This book was written by Ron Newman and proof read by Ray Prouty in 1999. Ray is world renowned as a helicopter design engineer, but he is more widely known for his articles in ROTOR & WING and his many books aimed at design engineers, but his most popular books for engineers and pilots are PRACTICAL HELICOPTER AERODYMICS, MORE HELICOPTER AERODYNAMICS, and EVEN MORE HELICOPTER AERODYNAMICS.
When you read the following information I gathered from the internet on Mr Timothy Tucker, you will see why I value his letter so much.
As a courtesy to Tim, I have blacked out his address and phone number.
Cpt. Timothy Tucker is a 35 year aviation veteran, with over 20,000 flight hours and over 6,500 hours in the Robinson R22 helicopter alone. He has type ratings in the R22, R44, BH206, and the BH205.
As a former pilot in the U.S. Army, Timothy served as an Instructor Pilot, Standardization Instructor Pilot, and Instrument Flight Examiner. While serving his country, he was awarded the Silver Star, the Distinguished Flying Cross, the Bronze Star, and 28 Air Medals.
Since 1982, Tim has been the Chief Instructor for Robinson Helicopters and during that time he designed and developed the Robinson Safety Course. He has authored numerous articles for magazines and newsletters, and composed the Robinson R22 Flight Training Guide, which has become the foundation for R22 flight training around the world.
In 1992, he began conducting Robinson Safety courses outside the U.S.
This course has had a tremendous impact on reducing helicopter accidents by targeting helicopter owners and operators, and focusing on the main causes of fatal helicopter accidents. Over 1,500 pilots a year now attend the course, which is held monthly at the Robinson factory.
In 2000, he received the Helicopter Association International’ (HAI) Flight Instructor of the Year Award .
In 2005, he received HAI’’s Joseph L. Mashman Safety Award, which acknowledges outstanding individuals with contributions in the promotion of safety and safety awareness throughout the civil helicopter industry.
Today, Tim still serves as the Chief Instructor for Robinson Helicopters, and travels throughout the world presenting the Robinson Safety Course in 21 different countries. Obviously, Tim is a Certified Flight Instructor, and he is also a FAA Designated Pilot Examiner and an Airline Transport Pilot.
Tim’s helicopter career, spanning 34 years and over 20,000 hours, along with his dedication to safety, has allowed him to make a large and important contribution to the helicopter industry, and his educational contributions have greatly impacted on helicopter safety around the world, raising the standards and saving lives in the process.
Where you see this – ::::::::::::::::::::::::::::::: – it means text from that paragraph in the book has been removed from this short insight into this book.
2.5 – LIFT
Lift is produced when an aerofoil’s moved through the air at an appropriate angle and speed.
In a helicopter, lift and thrust are, for all practical purposes, one and the same thing in regard to the way they’re produced. Basically, the force produced by a rotor blade is called lift, whereas the force produced by a rotor is called thrust.
The total amount of thrust being produced by a rotor system (or a propeller or jet engine) is best described by the momentum theory (Chapter 2.2).
When discussing the lift of a rotor blade, we’re discussing the force that acts at right angles to the relative airflow.
When discussing the weight that a rotor system can support (which in some publications is also called lift), we’re discussing the vertical component of the total rotor thrust (Chapter 2.12). It’s this vertical component that opposes the weight of the helicopter and keeps it airborne.
The lift being produced by an individual rotor blade is generally referred to as the sum of the difference between the low pressure areas above the aerofoil and the high pressure areas below the aerofoil (conventional thinking). According to this principle, lift is the mathematical sum of the difference in the pressure above and below an aerofoil.
Clearly, the pressure changes around a rotor blade are aerodynamic forces caused by the rotor blade acting on the surrounding air. According to the momentum theory, because the air’s being deflected downward as a result of interference by a rotor blade, the blade receives an upward force from the reaction of deflecting the air downwards. In a helicopter, this downward flow of air coming from the rotor is called ‘rotor downwash’.
