Hovering should be simple, right? It’s not like you want to go anywhere, you just want to fly a few feet above the ground in the same spot. If only it was that easy. In this article I will try and explain why you have so much trouble going nowhere, and hopefully you will gain a little more understanding about what you’re doing and why.
Most people crash and burn pretty quickly trying to hover for the first time. Control is usually lost within a couple of seconds. Constant control input is required in order to maintain a constant position and height above the ground. So what can we do about it? The answer is not so simple, we have to combine a little knowledge with a lot of practice. Let’s face it if Helicopters were easy to fly we would all have one in the back yard. So be patient and practice a lot.
This is made difficult in a 2 bladed teetering rotor system (R22 Bell 206) because the fuselage reacts very slowly to cyclic inputs (but the rotor disk reacts right away). The slow response tricks new pilots into over controlling the aircraft. This means you react and then feel the need to react more because there is a lack of notable response. The result is ever increasing over controlling of the helicopter until you make unwanted contact with the ground.
The problem is that the cyclic is not a position control. You don't move the cyclic 1 inch to the right to move the helicopter 1 inch to the right. The cyclic is an acceleration control. You move the cyclic a little to the right if you want to slowly accelerate to the right. You move the cyclic more to the right if you want to accelerate rapidly to the right. When hovering we don’t want to move at all and so the tendency is to not move the controls. Until we notice it moving by its self and then we restart the cycle of over correction again.
To fly a helicopter you must always be imputing something into the controls. If you want to go forward you will push the nose down a little. Then you will need to increase lift to maintain height, and then you will need to increase anti torque on the tail rotor to counter for the increase in engine torque. It’s a never ending cycle from the moment you climb into the cockpit. Control input into one stability mechanism will almost always lead to control inputs in all the others as well. It’s kind of like balancing two balls on top of each other on a windy day. Before getting in the Helicopter you know you’re going to be busy.
So how do you pin point the location on earth you want to be and get the helicopter go there.
You need to start out slow, baby steps and the slower the better. Because today you have looked out the cockpit window and decided that a spot 1 meter away is a far better place to park your new helicopter. It’s further away from the trees and that can’t be a bad thing given the last time we went flying. We are already in the hover and we have chosen our touchdown point. So we are ready to go. First we input some right cyclic to start a bank to the right. Then we judge the acceleration to determine our speed and how fast we would like to get there. When the helicopter has accelerated to the correct speed we Center the cyclic. As we get closer to our landing spot move the cyclic to the left, judge the deceleration and as we come to a stop, again Center the cyclic. If you find your self a little off target you have two choices. One land there anyway as if it was the intended location and hope nobody notices. Or two start the process all over again until you hit the spot.
The pilot has to learn what pitch attitude will give him zero airspeed flight, and then simply maintains that attitude. If the helicopter drifts forward or back, changes to attitude will need to be made in order to attenuate any speed caused by pitch attitude until the desired position over the ground is achieved and maintained.
Collective Control
The collective control adjusts the altitude of the aircraft during hovering flight. This is a simple control because it is a position control. For instance, if you are in a 10 foot hover, and you lower collective a little, the helicopter will descend and then stop. There is a direct link between how far you move the collective and how much your altitude will change. Move the collective down 5cm and you will descend roughly 5 feet and stop. This is most notable when hovering in ground effect.
Most collective controls also affect engine power output. This is because the change in pitch angle of the main rotor almost always requires a change in engine power to maintain a constant main rotor RPM.
The most common method now these days is by connecting the collective and engine with a governor. This is a device which actively tries to maintain rotor RPM at a preset value. A governor would typically sense the change in main rotor RPM caused by a collective pitch adjustment, and would increase or decrease throttle as required to maintain desired RPM. Some governors sense collective movement and start adding or removing fuel right away in anticipation of the effect the movement will have on RPM. This is called a "compensator" and is typical in turbine engines which are slow to accelerate and decelerate (and therefore would experience large RPM fluctuations without the compensator).
Confused yet?
Anti-Torque Pedals
The anti-torque (no they are not called rudder pedals!) pedals are used to yaw the helicopter during hovering flight. The left pedal yaws the nose to the left, the right pedal yaws it to the right, just as do rudder pedals in airplanes. See this is easy.
The tail rotor will use one or both of the following methods to maintain control. You can either change the pitch angle of the tail rotor blades. Just like the main rotor when you want to increase lift. Or you can change the tail rotor RPM both methods have pros and cons but I will leave those for another article.
During the hover the anti torque pedals are a rate device. Pushing on the left or right pedal a certain amount will cause the helicopter to yaw right or left at a particular rate. The more you push the pedal, the faster the helicopter will yaw.
On a calm nil wind day, you will not need the anti torque pedals for anything but deliberate yaw and torque corrections. On a windy day however you will be kept very busy with constant peddle input due to wind and rotor wash.
Changes in tail rotor thrust will also cause a right or left rolling tendency depending on which pedal is being pushed in most helicopters, and whether the tail rotor is mounted above or below the CG. The rolling tendency has to be countered by the pilot moving the cyclic control. This cross coupling adds to the pilot's workload.
Disclaimer
This article "The Hover" is intended for intermediate pilots. I make the assumption most readers would have a good knowledge of aviation terminology. For definitions of some of the words used in this article you can go to the
AusFlightSim Encyclopaedia to learn more.
Source
· Helicopter Basics by Rowan Arnold
· Helicopter AGK by Flight Theory Center UK
Article By Gavin Gillett (Gavin)
