Helicopter Model Basics

TX Helicopter Model Basics

 Your instructor: Craig (Fig ArchmageAU on RCGroups and HeliFreak)

  Introduction

So, you have an OpenTX radio and have decided to try the world of RC helicopters. This is a primer to let you know the basics of how the control of an RC helicopters is different to all other forms of aircraft and what this means to the programming of a transmitter.

RC Helicopters are arguably the hardest to learn to fly. Mistakes are quickly punished. You also cannot take your eyes off them for a second as they generally will not fly themselves.

Without further ado, let’s jump in.

Basic Helicopter Controls

This section deals with the basic controls (telling it what to do). The next section deals with how a helicopter works (some of the mechanics), and finally the last section is on flying modes (conventions used in controlling a helicopter which helps to some knowledge of the mechanics).

A helicopter can move in all basic directions. Up/down – Left/right – Forward/backward. However, there is only one main source of power, the main rotor.

The helicopter will only go where the main rotor is pointed. You can tilt the main rotor forward/backward (pitch) or tilt it left/right (roll). It’s uncomfortable to travel backwards, so you can also turn the body of the helicopter left/right to point in any direction (yaw).

The other way to think of it is rotation. If you put a rod vertically though the main shaft of the helicopter turning the rod left and right is yaw. If the rod is through the helicopter from nose to tail, turning the rod is roll. If you put the rod through the heli from left side to right side, turning the rod is pitch.

Next is how much lift/downforce the main rotor can make. This can be controlled 2 ways. First is by increasing or decreasing the motor speed (throttle). Second is by changing the angle of the rotor blades (collective).

Collective pitch helicopters can change the angle of the blades relative to each other. They effectively twist, changing the angle that the airfoil shaped blade meets the air, thereby changing the amount of lift the blade produces. RC collective pitch helicopters can change this angle so far that the blades can be commanded to push down instead of up (often allowing the helicopter to fly upside down) without changing the direction of the motor.

Fixed pitch helicopters have blades at fixed angle to each other (and can generally only fly upright). Their thrust can only be controlled by blade speed (throttle).

Coaxial helicopters have 2 counter rotating main blades on a single shaft (usually fixed pitch) and are very stable. Like fixed pitch, thrust can only be controlled by throttle.

Mulit-rotors (quads, drones, octo-copters, etc…) tend to have multiple fixed pitch propellers, and fly with the same controls as a fixed pitch helicopter. Some advanced multi-rotors can reverse the motors to spin the propellers backwards and thereby fly upside-down.

So in summary, a helicopter has up to 5 basic controls / channels:

  • Aileron (roll)
  • Elevator (pitch)
  • Throttle
  • Rudder (yaw)
  • Collective (nick) – for collective pitch models only.

This is one more control than we have sticks axes. (axes is the plural of axes….weird English language).

Aileron and Elevator are known collectively as cyclic. Remember the earlier description of the rotor movement (tilt forward/backward or tilt left/right).   The cyclic are the main control of the tilt direction of the main rotor, and such are often grouped together.

Collective is the unusual one. As discussed earlier, collective is the angle (pitch) of the main rotor blades. Helicopters with fixed pitch blades do not require collective control (as their blades do not twist relative to each other).

From here on I will assume you are interested in collective pitch helicopters.

Rudder controls the turning of the helicopter round the main shaft. Think of it like steering a car. If looking from the back, left should make the front turn left. Right should make the front turn right. The mechanics of how rudder works is different between different type of helicopters (single rotor (including collective and fixed pitch), coaxial (described earlier) and multi-rotor). How various helicopter controls work will be discussed later in this lesson.

Since we only have 4 stick axes and 5 controls, the normal way to control a helicopter is to make the throttle axis control both throttle and collective. This is done by assigning two curves to the throttle stick. First one is a throttle curve, the second is a pitch (collective) curve.

These curves allow the same stick position to transmit different values on different channels. Different flying modes can change the curves assigned to the stick while in flight to allow the helicopter to fly the way you (the pilot) wants. This is useful to allow a gentle takeoff, then change to a more aggressive flight mode to zoom around, then back to gentle to land.

 How a Helicopter flies

** Upfront warning, this is going to get a little rough (and it's only the basics). You can skip this if you already know how a helicopter flies. Chances are you may learn something though. **

glider_text_boxAs you can imagine, a helicopter is anything but subtle. The only more brutal flying machine is a rocket (and it needs to work out how to bypass the air instead of use it).

So how is this brutality administered?

I’ll start simple. On all rotary flight craft (helicopters, multi-rotors), there is a throttle. This controls the speed of the lifting rotor (or rotors).   It’s pretty simple. Raise the get-louder control and the motor(s) make more noise and start their assault on the air.

Now onto some specific differences. Let’s start with the simple 3 channel coaxial helicopter (two counter rotating blades on the same shaft, tail is a horizontal propeller).

