How To – The Art Of Autos
In this, the latest in his series of articles on aerobatic training, John Parker gets stuck into the art of autos.
Hopefully, you’ll be getting pretty confident now, doing those loops, stall turns, and rolls that we’ve been teaching you to perform in this series. Time to move on and consider attempting an auto.
For the purposes of this article, autorotation can be described as the collecting of energy, followed by the storage of it, and finally the act of expending it, in order to achieve a controlled landing. As a schoolboy, I always remember throwing sycamore seeds into the air and watching them rotate helicopter style down slowly; this happens because the wing on the seed is actually in autorotation, just like a real helicopter in the descent. It has a negative angle of attack, and is aerodynamically capable.
First of all we must ask, is your helicopter capable of autorotation? Looking at the more modern machines, you will have one of two types. First is the non-driven tail heli, which means that, if you turn the rotor blades in the direction of travel, they rotate very freely, and will drive no other part on your machine. On the other hand, a driven tail helicopter is fitted with a split gear, thus when the rotors are turned in the direction of travel, the tail goes with it. Note, though, that the split gear will disengage the main gear and clutch stack from the drive system by means of a one-way bearing, which reduces drive-train drag.
Any capable pilot should have the autorotation in his bag of tricks, and here’s why. First of all, it’s an emergency manoeuvre in the event of partial mechanical failure; then, there’s always the possibility that you could run out of fuel halfway through your flight – in such an event, the auto may be your last chance to save the model. The full-size, vintage (cloth bombers) flying fraternity have a saying: “There are two types of pilots – those who’ve had an engine failure, and those who are going to have an engine failure.” Yes, I think this saying holds true for us helicopter pilots too.
Having fun is the other main reason for learning the auto, because I guarantee you will enjoy bringing your helicopter down with no engine, whereupon you have a one-time chance of a good landing.
To execute autos properly, your helicopter needs to be set up correctly. To begin with, activate the throttle-hold switch, and programme an appropriate pitch curve that can be assigned to it. Consider buying some glass or carbon blades too because, although they’re not a necessity for learning autos in the early stages, you will want them later.
When switched in, throttle-hold will keep the throttle at a pre-set position; for the competent flier who can auto, this will hold at tick-over or idle, while still allowing full collective (pitch) travel at the stick. Since I fly 3D, my own pitch curves are all set the same, i.e. my normal, idle-up 1, idle-up 2 and throttle-hold are all plus and minus 10° with zero at centre stick. I’m very aware of my stick position and ‘feel’ of the helicopter in flight, and with these parameters all being the same, there is no noticeable jump as I switch between modes.
To begin with, copy your normal pitch settings to the throttle-hold programme – this will give you a starting point that can be used as a basis for some fine-tuning. In the ‘hold’, you need as much positive pitch as you can safely achieve; I would think 10 to 12° is obtainable from our modern machines. With the pitch gauge on, the bottom stick position only needs to be about -2 to -3°.
I learnt autos on my own, after reading books, and magazine articles like this one. There is no doubt helicopter flying starts with the hover and will always end with the hover, so I started to auto FROM the hover, with hover chops. Nope, I’m not talking about some sort of oriental cuisine – ‘hover chops’ is a procedure I used to gain the feel of the helicopter in a power-off condition, at a low and safe altitude. On the hover chop note, anyone seeing me fly will notice I never seem to land with the power on; I always chop the throttle in the hover, a common habit for heli pilots because the control system becomes much softer as the head speed is reduced.
Right, back to our hover. If you were hovering at, say, only 1ft, and switched into hold, what would happen? Well, providing your ‘hold’ pitch curves are a copy of your flying curves, albeit stretched in the positive and reduced in the negative regions, with the throttle set to idle, your helicopter should – if left alone – sink softly to the ground. What happens is, at the very low altitude in which we chop the throttle, all the stored energy (inertia) in our blades decays to such an extent that no more energy is left to support our model at the positive pitch we use to hover.
