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How to keep a stable hover when operating in the air cushion

Hovering

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#1 Weads

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Posted 02 December 2017 - 11:25

When landing from people often say get a steady controlled descent, when you hit the air cushion it will slow your descent to allow for a smooth landing or just keep it coming down. I know there are many talented people on here thats why I pose the question, what techniques do you use to stay smooth when only inches above the ground? I.e trailer landings, landing on platforms. Are movements more smooth and slow as opposed to quick short counter movements in a hover? Thanks in advance!

#2 Eric Hunt

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Posted 03 December 2017 - 17:28

Assuming you have a steady hover, look at this:

 

Lower the lever a little, maybe 1cm, and wait - after a small amount of time, the machine will settle down a bit, but "bounce" back up a little in the ground cushion. The real pros will "peg" the machine at the bottom of its partial descent by lowering the lever again just a bit to stop the bounce. If you don't peg it, you will find that on the fourth or fifth lower-wait-sink it will touch the ground but bounce back anyway, and you will panic and jerk the lever. 

 

Down a bit - wait. You gotta wait, for it to settle.

Down a bit, wait.

Down a bit, down a bit, down a bit- heels touch, down a bit more to peg it, oh look, we are on the ground.

 

When the skids are 2" off the ground, you can't afford to stuff around too much, you might drift backwards and catch the heel on something, so when you get down to around 6", start the "Down a bit, down a bit, down a bit" until on the ground.

 

I know that I am keeping my eyes well out the front to hold the attitude dead steady, with glances to the quarter area to ensure no drift. Do not look down at the ground in front of you in these early stages of learning - stay on the hover attitude at the horizon. A learner will not be putting choppers onto trolleys without some serious supervision - a very experienced friend (8000hrs, Huey, Chinook, multiple smaller machines) was killed when a B47 caught a heel on the edge of a trolley and rolled over.


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#3 WolftalonID

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Posted 03 December 2017 - 23:46

When you feel those heels touch lower the collective about half the width of the collective handle immediately. This anchors the skids, yet your still light so keep flying that cyclic..continue to smoothly lower the collective as you keep the ship stable with cyclic until the collective is neutral.
Sometimes we think we know it all....only later to discover we only knew all we had learned. Never stop learning.

#4 Wally

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Posted 04 December 2017 - 11:10

Landing is just like a stationary hover in my mind, except I'm not maintaining altitude.  Concentrate on being stationary and gradually being lower, but stationary, lower but stationary- until the collective bottoms out.  That's it- stationary but descending as slowly as you can.

 

NEVER EVER FEEL FOR GROUND CONTACT!  Concentrate on holding the aircraft stationary.


Just a pilot (retired, so I have a LOT of time)...


#5 Hand_Grenade_Pilot

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Posted 04 December 2017 - 13:13

When landing from people often say get a steady controlled descent, when you hit the air cushion it will slow your descent to allow for a smooth landing or just keep it coming down. I know there are many talented people on here thats why I pose the question, what techniques do you use to stay smooth when only inches above the ground? I.e trailer landings, landing on platforms. Are movements more smooth and slow as opposed to quick short counter movements in a hover? Thanks in advance!

Theres more then one way to skin a cat; youll eventually figure out what works best for you through trial and error. Here are some tips that have worked well for me.

Once in a stable hover, make a slight reduction in collective and hold it. As the helicopter settles, your sink rate will decrease as ground effect becomes more and more effective. If you execute it perfectly, you will only make two inputs; one to initiate the sink and then another to smoothly bottom out the collective once skid contact is made.What you want to avoid is over-controlling or chasing the collective. The more adjustments you make to the collective, the more cyclic and pedal inputs are required and the more you will be fighting the helicopter.

It is possible to hover making micro or macro inputs, as long as the correct counter inputs are made. However, it is best to make very slight cyclic movements. I would do a demonstration with students that were struggling with hovering and over controlling; I would maintain a hover over a fixed spot, and make aggressive cyclic inputs. The attitude of the aircraft changes significantly, but if counter inputs are made quickly enough, you can still stay over the same spot. It is however a lot of extra work. I would then demonstrate hovering with very, very small inputs. The attitude changes of the helicopter are much less noticeable and creates a more pleasant hovering experience.

So cyclic inputs in a hover can basically be described as very quick and very small. To do it with finesse, fly with your fingers, not your fist (if that makes any sense). Keep your right arm propped on your leg and use only your wrist and fingers to make cyclic inputs.

As for dolly/platform landings; using a very slight amount of forward movement while setting down gives you a lot of extra stability and helps counter over-controlling. If you dont like how the set down is going, simply rise back up to normal hover height and reset. Meaning hover taxi around the dolly so that it is in front of you again, and then slowly move forward while descending onto it.

While you will most likely compensate subconsciously, a few more things to keep in mind for helicopters with counter-clockwise rotating rotor systems:

You can expect the left skid to touch down first. The helicopter hangs left skid low because the rotor disk will be tilted slightly to the left to compensate for translating tendency (tail rotor thrust pushing the helicopter sideways to the right). Helicopters with a rotor mast rigged forward (such as the Bell 407) hover nose high. As a result the aft part of the skids will make contact first, and then the nose will come down.

As you reduce collective you will also need a bit of right pedal input to compensate for the decrease in torque. If you notice that the nose is off-center to the left after setting down, this was your culprit).

Once you put in right pedal, the helicopter will drift slightly to the left and you will have to compensate with a bit of right cyclic (this is due to a decrease in translating tendency as you decrease anti-torque). If you are consistently setting down left of your spot, thats why.