Each blade on this helicopter is like a propeller. The faster they spin, the more air they push down. Because on a coaxial, the 2 blades rotate in opposite directions, the torque (rotational force opposite to the spinning blade – ie. for every action, there is an equal and opposite reaction—Sir Isaac Newton lives!) cancels out.

Why does it not tip over? On the top blade is usually a flybar.

What’s a flybar? A flybar is a counterweight system (either weights or paddles) that spins with the main blade tilting it on each rotation to keep it stable. The flybar works like a spinning top, if it starts to tilt one way, it makes the blades produce a force to tilt it back. It’s effectively a mechanical gyroscopic stabilization system.

If both the upper and the lower blade produce the same torque, the main body of the helicopter remains pointing in the same direction. Speed up one blade or the other, the torque can turn the helicopter. This is what the rudder control on these helicopters do (adjust the difference in the blade speeds).

(As a slight digression, this is how a multi-rotor turns too. By having propellers that turn in different directions (some clockwise, some counterclockwise), it can speed some propellers up and slow other propellers down to produce torque to allow the multi-rotor to spin. Cool huh?).

To make the coaxial helicopter go forward or backward, the tail motor provides lift (or downforce) to tilt the blades. It fights the flybar that is keeping the helicopter stable. If the tail motor lifts the back of the helicopter, the thrust is now downward and slightly backward, so the helicopter goes forward (this is the elevator control for this type of helicopter).

How do you roll left/right on this type of helicopter? In short you can’t (unless you have outriggers/side propellers), the stabilization system (flybar) keeps the left/right roll perfectly stable.

What about a single rotor helicopter? I’m glad you asked.

A single bladed helicopter has a vertical tail blade which counters the main blade torque (remember Newtons 3rd law-see above). Because the tail is so long, the leverage principle (Archimedes this time) means that only a small tail rotor is required to counter the torque from the much larger main rotor. (What about the tail rotor induced torque? It’s there, but is so minor that you never feel it when flying).

As the helicopter goes up and down, the torque from the main rotor changes. The tail needs to adjust its push dependent on the torque it needs to counter. If the tail rotor fails (stops spinning), or is unable to provide enough counter-torque, the body of the single bladed heli will spin. (opposite to the spinning direction of the main blades). As a curious note, if there is no power to the main blades (even if they are still spinning), no torque is applied to the body, so on tail failure you can reduce power to slow/stop the spin.

Tail rotors on helicopters can either be powered by a separate motor or driven by the main motor. Separate motor tails change thrust by adjusting the tail motor speed. Drive motor tails change thrust by adjusting the pitch of the tail rotor blades.

How does the tail supply just the right amount of thrust to counter the main rotor torque? This is where the gyroscope (gyro for short) comes in.

A gyro (when spinning vertically) will produce a reaction if you attempt to twist it left or right. The strength of the reaction is directly related to the speed of the twist. Fast twist, strong reaction, slow twist gentle reaction. This reaction can be measured and used to tell the tail how fast the helicopter is trying to spin. From this measurement the tail can be controlled to try counter this spin. This measure, control loop (feedback loop) is done many times a second. Gyros are VERY sensitive and can measure even minute twists.

Most old mechanical gyros have now been replaced by highly sensitive electronic gyros. The down side of a gyro is it’s very sensitive to vibration (bit of a problem when running on a machine full of rotating parts). Proper gyro mounting is important. A gyro needs to be solid enough to detect minute twists, but insulated/dampened enough to not be affected by vibration.

Tail gyros work in two modes. Rate mode or heading hold.

In rate mode, the tail is allowed to drift (move without command), but within a very tight tolerance (maybe up to a rate of no more than 30 degrees per second). This means in wind, a rate mode helicopter will drift so that the nose of the helicopter will point into the wind (weathervane). In forward flight this is good thing, but bad if you are trying to land in a cross wind.

In heading hold mode (also known as AVCSAngular Velocity Control System), the gyro remembers the direction the helicopter is pointing and rigidly keeps it point in that direction unless commanded to do otherwise. This is the normal mode most helicopter gyros work in. This mode is ideal if flying in windy conditions or doing aerobatic flight (like backwards or inverted).

So, when you control the tail of a helicopter, you really command the gyro. This is an important note. When you command “turn at this speed”, you tell the gyro to adjust its anticipated heading by a certain amount. The gyro then tells the tail how to get there at the commanded speed. This is the same for coaxials and multi-rotors. You tell the gyro how hard to turn, and it commands the turn mechanism to turn the craft at the rate required until you tell it to stop.

That’s all well and good for the tail, how does it tilt and roll with no tail or side propeller to push it up and down? This is where the swashplate comes in.

