By Friedrich Budig, Berlin-Grünau.
The basic features of the Budig-Lekht aircraft (Plate IV) are reminiscent of the design of the brothers W. and O. Wright. In Europe, the design was first adopted by Voisin and Farman
and further developed through the application of Penaud stabilization. The aircraft's wings were arranged more staggered than necessary, because the updraft and downdraft in front of and behind the wings were not taken into account at the time. As a result, the tailplanes were subjected to far too much stress, and
they caused considerable drag. In addition, under the influence of the propeller and atmospheric turbulence, disturbances in the tailplane's air force were bound to occur, making flight unstable and often endangering it.
Even before the aforementioned problems with the canard aircraft had been resolved, the use of today's standard type was adopted. Over time, it became clear that loaded tailplanes were a disadvantage for this type. This realization was promoted by the frequent use of adjustable tailplanes.
The tail surfaces are said to be loaded when they are set at a greater or lesser angle of attack to the wind direction and produce a greater or lesser aerodynamic force in the airflow.
During the war, I had the opportunity to observe the shape of the disturbances on tail surfaces subjected to varying degrees of load while conducting fin pressure measurements in flight using liquid manometers. The pressure measurements carried out at various longitudinal positions showed that as the angle of incidence of the wings decreases,
an increase in the downward air force on the rear horizontal stabilizer occurs, which satisfies static stability. Unsatisfactory changes in the air force were revealed by measurements in gusty weather. It was shown that gusts striking from the side greatly increase the air force exerted on one side of the horizontal stabilizer and decrease it on the other.
The force impulse of this disturbance is directly proportional to the aerodynamic load on the tail surface. The lateral tilting force detected on the loaded tail fin is distributed over the entire surface of the fin and is more severe than the tilting force measured on the wings; the former occurs simultaneously and in the same direction as the latter. I have already reported on tilting forces on the wings in the Zeitschrift für Motorluftschiffahrt, Issue 16 of August 28, 1925. It was demonstrated there that tilting forces on the wing are essentially limited to the trailing edges of the wings. Lateral stability is therefore most effectively maintained by deflecting using the trailing edges of the wings.
In addition to the lateral tilting forces detected on the loaded horizontal stabilizer, influences were observed on the pressure gauge before landing in gusty weather, which indicated that loaded stabilizers make landing more difficult for the pilot. If, during flight at a high angle of attack, a gust of wind increases the lifting force on the wing, the downward air force on the horizontal stabilizer also increases. The wing is thus lifted by the gust, and the horizontal stabilizer is lowered by it. The automatic forward tilt of the aircraft associated with this movement poses a danger during landing and demands the pilot's skill.
These results, obtained from air force measurements, prompted the pilot to eliminate the identified disturbances by completely unloading the horizontal stabilizer.
Towards the end of the war, I adjusted the tail of a Rumpler CV aircraft to match the airflow angle behind the wings. The correct setting was tested with the help of air pressure measurements until the position in which the horizontal stabilizer was unloaded was found. The aircraft was rebalanced by loading ballast into the tail of the fuselage. During the climb, the aircraft then rested on the control surfaces without pressure. In horizontal flight, with the wings' angle of incidence decreasing,
a nose-heaviness became noticeable, which had to be compensated for by pulling on the elevator. Pressure measurements taken on the horizontal stabilizer during this period showed that the increase in downforce previously observed on the loaded horizontal stabilizer in the same case had not reappeared, meaning that the unloaded horizontal stabilizer had not exerted any stabilizing effect. When gusts of wind hit during horizontal flight, the aircraft became unstable and teetered forward.
The final result of these experimental stability studies, promoted by Dr. Rumpier during the war, was that loaded tailplanes fully met static stability requirements, but inadequate dynamic stability. Completely unloaded tailplanes, by reacting passively to air turbulence, contribute to improving lateral stability and landing capability, but do not meet the static stability requirements, nor do they satisfy all of them.
In order to determine the actual load on the horizontal stabilizer in flight, it is necessary to examine the horizontal stabilizer using a pressure gauge. In the Journal of Flight Technology and Motor Aircraft, Issue 15, dated August 14, 1925, Mr. H. B. Helmbold shows how the tail assembly arrangement can be determined by calculating the downwash.
The experiences gained in flight were taken into account in the design of the Budig-Lekht aircraft. The moment compensation of the
pressure center shift is achieved, as far as static stability is concerned, in this aircraft by means of a self-acting stabilizing surface. This makes it possible to completely relieve the rear tail assembly in every flight condition.
