for its time (1915) in having a modern pump-pressurized oil feed system (among other features).  By pressurizing the oil going to all engine bearings, friction was reduced and the engine could be tuned for higher output without risk of seized bearings.  Both allowed sufficient power output to make the SPAD the favorite warplane of the Allied aces.  The block was of one piece unlike its contemporaries, and made of aluminum like the best of today's car engines.  Both factors allowed an extra-low weight which by itself increased the SPAD's ability to climb.  The final version, which entered production just at the war's end, produced 300 hp, which was far more than any other engine of the war (the '97 Corvette, by comparison, has 345 available to its wheels).

Germany's Fokker DVII biplane used a Mercedes inline six cylender.  This excellent water-cooled engine had been claimed for exclusive use early in the war by the Albatross, a composite plywood fighter plane.  It was a good plane, but not as good as Anthony Fokker's DVII.  Out of anger at the special deal between the Albatross manufacturer and Mercedes, he had the war's aces test fly both planes against each other in a competition for exclusive use of the Mercedes, with the result of the Albatross factory having to make DVIIs for Mr. Fokker, under license to him (after some midnight oil being burned to lengthen the DVII at one ace's secret recomendation).

OX-5 V8 Barstormer's Paradise

Wartime overproduction of primitive OX-5 V8s led to the postwar popularity of the Curtiss JN-4 "Jenny," the plane of choice for '20s barnstormers (started out as an official WWI aircraft of the U.S. Expeditionary Forces).  The engine produced so little power, yet weighed so much (like a mid '80s Ford OHC 2.3L: 90hp, about 370 lbs. and much shorter-lived) that huge, massively braced wings were needed to achieve useful flight when using it for propulsion.  All that bailing wire and those wooden braces and under-wing loops (for preventing lower wingtip damage when the plane rocked on its narrow landing gear in a crosswind) made for a wingwalker's paradise.  That was the plane that introduced America to the airplane, courtesy crappy but affordable war surplus engines.Curtiss JN-4

It used a single camshaft and pushrods for valve actuation, like '56 to present General Motors V8s.

Also during the '20s, some European car builders were using engine designs we normally consider to be recent technology.  Some used separate, overhead camshafts to operate overhead intake and exhaust valves slanted at an angle for optimum combustion efficiency.  Other than there still being only a single exhaust and intake valve per cylender, that is the same DOHC valvetrain design first introduced in modern economy cars in mid-80's Japanese cars.  Unfortunately, the poor metallurgy of the day made for very short valvetrain life,  and the Depression killed off all but established, inexpensively made automotive valvetrain designs.  The SPAD's Hispano-Suiza V8 used a single overhead cam per cylender bank to operate its 16 (overhead) valves.

Emergence of the Stationary Radial over the Rotary

The centripetal forces limiting the maximum size of the rotary, plus the gyroscopic effects (plus I'm sure crankshaft bearing problems, since it held the entire engine) led to the phasing out of the rotary, to be replaced by the stationary radial.  Finally, the axle was connected to the propellor and the engine firmly bolted to the airframe.

'20s biplane Single row Wright radial
1930s Wright single-row radial


 


Cooling was dealt with well enough that the last of the radials, at the dawn of the jet age, successfully used up to four sections of nine cylenders each around a common crankshaft (the R-4460 "corncob radial").  Those engines also used supercharging to increase output (use the engine to drive an air pump whose output all goes into the engine, boosting its output), increasing cooling loads.  It was found in the late twenties that a cowling surrounding the engine signifigantly decreased drag, increasing range about 10%.  The more the cowling curved inwards towards the propellor shaft, the lower the drag. Fw190F-8This also managed to simultanously increase the amount of air getting to the engine for cooling, even with the early, simple Townend airfoil-shaped ring.
 


The Focke-Wulf 190F-8 fighter-bomber


 


During WWII, the excellent, radial-powered Focke Wulf 190 fighter plane used a cowling that turned inwards extra far compared to other radial-powered aircraft to minimize drag, and used a fan mounted just inside the cowling, turning at twice the crankshaft speed, to keep the BMW 801 2000hp radial from overheating.  Like the rotary, typically a single camshaft turning around the crankshaft (geared to half its speed) moved pushrods which opened a radial's overhead valves via rocker arms.

The Wright (the original Wright brothers' company) Whirlwind was the first truly reliable and powerful radial engine, showcased by Charles Lindbergh's Spirit of St. Louis by its trouble-free crossing of the Atlantic in 1928.  An ex-Wright engineer greatly helped create the first Pratt & Whitney radial engines, clones of the Whirlwind.  Both makers constantly improved their designs, fueling the development of the aircraft and air travel industries in the 30's.  The Electra 10E used by Amelia Earhart was one of our first mass-assembled "modern" airliners, each of which could compete with Europe's finest custom-built single-purpose specials.

There was a long-distance trophy race in the late 1930s.  England entered a fairly small and fast flying fuel tank, basically, custom made for the race, with two liquid-cooled engines.  America entered standard production versions of the Boeing 247 and the Douglas DC-2 twin engine airliners, with the latest in commercially available radio gear and Wright-derived single-row radials.  England's racer won, but the much larger, hugely more spacious American airliners were right behind, in second and third places.  The English design evolved into the excellent (mostly wooden) Mosquito, the 247 helped with B-17 development, and the DC-2 soon evolved into the DC-3 (C-47), which was still used for many American commuter flights until at least the late 1980s.


