Vortex Ring – Settling With Power

Updated on 5th February, 2017
Settling With Power

What is Vortex Ring – Settling With Power?

Vortex ring is commonly called settling with power in the USA. Settling with power is a better descriptive name but vortex ring is technically more correct.

When the helicopter is descending into its own induced flow, there is a risk of vortex ring developing. If the rate of descent is high enough, raising the collective to reduce the rate of descent will make matters worse and the rate of descent will actually increase.

Vortices are always present at the tips of all rotor blades. These vortices can be reduced by good design but they can never be completely eliminated.

Vortices decrease the efficiency of the blades. The outer one third of the main rotor blades produce most of the lift. Losing any lift at the tips due to vortices has a large effect on the total lift generated by the blades.

During vertical descents, the opposing airflow from below changes the relative airflow and the angle of attack of the main rotor blades is increased. Due to washout (blade twist) on the main rotor blades and also the low rotational velocity of the blades close to the hub, the angle of attack is increased so much that this part of the blade starts to stall causing a vibration in the helicopter.

Raising the collective to reduce the rate of descent while the helicopter is in vortex ring has two effects:

  1. The vortices get bigger and move inboard from the tips of the blades thus reducing lift further and causing the helicopter to descend faster.
  2. The stalled area of the blades close to the hub starts to move outboard and lift is further reduced causing the helicopter to descend even faster.

Conditions For Vortex Ring – Settling With Power

Do you remember what the conditions are for entry to vortex ring? I have come across quite a few qualified pilots who know how to recover from vortex ring and what it is but they do not know all of the conditions that cause vortex ring. Do you?

To get a helicopter into vortex ring, three elements must be occurring at the same time. These are:

  • High rate of descent (> 500 ft./min)
  • Low indicated airspeed (< 30 kts)
  • Power applied (> 20% of power available)

All three of these elements MUST be present.

Symptoms of Vortex Ring

When vortex ring starts to develop you will have a high rate of descent and a low indicated airspeed. As the inboard part of the rotor blades start to stall, a vibration will be felt throughout the airframe. If you have not already noticed the rate of descent and airspeed, this should make you take notice and give you time to recover.

When it develops further, the vortices from the main rotor tips start to interfere with the efficiency of the tail rotor. This can cause random yawing and it must be controlled by the pedals. The pedal movements can be quite large. Start the recovery as soon as possible.

Recovery From Vortex Ring

The recovery is fairly simple. Lower the collective slightly and use forward cyclic to increase airspeed. When the airspeed reaches 30 kts, vortex ring will no longer be present. Entering autorotation (removing “Power” from the equation) will also stop vortex ring as there will be no induced flow but this is not a practical recovery technique. At some stage the rate of descent must be reduced and if the collective is raised, an induced flow is created. This has the effect of re-establishing the helicopter into the vortex ring state again (probably worse than before).

Vortex Ring Scenarios

Scenario 1 Imagine you have just passed your flight test and you now have a brand new PPL(H) licence. Your friends have helped you celebrate and you have promised Alan a free flight. You brief him and take off to go view his house as he wants to take a few photos. As you approach the house, you slow down to 40kts and hold 500’ AGL. Alan asks you to slow down so he can get a photo of his wife in the back garden and then he asks you to hold the helicopter steady in the hover. (Everyone knows that all helicopters hover – right). You come back on the cyclic and lower the collective to stop the helicopter climbing. As the helicopter slows down, it starts to lose translational lift and descend. You have very few visual ques to see this and it goes unnoticed. As the helicopter descends faster you start to feel a vibration and then you notice the high rate of descent. You naturally raise the collective to reduce the rate of descent but this only makes it worse. If you had altitude you could recover easily like you did during training. There is not enough altitude to recover and you hit the ground hard!

