Puma variants vs sloping ground
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Puma variants vs sloping ground
Heard a story from a mate who said he had once landed on lateral sloping ground in a Puma then not been able to get back off again due to running out of lateral cyclic. Anyone done similar? Wondering if AUM, lateral CG or crosswind were factors in any instances you've heard of. Theory goes that aircraft settled on downslope oleo as power reduced giving different conditions between landing and take off. Which then begs question as to how you apply limits...
Lateral Slope Qualification
GM, this is the qual test standard that the US Army applied to the UH-60 model line: For lateral slope landings, up to the limit ( in this case it was 15 degrees ) full lateral control is allowed. In fact for the left wheel up slope for that machine, the cyclic was on the stop with the right main about 6-12 inches off the down slope. We dropped it in with the stick on the stop and the Army approved it. Right wheel up slope had adequate control all the way to left wheel touchdown. If you are interested in why there is a difference, send me a PM.
Oleo settling: Not theory, but reality. The slope is measured with a protractor/level on a straight board or similar surface. In the above case it was 15 degrees. Once the landing is complete, the controls centered and the shutdown is accomplished, the roll attitude will indicate 2-3 degrees higher, and that is somewhat dependent on vertical CG as well as oleo design/gear geometry. I'm not familiar with the slope limits on the various Puma models, but would guess that a qualification flight test to whatever their limits are, has been accomplished in a somewhat similar manner to the above. Same oleo compression effects would be expected.
Would crosswind etc impact the results? Certainly. How then are these limits to be applied? Excellent question, as no pilot in the field has any way of eyeballing a slope and knowing whether it is within the approved envelope. The answer is in training the pilot in the procedure on a real measured slope so that he will know and be familiar with the aircraft and control system behavior under the conditions of maximum slope. Not sure the simulators are up to this yet.
Oleo settling: Not theory, but reality. The slope is measured with a protractor/level on a straight board or similar surface. In the above case it was 15 degrees. Once the landing is complete, the controls centered and the shutdown is accomplished, the roll attitude will indicate 2-3 degrees higher, and that is somewhat dependent on vertical CG as well as oleo design/gear geometry. I'm not familiar with the slope limits on the various Puma models, but would guess that a qualification flight test to whatever their limits are, has been accomplished in a somewhat similar manner to the above. Same oleo compression effects would be expected.
Would crosswind etc impact the results? Certainly. How then are these limits to be applied? Excellent question, as no pilot in the field has any way of eyeballing a slope and knowing whether it is within the approved envelope. The answer is in training the pilot in the procedure on a real measured slope so that he will know and be familiar with the aircraft and control system behavior under the conditions of maximum slope. Not sure the simulators are up to this yet.
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Issue is that we have crosswind limit, lateral CG limit (vertical not the limiting factor in this case) and slope limit. Individually fine but cumulatively bad so what do you say to pilot in limits section of RFM so he doesn't kill himself and his crew? Cannot give them a slope angle limit in isolation because crosswind and lateral CG might make slope really bad. I am all for giving pilot rope to exercise judgement but is saying "beware crosswinds from retreating blade" enough?
John, presumably the difference on UH60 (and other types) between left and right wheel up slope is to do with lateral cyclic control margins not being equal?
What I mean is that whilst the cyclic may be central at MPOG on flat ground, you immediately lose some cyclic control margin on the retreating side because of the need to counter TR drift - this would give reduced margin to the left in the case of a counter-clockwise rotor.
A crosswind from the left would just aggravate the situation as you have to use more left cyclic just to maintain the hover - leaving less available for in-slope cyclic on sloping ground landings with left wheel up.
The converse would be the case with the Puma with a clockwise rotating MR.
We teach the use of the AI (AH) to assess sloping ground limits (or roll indications in nav kit which are usually from a VG anyway).
If you make an 'operational' style of landing, you might inadvertently exceed the slope limits whereas with a slower, academic style landing, the AI should be monitored all the way to MPOG to prevent exceeding the limits - that way the situation described by GM shouldn't occur.
The Lynx 7 however, was more uncomfortable as the fuselage had a 4 degree either way roll between the fuselage and the skids such that on a 12 degree lateral slope, when the controls were centralised, the AI could read 16 degrees of roll even though the skids were at 12 and therefore in limits.
The only worry about lack of lateral cyclic would be dynamic rollover which shouldn't be an issue if you are right wheel up in a CCW rotor as you are closer to the hover attitude (opposite for Puma).
What I mean is that whilst the cyclic may be central at MPOG on flat ground, you immediately lose some cyclic control margin on the retreating side because of the need to counter TR drift - this would give reduced margin to the left in the case of a counter-clockwise rotor.
A crosswind from the left would just aggravate the situation as you have to use more left cyclic just to maintain the hover - leaving less available for in-slope cyclic on sloping ground landings with left wheel up.