Link to Special Section:  Slow and Old, but Steady and Rugged:
 


The Boeing B-17 Flying Fortress


 




WWII Engines: Radials

Many of the highest-performance WWII fighter planes, like in WWI, were liquid-cooled.  The main exceptions were Pacific carrier-based planes, whose maintenence requirements mandated simple, rugged radials.  If the plane is expected to be able to absorb lots of cannon shells and gunfire without losing an engine, such as heavy, anti-aircraft artillery (AAA)-susceptible ground attack platforms and huge, lumbering bombers, radials are better than inlines.  Radials will still run with entire cylender heads (they are all separate) blown clear off the engine, with the piston bobbing up and down with its top (crown) exposed to the atmosphere.  Liquid-cooled engines require plumbing and heat exchangers for their coolant.  One well-placed cooling system hit to an overwise rugged V12-powered P-51 Mustang, and there are only four seconds before the engine dies from overheating.

The Focke-Wulf 190 is again a curious exception, as a high-performance Western European radial plane.  It couldn't climb or turn as tightly as a P-51, but it was otherwise the only land-based radial-powered fighter plane of the European Theater comparable, performance-wise, to the highly successful P-51 (or to the Supermarine Spitfire).  Its designer, Kurt Tank, made a special final version that used a V12 and a lengthened fuselage (main body).  He designed a P-51-like remote underbody heat exchanger group for it, but the Reich didn't want so many airframe design changes for an engine change.

The resulting Fw190D-9 ("Dora Nine") Fw190D-9 had an annular radiator, in a disc around the propellor shaft between the engine and the propellor, neatly tucked into a still cylendrical radial-style cowling (like the WWI V8-powered SPAD).  This allowed the airplane to still have an easily serviced and replaced frontal "power pack," unlike most other fighter planes.

Another wartime benefit of radials is that flak, cannon shells, bullets and debris blown up into the sky by the plane's weapons into its own path can just pass between the cylenders, often not damaging anything.

The Alexander Seversky's P-47 is considered the ancestor of the modern multi-role fighter / bomber.

The heavy but rugged P-47 Thunderbolt (named the "Flying Brick" and the "Jug") excelled at ground attack missions partly because of that.  The P-51 had a special engine oil cooler with six separate chambers ahead of the (conventional) radiator.  If one chamber developed a leak and hence lost pressure, it would automatically seal itself from the oil lines.  That improved its ruggedness, but when forced into ground attack roles during the Korean War its coolant system caused losses to ground-based AAA.Attack, Douglas, 1 It was used instead of the P-47 only because the latter had already had its tooling destroyed.

The Douglas AD-1 (nicknamed "Able Dog" for its reliability) was the answer, like an improved, even more powerful  P-47 in concept.  It retained use of a rugged  but performance-limiting radial when the fighter planes high above it were all using turbojets.  For many ground attack missions, you want limited speed anyway.  It served well, in both Korea and Vietnam.

WWII Engines: Liquid-Cooled High-Performance V12s for Fighters

The liquid-cooled engines were best for nimble fighter planes, which could simply dodge enemy fire instead of having to fly right through it.  The use of liquid (a glycol mix, like today's car engine coolant) to cool the engine meant internal thermal stresses could be more localized and more intense than a radial's.  This in turn allowed higher supercharger boost pressures, more and smaller valves per cylender (they open and close quicker but it's harder to keep the head cool), and higher engine speeds.  All these improved airflow through the engine, increasing output without a similar increase in engine weight or fuel useage at cruise speed.  The P-51 Mustang, with its low frontal area V12, used about half the fuel per mile as the radial-powered P-47 Thunderbolt while also being more nimble.  The P-51B, C and D had 1490 hp while the P-47 had 2,000+ hp, yet they had nearly identical top speeds and maximum altitudes.  The P-47 hadn't been designed for long-range escort duty, but was all that was available and practical until the more economical and agile P-51 Mustang came along.  (It did okay partly via its turbocharging helping it gain speed with altitude, not gradually lose manifold pressure with altitude as with crankshaft-driven supercharging.)  The P-47 "Flying Brick" saw strictly ground-attack duties upon the P-51B's arrival, which it did very well at.


Most Successful Engine in the European Theater of Operations: Rolls-Royce Merlin

The best and most successful V12, or possibly any type of engine, of the war was the Rolls-Royce Merlin. Rolls-Royce Merlin 1650 It was used by the RAF (Britain's Royal Air Force) in their sleek Supermarine Spitfire (for which it was designed), the Hurricane (an easy to make, half wood fighter that complimented the complex, very time-consuming to build Spitfire, especially in the Battle of Britain), and many other RAF planes.  The Mosquito (also half wood) was a very successful high speed twin engine fighter / bomber, and the Lancaster twin-engine heavy bomber was the nighttime partner to our heavy bombers, but with a much higher payload capacity.  Our North American (merged with Rockwell later on) P-51B, C and D series Mustang used a Packard-built, "Americanized" version labelled as the 1650 series, thanks to a safe and secure crossing of the Atlantic with the Merlin's blueprints.

Every Merlin had two separate heads, each with a single overhead camshaft operating 24 valves, with 12 spark plugs.  Except for early versions (i.e. in Battle of Britain Spitfires), they also had a high-pressure bilevel centrifugal supercharging to maintain that performance at bomber escort and dogfighting altitudes.  It also used separate liquid cooling system for an intercooler, used to remove as much as possible of the heat added to the engine intake air by the supercharger's highly pressurizing it.  Air heats up as it is compressed, especially when compressed to an extra 2/3rds atmosphere normal full power (about 1500 hp), or 1 1/3 atm. on emergency overboost (1720 hp for 15 min. cumulative, maximum, usually requiring engine replacement upon landing).  Maximum mission duration could be nine hours, with the Merlin requiring a valve lash adjustment as part of every post-mission maintenence, and a 20 gallon oil change after every 200 hrs.


