This is my second book, published in 2010. It has a larger format (205 cm x 270 cm) and contains over 250 pages with 178 colour illustrations.
Whereas the first book HELICOPTERS WILL TAKE YOU ANYWHERE covered a lot more topics (as shown above), this one concentrates on the technical, aerodynamic and performance aspects of a helicopter (as the title suggests), but deals with them to a much greater depth.
The book is divided into the following sections :-
- 1 – STRUCTURES
- 2 – POWER TRAIN
- 3 – MAIN ROTOR SYSTEMS
- 4 – ANTI TORQUE SYSTEMS
- 5 – CONTROLS
- 1 – ATMOSPHERE
- 2 – BASIC AERODYNAMICS
- 3 – HELICOPTER AERODYNAMICS
- 4 – AERODYNAMICS OF POWERED FLIGHT
- 5 – AERODYNAMICS OF AUTOROTATION
- 6 – MAIN ROTOR AERODYNAMICS
- 7 – ANTI TORQUE AERODYNAMICS
- 8 – STABILITY & CONTROL
- 9 – AERODYNAMIC HAZARDS
- 1 – PERFORMANCE – GENERAL
- 2 – HELICOPTER PERFORMANCE
- 3 – FLIGHT MANUALS
- 4 – PERFORMANCE CHARTS – GENERAL
- 5 – LOADING & CENTRE OF GRAVITY
- 6 – PERFORMANCE & LOADING BELL 206 L1
- 7 – PERFORMANCE & LOADING ROBINSON R22
- 8 – PERFORMANCE & LOADING ROBINSON R44
- 9 – PERFORMANCE & LOADING FAA EXAMS
- 10 – PERFORMANCE ORIENTED FLIGHT OPERATIONS
With the exception of sections 3.1 & 3.4, each one has 10 questions at the end the section and the answers are on www.OnlineAviationTheory.com using the access codes from the book.
I have listed a few examples below to show the quality of the illustrations and the associated information, there are another 244 pages of similar quality in this book.
1.4.3 – DELTA 3 HINGES
Most tail rotor flapping hinges are of the Delta 3 format to prevent excessive flapping (which would increase the vibration level) and keep the tail rotor output shaft shorter and stronger.
Some French publications refer to this hinge arrangement as the ‘K Coupling’ (pitch/flapping coupling).
A Delta 3 hinge has the hinge set at an angle to the chord of the blade.
A Delta 3 effect can be achieved by having the pitch horn on a different plane to the flapping hinge.
In either of these cases, the pitch angle of the blade is mechanically altered as it flaps by using the pitch horn/pitch link as a pivot point. This eliminates the need for the blade to flap any further in order to control dissymmetry of lift aerodynamically.
If you take a stationary tail rotor blade with a delta hinge and move it through its flapping range, you will see its pitch angle change quite dramatically as you manually flap the blade.
2.2.15 – CHANGES EFFECTING DRAG
If the weight increases, the collective must be raised in order to increase the TRT to offset the increase in weight. This causes the induced drag curve to move up, which moves the total drag curve up and to the right (up because of the increase in power required, and to the right in order to remain above the intersection of the two drag curves).
Changing Density Altitude
As the density altitude is increased, a number of conflicting changes occur as follows : –
- The rotor has to work harder to produce the same amount of Total Rotor Thrust, which increases the induced drag.
- The parasite drag of the fuselage decreases in the thinner air, which decreases the requirement for power at moderate to high airspeeds.
- The profile power of the main and tail rotors decrease in the thinner air, which also decreases the requirement for power at a given TAS. This has a much lesser effect that the decrease in parasite drag, and is relatively uniform across the entire airspeed range.
If the density altitude increases, the lift decreases so the collective must be raised to provide the same amount of TRT, which moves the induced drag curve up. In the thinner air, the parasite drag decreases more and more as the airspeed is increased (the V2 in the drag formula), which effectively rotates the curve clockwise around its static zero position, which moves the intersect point even further to the right.
This increases the minimum drag speed (Best Rate of Climb, Minimum Rate of Descent and Best Endurance speeds), it also increases the speed for the best angle of climb.
Depending on the speed of the aircraft, the decreasing parasite drag may mean less power is required to cruise at altitude.
The table in Figure 2.2.17.a shows how the speed and Rate Of Climb for a typical light helicopter varies with the density altitude.
2.4.15 – INHERENT SIDESLIP
Sideslip is a lateral airspeed (a side wind). It occurs whenever the aircraft is travelling through the air in a direction other than the direction in which it is pointing. It is generally used to describe a turn where the aircraft slips downhill. In an aeroplane, this manoeuvre is often used to increase the rate of descent without gaining excessive speed.
In the ‘Dictionary of Aeronautical Terms’, a sideslip is defined as ‘a manoeuvre in which the aircraft continues to point straight ahead, but slips to the side’.
In forward flight, the combined thrust from the tail rotor and vertical stabiliser continue to push the helicopter sideways to the right. If you keep the balance ball in the middle you will need to point the nose to the left of track in order to maintain the required track.
