Article by Dr Igor B. Bensen, P.E.
Most students of aeronautical progress don’t realize what tremendous amounts of work have gone into rotary-wing aircraft during past decades. The possibility of producing lift while standing still has intrigued inventors with the magnetic fascination of inventing a wheel. If the wheel revolutionized ground transportation, they reasoned, the rotor could do the same for aviation.
Unfortunately for all of us, the rotor turned out to be a lot tougher nut to crack and to this day continues to defy most brilliant minds. Many fortunes have been staked and lost and many lifetime careers spent pursuing this stubbornly elusive dream. Only a few succeeded in achieving relative success.
Today, the rotor is still far from gaining the universal acceptance of the wheel. Rotary-wing aircraft, or rotorcraft, actually have a longer history than fixed-wing aircraft. Many years before Wright Brothers flew the first powered aero plane, Inventors were building and testing full-scale helicopters. None of them succeeded because of unavailability at that time of sufficiently light and powerful engines. Even Wright Brothers had to build the 12-hp engine for their 700-lb. machine.
While aeroplanes retained their overall aerodynamic configurations through the years - with fixed wings for lift and separate forward thrusters for propulsion - rotorcraft designers have explored a fantastic array of configurations that are difficult to even catalogue. This only serves to illustrate the tremendous challenge that still remains to build a truly successful rotary-wing vertical lift machine.
In today’s Design Classroom we will begin to survey the work done by previous designers so we will absorb the knowledge that has been already gathered. We will not attempt to make this a history lesson or a detailed design study. We will simply catalogue the many explored designs most of which have actually achieved successful flights status.
All rotorcraft designs can be divided into three broad categories: Gyroplanes, Helicopters and Convertiplanes.
Gyroplanes obtain lift from a free wheeling rotor while propulsion is obtained from a forward thrusting propeller.
Helicopters have powered rotors and obtain flit by sucking the air from above and blowing it down.
Convertiplanes are hybrid combinations, which can hover as helicopters, but in forward flight derive lift from fixed wings and/or unpowered rotors.
Gyroplanes, also known as Autogyros, Autogiros and Gyrocopters, were first to achieve practical success as rotary-wing aircraft. First flown in 1923 by Cierva in Spain, 20 years after the first aero plane flight, they paved the way to the helicopter 16 years later.
All gyroplanes in the past had single rotors and single forward thrusting propellers. Cierva’s early “Autogiros” had auxiliary wings which were removed in later designs. All but one early gyro used tractor propellers located in front of the pilot seat. Only Buhl in U.S. designed his as a pusher. The majority of modern-day Gyroplanes are pushers and some use shrouded propellers for augmented thrust.
The rotors were usually 4 or 3 bladed and were controlled by the “direct” tilting head control principle. Blades were usually fixed pitch and started by hand. Later, power starting and collective pitch controls were added to obtain short field and “jump” takeoff. Collective pitch was not used in landing; landings were made with a cyclic “flare” followed by a short roll, just as today.
In the pusher group, Pliasecki’s “Path finder”, is especially interesting. It looks and flies like a Gyroplane, but qualifies also as a helicopter. Its ducted propeller in the tail has deflector vanes which produce enough side thrust to overcome the torque of the rotor when the machine is hovering.
Gyroplanes’ strong points are:
1. Their low weight.
2. Low cost and simplicity compared to helicopters.
3. Good gilding capability power-off because of their low disc loading.
4. Simpler controls, as the machine flies more like a stall-proof slow-flying aeroplane than a helicopter.
1. Inability to hover.
2. Limited maximum speed because of high aerodynamic drag.
There are so many helicopter designs, we won’t be able to cover them all in one classroom session. So many designers and inventors conjured up and built so many different configurations, we won’t even try to name them all. We will just mention the ones that have left the deepest marks on technology.
