Helicopter Flight Information
Understanding Gyroscopic Precession and the Helicopter Rotor as a Gyroscope
Have you ever held a gyroscope in your hand and played with it? If not, perhaps you should, it may help your understanding of what we will discuss here. The gyroscope resists movement regardless of the direction from which it is applied. When this phenomenon is applied to helicopters, the only thing that is usually taught or mentioned are the issues of gyroscopic precession and control rigging. Here I will show that where helicopters are concerned, the principles of the Gyroscope are far more significant and important than perhaps you had imagined. Your full understanding of the effect of the main rotor as a gyroscope and the consequent reaction to control inputs will help you understand what you will go through while learning to fly helicopters.
Gyroscopic precession is rather simple in and of itself; it is a simple fact that when an outside force is applied to a rotating body, the result of the outside force will occur approximately 90 degrees later in the plane of rotation. It is not exactly 90 degrees later, but that is close enough for what we are concerned. This can be demonstrated by making a simple devise such as spinning a plywood disk on a pointed nail or similar support. As the disk spins, apply light finger pressure to any point on the disk; the disk will tilt forward approximately 90 degrees later in the direction of rotation.
What many don't quite understand is how the control rigging in the helicopter compensates for gyroscopic precession. In helicopters, the controls are rigged is such a way that when forward cyclic is applied, the helicopter moves forward, likewise for aft, etc. To accomplish this, the pitch horn is offset 90º to the rotor blade. The controls still tilt the swash plate in the same direction as the control input is made, but due to the pitch horn placement, the input to the blade occurs 90º earlier in the plane of rotation. The device to the left is a spinable disk which is effective to demonstrate gyroscopic precession.
To view the control rigging, turn the rotor so that one blade is to the left of the cockpit, and input forward cyclic. Note that the swash plate tilts forward, but the control link is over the aft portion of the swash plate, increasing the angle of attack on the left blade. When the rotor is in motion, the reaction to this increase in the angle of attack on the left side will result in a climbing blade that will reach maximum upward defection when the blade is in the aft position, the blade on the right side will reach minimum deflection in the forward position, decreasing the angle of attack as it moves forward reaching minimum deflection when it is in the forward position and the disk will be tilted forward.
Really that was simple enough to understand wasn't it? With respect to gyroscopic precession, that is really all we care about; the way the controls are rigged in the helicopter to compensate for it. But now let us talk about the complications of the main rotor, as a gyroscope, which is exactly what it is. The essence of a gyroscope is a spinning wheel (the main rotor) on an axle (the mast).
The Helicopter Rotor as a Gyroscope: Wow, that is an interesting thought isn't it? The rotor (main or tail) is a spinning disk and is therefore a gyroscope hence all laws of the gyroscope apply. The tail rotor as a gyroscope is rather insignificant so we will not discuss it any further, the main rotor however is another story. First, we must remember that a gyroscope will resist reaction to outside forces. The gyroscope prefers to remain in motion as it were; it does not want to move the way the outside force intends it to. As a result, the action intended through control inputs made by the helicopter pilot will not be quite as simple, or absolute as he/she thought they would. This is especially frustrating to the new helicopter student since the reaction of the helicopter to control inputs are not precisely what the student intended. The picture to the left is of a simple gyroscope I made to demonstrate to students the reaction of the rotor to control inputs so they understand why the helicopter responds the way it does.
Some examples of these unanticipated reactions to control inputs are demonstrated in the text below. But first let us establish the fact that any change to any control position will affect the rotor disk with the complications of gyroscopic action/reaction. This means that if the pilot moves either the collective or the cyclic, the helicopter will respond with the effects and/or tendencies of gyroscopic reaction. Pedal inputs alone do not have any gyroscopic effect on the main rotor; interestingly enough however there is no such thing as pedal inputs alone. If you input pedal, this directly effects torque and consequently other control inputs must be made which do have gyroscopic complications. Note that increasing tail rotor pitch requires an increase in power (collective) while decreasing tail rotor pitch requires a reduction in power.
As the cyclic is moved gently forward to initiate a departure from a hover, there is little noticeable effect of gyroscopic influence, but as the cyclic is input forward to counter the climbing tendencies of effective translational lift, the helicopter will roll slightly to the left, and the pilot must input slight right cyclic to over come this left turning tendency. Most will notice this as the helicopter leads to the left of the runway rather than to track the centerline. Student pilots will simply allow this to occur as they are letting the helicopter fly them rather then them flying the helicopter.
