Hydromatic Propellers |
Basic Operation Principles : |
Piston movement causes rotation of cam which incorporates a bevel gear (Hamilton Standard Propeller) . The oil forces which act upon the piston are controled by the governor |
Single Acting Propeller: The governor directs its pump output against the inboard side of piston only, A single acting propeller uses a single acting governor. This type of propeller makes use of three forces during constant speed operation , the blades centrifugal twisting moment and this force tends at all times to move the blades toward low pitch , oil at engine pressure applied against the outboard side of the propeller piston and this force to supplement the centrifugal twisting moment toward the low pitch during constant speed operation., and oil from governor pressure applied against the inboard side of the piston . The oil pressure from governor was boosted from the engine oil supply by governor pump and the force is controlled by metering the high pressure oil to or draining it from the inboard side of the propeller piston which balances centrifugal twisting moment and oil at the engine pressure. |
Double Acting Propeller: The governor directs its output either side of the piston as the operating condition required. Double acting propeller uses double acting governor. This type of propeller , the governor pump output oil is directed by the governor to either side of the propeller piston. |
Principle Operation of Double Acting : |
The Feathering System |
Feathering : For some basic model consists of a feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light. The propeller is feathered by moving the control in the cockpit against the low speed stop. This causes the pilot vave lift rod in the governor to hold the pilot valve in the decrease r.p.m. position regardless of the action of the governor flyweights. This causes the propeller blades to rotate through high pitch to the feathering position. |
Some model is initiated by depressing the feathering button. This action, auxiliary pump, feather solinoid, which positions the feathering valve to tranfer oil to feathering the propeller. When the propeller has been fully feathered, oil pressure will buildup and operate a pressure cutout switch which will cause the auxiliary pump stop. Feathering may be also be accomplished by pulling the engine emergency shutdown handle or switch to the shutdown position. |
Unfeathering : Some model is accomblished by holding the feathering buttn switch in the out position for about 2 second . This creates an artificial underspeed condition at the governor and causes high-pressure oil from the feathering pump to be directed to the rear of the propeller piston. As soon as the piston has moved inward a short distance, the blades will have sufficient angle to start rotation of the engine. When this occurs , the un-feathering switch can be released and the governor will resume control of the propeller. |
Aviation notes
Wednesday, June 20, 2018
Aircraft Propellers | Control and Operations (Part 3)
Aircraft Propellers | Control and Operations (Part 2)
Governor Operation Condition |
On-Speed Condition |
The pressure oil from the pump is relieved through the relief valve (6). Because the propeller counterweight (15) force toward high pitch is balanced by the oil force from cylinder (14) is prevented from moving, and the propeller does not chang pitch |
Under-Speed Condition |
Over-Speed Condition |
Flight Operation |
Takeoff : Placing the governor control in the full forward position . This position is setting the propeller blades to low pitch angle Engine r.p.m. will increase until it reaches the takeoff r.p.m. for which the governor has been set. From this setting , the r.p.m. will be held constant by the governor, which means that full power is available during takeoff and climb. |
Aircraft Propellers | Control and Operations (Part 1)
AIRCRAFT PROPELLER CONTROL AND OPERATION |
Control and Operation |
Propeller Control |
basic requirement: For flight operation, an engine is demanded to deliver power within a relatively narrow band of operating rotation speeds. During flight, the speed-sensitive governor of the propeller automatically controls the blade angle as required to maintain a constant r.p.m. of the engine. |
Fundamental Forces : Three fundamental forces are used to control blade angle . These forces are: |
Constant Speed, Counterweight Propellers |
Governor Operation (Constant speed with counterweight ) the Governor supplies and controls the flow of oil to and from the propeller. The engine driven governor receives oil from the engine lubricating system and boost its pressure to that required to operate the pitch-changing mechanism. It consists essentially of : |
Principles of Operation (Constant Speed with Counterweight Propellers) |
Aircraft Propellers | Type of Propellers
Aircraft Propellers | General Information
Monday, June 18, 2018
Introduction to Helicopters
A
helicopter can be defined as any flying machine using rotating wings (i.e.,
rotors) to provide lift, propulsion, and control forces that enable the
aircraft to hover relative to the ground without forward flight speed to
generate these forces. The thrust on the rotors is generated by the aerodynamic
lift forces created on the spinning blades. To turn the rotor, power from an
engine must be transmitted to the rotor shaft. It is the relatively low-amount
of power required to lift the machine compared to other vertical take off and
landing (VTOL) aircraft that makes the helicopter unique. Efficient hovering
flight with low power requirements comes about by accelerating a large mass of
air at a relatively low velocity: hence we have the large diameter rotors that
are one obvious characteristic of helicopters. In addition, the helicopter must
be able to fly forward, climb, cruise at speed, and then descend and come back
into a hover for landing. This demanding flight capability comes at a price,
including mechanical and aerodynamic complexity and higher power requirements than
for a fixed-wing aircraft of the same gross weight. All of these factors
influence the design, acquisition, and operational costs of the helicopter
Besides generating all of the vertical lift, the rotor is also the primary source of control and propulsion for the helicopter, where as these functions arc separated on a fixed-wing aircraft. For forward flight, the rotor disk plane must be tilted so that the rotor thrust vector is inclined forward to provide a propulsive component to overcome rotor and airframe drag. The orientation of the rotor disk to the flow also provides the forces and moments to control the attitude and position of the aircraft .The pilot controls the magnitude and direction of the rotor thrust vector by changing the blade pitch angles (using collective and cyclic pitch inputs), which changes the blade lift and the distribution of thrust over the rotor disk. By incorporating articulation into the rotor design through the use of mechanical flapping and lead/lag hinges that are situated near the root of each blade, the rotor disk can be tilted in any direction in response to these blade pitch inputs. As the helicopter begins to move into forward flight, the blades on the side of the rotor disk that advance into the relative wind will experience a higher dynamic pressure and lilt than the blades on the retreating side of the disk, and so asymmetric aerodynamic forces and moments will be produced on the rotor. Articulation helps allow the blades to naturally flap and lag so as to help balance out these asymmetric aerodynamic effects. However, the mechanical complexity of the rotor hub required allowing for articulation and pitch control leads to high design and maintenance costs. With the inherently asymmetric flow environment and the flapping and pitching blades, the aerodynamics of the rotor become relatively complicated and lead to unsteady forces. These forces are transmitted from the rotor to the airframe and can be a source of vibrations, resulting in not only crew and passenger discomfort, but also considerably reduced airframe component lives and higher maintenance costs. However, with a thorough knowledge of the aerodynamics and careful design, all these adverse factors can be minimized or overcome to produce a highly reliable and versatile aircraft.
