Flying Sieves

By William S Friedman

"Shooting down one of these four-engined American bombers is like taking a fortified position — it is a task for ordnance; the heavier the better!" Fieldmarschall Schperrle, Nazi air bigwig, back-handedly paid the B-17 her greatest compliment when he made this statement. He was, according to the Swedish aviation publication Flygning, arguing for heavier guns on future interceptor aircraft. Indications are that, he will get it, but whether anything less than a field gun will be effective against the B-17's miraculous structure is a question.

The B-17 can take it. Event after event prove that this four-engined production air cruiser can absorb punishment that could split a corvette and possibly send a destroyer to the bottom.

The Fortresses' toughness just didn't happen. It was engineered into B-17s long before the first one was more than a military idea. When even American air strategists thought four-engined planes were impractical, the Fortress' structural ancestor, a single-engined mailplane called the Boeing Monomail, flew five passengers and 750 lbs of cargo at a cruising speed of 140 mph on a 575-hp air-cooled engine. In an era where wooden wings and fabric covering were still good conservative engineering, a smooth-skinned, flush-riveted semi-monocoque airplane was an unheard-of innovation. Conservative operators despised the craft, and conservative legislators wanted to know what was wrong with biplanes for flying the US Mail.

Nevertheless, the Monomail proved that it could fly faster, farther and with greater loads. Its rigid, stressed-skin structure could carry its load with less weight invested in structure and with less maintenance grief than the conventional airplane.

The Boeing Monomail begat two offspring; one was destined to be popular and famous in its own right, the other to quietly revolutionize military aviation. The popular child was the Boeing 247 twin-engined all metal transport. For the pilot, here was a ship that had two engines for top performance, and a power reserve for good flying on one engine. To the operator, here was an airplane with a sturdy airframe that would need little or no maintenance for its entire lifetime.

The military offspring was another twin-engined low-wing monoplane not quite as pretty, but more revolutionary in its field. Its maximum speed with full military load was 186 mph, and it went by contemporary combat planes as though the fighters were parked. This Boeing Death Angel marked the doom of many reactionary ideas in design. It kissed farewell to the biplane as a military entity, it doomed the fixed landing gear, and started a world-wide design competition for better ships, tougher structures and higher cruising speeds.

There is a lot of history between the B-9 and the B-17 including the decision that a proper defensive airplane for the nation must be one that can reach into the middle of either ocean and drop enough weight to sink a battleship. It must have enough stamina to take any beating defensive aircraft can hand out in order to reach the bomb release line. The Fortress has this stamina — it has proved itself more times than has ever been recorded for public observation. Like the predecessor, the Fortress is a semi-monocoque structure, one in which the skin or outer shell, reinforced by a widely separated supporting structure, carries the weight. Compare this with the skeleton structure used a decade or so back, and still prevalent in light airplanes. This frame, usually made of welded steel tubing, is faired off with wooden or metal spacers, and covered with doped fabric. In non-combat aircraft, this is fine, but in a military plane, it is unsatisfactory, for if a major or even a supporting member is blown away, the entire structure will collapse.

The secret of the Fortress' structural strength is decentralization of stress. No single part of a Fortress carries more than a just proportion of the total load, so there seems to be no Achilles heel. To knock a Fortress down, one must kill three engines, fire the fuel tanks or blow off the entire tail or entire wing. This is admitted, even by the Luftwaffe experts.

The evidence supporting it seems to be their willingness to surrender speed and performance to carry enough fire power to down a Fortress. Look at the planes they send against them. Fw-190s armed with four 20-mm cannon, special interceptor versions of the Junkers Ju-88, with four 20-mm cannon in the nose and 13-mm machine guns in the top and bottom turrets. Special "souped up" versions of the Dornier 217 have attacked them with compound attack, using 88-mm rockets and a new type 30-mm cannon.

Goebbels may mouth, and the critics of the old Boeing may hunt for new epithets, but the enemy still pays her the greatest compliment — heavier ordnance — big guns to storm a Fortress.

And the Fortress will stand the assault.

This article was originally published in the February, 1944, issue of Air News magazine, vol 6, no 1, pp 24-25.
The PDF of this article [ PDF, 9.1 MiB ] includes 19 small captioned photos, most showing battle damage to B-17s.
The original was printed on 9½ by 12¾ inch paper. The pages in the PDF were reduced to print on letter-size paper.
Photos credited to Signal Corps, USAAF.