The stabilizing surface is arranged in front of the wings. In slow flight, the stabilizer has little bearing and is almost in the direction of inclination of the updraft. As the flight speed increases, the profile of the stabilizing surface is adjusted by the airstream, so that a significant lifting force is exerted on the stabilizing surface. Fig. 1 shows the profile cross-section of the stabilizing surface at slow, normal, and fast flight speeds.
A characteristic of far-reaching importance of the new stabilization method is that, in addition to the static stabilization of the aircraft by changing the shape of the stabilizing surface, the aircraft automatically adjusts the angle of pitch between the wing and the stabilizing surface to suit the current flight condition.
This circumstance influences flight in a turbulent atmosphere, i.e., the dynamic stability of the aircraft, and is substantiated by the following test results.
If the aircraft's aerodynamic arrangement, shown in Fig. 2, experiences an increase in lift forces due to gusts during slow flight, the lift force on the wing is increased more strongly by the gust due to the higher load per unit area, and the wing is therefore lifted faster than the stabilizing surface, which has a lower load per unit area. The aircraft therefore pitches forward, gaining altitude, with the final movement being dampened by the unloaded tail fin.
By pitching forward, the aircraft transitions to the fast flight condition and remains in equilibrium in the new attitude because the stabilizing surface has meanwhile assumed the shape appropriate for this flight condition. With its now slender profile shape, the stabilizing surface receives slightly more lift per unit area due to the rapid movement than the wing unit. The aerodynamic forces encountered by the aircraft during flight due to the moving atmosphere must therefore influence the aerodynamic forces on the aircraft differently than in the case shown in Fig. 2. The gusts, which increase the lift forces on the unloaded elevator, now increase the lift force on the stabilizing surface per unit area slightly more than the unit area of the wing, so that a forward pitching during fast flight due to the gust is prevented. Thus, in flight, the aircraft, at a small angle of attack, remains firmly in a longitudinal position against gusts.
Reductions in the lift forces on the wings and stabilizing system due to gusts are initially noticeable in every flight condition by a sinking of these surfaces.
The sink rate of the surfaces is closely related to the decrease in lift force, which is why the sink rate of the unloaded horizontal stabilizer remains the lowest. The rear-mounted stabilizer therefore causes the aircraft to pitch forward and accelerates it, while the stabilizing surface is shaped by the airstream to match the resulting flight condition.
The way I describe the gust attack in flight is the prerequisite for the usability of the stabilization system. Extensive observations of the gust effect in flight using liquid manometers allow me to characterize the gust effect on the wing simply as an increase or decrease in the air forces encountered on the wing. On the wings, both the positive pressure generated by the aircraft's forward motion and the negative pressure with an increase in dynamic pressure in the gust are explosively increased. Only in one case, due to the influence of crosswind, does a deviation from this rule occur at the trailing edge of the wings and at the loaded, rear-mounted tail unit, as already shown. Effects such as those imagined as a result of periodic changes in the direction of the airstream under the influence of gusts have not been observed. All gusts manifest their effect as if they all arrived in the plane of travel of the aircraft. Using sheet metal discs located in the flight plane (so-called crosswind sensors), upwind, downwind, or crosswind can be measured outside the wing area.
The engine power was just sufficient for flight at a high angle of attack, and only the external wind supply could bring about flight at a low angle by lifting and pitching the aircraft forward. In the stalled flight condition, gusts did not cause the aircraft to crash, but rather brought it back to normal flight by lifting and pitching without the pilot's assistance. It is clear that this flight characteristic derives performance from wind fluctuations on the level, which increases with decreasing wing loading. Observations and measurements of this kind were the goal of the tests, which is why the low engine power had to be maintained. Almost universally, sports witnesses present at the flight tests expressed the opinion that the stalled flights in the wind represented acrobatic feats. Previous statements justify the view that the aircraft, and not the pilot's skill, ensured flight safety.
When using an automatic stabilization device, it should be possible to expect the same operational reliability from it as from a fixed tail assembly. The design of the Budig stabilization system fulfills this requirement. This device is characterized in that the stabilizing surface (1), as shown in Fig. 4, is hinged to a fixed surface (2) connected to an air suction device and is connected to it by a dense fabric in such a way that during flight, due to the suction force occurring at the gap (3) and the associated air thinning inside the suction chambers (4), the stabilizing surface (1) is moved against the fixed surface (2). This creates a lift-generating wing profile as the speed increases. As the speed decreases, the stabilizing surface (1) is moved upwards around the pivot point (5) by means of the spring (6) and the lifting force acting on the stabilizing surface. The lifting force acting on the stabilizing surface in addition to the spring (6) during flight ensures that the power source found in the airstream can exert its activity independently of the aircraft's altitude. The dense fabric between the movable stabilizing surface (1) and the stationary surface (2) collapses like a bellows as the speed increases.