Helicopters come in many sizes and shapes, but most share the same major components. This component include a cabin where the payload and crew are carried; an airframe, which houses the various components, or where components are attached; a power plant or transmission, which, among other engine; and things, takes the power from the engine and transmits it to the main rotor, which provides the aerodynamic forces that make the helicopter fly. Then, to keep the helicopter from turning due to torque, there must be some type of ant torque system. Finally there is the landing gear, which could be skids, wheels, skis, or floats.
Besides generating all of the vertical lift, the rotor is also the primary source of control and propulsion for the helicopter, where as these functions arc separated on a fixed-wing aircraft. For forward flight, the rotor disk plane must be tilted so that the rotor thrust vector is inclined forward to provide a propulsive component to overcome rotor and airframe drag. The orientation of the rotor disk to the flow also provides the forces and moments to control the attitude and position of the aircraft .The pilot controls the magnitude and direction of the rotor thrust vector by changing the blade pitch angles (using collective and cyclic pitch inputs), which changes the blade lift and the distribution of thrust over the rotor disk. By incorporating articulation into the rotor design through the use of mechanical flapping and lead/lag hinges that are situated near the root of each blade, the rotor disk can be tilted in any direction in response to these blade pitch inputs. As the helicopter begins to move into forward flight, the blades on the side of the rotor disk that advance into the relative wind will experience a higher dynamic pressure and lilt than the blades on the retreating side of the disk, and so asymmetric aerodynamic forces and moments will be produced on the rotor. Articulation helps allow the blades to naturally flap and lag so as to help balance out these asymmetric aerodynamic effects. However, the mechanical complexity of the rotor hub required allowing for articulation and pitch control leads to high design and maintenance costs. With the inherently asymmetric flow environment and the flapping and pitching blades, the aerodynamics of the rotor become relatively complicated and lead to unsteady forces. These forces are transmitted from the rotor to the airframe and can be a source of vibrations, resulting in not only crew and passenger discomfort, but also considerably reduced airframe component lives and higher maintenance costs. However, with a thorough knowledge of the aerodynamics and careful design, all these adverse factors can be minimized or overcome to produce a highly reliable and versatile aircraft.
Helicopters come in many sizes and shapes, but most share the same major components. This component include a cabin where the payload and crew are carried; an airframe, which houses the various components, or where components are attached; a power plant or transmission, which, among other engine; and things, takes the power from the engine and transmits it to the main rotor, which provides the aerodynamic forces that make the helicopter fly. Then, to keep the helicopter from turning due to torque, there must be some type of ant torque system. Finally there is the landing gear, which could be skids, wheels, skis, or floats.
The major components of a helicopter are the Cabin, airframe, landing gear, power plant, transmission, main Rotor system and tail rotor system. |
The Fundamental Technical Problems in Early Attempts at Vertical Flight
Six fundamental technical problems can be
identified that limited early experiments with helicopters. These problems are
expounded by Sikorsky (1938, and various editions) these problems were:
1.Understanding
the aerodynamics of vertical flight. The theoretical power required to produce a fixed
amount of lift was an unknown quantity to the earliest experimenters, who were
guided more by intuition than by science.'
2.The lack
of a suitable engine. This was a problem that was not to be overcome until the beginning of
the twentieth century, through the development of internal combustion engines.
3.Keeping
structural weight and engine weight down so the machine could lift a pilot and
a payload.
Early power plants were made of cast iron and were heavy.
4.Counteracting
rotor torque reaction. A tail rotor was not used on most early designs: these machines were
either coaxial or laterally side-by-side rotor configurations. Yet, building
and controlling two rotors was even more difficult than for one rotor.
5.Providing
stability and properly controlling the machine, including a means of defeating
the unequal lift produced on the advancing and retreating blades in forward
flight. These
were problems that were only to be fully overcome with the use of blade
articulation, ideas that were pioneered by Cierva, Breguet, and others, and
with the development of blade cyclic pitch control.
6.Conquering
the problem of vibrations. This was a source of many mechanical failures of the rotor and
airframe, because of an insufficient understanding of the dynamic and
aerodynamic behavior of rotating wings.
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