Scenario 2 You are enroute from A to B. The weather forecast at A and B is good but you fly into un-forecast bad weather. As the cloud base gets lower you descend to remain clear of cloud (you know how dangerous it is to fly into cloud). The visibility is deteriorating and you slow down to compensate. You are now following a truck on the motorway below you and you notice that its lights are on. You slow down further as you are aware of hills in the vicinity. You are now flying at 500’ AGL and 40kts with 1000m visibility. There is so much drizzle and moisture in the air that clouds are forming and dissipating around you. Suddenly a cloud forms just in front of you and you start to lose sight of the truck. You come back on the cyclic to slow the aircraft down to avoid entering the cloud. Realising that this is not going to work, you lower the collective to lose altitude. You now have all three conditions for vortex ring and not enough altitude to recover.

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Tail Rotor Failure

Updated on 6th March, 2016

Tail Rotor FailureDifferent Types of Tail Rotor Failure

While training for a JAR PPL(H) licence, you will be told about tail rotor failures. There is no flight exercise for tail rotor failures under the JAR system however I like to demonstrate the different types of tail rotor failure that can occur during flight. They are as follows:

  • Tail rotor failure in the hover
  • Tail rotor failure in forward flight
  • Stuck left pedal
  • Stuck right pedal

Although tail rotor failures are extremely rare, I have been unfortunate enough to have experienced one. Because of my training, I was able to identify it and land safely. If you have a sufficiently large, flat area, there is no reason why you should not be able to do the same should this ever happen to you.

Some books that cover this topic are:

Tail Rotor Failure in the Hover

When the tail rotor fails while hovering, the nose of the helicopter will yaw rapidly to the right (in helicopters that have a rotor that turns anti-clockwise when viewed from above). You will automatically apply more and more left pedal to try and compensate but this will have no effect and you will eventually reach a point where you have full left pedal applied but the helicopter is still yawing to the right. You should now have realised that the tail rotor is not working.

You must immediately roll off the throttle and perform an engine off landing from the hover. By rolling off the throttle, you will have eliminated the torque and the helicopter will stop yawing immediately. Keep the helicopter level and let it settle. Just before the skids touch the ground, start raising the collective to reduce the rate of descent.

If your helicopter type uses a correlator to adjust the throttle (e.g. R22, R44, H269) make sure that you roll the throttle past the detent to prevent the correlator from opening the throttle up again as you raise the collective.

Tail Rotor Failure in Forward Flight

In forward flight, a tail rotor failure may not be noticed right away if you are traveling at high speed. At lower speeds it is noticeable by a nose right yaw (in helicopters that have a rotor that turns anti-clockwise when viewed from above). If airspeed is reduced, the helicopter will yaw further to the right.

The only option here is to eliminate the torque by lowering the collective and closing the throttle. Obviously you will have to perform an engine off landing but this should not be a problem to you if you have flat ground below.

On touch down, try to run on slightly. The helicopter will try to turn to the left as it slides along the ground. This could cause the helicopter to roll over but there is a way that you can prevent this from happening. If you open the throttle slightly, the increased torque will turn the nose to the right and if you close the throttle, the decrease in torque will turn the nose to the left.

I use a simple trick for this. Stick your left index finger straight out as you grip the collective. When you open the throttle, your index finger points right. This is the direction the nose will yaw and it will yaw by approximately the same number of degrees as the movement your index finger made. (Vice verse for closing throttle). So, by adjusting the throttle, you can prevent the aircraft from rolling over.

Stuck Left Pedal

It is possible that you may find yourself in the position that the left pedal is stuck in position (to the left) during flight. This could occur due to different reasons and it has been known to happen. The pedals will be locked in position. How do you land? Do NOT enter autorotation as you would for a normal tail rotor failure. Instead, experiment with different speed and power settings to get a feel for how much the nose yaws at different speeds. Set yourself up for a shallow approach (preferably to a concrete or tarmac runway where the helicopter can slide easily and the skids will not dig in). As you lower the collective for the descent, the nose will yaw even further to the left. Don’t worry about this. Gradually wash off the speed and be prepared for the helicopter to be yawing up to 30 degrees to the left. As the aircraft loses speed it will eventually lose translational lift and start to sink. You should only be a few feet above the ground at this point. As you raise the collective up to reduce the sink rate, the increase in torque will make the nose yaw right and the helicopter starts to straighten up. when the nose is almost straight, accept the speed and by using a combination of slightly forward cyclic to pitch the nose slightly down and also adjusting the throttle and collective; squeeze the helicopter down onto the runway. With a little practice, this is quite easy to do – but it does take practice.

