Chapter 4: Flight in X-Plane Helicopter

The following is a description of how helicopters are flown in the real world, along with the application of this in X-Plane. As you are about to see, flying a helicopter is very difficult and much more demanding than flying a fixed-wing airplane.

Note that unfamiliar terms may be found in the Glossary.

Though all manner of different helicopter layouts can be found in reality, we will discuss only the standard configuration here—a single overhead rotor with a tail rotor in the back, like the Robinson R22 Beta. Here's how it works: First, the main rotor provides the lift needed to support the aircraft, exactly in the same way that an airplane's wing supports its weight, or its propeller pulls it through the air. Quite unlike an airplane propeller, though, a helicopter's rotor spins at the same RPM in all phases of flight. Whereas an airplane propeller speeds up in order to generate thrust, the amount of lift generated by a helicopter rotor is controlled by adjusting the pitch of the main rotor blades. This is done using the collective control.

Imagine the one and only operational RPM of a helicopter is 400 RPM. When the craft is sitting on the ground, the rotor is turning 400 RPM, and the pitch of the rotor’s blades is about zero. This means that the rotor is giving about zero lift. Because the blades have zero pitch, they have very little drag, so it is very easy to move them through the air. In other words, the power required to turn the rotor at its operational RPM is pretty minimal at this point. Now, when the pilot is ready to go flying, he or she begins by pulling up on a handle in the cockpit called the collective. When this happens, the blades on the rotor go up to a positive pitch. All the blades on the main rotor do this together at one time—"collectively."

Of course, they are then putting out a lot of lift, since they have a positive pitch. Equally apparent is the fact that they are harder to drag through the air now, since they are doing a lot more work. Since it is a lot harder to turn the blades, they start to slow down—if this were allowed to happen, it would be catastrophic, since the craft can’t fly when its rotor isn’t turning! To compensate for this, the helicopter's feedback sensors will increase the throttle as much as necessary in order to maintain the desired 400 RPM in the rotor. In X-Plane Helicopter, the effect of this throttle governor can be seen in the slider on the right side of the screen, as highlighted in the following screenshot.


 * [[Image:Throttle_helicopter.png]]

So, increasing the collective pitch of the main rotor will increase the lift generated by it, thus pulling the helicopter off the ground. However, because of the main rotor's inertia and its increasing drag as its pitch increases, the force required from the engine to spin the rotor also increases. When the throttle governor increases power to meet this need, the torque delivered from the main rotor blades to the fuselage of the helicopter will change. This torque (and the fact that it is continually changing) must be compensated for in order to keep the craft flying straight.

This compensation comes in the form of the tail rotor, controlled with the foot pedals in a real helicopter. The pilot must continually be making small changes with his or her feet (changing the pitch of the tail rotor just like the collective control does to the main rotor) in order to correct for the torque of the main rotor. This torque itself is continually changing depending on the amount of power delivered to the main rotor by the engine.

X-Plane will try to stabilize the tail rotor automatically. However, in most cases, it is a good idea to manually control it using the TR slider, found at the bottom of the screen (see the screenshot below). Remember, move this left to turn the helicopter left, and move it right to turn right. This will need to be moved back and forth as the collective pitch of the main rotor is changed (thus changing the power delivered by the engine and the energy absorbed by the rotor).


 * [[Image:Tail_rotor_helicopter.png]]

Incidentally, the tail rotor is geared to the main rotor so that they always turn in unison. If the main rotor loses 10% RPM, the tail rotor loses 10% RPM. The tail rotor, like the main rotor, cannot change its speed to adjust its thrust. Like the main rotor, it must adjust its pitch, and it is the tail rotor’s pitch that is being controlled with the anti-torque pedals (that is, the TR slider in X-Plane Helicopter).

Once the craft is in the air and the collective pitch of the main rotor is being adjusted (in X-Plane Helicopter, using the sliding control on the left side of the screen), the helicopter pilot in the real world will use the cyclic control (the joystick) to tilt the craft left, right, down, or up. In X-Plane Helicopter, this is controlled by tilting the iPhone/iPod left, right, fore, and aft.

The cyclic control works like this: If the cyclic is moved to the right (corresponding to the device being tilted to the right), then the rotor blade will increase its pitch when it is in the front of the craft, and decrease its pitch when it is behind the craft. In other words, the rotor blade will change its pitch through a full cycle every time it runs around the helicopter once, continually going to a higher or lower pitch. If we use the example from before, this means that the rotor would change its pitch from one condition to the other, and back again, 400 times per minute (7 times per second), because the rotor is turning at 400 RPM. Pretty impressive, especially considering that the craft manages to stay together under these conditions! The fact that moving the stick sends the blade pitch through one cycle every rotation of the rotor blades is why we call the control stick the cyclic.

