MD Explorer
Blade Sailing
Vfr,
This is correct and quite normal, it's known as blade sailing. Rotor blades rely on centrifugal force in order to maintain sufficient rigidity in the flap-wise direction. As the rotor speed decays, the centrifugal force reduces more rapidly than the aerodynamic force and therefore very large flapping motions can result. The effects are most pronounced for ship-borne helicopter operations where high winds and the turbulence around the ships superstructure result in rather extreme conditions.
Hope this helps
CRAN
This is correct and quite normal, it's known as blade sailing. Rotor blades rely on centrifugal force in order to maintain sufficient rigidity in the flap-wise direction. As the rotor speed decays, the centrifugal force reduces more rapidly than the aerodynamic force and therefore very large flapping motions can result. The effects are most pronounced for ship-borne helicopter operations where high winds and the turbulence around the ships superstructure result in rather extreme conditions.
Hope this helps
CRAN
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Don't these guys normally shut down or disengage the rotors before loading casualties?
I guess the medics can unload with the ship held at full power, which would not be right on top of a casualty site anyway??
h-r
I guess the medics can unload with the ship held at full power, which would not be right on top of a casualty site anyway??
h-r
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Good morning Cran,
Yes I know about sailing, but what I was trying to find out was are those blades on the MD so flexible as to be able to actually come down far enough to make contact with any crew or ground crew, from the picture that I have been able to drag up showing the MD the blades look quite short and ridgid, and don't appear to be able to flex that far!
Peter R-B
Vfr
Yes I know about sailing, but what I was trying to find out was are those blades on the MD so flexible as to be able to actually come down far enough to make contact with any crew or ground crew, from the picture that I have been able to drag up showing the MD the blades look quite short and ridgid, and don't appear to be able to flex that far!
Peter R-B
Vfr
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Vfr
The Explorer blades are no better or worse than other types. The max windspeed for starting/stopping rotors is 50 kts.
You can start/stop the machine so quickly, there is no benefit in loading rotors running.
In any case,crew should not be entering or leaving the disc whilst starting or stopping!
The Explorer blades are no better or worse than other types. The max windspeed for starting/stopping rotors is 50 kts.
You can start/stop the machine so quickly, there is no benefit in loading rotors running.
In any case,crew should not be entering or leaving the disc whilst starting or stopping!
Again it is difficult to answer properly without some sums, but my guess is that under the correct conditions all types of helicopter can experience blade sailing sufficient to harm by-standers. (Even those with hingeless hubs!) So in answer to your original question, yes I still think the report is credible and I don't think it is limited to the MD900 range.
Hope this helps
CRAN
Hope this helps
CRAN
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Rotor blades can flex sufficiently to endanger anyone under them when RRPM is less than flying rpm. There is, and unfortuately I do not have a copy, a photograph of a Wessex rotors running on a carrier with the forward rotor blade flexing to about 4 feet of the deck. Blade strikes of tailbooms are not uncommon during starting and particulary stopping rotors during windy/gusty conditions. Hiding behind a building to be out of the wind is not the best place to close down as eddys and gusts can be more destructive in these places - stay out in the open if possible where the wind is more constant.
Purveyor of Egg Liqueur to Lucifer
This'll make your eyes water;
Add a bit of rotation a sniff of wind and stand well back!!!
Ok so a bit of an extreme case the old Chinook, but I would hazard a guess that with a slow-med rotation speed and a reasonable wind, the blade sail for even something like the Lynx/135 types would still be a good couple or three feet.
Bearing in mind the original post relates to the GNAA which will be operating in, I would suspect, uneven ground a lot of the time, no matter which type of machine they use, the risk of a blade coming down to head/'arm holding drip up' height, would be reasonably high. Definitely high enough of a risk to warrant mention in a brief, it even if they had a 135!
Slow rotor speed + incoming crew/pax = high risk of mess!
