I love the billion dollar blender.
Introduction
Helicopters, or as more accurately termed Rotary Wing Aircraft are (unless your are totally dense) radically different from conventional fixed wing aircraft. In a fixed wing aircraft, the movement of air over the surfaces of the wing is what produces the lift necessary for the airplane to remain in flight.
Simply put and in a perfect world, as long as sufficient velocity (and space to continue moving forward) exists the aircraft will remain airborne. There are of course trades being made at all times with respect to the flight of an aircraft.
These aircraft are particularly strange beasts as they “beat the air into submission.” In addition to the enormous torques generated by the revolution of the rotor head and tail there exist all of the normal problems associated with keeping a fixed wing aircraft aloft.
At the end of this node it is the hope of the author that, you, the reader, will come away with some understanding of the basic function of a rotary wing aircraft. What this material will not tackle is a history of the helicopter, as we are going to have a lot to discuss just about the way the things work.
Aerodynamics for Beginners
There are four basic forces acting on an airframe whenever it is in flight. These are:
Lift- generated by the wings and the force that keeps the airframe in the air.
Weight- gravity acting on the airframe and pulling it back toward the surface of the Earth. This, obviously, is the opposite of lift.
Thrust- the force generated by the motor, rotor system, propeller, or engine.
Drag- the resistance generated by the airframe while moving through the air.
Types of Drag:
Drag itself is broken down into several categories, including: skin drag (the resistance to airflow generated by the surface of the aircraft,) form drag (the resistance to airflow due to the shape of the object,) and parasitic drag (the resistance to airflow due to various antennas and other objects protruding from the airframe.) A good example of this would be the EP-3E. This is a military aircraft with a matte paint scheme (high skin drag,) with a blunt shape (high form drag) and an enormous number of antennas poking out at various places (high parasitic drag.)
Flight Axes
There are also three axes through which an aircraft moves, which are:
Pitch (Y)- The movement of the nose up and down where as the nose moves up the pitch angle increases. It would also bear to mention right now that this also affects a condition known as “Angle of Attack/AOA” or “Angle of Incidence/AOI.” (For our purposes and since I was taught as Angle of Attack, this is the term that we will use here.) This is movement of the aircraft around the lateral axis.
Angle of Attack:
AOA is defined as the movement of wind over the surface of the wing relative to the chord line of the wing. To find the chord line of a wing, start at the peak of the leading edge (front) and draw a line straight back to the tip of the trailing edge (back.) This theoretical line is the “chord line” of the wing. Angle of Attack is determined by looking at the angle of the air as it approaching the chord line.
a
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Lift
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a-Thrust- @--------------------------------------------------------------------------& -Drag-a
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Weight
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a
Crude ASCII Diagram of a Wing Cutaway,
(As viewed from wingtip toward fuselage)
@ = Leading Edge ------ = Chord Line & = Trailing Edge xxx = Wing Surface
a--- = Forces acting on a wing in flight.
Note that lift, weight, thust and drag are also occasionally
referred to as 'vectors.' When you have a Lift Vector greater
than your Weight Vector, the aircraft will rise. Similarly,
Thrust is greater than drag, the aircraft accelerates.
Understanding this is not nearly as difficult as it may seem. A simple experiment can be conducted using a household fan and your hand. Set the fan on ‘High’ and then place your hand and arm approximately eight inches in front of the guard, parallel to the floor and perpendicular to the flow of air. Now slowly rotate your hand so that the leading edge is higher than the trailing edge, just a little, perhaps fifteen degrees. This is called a positive Angle of Attack and the opposite would be a negative Angle of Attack.
Now you should notice that air is flowing over the upper and lower surfaces of your hand. Now increase the angle of your hand to the point where there is no more air moving over the top surface. This is called a stall, or where the air flow over one part of the flight surface is no longer moving in one cohesive direction parallel to the flow of air on the opposite side.
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>>>>>>xxxxxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>xxxxxxxx xxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>>>>>>>>>>>>
>>>>@--------------------------------------------------------------------------&>>>>>>>>>>>>>
>>>>>xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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AOA of Zero Degrees
Banal ASCII Diagram of a Wing Cutaway,
(As viewed from wingtip toward fuselage)
@ = Leading Edge ------ = Chord Line & = Trailing Edge xxx = Wing Surface
>>>> = Airflow over wing surfaces
Note that less air is flowing (at lower pressure) over the top
of the wing. This area of low pressure is what allows lift
to be generated and for the aircraft to rise into the sky.
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>>>>>>xxxxxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>
>>>>>>xxxxxxxx xxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>>>>>>>>>>
>>>>@--------------------------------------------------------------------------&>>>>>>>>>>>>>
>>>>>xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>>
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>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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>>>
>
Positive AOA
Inane ASCII Diagram of a Wing Cutaway,
(As viewed from wingtip toward fuselage)
@ = Leading Edge ------ = Chord Line & = Trailing Edge xxx = Wing Surface
>>>> = Airflow over wing surfaces
Note reduced airflow over top of wing.