Link to Special Section:  Most Succesful, and Famous, Fighters of the War:
 


The North American Aviation P-51 Mustang & the RAF's Supermarine Spitfire


 




P-51 Mustangs normally achieved 4.5 to 5 mpg, but by leaning out of the fuel to air ratio, increasing supercharger boost and reducing engine speed, 8.3 could sometimes be reached (for extra-long missions, like Pacific island-hopping over Japanese-held territory.)  Charles Lindbergh (the first to cross the Atlantic Ocean), as a civilian, taught P-38 pilots the same techniques for Pacific island-hopping, such as to Guam.  He usually taught them during actual missions, flying his own P-38 as part of the rest of the squadron.  The pilots were all convinced the engines would blow from preignition, but thanks to Lindbergh's work with Allison, the P-38's engine manufacturer, they didn't.  Mr. Lindbergh, according to some reliable accounts, during one such mission shot down an excellent Mitsubishi Zero, but since he was after all a civilian, he actually didn't do that because it would have been an act of murder.

Engine Longevity and Power Extenders

Both engine types used rich fuel:air intake mixtures to boost engine cooling at very high power levels, such as during dogfights and take-offs.  Evaporating gasoline has a great cooling effect, but the engine could consume twice the fuel at full, emergency-level power that it would at 80% power.  Too much power useage, and either empty tanks or worn-out / blown / burned-out engines would lead to landings either into the English Channel or onto hostile ground.

The WWII Luftwaffe aircraft and the Korean War-era P-51H (Allison V12), used a methanol and water mix, injected into the intake air, as a combination coolant and octane booster to increase maximum power output by allowing higher (temporary) supercharger boost (a deliberate "overboost" condition).  The water would very efficiently convert the high but low-duration peak temperatures and pressures that preignition occurs at for longer-duration pressure but much lower peak temperatures, removing preignition and its tendency to hammer apart an engine.  The methanol would act as power booster and antifreeze for the water.  Without an automotive-style long exhaust system, the steam from the water injection couldn't cause self-defeating excessive exhaust backpressure which itself can lead to overheating.  Pursuing aircraft could always tell when the water or MW50 injection was turned for overboost by an extra-thick pure white vapor trail suddenly pouring out from the engine.

Night fighter planes, usually twin-engine light bombers with radar gear on board, would have to put manifolds on their engines.  Otherwise the flames and occasional sparks that came out of the engines at full power would visually give away their location.  When used, the engines' output would drop over 10%, allowing the faster, twin Merlin-powered Mosquito night fighters to reach them (even when they too were using manifolds).

British planes first used active radar sets, but when Luftwaffe passive (receive-only), radar sets used that signal to hunt the planes emitting it, a detection tactics war began.)


Bf109 inverted V12The Daimler-Benz inverted V12 used in the Messershmitt Bf109E "Emil" fighter plane used in the Battle of Britain was roughly equal in power output to the Merlin, but required much larger cylenders to produce that output.  It used fuel injection (mechanical, similar to an older diesel's system) while the Merlins all used pressurized carburetion, right through to the war's end.  The 109E was able to pull away from an early Merlin-powered plane by diving with negative Gs.
 


One of Rommel's Bf109EsA Messerchmitt Bf109E "Emil" in camoflauged desert duty under Rommel, the "Desert Fox."  Germany ended up having extreme trouble getting supplies to him, and then he was implicated by the Gestapo for supposed involvement in an assasination attempt against Hitler.)


 


The Merlin-powered plane following, upon pulling negative Gs as well, would toss the fuel bowl's contents to the top of the bowl, with the float following.  While the engine flooded while belching black smoke out the exhaust, the 109 would be smoothly accelerating away at full power.  Later, a restrictor disc, with a small hole in the center, was put in the Merlin's fuel line to prevent fuel from instantly filling the bowl upon use of negative Gs.

The 109 was more a transitional plane, obviously a monoplane vs. a biplane, but without the durability of later monoplanes like the P-47D or the Fw190.  The wing roots were notoriously weak throughout the plane's lifespan, right to the war's end.  Me109 pilots in the Battle of Britain would sometimes prefer evading Spitfires by pulling moderate negative Gs rather than risk having a wing break off by pulling high positive Gs, although this was also likely a tactic against the early Spitfire's negative Gs -incompatible carburetion.

During the the Battle of Britain, the Me109 was without drop tank capability.  15% of them ran out of fuel before reaching home base.  On some missions, some squadrons would mostly falll into the Channel.  The pilots would either freeze, or be lucky by either being picked up by rescue craft and/or find one of the float platforms the Germans put in the channel just for them.Me109B
 


An early 109, the Me109B: Willy Messerchmitt's first fighter design, part of the coming out party of Nazi Germany in the mid to late '30s in blatant violation of the strict, punitive Versailles Treaty from the end of WWI.


 


The first two versions of the Messerschmitt 109, the one that ran away with the awards at the 1936 Berlin international airshow and the Me109B battle-tested in the Spanish Civil War, used either a Jumo inverted V12 or a Daimler-Benz inverted V12.


The Offbeat:

Jumo Aero Turbodiesels

Jumo Opposed-12 turbodieselBefore and during the war, German Jumo engines powered many Luftwaffe aircraft, from the Bf109 fighter plane to cargo planes.

The Hugo Junkers -designed Jumo opposed-6, 12-piston, two-stroke turbodiesel used in freight transport aircraft, at the Paul E. Garber Restoration & Storage Facility (click on image for choice of either 56kB normal-size JPEG or 189kB "mural" JPEG.)