Either of these explanations is more than adequate to explain how an aerofoil produces lift. From a pilot’s point of view, all you really need to know (and it’s essential that you do know it) is that after the designer has done his job, all you can do to alter the lift that an aerofoil produces is :-
- vary the speed at which it passes through the air (in our case, it’s the R/RPM);
- vary its angle of attack;
- keep the aerofoil clean (reduce its profile drag); and
- be aware of the effect that density altitude and humidity has on lift, regardless of the above parameters.
2.19 – FLAP-BACK
Flap-back (sometimes called ‘blow back’) occurs during acceleration or deceleration (flap down). It also occurs whenever the collective’s raised or lowered in forward flight; it’s caused by the manner in which dissymmetry of lift is automatically corrected by blade flapping.
If the airspeed of the helicopter’s increased, the advancing blade gets an increase in relative airspeed whilst the retreating blade suffers a decrease in relative airspeed (Figure 2.18a), thus creating dissymmetry of lift.
To eliminate this dissymmetry of lift, the blades flap; as the advancing blade flaps up, its angle of attack is decreased, and as the retreating blade flaps down, its angle of attack is increased, thus eliminating the dissymmetry of lift.
Because of gyroscopic precession (Chapter 2.13), the advancing blade flaps up over the nose and the retreating blade flaps down over the tail. This causes the disk to flap back, which makes the nose of the helicopter rise (static or speed stability).
The pilot sees this occurring and has to move the cyclic forward to regain the original attitude and airspeed; this cyclic movement decreases the pitch angle of the advancing blade and increases the pitch angle of the retreating blade. The pilot continues to move the cyclic forward until the nose of the helicopter stops rising; at this point, the lift on the advancing and retreating blades has been equalised by the pilot mechanically altering the pitch angle of the blades, and they no longer need to flap in order to equalise lift aerodynamically.
MAIN ROTOR SYSTEMS
4.1 – MAIN ROTOR HEADS
The rigid rotor has no flapping or drag hinges but, although the rotor head is rigid, the blades are made progressively more flexible as they extend out from their root end. Therefore, the blades bend at a point well away from the hub to absorb the loads imposed by the various aerodynamic forces. This has the same effect as a huge offset in the flapping and drag hinges.
This control force is such that, on the rigid rotor Bolkow BO-105 and BK-117, a strain gauge is fitted to measure the bending force on the rotor mast, and warn the pilot if they exert an excessive force on the mast by using too much cyclic whilst on the ground (when the helicopter can’t respond), or by over-controlling in flight.
When the rotor disk of a semi-rigid rotor system is tilted, the only lateral force that’s transmitted to the rotor mast comes from the Htrt.
In a rigid or fully-articulated rotor system, the rotor head’s rigidly attached to the rotor mast and the Htrt that’s present whenever the disk is tilted is assisted by the centrifugal force from the rotor blades trying to align the rotor head with the rotor blades (Figure 2.30.a). The greater the offset of the flapping hinges (a larger diameter rotor head or a rigid rotor head with ‘soft’ blades), the greater this control force is; it can be up to ten times the Htrt that results from tilting the lift vector.
This force tends to align the rotor mast at right angles to the rotor disk, which in turn, makes the helicopter follow the disk with less cyclic input (more controllability). More information in Chapter 2.30 – controllability.
ANTI TORQUE SYSTEMS
All helicopters need a means of counteracting rotor torque and providing directional control. The most common method of achieving this is by using a tail rotor.
Two fundamental differences between a tail rotor and a main rotor are that the tail rotor has no cyclic pitch control, and its collective pitch range goes from negative pitch to positive pitch.
It’s commonly stated that the function of the tail rotor is to counteract ‘engine’ torque and, although not technically correct, this statement’s close to being true in relation to conventional helicopters.
If you refer to Chapter 5.2 – counter-rotating main rotors & tip jets, you’ll see that, although these helicopters have engine torque, they have no need for torque correction, and the torque from a horizontally mounted engine would cause the helicopter to roll, not yaw. Therefore, it’s ‘rotor torque’ and not engine torque that has to be balanced by tail rotor thrust. More information in Chapter 2.32 – torque reaction.
