Aircraft Evolution- A minor case study


This is the F-15E Strike Eagle.

Beagle

When the original F-15 was developed at the tail end of the Vietnam War, the Air Force, for the first time in more than a generation, focused the design of a fighter exclusively on its air-to-air capabilities. For 2o year prior to this, fighters for Tactical Air Command1 had been focused on the long range strike mission, via either conventional bombing, or the nuclear strike role. But given the appalling performance of TAC aircraft in air-to-air combat in Vietnam, the powers that be at the Air Force decided that the FX design philosophy would be “not a pound for air-to-ground.” And so the F-15A rolled out with no ability, nor tasking, to drop bombs. Instead, it had combination of speed, altitude, maneuverability, range, outstanding radar, and large air-to-air missile payload that made it the benchmark by which all other fighter aircraft were measured.

A decade later, as the Air Force cast around for a replacement for its aging F-111 strike fighter fleet, the same basic properties that made the F-15 an excellent airplane also made it quite adaptable. Its thrust to weight ratio meant it had great load carrying capability. It’s large internal fuel tanks gave it good range2. Two seat operational trainer versions were already in service, so the layout needed to accommodate a weapon systems officer was already in place. And the Eagle’s excellent, large radar could be developed into a fine air-to-ground sensor while still retaining its air-to-air capability.

It took a while, but eventually the “Beagle” came into being, and quickly rose to become the Air Force’s premiere long range strike airframe, operating with great success in Desert Storm, and every aerial conflict since.

This evolution from air-to-air fighter is quite common in history. The Republic P-47 Thunderbolt was such a formidable ground attack airplane, Republic eventually graced it’s A-10 with the same moniker. But the P-47 had been designed from the outset as a very high altitude, long range escort fighter.

As time goes on, virtually every fighter develops at least some significant air-to-ground capability. Even the F-104 and F-14, both designed with only air superiority in mind, went on to become effective strike aircraft.

But how often do you hear of a bomber being developed into a fighter? It’s pretty rare. Or rather, it’s pretty rare for it to be successful. And that is where our story today begins.

The 1930 were a time of great advancements in aircraft design. From the end of World War I until the storm clouds of war began to gather over Europe, design improvements had been quite incremental. Externally, aircraft still greatly resembled their forebears- biplanes with fixed landing gear, fabric covered frames, wooden fixed pitch propellers and open canopies. To be sure, there had been improvements. Steel tubing replaced wood as the choice for the aircraft frame. Engine power, spurred by interwar years air racing, had grown quite steadily. But underlying design philosophies of WWI still held sway.

But as tensions mounted in Europe with the rise of Fascism and Nazism, nations began to consider rearmament on a large scale. As as money began to trickle into the defense industries, airplane designers finally had enough funding to take some risks. Simple steps at first, such as using enclosed canopies. Others switched to very thin aluminum skins over steel tubing. More radical was the introduction of retractable landing gear. And the old fixed pitch wooden propellers began to give way to variable pitch propellers, better able to utilize the increased horsepower of even more powerful engines.

One of the biggest advances was the abandonment of the steel tubing frame for semi-monocoque airframes. In this, aluminum ribs and stringers, directly attached to the aluminum skin itself provided the structural strength of the frame. For the first time, the the skin itself provided a part of the structural strength. For this reason, semi-monocoque designs are sometimes called “stressed skin” designs. This approach gave greater strength at a lighter weight. It also lent itself to the new emphasis on greater streamlining.

In Great Britain, among the very first airplanes to meld all these advances into one model was the Bristol Blenheim light bomber. First flying in 1935, and introduced into squadron service in 1937, the Blenheim was light years ahead of other aircraft of the day. A twin engine light bomber, with stressed skin construction, excellent streamlining, retractable gear, and variable pitch three blade propellers, the Blenheim had a three man crew- pilot, navigator/bomb-aimer, and gunner. It had a healthy bomb load of up to 1200 pounds of bombs over a range of 1400 miles, giving it a combat radius of about 600 miles. Its blistering top speed of 231 knots led the RAF to believe that it could outrun any fighter opposition3, thus obviating the need for any fighter escort.

In fact, the Blenheim’s performance was so good, the RAF bought a version for use as a long range fighter aircraft, intended to go into enemy skies and defeat him over his own turf.

