Gyroscopic Precession and the 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 on this topic. The gyroscope resists movement regardless of the direction in 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 when it is much deeper than that. The principles of the Gyroscope are far more significant and important than most pilots understand. A better 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. (In the photo to the left, I made this gyroscope to demonstrate the effect of rotor resistance to control inputs, simulated by tilting the shaft which the spinning disk is mounted to.)
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's 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 (example in the photo to the left). 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.
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.
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 deflection when the blade is in the aft position, while the blade on the right side will reach minimum deflection in the forward position.
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, with respect to the main rotor, it is a little more complicated than that. I get some argument on this from time-to-time, but that doesn't change the fact that the main rotor, in rotation functions as a gyroscope. The essence of a gyroscope is a spinning wheel (the main rotor) on an axle (the mast). There you have it, and yes it does work that way.
The Helicopter Rotor as a Gyroscope: Wow, that's an interesting thought isn't it? The rotor (main or tail) is a spinning disk and is therefore a gyroscope. Though once in motion, we have gyroscopes, but the blades are still blades aerodynamically. The tail rotor as a gyroscope is rather insignificant so i'm not going to discuss it any further in this article, the main rotor however is another story.
First, let's remember that a gyroscope will resist reaction to outside forces. The gyroscope prefers to remain in motion as it were, despite the fact that we have made a control input. 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 anticipated; this can be frustrating, but we're going to fix that.
Be aware that any change to any control position will affect the rotor disk with the complications of gyroscopic resistance. 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 resistance. 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 the complications of gyroscopic resistance. 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 resistance, but as the cyclic is input further 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 roll. 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 making the correction to track the centerline.
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 the 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 resistance. 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 caused by gyroscopic resistance 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 resistance 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 effect of the gyroscopic. 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 effect on the rotor, and the consequent reaction of the helicopter to control inputs will just learn to overcome these tendencies through experience not ever knowing what was taking place. 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 effects 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.
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 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 should understand that a helicopter rotor is a disk once it is turning; though aerodynamically it is blades. 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. You must keep the cyclic into the wind to prevent the helicopter from drifting with the wind. 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 due to the pendulum action of the fuselage hanging on the disk. Learning to anticipate what is going to happen, and reacting with the proper control inputs in advance is a challenge. This is one of the reasons for a distant focus; you can sense a movement of the helicopter much quicker when you are focused more 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 piece of tape, don't use dry erase marker as they can stain the windshield. 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 removed 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 a fraction of 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 a fraction of 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 is by now 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 then 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. 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.
Some time ago I was on an Ag job with some friends of mine. We were flying a total of 9 helicopters on the morning of my arrival, but only 8 by that evening. Unfortunately, a very good friend of mine found himself rolled over in an irrigation ditch in the afternoon; the helicopter was a total loss, but he only had skinned knuckles out of the ordeal. He had made a very simple mistake that resulted in the accident. Actually, as with all accidents, he had made a series of mistakes which culminated in the accident. But this was his first Ag job, and he had minimal training at best.
His series of mistakes began with a right turn into a downwind with insufficient altitude to recover. Well, many would say that a right turn takes less power so that should have worked. The fact of the matter is however, that it takes a much more aggressive pedal input to stop a turn than it does to execute the turn, therefore as was the case with my friend, when he came in with that strong left pedal, the main rotor rpm drooped, and he of course lost lift without the altitude cushion he needed.
As he began to sink, he also began to milk the collective. He hit the ground rather hard with forward motion and slid off of a dirt road into an irrigation ditch. I have always maintained that milking the collective will not work. I have encountered some argument in that regard, but I say unjustified. Just do the math, it can not work better or quicker than a simple reduction of the collective. Remember, "if it don't give milk, don't milk it", and, "milking the collective equals accident report".
There were a couple of things he could have done if he had had the knowledge that this would occur. He was in a piston aircraft so he could have led with throttle immediately prior to the left pedal application which would have eliminated the rpm droop (this is even very effective in the Robinsons which have an excellent governor, but that which can not keep up with aggressive pedal application in a high powered situation). He could have climbed just a little bit higher in the turn, which would have given him more altitude to make a low powered dive back into the field. He could have kept the collective down while maintaining level flight and just let the aircraft settle slowly into the field as it built rotor rpm.
Why does this happen? Don't ever forget the Flywheel Effect! True in all helicopters, but especially so if you are flying a piston powered helicopter. An abrupt left pedal input will get its power from the main rotor where a tremendous amount of energy is stored. You can experiment with this safely in a hover, but don't be overly aggressive. With a fixed collective setting into the wind and in a calm wind situation, notice that when you input left pedal, the helicopter will settle. When you input right pedal the helicopter will ascend. In the Robinson's, with the governor on, you will notice the delay in the governor application, and the corresponding droop or increase in the main rotor rpm. Don't get to aggressive with this practice, but toy with it a little bit. You can easily see why you may need to lead with throttle control even though there is a very good governor in the Robinson's. The throttle issues are not relative in turbines since the throttle is always full-open. You can still have rotor rpm droop, you just have to be aware of it and fly accordingly, in turbine helicopers it would be necessary to lower the collective to correct such situation.
Low Rotor RPM
Low rotor RPM is a dangerous situation which can quickly turn into an emergency if not caught and corrected early. You must be aware and remember that for each percentage of rotor rpm lost, you also lose that same amount of power available to recover that lost rpm. This means that if you lose 3% rotor rpm, you have also lost 3% power available to recover. You must also be aware of, and remember that for each 1% main rotor rpm lost, you also lose 5-7% tail rotor thrust available which can easily lead to an LTE event.
Rotor RPM is directly controlled by the throttle when there is engine power available. When engine power is not available, the RPM can be controlled by the collective through disk loading, and/or with the cyclic by disk G-loading.
In the event of an engine failure, rotor RPM can only be controlled by the collective and/or the cyclic. The collective must be reduced to the point required to achieve rotor RPM within the acceptable range specified by the particular aircraft flight manual. If a condition exists where full down collective still will not provide satisfactory rotor RPM, it is possible to increase the RPM by increasing the G-load on the rotor disk by accomplishing S-turns, or a spiraling descent.
If there is a loss of rotor RPM while engine power is available, the corrective action will be to lower the collective while simultaneously applying throttle as necessary to recover the RPM. In the event that full-throttle has been reached, then the only means of increasing rotor rpm will be through a reduction of collective pitch. In cases where the engine power has reached its maximum, it may become necessary to control the RPM with the collective even under powered flight. This technique is common at high altitude when the engine has reached maximum power output (full throttle).
If a loss of rotor RPM occurs while in a hover, the proper corrective action is simply to land the helicopter; terrain permitting. Never raise the collective to maintain hover height while trying to recover the rotor RPM in a hover (unless you are over a mine field, then you might try some creative thinking). It is possible when over rough terrain to regain rpm by touching down any portion of the helicopter landing gear which will result in unloading to some degree. In such a case, early recognition is necessary to avert disaster by waiting until the condition is extreme and it is no longer possible to balance the helicopter on one skid while rpm returns.
The recognition of a low RPM condition is best by noticing a change in the tone of the rotor and/or engine noise; and secondarily by any associated warning lights or horns.
Avoidance requires close monitoring of the RPM so that it is maintained within the acceptable range at all times, and by being aware that you are flying in conditions conducive to limited engine power. END Jump to Top