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December 12, 2009

Jets: Freedom of Placement

Donald Pittenger writes:

Dear Blowhards --

As noted from time to time here, the advent of new technology tends to create a burst of experimentation, the testing of new configurations in the hope of finding the best one. (That's "best" in terms of a compromise dealing with functional efficiency, ease of manufacturing, cost, customer acceptance and others.) Eventually a configuration evolves that fills the bill, though competing products embody small variations from the norm.

Of course, small changes in technology will keep the "best" or "ideal" form changing or drifting over time. The exception is when a large technological shift occurs. Then everyone dealing with the product has to scramble.

Effects of these sudden changes can be interesting to watch. Just for kicks, consider the early effects of the introduction of jet engines to aircraft design.

In the propeller era, the arc of the blade was a significant factor in shaping the configuration of the aircraft. For instance, the propeller and (usually) the engines had to be placed so that the tips of the blades wouldn't touch the ground or other parts of the aircraft. This contributed to a lot of head-scratching by engineers regarding wing placement (high, medium or low relative to the fuselage center-line), length of landing gear assemblies and a number of other issues.

Jet power eliminated the propeller (if turboprops are disregarded), so planes could now be designed without regard to propeller arc. Freedom!!

Well, not quite. There was the matter of ducting air to the turbine while taking into account pesky details such as boundary airflow and the fact that long exhaust ducts tended to reduce propulsive efficiency -- that is, short tailpipes would be nice to have.

Still, the comparative freedom created by the jet engine led to a good deal of experimentation in aircraft shapes from mid-World War 2 well into the 1950s, the greatest burst in the late 1940s. Some examples are shown below.


In an effort to get a jet fighter into production, the USSR's Yakovlev design bureau used a piston-engine design with a jet engine placed in the front where the piston motor would have been. Actually, the jet engine had to be placed lower to allow for a short tailpipe. Front-mounted jet engine layouts proved to be impractical. (The U.S. firm Republic considered adapting its P-47 prop fighter to jet power, but didn't pursue this approach beyond the paper stage.)

Bell XP-83
This is a scaled-up, long-range version of America's first jet fighter, the Bell P-59. The engines are tucked under the wings and have minimal ducting, a nice thing in the days when jet engines didn't create much power. On the other hand, the placement combined with the width of the engines added the the plane's frontal area and, therefore, drag -- resulting in lowered performance.

McDonnell FH-1 Phantom
The Phantom was the U.S. Navy's first operation jet fighter. Like the XP-83, it had two engines, but these were of the thinner, axial-flow variety, resulting in less air-resistance. Note that the engines are "buried" in the wing roots, another factor making the FH aerodynamically "cleaner" than the XP-83.

Republic P-84
After abandoning the concept of jet-powering its P-47, Republic came up with the P-84. As the F-84 (creation of the U.S. Air Force resulted in dropping the P-for-pursuit for the F-for-fighter designation), it served as a fighter-bomber in the Korean War. The ducting comprises the entire fuselage length. This resulted in less propulsive efficiency, but drag was less because the aircraft had a comparatively small cross-section. Several important fighters circa 1950 used this configuration: examples are the MiG-15, F-86 Sabre and the Dassault Ouragan and Mystère.

Convair XF-92 early mock-up
The XF-92 that actually flew looked far different from this early mock-up, the main commonality being the delta-shaped wings. Like the P-84, the ducting is extensive. But the obviously radical feature is the placement of the cockpit at the extreme front, immediately behind which is an annular air intake. This arrangement was used by the French Leduc ram-jet experimental planes which were probably known to the Convair designers.

Boeing early version of jet bomber (model)
As World War 2 was winding down, Boeing began experimenting with designs for turboprop and turbojet powered bombers. Shown is a model of a stillborn design incorporating the then-radical swept-back wings. Wings aside, the design looks clumsy with the bulbous fuselage and the placement of the engines atop the fuselage; at first glance, this design seems to have potential center-of-gravity/lift problems. Note that the intake ducts originate at the plane's nose -- the ducting seems to have been planned to pass under the cockpit floor and then curve upwards to the engines. Boeing's ultimate solution of putting engines in pods hung under the wings is found on most of today's jet transports.

Martin XB-48
Six engines were needed to give the XB-48 enough power, so Martin designers simply clumped them and boxed three under each wing. Those boxes incorporated air passages that, in theory, would minimize engine-area drag, but this solution didn't work out well in practice; Boeing's B-47 went into production instead.

Martin P6M Seamaster
Sometimes jet engines actually do need to be placed high. This was the case for Martin's Seamaster flying-boat bomber where the engine's air intakes had to be placed away from the risk of being clogged by spray when the aircraft was still water-borne.

Sud-Ouest Tridant
The Trident was an experimental interceptor whose primary power was a rocket motor mounted at the rear of the fuselage. It also incorporated two small jet engines mounted on the wingtips that allowed the plane to return to base after a mission. The jets were too puny to provide take-off power, however. While it's technically possible to mount engines on wingtips, this practice is normally avoided because of the danger of "power-out asymmetry" -- if one engine fails, the other will tend to yank the plane around. So if a plane has two wing-mounted engines, designers try to place them as near to the fuselage as possible (landing gear and fire hazard are two factors calling for at least some separation) to keep the asymmetrical power situation correctable by use of the plane's rudder.

Since the late 1950s other engine configurations have been used, but the era shown above is most interesting due to its intensity of experimentation.



posted by Donald at December 12, 2009


Do you have any photos of the first jet fighters i.e. the German and British ones?

Posted by: dearieme on December 13, 2009 6:22 AM

dearieme -- Michael just grabbed images off the Internet and I follow the same practice. Just Google a topic such as "de Havilland Vampire" and then click on "Images" on the top bar to see what's there.

Someday I might do a post on the Vampire, Meteor, Volksjäger & Sturmvogel."

Posted by: Donald Pittenger on December 13, 2009 12:50 PM

I love that Martin Seamaster. It's one of the Revell model kits I didn't build when I was a kid, but I used to look at these cards with the box art they included with their kits and dream about the other planes.

You should do a post sometime about model kit box art. There have been some books collecting the art published recently.

Posted by: Peter L. Winkler on December 13, 2009 3:45 PM

Note that the nose intake on single-engined fighters went away at least in part because radar needed to be placed there. I also find it interesting how the old ways of smoothly fairing intakes against the fuselage (e.g XP-83, etc.) went away as aerodynamic knowledge increased, leading to designs placed a bit away from the fuselage for less turbulent air. E.g. F-4, F-16

Posted by: PapayaSF on December 15, 2009 1:35 AM

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