Lesson 6: Hovering
Now we are ready to take advantage of one of the unique capabilities of the helicopter: the ability to remain in the air while staying over a point on the ground, also called hovering. This exercise will build on the coordination of controls that we worked on in basic maneuvers and approach/departure, and really rely on your skills because we will be so much closer to the ground.
Hovering involves more than just keeping the aircraft stationary over a point on the ground, but rather encompasses just about all the activities involved when operating in close proximity to the ground. Except for an occasional glance at your engine instruments, your eyes should be focussed outside the cockpit while performing these ground reference maneuvers. These maneuvers include: picking up and setting down of the aircraft, actual in-ground-effect hovering, forward, rearward, and sideward translation, pedal turns, and turns about the tail rotor.
For these exercises we will assume a helicopter of western manufacture. That means that, as viewed from above, the rotor will turn counterclockwise. Russian and French helicopters (and some homebuilt helicopters) have rotors which turn clockwise, and this makes a difference when applying pedal changes with changes in power or lateral trim during speed changes because of dissymmetry of lift. We can expect to apply left pedal with increases in power, and right trim with increases in speed.
Hover In Ground Effect
If you ever review the specifications for a helicopter, you may come across the acronyms HIGE (for hovering in ground effect) and HOGE (hovering out of ground effect). These terms are typically applied when referring to operating ceilings for aircraft, and relate to the power available from the engine(s). Just like the wings of an airplane, there is a ground effect associated with operating a helicopter resulting from the interaction of the downwash from the rotor with the ground. To some extent, this offers an induced power increase and we will make use of it. Ground effect is typically present within an altitude of one rotor diameter, and is less effective when over water or high grass because these surfaces absorb some of the downwash energy.
You may wonder why you should be concerned about a HIGE ceiling, but if you are operating at a high gross weight or at a high altitude airport or in high temperature situations, it is the limiting factor to your ability to get off the ground. You may attempt a running takeoff (which can involve dragging your skids along the ground for a good distance) to exploit the effects of effective translational lift in these circumstances. Just remember to control your direction with the pedals. We will briefly address HOGE later, although the utility of this maneuver is somewhat lost in a simulation.
X-Plane sets you up at zero power at the end of a runway. Now what? Obviously we need to get the aircraft up into the air. For this first time, slowly increase collective to about half of total power. Move the cyclic very slightly and note the aircraft response. The object here is to determine just how light the aircraft is on its landing gear. Get some idea of how this feels and how far along the aircraft is. Continue to increase collective and add pedal proportionately. Note that the aircraft may not leave the ground evenly. That is, you may have one part of the landing gear still in contact with the tarmac after the rest has broken free. This is normal, and you shouldnt try to compensate for this. You should be concerned with how fast you are climbing (not very) and whether you are moving in any particular direction.
It should take you at least 5 seconds to get power up to half way, and another 5 seconds to get power up to where the aircraft breaks free of the ground. Breaking the aircraft free of the ground is similar to two effects in driving: backing out of a parking spot and engaging the clutch, only you are working in four directions that must be coordinated precisely.
Remember how when you back a car up from a start and you really have no idea how the wheel is turned? You must figure this out before you are going too fast so you dont hit the car next to you. Similarly, when lifting off the ground you must figure out where the cyclic and pedals located compared to where they need to be. Will you roll left or right, move forward or backward? Will you begin yawing left or right? You must figure this out just as the aircraft leaves the ground.
When engaging the clutch on a car, you must figure out if you have the right amount of accelerator depressed so you dont stall the engine or cause it to overspeed and jerk. Similarly, you must ensure that you have a very smooth application of collective to avoid jerking the aircraft into the air and possibly drooping the rotor speed.
So now we have successfully brought the aircraft into the air. Try to achieve a hover at about 5 feet (the radar altimeter may come in handy here, if you choose to employ it). Just try to stay at a constant location over the ground. There should be no forward or rearward motion, and the aircraft should not be drifting to one side or yawing. Make very small corrections and stabilize yourself in the hover.
Now lets try this again. You can try setting the aircraft down on the ground (we will discuss this in a bit) by reversing the above control movements just make sure you are moving forward a bit as you touch down, or re-start your situation from the menu bar. This time around you should have a better idea of where the controls will be as you break free of the ground, so you can spend less time playing with cyclic and pedals as you raise collective.
Part of the fun in flying helicopters is the ability to land on and depart from sloping ground. Each particular helicopter will have limits based on control authority and landing gear capability that will limit the degree of slope that you can operate on. We will address this a little more when we talk about set down, but it isnt an essential skill for simulator flying.
Since you have mastered lifting off the ground and stabilizing the aircraft, you can essentially hover. From your position five feet off the ground, slowly apply right pedal. The aircraft should turn to the right. Add some more pedal and make it turn a little faster. Now reverse pedal motion to stop your turn and try stabilizing in a heading 90 degrees from where you started.
One thing you may have noticed is that the aircraft gained a little altitude. There are two reasons for this. The first is that the right turn is accomplished because you have reduced the power required by the tail rotor. That is, a right turn is the natural reaction to the torque provided by the rotor (remember how you apply left pedal as you increase collective?), so you are reducing the tail rotor pitch. With a constant collective setting, this leaves a small amount of extra power to be devoted to a climb. Also (and this is a small effect), you are adding to the effective rotational speed of the rotor, increasing lift. Climb speed records for helicopters have typically taken advantage of this and the pilots have knowingly not applied anti-torque as collective is applied.