To begin with, we must sort the configurations by class:
1.Single rotor, torque driven
2.Single rotor, torqueless drive
4.Three or more rotors
We will start with today’s leading design - the single rotor, torque driven helicopter. This is generally considered to be the first practical helicopter and is usually credited to Igor Sikorsky (PRA Honorary Member 1033). Since a shaft-driven rotor produces torque, some means must be used to produce an equal and opposite torque, Sikorsky chose a vertically turning tail rotor. Actually, the idea first occurred to the Dutch designer Baumhauer, who placed on the tail a side-facing propeller with its own engine. The machine was tail-heavy, but flew in ground effect, although torque compensation by a separate engine proved too difficult for a pilot to control. Sikorsky’s major contribution was in connecting the tail rotor to the engine driving the main rotor - at the expense of added power transmission and collective pitch mechanisms to drive and control the tail rotor.
Concurrent with Sikorsky’s work but independent of it, another pioneer designer, Art Young was developing a similar tail-rotor helicopter for Bell Aircraft Corp. The Bell machine was successful and has the distinction of being the first helicopter design certified by CAA for production.
With a few exceptions, flight controls of helicopters in this class are cyclic and collective pitch controls for the lifting rotor, and collective pitch for the anti-torque rotor. Cyclic pitch is accomplished by means of a swashplate, since the shaft of the rotor remains fixed with respect to the airframe.
SINGLE ROTORS TORQUE DRIVEN
Rotor blades, with a few notable exceptions, are hinged to flap up-and-down (this was Cierva’s contribution that came from gyroplanes) as well as back-and-forth. The blades can also rotate in pitch around their feathering axles in response to commands from the swashplate. This type of blade attachment is known as “full articulation”.
The exceptions are:
Bell’s “semi-rigid” 2-blade teetering rotor using no lag (in-plane) hinges.
Lockheed’s “rigid rotor” with feathering control but neither flapping nor lag hinges.
Doman’s hingeless blade attachment with free-floating rotor head and limber tubes (flexible enough to simulate hinges) connecting the blades to the hub.
Brantly’s double flapping hinges - one near the hub of each blade and the second at about one-third radius.
All these hinges are used to reduce the stresses in the blade attachments, so that lighter structural member can be used. Tail rotors have either teetering, (if 2-bladed), or flapping hinges, but no lag hinges. Pitch horns are so connected that flapping motion produces strong unpitching effect. This is known as the Delta-3 effect, and its purpose is to reduce flapping amplitude.
Some designers sought to avoid the complexity of powering and controlling the tail rotor by replacing it with fixed surfaces. In the configuration No. 2b rotor slipstream itself was used to produce the opposite torque. This system was marginal in performance and had forward flight problems that were difficult to overcome.
Hirtenberger in Austria wanted a more positive torque control and obtained it by installing a controllable rudder in a pusher propeller slipstream (scheme No. 2c) not unlike the modern Gyrocopter. The idea worked but torque control was very tricky in some flight regimes because he used two different engines to power the rotor and propeller.
Anton Flettner, in the scheme No. 2d went one step further and used two forward facing propellers with reversible pitch to produce anti-torque action, as well as forward propulsion. The main advantage of this idea was that the counter-torque produced by the propellers was a pure couple around the centre of gravity. Thus there was no side force to compensate by an opposite tilt of the rotor and none of the yaw-roll coupling that exists in tall-rotor helicopters. An additional advantage was that no horsepower was wasted on the tail rotor in forward flight, since both propellers were doing useful work by pushing forward.
The logical conclusion of this series was Fairey’s “Gyrodyne”, which eliminated one of the forward thrusting propellers. This was an eminently successful machine, flew very well and even held a world speed record for helicopters for some time. In spite of this, it “did not take hold” because of piloting difficulties. The machine hovered with severe nose-up attitude and had control couplings between yaw, pitch and roll which were impossible to eliminate.
The advantages of shaft-driven single rotor helicopters are manifold. Such machines require fewer parts, rotor blades and power transmission assemblies and are therefore less costly and complex than multi-rotored configurations. Bell and Hiller, with two-blade lifting and anti-torque rotors, are about as simple as this type of aircraft can get. Bolkow in Germany recently tried to do them one better by using counter-weighted one-blade main and tail rotors but the design was never put into production.
