Autorotation - Beginners Guide

Introduction

Autorotation is a manoeuvre that should be learned fairly quickly after you start flying circuits with your Swift. The reason for this is that should anything go wrong being competent to perform an autorotation could well save you from an expensive crash. However, whilst this is the logical reason for learning to autorotate a far better reason is that it is a very satisfying flight manoeuvre once you have mastered it.

Before embarking on learning this manoeuvre the first question really ought to be "how well does the Swift auto rotate?". The answer to this is that the Swift is capable of excellent autorotation's but there are a couple of things to be aware of before making an attempt.
The first of these is that the Swift does not have a driven tail during an autorotation. This means that during the manoeuvre you will not have tail rotor control. The Swift will naturally weathercock into wind or into the direction of it's flight.
The second is that in order to improve your chances of success you must run a good blade that will give you plenty of inertia when you come to flare out and land. The wood blades in the kit are capable of autorotation but it is generally accepted that carbon blades will give you more inertia (thinking time) at the crucial endpoint of the autorotation.

For the purposes of this article I am using MS-Compsit 515 mm carbon blades. My experience with this blade have been that it offers very good performance for autorotation purposes.

Autorotation Theory

This is an area of much misconception. Most people understand autorotation in the following way:

In order to autorotate the engine/motor is cut and negative pitch is applied to the blades. The helicopter falls through the air and the air passing through the blades speeds them up. The inertia built up in the blades is then used to land the helicopter at the end of the autorotation.

Whilst this appears to be what is happening and seems like a logical explanation it is not quite correct.

Applying negative pitch to the blades does allow the helicopter to fall through the air and the act of doing so applies a force to the blades which keeps them spinning (like a sycamore leaf). However, in order to obtain an increase in speed on the blades the requirement is to convert the energy in the falling helicopter into a stronger force driving the blades. This is done by increasing pitch towards positive pitch near the end of the autorotation. In fact the most effective autorotation pitch angle for maximum blade speed is around 0.5 degrees positive pitch. This increased blade speed and extra inertia is then utilised with larger amounts of positive pitch to bring the helicopter to a halt, hover briefly and then land.

Applying high degrees of negative pitch will cause the helicopter to fall faster, therefore when the small amount of positive pitch is applied there is more energy to be converted into blade inertia. It is not the act of applying large amounts of negative pitch that creates blade speed/inertia it is the act of converting that rapid rate of descent (energy) back into blade inertia at the closing stages of the autorotation.

In practice it is very unlikely (as a beginner) that you will be thinking about this but as you become more proficient it is worth remembering and utilising this knowledge to perform better and safer autorotation's.

For beginners it is sensible to limit the amount of negative pitch that can be applied to something like -3° as anything more than this will induce an increasingly rapid descent. A beginner to autorotation will very often panic when faced with this situation. Having less negative pitch available means that the helicopter will fall more slowly and therefore there is less energy to convert back into blade inertia. This is preferable to the beginner who will need more thinking time on the way down to plan their landing. This also means that there is more time to abort the autorotation if it isn't looking good.

Autorotation Setup

Setting the Swift up to practice autorotation is really quite straightforward. We do not need to worry about throttle curves as during an autorotation the motor is switched off. Therefore the two main areas to concentrate on our the pitch curve and the throttle hold switch. A further area of importance is the electronic speed controller but I will cover this separately later.

Most helicopter transmitters have a throttle hold switch. This switch should be enabled and its function should be checked to make sure that when the switch is toggled the motor is cut. Usually the throttle hold setting on the transmitter allows you to set a percentage for the throttle hold. On electric models the value should be zero or 0%. Values above this are only used on nitro machines where you want the engine to idle but not cut completely.

As a beginner you may already have a shallow pitch curve that doesn't have more than -3° of pitch. If this is the case then there is no need to adjust your pitch curve.
If you are running a full pitch curve that goes from (as an example) -10° to +10° then you will need to adjust your pitch curve. Many transmitters have a separate pitch curve that you can utilise when the throttle hold switch is engaged. If you're transmitter does not have this function then you will need to alter your standard pitch curve. The only change required is to make sure that from 0° pitch downwards you have a straight line linear pitch curve to -3° pitch. The pitch curve from 0° upwards should be left untouched.

Below two pictures showing this pitch curve modification.

Of course having made this change you should check with a pitch gauge that you do have the required pitch range of -3° to +10°. If not then adjust accordingly. Having made your changes check on the bench that you are getting the desired results when you toggle the throttle hold switch. You can also check that the hold switch is working correctly at the field by spooling the helicopter up and hitting hold whilst it is still on the ground.

Electronic Speed Controller Setup

Most helicopter electronic speed controllers have a soft start option. This option makes your helicopter spool up very slowly. Very often this effect will also be induced by using the governor function on a speed controller.

When practising autorotation's it is undesirable to have this soft start enabled. The reason is that if you have cut your motor in order to do an autorotation and you decide you want to abort, you do not want to have to wait for a slow spool up in order to get the power back on. Therefore it is preferable for autorotation practice that "slow spool up", governor mode or soft start is disabled. Otherwise you may be completing your first auto long before you are ready.

One thing to make sure of having disabled soft start is that your blades are quite tight in the blade grips. We don't want the blades folding in the grips if the motor start is now a bit rapid. On some ESC you might get away with just 'soft start' rather than 'very soft start' but I would be very careful in what you select as you need to be able to power up immediately if an auto needs to be aborted.

Low Practice

Okay, you have setup your Swift, installed some nice carbon blades and you are ready to start practising. Where do you begin?