As rotor torque is a function of the amount of power being transmitted through the main rotor shaft to the main rotor, you can see how it can easily be confused with engine torque in helicopters with a single, shaft-driven, main rotor.
Remember, torque is a force that’s turning or attempting to turn a body.
Although rotor torque’s generally thought of as being transmitted through the main rotor drive shaft from the main gearbox up to the rotor head, this is only true in powered flight.
Stability and controllability are inter-related; the more stability an aircraft has, the more resistance there is to the pilot’s attempts to change its attitude, and therefore its controllability suffers (and vice-versa). This is an important juggling act for the designer, and the intended use of the aircraft dictates which attribute is more important.
Maximum controllability (without excessive instability) is of paramount importance in a military attack helicopter, whereas stability’s more important than controllability in a corporate helicopter.
7.7 – POWER CHECK
If you don’t have a system to use whilst you’re in-flight that’ll enable you to establish the power you’ll require to hover at your intended landing place, you can make up your own power check procedure for any helicopter as follows :-
- establish an IGE hover in nil wind, at the RPM you intend using for a landing, and note the power being used;
- take off and climb to a height of 500’ AGL;
- set and maintain a selected speed; and
- note the power required to maintain height at this speed.
I use 50 knots for the Hughes 300 and the R-22 and 80 knots for the Jet Ranger.
It’s essential that you monitor the height and speed for at least 20 seconds to ensure that your power check is accurate.
Repeat this check over several days, at several gross weights, and select a power margin that errs on the safe (high) side of the difference in the readings taken.
This difference between the power required to maintain height in the hover and the power required to maintain height at the selected speed can then be used to establish hover power on future flights by flying at the selected speed approximately 500’ above the intended landing place, noting the power required, and adding the margin established by the above checks.
You’ll need more power than this to take off.
7.8 – MAXIMUM ALL-UP WEIGHT
The maximum all up weight (AUW) of the helicopter is set by the manufacturer, and then reduced by hover performance graphs in the Flight Manual for increases in density altitude.
The maximum AUW’s usually determined by performance criteria, but sometimes it’s determined by structural limitations. Any helicopter that has a higher allowable AUW with a hook load that can be jettisoned in flight (and most helicopters do), obviously doesn’t have its AUW limited by power-on considerations.
Think about the consequences of this if you ever consider overloading a helicopter. It might handle the take-off under power, but will it handle an autorotation?
In the Bell 47-J series, Bell instigated a modification where a 10 lb weight was installed inside the hollow spar at the tip of each main rotor blade. This increased the maximum AUW of the helicopter by 100 lb (45 kg) with no other modifications, purely because the autorotation characteristics of the helicopter improved dramatically, and yet any increase in its performance in powered flight due to the reduced coning angle wasn’t noticeable.
If a customer asks you to operate over maximum AUW, which has happened to me several times, tell them you’re not sure if the helicopter’s capable of being landed safely in the event of an engine failure and you’ll find that the request will be rapidly withdrawn, particularly if they’re going to be on board – Refer to the summary of this book for more information.
Remember – if you don’t run out of airspeed or power you’re unlikely to get into trouble, and if you’re getting low on either one, it’s much better to know you’re approaching a problem situation than to suddenly find you’re in one.
8.4 – NORMAL TAKE-OFF AND LANDING
Where possible, all approaches and landings should be conducted over an area suitable for a forced landing, and at a height and speed that keeps the helicopter outside the height velocity curve for as long as possible.
If you commence the take-off by :-
- lowering the nose slightly, and then
- wait for the helicopter to respond before lowering it any further, and then
- hold your height as you accelerate gradually until you get a comfortable accelerating attitude, and then
- commence climbing, using only enough power to get a moderate ROC until you reach a safe height and a safe airspeed,
you will have a take-off profile that remains outside the H/V curve, and you will use the minimum power possible for a take-off under these conditions.
Pilots of twin-engine helicopters operating from ground level helipads adopt these take-off and landing procedures for ‘Category A’ operations, where they have to maintain a flight profile that allows a safe landing to be made at all stages of the flight in the event of the failure of ONE of their engines.