Add a bit of rotation a sniff of wind and stand well back!!!
Ok so a bit of an extreme case the old Chinook, but I would hazard a guess that with a slow-med rotation speed and a reasonable wind, the blade sail for even something like the Lynx/135 types would still be a good couple or three feet.
Bearing in mind the original post relates to the GNAA which will be operating in, I would suspect, uneven ground a lot of the time, no matter which type of machine they use, the risk of a blade coming down to head/'arm holding drip up' height, would be reasonably high. Definitely high enough of a risk to warrant mention in a brief, it even if they had a 135!
Slow rotor speed + incoming crew/pax = high risk of mess!
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SS - I'd be reluctant to compare Lynx with EC135.
The Lynx starts with a lower head height and more flexible blades than the 135 and they can sail enough to remove an average head from its shoulders in the wrong conditions. When operating a Lynx at sea therefore (or any other navalised helicopter), one engine is started against the rotor brake - the other (in a twin) will have been started in accessory drive to power hydraulics, electrics etc. The rotorbrake is then released and the blades accelerated with some significant (60-70%) torque, to avoid possible blade sail. Stopping is the reverse, the rotor brake coming on at the earliest opportunity and stopping the blades significantly faster than any civil heli - full hydraulic pressure (103 Bar, if memory serves me right, in the case of the Lynx) applied at 47% Nr causes the whole aircraft to twist, with a torque reaction that feels as if it would tear the gearbox out of many lesser machines!
The 135, on the other hand, with it's higher head height and rigid head & blades, hardly sails at all and never (in my experience) enough to worry anyone at the edge of the disk on level ground. Whilst I would never allow anyone into/out of the disk whilst starting/stopping, (long established flight safety principles are worth sticking to), it is one of the few aircraft where it's theoretically possible without danger.
Concur with allthepies - when you can stop & start so quickly, there's no point in risking it for an extra few seconds.
I seem to remember an accident report, a year or five ago, where a B206 had landed rather heavily (Brazil, I think), pilot injured & unable to tell the pax to stay where they were until all had stopped. CEO & his PA (pax) jumped out, relieved to be alive, only to both be de-capitated by a decaying rotor which had been further lowered by splayed skids. A salutory lesson to all!
The Lynx starts with a lower head height and more flexible blades than the 135 and they can sail enough to remove an average head from its shoulders in the wrong conditions. When operating a Lynx at sea therefore (or any other navalised helicopter), one engine is started against the rotor brake - the other (in a twin) will have been started in accessory drive to power hydraulics, electrics etc. The rotorbrake is then released and the blades accelerated with some significant (60-70%) torque, to avoid possible blade sail. Stopping is the reverse, the rotor brake coming on at the earliest opportunity and stopping the blades significantly faster than any civil heli - full hydraulic pressure (103 Bar, if memory serves me right, in the case of the Lynx) applied at 47% Nr causes the whole aircraft to twist, with a torque reaction that feels as if it would tear the gearbox out of many lesser machines!
The 135, on the other hand, with it's higher head height and rigid head & blades, hardly sails at all and never (in my experience) enough to worry anyone at the edge of the disk on level ground. Whilst I would never allow anyone into/out of the disk whilst starting/stopping, (long established flight safety principles are worth sticking to), it is one of the few aircraft where it's theoretically possible without danger.
Concur with allthepies - when you can stop & start so quickly, there's no point in risking it for an extra few seconds.
I seem to remember an accident report, a year or five ago, where a B206 had landed rather heavily (Brazil, I think), pilot injured & unable to tell the pax to stay where they were until all had stopped. CEO & his PA (pax) jumped out, relieved to be alive, only to both be de-capitated by a decaying rotor which had been further lowered by splayed skids. A salutory lesson to all!
The head on the 902 is not semi rigid and employs a form of droop stop so there is always a danger of blade sail. I have shut the aircraft down in winds up to 50kts and there is a lot of movement especially if it is gusty.