The sideslip created by this manoeuvre is called inherent sideslip because it is inherent in all shaft driven single rotor helicopters. The amount of inherent side slip varies with the weight of the helicopter, the power setting, and the position of the C of G
There are two ways to overcome this problem and keep the helicopter on the desired path through the air :
- Keep the helicopter level (balance ball in the middle), which is best for passenger comfort, and apply left pedal to point the nose a little to the left of the desired track. The crabbing action generated by pushing the helicopter sideways increases the drag slightly. or
- Keep the nose pointed along the desired track (slip string in the middle) and apply left cyclic to maintain track. Although this manoeuvre minimises the drag, it is less comfortable for the pilot and passengers.
If the helicopter is left side heavy, with the first method you would need right cyclic to keep the helicopter level and this lateral thrust would cause the helicopter to track further out to the right, which would require more left pedal (more inherent sideslip). With the second method, you would reduce the amount of left cyclic applied.
If the helicopter was right side heavy, you would need a less right cyclic with the first method, and more left cyclic with the second method.
2.7.6 – TAIL ROTOR DRIFT
Any method of torque control that creates a sideways thrust on the tailboom will cause the entire helicopter to drift sideways; this is sometimes called translating tendency.
Although this is most noticeable in the hover, it is present in all powered flight and it must be countered by tilting the main rotor thrust in the opposite direction in order to prevent sideways movement of the helicopter.
In some cases the manufacturer mounts the main transmission with the main rotor mast offset to the left, or they rig the control system to tilt the swashplate to the left when the cyclic stick is neutral.
These inbuilt fixes are only accurate for one value of tail rotor thrust, which is based on the manufacturer’s estimate of the average power that will be used to hover. Any change to the power required to hover requires a different tail rotor thrust to counteract it, and consequently a different lateral cyclic position to eliminate tail rotor drift. The higher the power being used (such as a maximum performance take-off out of a confined area), the more drift there will be, and the more cyclic that will be needed to correct it.
From a pilot’s point of view, it doesn’t matter whether the manufacturer has incorporated any of these fixes or not, the pedals and the cyclic have to be applied as necessary to maintain the required heading and position over the ground.
2.9.5 – LOSS OF TAIL ROTOR EFFECTIVENESS
In my opinion, loss of tail rotor effectiveness (LTE) is an aerodynamic tail rotor failure; however some publications refer to it as anything that causes an uncommanded yaw that does not correct itself.
Some publications state that LTE can occur at speed below 30 knots, and although I cannot argue against it, most LTE accidents have occurred when the airspeed has been below ETL speed.
When operating with a tail wind coming from an arc extending 60o either side of the tail, the helicopter will attempt to ‘weathercock’ into wind as it tries to take up its most streamlined position. This is more noticeable with high power settings, and if the helicopter is allowed to yaw, the wind on the vertical stabiliser, which is now angled in relation to the tail wind, will increase the rate of yaw.
In my opinion this is a Loss of Tail-rotor Authority (LTA), not LTE. It is a normal occurrence that can be corrected by timely pedal inputs by the pilot.
Some helicopter Flight Manuals also have a restriction on AUW and/or density altitude when operating with crosswind from the right. This is to prevent running out of left pedal whilst attempting to maintain heading, and to allow for the additional power that is required when more left pedal is applied under these conditions.
In my opinion, this is a control limit problem, and should not be confused with LTE.
LTE should be caused by:
- Wind from the left front (285-315o) carrying the main rotor tip vortices from the retreating blade into the tail rotor. or
- A crosswind from the left (210-330o) causing a tail rotor vortex condition.
When flying slowly with the nose cocked off to the right (a common occurrence in aerial filming) the tip vortices from the retreating main rotor blade can enter, and oppose, the airflow through the tail rotor and cause it to lose its effectiveness.
Remember, tip vortices are a product of the pressure differential that exists above and below the blade, and as the collective is higher at slow speeds and the retreating blade has the highest angle of attack, it obviously has the highest pressure differential, and therefore creates the most tip vortices.
If the yaw is uncontrollable you MUST close the throttle to eliminate the rotor torque that is causing the helicopter to turn out of control, and the spin from a fully developed LTE is not controllable, regardless of what the Flight Manual states (This statement comes from personal experience).
Some helicopters are more prone to LTE than others, and one of the models effected is the early Bell 206B with the small (62”) tail rotor.
I had an occurrence of LTE whilst flying with the nose cocked off to the right in an early model Bell 206 to film a murder scene during my time in the Victoria Police.
The published procedure is to lower the nose and gain speed – Let me tell you, you cannot lower the nose when the helicopter is spinning that violently, and that irregularly that you are using full cyclic in all directions to keep it level. After it had spun three times, I decided enough was enough, and closed the throttle.
The observer sitting in the front passenger seat had a wooden clipboard on his knees, and this shot off his knees in a horizontal direction and crashed into the chin bubble, still at knee height. This is a distance of 1.5 metres (5′) which gives you some idea of the severity of the yaw.