The vast preference of manufacturers for this single rotor, torque driven configuration points to its current superiority over other helicopter designs. Still, there are some glaring disadvantages to this configuration.
The leading disadvantage is mechanical complexity compared to other types of aerial vehicles. This translates itself as high costs of manufacture and maintenance. Piloting skill required is of higher order than needed to pilot a plane, or a Gyroplane. Many high-time fixed-wing pilots flunk out on helicopters.
The field is still wide-open for brilliant designers. Will final success be with the same tail-rotor design? Or with some other idea? Here is a place where you, the reader may have the last word. The rewards are immense, and the challenge is great. Let’s see you go to work on it!
Long before Sir Isaac Newton said, “action must equal the reaction,” birds and insects followed nature’s inviolate law of “conservation of momentum” by beating down the air in order to stay aloft. Whether you fly an aeroplane, an autogyro or a helicopter, you must do the same thing.
It does not matter what type of aircraft is moving through the air, the net effect on the mass of air is the same. Streamline layers of air in front of the aircraft move horizontally relative to it and are deflected downwards by the lifting surfaces as the craft passes over them.
In case of an aeroplane wing the mechanism is straight-forward. The air flows over and under the wing uniformly, and the lift produced by it can be readily calculated by the well-known two-dimensional formula:
Lift = ACL P/2 V2, lb.; (1)
Where A is total area of the wing in sq. ft.; CL is the effective lift coefficient of the wing at the particular angle of attack; P (Greek Rho) is air density, equal to .00238 at sea level; and V is velocity, or airspeed, in feet per second.
It can be readily seen that in level flight there are only two variables, CL and V2 , which must vary inversely to maintain constant lift. Thus, angle of attack of the wing must be decreased when the airspeed is increased, and conversely, the craft must be nosed up as it slows down. Every pilot knows this. This holds true until CL reaches its maximum and breaks down when the wing “stalls”.
Curiously, it does not matter to the surrounding air whether the craft flies fast or slowly, the downward push on it or momentum, remains the same. It can be expressed by the formula:
Lift = QP Vi/g. lb.; (2)
Where Q is the volume flow of air accelerated downwards by the wing, cu ft/sec; P is same air density as in formula (1) above; g is acceleration of gravity, 32.2 ft/sec sq: and Vi is downward velocity imparted to the air by the wing in ft/sec.
This formula therefore expresses what happens to the surrounding air when it supports an aircraft.
Action equals the reaction. Aircraft push down on the air. Air pushes up on the aircraft.
Without going too deeply into the theory of aerodynamics, it might be mentioned here that greater lifting efficiency is obtained when Q is high and Vi is low.
This is why the sailplanes with wide wingspans are more efficient flying machines than shorter wing aeroplanes. The same principle holds true for rotorcraft.
When we replace the fixed wing with a rotor, there are some local changes in the airflow pattern, but the overall mechanism remains the same. However, there is a notable difference in the way the air passes through the rotor depending on whether it is powered or unpowered.
In case of an Autogyro rotor, the air must pass through it from below in order to keep it turning. The rotor is tilted backward some 10 degrees and acts somewhat like the wing of an aeroplane. The main difference is that the air goes through it, while in case of a wing, it goes around it. The air in their wake still turns down as usual.
A powered helicopter rotor acts quite a bit differently. It sucks the air from above and blows it downwards.
Thus the airflow through the helicopter rotor disc is exactly opposite to that of an autogyro. Yet the net effect on the surrounding air mass ends up the same.
FLOW THROUGH ROTOR
As you might have suspected, the airflow through the rotor is nowhere near as simple as the airflow around the wing. To begin with, by the very nature of the beast, blade tips of the rotor travel faster and impart much more energy into the air stream than the inboard sections of the blade. The inner third radius of the blade is quite ineffective as an aerodynamic surface because of its low energy content and low velocity.