One unusual design was a two-stroke, highly turbocharged 6 cylender, 12 piston diesel.  It used a single bank of 6 cylenders, with two separate crankshafts at the top and the bottom connected to pistons facing each other. They shared combustion chambers: one piston would move up while the other moved down and vice-versa, so the high compression ratio required for diesel operation was possible with minimal piston movement.  It was a simple two-stroke, with the one piston uncovering the exhaust ports while the other would uncover the intake ports, eliminating complex valvetrain systems and their maintenence requirements.  In the final 207B-3 version, the combined turbocharging and supercharging helped it provide 750hp from sea level all the way to nearly 40,000ft, with 1000hp available on take-off.

Napier Sleeve-Valve U16s

The RAF's Tempest and Typhoon fighter/bombers used heavy but powerful Napier-Sabre U-16 -layout sleeve-valve engines.  They used separate crankshafts for each bank of eight inline cylenders to avoid breakage and minimize twisting.  Compared to flying a plane with a pair of great big engines bolted to each other in the nose, the sound and power of the Tempests were beyond that of more conventional aircraft.  They lacked some maneuverability compared to them, but their 2,500 hp and decent aerodynamics gave them the speed (400+ mph) to escape most danger after launching devastation upon the Germans comparable only to the Thunderbolt's abilities.


The Development of Supercharging

During the 1920s, research was done into supercharger design.  This allowed aircraft to fly higher, into the thinner air at altitude, relying on the supercharger to pump whatever air it could find into the engine.  That in turn allowed pilots to not only fly above bad weather, but also take advantage of high-speed air currents like the Jet Stream that exist only at high altitude, shaving sometimes 25% off their travel time (and hence reducing fuel requirements and increasing range).  It also allowed a strongly-built, but normal-size engine to produce the power of a much larger unboosted engine.  The first designs used a centrifugal blower, like a hairdryer, geared to the crankshaft to pump extra air into the engine (centrifugal supercharger).

Development of the Turbo(super)charger

Evolution of the impellor used allowed a second design, the turbo supercharger, normally just called a turbocharger, to be developed.  This used a second impellor mounted in the exhaust stream attached via a short axle to the first impellor.  Instead of relying on gearing that would be damaged by sudden engine speed changes, a turbocharger uses exhaust pressure.  The greater the intake pressurization, the greater the exhaust pressure against the exhaust turbine impellor, thus the greater the intake pressurization by the intake impellor, etc.  The time that cycle takes to build signifigant boost pressures is dependant largely on how small and therefore low-momentum the turbocharger impellors are made.  If they are too small for the engine they're attached to, they take very little time to build up speed and therefore pressure, but have a lower maximum pressure building capacity than larger, slower to spool up (longer "turbo lag") impellors.


Too Fast for Its Day?: Lockheed P-38 Lightning

Since the exhaust turbine makes an excellent muffler, turbocharged engines are quieter than both supercharged engines and even non-pressurized engines.  This is why the WWII Lockheed P-38 Lightning fighter / bomber,Lockheed P-38L Lightning with two turbocharged and very durable (American) Allison V12s, was quieter than the supercharged single-V12 fighters like the P-51 Mustang and the Spitfire. The Germans supposedly called it the "Fork-Tailed Devil" because of supposedly not being able to hear it coming towards them as well as they could hear the others, although I've been told that was only wartime propoganda.  To the pilot inside a closed canopy, a  Mustang's Merlin V12 at full power sounds like a "bull in pain" with a headset on, and like "putting a stethescope to a tin roof in ball-bearing storm" without.  P-51 pilots' ears would ring for hours after a long mission.  The main reason the P-38 wasn't used more in European skies (it had excellent low to normal-speed handling qualities, and could powerdive with the best) was that its cockpit wasn't heated enough for very cold elevations.  Before the P-51 Mustang came into being, P-38s that saw bomber escort duty had to have their frozen pilots extracted from the cockpit upon landing.  At non-freezing lower altitudes over the Pacific, however, it excelled as a long-range Zero-killer (had the most kills, more than carrier-based planes, partly due to its large numbers).

The P-38 was introduced early in the war, before the Republic P-47 Thunderbolt.  At that time, little was known about high-speed aerodynamics.  That led to the Lightning having often fatal trouble pulling out of high-speed dives.  Buffetting from the cockpit nacelle extended at extreme speed (over 420 mph) back to the elevator panel with enough force to sometimes destroy it, making the plane totally uncontrollable.  The faster the airspeed, the farther back the "center of lift" is (like center of gravity - a balancing point, but of lift forces, not gravity).  That led to the elevator panel being tilted 0.5 degrees downwards so the elevator's range of motion would be sufficient to pull out of an extended power dive.  There were also airbrakes later installed to simply keep the speed in more familiar territory.

Since the P-38 was ground-based, not carrier-based, when the Pacific war had come to a close and our P-38s were then closer to distant Japan than the U.S., I have read that it was decided for economic reasons to use hundreds of them as landfill at Clark airbase in the Philippines, although I cannot confirm that.


Under sustained boost, automotive turbocharged engines' exhaust manifolds glow red to orange hot from the extra-hot extreme exhaust backpressure caused by the turbocharger.  A turbocharger's exhaust turbine has to withstand extreme centripetal forces and very high temperatures simultaneously.  Its bearings have to withstand extreme temperatures and axle speeds without leaking its lubricating (and cooling) oil.  Typical modern turbocharger impellors can spin at at least 150,000 rpm, compared to a 5,500 to 7,500 rpm automotive crankshaft redline, or under 4,500 rpm for normal piston aircraft.   Peak turbocharger pressures normally range from five to 15 psi on the intake side (14.7psi being the same as sea-level atmospheric pressure), and about five to 15% over that for the exhaust side.