Stuck Right Pedal

If it is the right pedal that is stuck, again, you must not enter autorotation. As with stuck left pedal, set yourself up for a shallow approach. Lowering the collective to start a descent will automatically make the nose yaw left due to the reduction in torque. You will have time to experiment with different speed and power settings to see what keeps the nose straight. Find a speed and power setting that gives a slight rate of descent at 10 – 30 knots while also keeping the nose straight. Do your approach with the intention of setting up these parameters before touching down. If it does not feel right on the approach, feel free to go around and try again. At a few feet above the ground and with the nose slightly yawing to the right, gently lower the collective. This brings the nose straight again and also makes the helicopter descend. As you touch down, use the throttle to control heading. Simple.

Instruction

Whether you are a student or a qualified pilot, it is beneficial to get an instructor to show you these tail rotor failure procedures every now and again. You never know when it might come in handy to be able to handle tail rotor failures.

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The Helicopter Engine (Piston)

Updated on 11th February, 2017

Piston EngineThe Piston Engine

Piston engines are the most common type of engine to be found in modern, light helicopters. Normally there will be 4 or 6 cylinders in a horizontally opposed configuration. For information about turbine engines, visit the Turbine Engine post on this website.

Pistons move back and fourth inside the cylinders. Inside each cylinder, fuel is mixed with air and ignited. The energy produced by the combustion of the fuel air mixture causes the gases to expand and drive the piston down into the cylinder.

The piston is connected by a connecting rod “con-rod” to a drive-shaft which is forced to turn due to the movement of the piston.

Piston engines may operate on either a two stroke cycle or a four stroke engine cycle.

The Four Stroke Engine Cycle

A complete cycle of the four stroke engine comprises four strokes of the piston moving within the cylinder. This cycle is also known as the “Otto cycle” after its inventor in 1876.

The four strokes are:

  1. Induction
  2. Compression
  3. Combustion (or Expansion) (or Power)
  4. Exhaust

Induction

Intake Stroke

The Intake Stroke

During induction, the fuel and air mixture is sucked into the cylinder through an open intake valve (on the right) as the piston moves from the top of the cylinder to the bottom of the cylinder.

The exhaust valve (on the left) is closed.

 

 

 

 

 

 

 

 

Compression

Compression Stroke

The Compression Stroke

Early in the compression stroke, the inlet valve closes and the fuel/air mixture is trapped in the cylinder. The piston then moves back up to the top of the cylinder.

This compresses the mixture and causes the temperature and pressure of the fuel/air mixture to rise.

As the piston reaches the top of the cylinder and completes its compression stroke, the fuel/air mixture is ignited by a spark from the spark plug. This causes combustion which causes the gases in the cylinder to expand.

 

 

 

 

Power

The Power Stroke

The Power Stroke

As the piston has passed the top of its stroke, the expanding gases force it back down the cylinder.

This is called the power stroke as the heat energy provided by the combustion process is now converted into mechanical energy.

Just before the piston reaches the bottom of its stroke, the exhaust valve will open.

 

 

 

 

 

 

Exhaust

The Exhaust Stroke

The Exhaust Stroke

As the piston returns to the top of the cylinder again, the burned gases are forced out of the cylinder and into the atmosphere through the exhaust manifold.

As the piston nears the top of its stroke and while the last of the burned gases are being expelled, the inlet valve opens in preparation for the next induction stroke.

 

In one complete Otto cycle, only one of the four strokes provides power – but the crank-shaft has rotated two times.

Engine manufacturers increase the power of the engine by adding more cylinders. This has the added bonus of making the engine run smoother. Each cylinder will have the power stroke occurring at different positions during the rotation of the crankshaft to try to even out the power impulses.