Movement of the stabilizing surface (1) can only occur if air is either drawn into or drawn into the chamber (4). To ensure the effective supply of air to the chamber (4) as the speed decreases, compressed air is continuously supplied to the chamber through the opening (7). The amount of compressed air is determined by the size of the opening so that the long gap (3) can expel the incoming air during fast flight
while still maintaining the air rarefaction in the chamber (4). On the other hand, the influence of the opening (7) slows down the movement of the stabilizing surface as the speed increases. This is also desirable because it allows the stabilizing surface to retain its current shape, insensitive to the twitching of gusts. The stabilizing surface and wing, viewed together, present a rigid form to the gust attack at any moment, which, depending on the flight condition, is correctly adjusted, making the entire aircraft a gust sensor.
The shape of the stabilizing surface is visible to the aircraft's occupants in flight at the level of the rear bellows wall, thus also providing an advantage as a convenient indicator of the flight condition. Fig. 5 shows various positions of the bellows wall, photographed in flight.
In the new design (Plate IV), the stabilizing device is designed as a self-contained unit, with the hollow wing, equipped with an extended gap (3), connected by air ducts to the solid surface (2), forming a rigid bridge. This bridge is supported between two lightweight fuselages by four screws.
The stabilizing surface and the front section of the fuselage can be quickly removed from the aircraft, allowing it to be flown as a normal aircraft without the device. This option is provided in case the nose assembly breaks against an obstacle, preventing major damage. The stabilizing system, including the front fuselage and struts, weighs 18 kg, and this weight is supported in normal flight by the stabilizing surface, which, because it is favorably positioned in the updraft, causes little drag.
With the removal of the stabilizing system, the resulting flight characteristics are naturally lost. The rear elevator must then be loaded from above by air force for well-known stability reasons, or the pilot must compensate for the pressure fluid movement on the wing by adjusting the elevator on an unstable aircraft. During and after landing, the removal of the nose assembly would also compromise the aircraft's advantages.
The basic features of the Budig-Lekht aircraft (Plate IV) are reminiscent of the design of the brothers W. and O. Wright. In Europe, the design was first adopted by Voisin and Farman
and further developed through the application of Penaud stabilization. The aircraft's wings were arranged more staggered than necessary, because the updraft and downdraft in front of and behind the wings were not taken into account at the time. As a result, the tailplanes were subjected to far too much stress, and
they caused considerable drag. In addition, under the influence of the propeller and atmospheric turbulence, disturbances in the tailplane's air force were bound to occur, making flight unstable and often endangering it.
Even before the aforementioned problems with the canard aircraft had been resolved, the use of today's standard type was adopted. Over time, it became clear that loaded tailplanes were a disadvantage for this type. This realization was promoted by the frequent use of adjustable tailplanes.
The tail surfaces are said to be loaded when they are set at a greater or lesser angle of attack to the wind direction and produce a greater or lesser aerodynamic force in the airflow.
During the war, I had the opportunity to observe the shape of the disturbances on tail surfaces subjected to varying degrees of load while conducting fin pressure measurements in flight using liquid manometers. The pressure measurements carried out at various longitudinal positions showed that as the angle of incidence of the wings decreases,
an increase in the downward air force on the rear horizontal stabilizer occurs, which satisfies static stability. Unsatisfactory changes in the air force were revealed by measurements in gusty weather. It was shown that gusts striking from the side greatly increase the air force exerted on one side of the horizontal stabilizer and decrease it on the other.
The force impulse of this disturbance is directly proportional to the aerodynamic load on the tail surface. The lateral tilting force detected on the loaded tail fin is distributed over the entire surface of the fin and is more severe than the tilting force measured on the wings; the former occurs simultaneously and in the same direction as the latter. I have already reported on tilting forces on the wings in the Zeitschrift für Motorluftschiffahrt, Issue 16 of August 28, 1925. It was demonstrated there that tilting forces on the wing are essentially limited to the trailing edges of the wings. Lateral stability is therefore most effectively maintained by deflecting using the trailing edges of the wings.