Let's talk more about the cyclic. When the stick is moved to the right, the rotor increases pitch when it is in the part of its travel that is in front of the helicopter. Surprising, right? One might have assumed that if we wanted to turn right, we would increase lift on the left side of the helicopter, thereby lifting the left and causing the helicopter to roll right. This isn't how it works, though, due to the fact that gyroscopic forces are applied 90 degrees along the direction of rotation of the gyroscope, which is why the lift must be changed in front of the helicopter to enact a change on the left of the machine.

Here’s an amazing experiment that you can try on your own to see how this works. Sit on a free spinning (low friction) bar stool with a bicycle wheel in your hands. Have a friend spin the tire as quickly as possible while you hold the wheel stationary with one hand on each side of the axle. Now, after your friend backs away a bit slowly rotate the axle about the lateral (fore and aft) and roll axes and you will be surprised at how you can control your spinning motion on the stool by making controlled movements with the bicycle wheel. Cool!

Here’s something else that is surprising—the helicopter’s rotor doesn’t directly pull the aircraft to change its flight path. To turn right, the helicopter must increase the lift on the front of the rotor, which causes the left side of the rotor to come up (tilt to the right). But the rotor doesn’t force the helicopter to roll to the right; only the angle of the rotor itself is changed. The resulting change in the direction of lift is what actually changes the flight path of the helicopter Once the rotor (and thus the helicopter's thrust vector) is tilted to the right, it will drag the craft off to the right. In fact, the thrust vector from the main rotor can be broken down into two components, vertical lift (which supports the weight of the helicopter) and horizontal lift (which causes the helicopter to accelerate to the right).

We said that the helicopter's rotor doesn't directly pull the craft to change its flight path. This is because the rotor on many helicopters is totally free-teetering; it has a completely "loose and floppy" connection to the craft. It can not conduct any force (left, right, fore, and aft) to the body of the helicopter. Maneuvering is only achieved by the rotor tilting left, right, fore, and aft, dragging the top of the craft underneath it in that direction. The helicopter body is dragged along under the rotor like livestock by a nose ring, blindly following wherever the rotor leads.

To summarize, this is the sequence for getting a helicopter to fly (in the real world, as well as in X-Plane):


 * 1. While on the ground, the collective handle is flat on the ground. This means the rotor pitch is flat, with minimum drag and zero lift. In X-Plane Helicopter, a flat collective corresponds to the collective control (found on the left side of the screen) being at the top of its range of motion. The automatic throttle in the helicopter is obsessively watching the rotor’s RPM, adjusting the throttle as needed to hold exactly the design RPM (which varies from helicopter to helicopter). On the ground, with the collective pitch flat, there is little drag on the blades, so the power required to hold this speed is pretty low.


 * 2. When the user decides to take off, s/he does so by raising the collective up by pulling it up from the floor of the helicopter. In X-Plane Helicopter, this is done by easing the sliding bar collective control down toward the bottom of the screen. This increases the blade pitch on the main rotor and therefore increases its lift, but it also increases the drag on the rotor a lot. The rotor RPM begins to fall below its operational speed, but the auto-throttle senses this and loads in however much engine power it has to in order to keep the rotor moving at exactly the required RPM.


 * 3. More collective is pulled in until the blades are creating enough lift to raise the aircraft from the ground. The auto-throttle continues adding power to keep the rotor turning at its operational RPM no matter how much the collective is raised or lowered.


 * 4. The tail rotor is actively controlled to keep the helicopter from spinning due to torque and gyroscopic effects. Any change made by the pilot or nature will require input in the other two controls. Thus, the pilot must continually be making small adjustments to the cyclic (the control stick—controlled with the iPhone or iPod's tilt), collective (the lever to adjust main rotor pitch—controlled with the COLL slider) and anti-torque pedals (to adjust tail rotor pitch—controlled with the TR slider) to account for changes based on moving any of these controls.

Use the above information to hover perfectly. Once that is mastered, push the nose down to tilt the rotor forwards. The lift from the rotor acting above the center of gravity of the aircraft will lower the nose of the helicopter, and the forward component of lift from the rotor will drag the craft forward as it flies along.

Review
To review, flight in X-Plane Helicopter follows these steps:


 * 1. The collective slider, found on the left side of the screen, is dragged down gently in order to increase the pitch of the main rotor. This increases the lift generated by the rotor. As this is done, the throttle, shown on the left side of the screen, will automatically increase. This is caused by the throttle governor automatically increasing the engine power to maintain the desired rotor RPM as the rotor load is increased (caused by increasing the rotor’s pitch). This is how the real helicopters work, and thus how they are simulated in X-Plane.


 * 2. The TR slider (controlling the tail rotor) is dragged left to turn the helicopter left (or to counter rightward torque). It is dragged right to turn the helicopter right (or to counter leftward torque).


 * 3. The iPhone/iPod is tilted left, right, forward, and back to move the craft in each respective direction. This corresponds to input from the stick (the cyclic control) in a real helicopter. Remember that what is actually happening with the cyclic is that the rotor is being made to roll and pitch around the helicopter, thereby changing the thrust vector. This thrust is what causes the helicopter to accelerate either forward, rightward, rearward or leftward.