Remember how a flybar can tilt a blade as it spins to keep it level? A swashplate commands the blades to tilt to make them not level. So, just as a flybar can put the right tilt on a propeller to make it push opposite to the way it started to lean, the swash can create a tilt.

A swashplate is essentially 2 discs stacked flat one atop the other and locked together so they cannot separate. Easiest way to picture this is stack two paper plates, one atop the other and punch a hole through the middle of both plates with a pencil.DFC_Head_250pics

The top disc rotates as the rotor head rotates (locked to the pencil), the bottom disc does not rotate and remains locked to helicopter body’s orientation. The swashplate has a bearing in the middle and sits around the main rotor shaft of the helicopter (so the swashplate can spin on the main shaft). The swashplate bearing is special as it allows the swashplate to tilt relative to the main shaft while remaining perfectly centered. If you tilt the bottom disc, the top disc also tilts (as they are locked together but slide over one another).

The top disc of the swashplate is connected to the rotor blades and can tilt the individual blades. So, on a tilted swash, a blade will have more lift on one side than another. (Swashplate image courtesy of: RC Helicopterfun)

This is where things go a little crazy. If you want to tip a rotor disc toward the nose of a helicopter (to go forward), you need to apply maximum lift “approximately” 90 degrees before it’s needed. Assume the heli is facing North and the blades are spinning clockwise. To tip the North side of the rotor disc down, maximum lift must occur at the East point so the South side rises (and the North side dips). Don’t panic, the swash is usually engineered so that if you want the South side of the rotor to rise, you lift the South side of the swashplate.

(Interesting note: It is a common misconception that the “approximately” 90 degree lift/tilt angle of a helicopter is due to gyroscopic precession. In short it is not (which is why “approximately” 90 degrees). The lift mechanics of blade are more than 10 times that of any gyroscopic precession forces. Since the blade is not a rigid ring its actually lift resonance of the main rotor that tilt the main shaft. Gyroscopes cannot precess (tilt axis) “out of phase” (less or more than 90 degrees to the force on the disc). Helicopter rotors can – so it’s not precession)

The whole point of this is that on a single rotor blade helicopter, the tilt control (roll and pitch / elevator and aileron / cyclics) are commanded by the swashplate. In a collective pitch helicopter, the blade pitch is also controlled by raising and lowering the swashplate (up and down the main shaft – this is why the swash is not affixed to the main shaft). Therefore control of the swashplate is crucial to the control of a helicopter.

Normally the swashplate is controlled by directly linking it to servos and 2 or 3 points (sometimes 4). Only two servos are required for fixed pitch (as the swashplate does not move up and down. Three or more are essential for collective pitch.

For the following example, I will talk about a swashplate with servos evenly connected at 120 degree intervals (120 x 3 = 360) for a collective pitch helicopter. The location of the servos are; one servo atservo_placement helicopter nose (North – cyclic 1), one servo back right (South East – cyclic 2), one servo at back left (South West – cyclic 3).

Let’s say you want lift the swash (add positive pitch to the blades so the helicopter can take off). You need to command all 3 servos to lift. If you want to tilt the swashplate forward (to make the helicopter tilt forward to go forward), the North servo (cyclic 1) needs to go down, but the other 2 servos (cyclic 2 and cyclic 2) need to lift. To control the 3 servos with the inputs of aileron, elevator and collective, you need to mix the controls so they produce signals for cyclic 1, cyclic 2 and cyclic 3. This is the magic known as CCPM (cyclic collective pitch mixing). This CCPM may be implemented in the transmitter, or in an on-board control unit. In short, this is as much as you need to know about that at this stage.

Because all the in-flight controls get a little complicated, often an on-board flight controller is used (often a FBL (flybarless) controller). This flight controller usually does the swash (CCPM) mixing. It also often does the tail control. Lastly it may do the stabilization (which is essentially a heading hold gyro control for the swash to instruct it how to stay or move to a commanded angle – This is really a FBL unit’s main function).

Lastly let’s revisit throttle (the simple start air brutality control). On advanced helicopters, a governor can be used. This is an extra control to keep the motor spinning at a set commanded speed. As a helicopter moves up and down (or tilts), the movement puts varying strain on the rotor and the motor. The governor controls the throttle (like a gyro controls the rudder) to keep the rotor at the commanded speed regardless of changes by changing motor speed.

As you can see, there is a LOT going on at once when you’re flying a helicopter. This may well be the origin of the belief by some fixed wing Luddites who claim, “Helicopters don’t fly. They beat the air into submission.” Thankfully, on-board systems and programmable transmitters such as the Taranis with OpenTX firmware have allowed us to automate many of those functions making a helicopter much easier to fly.

** Hopefully you are not too overwhelmed. The rest is easy. **

 Helicopter Flight Modes

Now that you know the basics of how a helicopter works, we can get onto other things useful in flying them.

RC helicopters fly generally in one of two modes. Normal or Idle Up.