The Beginning of the Jet Age

The development of how to make parts that can accept those conditions led to the centrifugal turbojet, the first practical jet engine type.  It's basically a turbocharger, except instead of having the fuel burn in an internal combustion engine, it burns in cone-shaped "cans" with spark plugs near their wide inlet.  The cone shape makes more exhaust leave via the "nozzle" tip than the wide end, increasing air:fuel mix flow and therefore rate of combustion.  Like in a car engine's turbocharger, the exhaust gases rush over the high temperature turbine, turning the compressor turbine faster, increasing "can" intake pressure, etc.   The exhaust gases leave the high-temperature turbine, called the power turbine or "core," via a nozzle at a high enough speed to push the craft forward (actually most "thrust" comes from pull from the air intake).  The higher the ratio of "can" inlet pressure to power turbine exhuast pressure, the more efficient the turbojet, and the higher the power turbine temperatures.

F/A-18 post-ScudModern fighter jets have variable-diameter exhaust nozzles to optimize exit flow speed under a wide range of power settings, further increasing efficiency.
 


F/A-18 engines after a missile hit during Desert Storm.  Having a backup can sometimes help...


 


Centrifugal turbojets have a highly curved, restrictive passage for air to flow through the engine.  The "cans" surround the power turbine, allowing the impellors to be close together like in a turbocharger.  Model turbojets for radio control aircraft are generally automotive turbochargers with totally redone cases.  Combustion cans get spliced in between them, wrapped around the exhaust turbine, and a nozzle is attached to the exit, making them miniature centrifugal turbojets.


First of the Fighter Jets: the Messerschmitt 262a

The first mass-assembled turbojet warplane to see active service in noticeable numbers was the Messerschmitt 262a interceptor / fighter / bomber.  It used two 2,000 lbs. thrust axial Jumo turbojets, and was delayed in its design for approximately two years by the Reich command (which basically wanted to get the war over before developing completely new aircraft, fortunately).  In 1941 a senior Reich official stopped over at Mr. Messerschmitt's home to order him to stop work on it.  By the time it was finally mass assembled, supply problems cut short the nickel and chromium needed for the bearings.  That and drastic petroleum shortages via Allied bombing of Romanian oilfields and refineries led to both increasingly reduced flight, high power setting use and pilot training times, preventing pilots from getting full use of the new technology.  The Allied bomber stream did such damage to the normal German military infrastructure that some were assembled in clearings in the woods, and many others underground.  (Despite Allied bombing, German war production, against all odds, continued to increase as the war continued, nearly to the horrid end.)  The Me262's airframe also became unstable as it approached its top speed due to not well understood near-Mach 1 aerodynamic effects.

Compressibility factors are aerodynamic disasters-in-waiting that can appear as Mach 1, the speed of sound, is approached.  Shock waves start over the leading edges of every surface.  Near Mach 1 these shock waves will extend themselves from the leading edge to the back of the control surfaces, making movement of those panels irrelevant until the plane finds itself several thousand feet lower, in denser air.  The effect has been described as the flight stick being rooted in cement.  All the later (meaning fastest) WWII fighter planes experienced signifigant trouble with compressibility effects when pulling out of extended power dives.

During the later war years, once the Reich finally noticed that drastic measures were required yesterday to prevent approaching annhilation by the gargantuan galloping American "arsenal of democracy," a push for advanced "super-weapons" was launched.  The Me262 project was made active again, and much research into high speed aerodyanamics was done with the help of new advanced, large-scale high-airspeed wind tunnels.

During the later design phase of the 262, the wings had to be angled back to match the center of lift to the center of gravity (a balance correction).  It was later found, before the war ended but after the 262 design was finalized, that sweeping the wings back delays the formation of and helps "shed" shock waves that otherwise would cause compressibility effects.  Without that coincidental balance correction, the 262 would have been probably been totally uncontrollable near its top speed (540 mph, vs. 437 to nearly 500 mph for the fastest prop planes).  It was wobbly near that speed with the angled wings.

Their bearing design was a guess in the dark, and the nickel and chromium that was found to work the best became unavailable as the Allies cut off Axis shipping routes.

The use of two engines out on the wings vs. a single engine in the nose, like the prop fighter planes fighting it, caused its roll rate (ability to quickly bank into a turn) to be signifigantly less than the latter.  As long as the 262 didn't simply run away from you with its sheer speed, outmaneuvering it and/or sustained overtaxing of its engines could let a good pilot of a good prop warbird bring it down.

There is a current project to build Me262 replicas using GE turbojets.  The GE engines are smaller than the original Jumos (but have an extra 1,000 lbs. of thrust), so they are getting cases identical to the size and appearance of the original Jumos (and of course open up clamshell-style for genuine engine servicing).


In the early to late '40s, axial-flow turbojets were developed.  They have a longer distance between the compressor and the power turbine.  The  original turbocharger-style impellors are replaced with mutli-bladed, multi-row fans, and the  combustion cans are added inline (in between them), so the airflow never has to turn corners.  The reduced internal airflow speed requirement of turboprops allows them to use shorter centrifugal turbojets to turn the propellor via a gearset.

Fanjets attach an oversize fan to the compressor section, right in front.  The fan is large on airliners and slow military planes, like cargo planes and the A-10 Warthog / Thunderbolt II tank killer.  It's smaller (but still extends outside the turbojet housing) on high-speed fighter jets, which can't afford the drag of an airliner-style "high-bypass" fanjet, making them "low-bypass" fanjets.  The longer and wider the fan blades, the more efficient the thrust generation since its increasing diameter will make it turn slower to generate the same thrust.  The faster the fan blade (or propellor), the greater the turbulence generated by it and then passed on from one blade to the next, and hence the lower the efficiency.