With practice, hover chops can be executed to a height of, say, 4ft. quite safely. Any higher than this will require flying into a breeze, or gaining some forward speed.
Cut And No Thrust
At this point then, using the above information as a guide, you should be able to start learning to cut the throttle in the hover, whereupon you land with some control on the collective. The higher you hover and cut the throttle, the more you will have to use the collective to get her down. You’ll not be able to hold all that positive on at the greater heights, because the energy will be lost far too early before landing – at this stage, you have to reduce collective early on when high, then re-apply when into the flair and landing stage.
The textbook standard approach for an autorotation is this: fly into wind from your favourite side, at a comfortable height of perhaps 50 metres, and moving at a medium speed. With the helicopter still just behind you, level its attitude, and switch into hold. As the switch is thrown, softly lower the collective to -2 or -3°, with the engine now at idle. At this point, it is very important that you keep the attitude of the model level, because this will control your approach speed in the descent – with only -2° on the collective, you should achieve a nice steady pace on the way down, with no dramas.
At something like 10 metres from the bottom, bleed some of that forward speed off without losing much, if any, of the stored energy in those turning rotors – this is what we call the ‘flair’. At this point, you are making the transition from forward flight to the familiar hover. Steadily apply some back-cyclic with the collective still at -2°, and arrest the descent, using the reserve of energy in those blades to do so. With the forward speed greatly reduced, and at a low altitude, remember to execute those throttle chops, slowly apply the collective, and land.
Reality time: we are all human and suffer from nerves, which can make the above a lot harder in reality than it appears on paper. But, there is a nice little trick you can use to help your confidence, and build up some ‘feel’ for the helicopter in the auto…
Set the throttle on your hold function higher than idle. In fact, you could set it so the engine is still driving those blades around, but not enough to provide a hover in the hold condition. You will be amazed how well this works – try the hover chop from 5ft., and you will find plenty of time to land. Then, go out higher and try a proper approach with this new set-up. Now, you should start to gain a feel for the helicopter when making an approach with reduced power. When learning, I used this technique myself, gradually reducing the throttle to idle as my confidence and ability grew.
Auto-matic For The People
Try to pick the right day for practicing autos. As you become confident with the standard manoeuvre in light-wind conditions, be wary of very still days, where you will have little or no transitional lift (an increase in rotor efficiency, due to the rotor disc acting like a solid wing in forward flight when hovering). Remember, a helicopter requires less power to fly forwards than it does to hover; as a result, autos are a lot easier when executed facing into a breeze. If you fly a non-driven tail helicopter, you’ll notice the heli swinging round in the direction of the blades upon landing – because the tail is not driving in the auto, it cannot counteract drag on the rotor system, so chances are your tail blades will stop turning on the way down. Don’t worry though, because the forward momentum and tail fins will stop your helicopter from turning around in flight.
Beware of bad habits. One of my old ones was diving the helicopter upon switching into hold, which meant I was arriving unnecessarily fast, with too much workload for a learner. Still, I guess it gave other people a lot of fun, watching me wrestle with the model! We got there in the end, and I must say that autos became addictive. One thing I did do was to buy myself some semi-symmetrical blades early on because, for aerobatic inverted autos, these double the time you can spend in the descent when compared to wooden equivalents, plus the flair is greatly extended time wise.
As you master the standard auto, try adding some more negative pitch, because this enables you to keep an approach on the right track by starting to fully control your machine all the way down. If the day is reasonably calm, your descent path will be, say, only 40° from horizontal; however, on a windy day, the path will change up to 60° from the horizontal. If you are really clever, on breezy days a near vertical descent is achievable.
Generally speaking, the auto will play a part in most pilots’ every day flying – indeed, once mastered, the hold can be switched in at almost any altitude. My own favourite trick is, when coming into land quite fast and at, say, 30ft, I level off, and then flick into hold before cruising in at 0° pitch; finally, I lift the nose with some negative pitch to lose the speed without gaining any height, then re-apply pitch and land.
Well, all that’s left is for you to go out and try. Like any manoeuvre, it’s practice that makes perfect.