Slope landings.

[loachboy]

I am wondering what the preferances and the pros and cons of landing on sloping ground are.
Assuming there is no wind and there are no obstructions, and the angle of the slope will allow the four options below.

Example:
In a left skid low heli, what would the preferance be and why ?

1. Left skid on high side of slope.
2. Left skid on low side of slope.
3.Nose facing up the slope.
4.Tail facing up the slope....



Thanks in advance,

Loachy

:cool:

[teeteringhead]

Has to be "2"

Best to avoid "up" or "down" on skidded helis - the brakes are never very good!!

Being closer to the hover attitude (ie left skid down for left skid low) means less control deflection used (and therefore more deflection/control power remains available) for landing and take off.

Must also consider (you didn't exclude) pilot position. If you're flying from the "downslope" side then you move through a longer arc which can add to psychological effects....

[Thomas coupling]

After shutting down - make sure the handbrake is ON :D

[B Sousa]

Want something to make you nervous. Cobra on slope landings where you can look out and see air between you and the ground................

[NickLappos]

The slope landing needs the most lateral stick, which depends on how the design is rigged. An easy way to see which direction is best is to locate the stick in a no-wind hover, relative to the lateral stick range (a clipboard held over it, referenced to the side of the window frame helps). Then shutdown and work the stick to each lateral stop. Usually, the stick is rigged so that there is slightly more right stick than left stick but the real answer is different for each helo. If there is more right stick, this favors the left skid downslope, since both the stick and the "natural slope" of the helo favor that direction.

The side with the most lateral stick travel from hover position to the stops is usually the best side to put upslope.

If you have the maintenance manual, check the lateral stick rnge in degrees. For each degree of lateral stick from the hover point, you can get about 1 to 1.25 degrees of slope. If there are 8 degrees from hover to the right stop, you can get about 8 to 10 degrees of slope in that direction.

[N Arslow]

Notwithstanding the above, and if nothing else prevents it I also offset the tail slightly down the slope - just gives me a warmer feeling about the clearance, although this is not particularly scientific

[Disguise Delimit]

Skids are designed for forward stresses, not rearward. Example: running landings are fine, but catch the back of the skid on something as you wheel it backwards into the hangar and the skid crossbar-to-fuselage brackets can suddenly break off. Been there, seen that.

Therefore: don't park facing uphill.

Skids are also designed to allow easy slipping in the forward direction.

Therefore: don't park facing downhill.

[Grainger]

Also, park facing downhill = no more tail rotor :eek:

[ylhelico]

Thanks NL for the great explanation,

Some helo models like the 520N are pretty tail heavy. If you land them facing uphill, it might fall on the tail skid once the pilot gets out of the cockpit. Or when he comes back from a nice walk in the forest.:eek:

Also, dont forget, if you are going to spend a couple hours waiting there, ask yourself if you will be confortable sitting and waiting in the helicopter. :hmm:

But first of all, take your time, if you dont feel comfortable with it, take it back up. Try it differently.

Cheers,

YL

[Hughes500]

Depends upon which way round the blades go. US helis that turn CClockwise ( when viewed from above) hang left skid low. therefore park it right skid up the slope. The reverse is true for Eurocopters. Most machines will take 10 deg x slope and about 12 deg upslope.
Good tip, on soft ground do not get out of the machine with blades turning. It is embarrasing to walk away and watch the tail hit the ground as the rear of the skids sink in on an upslope landing. When getting out stand on the skids if you are unsure and gingerly step off !!!

[ShyTorque]

The best slope to land on (i.e. easiest for the aircraft and the pilot) is the one that matches the hover attitude.

Normally this means a combination of low skid downhill and slightly nose up - so you land diagonally. This means the least control movement.

Whatever you do, take it very carefully and if you reach the limit of cylic movement, you must lift carefully off the slope and find somewhere else to land because you have just found the aircraft limit. A crosswind, the C of G and sometimes a fat boy or a daddy-long-legs in the other seat will have a bearing on the limit of cyclic, as well as the steepness of the slope. No names mentioned, but I have flown with a few folk who bodily blocked the full movement of cyclic - a steep slope is the time you find this out....

[ylhelico]

Question for Nick, (or anybody else that knows)

You explained us how we could calculate the number of degrees of the slope we could land on according to the control rigging.

Wouldnt the width of the skids and the height of the CG play a part in the equasion? Or, are every designed aircrafts rigged to make sure we can not land on a slope where a static roll over would be likely to occur?

What are the RFM limits based on?

Thanks

YL:hmm:

[puntosaurus]

I think your assumption is correct, static rollover occurs outside the range we are talking about for slope landings. However I'm surprised Nick is focussing exclusively on stick movement, I've always followed the Shy Torque philosophy.