Compression Ratio

This is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (bottom dead centre BDC) compared to the top volume of the cylinder when the piston is at the top of its stroke (top dead centre TDC).

The compression ratio is designed to suit the type of fuel used. If the compression ratio is too high, the fuel may ignite early and excessive wear will occur.

Valves

Piston Engine Valve timing

Piston Engine Valve Timing

Both the inlet valves and the outlet valves must open and close at the correct times in relation to the movement of the piston. The timing of the valve operation is controlled by a camshaft. The camshaft only rotates at half the speed of the crankshaft. The camshaft operates rocker arms and pushrods that push to relevant valve open. When the camshaft releases the pressure, a spring returns the valve to the closed position. A typical helicopter piston engine speed in flight is 2700 revolutions per minute (RPM). Each inlet valve and exhaust valve opens once during the four strokes of the Otto cycle. I.e. once in every two revolutions of the crankshaft. This means that at 2700 RPM, each valve will open and close 1350 times per minute = 22 times per second. This is a very short time to get the fuel/air mixture into the cylinder and exhaust the burnt gases again. To increase the efficiency of the fuel/air mixture induction, the inlet valve opens before the piston reaches top dead centre (TDC). This allows maximum time to induce the fuel/air mixture into the cylinder. It is referred to as valve lead. Similarly the exhaust valve opens before the piston reaches bottom dead centre (BDC) on the power stroke. It is worth noting that for a very short time at the start of the induction stroke, the exhaust gases are still exiting through the open exhaust valve while the fuel/air mixture is being forced into the cylinder through the open inlet valve. This period of overlap when both the inlet valve and the exhaust valve are open at the same time is known as valve overlap.

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The Helicopter Engine (Turboshaft)

Updated on 12th February, 2017

Helicopter EnginesHelicopter Engines

One of the most common helicopter engines used today is the turboshaft engine. A turboshaft engine is effectively a jet engine and normally runs on a kerosene based fuel. There are many variants of turboshaft engines but they all follow basic concepts and principles. To keep things simple, I will be referring to the engine used by the Bell 206 JetRanger. The Rolls Royce Allison 250/c20J.

The working cycle of the turbine engine is similar to that of the piston engine. There is induction, compression, combustion and exhaust. The major difference is that unlike the piston engine, the turbine engines cycles are continuous. Unlike airplane jet engines where the exhaust gases provide the thrust, the exhaust gases for the helicopter turbo shaft engine are intercepted by a turbine that transfers the energy from these gases to a gearbox that provides power for the helicopter.

It is vital that you learn not to over-torque or over-temp the engine as this can have disastrous consequences for someone flying it, weeks or even months later. Misusing the engine will not necessarily cause a problem right away.

Starting the helicopter engine causes more stress to the components than just about any other operation due to thermal shock and wear. For this reason, the start cycles are counted and recorded in the technical log after each flight. As helicopters tend to fly for short periods compared to airplanes, it is possible that the engine could reach its start count limit before it reaches its useful life hour limit. Start-ups are expensive and so it is better to leave the engine running for ten minutes while waiting for a passenger rather than shutting the engine down and then starting up again.

The engine relies totally on the battery or Auxiliary Power Unit (APU)  for startup. The engine has to be turning fast enough to keep enough cold air flowing through the turbine otherwise it will become very hot and damage the internal components. A weak battery could run out of power before the combustion process becomes self sustaining and this would be disastrous.

A “Hung Start” can happen if the engine fails to accelerate and the RPM stays constantly low. This uses up your battery power and if the battery runs out of power then the engine slows down, less and less cold air is drawn through the engine and subsequently the temperature in the combustion chamber becomes really hot within a few seconds causing a lot of damage.

A “Wet Start” is the equivalent of a flooded engine and the igniter has failed to light the fuel. Wait for at least 5 minutes and then vent the engine. Because the battery has already been partially drained by the failed start and the venting, it is probably a good idea to get an external start using an APU to make sure you do not run out of power on your next start attempt.

