Slope landings.
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XNR, Shytorque says it right, oleos don't really harm things much, in fact, the softer touchdown helps you plant the machine more gently.
I find a wheeled helo much easier to land on slopes. The three points almost always find a solid touchdown, unlike the rocking that skids often assume (since the skids represent two parallel lines that must both find contact, on any uneven ground, the skids rock somewhat. A tripod can almost always find solid purchase.)
The uphill oleos do extend slightly when you go flat pitch on the slope, adding maybe 2 degrees to the indicated slope, but since most articulated systems have great control power even at low collective, this does not detract from the slope capability. Since you get this 2 degrees extra only when you neutralize the cyclic, it actually does not detract from controllability. I have often landed on 12 to 16 degree slopes in S-76 and Black Hawk, values that are almost beyond practical, since people tend to fall down alot when on such slopes. IMHO, about 8 degrees is the steepest practical slope for loading people or cargo, I think, and 4 to 6 is closer to comfortable.
Once in the Phillipines, a Colonel told me that he thought wheels were not suited to rough areas, and impossible for slopes (he had been told that by the local Bell rep). I took him out and landed on a 16 degree slope, and asked if he wanted to get out and walk around. He looked at me with eyes larger than dinner plates, and said, "Its too steep to walk on!"
I find a wheeled helo much easier to land on slopes. The three points almost always find a solid touchdown, unlike the rocking that skids often assume (since the skids represent two parallel lines that must both find contact, on any uneven ground, the skids rock somewhat. A tripod can almost always find solid purchase.)
The uphill oleos do extend slightly when you go flat pitch on the slope, adding maybe 2 degrees to the indicated slope, but since most articulated systems have great control power even at low collective, this does not detract from the slope capability. Since you get this 2 degrees extra only when you neutralize the cyclic, it actually does not detract from controllability. I have often landed on 12 to 16 degree slopes in S-76 and Black Hawk, values that are almost beyond practical, since people tend to fall down alot when on such slopes. IMHO, about 8 degrees is the steepest practical slope for loading people or cargo, I think, and 4 to 6 is closer to comfortable.
Once in the Phillipines, a Colonel told me that he thought wheels were not suited to rough areas, and impossible for slopes (he had been told that by the local Bell rep). I took him out and landed on a 16 degree slope, and asked if he wanted to get out and walk around. He looked at me with eyes larger than dinner plates, and said, "Its too steep to walk on!"
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Thanx Nick
I also believe the wheeled oleo aircraft to be better for off level operations.
Have also had the S76 on a 15 degree slope although I prefer to take that slope nose uphill if possible.The 76 is a stable platform for slopes.
In my mind an oleos sole purpose in life is to try to entend and in that it performs the rest of the duties that make life easier and our landings so nice.
Just as an aircraft settles on its oleos as it is being refueled, I would assume the opposite to be true. (extend if being defueled)
Accepting sloping terrain laterally, the aircraft weight first squashes th uphill oleo. As the downhill oleo makes contact and starts to accept the weight of the aircraft it also squashes. I was just wondering if the uphill oleo may get to a point to which the weight transfer to the dowhill oleo may unload the uphill oleo to the point where it may extend.
From your answer above you would say no if I understand correctly. (a couple degrees maybe)
Have you ever shut down on such a slope (15 degrees). I would not feel comfortable in doing so.
I also believe the wheeled oleo aircraft to be better for off level operations.
Have also had the S76 on a 15 degree slope although I prefer to take that slope nose uphill if possible.The 76 is a stable platform for slopes.
In my mind an oleos sole purpose in life is to try to entend and in that it performs the rest of the duties that make life easier and our landings so nice.
Just as an aircraft settles on its oleos as it is being refueled, I would assume the opposite to be true. (extend if being defueled)
Accepting sloping terrain laterally, the aircraft weight first squashes th uphill oleo. As the downhill oleo makes contact and starts to accept the weight of the aircraft it also squashes. I was just wondering if the uphill oleo may get to a point to which the weight transfer to the dowhill oleo may unload the uphill oleo to the point where it may extend.
From your answer above you would say no if I understand correctly. (a couple degrees maybe)
Have you ever shut down on such a slope (15 degrees). I would not feel comfortable in doing so.
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XNR,
The upper oleo partially extends as the weight on it reduces when you center the cyclic. The extension is somewhat proportional to the reduction in weight, since the oleo compresses the nitrogen in it as it is pressed down. More weight on the oleo is equalized by the oleo compressing the gas a bit more as the oleo piston squeezes the gas into a smaller volume. So we don't think of the oleo fully extending as the load changes. On a slope, the uphill oleo does partially extend when you center the cyclic, in my experience about 2 degrees worth (since the oleo is about 5 feet from the centerline of the S76, the oleo will extend about 2 inches for 2 degrees - 1 inch per 57 inches is one degree.) I have not shut down on a 15 degree slope, but I am sure the aircraft is stable there, and so should the rotor be stable, as long as you have the stick reasonably neutral.
