Takeoff to a Hover from a Slope

December 2nd, 2009 Leave a comment Go to comments

Takeoff to a Hover from a Slope

This maneuver is used to transition the helicopter from a parked position on a slope, into a normal hover.

Maneuver Description

With the RPM within the normal operating range, the pilot displaces the cyclic toward the slope. Depending on the circumstances, he might put just the amount he thinks is required, or on a steeper slope he may elect to put all available cyclic into the hill to start with. The intent is not to tip the rotor toward the hill, but to have the main rotor disk level with the horizon, or tipped just slightly into the hill.
As power is increased, the downhill skid will eventually lift up. During this phase of the maneuver, collective is controlling the height of the skid, and cyclic is simply trying to maintain the rotor system level with the horizon. As the fuselage rolls uphill, the swashplate and therefore the rotor system tip with it, and the pilot has to take out some of his uphill cyclic in order to maintain the rotor level with the horizon.
The collective should be slowly raised until the downhill skid is level with the uphill skid. Cyclic should continue to be manipulated to maintain a level rotor system. It is critical that the downhill skid does not get raised above the uphill skid. Doing so starts biasing the equation toward dynamic rollover a lot. This is because not only may some main rotor thrust be trying to roll us uphill, but the CG is shifting toward the uphill skid, and thus any restoring force preventing dynamic rollover is being reduced.
With the RPM within the normal operating range, the pilot displaces the cyclic toward the slope. Depending on the circumstances, he might put just the amount he thinks is required, or on a steeper slope he may elect to put all available cyclic into the hill to start with. The intent is not to tip the rotor toward the hill, but to have the main rotor disk level with the horizon, or tipped just slightly into the hill.
As power is increased, the downhill skid will eventually lift up. During this phase of the maneuver, collective is controlling the height of the skid, and cyclic is simply trying to maintain the rotor system level with the horizon. As the fuselage rolls uphill, the swashplate and therefore the rotor system tip with it, and the pilot has to take out some of his uphill cyclic in order to maintain the rotor level with the horizon.
The collective should be slowly raised until the downhill skid is level with the uphill skid. Cyclic should continue to be manipulated to maintain a level rotor system. It is critical that the downhill skid does not get raised above the uphill skid. Doing so starts biasing the equation toward dynamic rollover a lot. This is because not only may some main rotor thrust be trying to roll us uphill, but the CG is shifting toward the uphill skid, and thus any restoring force preventing dynamic rollover is being reduced.

Once the skids are level, remove any remaining uphill rotor thrust by moving the cyclic away from the hill. It is normally very apparent when there is no main rotor thrust into the hill, because the helicopter will suddenly become much less stable on the hillside. Continue to center the cyclic, and increase power to cause the helicopter to lift straight up. Continue up to your desired hover height.

Typical Mistakes

Not using enough uphill cyclic

If you don’t have enough uphill cyclic, such that the rotor is tipped downhill, the uphill skid may be the first to lift off, or the helicopter may try to slide downhill, or it may simply dynamically roll downhill. Needless to say, none of these are fun, and it’s probably safer to carry too much uphill cyclic at the start of the maneuver rather than not enough uphill cyclic.

Using too much uphill cyclic

This usually happens because people put in a certain amount of uphill cyclic, and then as the helicopter rolls to a level attitude they either don’t take out any of the uphill cyclic, or they just don’t take out enough. This leaves you with a lot of thrust toward the uphill side, which can easilly turn into a dynamic rollover uphill. Practice will show you just the right amount of uphill cyclic you should be holding when skids level.  

Rolling uphill too fast

Hamfisted manipulation of the collective can induce a very fast uphill roll which may be difficult to arrest. In extreme circumstances, there may be enough uphill roll momentum to cause an uphill dynamic rollover. The downhill skid should be brought up very slowly. I usually teach bringing it up a few inches and pausing, then a few more inches and pausing, and so forth until the skids are level. This insures no roll momentum gets built up.

Overcontrolling the cyclic

People who are nervous on a slope will tend to overcontrol. Moving the cyclic forward and backward will tend to unlock the uphill skid, making the helicopter unstable on the slope. This happens when only one part of the skid is left in contact with the ground. This could be either a heel or a toe, and creates a pivot point which requires lots of pedal work to handle. A properly locked in uphill skid has both toe and heel planted, and provides a very stable platform with almost no pedal work required.

Overcontrolling the cyclic in roll almost always means the person is confused about what control commands the height of the downhill skid. The cyclic can wobble the downhill skid up and down a bit, and this may lead people to think they are on the right track, especially since this is the proper control input in normal flight. It is important to realize that as long as one skid is planted, the height of the other skid is controlled by rotor thrust, i.e. the collective.

Not centering the cyclic before vertical liftoff to a hover

If the cyclic is still displaced into the hillside when a vertical liftoff is attempted, the helicopter will perform a distinct wobble as it leaves the ground. While not particularly dangerous in small amounts, it leaves some doubts as to the ability of the pilot to properly operate on a slope.

Allowing the tail rotor to swing toward the slope

Once in a hover, the pilot has to remain concious of the tail rotor and avoid swinging it toward the hillside.

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