Thrust
The propulsion is in the physics of drive technology and related topics such as biomechanics, the effective force acting in the direction of movement on the body or system.
Negative propulsion means braking force.
General:
- 2.1 In the case of floatation
- 2.2 at the support surface
- 2.3 In case of jet engines
Vehicle Technology
Vehicle resistance, resistance to draw
Vehicles are subject to two basic resistors: The air resistance and soil resistivity ( road or path resistance, rolling resistance of the wheels ), the friction losses inside the vehicle between the prime mover and the driven wheels are thereby the efficiency of the propulsion machinery attributed ( bearing friction resistance, engine and transmission resistors ), and separating the vehicle into the drive and the driven side, of which only the latter is considered.
The air resistance can be calculated from:
The air resistance is thus zero on the stationary vehicle. It increases with the square of the speed.
- Is 0.6 for a convertible, and about 0.25 for a modern passenger car, the old VW Beetle had 0.42, for a flatbed truck at 0.7 and 1.1 for a tractor-trailer. For passenger trains is expected approximately 0.10-0.20 per each intermediate cars, and 0.3-0.5, respectively for early car / locomotive and end cars.
- Is 1.5 m on a sports car at 10 sqm for a truck (4 m × 2.55 m according to traffic regulations ), in rail vehicles at 10-15 sqm (European standard: 4.30 m × 3.25 m maximum).
The soil resistance is caused by rolling friction and managers, and is composed of the frictional resistance between the tire and rim and track and the rolling resistance ( the resistance to deformation of the wheels and the road surface / rail) together.
- The rolling resistance is calculated from the rolling resistance coefficient cRo of 0.001 for the railroad and about 0.006 to 0.015 for car tires on asphalt, is around 1% for rail vehicles and passenger cars, but reached on bad roads typically 3-5%, in commercial vehicles significantly more.
- The frictional resistance is at Wi roles in general zero, and acts only in curves (slip), and during hard acceleration ( traction τ ) to the drive wheels, to about 10% of the maximum force is transmitted ( power circuit). It can thus be generally neglected with respect to the rolling resistance. Otherwise, however, on the slope and on slippery roads (strongly reduced soil resistance). Then the adhesion limit is exceeded quickly (especially when braking ) and other mechanical conditions prevail - is shown on the Kamm's circle
The soil resistance is total of rail vehicles with rigid axle, steering and lack of low static friction of steel on steel in cornering but quite relevant ( sheet resistance). Both resistors are part of the speed quite independent but proportional to the weight of the vehicle.
Additional resistors heard about the longitudinal slope resistance by the elevation of the road or the track body in the curve, flood resistance, water resistance when wet, which is a function of speed (see aquaplaning ) and others. For anfahrende vehicles and the increased rolling resistance due to the higher static friction is taken into account.
In the simplest form so the following applies:
Overall, the vehicle resistance of about 14% results in a 40-ton truck loaded at 80 km / h without load of 31%, and 4% for a 1800 -tonne freight train ( four-axis ) at the same pace. The contribution of the resistance is higher for empty vehicles, because a large part of the total resistance of the weight is independent of the much cheaper, because these values are relative to the necessary power per load weight while train.
Acceleration and performance
Now, the acceleration is, however, a question of the applied work, so the difference in energy, and in addition to the kinetic energy of the vehicle itself is also taken into account, the energy stored in the rotating wheels. This is speed dependent, ie increases with acceleration. To do this takes the concept of reduced mass:
Then for the system vehicle the Newtonian law in the form:
Because the vehicle but must be in addition to the balance of forces are also in the instantaneous equilibrium, and the jacking attacks in the vehicle center of gravity, but is mediated at the contact point with the ground, and have the drive in the axes of the wheels, and all of these forces with different lever arms against each other, is addition to the pure power of the engine, which controls the speed and the torque of the engine defining characteristic: This determines the pulse and thus the acceleration ability ( the impulse ).
And the power that is needed for velocity:
Wherein P is a variable which is dependent on the maximum speed in the cube. Therefore, the maximum speed is highly dependent on the input power, and the acceleration performance decreases with the square of the speed dramatically.
Aviation
In aviation, the air resistance is only of importance (except during takeoff and landing ). In air are in the general direction of movement, and drive force in a line, and because the flow velocity depends primarily on the speed of flight, and the resistance at the hull. The air resistance can be in a form of resistance, disconnect the parasitäreren resistance, and resistance induced by the buoyancy.