Of course, the real reason for the oleo is to create a velocity damping, so that ground resonance is prevented.
The upper oleo partially extends as the weight on it reduces when you center the cyclic. The extension is somewhat proportional to the reduction in weight, since the oleo compresses the nitrogen in it as it is pressed down. More weight on the oleo is equalized by the oleo compressing the gas a bit more as the oleo piston squeezes the gas into a smaller volume. So we don't think of the oleo fully extending as the load changes. On a slope, the uphill oleo does partially extend when you center the cyclic, in my experience about 2 degrees worth (since the oleo is about 5 feet from the centerline of the S76, the oleo will extend about 2 inches for 2 degrees - 1 inch per 57 inches is one degree.) I have not shut down on a 15 degree slope, but I am sure the aircraft is stable there, and so should the rotor be stable, as long as you have the stick reasonably neutral.
Of course, the real reason for the oleo is to create a velocity damping, so that ground resonance is prevented.
Nick,
An observation re your earlier reply: not all wheeled helicopters are equal(!). Some do not share the advantages of the tripod-layout undercarriage, as RAF students of Shy- and my vintage used to regularly discover. The Whirlwind/ S55/ H19 aircraft was effectively always presented with a compound slope, owing to the layout where the rear 2 Dunlops had a wider wheelbase than the front 2. Oldbeefer and his workmates earned their massive instructional pay increase(?) watching us struggle to land nose-up in an aircraft with no brakes on the nosewheels.
Chinook works a treat on slopes, and is the only aircraft I operated with a genuinely useful nose-down capability. Any Kamov operators out there with observations on their aircraft's foibles?
My current type, the EC135, is one of the most 'interesting' slope-landing types I've seen. The Mast Moment Indicator limit is usually reached (using traditional CFS(H) techniques) around 6 degrees right-skid-up slope, requiring a combination of off-slope cyclic and collective to complete landings up to the aircraft limits which - for an old git - takes a little getting used to.
An observation re your earlier reply: not all wheeled helicopters are equal(!). Some do not share the advantages of the tripod-layout undercarriage, as RAF students of Shy- and my vintage used to regularly discover. The Whirlwind/ S55/ H19 aircraft was effectively always presented with a compound slope, owing to the layout where the rear 2 Dunlops had a wider wheelbase than the front 2. Oldbeefer and his workmates earned their massive instructional pay increase(?) watching us struggle to land nose-up in an aircraft with no brakes on the nosewheels.
Chinook works a treat on slopes, and is the only aircraft I operated with a genuinely useful nose-down capability. Any Kamov operators out there with observations on their aircraft's foibles?
My current type, the EC135, is one of the most 'interesting' slope-landing types I've seen. The Mast Moment Indicator limit is usually reached (using traditional CFS(H) techniques) around 6 degrees right-skid-up slope, requiring a combination of off-slope cyclic and collective to complete landings up to the aircraft limits which - for an old git - takes a little getting used to.
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Interesting subject; I'll add another thought into the pot. As a right-hand articulated rotor chap (S330J) left wheel up landings are not always the best due to dynamic roll-over into slope. The process is exacerbated by tail rotor thrust into slope and a right to left wind when viewed from above which can assist an unrecoverable build up of rolling momentum when trying to take off (or controlling a mislanding). It's not an issue with a teeter due to the lower roll control power associated with non-articulated heads.
Rotordoter - I suggest you look at how many R22s have been totalled due to dynamic rollover - the low control power means that when it starts to roll at rates that an articulated head could stop, the teetering one keeps on going with lowering the collective being the only way of stopping the roll.
Whilst inappropriate contol inputs in an articulated or semi rigid rotor can start a roll rate which, if left uncorrected, can lead to DR, you can oppose that roll rate with opposite cyclic as well as lowering the lever.
Dynamic rollover is a popular subject in the RAFSARforce since we have lost 2 aircraft (one thanks to Boscombe Down) proving that dynamic rollover can happen on level ground as well as slopes.
Whilst inappropriate contol inputs in an articulated or semi rigid rotor can start a roll rate which, if left uncorrected, can lead to DR, you can oppose that roll rate with opposite cyclic as well as lowering the lever.