Normal mode is a bit of a misnomer. The reason is because most experienced collective pitch RChelicopter pilots fly almost 100% of the time in Idle Up mode. In Normal mode, a low throttle stick commands the engine to run at idle speed or to turn off. As the throttle stick rises, the engine speed increases.

setup16Normal Mode Curve

This usually happens quickly at first, so the first ¼ stick has the throttle near flying speed. Beyond the first 25%, the throttle increases slowly, but the blade pitch increases to allow upright flight.

Idle Up mode is what most collective pitch RC helicopter pilots fly. In this mode, the rotor spins at a high speed regardless of the throttle stick position. Altitude is governed by the angle of the main rotor blades. If a governor is in use, the throttle curve is a constant value. Other setups may use a V curve to provide more power at the extremes where the blade resistance is the greatest. The most common Idle Up mode has a linear pitch curve (straight line from minimum to maximum) and a flat (constant speed) throttle curve.

So, if the motor in Idle Up is always on, how do you stop it? The answer is Throttle hold.

Idle Up is the reason why you should not get into the habit of rapidly lowering the throttle stick to stop the motor. This is known as “chop throttle”. Chop throttle on an Idle Up collective pitch helicopter will violently crash your expensive model. It is recommended to use throttle hold to stop the throttle on even fixed pitch or coaxial helicopters.

idle-up-flat

Idle Up Flat Curve

 Throttle hold sets the throttle to low value (usually -100) irrespective of the position of the throttle stick. Normally there is a separate pitch curve for throttle hold (linear at -100). Throttle hold is often seen as another flight mode. For internal combustion (nitro or gasser), there is also a separate throttle cut to stop the motor (On the Taranis, I’d recommend using a safety channel and extended limits to enable this). On an electric helicopter, hold should simply stop the motor (after all, we have a way to restart it if we wish).

As an interesting side note, both electric and internal combustion collective pitch helicopters can land by autorotation. This means they can land without main rotor power. To enable this, a one-way bearing is used (like in a bicycle but without the clicking). This allows power from the motor to turn the rotor, but when the power is off, the rotor can spin freely. As the helicopter descends, negative collective pitch can keep the blades turning (like blowing air through a fan). By adding positive collective pitch near the ground, the momentum in the spinning blades slows the descent, but also slows the blades so it must be done quickly.

There are two other common channels used in RC helicopters. Gyro and Governor.

Gyro modes (rate mode and heading hold) were previously discussed. In most RC helicopters gyro settings from -100 to 0 are for rate mode and 0 to +100 are for heading hold. The closer to the extreme, the more aggressive the gyro feedback corrections on the tail position. Too gentle and the tail will wander around like a lost kitten. Too aggressive and the tail will wag like a happy puppy.

Lastly there are governors. Governors keep the rotor speed constant and were also discussed in the last section. The settings for how individual governor’s work vary from model to model. Some require an extra channel, some do not. If using a governor on your RC helicopter model, please consult the manuals to understand how to use it from the transmitter.

Review

So lets see if you have been paying attention:

1) What are the 5 main channels of a collective pitch RC helicopter?
2) What 2 channels are known as cyclic?
3) How do you control 5 channels with 2 sticks?
4) What are the 2 main flight modes of a collective pitch RC helicopter?
5) How do you avoid “chop throttle”?
6) What tail gyro mode assists in backward flight or windy conditions?

 

 

Answers (none of these should be surprise).

1)      Aileron, Elevator, Throttle, Rudder, Collective.
2)      Aileron and Elevator.
3)      Throttle and collective are handled by curves on a single stick axis.
4)      Normal and Idle Up.
5)      Always use throttle hold to stop the motor, even on fixed pitch and coaxial helicopters.
6)      Heading Hold (also known as AVCS – Angular Vector Control System).

 Resources:

Here are some helpful links for you:

“Newbies guide to the DX6i for RC helicopters” – http://www.helifreak.com/showthread.php?t=580508

“Beginners Taranis programming guide for RC helis” – http://www.helifreak.com/showthread.php?t=598718 (Although this is for another make of transmitter, it covers all the concepts as to how an why a transmitter works in relation to an RC helicopter).

“HeliFreak” – RC helicopter specific resource on the net – http://www.helifreak.com/

“HeliFreak OpenTX forum” – http://www.helifreak.com/forumdisplay.php?f=353

“RC Groups” – Lots of discussion on RC aircraft and helicopters – http://www.rcgroups.com

“FrSky Taranis with OpenTX 2.0 and beyond” – http://www.rcgroups.com/forums/showthread.php?t=2178865

“FRSKY Taranis ‘How to’ Thread” – http://www.rcgroups.com/forums/showthread.php?t=1914834

The next class is how to quickly program the OpenTX for a helicopter (after that you can go fly).

 


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