When Planes Hit Puberty
How the Jet Engine Changed Planes
Modern planes have jet engines, but the first jet planes weren’t modern. They were propeller planes that happened to have a jet engine. The jet engine’s raw power was necessary to build the modern plane. But it wasn’t enough. Innovations in aerodynamics, safety, and operations turned jet-powered propeller planes into modern jetliners.
This is the story of the decade it took planes to catch up with their engines. Of fumbled experimentation with awkward bodies. Of when planes went through puberty.
The Modern Plane
The Boeing 707 is a modern plane. A 707 taxiing at JFK today wouldn’t catch your attention. What keeps a 60 year old design looking contemporary? Four features the 707 shares with the latest planes: Jet Engines, Swept Wings, Rounded Windows, Podded Engines.
But to understand how we got modern planes, we need to consider the planes they replaced.
The Pre-Modern Plane
In 1937, the DC-3 certainly seemed modern.
- All-metal body (wood construction had recently caused accidents).
- Large enough to make passenger service economically sustainable (previously airlines depended on airmail subsidies).
- Propellers driven by the latest engines flew it at twice the speed of the Ford Trimotor it replaced.
But what made the DC-3 (and the B-17, which was to bombers what the DC-3 was to airliners) so significant is not that it was a great plane. There had been lots of great planes. It was a great recipe. Want a larger plane? Just double all the ingredients of a DC-3 and you end up with the larger DC-4. And the DC-4 begat the DC-5, which begat the DCs-6 and -7. In the span of 20 years, the engineers at Douglas (by applying prodigious engineering talent) scaled the same basic formula up from 25,000 pounds to 150,000 pounds.
A New Engine
Planes were powered by piston engines that drove a propeller. A piston engine is the kind you have in your car, but turning a propeller instead of a wheel. This arrangement worked, but:
- piston engines are heavy
- more power requires more pistons, means more moving parts, means more maintenance and breakages
- turning propellers faster is less efficient (more of the power goes to chopping the air than to propelling the air backwards). You can ameliorate this somewhat by adding more blades, but only so far. (Anything above 4 blades is just silly.)
The Jet is a better engine. It burns fuel to expand air, which turns a turbine to compress air, which makes the fuel that’s burnt expand even more and shoot out the back with extreme force. Fewer moving parts. Avoids the waste of turning linear energy (of a piston) into rotational energy (of a crank shaft and propeller) into linear energy (of the plane’s forward motion). This makes jet engines twice as powerful, pound for pound, as propellers. And where propellers become less efficient as they go higher and faster, jets become more so.
Independent experiments in Germany and England created it at about the same time in the late 1930's. Early jet engines were temperamental. There were fewer moving pieces, but designers hadn’t perfected them yet. The moving pieces moved so much faster that when things went wrong, they went much wronger. And jets were more powerful per pound, but they were so much smaller that jets couldn’t match the output of the largest piston engines.
Fumbling Experiments
The jet was the future, but instead of waiting for it to mature, plane builders were pressured to make it the present. The results were awkward, bordering on useless. Some lowlights:
(FR Fireball)
- The Heinkel He 178 (1939) could last for 10 minutes at altitude and speed.
- The US Navy wanted a plane with the power of a jet but the reliability of a propeller, so the FR Fireball (1944) had one of each.
- The mammoth B-36 (1946; 200 tons, meant to carry bulky early atomic bombs) was built with 6 propellers. After building 80 planes, they decided it needed more power and added 4 jet engines to the blueprint (and then went back and modified the already-built airplanes). This turned a huge, slow plane into a huge, pokey, confused plane.
- The F7U Cutlass (1948) had a “semi-tailless” design and a lack of power that led to the nickname “Gutless Cutlass.” Trivia: “Whitey” Feightner’s emergency landing of a Cutlass was the first landing at Chicago’s newly-renamed O’Hare airport; Butch O’Hare (the airport’s namesake) had been Whitey’s squadron commander in WWII.
Growing Into Great
The first generation of jet planes were propeller planes with jets instead of (or in addition to) propellers. These planes were faster; they did fly higher. But they weren’t great. That would take three refinements to the pre-modern plane it took jet engines to reveal.
Sweep the Wing
Move close to the speed of sound, and air can’t get out of the way fast enough. This is called compressibility (by people who understand physics), and the Sound Barrier (by people who understand poetry). With straight wings, much of the extra power of jet engines wasn’t pushing planes faster; it was just pushing air.
The solution is a mix of physics and trigonometry. Pre-modern planes had the wing pointing straight out. If it instead angles backwards, the wing will be slicing the air at an angle and appear to be going under the speed of sound. This trick actually fools the air and lets the power of jet engines go towards speed, not waste.
Rounded Windows
Even propeller planes could fly higher than humans are able to breathe. In cargo or military planes, the crew could breathe from oxygen masks. But no passenger would pay for the privilege to travel in thin, frozen air. The solution is to use the skin of the airplane as a giant balloon you can inflate to a reasonable pressure. (Modern airplanes hold the cabin at the equivalent of 8,000 feet, the height of Machu Picchu.)
Several propeller planes had pressurized cabins (the B-29, DC-7). But it took the tragedy of the Comet to perfect them. The Comet was an almost-modern airplane, the Neanderthal to the 707's Homo Sapiens. It was the first jet-powered airliner, flying from England to Rome, India, and Africa. Within two years, 3 Comets had blown up in mid-air. After initially blaming terrorism, the culprit was found: each flight pressured the cabin, pushing the windows out which had to be held in by the metal fuselage. The stress on the metal was highest at the corners, which fatigued too rapidly and buckled in flight.
The solution: round the windows and you apply the stress evenly. Every jetliner since has had the same design. But it was too late to save the tarnished reputation of the Comet, and the 707 became the first modern plane.
Podded Engines
Jet Engines had fewer moving parts, which should have made them more reliable. But those early jet engines were still fiendish as manufacturers were learning how to build those parts to such high tolerances. Jet engines would have to be accessed and repaired far more frequently than propeller engines. And what few moving parts there were moved at much higher velocities. If any piece broke, the remaining pieces would fly out at dangerous speeds.
Plane designers moved engines from the wing to “pods” that hang under the wing. The result is slightly less aerodynamic (which is why high-performance military planes keep them in the body or wing) but much safer. The engine is also easier to take out (just put a forklift underneath and take out a few bolts), which makes it easier to do maintenance or replace them entirely.
The Moral
This is the story of engineering: when you fix one problem, you reveal another. The extra power of jet engines was wasted when straight wings held them back. In buildings, the extra strength of improved masonry was wasted when no one wanted an eighth-floor walkup (the elevator solved that). When you meet a new problem, don’t be discouraged: it means you just solved your last one.
