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stanmorek · 05-11-2008, 09:29 AM

Running the risk of boring members of this board with a long post, I just wanted to mention the story of the German Fokker D8 an advanced monoplane fighter in 1917. I got this story from a good structures book of mine recounting how early aircraft designers had not recognised the effect of torsion on aircraft wings. Due to the critical tactical situation, the Germans issued the D8 to several fighter squadrons without adequate test flying. But under combat conditions when the D8 pulled out of a dive the wings came off. Much to the grave concern of the German air force authorities they were losing some of their best and most experienced pilots.

The internal structure of the fabric covered wings were two wooden spars or cantilever beams projecting sideways from the fuselage and interconnected by a series of wooden ribs every few inches. Naturally the authorities ordered load testing of a complete aircraft by way of mounting it upside down in a test frame and loading up the wings with shot bags to simulate aerodynamic forces which occur during flight.

During tests no weakness was observed until the wings failed at a loading 6 times the equivalent self weight of the aircraft. This load was considered to be higher than would occur during the worst combat conditions. In other words the D8 should have been perfectly safe. However, observations during testing saw failure begin in the rear spar. As a result the rear spars of all D8s were strengthened by thickening. Unfortunately, after this had been done, the number of accidents did not decrease but increased. The effect of adding more structural material to the wing had actually made it weaker.

Unofficially, aircraft engineer Antony Fokker under his own supervision loaded up another D8. This time he took incremental measurements of wing deflections as the wings were loaded gradually. What he found was that the wing deflected in bending as it was loaded and that the wing tips would rise up relative to the fuselage. This was expected, but in addition he also discovered that the wings also twisted at the same time although no obviuous twisting loads were being applied. Importantly, the direction of twisting was such that the angle of attack of the wing was increased significantly.

Fokker had discovered something called a 'divergent condition' which was very lethal to aircraft of mono wing design. He realised that when the pilot pulled back the control stick the nose rose increasing load on the wings. But at the same time the wings twisted and so air loads on the wing increased causing the wings to twist more and so on, until the wings were loaded disproportionately and the pilot no longer had control.

The solution to the problem lay in understanding what was happening in terms of elasticity and the stiffness of the tandem wing spars. If the air load was applied to two identical spars at a position exactly midway between the spars, the load will be equally shared and they will be both deflect in equal amounts. If the load was applied at any other position there would be an unequal share in loading between the spars and consequently unequal amounts of delflection. The most heavily loaded spar would deflect the most and cause the wing to twist. The position of loading that causes no twisting is known as the flexural centre of the wing. The theory of elasticity shows that every beam or system of beams has an associated flexural centre. If the wing had more than two spars or the spars were of different thicknesses the flexural centre would not correspond with the mid point but would lay at some other position relative to the chord line. This could cause greater or less twisting in addition to the normal bending deflection of wings during flight.

Naturally the aerodynamic forces of flight occur all over the wing of an aircraft keeping it up in the air but mathematically all forces can be considered to be acting at a single point. The position of this single point is known as the centre of pressure (CP) of the wing. It is well known that the CP is located at 25% of the chord behind the leading edge or the 'quarter chord' position. It follows that to avoid wing twisting the centre of flexure ought to be close as possible to the CP. The wing's resistance to failure by twisting is its torsional stiffness of the spars. The D8's wing torsional stiffness depended on the relative bending stiffness of the wing spars and the flexural centre lay near mid chord. This was a long way aft of the CP and the wing spars had insufficient torsional stiffness to resist twisting forces.

The modification of increasing the thickness of the rear spar had caused the flexural centre of the wing to move even further away from the CP introducing greater twisting of the wing. Once Fokker realised this he took the step of reducing the thickness of the rear spar thus moving the flexural centre of the wing closer to the CP. Once this was done the D8 was a much safer plane.

The wing torsional stiffness of D8 like for most early fabric covered planes relied on the internal structure of the main spars and ribs. Modern aircraft wings use a continuous covering of metal turning the wing into a large torsion box or tube to give it torsional strength. Because of this heavy skin a comparatively high proportion of the weight of the structure of a modern aircraft is devoted to resisting torsion.