Seems to me the tilt of the heli at hover ought to be an issue. Let's say your skids are 5ft (60in) wide and you hover with one skid 3in low, that looks like a 3deg built in slope you have to overcome in one direction. If you're talking about stick movements of 8-10deg, that's quite a significant factor.

Maybe manufacturers always allow pro rata extra movement in the TR drift avoidance direction to equalise this, but it's still easier to go with the tilt IMHO. Minimises that rather uncomfortable half on half off time.

[NickLappos]

puntosaurus and ylhelico,

The flight manual limits are established by landing on ever increasing slopes until the lateral stick stops are contacted. I have never heard of static rollover margin being lower than stick margin.
The Static rollover angle for most helos is far greater than the slope limit due to stick margins, almost never is the static rollover the limit to slopes. A typical stick margin limit might be 8 to 16 degrees (measured as the maximum slope one can land on without touching the lateral stick stops). A typical static rollover angle might be 30 degrees. One can calculate the static rollover angle by simply drawing a cross section of the helicopter, with a line through the cg extending thru the skid or wheel. Here are two examples:

http://webpages.charter.net/nlappos/staticrollover-b206.jpg

http://webpages.charter.net/nlappos/staticrollover-s76.jpg

Remember, the actual slope capability on any given day can be less than the flight manual limit, because the wind and lateral CG as well as the slope material (friction coefficient) all affect the true capability.

[ShyTorque]

Thanks for those diagrams Nick, they are very useful - I wish I had similar when I was instructing on the military Puma!

The SA330J Puma is notorious for its propensity to "lift her inside leg" on the ground due to it's high C of G and narrow track u/c. That aircraft has a shim pack underneath the cyclic to prevent excessive lateral movement by the the pilot. Even so, I know of one case where quite an experienced RAF pilot (ex Wessex, IIRC) rolled the aircraft onto its side whilst enthusiastically turning the aircraft onto the runway for what should have been his first solo takeoff....

I still sometimes get laughed at by non-Puma pilots because a dozen years or so after I last flew that type, I still cannot help myself applying in-turn cyclic during ground taxying "for the wife and kids" - even on fixed wing at walking speed! Self preservation rules OK :O

[Gerhardt]

Thanks for the diagrams!

[PPRUNE FAN#1]

Shy Torque:I still sometimes get laughed at by non-Puma pilots because a dozen years or so after I last flew that type, I still cannot help myself applying in-turn cyclic during ground taxying "for the wife and kids"Why would you get laughed at?

Back when Pan Am was running a schedule helicopter service between Manhattan to JFK, the ships would land out on a taxiway and then have to ground-taxi to Pan Am's Worldport. I used to see those little 222's hauling ass inbound, coming around the corner with the inside wheel completely off the ground. I used to watch them from my airplane sometimes and think to myself, "Doesn't that lean to the outside of the turn bother them? Can't they feel that?" As a motorcyclist, I wouldn't be able to stand it. And I used to wonder when they were going to roll one over. Never did though AFAIK.

Into-the-turn cyclic sounds like a pretty good idea to me, Shy.

[Letsby Avenue]

The most expeditious way of landing on a slope is to land (in you case) slightly upslope and slightly left skid low at a 45 degree angle, a small (1-2Kt) run on makes life easier. There is a lot of advice out there about these things but this should be akin to falling off a log... If you are flying a rigid rotor then limit your inslope cyclic to the minimum required.

Teeteringhead is correct in what he says reference psychological effects but I assume you fly from the RHS - Practise, Practise, Practise...

[Xnr]

Always wondered:

Flying a machine with oleo equipped gear (S76), could you transfer enough of the aircraft weight to the downhill oleo unloading the upslope one. This upslope oleo could then extend causing you to exceed the lateral cyclic limits.

Maybe this one's for you Nick.:hmm:

[ShyTorque]

Having spent a few years using the S-76 for SAR and mountain flying training, I can say that in common with most other wheeled helicopters you can do anything wrong if you try hard enough.

However, a good, sensible pilot will take the landing slowly and keep flying the heli all the time, monitoring the AI to check the RFM slope limits aren't exceeded, so this shouldn't be a problem.

I can only see this happening in practice if the downhill oleo "stuck" extended, then suddenly gave, as the collective was lowered further. BTW, it helps settle the oleos if the pilot gently makes the aircraft "jiggle" a little.

[NickLappos]

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!"

[Xnr]

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.

[NickLappos]

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.

[Thud_and_Blunder]

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.

[rotordoter]

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.

[[email protected]]

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.

[Matthew Parsons]

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.

[NickLappos]

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.

[[email protected]]

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.

[Matthew Parsons]

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.

[HEDP]

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

[Matthew Parsons]

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.

[[email protected]]

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.