If you follow the check list exactly then you should have no problem starting the helicopter engine every time. The most common reason I have seen for hot starts is low time pilots starting the engine with the throttle already partially (or fully) open.

Therefore it is vital for you to double check that the throttle is fully closed before pressing the starter button.

Turbine engines take time to “Spool up” or “Spool down” when you make power changes. For this reason you should operate the collective or throttle very smoothly to prevent engine “surging”. Surging can happen when the airflow over the compressor blades becomes disturbed causing them to stall. This leads to loud banging noises (similar to a shotgun going off) and a very noticeable vibration from the engine. Shut down immediately if this happens during startup.

Turboshaft Engine

The Air Inlet

The air inlet is designed to stabilize the air before it enters the compressor. A particle separator may be fitted at this point to remove any foreign matter such as dust or sand which could cause erosion of the compressor blades.

The Compressor

The compressor is made up of a series of blades and an impellor (or centrifugal) compressor. It is designed to take large quantities of air and compress it before directing it to the combustion section.

The engine in the JetRanger has a six stage axial flow compressor and a single impellor compressor.Helicopter Engine 01

In the axial compressor, each stage is separated by stator vanes to make sure the air hits the following blades at the correct angle. As the air flows past these blades it becomes compressed more and more. The pressure rises and its velocity decreases.

The centrifugal compressor diverts the air outwards into channels that lead to the combustion section while compressing the air further.

During startup there is a bleed valve that opens and allows some of the air to escape from the compressor. This makes it easier to get the engine up to speed and takes less power from the battery. When the engine reaches a sustainable speed the valve closes automatically.

Because of the high temperatures of the compressed air in the compressor section (up to 250 degrees celsius), this air is used to heat the cabin and for anti-icing. Anti-icing uses air from the rear of the compressor and directs it through the compressor casing and the inlet guide vanes to prevent ice forming there. When anti-icing is used there will be a small rise in Turbine Outlet Temperature (TOT).

The Combustion Section

In this section the fuel is mixed with the air and ignited. The air ducts are shaped in such a way that the flame never comes into contact with the metal casing but instead is contained within a shroud of cooler air. A large proportion of the air is used for cooling. Once the fuel is lit and the engine is up to running speed, the combustion is self sustaining. The engine is turned initially by a starter. When there is enough airflow through the engine to keep everything cool, the fuel is ignited by the igniter plug. The fuel enters the combustion chamber through a fuel nozzle that atomizes the fuel. On ignition the gases expand and flow to the turbine section at an increased velocity. Approximately 60 to 80% of the air entering the combustion chamber is used to keep the liners cool. The fuel nozzle is highly polished and engineers have to handle it very carefully as the tiniest scratch will disturb the spray pattern and cause hot spots which will eventually damage the turbine blades. It has holes for delivering fuel.

The Turbine Section

This section creates the power. The turbine inlet is the hottest part of the helicopter engine and it is too hot for temperature sensors to survive here. Temperature is therefore measured between the turbines by thermocouples (the readings are averaged and displayed in the cockpit instrument display) and is called the Turbine Outlet Temperature (TOT). The heat is kept to a manageable level by cold air extracted from the compressor which is driven through a connection by the turbine. The gases are directed through the compressor turbine blades (N1) thus ensuring that the compressor is continually powered. From there the gases pass through a two stage “Free Turbine” (N2). As the free turbine is not directly connected to the compressor, the engine is easier to turn during startup. The free turbine is connected to the Accessory gearbox which reduces the high speed of the turbine to a more manageable level. When more power is required, the compressor speed (N1) increases to supply more air. At the same time more fuel enters the combustion chamber and therefore N2 is maintained at a constant speed. The turbine blades are operating in a very hostile environment. As the temperatures are so high and the blades are spinning so very fast, centrifugal force causes the blades to stretch (blade creep). This is normal, however if the engine has had a hot start, the blade creep becomes much larger than normal and becomes permanent. The blades can make contact with the sides and expensive repairs will be required. Higher TOT temperatures are permitted during startup as the turbine is spinning relatively slowly. The N1 turbine is doing more work than the N2 turbine. It is also exposed to hotter gases. For these reasons the N1 turbine has only half of the service life of the N2 turbine.