Dynamic rollover is a popular subject in the RAFSARforce since we have lost 2 aircraft (one thanks to Boscombe Down) proving that dynamic rollover can happen on level ground as well as slopes.
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crab,
I think you'll find that if you're approaching a rollover without sufficient control power due to a teetering head that lowering the collective further reduces your control power and allows a greater roll rate. Increasing collective increases control power and reduces the roll rate.
Dumping collective to get the skids planted firmly is a bit of a gamble. I would opt for the controlled manoeuvre.
Matthew.
I think you'll find that if you're approaching a rollover without sufficient control power due to a teetering head that lowering the collective further reduces your control power and allows a greater roll rate. Increasing collective increases control power and reduces the roll rate.
Dumping collective to get the skids planted firmly is a bit of a gamble. I would opt for the controlled manoeuvre.
Matthew.
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Matthew is quite right, lowering collective rapidly reduces the control power of the cyclic, and if you still have roll control when you start the collective down, makes it more likely to cause dynamic rollover. Once in dynamic rollover, lowering the collective is the only escape you have, because you have let it go far enough to be truly out of control.
Dynamic rollover is caused when you get a roll rate that your controls can't stop. That rate can carry the aircraft past the tipping point, where the controls cannot help stop the roll, and even further where the controls actually help the roll. But all dynamic rollovers start where the pilot makes the roll rate high, sometimes on a slope and sometimes in a running landing when a yaw develops.
Dropping the aircraft onto a slope is one way to cause dynamic rollover if you are close to control limits.
Thud_and_Blunder,
You are quite right about the MM meter, your technique is perfect to get the most out of your machine. Slopes are one of the problems for rigid heads, since a given angle causes higher moments than an articulated head. It is the price paid for that superb control in normal flight.
Dynamic rollover is caused when you get a roll rate that your controls can't stop. That rate can carry the aircraft past the tipping point, where the controls cannot help stop the roll, and even further where the controls actually help the roll. But all dynamic rollovers start where the pilot makes the roll rate high, sometimes on a slope and sometimes in a running landing when a yaw develops.
Dropping the aircraft onto a slope is one way to cause dynamic rollover if you are close to control limits.
Thud_and_Blunder,
You are quite right about the MM meter, your technique is perfect to get the most out of your machine. Slopes are one of the problems for rigid heads, since a given angle causes higher moments than an articulated head. It is the price paid for that superb control in normal flight.
Matthew, the whole point of lowering the lever is that you reduce your total rotor thrust and in particular, the horizontal element that is rolling you over - since the control power on a teetering head is relatively low, using opposite cyclic is almost ineffective and lowering the lever quickly is your only chance of stopping the roll rate - but even then it may be too little too late.
Whilst your C of G remains inside the point of contact with the ground (pivot point essentially) there is a restoring moment thanks to the static stability of the aircraft - the roll couple acting about the C of G that is accelerating you towards dynamic rollover (whether through mishandling or whatever) can be opposed with opposite cyclic (effective unless the roll rate is already too high or your control power is low) but lowering the lever a. reduces the cause of that accelerating roll by reducing rotor thrust is all directions and b. transfers the weight of the aircraft from the rotor to the skids, thus helping the restoring moment.
Prevention of dynamic rollover is best by using the correct take-off technique and being aware of when aggravating factors are present (high C of G due to low fuel weight, crosswind flapping the disc towards the slope, TR thrust acting as a rolling moment and nose down attitude lifting the TR and increasing that moment about the C of G). Add this to a situation where a disc level attitude is hard to assess (night for instance) and you have a recipe for turning a normal take-off into a broken helicopter.
Whilst your C of G remains inside the point of contact with the ground (pivot point essentially) there is a restoring moment thanks to the static stability of the aircraft - the roll couple acting about the C of G that is accelerating you towards dynamic rollover (whether through mishandling or whatever) can be opposed with opposite cyclic (effective unless the roll rate is already too high or your control power is low) but lowering the lever a. reduces the cause of that accelerating roll by reducing rotor thrust is all directions and b. transfers the weight of the aircraft from the rotor to the skids, thus helping the restoring moment.
Prevention of dynamic rollover is best by using the correct take-off technique and being aware of when aggravating factors are present (high C of G due to low fuel weight, crosswind flapping the disc towards the slope, TR thrust acting as a rolling moment and nose down attitude lifting the TR and increasing that moment about the C of G). Add this to a situation where a disc level attitude is hard to assess (night for instance) and you have a recipe for turning a normal take-off into a broken helicopter.
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Sorry crab, I can't agree with that.
If there is a horizontal element to your rotor thrust that is causing you to roll over even with full opposite cyclic and close to zero contro power, then things are very bad. Essentially you can't level the disc prior to take off from the slope. The question is, how did you get there in the first place?