More significantly, during final approach when the aft cyclic input for deceleration is made, the helicopter will drift to the right unless significant left cyclic input is made to counter this drift. This right drift is complicated even more as the collective is increased more aggressively at the bottom to terminate the approach to the hover and on top of that, a large left pedal input must be made to counter the torque increase and as a result, translating tendency is also very powerful. Remember that when the approach was initiated, right pedal pressure was increased to compensate for the decrease in torque as the collective was reduced for the descent.
The drifting tendencies as described in the above paragraphs are a direct result of gyroscopic action. This is easily understandable as the cyclic directly inputs tilting forces on the rotor gyroscope. Think about it, as you move the cyclic forward, the rigging causes the rotor to tilt forward, but the follow through effect of gyroscopic action causes the left rolling tendency. And likewise to the right when aft cyclic inputs are made.
Gyroscopic influence also acts on the helicopter when a turn is entered from level flight in either direction (left or right). When the helicopter is rolled into a right turn, the disk is tilted to the right through cyclic rigging, but the disk will have a further tendency to tilt forward and the nose of the helicopter will sink and unless corrected immediately by the pilot, an increase in airspeed will also occur. When a turn to the left is entered, the disk will have a further tendency to tilt aft, the nose will climb, and the airspeed will decay. This is why all students notice increasing airspeed when they bank right, and a decreasing airspeed when they bank left.
Interestingly enough, collective inputs also respond with gyroscopic influence. The effects of gyroscopic action from collective inputs can be demonstrated from level flight; as the collective is lowered, the angle of attack is decreased on all blades collectively (at the same time), and the helicopter will roll to the left. Remember that the disk is already tilted forward through cyclic input for forward flight, and this tends to follow through in the direction of rotor rotation when the collective is decreased resulting in a left turning tendency.
The affect of collective inputs on the rotor are also very noticeable in the termination of the approach to a hover when the helicopter rolls aggressively to the right. The right roll is caused by the rotor disk being tilted aft, but more so by the aggressive raising of the collective to terminate in a hover, and the follow through affect of gyroscopic action. There is a secondary affect which is the result of "Translating Tendency" during this phase of the approach as well. This is due to the fact that as the collective is more aggressively increased for the termination, left pedal is also applied rather aggressively.
Compound Control Inputs: Eventually those students not taught about the gyroscopic action of the rotor, and the consequent reaction of the helicopter to control inputs will just learn to overcome these tendencies through experience, but often they will not know how or why. If the student is taught in the beginning that a turn to the left or right requires a compound input (left with slight forward or right with slight aft), perhaps they would not have to work so hard to try and understand why the helicopter reacts the way it does. What is little understood is the fact that although the helicopter rotor is rigged in such a way that the controls work properly, there are still gyroscopic actions/reactions that the pilot must either be taught to overcome or else he/she will just have to learn the hard way without knowledge and will therefore not be able to pass that knowledge on to students whom they may teach in the future.
Interestingly enough, when a student begins flying the helicopter, the ultimate objective is that the student learns to detect very early what the helicopter is going to do next, and fix it before it happens. That is a significant challenge in itself. The student in this process must also learn all the requirements of compound control inputs. When flying a helicopter it is a very rare occurrence when an input to one control is all that is necessary since an input to any single control will certainly have some affect on all of the others.
Flying the Disk
Helicopter pilots must understand that a helicopter rotor is a disk once it is turning; they must not think of it as blades. Pilots must also be aware that the helicopter hangs on the mast like a pendulum from the main rotor disk. With this knowledge, perhaps it should be easier to understand how the helicopter reacts to control inputs to the rotor disk.
Knowing that the rotors are disks, it should be obvious that the effect of the wind on these disks will be similar to that as if you were carrying a large flat object in the wind. If you were carrying this object over your head, and you let the wind get under it, the wind would tilt it in that direction therefore you must tilt this object into the wind to prevent it from blowing you around; cyclic inputs to the main rotor on the helicopter are the same. Likewise if you were carrying this object (a large piece of plywood for example) vertically, the wind would blow you sideways. As a pilot, you really can't do much about the wind effect on the tail rotor except to understand that this is one of the most significant causes of the 'weather vane' tendencies of the helicopter, and that your pedal workload will be greatly increased as a result.