The Accessory Gearbox

The accessory gearbox converts the high speed of the free turbine (N2) to a more manageable level. It has a drive-shaft powering the main rotor gearbox, a rear drive-shaft powering the tail rotor, a freewheel unit and attachment points for all the accessories such as fuel pump, tachometers, generator etc.

Compressor Stall

Compressor stall can occur on any turbine engine if the conditions present themselves. In order to meet the design requirements, the engine must have a relatively high power output, good fuel consumption and fast acceleration characteristics. For these reasons it is beneficial to operate as closely as possible to the stall angle of the compressor blades. Operating close to the stall angle has the following benefits:

  • The volume of air passing through the engine is increased.
  • The pressure ratio of the engine is increased thus increasing the power output.
  • Turbine temperatures can be increased because of the greater airflow.
  • The efficiency of the compressor and turbine sections are increased.

To reduce the risk of the compressor stalling during startup or acceleration, the fuel flow is carefully regulated. So what is compressor stall? Many pages could be devoted to explaining this but the following explanation should help answer the question. Compressor blades and vanes are aerofoils. The airflow over an aerofoil will separate and become turbulent if either of the following occurs:

  • The velocity of the air passing over the aerofoil is too low.
  • The angle of attack is too high.

If the airflow over an aerofoil separates then the aerofoil stalls. Approximately 80% of the air entering the engine is used for cooling. This means that much more air has to enter the engine than what is needed for cooling. The cooling air is used to control the length of the flame in the combustion chamber and prevent it from touching the sides of the container. The hot combustion gases are cooled by the cooling air and the cooling air is also heated by the combustion gases. This keeps the gases at an acceptable temperature as they mix and enter the turbine section. If too much fuel is supplied to the burner, there will be more than enough air to allow proper combustion. However as extra air is used during this combustion, there will be less air available for cooling and therefore the temperature inside the combustion chamber will rise. As the temperature rises, there will be more gases to be exhausted. It is possible that the volume of gases to be exhausted may exceed the capacity of the turbine and the turbine will “choke”. When this happens, the pressure inside the combustion chamber will rise rapidly and may equal or exceed the pressure that the compressor is producing. If the pressure in the combustion chamber is equal to the pressure of the compressor discharge air, then the compressor will stall. If the pressure in the combustion chamber exceeds the pressure of the compressor discharge air, then not only will the compressor stall but also the hot gases will flow from the combustion chamber into the compressor section. Both of these conditions will result in a loss of air into the combustion chamber. The flame will not have enough oxygen and will die, resulting in a rapid drop in temperature. As the temperature drops, the expansion is stopped (or greatly reduced). The turbine is no longer choked and the combustion chamber pressure drops to a very low value. The low pressure in the combustion chamber means that air can flow in the proper direction again. The compressor is no longer stalled and a “Surge” of air flows back into the combustion chamber. This extremely fast movement of air elongates the flame downstream and through the turbine causing another rapid expansion of the gases. The cycle repeats itself at approximately 120 times per second. Compressor stalls may or may not have an audible sound but there will often be a vibration. If the stall is severe a flame may emanate from the exhaust or a very loud backfire may be heard. Smoke may also be seen. If the proper corrective action is made immediately then it is unlikely that any damage will occur. The actions to be taken are:

  • Reduce the throttle to flight idle.
  • If the stall remains then close the throttle completely and shut down the engine.

Rapid throttle movements may induce stalling therefore it is a good idea to make smooth, slow throttle movements.

Summary

Helicopter engines have become very sophisticated. turboshaft engines are extremely reliable as long as they are maintained and operated correctly. This post has only just touched on the subject of turboshafts but if you would like more information then please let me know.

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