Controllable pitch propeller
Variable-pitch propellers are propellers in which the pitch angle of the propeller blades is variable, so that the pitch angle of the blades can be adapted to different operating situations. Variable-pitch propellers are used as propellers and ship propellers.
aviation
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The variable pitch propeller used in aviation is a propeller whose pitch can be adjusted on the ground or during flight.
Variable pitch propellers in aviation go back to developments in the 1930s, when, due to the increasing speed range in aircraft, the fixed pitch propeller increasingly appeared to be too limited in its performance. The propellers that were initially used, which were adjustable on the ground, were unsatisfactory, as they could only be adapted to the launch site or the load situation. Propellers, which could also be adjusted in flight, made it possible to adapt the propeller to the ambient conditions during take-off and also during cruise. The great advantage of the variable pitch propeller is the possibility of adapting the pitch of the propeller to the actual speed, comparable to a continuously variable transmission in an automobile ( CVT transmission). The problem of starting before the variable pitch propeller era is roughly comparable to trying to start a car in fifth gear. That is why, to a certain extent, "third gear" was selected for aircraft and a lower top speed was accepted.
With the introduction of the controllable pitch propeller, the performance data of the aircraft has made great progress: A Boeing 247 with controllable pitch propeller needed 20% less take-off distance with the same engine power, had a 22% better climb rate and a 5.5% higher cruising speed. In addition, the safety of multi-engine aircraft increased, because if the engine failed, the corresponding propeller could be brought into feathered mode. As a result, the engine no longer forcibly rotated, which could previously lead to the crankshaft breaking out of the engine housing, and the air resistance was significantly reduced - the remaining engines needed less power to keep the aircraft in the air. Variable-pitch propellers with sail position can also be used on touring motor gliders in order to achieve lower air resistance in motorless flight.
In principle, automatic or manual adjustment is possible. The controllable pitch propeller can also enable a stepless change in the pitch or only certain steps. The highest level of development of the variable pitch is the constant speed propeller ( constant speed propeller ) in which the motor speed remains constant and the thrust force is only controlled by the adjustment of the propeller blades. This offers the advantage of having the full engine power available immediately if necessary, without the need to accelerate masses, and a reverse thrust is also possible, as well as a feathering position that offers the lowest possible air resistance when the engine is at a standstill. This goes so far that model airplanes can be stopped in the air during 3D flying and briefly flown backwards.
advantage
More thrust during take-off and climb, less stress on the machine, resulting in significantly less fuel consumption. The ability to set the sails in the event of a defective machine and reverse thrust (for braking when landing and maneuvering on the airfield) make modern flight operations with fan guns possible.
disadvantage
Additional maintenance and control effort, but ultimately only slow small or training aircraft and nostalgic machines without controllable pitch propellers are operated among the propeller-driven aircraft. Even higher-powered versions of the standard Cessna 172 and Piper PA-28 aircraft have controllable pitch propellers, and recently even individual ultralight aircraft (e.g. Flight Design ).
Propeller adjustment lever
The propeller adjustment lever is a lever as part of a mechanical, electrical or hydraulic system for the manual adjustment of the setting angle of the propeller of aircraft with an adjustable propeller. This allows in different flight situations such. B. when taking off, climbing and cruising or when landing, the angle of attack of the propeller is changed and thus, in conjunction with the engine speed, optimal use of the propeller power can be achieved.
Basics of propeller adjustment
Pulling or pushing the propeller adjusting lever changes the setting angle of the propeller at the hub, which means that the profile of the propeller blade assumes a different angle of attack in relation to the incoming air. When stationary, the angle of attack is equal to the given angle of attack. The air flowing through the propeller circle is accelerated backwards. As a result, the accelerating propeller blade, and thus the aircraft connected to it via the crankshaft, is accelerated in the opposite direction, i.e. forwards, according to the reaction principle.
With increasing speed, e.g. B. when cruising, the angle of attack of the air flowing onto the propeller decreases. This reduces the lift that can be generated by the blade in the form of propeller thrust and drag, the speed of the engine increases with the same throttle position and fuel consumption increases. In order to achieve the optimal propeller advance again at a lower speed in this situation, the setting angle of the propeller is adjusted according to the specifications in the flight manual of the respective model.
The adjustable angle depends on the aircraft type and can be set from the specified setting angle up to + 90 ° (sail position) or also to negative values, which can be used to reverse thrust . The propeller position is specified here as the blade angle at 3/4 radius of the propeller.
Mode of action
The blades of the variable pitch propeller are rotatably built into the hub via ball, roller or needle bearings. The components necessary for the adjustment are accommodated in the front part of the hub, including an adjusting piston and an adjusting cylinder, either piston or cylinder being movable. One possible structure is the Hamilton standard propeller design. The propeller blade sits on a bevel gear, which in turn engages with another bevel gear that is perpendicular to it. This in turn has a control cam which is embedded in a control cam within the bevel gear. By actuating the propeller adjusting lever, oil is pumped into or out of the adjusting cylinder and the pressure is exerted on an adjusting piston, which in turn pushes the control cam connected to it over the control cam in the bevel gear and thus rotates it. The oil required for this is taken from the engine lubricant circuit and fed to the adjusting cylinder through a control valve. If the pressure generated in this way is not sufficient, additional pressure can be generated using an additional pump.