Fighter jets also have a wider range of speeds they travel at.  The most powerful propellor aircraft  have relied (since the late 30's) on a governor-controlledConstant-speed prop blade pitch (angle) altering device to maintain fairly constant, optimal engine speed over their fairly limited airspeed range, the way a sports car uses a manual transmission to exactly optimize otherwise changing engine speed.
 


Early Hamilton Standard constant-speed governor assembly


 


 The way an automatic transmission has a fluid clutch that reduces the need for lots of different gear ratios, low-bypass turbojets deliver fairly constant thrust (push) over a wide airspeed range.  Like many turbocharged car engines, they are actually stronger at high speeds than low.  High-bypass fanjets are therefore a good, efficient middle ground.

Low-bypass fighter jet turbojets also use afterburners, which inject large volumes of raw fuel directly into the hot, but mostly fresh perimeter air of the exhuast stream where it explodes, adding to the thrust.

At low speeds, especially on early turbojets, rapid power setting increases could cause a "flameout," in which exhuast pressure is so much higher than intake pressure or compressor exit pressure that the exhaust gases begin to use the intake path as an exit.  When that happens, the efficiency and thrust go to zero and flame retardance systems must be used immediately.  High power use at too low a speed also risks flameout, since the reduced airflow speeds and pressures present could allow that combustion and backpressure farther back up the engine than is safe.


The Russian Approach

Modern Russian fighter jets have the ability toMiG-29 at extreme Gsuse the entire wing as an airbrake by maintaining forward momentum, but having the plane go from forward-pointing to vertical and slightly beyond, and then back to forward-pointing again.  It's called the Cobra maneuver, and each Russian fighter jet since the MiG-29 has allowed a new variation of it.
 


MiG-29 yanking enough Gs to condense water vapor from extreme upper-surface vacuum


 


The maneuver demands that the turbojets be able to continue providing thrust while air is trying to flow backwards through the engines.  Russian turbojets don't last as long as ours, but trying a CobraRussian Knights: Su-35?in say, an F-16, would (if the onboard computers messed up by allowing it at all) first of all induce a snap-spin and secondly stall the engines since our turbojets aren't expected to have to put up with such maneuvers.
 


The Russian Knights (demonstration team; probiscus is sensor array for the Sukhoi's rear-firing missiles)


 


Russia's Saturn AL-31 fighter turbofan was designed with the Cobra maneuver in mind (specifically for the Su-27 Flanker), and for many years at least it has been the only engine is existance to allow the maneuver without stalling.  It has an extra-low fan bypass ratio of 0.6, probably to avoid having the Cobra maneuver excessively slow a more airspeed-sensitive larger fan, which could lead to engine stalling since thrust output is largely proportional to rpm.

The Cobra and most other uniquely Russian maneuvers are extreme, desperate measures designed as last-ditch attempts to destroy the enemy, perhaps even at the cost of one's own death, when normal methods have all failed.  A plane's combination of speed and altitude, called its energy state, is all-important in dogfighting.  Normally too low an energy state vs. your opponent and you're dead because the opponent has the abiltiy to outmaneuver and outrun you.  The Cobra is the most effective method ever created for rapidly minimizing one's energy state.MiG-29 crash at Paris '89 But it will get the MiG or Sukhoi pilot behind an opponent that had just been behind him, allowing a rapid missile or guns kill, but if there are other opponents in the area the MiG / Sukhoi driver will be in a vulnerable state indeed.

The MiG-29 concludes its introduction at the 1989 Paris Air Show.  The ejection devices worked well enough for the pilot (not his error) to survive the <300' above ground bail-out.


The Blackbird CIA Spy Plane

The Mach 3.2+ BlackbirdThe SR-71 (strike/recon) Blackbird's turbojets, however, were designed to turn into "ramjets" at extreme speeds.  The turbines are reduced in their importance and the entire engine acts like a combustion "can" by having six large pipes bypass the compressor section which would otherwise just be in the way.  The force and speed of the intake air alone virtually eliminates the need for a compressor stage at its cruise speeds anyway.

The plane's design was a reaction to the shooting down of Francis Gary Powers.  His U-2 spyplane was designed for 90,000 ft. altitudes but was slow and fragile enough to be potentially shot down by a Mach 3-capable MiG 25, by firing its missiles slightly upwards.  The U-2 went from being an officially undocumented, unbudgeted "Black Project" CIA "nonplane" to collected debris displayed for the world to see from downtown Moscow after a surface-to-air SA-2 missile shot him down.

The Blackbird's engine design started as a hydrogen-fueled semi-spaceplane ultra-secret Black Project codenamed Project Suntan.  After at least $250 million, in late '50s dollars, it was decided that a petroleum-based fuel would work better after all for a semi-earthbound engine.  The research into hydrogen propulsion use did help NASA's rocket design research however.

The Blackbird can travel at in excess of Mach 3.2, faster than missiles.  (The engine is designed for a Mach 3.35 maximum, and most of the "thrust" comes from intake section pull, with only a little added by the afterburning exhaust nozzle.)  Since the plane is designed for speeds that make the airframe edges glow red hot from air friction, when the fuel tanks are cold they have leaks that thermal expansion close up upon sustaining cruise speed.11 Blackbirds, just before decommission The recent reactivation of three SR-71s involved the selection of which three to reactivate based on how little fuel they leak on the ground.   The fuel when cold is a special low-flashpoint gel, but still leaks.  (You can spot the active airframes in the above photo, taken just before their early 90's deactivation, by the fuel leakage under them.)  They also have to take off with only a partial fuel load and then finish fuelling up in midair from a tanker plane, due to excessive stress on the landing gear otherwise.

It has special "radar-eating" structural elements - metal triangle framing with radar absorbant material filling the void, plus other techniques, making it partly stealth.  Its main purpose, or raison d'entre was to spy on Russia thoughout the Cold War.  The photos it returns are at least as sharp as any satellite's.