If youre able to master hovering, youll get to the point where you can set the helicopter down smoothly while looking through the side window or hanging out the doorframe and referencing the skid. This is done a lot in utility flying, where ground obstructions or a cliff ledge can make exact skid placement critical.

Edited by Hand_Grenade_Pilot, 04 December 2017 - 13:20.

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#6 Whistlerpilot

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Posted 11 December 2017 - 02:54

Its way simple. Just dont move the controls and the helicopter wont move either. Be in the sweet spot. Use pressure not movement. Less is more, IMHO. Practicing no hover spot landings helps to get the confidence up for the tricky ones. Also practice precision landings in easy spots like onto taxi and runway markings. Every landing should be better than the last. Practice makes perfect. Dont accept just good enough. There aint just one way to do it but watch the master pilots and try to imitate. The average pilot is sloppy on landing. Dont be average. It just takes effort to improve and a give a sh$t factor. If I dont make the effort every time my landings are good enough.
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When life's path is steep keep your mind even.


#7 Whistlerpilot

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Posted 11 December 2017 - 02:55

Oops

Edited by Whistlerpilot, 11 December 2017 - 02:56.

When life's path is steep keep your mind even.


#8 Wally

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Posted 11 December 2017 - 20:43

Its way simple. Just dont move the controls and the helicopter wont move either. Be in the sweet spot. Use pressure not movement. Less is more, IMHO. Practicing no hover spot landings helps to get the confidence up for the tricky ones. Also practice precision landings in easy spots like onto taxi and runway markings. Every landing should be better than the last. Practice makes perfect. Dont accept just good enough. There aint just one way to do it but watch the master pilots and try to imitate. The average pilot is sloppy on landing. Dont be average. It just takes effort to improve and a give a sh$t factor. If I dont make the effort every time my landings are good enough.


What he said, all caps, bold and underlined. There is more headwork than handwork in flying, but trying to learn and improve goes a long way in all aspects. My favorite landing exercise was putting the heel on a spot...

Edited by Wally, 11 December 2017 - 20:45.

Just a pilot (retired, so I have a LOT of time)...


#9 Weads

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Posted 06 February 2018 - 13:02

To elaborate on my question a little more; have you ever seen anyone hold a hover whole operating in the air cushion region approximately 2-6 inches off the ground? Everyone says, “just push through it,” “allow the helicopter to continue settling,” as if it’s not even possible to hold a steady hover in that region? That’s really the basis for my original question. I have heard where the root vortices of the blade are what causes disturbances when extremely close to the ground? Can anyone expand a little on this? Thanks

#10 iChris

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Posted 09 February 2018 - 20:14

To elaborate on my question a little more; have you ever seen anyone hold a hover whole operating in the air cushion region approximately 2-6 inches off the ground? Everyone says, just push through it, allow the helicopter to continue settling, as if its not even possible to hold a steady hover in that region?
 
Thats really the basis for my original question. I have heard where the root vortices of the blade are what causes disturbances when extremely close to the ground? Can anyone expand a little on this? Thanks

 
Remember your first attempt to hover a helicopter. Why did it take you more than a few hours to learn to hover? Nothing most of us were familiar with prior to helicopters had that type of control response and sensitivity. The problem, helicopters are inherently unstable, period. Helicopters respond to control motions slower than most vehicles we're familiar with; moreover, helicopters produce their own destabilizing and gusty air flows even on calm days.
 
In fact, the helicopter is said to possess positive static, negative dynamic stability. Meaning it has oscillatory motion, which is a characteristic of positive static stability; however, the amplitude of the oscillations increase as time passes, indicating negative dynamic stability. 
 
Technically, the helicopters behavior with respect to hovering is defined by its dynamic stability. Dynamic stability refers to the movement of an object with respect to time. In this case, were referring to the forces that cause the movement of the helicopter as it hovers with respect to time. We can categorize these forces under two categories, aerodynamic forces and inertia forces.
 
The aerodynamic forces include main/tail rotor thrust, rotor downwash, wind, rotor vortex, ground effect and its interactions with the airframe, tail boom, and tail rotor. The inertia forces are these mainly related to the aircrafts mass, rotor inertia, and centrifugal forces. Inertia forces effect rotor response, pitch and roll rates, pitch and roll acceleration, yaw rates, and yaw-roll coupling.
 
These characteristics and dynamics mean the pilot must constantly make small control inputs to maintain hover position over a given point. The idea of holding a hover is a misnomer, the fact being no one control position or setting would maintain a given position over the ground. In developing the ability to hover, the pilot must learn to anticipate the motion and put in just enough control without overreacting. This is all complicated by the helicopters failure to react instantaneously to control inputs.
 
Theres a time lag between control input and the maximum aircraft movement. This is due directly to the magnitude of the control moment caused by flapping that always leads fuselage movement. The control moment also being a function both of the height of the rotor above the center of gravity and the distance the flapping hinges are offset from the center of the hub. Thats why some find the S-300 an easier learn to hovering vs. the R22.
 
If the pilot is impatient, too much control input will result in overshoot and over correction. That, coupled with the normal instability and the need to control altitude and heading at the same time, soon gets the pilot in an unwanted oscillation. Because of the time it takes for messages to go from the eye to the brain to the hand, the pilot is now doing the wrong thing at the wrong time, pilot induced oscillation.
 
Eventually the pilot puts all this together and can hover over a spot and land with precision under almost any condition, in any helicopter. Based on practice and experience, the pilot learns to analyze and resolve the inherent instabilities of a hovering helicopter. This all works just as it did during that most difficult period when you first learned to hover; however, due to your additional knowledge and experience, it all fits together much faster.

 
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Edited by iChris, 03 March 2018 - 16:53.

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Regards,

Chris





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