Autorotation (auto for short) is the method full size helicopters use to glide down safely and under control when there is a loss of power. Most collective pitch RC helicopters can also "auto" so if the engine flames out or the motor quits for some reason, you will be able to save your bird.
OK – I hear some people saying helicopters can’t glide, they drop like a stone if the engine quits – that is true if you don’t have autorotation capability on your collective pitch RC helicopter, but if you do, you can glide your RC heli in for a perfect landing as seen in the simulation below.
So how does something without wings glide you ask? Our helicopter does have wings – they are called rotor blades. Rotor blades are simply rotating wings, and as long as they are rotating fast enough, they provide lift. The question is how to keep them rotating fast if there is no engine power?
You need two things to achieve this.
First - do you remember me talking about the other benefits of
in the Fixed Pitch or Collective Pitch section? Well, here is one of those benefits. Yes you need collective pitch control for autorotation to work. This is what allows you to keep the rotor blades spinning fast.
How? When your engine quits, you immediately change the pitch angle of the main rotor blades from positive pitch to negative pitch. By doing this the rotor blades will continue to keep spinning in a forward direction as the air rushes by them while the helicopter is descending.
The whole idea behind this is to build up rotor speed as the heli descends and then as the helicopter nears the ground you change the pitch angle of the rotor blades back to positive. The kinetic energy built up in the rotor allows you enough power to stop your descent and make a soft landing.
This is a bit tricky. Having the right amount of negative pitch and positive pitch is very important (generally -3 degrees to +12 degrees but it depends on your helicopter). Auto rotations work much better if you have some forward speed providing "clean" air (also referred to as
) preventing the heli from descending too quickly while still generating lots of energy in the rotor. This is generally referred to as "glide path" and a forward glide path of about 45 degrees provides a good slow descent rate while building lots of rotor energy.
The key to a successful autorotation is all in the timing and knowing when to slow your descent and forward speed by flaring with a rear cyclic command, applying positive collective pitch, and then leveling out the heli just before touch down with forward cyclic to land gently. All that without dissipating all the energy in the rotor before you land - remember, you only get one shot at this.
Practice makes perfect and this is where your trusty
can help with the dynamics of what all is going on.
If you had fixed pitch or kept the rotor blades at a positive pitch angle, they would soon stall and stop rotating. Yup - the helicopter has now become an expensive rock. This is why autoroations rely on collective pitch.
The second thing your helicopter needs to perform auto rotations is a way to disconnect the main rotor shaft from the rest of the drive or gear assembly (the engine/motor and in some cases, the tail rotor). If the rotor shaft stayed connected to these other drive devices, it wouldn’t keep spinning. Plus you don’t need the tail rotor spinning during an auto rotation since there is no more torque being developed with no power going to the main rotors. That said, there are many aerobatic RC helicotpers that do keep the tail rotor spinning for yaw control during the auto. Keep in mind however, the more tail input you give, the more power you are robbing from the rotor disc so keep tail inputs small when first practicing.
We don’t actually physically disconnect the rotor shaft from the rest of the drive components - but we do allow it to spin freely, by using one way bearings on the rotor shaft. This is the same idea as a bicycle’s rear wheel. While you are pedaling, there is power going to the rear wheel - but when you stop pedaling, the rear wheel is free to spin. If this wasn’t the case, the pedals would keep rotating and you couldn’t coast.
This is exactly the same principle that is used in a RC helicopter with autorotation capability. As long as the engine is supplying power, the energy is transmitted to the main rotor shaft through the locked one-way bearings. If the engine quits, the bearings now rotate and the rotor shaft is allowed to spin freely – just like coasting on your bicycle.
With no engine noise, the only thing you can hear is the rotor slicing through the air. Hearing how the noise changes as the blades are changed from negative to positive pitch just before touch down is very cool to experience and is why the auto has become a crowd favorite and is so popular in RC Heli competition events.
Here's a pretty neat video of a full size S-92 heli performing an auto rotation. Note the huge amount of flare required to dissipate forward speed.
The Setup & Tips For Electric Collective Pitch RC Helicopters e-book is all about setting up a collective pitch RC helicopter with a computerized radio to turn it into the perfect trainer. Click on the image of the book to find out more about what information is covered in this, my most popular & #1 selling e-book.
The 120/140 Degree Swashplate Setup & Levelling e-book shows how to properly setup and level a 120,135, or 140 degree electronically mixed swash to attain perfect interaction and a trimmed out bird off the bench. Click on the image of the book to learn more about it.
RC helicopter training gear is the essential training aid used when learning to hover and fly most RC helicopters, this e-book shows how to build them. If you want to learn more about what's inside, click on the image of the book.
Thinking of getting into planes too, or trying to decide between RC helicopters or RC airplanes? If so, the Beginner's Combo deal which includes both the Beginner's Guide To Flying RC Helicopters and the Beginner's Guide To Flying RC Airplanes e-Books, represents excellent value for anyone looking to get started safely and quickly in either or both forms of radio control flying.