The converse would be the case with the Puma with a clockwise rotating MR.
We teach the use of the AI (AH) to assess sloping ground limits (or roll indications in nav kit which are usually from a VG anyway).
If you make an 'operational' style of landing, you might inadvertently exceed the slope limits whereas with a slower, academic style landing, the AI should be monitored all the way to MPOG to prevent exceeding the limits - that way the situation described by GM shouldn't occur.
The Lynx 7 however, was more uncomfortable as the fuselage had a 4 degree either way roll between the fuselage and the skids such that on a 12 degree lateral slope, when the controls were centralised, the AI could read 16 degrees of roll even though the skids were at 12 and therefore in limits.
The only worry about lack of lateral cyclic would be dynamic rollover which shouldn't be an issue if you are right wheel up in a CCW rotor as you are closer to the hover attitude (opposite for Puma).
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It's twenty years since I instructed on the Puma HC1 but I think the limits were 6 RWU/8 LWU/12 NU degrees.
The cyclic lateral movement is limited by a shim box under the floor. There was no mistaking when the lateral limit was reached - CLUNK! The safest way to land a Puma on an untested slope was a combination of left wheel and nose up.
The slope often tends to apparently increase on soft ground after landing because the downhill wheel tends to be more heavily loaded, causing it to sink in more.
The cyclic lateral movement is limited by a shim box under the floor. There was no mistaking when the lateral limit was reached - CLUNK! The safest way to land a Puma on an untested slope was a combination of left wheel and nose up.
The slope often tends to apparently increase on soft ground after landing because the downhill wheel tends to be more heavily loaded, causing it to sink in more.
S-70 Lateral Control
Not unequal control margins per se, Crab. Lateral rigging is +/- 8 degrees . What complicates the situation is the collective to lateral coupling, which cuts off some of the left control envelope at reduced collective, which is where you find yourself when doing the max left wheel up slope landing.
Thanks John, that makes sense
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Not just on muddy ground - the downhill oleo compresses more than the uphill one. The frustrating thing is even though you can define a nice slope limit as per shytorque says, you add lat CG loading plus a crosswind and then the AI reading the pilot reads is not giving him any useful hint about whether he's about to get himself in trouble. And a limit which factored in slope, lat CG and crosswind would be ridiculous. Not looking for a solution just venting....
Bear in mind GM that those sloping ground limits were determined by Boscombe and they will have added a safety margin to allow for variables such as C of G, crosswind etc and also made them acceptable for an 'average' squadron pilot of whatever level of experience. So for an ace pilot on a perfect slope in ideal conditions, the slope limits are probably higher than the RTS limits.
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Sloping ground testing is done on carefully calibrated concrete slopes (Pax River and Boscombe Down).
Have you ever seen a sloping piece of concrete anywhere else?
Limits take no account of slippery slopes, high vertical CG and so on. It's a place where every helicopter pilot become a test pilot for a short while.
I've heard of a helicopter that had a sloping ground limit of 3° - when I asked the company said it was done at the worst lateral CG with the maximum wind from the side... (I got them to change the test method).
So, it's another case where we measure with a micrometer, mark with a chalk line and cut with an axe.
Have you ever seen a sloping piece of concrete anywhere else?
Limits take no account of slippery slopes, high vertical CG and so on. It's a place where every helicopter pilot become a test pilot for a short while.
I've heard of a helicopter that had a sloping ground limit of 3° - when I asked the company said it was done at the worst lateral CG with the maximum wind from the side... (I got them to change the test method).
So, it's another case where we measure with a micrometer, mark with a chalk line and cut with an axe.
Slope testing
Shawn's statement is a bit too all-inclusive.
" Sloping ground testing is done on carefully calibrated concrete slopes (Pax River and Boscombe Down)."
Not Always so, and for good reasons.
The Not Always so: UH-60 and S-92 were done on dirt ( and since we only had dirt, the 76 must have been done that way-just can't recall doing them. Nick will remember.)
The 53K will be done on a new slope landing complex they built at the West Palm facility after I retired. Grass Slopes 3 to 15 degrees with a concrete sub-strata.
Reason why is not only replication of the real world, but that replication is also important from getting the mechanical stability part of the equation correct, or at least highly representative. More important for an articulated rotor, to be sure.
NAS PXT built a 9 degree 50' x 50' concrete slope for the SH-60B, but that was for doing the 12 ft/sec hard landings. At the time, that was all they had.
Thanks,
John
" Sloping ground testing is done on carefully calibrated concrete slopes (Pax River and Boscombe Down)."
Not Always so, and for good reasons.
The Not Always so: UH-60 and S-92 were done on dirt ( and since we only had dirt, the 76 must have been done that way-just can't recall doing them. Nick will remember.)