~~~~~~~~~~~~~~~~~~~~~
~~~~~~xxxxxxxxxxxxxxxxxx~~~~~~~~~~~~~~~
>>>>>>xxxxxxxx xxxxxxxxxxxxxxx~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>xx xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>~~~~~~>>>>>>>>>>>
>>>>@--------------------------------------------------------------------------&>>>>>>>>>
>>>>>xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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Extreme Positive AOA
Blah ASCII blah blah blah blah blah
(As viewed from butt toward elbow)
@ = Leading Edge ------ = Chord Line & = Trailing Edge xxx = Wing Surface
>>>> = Airflow over wing surfaces ~~ = Area of disrupted airflow
Note that this would be a stall condition and that
the wing cannot generate lift.
Roll (X)- Movement of the aircraft about the longitudinal axis, meaning either left and right wing up or down. When an aircraft turns or ‘banks’ this would condition would be a use of the roll axis.
Yaw (Z)-Movement of the aircraft about the horizontal axis. This would be the equivalent of simply turning the nose of the airplane to a new direction without banking or rolling.
Some Parts
Helicopters differ from fixed wing airplanes in that the wing is actually rotating above your head the entire time. This is why from time to time you will hear helicopters referred to by their more technically accurate nomenclature of “Rotary Wing Aircraft.” Basic helicopter parts consist of a main rotor and a tail rotor used to overcome the torque generated by the turning main rotor.
Rotor System Types:
This does not tackle (in detail) different types of rotor systems, which would be counter-rotating versus more conventional rotor types.
A good example of a common counter-rotating design would be the CH-47 'Chinook'. This aircraft is a mainstay of Army logistics support, and is vaguely reminiscent of a banana and has one rotor head at the front and one at the rear.
Alternatively, a manufacturer named Kamov (Russian) utilizes a counter-rotating design whereby two rotors share the same central hub. These aircraft have no conventional tail rotor and look like a large box with two sets of main rotors, one stacked upon the other. (The Ka-29TB ‘Helix’ attack helicopter (see www.fas.org or www.google.com) built by Kamov is an outstanding example of a counter-rotating design.)
Where’s Your Head At?
Static and Dynamic Heads:
There are basically two main types of rotor head: static and dynamic. (These terms are relatively standard and what the U.S. military utilizes, which given my experience is why they are reproduced here.) Static heads are very uncommon due to their inherent complexity and difficulty in servicing. The Kaman (different company from Kamov) SH-2F/G (which the Royal Australian Navy employs,) uses a static head. These are designs where the rotor blade remains fixed and a small “flaperon” or similarly termed flight surface (mounted toward the end of the blade) moves to cause deflections in the aircraft’s attitude to occur.
Dynamic heads are far more common and involve a system where the entire blade moves with actuation of the flight controls. Dynamic heads can be seen on (my very favorite Sikorsky product) the H-60, Mil Mi-24 'Hind', and others.
The important distinction that is made here is that static head rotor blades are fixed in pitch and do not move, whereas with a dynamic head the entire blade moves.
Types of Dynamic Heads:
There are several different types of Dynamic rotor heads. These are: Rigid, Semi-Rigid, Semi-Articulated, Fully Articulated and a host of other slightly more esoteric forms. You are most likely to encounter Semi-Rigid types as these are the most common. You can find Semi-Rigid heads on the Bell Jet Ranger and almost all other two-bladed rotor systems. The distinction is made by the way that the rotor blades are mounted to the head and the way that they compensate for the forces acting on the blade in flight.
(Author's note: I apologize for the lack of information concerning mounting and other distinctions between these types. I chose not to pursue this further here as a w/u greater in length than this one is required to properly discuss this particular facet of rotary wing aviation.)
Putting the Pieces Together
Confused yet? Hope not, we still have swashplates to talk about. The swashplate is what allows a helicopter to function as it does, and to induce pitch in the rotor blades. We are going to for the purposes of simplification stick to dynamic heads and chuck all that business about static rotor heads from earlier clean out of the window.
Think of a donut. (If you do not know what a donut is, I encourage you to find out.) Now imagine if you were to cut the donut so that the inside ring was separate from the outside, and this cut was perfectly parallel to the inside and outside edges.
The inside of the donut-swashplate remains stationary and does not move. This is where the flight control servos or hydraulics are connected. The outside of the donut moves and is connected to the moving rotor blades by what appear to be metal rods running vertically and parallel to the central hub. These we will call pitch control rods.
In the center of the donut you have an item called the “Uniball” or “Hub.” This functions as a bearing and allows the donut to pivot around when moved by the flight controls. This in turn induces a pitch change in the outside of the donut and changes the angle of the blade affected by which side of the donut has risen and fallen according to the input provided to the flight controls.