Nowadays, hydraulic controllable pitch propellers are almost exclusively found in aircraft.
The propellers that were initially only used manually on the ground (before take-off) were unsatisfactory, as only a one-time (static) adjustment was possible here.
shipping
In contrast to the conventional propeller with a fixed pitch, which today is usually made from a single cast, the propeller blades of the controllable pitch propeller are rotatably attached to the hub . This means that the pitch can be continuously adjusted from zero thrust towards forward or backward. Variable-pitch propellers are used in units that require good maneuverability , generator operation and / or very different continuous speeds, e.g. B. Ferries , passenger ships , feeders .
history
Variable pitch propellers have existed since at least around 1850. However, their complex mechanical construction stood in the way of their success for a long time. The principle of the modern controllable pitch propeller goes back to the construction of water turbines with adjustable blades in Kaplan design. The Swiss company Escher-Wyss brought out the first modern-design variable pitch propeller in 1934 - the Etzel , which operates on Lake Zurich , was the first ship to be equipped with it. In the mid-1930s, the engineer Elov Englesson of the Swedish company KaMeWa also worked on the fundamentals for the technical implementation of hydraulically controllable controllable pitch propellers and in 1937 tested the Rane motor glider with a prototype. After further individual constructions in the tanker Dalanäs (1939) and the tug Herkules (1940), a series of 20 minesweepers for the Swedish Navy followed in the early 1940s. In 1944, the first seagoing ship with a controllable pitch propeller, the general cargo ship Suecia of the Swedish shipping company Nordstjernan, started up with a controllable pitch propeller system from KaMeWa.
advantages
The drive can be switched from “ahead” to “back” while the ship's engine is running , which is associated with considerable time savings, as the machine no longer has to be stopped and also does not have to be run down to minimum speed. This significantly improves maneuverability.
The machine is started at zero thrust and run up to cruising speed, it is not additionally loaded by the drive torque when starting . The vehicle does not immediately start moving when the engine is started. The propeller, which is set to zero thrust, prevents the shaft and therefore the engine from spinning due to the current (e.g. ships passing in port). Ships with controllable pitch propellers do not have a reversing gear , at most a reduction gear for high-speed engines. This eliminates a major weak point in the drive system. The efficiency is more favorable at different speeds than in the case of a fixed pitch propeller.
disadvantage
A controllable pitch propeller is mechanically much more complex and expensive to manufacture and at the same time less robust than a propeller with a fixed arrangement of the propeller blades. The adjustment mechanism requires a certain amount of maintenance and is an additional system that can fail. The power transmission is weakened to a certain extent due to the multi-part construction of propeller blades and a separate hub. The energetic efficiency of the propulsion is good over large parts of the adjustment range, but not as optimal as the efficiency of a fixed pitch propeller that works at its nominal torque.
business
In the case of large ship propellers, the adjustment takes place hydraulically, the necessary lines are guided axially through the shaft. Small systems can also be adjusted mechanically. The motor is driven to its nominal speed without load (wing position zero). Once this has been reached, the propeller can be used for maneuvering. Generator operation is possible at constant speed. In the case of so-called combinator operation, the load control follows the speed / gradient characteristics that are parallel over a wide range.
Related systems
A technical equivalence to controllable pitch propellers can be found in hydropower with Kaplan turbines ; these allow a similar adjustment of the turbine blades. If this setting option is missing, they are referred to as propeller turbines.
Modern wind turbines have variable angle of incidence of the rotor blades in order, on the one hand, to deliver a constant nominal power by changing the speed in different wind conditions and, on the other hand, to enable power limitation without stalling .
Web links
Individual evidence
- ↑ Winfried Kassera: Motorflug compact . 1st edition 2015. Motorbuch Verlag, Stuttgart, ISBN 978-3-613-03443-3 , pp. 82 ff .
- ↑ Dieter Meschede (Ed.): Gerthsen Physik . 25th edition. Springer Spectrum, Berlin / Heidelberg, ISBN 978-3-662-45977-5 , p. 25 .
- ↑ Winfried Kassera: Flight without a motor . 1st edition 2013. Motorbuch Verlag, Stuttgart, ISBN 978-3-613-03574-4 , pp. 76 .
- ↑ Rossow, Wolf, Horst (Ed.): Handbuch der Luftfahrtzeugtechnik . 1st edition 2014. Carl Hanser Verlag, Munich, ISBN 978-3-446-42341-1 , pp. 477 .
- ↑ Eugen Greiner: Construction and operation of controllable pitch propellers . In: Yearbook of the Shipbuilding Society . tape 47 , 1953, ISSN 0374-1222 , p. 151-166 .
- ^ The Motor Ship Reference Book . 19th edition. Temple Press, London 1950, p. 24 .
- ^ Andrew Rice: 80 years of revolution . In: Indepth . tape 31 . Connect Publications, Paisley September 2017, pp. 22/23 .