The 53K will be done on a new slope landing complex they built at the West Palm facility after I retired. Grass Slopes 3 to 15 degrees with a concrete sub-strata.
Reason why is not only replication of the real world, but that replication is also important from getting the mechanical stability part of the equation correct, or at least highly representative. More important for an articulated rotor, to be sure.
NAS PXT built a 9 degree 50' x 50' concrete slope for the SH-60B, but that was for doing the 12 ft/sec hard landings. At the time, that was all they had.
Thanks,
John
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Shawn that's perfect - the bit about the 3 deg slope limit being due to the test being done worst case. I know at least one other system that was tested like that and it was absurdly restricted 99% of the time. I suppose there is a balance between simplicity (3 deg - easy to remember always safe) and complexity (6 deg with no crosswind, 4 deg with lat CG and light wind, 3 deg on a Wednesday - impossible to remember but useful on certain days of the week).
I like the idea of a calibrated grass slope. The 12deg at a certain test establishment is a little unrepresentative.
Crab - love the way you point out slope limits determined "by Boscombe". Surely not those hallowed souls they call Test Pilots? - I wondered where all the average pilots had ended up
(Light touch paper, retreat rapidly, enjoy)....happy Bonfire night.
I like the idea of a calibrated grass slope. The 12deg at a certain test establishment is a little unrepresentative.
Crab - love the way you point out slope limits determined "by Boscombe". Surely not those hallowed souls they call Test Pilots? - I wondered where all the average pilots had ended up
(Light touch paper, retreat rapidly, enjoy)....happy Bonfire night.
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Now....did someone mention Boscombe and the Puma? We'd been successfully operationally flying plastic bladed Pumas for years when they borrowed one of ours to play with, sorry I meant carry out some trials on, "updated" handling techniques for plastic bladed Pumas..... They soon bent it! From that we learned to fly it like we had been doing, not like they did!
Way back in 1978 when I left the RAF I went on to the North Sea on to the As330J. That was an upgraded Puma with plastic blades. The Air Force at that time were using metal blades with a 60 min inspection time.
I wrote to my old squadron commander extolling the virtues of these blades and as a result they found out that there was a set sitting in a hangar at Boscombe Down, and had been for some time. These were transported up to Odiham, fitted to an aircraft and the trials and conversions started there.
Boscombe had nothing to do with it.
I wrote to my old squadron commander extolling the virtues of these blades and as a result they found out that there was a set sitting in a hangar at Boscombe Down, and had been for some time. These were transported up to Odiham, fitted to an aircraft and the trials and conversions started there.
Boscombe had nothing to do with it.
Shy, they did the same with a Sea King to 'prove' dynamic rollover wasn't an issue with the old girl and to try and replicate the conditions of an accident at St Mawgan - they did, perfectly.....doh!
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Initial trials were conducted in France in 1976 with Boscombe/Istres. Other trials were conducted in 1979, and again in 1980. So, I think Boscombe WAS involved in CMRB flight testing.
It does not alter the fact that in 1977 and 1978 the UK Puma force was in deep trouble because of the unreliability of its rotor blades. Aircraft going from Odiham to Aldergrove had to cross the Irish Sea at Stranraer because the crossing from Liverpool to Ulster took too long. Some blades BIMs were indicating just over an hour after the last inspection. They were buying up second hand metal blades from everywhere. One, that had perforated trim tabs, lasted 30 secs. when I did the initial ground runs on it owing to the vibration. Should Boscombe had partaken in the testing of the plastic blades in 1976, a requirement for civil certification of the 330J, then they should have put their oar in; they didn't.
It was left up to Odiham to do Boscombe Down's job.
It was left up to Odiham to do Boscombe Down's job.
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I wrote to my old squadron commander extolling the virtues of these blades and as a result they found out that there was a set sitting in a hangar at Boscombe Down, and had been for some time. These were transported up to Odiham, fitted to an aircraft and the trials and conversions started there.
Boscombe had nothing to do with it.
Boscombe had nothing to do with it.
I'm referring to a later episode, it occurred in the mid to late 1980s.
"New" (well overdue) engines-off trials on a plastic bladed aircraft resulted in a damaged airframe. It proved what some of us already thought - if the Puma's tail "stinger" touched the ground, rather than preventing tail rotor damage, it was likely to pivot around the rear bracket, pulling off the riveted bracket attachment at the front end. It would then swing upwards and get tangled up with the tail rotor blades.
IIRC, back then the FRCs hadn't then been changed from the advice given for the old metal bladed aircraft i.e. a flare of up to 27 degrees nose up was advocated.
By then we all knew that the plastic bladed aircraft could be flown much like a big Gazelle and didn't need such an aggressive flare, due to the much better performance of the blades.
It was left up to Odiham to do Boscombe Down's job.