It should be expected therefore that the airflow would be different at the tips from the airflow at the hub.
Let us first examine a simple case of the airflow through the rotor traveling parallel to its axis, that is flying straight up, hovering, or straight down.
As the Figure 1 indicates, there are five distinct and different modes of airflow through the rotor depending on its regime of flight. Let us study them one at a time.
1.PROPELLER STATE: As the description suggests, this represents the condition similar to the aeroplane propeller, where all air is pumped through the rotor in one direction. Power must be added to the rotor to maintain the flow through it. There is no recirculation of air at the tips, and the streamlines characteristically contract downstream from the rotor plane. This occurs when a helicopter climbs straight up like an elevator. Vertically climbing aeroplanes would have a similar flow through its propeller. High positive pitches are typical for such blade sections, starting with 16 degrees and up depending on the design advance ratios.
2.HOVERING STATE: The chief distinguishing mark of this flight regime is that the rotor is stationary with respect to the surrounding air. Blade pitches are moderately positive, say, between 8 and 16 degree, and of course power must be added to the rotor to produce static lift. A tip vortex begins to form around the periphery of the rotor disc, which is a doughnut shaped recirculation of air. Some air that is pushed down by the blades comes around on the outside of the tips and goes through the rotor disc for another trip, then another etc.
As a producer of lift this form of recirculating airflow is quite inefficient, and fortunately, the tip- vortex in hovering flight is quite weak, accounting for no more than 5-10 percent of the power loss.
3.VORTEX RING: This unique airflow pattern occurs in partial-power vertical descents of helicopters and is characterized by a severe deterioration of aerodynamic controls. Helicopter pilots describe this regime as “descending into your own downwash” and shy away from it whenever they can. Actually, what happens is the tip vortex grows into the equivalent of a giant smoke ring, which grows up and engulfs the entire lift producing area of the rotor disc. As you know, the smoke ring is a stable self-contained airflow pattern that propagates through space as an independent body. With the rotor sitting in the middle of it and feeding energy into it, it reinforces its circulation as it accelerates downwards with the aircraft. A secondary vortex is formed in the centre of the rotor, circulating in the opposite direction, which further reinforces the outer vortex. The pilot soon realizes that the rotor no longer responds to control commands, as the craft sinks faster and faster. Even the application of full power and collective pitch does not pull him out of a well formed vortex ring, as the ship seems to “fall through” an invisible hole. Accident files of CAB are generously sprinkled with cases of crash landings when “the pilot was unable to arrest vertical descent”.
The only salvation from this predicament, providing the pilot takes action with enough altitude to spare, is to reduce the pitch and throttle to zero power and “fall out” of the vortex ring in vertical autorotation. Then apply cyclic control to gain some forward speed and to leave the vortex behind. Power can then be applied successfully, with rotor promptly responding to the controls.
Vortex ring state has another unpleasant by-product. Engine exhaust gases are recirculated within the inner vortex, being unable to escape to the outside air, and can do damage to the pilot and equipment with carbon monoxide and high temperatures.
In short, in partial-power vertical and near-vertical descents the helicopter rotor acts as a giant smoke-ring generator. Strong updraft and downdrafts in the ring can cause severe blade bending and flapping, giving the pilot a rough ride. For all these reasons the “vortex-ring’ state of flight is generally avoided by knowledgeable pilots.
4.AUTOROTATION: The chief distinguishing mark of this state of rotor operation is that power is neither added to nor extracted from the rotor. The twirling seed of the maple tree gently floating to the ground is an age-old auto-rotating rotor. When Juan de la Cierva invented the first autogyro in 1923, he intended the rotor to act as a built-in parachute for the entire aeroplane, should there be an engine failure over unsuitable terrain. History does not tell us whether the seed of the maple tree inspired him, but his keen mind was first to analyze its principle and to put it to practical use.
As the Figure 1 shows, the airflow through an autorotating rotor is preponderantly upward, although there is also present a weak tip vortex. This vortex is somewhat different from the hovering tip vortex in that the air double-reverses its flow in-board of the tips, as it starts up first and then is pushed down again by the tips.