05-11-2008, 09:29 AM	#11 (permalink)
stanmorek New Member Join Date: 04-26-08 Location: London Posts: 19 Country:	Running the risk of boring members of this board with a long post, I just wanted to mention the story of the German Fokker D8 an advanced monoplane fighter in 1917. I got this story from a good structures book of mine recounting how early aircraft designers had not recognised the effect of torsion on aircraft wings. Due to the critical tactical situation, the Germans issued the D8 to several fighter squadrons without adequate test flying. But under combat conditions when the D8 pulled out of a dive the wings came off. Much to the grave concern of the German air force authorities they were losing some of their best and most experienced pilots. The internal structure of the fabric covered wings were two wooden spars or cantilever beams projecting sideways from the fuselage and interconnected by a series of wooden ribs every few inches. Naturally the authorities ordered load testing of a complete aircraft by way of mounting it upside down in a test frame and loading up the wings with shot bags to simulate aerodynamic forces which occur during flight. During tests no weakness was observed until the wings failed at a loading 6 times the equivalent self weight of the aircraft. This load was considered to be higher than would occur during the worst combat conditions. In other words the D8 should have been perfectly safe. However, observations during testing saw failure begin in the rear spar. As a result the rear spars of all D8s were strengthened by thickening. Unfortunately, after this had been done, the number of accidents did not decrease but increased. The effect of adding more structural material to the wing had actually made it weaker. Unofficially, aircraft engineer Antony Fokker under his own supervision loaded up another D8. This time he took incremental measurements of wing deflections as the wings were loaded gradually. What he found was that the wing deflected in bending as it was loaded and that the wing tips would rise up relative to the fuselage. This was expected, but in addition he also discovered that the wings also twisted at the same time although no obviuous twisting loads were being applied. Importantly, the direction of twisting was such that the angle of attack of the wing was increased significantly. Fokker had discovered something called a 'divergent condition' which was very lethal to aircraft of mono wing design. He realised that when the pilot pulled back the control stick the nose rose increasing load on the wings. But at the same time the wings twisted and so air loads on the wing increased causing the wings to twist more and so on, until the wings were loaded disproportionately and the pilot no longer had control. The solution to the problem lay in understanding what was happening in terms of elasticity and the stiffness of the tandem wing spars. If the air load was applied to two identical spars at a position exactly midway between the spars, the load will be equally shared and they will be both deflect in equal amounts. If the load was applied at any other position there would be an unequal share in loading between the spars and consequently unequal amounts of delflection. The most heavily loaded spar would deflect the most and cause the wing to twist. The position of loading that causes no twisting is known as the flexural centre of the wing. The theory of elasticity shows that every beam or system of beams has an associated flexural centre. If the wing had more than two spars or the spars were of different thicknesses the flexural centre would not correspond with the mid point but would lay at some other position relative to the chord line. This could cause greater or less twisting in addition to the normal bending deflection of wings during flight. Naturally the aerodynamic forces of flight occur all over the wing of an aircraft keeping it up in the air but mathematically all forces can be considered to be acting at a single point. The position of this single point is known as the centre of pressure (CP) of the wing. It is well known that the CP is located at 25% of the chord behind the leading edge or the 'quarter chord' position. It follows that to avoid wing twisting the centre of flexure ought to be close as possible to the CP. The wing's resistance to failure by twisting is its torsional stiffness of the spars. The D8's wing torsional stiffness depended on the relative bending stiffness of the wing spars and the flexural centre lay near mid chord. This was a long way aft of the CP and the wing spars had insufficient torsional stiffness to resist twisting forces. The modification of increasing the thickness of the rear spar had caused the flexural centre of the wing to move even further away from the CP introducing greater twisting of the wing. Once Fokker realised this he took the step of reducing the thickness of the rear spar thus moving the flexural centre of the wing closer to the CP. Once this was done the D8 was a much safer plane. The wing torsional stiffness of D8 like for most early fabric covered planes relied on the internal structure of the main spars and ribs. Modern aircraft wings use a continuous covering of metal turning the wing into a large torsion box or tube to give it torsional strength. Because of this heavy skin a comparatively high proportion of the weight of the structure of a modern aircraft is devoted to resisting torsion.