When floats
- In the simplest case of a balloon, the driving force is absent, the lift is generated by displacement (static lift ), and the tunneling is the air resistance exactly opposite: The balloon goes wherever the wind blows, and so fast, how fast the wind is blowing ( stable flow required), the motion base is formed only by the movement of the medium
- When an airship lift is also generated by the buoyancy. For low speeds, air resistance is calculated according to the linear law of resistance, is proportional to the speed. The air resistance is less of an airship from its Spantfläche ( front surface ), but depending on its volume, so the ratio of length to diameter. Optimum values are included. The propulsion is calculated directly from the driving force to the propeller gondolas minus air resistance.
On the support surface
When flying with wings is the dynamic buoyancy which produce the wings, the decisive factor. Engaging the support surface in the resistance is the total resistance of the aircraft. In order to relate the resistance of the fuselage to the wing in which one determines the equilibrium of forces, one introduces a harmful area that the area of a square plate ( with a drag coefficient of 1.2 ) with the same resistance as the non- lift-producing parts of the aircraft complies, and is mounted to imagine the pressure point of the profile. This value is the wings just slammed shut.
Important quantity in calculating the balance of forces is the angle of attack.
- A glider is in a free of external forces of equilibrium: it "falls", but not in ( almost ) free fall, but gliding along the resultant of the forces of weight, drag and lift (the latter two together are called the air force). In this case, a balance between speed and airspeed case in direction of advance is one that requires the angle of attack, and thus the propulsion. As in the free case there is but one - gleitwinkelabhängige - limit speed at which the air resistance is too high to provide more free propulsion.
The latter two values are not related to the face, but the bearing surface of the blade at the pivot point, and depending on the angle α. They are typical of the wing geometry and are given in the polar diagram by Otto Lilienthal.
- For propeller-driven flight vehicles, the situation corresponds to a glider with an additional driving force. This is on the rotor blades is the same physical process instead of the wings, but here is the buoyancy of the rotor faces the drive ( the screwing ), and the driving of the rotor blade is an inhibiting force that overcome on the distance from the propeller center line of the torque of the engine must be. The screwing, pulls the aircraft forward (or pushes ), then apply an additional force vector as the center of rotation, and can accelerate the flying object about the slip limit speed addition.
Complicated the relationships are in the turn, because here weight and buoyancy no longer stand in a line: the plane tilts, until a balance between weight, buoyancy and speed-dependent centrifugal force are reached. Overall, the buoyancy is low, and therefore the curve flight more drive required. If the speed is too low, " lubricates " the plane sideways to curve Mitte.
The missile is in this calculation but not yet at the moment of balance, as the total resistance in the attack of the aircraft center of gravity, not that of the wing profile, and the driving force in the engine centerline axis. This can only be by the design of the machine to compensate, and therefore high-wing aircraft in unpowered flight are less stable because the resulting torque pulls the aircraft to descend. In modern machines, the moments of the engine nacelles and the fuselage around the pivot point of the wings of the same.
In jet engines
In reaction drive is called the driving force of the thrust (English propulsion ).
- For jets the same aerodynamic conditions as for propeller driven objects apply. For large commercial machines as well as in fast military aircraft of the air resistance of the fuselage hardly plays a role.
But shortly before reaching the sound barrier, the flow resistance coefficient increases strongly, but decreases again in supersonic flight. In these areas the Mach indicator ( speed through the speed of sound ) is an important parameter. The drag coefficient increases with in sometimes multiple values , and approaches for back to a stable value, which is near the subsonaren value.
- Rockets do not generate lift. The air resistance turns out to be the lowest when the rocket has roughly the shape of an elongated triangle, because she "rides " on the ( also laterally expanding ) exhaust jet, and there is no Sogwiderstand at the rear. The tunnel is almost exactly equal to the thrust. Allfähige wings are used mostly as flight stabilizers which prevent the missile rotates about the longitudinal axis, or begins to spin.
For all further calculations for aircraft and missiles in particular is to be noted however that the fuel makes up a good - until the missile mass majority, and therefore, the mass can not be regarded as a constant.
- When driving in a vacuum there is no more friction here without restriction applies the Rocket Equation:
The force of gravity is not zero, there is also everywhere in space gravity. Weightlessness occurs only when a stable orbit. Far away from the larger masses, the gravitational force so small that almost nurmehr the tunneling effect, and can create gravity- like conditions ( simple " artificial gravity " ) by driving (ie acceleration of the spacecraft ). At a constant speed, there is almost zero gravity in the spacecraft.