The swashplate acts as a bearing that allows the parts of the rotor head so that the helicopter can change pitch and roll by moving one side up and the corresponding side down. Think of the blade as it revolves around the head. If the left side of the swashplate rises then the right side will fall.
As the blade you are looking at rotates to the right side of the aircraft the pitch control rod will move down and then cause the angle of the blade angle to (deflect upward) change and bite into the air. Again, as the blade rotates to the left the pitch control rod will rise and cause the blade angle to (deflect downward) and bite into the air there. Since the forward and aft parts of the swashplate have not moved, the rotor will level as it approaches the front and back of the aircraft. As a result of all this, the aircraft will now roll to the left.
Control Inputs:
These single roll and pitch input is provided to the rotor head from the
Cyclic stick, which is the stick between the
pilot’s legs. If you were to say want to make all of the blades pitch up and therefore cause the helicopter to rise straight up in the air, you will pull up on the
Collective stick at the pilot’s left side. The
collective input causes the entire swashplate to rise which then leads to all of the blade angles to change simultaneously.
Easy way to remember? The
Cyclic stick “cycles” one or two movements at a time, whereas the
Collective stick “collectively” causes all of the blades to move.
Remember that the rotor head generates an enormous amount of torque that must be compensated for, lest the aircraft just start spinning around. I would suggest that you visit http://www.verticalreference.com/vertical_reference_videos.htm and you can see what exactly happens when you lose the tail rotor on landing. (Note: the individuals in this mishap and the seven passengers aboard at the time all survived without significant injury. This is entirely credited to the HC-2 Detachment 5 Pilots and Utility Crewman onboard the aircraft at the time of the mishap. Nice job, fellas.)
Input to the tail rotor is provided through the use of the rudder pedals. Press down on the left pedal and the aircraft will pivot left, depressing the right pedal will have the opposite effect. What this control input does is causes the tail rotor blades to change in pitch which either provides more or less torque to the tail. This therefore allows the torque from the main rotor head to overcome or be overpowered by the tail rotor and a change in aircraft heading (yaw) to occur.
Advancing and Retreating Rotor Blades, Blade Stall and Balling It Up
One of the principle speed limitations of the helicopter is that it cannot exceed the speed of sound. As the aircraft flies through the air and as the rotor head turns you have what are referred to as the advancing and retreating sides of the rotor disc. (Typically, rotors turn in a clockwise direction. The system that would be an exception to this general rule would be a counter-rotating head.)
You can begin to see why it is that helicopters cannot (currently) exceed the speed of sound due to this fact. Take for example an aircraft that is moving through the air at seventy-five knots. The speed of the advancing blades is actually the rotational speed of the head plus 75 KIAS (Knots Indicated Airspeed) and the retreating side would be the rotational speed of the head minus 75 KIAS.
Obviously, the faster the helicopter moves the greater the difference in the advancing and retreating blade velocities. When a conventional, jet-powered fixed wing aircraft begins to approach the speed of sound a shockwave forms at the nose of the aircraft, then moves aft, and eventually behind the plane as it moves through the air.
Let’s say you were to theoretically accelerate in a helicopter toward just shy of the speed of sound. Due to the fact that the airframe is moving at say 690 miles per hour, one side of the rotor disc is actually far above the speed of sound and the other side is actually far below that mark and could be relatively close to not providing any lift at all. This would induce not only a titanic amount of torsion stress on the blades and hub, but probably cause the aircraft to violently roll right and pitch down.
Furthermore, rotary wing flight is a fairly inefficient operation as far as types of aviation are concerned. Rotor blades are designed to provide maximum lift while rotating and this comes at a direct expense to the maximum speed that they can reach. Granted, aircraft such as the Royal Air Force's Westland Lynx have tested more efficient blade designs that have allowed this helicopter to exceed what were previously established limits. In the end, the shape of the blades and the advancing/retreating blade velocity differential is what limits helicopter speed.
Blade Stall:
Blade stall occurs when the aircraft blades begin to stall on one side of the rotor disc while continuing to provide lift in other areas. This is a rapidly decaying condition that will lead to an aircraft crash. This is usually due to a pilot exceeding maximum velocity or “dumping” (rapidly pressing down) collective to the point where the blades speed up and one side of the disc is no longer generating lift. It bears mentioning that entering blade stall on purpose while close to the ground (or in any other flight regime for that matter) is a pretty damn good way to figure out how to crash a helicopter.
Conclusion
Theoretically you have something to take away from here. You would be surprised at the number of people who are ignorant concerning this most complicated of aviation types, both from a mechanical and procedural standpoint. If you are interested in piloting a helicopter, the reader is strongly advised to seek suitable, qualified and certified instruction before attempting to do so solo. This is where fixed wing and rotary wing aviation truly differ. Any six-year old boob with a flight simulator can fly a fixed wing aircraft. It takes talent to fly a helicopter.
Sources:
-My Head and the Head of My Division Officer
-www.ntsb.gov
-www.verticalreference.com