Blade pitch of autorotating rotors may be anywhere from 0 to 6 degrees depending on the airfoil, disc loading and some other factors. ‘
“What turns the rotor in autorotation?”
This question is asked in million variations by the beginners, who insist that it should turn backwards since its blades are set at a positive pitch. This “common -sense” conclusion is not borne out by the nature’s behavior. True enough, if a rotor were to start from a standstill in a vertical descent it would begin to turn backwards, and the airflow pattern through it would then fall in the category of “wind-milling” which we will describe later.
But if the autorotation had begun in the right direction previously, the rotor will continue to rotate in the same direction even in a vertical descent. One may say that the same kind of forces push the blade forward that enables a sailboat to make headway against the wind. More precisely, these forces combine into a vector diagram that is shown in figure 2. The important ingredient is the velocity vector Vr, rotational speed of the airfoil, which is considerably larger than the inflow velocity Vi. When they combine, they produce the total velocity vector Vt that acts upon the airfoil at a fairly shallow angle.
Constructing now the Lift and Drag force vectors (perpendicular and parallel to the impinging air), we see that the resultant force R lies ahead of the axis of rotation of the rotor. The bulk of the force is transmitted through the main bearing to the Mast as a part of the total lift LT, but the small vector component of it, F, remains. This vector F in fact is the force that acts upon the airfoil to drive it forward.
Not all sections of the rotor blade have the same vector diagram since the magnitude of the Vr varies with the radius of the blade. Thus toward the tip for instance the angle between Vt and the airfoil becomes so shallow that the driving force F becomes zero or even falls behind the axis of rotation. In the latter case then it tends to decelerate the blade and consumes the driving power supplied by the inboard sections.
Much further inboard, on the other hand, Vr becomes small compared to Vi, and the angle between Vt and the airfoil becomes large enough to cause the airfoil to stall. As you know, the stall is characterized by a sharp decrease of L and increase of D, which can again reduce F to zero and even make it negative. Thus the inner sections of the rotor blade may consume the power supplied by its middle sections.
Two more things are worth mentioning before we leave the subject of autorotation. One is the observation that Vi can be at almost any other angle than shown, including vertical, or parallel to the axis A. So long as it is in generally upward direction and its magnitude is relatively small compared to the rotational speed Vr, it will always combine into a Vt that will sustain autorotation. In practice, the rotor RPM of modern Autogyros shows very little variation when airspeed is reduced from cruising speed to vertical descent.
The second noteworthy observation is the equilibrium diagram included in the Figure 2. It shows the location of the centre of gravity of the gyro with respect to the lift vector and the propeller thrust. Ideally all three force vectors, Lift, Thrust and Weight, should intersect at one point, which is the craft’s centre of gravity. When the engine is shut off, its thrust T becomes zero, and in vertical descent the lift vector Lt must become vertical to directly oppose the Weight W.
Much more can be said about autorotation, but it has more to do with forward flight and we will return to it later.
5.WINDMILLING: The chief characteristic of the windmilling state is the extraction of power from the rotor. Windmills have been used by human beings long before aviation was born, to mill the grain, to pump water and do a host of other physical chores. It’s a wonder why nobody thought centuries ago of tilting a windmill rotor on its side and making it into a kiting gyroglider!
Blade pitches on windmilling rotors are generally set at a considerably negative pitch, and such rotors will not turn fast even when unloaded. Tip vortex vanishes completely. The rotor acts more as a perforated drag plate than a rotating body, although here too, the centre area offers less resistance to airflow than the tips.
Windmilling state occurs very rarely in modern rotorcraft. Aeroplane pilots sometimes use this principle to restart their stalled engines by diving at high speed, which of course requires the reversal of the airflow through the propeller. In rotorcraft, windmilling was used only experimentally for such purposes as driving the rotors by torqueless means and other schemes which had more to do with rotor propulsion than with producing lift.