Al Schlageter
09-26-2011, 10:08 PM
Excerpt from http://www.vintagewings.ca/VintageNews/Stories/tabid/116/articleType/ArticleView/articleId/124/language/en-CA/Bounding-Clouds--Flying-the-Messerschmitt-Bf-109.aspx
Taxiing is the Messerchmitt’s opportunity to get you alone and to whisper a warning in your ear. There is a grotesquely high download on the tailwheel in the Bf-109; a situation made evident by the requirement for full rudder, hard braking, forward stick and a blast of power to effect a turn. Try that in a Spitfire and the propeller will chew dirt! While odd, it at least gave reassurance that even aggressive braking would be unlikely to result in a nose-over. Unfortunately it also meant that the center of gravity was very far aft of the main wheels. That is not a good thing. Recalling my misadventures in once trying to steer a shopping cart backwards down a hill, I made a mental note that the tail might try to pass me during the landing.
The geometry of the undercarriage is perhaps the most unusual feature of the Bf-109. A digression is in order to appreciate how its characteristics would manifest themselves during take-off or landing. Some sources claim that between 15-25% of the Bf-109s ever built were damaged or destroyed during take-off or landing accidents. I find this a remarkable figure for a combat aeroplane – especially one that served on the losing side of the war! Most contemporary histories of the Bf-109 attribute this to the narrow undercarriage track, however this misses the point. (The Spitfire’s undercarriage is just as narrow, and it doesn’t have any of the Bf-109’s quirks. It has its own quirks – but that’s another story.) Dr. Messerschmitt faced a challenge in the design of his first fighter. In the interest of simplifying transport and repair of the aeroplane, it was designed with the undercarriage attached to the fuselage, such that the wings could be completely removed with the aeroplane resting on its wheels. The undercarriage struts were attached to a complicated forging at the firewall aft of the engine mount. The narrow width of the fuselage structure necessitated installing the undercarriage legs splayed outwards. This feature became the aeroplane’s Achilles heel.
Another advantage of main gear design was that because landing gear, retracting through roughly an 85º angle, was attached to the fuselage, it was possible to completely remove the wings of the aircraft for major servicing without the need for additional equipment to support the fuselage. It also meant that the wing structure was able to be simplified through not having to carry the weight of the aircraft and not having to bear the loads imposed during takeoff or landing. However, this had one major drawback - the wheels had to be splayed outwards and this created an extreme tendency to ground loop and/or collapse.
Imagine that you have a bicycle wheel in your hands. Roll the wheel with the axle parallel to the ground. It goes straight. Now roll the wheel such that the axle is not parallel to the ground. The wheel turns. Let’s return to the Bf-109. Both of the tires are mounted “crooked”, rolling with a camber angle of about 25°. Consequently both wheels want to turn inwards under the aeroplane. When the aeroplane is rolling with an equal download on both wheels, symmetry prevails; both wheels fight to a stand-off, and the aeroplane rolls straight. Now imagine that something causes the download on the wheels to momentarily become unequal. In that case the rolling friction of the tires becomes uneven and the turning tendency of the “heavy” tire asserts itself. What might do this? Well, crosswinds. Or torque from engine power. However, the most dangerous culprit is turning. With the aeroplane’s centre of gravity situated high above the tires, a swerve will set loose large centrifugal forces that cause the aeroplane to try to roll over the tires. This is true of any aeroplane, but in this scenario the unusual camber of the Bf-109’s tires creates strong directional instability, requiring a different type of control strategy for take-offs and landings. Tight heading control or aggressive tracking of the runway centerline can set off abrupt directional divergence. Better for the pilot to relax, merely dampen heading changes, and accept small heading errors. Funny, I didn’t feel relaxed.
Taxiing is the Messerchmitt’s opportunity to get you alone and to whisper a warning in your ear. There is a grotesquely high download on the tailwheel in the Bf-109; a situation made evident by the requirement for full rudder, hard braking, forward stick and a blast of power to effect a turn. Try that in a Spitfire and the propeller will chew dirt! While odd, it at least gave reassurance that even aggressive braking would be unlikely to result in a nose-over. Unfortunately it also meant that the center of gravity was very far aft of the main wheels. That is not a good thing. Recalling my misadventures in once trying to steer a shopping cart backwards down a hill, I made a mental note that the tail might try to pass me during the landing.
The geometry of the undercarriage is perhaps the most unusual feature of the Bf-109. A digression is in order to appreciate how its characteristics would manifest themselves during take-off or landing. Some sources claim that between 15-25% of the Bf-109s ever built were damaged or destroyed during take-off or landing accidents. I find this a remarkable figure for a combat aeroplane – especially one that served on the losing side of the war! Most contemporary histories of the Bf-109 attribute this to the narrow undercarriage track, however this misses the point. (The Spitfire’s undercarriage is just as narrow, and it doesn’t have any of the Bf-109’s quirks. It has its own quirks – but that’s another story.) Dr. Messerschmitt faced a challenge in the design of his first fighter. In the interest of simplifying transport and repair of the aeroplane, it was designed with the undercarriage attached to the fuselage, such that the wings could be completely removed with the aeroplane resting on its wheels. The undercarriage struts were attached to a complicated forging at the firewall aft of the engine mount. The narrow width of the fuselage structure necessitated installing the undercarriage legs splayed outwards. This feature became the aeroplane’s Achilles heel.
Another advantage of main gear design was that because landing gear, retracting through roughly an 85º angle, was attached to the fuselage, it was possible to completely remove the wings of the aircraft for major servicing without the need for additional equipment to support the fuselage. It also meant that the wing structure was able to be simplified through not having to carry the weight of the aircraft and not having to bear the loads imposed during takeoff or landing. However, this had one major drawback - the wheels had to be splayed outwards and this created an extreme tendency to ground loop and/or collapse.
Imagine that you have a bicycle wheel in your hands. Roll the wheel with the axle parallel to the ground. It goes straight. Now roll the wheel such that the axle is not parallel to the ground. The wheel turns. Let’s return to the Bf-109. Both of the tires are mounted “crooked”, rolling with a camber angle of about 25°. Consequently both wheels want to turn inwards under the aeroplane. When the aeroplane is rolling with an equal download on both wheels, symmetry prevails; both wheels fight to a stand-off, and the aeroplane rolls straight. Now imagine that something causes the download on the wheels to momentarily become unequal. In that case the rolling friction of the tires becomes uneven and the turning tendency of the “heavy” tire asserts itself. What might do this? Well, crosswinds. Or torque from engine power. However, the most dangerous culprit is turning. With the aeroplane’s centre of gravity situated high above the tires, a swerve will set loose large centrifugal forces that cause the aeroplane to try to roll over the tires. This is true of any aeroplane, but in this scenario the unusual camber of the Bf-109’s tires creates strong directional instability, requiring a different type of control strategy for take-offs and landings. Tight heading control or aggressive tracking of the runway centerline can set off abrupt directional divergence. Better for the pilot to relax, merely dampen heading changes, and accept small heading errors. Funny, I didn’t feel relaxed.