You discussed lowering collective during a take off from a slope. I would agree with you on that, because flying cautiously you'd be able to identify reaching a control limit prior to lifting the first point. However, that should be recognized on the ground prior to any significant collective input and thus prior to having much control power on the cyclic. This is one of the overlooked benefits of teetering heads.
However, the topic is "Slope Landings". If during a landing you reach a cyclic control limit prior to landing, that is when you should raise collective. Lowering collective at this point would not be helpful. If you reach a control stop and still have an opposite roll rate, then raising collective gives you the control power to stop the roll rate. Of course, it will also increase lift and thus decrease the couple from the to uphill contact point which further helps to reduce the roll rate.
Lowering collective as a last ditch effort to prevent the rollover is preferable to knowing you'll roll over, but there is so much risk. You will immediately aggravate the roll rate. You will have no cyclic control power so could induce a pitching rate as well. If you make it to the ground with zero control power and a zero roll rate, you will learn immediately (& destructively) if you are not statically stable.
Matthew.
If there is a horizontal element to your rotor thrust that is causing you to roll over even with full opposite cyclic and close to zero contro power, then things are very bad. Essentially you can't level the disc prior to take off from the slope. The question is, how did you get there in the first place?
You discussed lowering collective during a take off from a slope. I would agree with you on that, because flying cautiously you'd be able to identify reaching a control limit prior to lifting the first point. However, that should be recognized on the ground prior to any significant collective input and thus prior to having much control power on the cyclic. This is one of the overlooked benefits of teetering heads.
However, the topic is "Slope Landings". If during a landing you reach a cyclic control limit prior to landing, that is when you should raise collective. Lowering collective at this point would not be helpful. If you reach a control stop and still have an opposite roll rate, then raising collective gives you the control power to stop the roll rate. Of course, it will also increase lift and thus decrease the couple from the to uphill contact point which further helps to reduce the roll rate.
Lowering collective as a last ditch effort to prevent the rollover is preferable to knowing you'll roll over, but there is so much risk. You will immediately aggravate the roll rate. You will have no cyclic control power so could induce a pitching rate as well. If you make it to the ground with zero control power and a zero roll rate, you will learn immediately (& destructively) if you are not statically stable.
Matthew.
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Mathew,
I think you will find that the deviation took place before Crabs post.
If you are discussing landings alone then you appear correct.
Crab was replying to an earlier comment about dynamic rollover which may happen as part of a poor landing or more predominently a take-off situation.
I would suggest that if you are finding yourself hitting cyclic control stops during a landing then perhaps the limits for the aircraft have not been necesarily properly determined.
IMHO
HEDP
I think you will find that the deviation took place before Crabs post.
If you are discussing landings alone then you appear correct.
Crab was replying to an earlier comment about dynamic rollover which may happen as part of a poor landing or more predominently a take-off situation.
I would suggest that if you are finding yourself hitting cyclic control stops during a landing then perhaps the limits for the aircraft have not been necesarily properly determined.
IMHO
HEDP
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HEDP,
In the worst combination of centre of gravity, slope, and wind I can imagine the cyclic envelope being the limiting factor. I wouldn't want a slope limit based on the worst wind because then I wouldn't be able to land on a slope if I made sure the wind was ideal.
You're right about the diversion of the thread, but the information given was dangerously leading towards a recommendation to using the wrong technique for a slope landing. I just couldn't let this one go by.
That being said, if you find yourself with an unstoppable downhill roll rate on takeoff in any type of aircraft, its the pilot technique not the rotor type that needs scrutiny.
Matthew.
In the worst combination of centre of gravity, slope, and wind I can imagine the cyclic envelope being the limiting factor. I wouldn't want a slope limit based on the worst wind because then I wouldn't be able to land on a slope if I made sure the wind was ideal.
You're right about the diversion of the thread, but the information given was dangerously leading towards a recommendation to using the wrong technique for a slope landing. I just couldn't let this one go by.
That being said, if you find yourself with an unstoppable downhill roll rate on takeoff in any type of aircraft, its the pilot technique not the rotor type that needs scrutiny.
Matthew.
Thanks HEDP you saved me a post. I was indeed talking about take-off.
To induce dynamic rollover on a landing would require some serious mishandling and poor technique and, in the event of a mislanding, I certainly wouldn't lower the lever, I would get back to the hover as quickly as possible.
To induce dynamic rollover on a landing would require some serious mishandling and poor technique and, in the event of a mislanding, I certainly wouldn't lower the lever, I would get back to the hover as quickly as possible.