The rotor brake on the 902 can be applied at 70% Nr which is as soon as you switch the engine control switches from IDLE to OFF. As there is no cooling period at IDLE with the P&W engines the aircraft can be shutdown within seconds of landing.
The rotor brake is one of the many good features of the aircraft and probably came off some American muscle car.
Having flown the Lynx and being current on the 135 and 902 I can safely say that the 902 is quicker to start and stop than either of them.
No doubt one of the 902 knockers will chip in that a good rotor brake isn't much use if the aircraft is always in the hangar!
FNW
The rotor brake on the 902 can be applied at 70% Nr which is as soon as you switch the engine control switches from IDLE to OFF. As there is no cooling period at IDLE with the P&W engines the aircraft can be shutdown within seconds of landing.
The rotor brake is one of the many good features of the aircraft and probably came off some American muscle car.
Having flown the Lynx and being current on the 135 and 902 I can safely say that the 902 is quicker to start and stop than either of them.
No doubt one of the 902 knockers will chip in that a good rotor brake isn't much use if the aircraft is always in the hangar!
FNW
Purveyor of Egg Liqueur to Lucifer
The Lynx starts with a lower head height and more flexible blades than the 135 and they can sail enough to remove an average head from its shoulders in the wrong conditions.
Point taken, but I was wondering about the 135s forward tilt in the 'wrong conditions'!
Sods law states that when 'in the wrong conditions', the Lynx ACC/MAIN drive actuator will go u/s!
Disagree with you tc and zorab, I have seen a BK117 disc tilt down far enough to trim shoulder hairs.
IMHO there should be a simple rule regardless of type when blades are turning:
ALWAYS get a thumb up from the pilot monitoring the controls (or a crewman in comms with him). ALWAYS bow to the helicopter. NEVER go in or out of the disc when between engine shutdown and rotors stopped.
NEVER enter or leave a control seat with someone moving under the disc.
Best way to keep your head really.
IMHO there should be a simple rule regardless of type when blades are turning:
ALWAYS get a thumb up from the pilot monitoring the controls (or a crewman in comms with him). ALWAYS bow to the helicopter. NEVER go in or out of the disc when between engine shutdown and rotors stopped.
NEVER enter or leave a control seat with someone moving under the disc.
Best way to keep your head really.
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The CH-47 can provide a very good example of this problem as a pilot induced problem.
While shutting down, with the RRPM getting slow enough that, with a little head wind, the blades of the forward rotor head that have been held up by the wind till about the 1 O’clock position, come down on the droop stops with some force.
The pilot, ever wanting to minimize the pounding of the droop stops, tries to "fly" the rotor blades off the stops by adding aft cyclic, which works very well, for a little while.
Then all at once, the blade is moving slowly enough the head wind no longer holds the blade up. The blade comes down with force and can flex low enough across the top of the aircraft to hit the tunnel cover over the drive shaft.
And another hard day of explanations has begun.
While shutting down, with the RRPM getting slow enough that, with a little head wind, the blades of the forward rotor head that have been held up by the wind till about the 1 O’clock position, come down on the droop stops with some force.
The pilot, ever wanting to minimize the pounding of the droop stops, tries to "fly" the rotor blades off the stops by adding aft cyclic, which works very well, for a little while.
Then all at once, the blade is moving slowly enough the head wind no longer holds the blade up. The blade comes down with force and can flex low enough across the top of the aircraft to hit the tunnel cover over the drive shaft.
And another hard day of explanations has begun.
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Helmet fire -
I can't claim familiarity with the 117, so can't comment on the rigidity of either head or blades.
Please note that I said "theoretically possible without danger." - we still warn people about the dangers of entering & exiting the disc & follow the long established principles of obtaining permission from the pilot etc. No change just because we've got a machine that appears relatively risk free.