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Bristol Blenheim

But such was the pace of aviation development that by the time World War II broke out in 1939, the Blenheim, after only two years of service, was badly obsolescent. Losses over France in 1940 were ghastly. Only in lesser theaters could the Blenheim be used without crippling losses.

Great Britain is an island nation, and had an intense interest in denying the use of the seas to any potential enemy. The single most effective aerial weapon against shipping was the air-dropped torpedo. Needing a torpedo bomber, the RAF turned to Bristol, who in turn developed a larger, more powerful evolution of the Blenheim, which came to be known as the Beaufort. More powerful engines, a larger fuselage, a fourth crewmember, and a slightly larger wing meant the Beaufort was a new airplane, not a mere modification, but its Blenheim roots were clear.  Despite being designed as a torpedo bomber, the Beaufort spent most of its brief career performing as a level bomber, a role for which it was only marginally suited. The RAF, having a chronic shortage of aircraft, knew this, but felt no alternative. The Beaufort was not a particularly successful design, coming as it did on the cusp of a second wave of evolution in airplanes4, but still provided yeoman service in the dark early days of the war before survivors were relegated to training duties. Bristol_Beauforts_217_Squadron_in_flight

Bristol Beaufort

The Beaufort may not have been particularly successful, but it did spawn a worthy successor. The RAF still had a strong need for a long range fighter, particularly a night-fighter with the capacity to carry the primitive early airborne radar sets (and just as importantly, the space to carry a radar operator).

The Beaufort may have been slow, but the basic design, especially the wing, was sound. By adding yet again more powerful engines, and designing a new fuselage that was smaller and lighter, and replacing the bomb bay with a tray for 20mm cannon, the Bristol Company introduced the two-seat long range Beaufighter. As a night fighter, the “Beau” was an immediate success. It may not have been faster than the day fighters of the era, but it was more than fast enough to chase down Luftwaffe bombers. Its four 20mm cannons, and up to six .303 machine guns gave it plenty of firepower to knock down even the toughest of airplanes.

Bristol_Type_156_Beaufighter_ExCC

Bristol Beaufighter

That same firepower quickly led to the Beau being used in the ground attack role, interdicting transportation targets in occupied France, and shooting up Luftwaffe airfields and other targets. Adding to its arsenal, up to eight rockets could be carried, or two 1000lb bombs.

The Beaufort’s lack of success as a torpedo bomber also meant RAF’s Coastal Command still needed an effective anti-shipping weapon. The Beau was soon adapted to carry a torpedo. Typical anti-shipping strikes would see Beaus armed with guns and rockets suppressing the flak of a target while torpedo armed variants came in to finish off the targets with torpedoes.

The Beaufighter was so successful a design that the US Army Air Forces, awaiting delivery of its own P-61 Black Widow night fighters, shunned its own P-70s5, and operated a Wing of Beaufighters in the Mediterranean. 

The Beaufighter served with distinction in almost every theater of war from its introduction in 1940 through the end of the war. Australia built Beaufighters for its own RAAF. Eventually, over 5000 Beaus were fielded.

From light bomber, to torpedo bomber, to fighter, to torpedo bomber to light bomber. What an evolution.

1. Air Defense Command, on the other hand, had developed a series of aircraft exclusively oriented to air-to-air combat. But these aircraft stressed intercepting Soviet bombers, and while they had great speed and range, their maneuverability against small Soviet fighters was sorely lacking.

2. External conformal fuel tanks were originally developed for the A-D model Eagles. They were never used operationally by them, but are standard fit on the E model.

3. In fact, they were pretty much correct… for 1937. The same advances that gave the Blenheim, applied to fighters, soon meant that assumption was quite invalid.

4. This second wave, with planes such as the A-26 and B-26 bombers, the P-47 and P-51 fighters were near the pinnacle of piston engine airplane development. They were incremental improvements over the immediate pre-war types, but it was a fairly large increment.

5. An underwhelming night fighter variant of the otherwise successful A-20 Havoc design.

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One response to “Aircraft Evolution- A minor case study

  1. I have always had great affection for Coastal Commands TorBeau version of the Beaufighter. A torpedo, 8 rockets, and 4 20 mm guns work just fine on freighters, thank you very much.