Messerschmitt Bf 109 - Design and Development - Design Features

Design Features

As with the earlier Bf 108, the new design was based on Messerschmitt's "lightweight construction" principle, which aimed to minimise the number of separate parts in the aircraft. Examples of this could be found in the use of two large, complex brackets which were fitted to the firewall. These brackets incorporated the lower engine mounts and landing gear pivot point into one unit. A large forging attached to the firewall housed the main spar pick-up points, and carried most of the wing loads. Contemporary design practice was usually to have these main load-bearing structures mounted on different parts of the airframe, with the loads being distributed through the structure via a series of strong-points. By concentrating the loads in the firewall, the structure of the Bf 109 could be made relatively light and uncomplicated.

An advantage of this design was that the main landing gear, which retracted through an 85-degree angle, was attached to the fuselage, making it possible to completely remove the wings for servicing without additional equipment to support the fuselage. It also allowed simplification of the wing structure, since it did not have to bear the loads imposed during takeoff or landing. The one major drawback of this landing gear arrangement was its narrow wheel track, making the aircraft unstable while on the ground. To increase stability, the legs were splayed outward somewhat, creating another problem in that the loads imposed during takeoff and landing were transferred up through the legs at an angle.

The small rudder of the Bf 109 was relatively ineffective at controlling the strong swing created by the powerful slipstream of the propeller during the early portion of the takeoff roll, and this sideways drift created disproportionate loads on the wheel opposite to the swing. If the forces imposed were large enough, the pivot point broke and the landing gear leg would collapse outward into its bay. Experienced pilots reported that the swing was easy to control, but some of the less-experienced pilots lost fighters on takeoff.

Because of the large ground angle caused by the long legs, forward visibility while on the ground was very poor, a problem exacerbated by the sideways-opening canopy. This meant that pilots had to taxi in a sinuous fashion which also imposed stresses on the splayed undercarriage legs. Ground accidents were a problem with rookie pilots, especially during the later stages of the war when pilots received less training before being sent to operational units. At least 10% of all Bf 109s were lost in takeoff and landing accidents, 1,500 of which occurred between 1939 and 1941. The installation of a fixed "tall" tailwheel on some of the late G-10s and 14s and the K-series helped alleviate the problem to a large extent.

From the inception of the design, priority was given to easy access to the powerplant, fuselage weapons and other systems while the aircraft was operating from forward airfields. To this end, the entire engine cowling was made up of large, easily removable panels which were secured by large toggle latches. A large panel under the wing centre section could be removed to gain access to the L-shaped main fuel tank, which was sited partly under the cockpit floor and partly behind the rear cockpit bulkhead. Other, smaller panels gave easy access to the cooling system and electrical equipment. The engine was held in two large, forged, magnesium alloy Y-shaped legs which were cantilevered from the firewall. Each of the legs was secured by two quick-release screw fittings on the firewall. All of the main pipe connections were colour-coded and grouped in one place, where possible, and electrical equipment plugged into junction boxes mounted on the firewall. The entire powerplant could be removed or replaced as a unit in a matter of minutes.

Another example of the Bf 109's advanced design was the use of a single, I-beam main spar in the wing, positioned more aft than usual (to give enough room for the retracted wheel), thus forming a stiff D-shaped torsion box. Most aircraft of the era used two spars, near the front and rear edges of the wings, but the D-box was much stiffer torsionally, and eliminated the need for the rear spar. The wing profile was the NACA 2R1 14.2 at the root and NACA 2R1 11.35 at the tip, with a thickness to chord ratio of 14.2% at the root and 11.35% at the tip.

Another major difference from competing designs was the higher wing-loading. While the R-IV contract called for a wing-loading of less than 100 kg/m2, Messerschmitt felt this was unreasonable. With a low-wing loading and the engines available, a fighter would end up being slower than the bombers it was tasked with catching.

A fighter was designed primarily for high-speed flight. A smaller wing area was optimal for achieving high speed, but low-speed flight would suffer, as the smaller wing would require more airflow to generate enough lift to maintain flight. To compensate for this, the Bf 109 included advanced high-lift devices on the wings, including automatically-opening leading edge slats, and fairly large camber-changing flaps on the trailing edge. The slats increased the lift of the wing considerably when deployed, greatly improving the horizontal manoeuverability of the aircraft, as several Luftwaffe veterans, such as Erwin Leykauf, attest. Messerschmitt also included ailerons that "drooped" when the flaps were lowered, thereby increasing the effective flap area (and later radiator flaps as well). When deployed, these devices effectively increased the wings' coefficient of lift.

Fighters with liquid cooled engines were vulnerable to hits in the cooling system. For this reason, on later Bf 109 F, G and K models the two coolant radiators were equipped with a cut-off system. If one radiator leaked, it was possible to fly on the second, or to fly for at least five minutes with both closed. In 1943, Oberfeldwebel Edmund Roßmann got lost and landed behind Soviet lines. He agreed to show the Soviets how to service the plane. Soviet machine gun technician Viktor M. Sinaisky recalled:

"The Messer was a very well designed plane. First, it had an engine of an inverted type, so it could not be knocked out from below. It also had two water radiators with a cut-off system: if one radiator leaked you could fly on the second or close both down and fly at least five minutes more. The pilot was protected by armour-plate from the back, and the fuel tank was also behind armour. Our planes had fuel tanks in the centre of their wings: that's why our pilot got burnt. What else did I like about the Messer? It was highly automatic and thus easy to fly. It also employed an electrical pitch regulator, which our planes didn't have. Our propeller system, with variable pitch was hydraulic, making it impossible to change pitch without engine running. If, God forbid, you turned off the engine at high pitch, it was impossible to turn the propeller and was very hard to start the engine again. Finally, the German ammo counter was also a great thing."