SS -
It doesn't have any to speak of - & don't forget the high skids add another 300mm or so to your diagram. Rotors stopped, tip height above ground is around 3m, when running add another 300mm or so - I'm not volunteering to measure it! Any blade sail that I've seen is not going to come lower than 2.5m - anyone that size won't fit in the door & is likely to put us over MAUW!
When this happens, there's a sodding great bang, the whole aircraft twists on the ground with an angry torque reation and the pilot will shut down immediately - and pray that it was a mechanical fault and not his carelessness with the switches! It doesn't happen often, but it has happened & results in a complete drive train change having overloaded the system somewhat - and normally rather a lot of humble pie/egg on face!
Disagree with you tc and zorab, I have seen a BK117 disc tilt down far enough to trim shoulder hairs.
Please note that I said "theoretically possible without danger." - we still warn people about the dangers of entering & exiting the disc & follow the long established principles of obtaining permission from the pilot etc. No change just because we've got a machine that appears relatively risk free.
SS -
Point taken, but I was wondering about the 135s forward tilt in the 'wrong conditions'!
Sods law states that when 'in the wrong conditions', the Lynx ACC/MAIN drive actuator will go u/s!
Purveyor of Egg Liqueur to Lucifer
zorab64;
Point taken about the 135s skid height.
2.5m! About the same height as a hand holding up a drip. Ok a bit of straw clutching there as y'all shut down anyway.
Reading your choice of my quote;
"Sods law states that when 'in the wrong conditions', the Lynx ACC/MAIN drive actuator will go u/s!"
This would in fact be a good thing the way you quote it. Why? because if you had the ECLs in the wrong configuration and flicked the switch, with the actuator u/s, nothing would happen. No big bang, just stuck in the same mode (main/acc) as when you flicked the switch.
Apparently, even with an 's' actuator there is now a system of microswitches that prevent the change-over if the ECLs are wrongly configured. yea right! Lets test that one out then!
In the context of the quote you gave, if you were on exercise for example and the actuator went u/s, the aircraft would be operated with main drive starts/stops, thus preventing the ability of going to ACC drive, rotors stopped, for pax embarkation/disembarkation.
By the way, did you hear about the guy who sat on the pan at GT for over an hour early one summers evening, between end of play and start of night flying, for no apparent reason until the duty NCO, lynx jockey, investigated?
Apparently the actuator wouldn't motor over to ACC. He sat there burning/turning waiting for the fuel to run out until Duty NCO carried out a main drive shutdown!
Point taken about the 135s skid height.
2.5m! About the same height as a hand holding up a drip. Ok a bit of straw clutching there as y'all shut down anyway.
Reading your choice of my quote;
"Sods law states that when 'in the wrong conditions', the Lynx ACC/MAIN drive actuator will go u/s!"
This would in fact be a good thing the way you quote it. Why? because if you had the ECLs in the wrong configuration and flicked the switch, with the actuator u/s, nothing would happen. No big bang, just stuck in the same mode (main/acc) as when you flicked the switch.
Apparently, even with an 's' actuator there is now a system of microswitches that prevent the change-over if the ECLs are wrongly configured. yea right! Lets test that one out then!
In the context of the quote you gave, if you were on exercise for example and the actuator went u/s, the aircraft would be operated with main drive starts/stops, thus preventing the ability of going to ACC drive, rotors stopped, for pax embarkation/disembarkation.
By the way, did you hear about the guy who sat on the pan at GT for over an hour early one summers evening, between end of play and start of night flying, for no apparent reason until the duty NCO, lynx jockey, investigated?
Apparently the actuator wouldn't motor over to ACC. He sat there burning/turning waiting for the fuel to run out until Duty NCO carried out a main drive shutdown!
ACC/MAIN
both engines running and stable
No2 95% plus, No1 a positive split
Pretty simple, I always thought, but every now and then someone gets it wrong.