Robert MacNamara, head of the U.S. Air Force during the SR-71's beginnings and during the Vietnam War, considered the SR-71 to be in conflict with his own pet projects and grossly in conflict with his governing philosophy of common parts for all USAF machinery, and so repeatedly tried to kill the program.  He had the tooling for assembly of them destroyed for some inexplicable reason, yet enough parts were made, enough retired airframes are still around and they are reliable enough that they can be in active service for many, many years to come.  The extreme heat-cycling that the airframes went through with each full flight actually made them become stronger with use.

Prior to the Persian Gulf War they were mothballed, largely for political reasons.  Management would deny it sensor and communication upgrades, citing the plane's age, and then say "without the modern sensor and commications capabilities that our more recent planes have, why should we continue flying this outdated plane?"  Increasing faith in spy satellites was also part, as was the design's high flight time cost vs. the still-used U-2.  At the onset of the Persian Gulf War, however, the first thing Gen. Shwartzkopf requested was at least one Blackbird for tracking Iraqi movements.  Three were recommissioned in the mid-'90s, and even deficit-minded Congress supported reactivation of two additional Blackbirds.  The active ones now had the original sensor upgrades, and leading-edge F-15E Strike Eagle-style sensor and communications upgrades were also planned for it.  As the highest-performance spyplane the earth has seen to our knowledge, certainly more versitile than predictable-location satellites but with equal or greater photo sharpness, able to outrun missiles, it will for the indefinate future continue to be considered a treasured national asset if allowed to be used.

Why the past tense?  Careers can't be built around a project that has already been completed, so again for political reasons, the Blackbird's relatively microscopic $39M FY'98 maintenence funding was yanked by President Clinton during a trip to Brazil.  (For comparison, $50M had apparently been earmarked for a Viagra program.)  With the world in post-Cold War semi-chaos, with U.N.-mandated monitoring programs in the Middle East, North Korea and the Balkans simultaneaously, plus the fragility of Iraq and Afganistan, and Russia very desparate to sell Cold War-era weapons technology for very needed hard cash to the nearest allowed bidder, the world's only spontaneous-mission intelligence photography source has been canned.  With lots of careers currently wrapped around the apparently slow development of new, very expensive to develop unmanned aerial reconnaisance vehicles (with a small fraction of the Blackbird's speed), the SR-71 is now permanently out to pasture.  NASA is to get three recently active airframes, with all others most likely relegated to museums.



 


F-15E Strike EagleThe F-15E Strike Eagle, with all the latest available sensor and communications gear.


 


Modern American fighter jets, starting with the F-15 Eagle, are all "fly-by-wire."  This means that by use of a full-time autopilot system, with the control stick being an input device for the autopilot system, the plane was able to be designed so far towards pure agility that just like a P-51 with an overfilled (behind the pilot) fuselage fuel tank, a computer is needed to keep the plane from flopping out of control and tumbling to the earth.  Picture trying to drive a car backwards at over 100 mph and you have the idea.

F-16 Fighting FalconThe F-16, with a very busy pilot (no Weapons Systems Officer), and one very reliable engine, was the "workhorse" of Desert Storm.

Despite the high natural instability, F-16 "Electric Jet" Falcon jocks report the plane to be "a sweet ride," "the Porsche of the jets."  The high speed with which the computer accurately senses G-forces acting on the plane and correspondingly adjusts the control surfaces makes it relatively easy to fly.

The computer uses "hard limits" to keep the plane under control when the pilot is asking for maximum performance.  It will go up to a very specific performance limit, i.e. maximum G-forces at speed x and so on, and then no farther.

Russian fighter jets use more advanced aerodynamics, and so program their flight computers to allow "soft" limits, so the pilot can pull some crazy stunt like a Cobra or a derivative of it.  For normal flight, our F/A-18 flies very similarly to the MiG-29 and later Sukhois.
 


Blue Angels' F/A-18sThe twin-engine F/A-18, used by the Blue Angels, is relied on for replacement of many more spacious, specialized and then- successful but aging and now retired Navy airframes.


 


Russians "admire" the single-turbofan F-16, but are too disaster-minded to not have a pair of engines in all their airframes.  The Pratt & Whitney F100-229 and F110-229 low-bypass augmented (afterburning) turbofans used in the F-16, however, are designed to go for several years of normal use without requiring replacement of even the hottest, most stressed parts.  (The F-15 gets the F100-229 design only, and the F/A-18 gets a pair of different engines, F404s.)


The Evolution of Stealth Technology
 

B-1B "Lancer" supersonic bomber

As mentioned above, the Mach 3.2+ SR-71 had some radar-absorbing technology built in.  The B-1 was started as the Tacit Blue black budget program in the '70s, two decades after the Blackbird's origins.  In the late '70s, the Carter Administration eventually decided it was a waste of resources and killed the B-1A project.  Reagan resurrected it as the B-1B.  Its goal isn't so much radar stealth, but radar evasion.  It is designed for supersonic travel at ultra-low altitudes to get below radar coverage, preferably with a mountain range between it and radar towers.  It uses a very quick-response terrain-following autopilot system for fully automated, and so far reliable to the best of my knowledge, crash avoidance.
 