When flying a single-engine helicopter, it makes sense to adopt these procedures at all times, obstructions permitting, in case your ONE AND ONLY engine fails.
8.6 – OPERATING WITH LIMITED POWER
At various times you may have to operate with limited power; this could be because of an increase in density altitude, a sling load that’s leaving you with very little power margin for manoeuvring, or it could be because of a mechanical malfunction such as a spark plug or magneto failure in a piston engine, or a stuck bleed valve in a turbine engine.
When operating with limited power :-
- the slower you go, the less efficient the rotor is, and the more power you need to maintain height;
- the more you lower the nose, the less Vtrt there is; and
- if you tilt the disk forward, as it’s tilting, more of it’s exposed to the horizontal flow of air, therefore increasing the induced velocity and decreasing the TRT until the airspeed increases. More information in Chapter 9.3 – mast bump.
These effects are particularly important at any time you’re flying ‘low and slow’, and are even more important if the power loss is significant.
If your power’s limited, you should remain above ETL speed whenever possible in order to minimise power requirements, and you must do everything possible to minimise attitude changes and/or collective applications.
Apart from wire strikes, one of the main causes of low-level accidents is turning down-wind and either consciously or sub-consciously using groundspeed as a reference.
As you turn down-wind, the groundspeed increases and the instinctive reaction is to raise the nose in order to maintain the same groundspeed, and then lower the collective to hold height. This is fine until the airspeed stabilises at a lower value and then, with the reduction in translational lift, the helicopter starts to descend.
If you have enough power to hold height when this occurs you don’t have a problem; however, this is not a good time to find out that you don’t have the required amount of power. It takes more power to arrest a descent than it does to prevent it occurring in the first place.
If you raise the collective AS you roll into the turn, you’ll be able to maintain your height and your airspeed. More information in Chapter 8.5 – turns.
If the groundspeed is then too fast and you’re overtaking the cow you’re chasing, or the geologist can’t count the gold nuggets in the cliff you’re following, look at your power margin before slowing down. If it’s marginal, make sure you either slow down in small increments, or don’t slow down at all. You should always know what your airspeed and power margin is.
8.11 – CONFINED AREAS
Make sure you have the correct RPM and AIRSPEED for the intended approach as you turn onto final, normally back to a maximum of 40 Kts by 300’ AGL, and don’t commence the descent until you can see your aiming point in the clearing over the trees at the approach end of the clearing.
Don’t come in too slow where you’re almost hovering down the approach path, but don’t rush into the clearing too fast. Fly in with a groundspeed that you can comfortably reduce to zero as you arrive at your approach point in the clearing.
There are two schools of thought on take-off profiles, and two schools of thought on how much power to use for the take-off. The two techniques are :-
- From a low hover, accelerate rapidly up to ETL speed and then raise the nose to hold that speed as you climb out of the clearing.
- Climb vertically until you confirm you have sufficient power to clear the obstacles and then commence a transition into forward flight.
The first take-off technique is the one most commonly taught, but the second is the one most commonly used in the bush.
With the first technique, what you’re doing is looking at the trees at the far end of the clearing and saying to yourself, “I reckon I can get out of here”.
In fact, what you’re actually saying is – “I bet my life (and the lives of my passengers) that I can get out of here”.
With the second technique, the lower you hover before commencing to climb, the more ground effect you have, and therefore, the more excess power you have available to initiate the climb. Once you get the climb established, you can maintain it with less power than you needed to initiate it because it takes more power to accelerate (horizontally or vertically) than it does to maintain a constant velocity (a constant speed and/or a constant ROC).
8.13 – NAVIGATION
I believe the most important things to learn in relation to aerial navigation are that you should look for major features that can’t be mistaken, and you should do your ‘what ifs’.
If that’s Mt. What’s-its-name, there should be a larger mountain to the left and a smaller one twice as far to the right – or – If I’m where I think I am, in 10 minutes I should be just to the left of a road intersection with one road running North/South and the other crossing it at an angle of around 30 0, etc.