In addition to the lateral tilting forces detected on the loaded horizontal stabilizer, influences were observed on the pressure gauge before landing in gusty weather, which indicated that loaded stabilizers make landing more difficult for the pilot. If, during flight at a high angle of attack, a gust of wind increases the lifting force on the wing, the downward air force on the horizontal stabilizer also increases. The wing is thus lifted by the gust, and the horizontal stabilizer is lowered by it. The automatic forward tilt of the aircraft associated with this movement poses a danger during landing and demands the pilot's skill.
These results, obtained from air force measurements, prompted the pilot to eliminate the identified disturbances by completely unloading the horizontal stabilizer.
Towards the end of the war, I adjusted the tail of a Rumpler CV aircraft to match the airflow angle behind the wings. The correct setting was tested with the help of air pressure measurements until the position in which the horizontal stabilizer was unloaded was found. The aircraft was rebalanced by loading ballast into the tail of the fuselage. During the climb, the aircraft then rested on the control surfaces without pressure. In horizontal flight, with the wings' angle of incidence decreasing,
a nose-heaviness became noticeable, which had to be compensated for by pulling on the elevator. Pressure measurements taken on the horizontal stabilizer during this period showed that the increase in downforce previously observed on the loaded horizontal stabilizer in the same case had not reappeared, meaning that the unloaded horizontal stabilizer had not exerted any stabilizing effect. When gusts of wind hit during horizontal flight, the aircraft became unstable and teetered forward.
The final result of these experimental stability studies, promoted by Dr. Rumpier during the war, was that loaded tailplanes fully met static stability requirements, but inadequate dynamic stability. Completely unloaded tailplanes, by reacting passively to air turbulence, contribute to improving lateral stability and landing capability, but do not meet the static stability requirements, nor do they satisfy all of them.
In order to determine the actual load on the horizontal stabilizer in flight, it is necessary to examine the horizontal stabilizer using a pressure gauge. In the Journal of Flight Technology and Motor Aircraft, Issue 15, dated August 14, 1925, Mr. H. B. Helmbold shows how the tail assembly arrangement can be determined by calculating the downwash.
The experiences gained in flight were taken into account in the design of the Budig-Lekht aircraft. The moment compensation of the
pressure center shift is achieved, as far as static stability is concerned, in this aircraft by means of a self-acting stabilizing surface. This makes it possible to completely relieve the rear tail assembly in every flight condition.
The stabilizing surface is arranged in front of the wings. In slow flight, the stabilizer has little bearing and is almost in the direction of inclination of the updraft. As the flight speed increases, the profile of the stabilizing surface is adjusted by the airstream, so that a significant lifting force is exerted on the stabilizing surface. Fig. 1 shows the profile cross-section of the stabilizing surface at slow, normal, and fast flight speeds.
A characteristic of far-reaching importance of the new stabilization method is that, in addition to the static stabilization of the aircraft by changing the shape of the stabilizing surface, the aircraft automatically adjusts the angle of pitch between the wing and the stabilizing surface to suit the current flight condition.
This circumstance influences flight in a turbulent atmosphere, i.e., the dynamic stability of the aircraft, and is substantiated by the following test results.
If the aircraft's aerodynamic arrangement, shown in Fig. 2, experiences an increase in lift forces due to gusts during slow flight, the lift force on the wing is increased more strongly by the gust due to the higher load per unit area, and the wing is therefore lifted faster than the stabilizing surface, which has a lower load per unit area. The aircraft therefore pitches forward, gaining altitude, with the final movement being dampened by the unloaded tail fin.
By pitching forward, the aircraft transitions to the fast flight condition and remains in equilibrium in the new attitude because the stabilizing surface has meanwhile assumed the shape appropriate for this flight condition. With its now slender profile shape, the stabilizing surface receives slightly more lift per unit area due to the rapid movement than the wing unit. The aerodynamic forces encountered by the aircraft during flight due to the moving atmosphere must therefore influence the aerodynamic forces on the aircraft differently than in the case shown in Fig. 2. The gusts, which increase the lift forces on the unloaded elevator, now increase the lift force on the stabilizing surface per unit area slightly more than the unit area of the wing, so that a forward pitching during fast flight due to the gust is prevented. Thus, in flight, the aircraft, at a small angle of attack, remains firmly in a longitudinal position against gusts.
Reductions in the lift forces on the wings and stabilizing system due to gusts are initially noticeable in every flight condition by a sinking of these surfaces.
The sink rate of the surfaces is closely related to the decrease in lift force, which is why the sink rate of the unloaded horizontal stabilizer remains the lowest. The rear-mounted stabilizer therefore causes the aircraft to pitch forward and accelerates it, while the stabilizing surface is shaped by the airstream to match the resulting flight condition.