HF,
I'd be interested in the circumstances behind a 117 disc getting that low: the MMI must have been firmly in the red
SS,
The Sea King Accesory Drive caught me (and the Boss: I was JJ, so his co pilot
) when we opted to shut down blades spread in Denmark, without the tedious business of retarding engines, etc.
"Watch this, Boss. All you do is switch off the gens, shut down, then put the Speed Selects in the right sequence to allow the AD actuator to motor, fool the system and Bob's your Mother's brother"
Great idea: especially when the battery cable had broken, so no generator, no power. No self exciting generators in those days, no way to shut down without wiggly amps: even retarding the manual throttles didn't flame the blasted engines out!!!!
Eventually the C130 landed with the support equipment and a ground power unit, but (as always) it was all My Fault. We sat there that long, that I wish we had run out of fuel to get away from the frosty atmosphere in the cockpit
I'd be interested in the circumstances behind a 117 disc getting that low: the MMI must have been firmly in the red
SS,
The Sea King Accesory Drive caught me (and the Boss: I was JJ, so his co pilot
) when we opted to shut down blades spread in Denmark, without the tedious business of retarding engines, etc."Watch this, Boss. All you do is switch off the gens, shut down, then put the Speed Selects in the right sequence to allow the AD actuator to motor, fool the system and Bob's your Mother's brother"
Great idea: especially when the battery cable had broken, so no generator, no power. No self exciting generators in those days, no way to shut down without wiggly amps: even retarding the manual throttles didn't flame the blasted engines out!!!!
Eventually the C130 landed with the support equipment and a ground power unit, but (as always) it was all My Fault. We sat there that long, that I wish we had run out of fuel to get away from the frosty atmosphere in the cockpit
Purveyor of Egg Liqueur to Lucifer
MightyGem;
From memory, Not strictly true!
And as you say, "every now and then someone gets it wrong".
SS
Isn't it something like;
Engines must be either both running or both stopped
No1 ECL must be at GRD IDLE
No2 ECL must be at MAX
If both engines are running, ensure No2 engine is above 95% Nf and No1 has a positive Nf split.
Post MOD 720, the Accessory drive switch can function only if the generators are both on-line or both off-line.
MG;
Famous last words! 
2 forward, 1 back
both engines running and stable
No2 95% plus, No1 a positive split
both engines running and stable
No2 95% plus, No1 a positive split
And as you say, "every now and then someone gets it wrong".
SS
Isn't it something like;
Engines must be either both running or both stopped
No1 ECL must be at GRD IDLE
No2 ECL must be at MAX
If both engines are running, ensure No2 engine is above 95% Nf and No1 has a positive Nf split.
Post MOD 720, the Accessory drive switch can function only if the generators are both on-line or both off-line.
MG;
Pretty simple, I always thought, but every now and then someone gets it wrong.
There are no limits
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EC135 Skid Height
I think that all EC135s in the HEMS role in UK are on the low skid. Add uneven ground and this could still be an issue for anyone. I fly a high skid machine and its one hell of a long way down! And one hell of a long way up to get a casualty into the side door - so we use the rear door.
BTW, MG was right about the acc drive checks on the mark 1 Lynx pre-mod, and I think SS was right about the checks on the post-mod mark 7. I never flew the 9 so can't comment.
You lot must be good to be able to remember that stuff, dumped it long ago to make room for later types! Still remember the downwind checks on the mighty De Havilland Chipmunk Mk 10 though!
MFFHHB
BTW, MG was right about the acc drive checks on the mark 1 Lynx pre-mod, and I think SS was right about the checks on the post-mod mark 7. I never flew the 9 so can't comment.
You lot must be good to be able to remember that stuff, dumped it long ago to make room for later types! Still remember the downwind checks on the mighty De Havilland Chipmunk Mk 10 though!
MFFHHB
Purveyor of Egg Liqueur to Lucifer
"Fuel on, Brakes on, Throttle closed, Switches off".
SS
SS