Once you’ve positively identified the features, you can work out where you are, or where you want to go, in relation to these features. Unless you can positively identify the features, do not change course.
It’s funny how some things really stick in your mind. In the early 1960’s, whilst with Ansett as a helicopter engineer, I was on a Geological Survey at Mt. Newman in Western Australia to establish if Mt. Newman was going to be opened up as a mine.
When we arrived on site we were told about a fixed wing pilot who got lost trying to find the camp and landed on a road almost out of fuel. I remember looking up at Mt. Newman and thinking, how could you get lost when the camp’s at the base of such a distinctive mountain as this. This has influenced my outlook on navigation ever since.
I had an amusing example of using major features for navigation when flying a Bell 47-J on a Geological Survey in the extremely flat and extremely desolate Woomera Rocket Range many years later.
The camp was at the base of the only hill in sight, and I left there on the first day with the Senior Geologist (who was a Private Fixed Wing pilot) and two other Geologists on board. They guided me from one rock outcrop to the next, where I would land and wait for them to finish looking at their rocks until, at the end of the day the Senior Geologist said, “Let’s go home”. Without picking up a map, I turned and headed for the hill, which was still in sight. The Geologist said, “A little more to the left” so, thinking he might have more rocks to look at, I asked him if he wanted to go home or did he have more work to do.
He said he wanted to go back to the camp and he was adamant that the camp was off to the left, so he navigated and I followed his directions. As we passed about 5 miles to the left of the camp, I told him he had another 10 minutes and then it was ‘my helicopter’ as we didn’t have enough fuel for any additional flying without cutting into my reserves.
After the 10 minutes had elapsed, and much to his dismay, I turned nearly 1800 without referring to a map and headed back toward the hill that was next to the camp.
When the camp came into sight over the hill, they were absolutely staggered and I wouldn’t tell them how I did it. I flew them around that area for the next three weeks and they never worked out how I could always turn directly towards the camp, regardless of how many twists and turns we made during the day’s flying.
They left that job shaking their heads and thinking that I was part homing pigeon, and I couldn’t see any reason to tell them otherwise.
HAZARDOUS FLIGHT CONDITIONS
The successful outcome of any emergency is greatly influenced by the pilot’s ability to quickly identify and analyse the symptoms, and then determine the type of malfunction that occurred in order to ascertain the most appropriate procedure to follow. You must then be able to fly accurately in accordance with your selected emergency procedure.
9.2 – DYNAMIC ROLL-OVER
Dynamic roll-over will occur if there’s an excessive application of collective while the cyclic is displaced laterally. When this occurs, one side of the undercarriage becomes a lateral pivot point and the helicopter rolls rapidly as it’s forced to pivot around that side of the undercarriage, instead of rolling around its more central C of G.
Unlike static roll-over, which occurs if the helicopter reaches an angle where the C of G is outside the low skid, dynamic rollover occurs when the rate of roll exceeds the force applied to stop it, and is generally unrecoverable well before the angle at which static roll-over would occur.
If the helicopter’s rolling at a rate where the inertia created is greater than the control force that opposite cyclic can impose, full cyclic will not stop the roll. The only means of preventing a roll-over at this time is to lower the collective; however, the roll normally occurs much too fast to take any corrective action.
A slope landing is very much an attitude control manoeuvre, and if you lower the collective slowly, you’re able to control the rate at which the helicopter rolls around the uphill skid. If you reach the cyclic limit before getting the downhill skid on the ground, pause on the collective, take stock of what’s happened, and commence a normal slope take-off.
If you’re going to land on a reasonably steep slope, think about what could happen, what you need to do if it does start to slide, and check your escape route. The chance of a slide occurring after you’ve thought about it is almost nil and, if it does happen, you‘ll automatically carry out the actions that have been pre-programmed in your mind.
The abort procedure is quite simple – just fly AWAY from the slope AS you raise the collective; but make sure you’ve thought about it before you need it, or it may be too late.
Like everything else, prior planning makes the difference.
This section is meant to supplement, not replace, the many books that are available on the theory of piston engine operation.