The way I describe the gust attack in flight is the prerequisite for the usability of the stabilization system. Extensive observations of the gust effect in flight using liquid manometers allow me to characterize the gust effect on the wing simply as an increase or decrease in the air forces encountered on the wing. On the wings, both the positive pressure generated by the aircraft's forward motion and the negative pressure with an increase in dynamic pressure in the gust are explosively increased. Only in one case, due to the influence of crosswind, does a deviation from this rule occur at the trailing edge of the wings and at the loaded, rear-mounted tail unit, as already shown. Effects such as those imagined as a result of periodic changes in the direction of the airstream under the influence of gusts have not been observed. All gusts manifest their effect as if they all arrived in the plane of travel of the aircraft. Using sheet metal discs located in the flight plane (so-called crosswind sensors), upwind, downwind, or crosswind can be measured outside the wing area.
The engine power was just sufficient for flight at a high angle of attack, and only the external wind supply could bring about flight at a low angle by lifting and pitching the aircraft forward. In the stalled flight condition, gusts did not cause the aircraft to crash, but rather brought it back to normal flight by lifting and pitching without the pilot's assistance. It is clear that this flight characteristic derives performance from wind fluctuations on the level, which increases with decreasing wing loading. Observations and measurements of this kind were the goal of the tests, which is why the low engine power had to be maintained. Almost universally, sports witnesses present at the flight tests expressed the opinion that the stalled flights in the wind represented acrobatic feats. Previous statements justify the view that the aircraft, and not the pilot's skill, ensured flight safety.
When using an automatic stabilization device, it should be possible to expect the same operational reliability from it as from a fixed tail assembly. The design of the Budig stabilization system fulfills this requirement. This device is characterized in that the stabilizing surface (1), as shown in Fig. 4, is hinged to a fixed surface (2) connected to an air suction device and is connected to it by a dense fabric in such a way that during flight, due to the suction force occurring at the gap (3) and the associated air thinning inside the suction chambers (4), the stabilizing surface (1) is moved against the fixed surface (2). This creates a lift-generating wing profile as the speed increases. As the speed decreases, the stabilizing surface (1) is moved upwards around the pivot point (5) by means of the spring (6) and the lifting force acting on the stabilizing surface. The lifting force acting on the stabilizing surface in addition to the spring (6) during flight ensures that the power source found in the airstream can exert its activity independently of the aircraft's altitude. The dense fabric between the movable stabilizing surface (1) and the stationary surface (2) collapses like a bellows as the speed increases.
Movement of the stabilizing surface (1) can only occur if air is either drawn into or drawn into the chamber (4). To ensure the effective supply of air to the chamber (4) as the speed decreases, compressed air is continuously supplied to the chamber through the opening (7). The amount of compressed air is determined by the size of the opening so that the long gap (3) can expel the incoming air during fast flight
while still maintaining the air rarefaction in the chamber (4). On the other hand, the influence of the opening (7) slows down the movement of the stabilizing surface as the speed increases. This is also desirable because it allows the stabilizing surface to retain its current shape, insensitive to the twitching of gusts. The stabilizing surface and wing, viewed together, present a rigid form to the gust attack at any moment, which, depending on the flight condition, is correctly adjusted, making the entire aircraft a gust sensor.
The shape of the stabilizing surface is visible to the aircraft's occupants in flight at the level of the rear bellows wall, thus also providing an advantage as a convenient indicator of the flight condition. Fig. 5 shows various positions of the bellows wall, photographed in flight.
In the new design (Plate IV), the stabilizing device is designed as a self-contained unit, with the hollow wing, equipped with an extended gap (3), connected by air ducts to the solid surface (2), forming a rigid bridge. This bridge is supported between two lightweight fuselages by four screws.
The stabilizing surface and the front section of the fuselage can be quickly removed from the aircraft, allowing it to be flown as a normal aircraft without the device. This option is provided in case the nose assembly breaks against an obstacle, preventing major damage. The stabilizing system, including the front fuselage and struts, weighs 18 kg, and this weight is supported in normal flight by the stabilizing surface, which, because it is favorably positioned in the updraft, causes little drag.
With the removal of the stabilizing system, the resulting flight characteristics are naturally lost. The rear elevator must then be loaded from above by air force for well-known stability reasons, or the pilot must compensate for the pressure fluid movement on the wing by adjusting the elevator on an unstable aircraft. During and after landing, the removal of the nose assembly would also compromise the aircraft's advantages.