Try a left pedal turn and note how the aircraft sinks a little because of the increased power draw of the tail rotor. I recall a recent accident where the crew attempted a departure from a confined area on a hot day with a heavy aircraft. They entered a high hover and applied left pedal at the top. As they watched rotor speed decay, all they could do was control attitude as the aircraft settled back to the ground, which it did rapidly.
Try turning 360 degrees in your direction of choice. Do this slowly, and in 90 degree increments. Be particularly careful as the tail rotor faces into the wind, as it will try to turn you around rapidly. You might try increasing the environmental wind and experimenting with the aircraft response here. In smaller helicopters, control will probably be lost as the wind exceeds 20 kts. Now try facing downwind. The aircraft will be even more prone to weathervaning. Keep making small pedal controls as required to keep the aircraft in control. Keep your yaw speed under control as you complete the turn to face into the wind, and apply opposite pedal to stop the turn. Repeat this in the opposite direction.
Sideward and Rearward Translations
From a hover, move the cyclic slightly backward. As always, make your inputs slowly, confidently, and deliberately. Maintain track using the pedals. Then apply forward cyclic to stop the motion and regain your hover.
Now move the cyclic a little to the left to translate the aircraft in that direction. Again, maintain your track using the pedals. Move the cyclic to the right to arrest your motion. Once you have regained the hover, move the cyclic some more to the right. Stop anytime you feel ready.
Here are a few exercises that will put the translations together. Select an area on the airfield that will give you good ground references. The apron area near a hangar or the end of a runway should be fine. The goal will be to define a square with the aircraft movement.
Face into the wind near an edge defining tarmac and grass. Move forward several feet. Arrest this motion and move across the tarmac. When you get to the edge, stop again. Now move rearward as much as you moved forward. Finally, move back across the tarmac to the start position. Now try doing this in the opposite direction.
Now try a sideward movement of the aircraft across the tarmac. When you reach the edge, do a 90 degree pedal turn so you face into the tarmac and continue moving sideward. Repeat this at each corner, and try it in the opposite direction when you reach your starting point.
Lastly, move forward along the edge of the tarmac. When you are ready, do a 90 degree pedal turn and move forward across the tarmac. At the far edge, do another 90 degree turn. Continue in this fashion until you reach the start, and repeat in the opposite direction.
If you are feeling particularly confident, try moving the aircraft in a circle with the nose pointed at the center. If you sense a need to truly be humbled, try moving the aircraft in a circle with the tail pointed at the center.
Before we can end our flight and pay our huge helicopter rental bill, we must set the aircraft back down on the ground. Start in a stable hover and slowly lower the collective just a bit. Notice how the aircraft descends, but might not quite be touching the ground. It is important to have no sideward or rearward motion here, and at most just a bit of forward motion. Wait until your hover stabilizes at this new power setting and lower collective a bit more. Just as in lift off, one part of the landing gear will touch down first. Anticipate this, and continue to slowly lower collective until the aircraft is firmly on the ground. Now you can lower collective all the way. Try lifting the aircraft up and setting it down with crosswinds and tailwinds.
When setting the aircraft down on a slope, it is important to do so with the aircraft facing across the slope. That is, the nose will not be facing either up or down the slope. Start by placing the uphill gear on the ground first. Begin lowering collective, and as the aircraft begins to roll downhill, apply cyclic control slightly uphill. Continue lowering collective and moving cyclic uphill until the downhill gear is on the ground. Keep the cyclic toward the uphill side as you completely lower collective.
I remember one instructor who asked if I was interested in seeing what it felt like to run out of cyclic control authority while landing on a slope. Of course, I declined. We picked a small mound of earth and he proceeded to do a slope landing. We ran out of cyclic before the downhill skid touched down, and hovered there for a few moments. He asked what I would do if I ever found myself in this situation, and I told him that I would find another spot.
Hover Out of Ground Effect
High performance helicopters typically only accomplish hovering out of ground effect. This operation is useful for handling external loads for construction work and helicopter logging, and for search and rescue. The maneuver is somewhat difficult compared to HIGE because you no longer have reference to the ground, but still want to remain over a spot on the ground. Specifications for US Army helicopters often require an ability to hover out of ground effect at 4000 feet on a 95 degree day at a specified weight.
There are two hazards associated with hovering out of ground effect. The first concerns an operating limit of the aircraft based on its ability to successfully enter an autorotation based on airspeed and altitude, and is defined by the height-velocity diagram for a given aircraft. This diagram is often referred to as the deadmans curve and we will review it when we talk about autorotations. The second concerns the aircraft falling into its own downwash. The situation is called power settling. If collective is lowered and the aircraft does begin settling, raising collective may not have any real effect so the aircraft will continue to fall.
The quick stop is a maneuver of little use, but it does give a good lesson in control coordination and this is a good place to discuss it. From a hover, accelerate to 50 kts into the wind and climb to 50 feet. When you are ready, begin decelerating and descending to a normal approach to your selected spot on the ground.
As difficult as hovering seems to be, the big aircraft have it a little easier. These aircraft may include systems that can set an aircraft up for an approach to a hover or even maintain a hover by taking advantage of automatic flight control systems, stability augmentation systems, and advanced avionics such as Doppler radar coupled to the flight controls. Some aircraft include displays that show hover-related information such as drift and accurate height. The pilot can maneuver these aircraft without having to look outside, or even with hands off the controls.