Piston engines come in a number of configurations, but all piston engines fitted to the current line of commercially produced helicopters are of the ‘horizontally opposed’ variety, and they’re all made by Lycoming. Therefore, apart from a brief look at a radial engine, I’ll restrict the discussion of helicopter piston engines to horizontally opposed engines made by Lycoming, but the basic principles apply to all engines.
HORIZONTALLY OPPOSED ENGINES
The following classifications are used to identify these engines :-
- O = Opposed
- H = Horizontally mounted engine designed specifically for a helicopter.
- V = Vertically mounted engine designed specifically for a helicopter.
- I = Fuel Injected.
- T = Turbocharged.
- S = Supercharged.
- 360 = Capacity in cubic inches.
Hence a HIO 360 is a 360 cubic inch, fuel-injected engine (I), with opposed cylinders (O), that’s specifically made to be mounted horizontally in a helicopter (H). This engine’s fitted to the Hughes 300-C.
The TVO 435 is a 435 cubic inch, turbocharged engine (T), with opposed cylinders (O), that’s specifically made to be mounted vertically (V) in a helicopter. This engine’s fitted to some of the Bell 47 series, as is the VO 540, which is a normally aspirated engine of 540 cubic inches that’s designed to be mounted vertically (V) in a helicopter.
Another engine is the O 320 that’s fitted to the Robinson R-22. It’s a 320 cubic inch engine with opposed cylinders that was designed for use in an aeroplane, hence there’s no ‘H’ or ‘V’ in its designation.
10.1 – MAIN COMPONENTS OF A PISTON ENGINE
GAS TURBINE ENGINES
This section is meant to help you understand how gas turbine engines operate, and hopefully give you a few tips on how to look after them. You’re not designing an engine, so I haven’t included any formulas, nor does it go into engine design criteria.
11.1 – BASIC TURBINE PRINCIPLES
When you see a large jet lifting off the runway and raising its nose to a 400 nose up attitude, and then accelerating while it’s climbing, you’re seeing Newton’s 3rd law of motion in action.
These engines are producing thrust in the same way as a helicopter rotor; they accelerate the air passing through them in order to produce thrust. The thrust generated is a product of the amount (mass) of air passing through the engine (or rotor), and the difference in the velocity of the air entering and leaving the engine (or rotor).
The mass of air passing through an engine varies with the engine RPM, the forward airspeed, and the air density.
Because the density of air varies indirectly with altitude, temperature, and humidity, it follows that the engine power will be less at altitude, or in hot and/or humid conditions.
Turbine engines and piston engines both use the same principles of operation (induction, compression, combustion, and exhaust). The main difference is that a turbine engine has a continuous cycle of events, whereas a piston engine has individual cycles of events.
The power (or thrust) created by a turbine engine is due to a large number of rapidly rotating blades in both the compressor and the turbine, and regardless of the power being produced,
The following explanations are based on the Allison 250, but most hydro-mechanical FCU’s operate in a similar manner.
Fuel Control Unit (FCU)
The FCU is driven by the N1 (compressor RPM) gear train to sense its RPM, both for fuel metering, including bringing the N2 (power turbine RPM) up to operating RPM, and to provide N1 overspeed protection. It also has a bellows that varies the fuel flow in accordance with compressor discharge pressure; this pressure varies with density altitude and ram effect in the compressor inlet, both of which affect the amount of air passing through the compressor.
Fuel enters the FCU via a filter and flows to the metering valve, which is moved in accordance with a signal from the PTG when it senses the N2 changing.
A pressure differential valve (not shown) maintains a constant pressure drop across the metering valve as it moves; this ensures that the amount of fuel flowing through the metering valve only varies in accordance with the size of the opening, and isn’t affected by a varying pressure differential across the valve.
Both the airframe and engine fuel filters have a by-pass valve that opens if the filter becomes clogged to ensure the engine continues to receive the necessary fuel (albeit unfiltered fuel). These filters are normally fitted with a pressure differential switch that activates a ‘FILTER’ light in the cockpit to warn the pilot that the filter is becoming blocked before the filter’s blocked to the point where it’s by-passed. More information in Chapter 8.1.