Initially, we want to learn what it is like to land the Swift without the motor running. This will give us a feel for what the Swift is like to control without the motor. It will also give us an understanding of how long the Swift will hover before sinking to the ground due to the blades slowing down. The way to learn this is a little step at a time - sometimes referred to as a throttle chop. First hover the Swift at knee height and then hit the hold switch. Don't worry, nothing exciting will happen other than it will hover briefly and then sink to a soft landing. Gradually increase the height that you hit the hold switch until you are doing it from approximately waist to chest level. Do not go any higher as you will definitely run out of blade inertia and land quite hard from above chest height. Make sure you are very comfortable doing this. The point is to get a good feel for how long you can hover before you have to land due to lack of blade speed. You will also learn how the helicopter behaves as the head speed decays. Typically helicopters become less stable as the head speed drops.

Once you are comfortable with this it is time to move on to high practice.

High Practice

Before attempting a practice autorotation from high up you should be competent to fly circuits high in the sky. You should also be able to fly your helicopter towards you and be able to hover the helicopter both side on and nose in.
The reason you need to be proficient in all of the above is that once you cut the motor you may find yourself in any one of these flight situations.

The Swift does not have a driven tail once you cut the motor. Therefore the Swift will either weather vane into wind or turn into the direction of its forward flight when you hit the throttle hold switch. If for any reason the helicopter turns you will not be able to correct it using the rudder. In fact after a second or so you will see that the tail rotor has stopped completely when you are autorotating.

Initially get yourself into the swing of things by doing powered descents from up high. Effectively you want to fly an autorotation but without having hit the throttle hold switch. Do two or three of these until you feel comfortable and are used to seeing the Swift dropping down in a fast descent.

Okay, so we are ready and are going to attempt an aborted autorotation. Effectively we are going to hit the hold switch and autorotate down to around 30 feet and then abort. Which means flicking the throttle hold switch and turning back on the motor. We then power up and fly a circuit to calm down. The sequence of events for this maneuver is as follows :

1. Fly up high (50 feet) or more to your start position making sure your are pointing into wind and in slow forward flight.
2. Toggle throttle hold and keep your finger on the switch ready to turn it off again.
3. Pull back on the collective pitch to enter autorotation (-3° picth)
4. At 30 feet turn off throttle hold
5. Raise the collective stick to power up and fly a circuit.

Practice this until you feel very comfortable aborting at 30 feet. Then reduce the abort height to 20 feet and then 10 feet. Do not come below 10 feet with an aborted autorotation.

Some things not to do :

Do not push the cyclic forwards in an attempt to gain forward speed. The helicopter will just dive and drop very quickly.
Do not try to autorotate downwind or crosswind, always into wind.
Do not pick a flat calm day to practice, you will get into trouble.

Something to definitely do :

Pick a nice windy day to practice, autorotation is much easier in a nice stiff breeze.

Putting it all together

At some point you are going to have to join together the high and low practice manoeuvres. Pick a day with a good stiff breeze and head to the field with the full intent to hit the hold switch and come all the way down.

Joining the high practice manoeuvre to the low practice manoeuvre is not that difficult. In order to make things as easy as possible for yourself you want to autorotate from up and behind your left or right shoulder down to directly in front of you. Effectively you are going to fly past yourself and land "tail in" in front of yourself. In order for this to work you must be facing into wind. Perform a couple of low-level practice autos and then do a couple of high-level aborted autos just to get a feel for the conditions on that day.

For your first attempt you are looking to introduce positive pitch and slow your helicopter as it hits head height. You want to be bringing in the positive pitch quite quickly as the Swift reaches head height. It should slow and enter a hover at chest to waist height and from here you will know exactly what to do. You will have some forward speed as you come past yourself and you need to be pulling back on the cyclic to slow and flare out as you bring in the positive pitch. Hang in there and don't be tempted to flick it out of hold only a couple of feet in the air. If you do run out of blade inertia then the resulting crash will do virtually no damage as there will be no inertia left in the blades. However if you attempt to flick it out of hold you might well still crash but it will be under full power and cause a lot more damage.

One Last thing to be prepared for is that you may get some rotation (nose to the right) in the last dying second of the auto. This is just part and parcel of not having a driven tail.

Update (26/11/2007):

I recently had a question on how to deal with the nose right rotation at the end of the auto as it can be very off putting. More importantly in certain circumstances you can end up trying to finish your auto nose-in, which really isn't desireable if you are already using all available brain power (and andrenalin) to do the auto in the first place.

OK, so here's the tips. The Swift always rotates nose right and the rotation is normally about 90 degrees. In extreme cases it can be 180 degrees if you hold the pitch on for too long. Therefore if you initiate your autorotation coming from the left to the right then in all likelihood your landing will be nose in. It could rotate beyond nose in. However, if you autorotate from right to left so that as you come into your flare out the left-hand side of the helicopter is facing you then the nose right rotation will put you into a tail in landing. When practising your first autorotations it is best to the only try them if you can do a right to left approach. The rotation of the helicopter will only occur once it has no longer got any forward speed. So it is essential to try to keep some forward momentum throughout the manoeuvre. If you are flying from a smooth surface or close mown field then you can avoid this phenomenon altogether by keeping a small amount of forward speed and doing a sliding autorotation. The last alternative which will only reduce this effect rather than eliminate it is to fit a larger vertical tail fin which has a greater surface area. This will provide more resistance to the torque rotation therefore reducing the amount that the nose swings to the right.

That's all there is to it ;-)

Obviously there are various types of auto but unless the Swift gains an upgrade to provide a driven tail the likes of backwards or 3D autos are somewhat limited. Personally I like it as it is as a driven tail would dramatically reduce it's current very good autorotation performance.

I will be adding to this article some videos showing the throttle cut, aborted auto and the full auto. Hopefully I can get the footage in a short period of time once I've found a willing camera person.