After ascertaining that the published procedure wasn’t possible and then closing the throttle and entering autorotation, the turn stopped within 90°; so if ever it happened to me again I would not waste time playing with it, I would close the throttle immediately and lower the collective; and if height and time permitted, I would then increase speed and re-introduce power.
The rapidity of the turn is understandable, but the irregularity is because you are still moving forward at this time, and you have an advancing and retreating tailboom to contend with (something like a fast spot turn in a 20 kt wind).
Each time the tailboom advances into the direction in which the fuselage is moving, its rate of rotation is retarded, and 180o later, its rate of rotation is accelerated, and each time the force on the tailboom alters, it alters the rate in which the fuselage is travelling, which puts a force on the bottom of the mast. This force causes the rotor disk to react 90o later, which would cause a rapid change in fuselage attitude unless an opposing cyclic movement was made to keep the fuselage reasonably level. If the fuselage attitude was allowed to get out of control, you would most probably suffer mast bump.
The faster you are travelling through the air and/or the faster you are rotating, the greater the alternating forces are on the tailboom, and the more cyclic you have to apply to maintain a reasonable fuselage attitude, and the more rapidly you have to apply it.
3.10.1 – FLYING FOR MAXIMUM RANGE
Aircraft engines have a maximum continuous power setting, and in piston engines, this is generally 75% of the rated power and therefore the HP (and the fuel flow) will not alter as the aircraft gains altitude as long as MCP settings for the current altitude are maintained during the climb. A different RPM and manifold pressures will be required to maintain this power, but the maximum continuous horsepower and the fuel flow to achieve it, will remain constant.
Best range speed is lower than the speed obtained by using maximum continuous power, and therefore in order to maintain best range speed, less power is required, which means less fuel and greater range.
As the altitude is increased, the power required curve moves up and to the right (next page), which means the best range speed increases, and although the power required increases, the ratio of the increase in speed verses the increase in power required (fuel flow) results in an increase in range.
Parasite drag is the predominant drag at high speeds, and some aircraft, depending on their shape, may experience enough of a reduction in parasite drag at high speeds in the thinner air to lower the high speed end of the power required curve, thus increasing range even further (Para 2.2.15).
The fuel consumption of a piston engine varies almost linearly with power changes, whereas a turbine engine is relatively inefficient at low power settings and therefore the fuel consumption does not drop as much when the power is reduced. For this reason, in most turbine helicopters, maximum range is achieved by using maximum continuous power.
In a twin-engine helicopter, if maximum range speed could be obtained by using single engine power, maximum range would be obtained by shutting down one engine. This would obviously defeat the purpose of paying all that money for a twin-engine helicopter, and would not be done unless the circumstances were desperate. In helicopters with 3 engines such as the EH-101 and the CH-53, it is a normal procedure to shut one engine down in the cruise.
In a piston engine helicopter under power, or any helicopter in autorotation, the speed for maximum range is set in accordance with the following graph.
Flying for maximum range requires flight at an airspeed that gives the best ratio of groundspeed to fuel consumption. This is the point where tangent from the origin touches the drag curve.
In nil wind, the point of origin is the zero speed/zero power corner of the graph. If there is a 20 kt headwind, the point of origin moves 20 kt to the right along the horizontal line. If there is a 20 kt tailwind, the point of origin moves 20 kt to the left along an extension of the horizontal line.
Point 4 on the graph represents the speed at which the drag coefficient/lift coefficient is at a minimum.
If the D/A or the gross weight of the helicopter increases, the power required curve moves slightly up and to the right.
This graph is not shown in many Flight Manuals, but the best range speed is usually given. If you use this speed for range flying in nil-wind and tail wind conditions, and add ½ the wind velocity to this speed when flying into a headwind, and subtract it in a tailwind, you will be very close to the mark.
Usually a helicopter that is more streamlined (less parasite drag), or one with a higher disc loading, will have a higher BROC speed.
Another speed that can be found from this graph is the speed that gives the best angle of climb.
You will see in the previous power required curve (Fig 3.10.1.a) that, as the power decreases, the airspeed must increase to obtain the best angle of climb. If the blue line was representing a twin engine helicopter with both engines operating, the pink line would be representative of the speed required to achieve the best angle of climb on one engine.
The definition of service ceiling varies in accordance with the rules of the governing authority and the category of flight (Day VFR, Night VFR, or IFR); it may be a zero ROC, a 100 FPM ROC or a 1% ROC. In multi-engine helicopters (and aeroplanes), a separate service ceiling is listed for normal operations and for single engine operations.
Service ceiling should not be confused with maximum operating altitude, which may or may not be determined by performance limitations; (e.g. the R 44 is limited to 9,000′ AGL, and being a height AGL, it is obviously not performance related). In this case, it is to allow the helicopter to land within 5 minutes in case of fire, which minimises the chance of structural damage occurring before a landing can be made.