The collective increases the pitch, or the angle of attack of all main rotor blades equally and at the same time regardless of the blade position in the plane of rotation (collectively). This action on the main rotor blades provides the thrust. Varying this thrust can be used to lose or gain airspeed, to lose or gain altitude, or any combination of airspeed and altitude.
The cyclic changes the pitch of a given main rotor blade in its cycle of rotation. If the cyclic is moved forward, the pitch is increased on the blade that is left of the fuselage, and the pitch on the blade that is right of the fuselage is decreased. Due to gyroscopic precession, this action takes effect 90 degrees later, and the disk tilts forward. This tilting of the main rotor controls the direction of main rotor thrust.
The pedals control the tail rotor blades in the same way that the collective controls the main rotor blades, but rather than a vertical lift vector, the tail rotor provides horizontal thrust but only to the right (nose left). The tail rotor counters the torque on the fuselage caused by the engine driving the main rotor. Note that there may be some right thrust ability of the tail rotor, but this is only for directional control in an autorotation and will not be used in powered flight. In essence, the tail rotor thrusts the nose to the left, but we only release left pedal pressure and let the helicopter torque to the (nose) right. In fact, if the pilot strapped his (left) foot to the left pedal, he/she could fly only by pressing and lifting his/her left foot.
When a cyclic input is made, there will be a time delay before the helicopter reacts to this input. The time delay is a fact of life that all pilots must learn to deal with but is most noticeable to helicopter pilots, and is largely due to the pendulum action of the fuselage hanging on the disk. The duration of this time delay will be relative to the speed that the rotor is turning, and also relative to fuselage weight. Consider a high-speed small diameter rotor like the R-22 with a tip speed of 672 feet-per-second, and compare that to a Bell 47 with a tip speed of 600 feet-per-second. Then consider the max gross weight difference of 1480 lbs (2850 – 1370). Obviously the reaction time of the R-22 will be noticeably quicker than the bell 47.
When flying a helicopter you have to learn to anticipate what is going to happen, and you must react with the proper control inputs in advance. This is one of the reasons for a distant focus; you can sense a movement of the helicopter much quicker when you are focused on the distant horizon using your peripheral vision around the helicopter. You can see a doorframe raise or lower laterally, while fore and aft movements can be detected by movements in the instrument panel against the horizon, or perhaps the compass, this is the attitude of the aircraft relative to the horizon, i.e. attitude flying.
When you are taking your first flight lessons in a helicopter, it may help to make a couple of marks on the windshield with a dry erase marker. Although I discourage this in most cases, I have had students where it helped. There should be no need for more than 2 marks, perhaps one for the hover attitude and one for the departure acceleration attitude. Other attitude references can be made to these marks. For example, the cruise attitude may be 1 or 2 inches above or below one of the marks. You should not look at these marks but rather look through them to the distant horizon. After a little time has passed these marks should be erased so that the student learns to fly without this handicap.
When you are flying the disk, you make a control input and then wait for a reaction (but only about a second), if it was to much, then adjust accordingly; if it was to little, then make a little more. These inputs should be small, and once an input is made, you must hold it and wait for the helicopter to react. In reality, only about a second elapses after the input is made, but it is enough time that students think their input was ineffective, and then they add more. Just about the time they add more, the helicopter reacts to the first input, which by now is extremely excessive, and a counter input must be made. Now cyclic chasing has begun, and it accelerates until the helicopter is swinging wildly like a ball on a rope, and the instructor must take the controls and get the helicopter back into a stable hover and give it to the student again for another attempt.
As you learn to fly, and throughout your career, just remember that the rotors become disks once they are in rotation, and not blades. Anticipate the action of the wind on these disks, and learn to stay ahead of the aircraft. The helicopter will only do what you let it do, or what you tell it to do through control inputs, but it will always follow the disk.
Note that a small helicopter such as the R-22, which has a relatively small tail rotor, still has a 42-inch disk on a 172-inch lever. That is a lever over 14-feet long measured from the mast centerline to the tail rotor centerline.
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