The final emergence of the Flying Wing concept: the F-117A Stealth Fighter

The F-117A is the first of the modern "flying wings" first proposed by Jack Northrup in the '30s.  The flying wing concept is highly aerodynamically sound, since it lacks the cylendrical fuselage other designs have, allowing the entire airframe to help generate lift.  Normal aircraft designs rely on engine placement fairly distant from the plane's center of gravity (its balancing point) to provide stability.  The WWII P-39 Aircobra and the B-26 Marauder were both tricky to fly from having the concentrated heft of the engine(s) too near the center of gravity.  The P-39 was mid-engined, with the engine between the tail and wing, and the B-26 had its engines only a bit forward of the fairly short wings.  In each case, stalling a wing would lead to a tight spin because of the engines' mass being near the center of gravity.  It could also help with agility, but made it no easier to fly.  It should however be noted that the Soviets loved the midengine P-39 Aircobra and supercharged P-63 Supercobra because of the resultant light, responsive controls, and the B-26 went on to improve its safety record to that of lowest loss rate of any bomber.

Flying wing designs have no structure other than one continuous wing.  If the engines are at the very front or rear, the aerodynamics are slightly spoiled.  The Northrup XB-49 of the late '40s was the first truly successful flying wing, with the the final version using four turbojets mounted at the far back edge of airfoil.  The engines' location combined with the airframe's large size & weight to give it enough pitch stability to be barely controllable without computer assistance.

The F-117A was designed to be both lightweight and agile with its turbojets mounted approx. midplane (for stealth reasons).  The fly-by-wire system, comparable in function to the F-15, -16 and -18 systems, is fast and effective enough to make what is otherwise as controllable as an oak seed fairly benign and responsive.  Its handling and flight duties are more in the Attack catagory, not the Fighter catagory populated by the likes of the super-agile, very highly aerodynamic, and very lightweight F-16 Falcon.

To achieve its sparrow-like radar signature, much of the design had to invented from scratch, assisted by early to mid '80s computer technology.  Until about mid-1997, the radar-absorbing paint was highly vulnerable to rain, causing the Navy to see no value in it at all.  Whenever it would start to rain at any airshow an F-117A attended, the flight / maintenence crew would toss a cover over it with a speed that would do racecar pit crew proud.

Jack Northrup's Dream as the USAF's Masterpiece: the Northrup B-2 Spirit

The B-2A is the most complex and expensive aircraft ever built.  It is the direct descendant of Jack Northrup's XB-49 prototype Flying Wing bomber, and is the first truly stealth and supersonic aircraft.  The relatively slow computers used to figure out the F-117A's shape caused its simpler, small number of flat surfaces, hindering its top speed to subsonic levels from the poor aerodynamics.  The B-2 however was designed later, with computers available with enough speed to calculate a succussful shape using a far greater number of surface planes, giving it its smooth aerodynamics, which in turn allows its far higher top speed.  Despite its great size, its radar signature is also approximately that of a sparrow.
 

The F-22 Raptor Advanced Tactical Fighter

Turbojet and turbofan engines normally cannot accept supersonic intake air due to shock effects.  Conventional fighters can only achieve supersonic flight with afterburner use, meaning extreme increases in fuel consumption per mile of travel.  The F-22 Raptor / Lightning II Prototype F-22 Raptor advanced fighterhowever is designed to "supercruise," meaning it can achieve signifigantly supersonic flight without fuel-guzzling afterburner use.  This increases maximum range, stealth interception range, and protection from heat-seeking missiles.  It will use thrust vectoring (like the recent Russian Su-35), which means high temperature and strength flaps will direct the turbofan output up to 20 degrees up or down.  This in turn will give the Lightning II extreme maneuverability and reflexes without resorting to use of oversize control surfaces.  Inboard weapons storage helps keep its aerodynamic drag and radar signature small.
 


Bibleography / Credits
You're thinking, "Well, that's fair amount of info there.  Now how the heck do I know any of it is true?"  I've been into aircraft since elementary school (mid-'70s).  My list of references for this site and aircraft identification for my grandfather's WWI photo album are:

The Aircraft Discussion List (over three years), merged with The Aerospace & Aircraft History List (since 1995)
Aircraft, by Kenneth Munson
Antique Airplanes Coloring Book, by Peter E. Copeland (esp. helpful for WWI pics)
Chuck Yeager's Air Combat and Aces Over Europe air combat simulator manuals
Classics, photography by Mark Meyer
Debt of Honor, by Tom Clancy
The Encyclopedia of Aeroengines, by Bill Gunston
Flight Handbook / Pilot Training Manual for the F-51D Mustang (reprint of 1/20/54 edition)
Fighter Wing, by Tom Clancy
Fighter, by Len Dieghton
The Great Planes, edited by James Gilbert
Milestones of Aviation, by the Smithsonian Institution and the National Air and Space Museum
World War II in photographs, by Robin Cross
The World's Worst Aircraft, by Bill Yenne
Spitfire, by Robert Jackson
1998, 2000 Ghosts WWII aircraft calendars
1999 Flying Legends WWII aircraft calendar
2000 daily aircraft calendar
Various contributions & corrections by

Yes, I know that Tom Clancy novels are not completely truth.  For instance at the end of Debt of Honor, an airliner's fuel load explodes violently in open air.  Jet fuel, which is basically kerosene, explodes only if  1. well vaporized,  2. mixed with just the right amount of oxygen,  3. heated, and  4. compressed,  so the scene relies entirely on Hollywoodization of fact.  (Some of the official findings on possible reasons for the explosion of the TWA Flight 800 take jet fuel's normal near non-flammability into account.)  On the other hand, esp. with the non-fiction Fighter Wing, there are loads of technical info which Mr. Clancy has no known motivation to Hollywoodize, and I can think of little reason to doubt it.

The Me190B picture, the Fw190D-9 sketch and picture, and the pictures of the damaged F/A-18 exhaust nozzles, the SR-71s, the F/A-18 and the greyscale backfiring "Memphis Belle" are signifigantly reduced versions of pictures elsewhere on the Internet or from AOL's library.  The rest of from my own photography.


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