Entering autos: discussion split from Glasgow crash thread
Ahhh, now I got it. Sorry was a bit slow.
I will amend my post to make it more precise, don't konw if it changes much, though.
G0ULI
If it had wheels, no big deal. If it had skids ... an interesting day! 
The maneuver you describe might be what we used to call a running take off. Or, a MGW take off. Not sure what it was you saw.
Gross weight versus DA, versus all other performance factors (to include how much wind and from where) influence the ability to hover in and out of ground effect.
If you can't hover without some droop, do you want to take off? If yes (there will some factors and rules and SOPs that drive that answer) you will want to get a bit of translational lift flowing through the rotor head to reduce power required.
Do you mean powered or unpowered?
DA influences everything a helicopter does. If your power available versus power required are an unfavorable balance, when you try to lift up into a hover you'll tend to see the NR droop, which is often a good performance flag to put it back down and either get rid of some weight, check that your performance planning didn't have an error, get into the wind ... depends on the situation.
My mentors who used to fly armed Hueys in Viet Nam used to tell stories of having to use an IGE take off run (get some speed and translational lift so they'd have enough power margin to begin a climb away from the forward operating base) when they'd come back to rearm/refuel so that they could actually take off and return to the fight. I am pretty sure SASless and a few others could tell us some real life experiences along those lines.
I remember years ago expessing surprise at seeing the police helicopter refuelling at a local flying club and doing a rolling takeoff.
The maneuver you describe might be what we used to call a running take off. Or, a MGW take off. Not sure what it was you saw.
Apparently with the temperature hovering around 30 degrees centigrade, they couldn't take off from their base, only about 250 feet higher but needing a vertical departure with the tanks more than a quarter full.
If you can't hover without some droop, do you want to take off? If yes (there will some factors and rules and SOPs that drive that answer) you will want to get a bit of translational lift flowing through the rotor head to reduce power required.
So clearly the weather can have huge effects on performance and I have to conclude that rotor stall would occur at a higher rotational speed in hot weather than on a cold day as just a single practical example of the variables that have to be taken into account.
DA influences everything a helicopter does. If your power available versus power required are an unfavorable balance, when you try to lift up into a hover you'll tend to see the NR droop, which is often a good performance flag to put it back down and either get rid of some weight, check that your performance planning didn't have an error, get into the wind ... depends on the situation.
My mentors who used to fly armed Hueys in Viet Nam used to tell stories of having to use an IGE take off run (get some speed and translational lift so they'd have enough power margin to begin a climb away from the forward operating base) when they'd come back to rearm/refuel so that they could actually take off and return to the fight. I am pretty sure SASless and a few others could tell us some real life experiences along those lines.
Last edited by Lonewolf_50; 16th Dec 2013 at 20:11.
GOULI - cock-all to do with rotor stall but lots to do with engine limits (usually T4/PTIT/T6 on a hot day) - the 'rolling takeoff' will likely have been a cushion creep transition to maximise power available in ground effect.
Gouli, by you living in Norfolk, it would have been a skidded police helicopter and they do very poor impersonations of rolling takeoffs, I would suggest.
Try again? What was it you actually saw and who told you he had DA issues?
Lonewolf: no please, please no more vietnam stories, please
Try again? What was it you actually saw and who told you he had DA issues?
Lonewolf: no please, please no more vietnam stories, please
TC, given where that flying took place, DA was a non trivial performance variable. Trying to provide helpful answer to our fixed wing friend, eh?
Thread drift from autorotations to max performance takeoff doesn't help the discussion intended, so objection sustained.
Thread drift from autorotations to max performance takeoff doesn't help the discussion intended, so objection sustained.
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Gouli,
I suggest that the hot-day density dependence reflects nothing more than the relatively marginal nature of helicopter flight. In exchange for those special capabilities, there is a compromise in the ability to carry great loads cheaply.
The power demands to haul vertically upwards are very significant, and when air density is down by about 10% at 30C, as compared with a freezing day, it gets tough. More rotor speed and angle of attack, and more power, wastefully beating up more intense tip vortices, and imparting rotation to the wake is required.
Then again, with fixed wings, the speed at which lift is adequate for flight also depends on the density, although the leeway between getting adequate lift and compressibility of the flow becoming an issue is likely greater.
While different outcomes might be achieved on different days, straying into territory where the outcome is that marginal is probably not a good idea. I would not encourage stalls to be considered as "chaotic". The airflow over the wing is chaotic post-stall, but when the flow breaks down for a certain set of temperature-pressure-humidity-turbulence conditions is very reproduceable. What about the Elmendorf C17 crash, where to paraphrase "the stall warning always sounds here"? It doesn't sound like a message you want to be giving.
Was that a "rolling take off" for a helicopter on wheels, or choosing/being required to accelerate in ground effect? I guess they're effectively the same thing.
I suggest that the hot-day density dependence reflects nothing more than the relatively marginal nature of helicopter flight. In exchange for those special capabilities, there is a compromise in the ability to carry great loads cheaply.
The power demands to haul vertically upwards are very significant, and when air density is down by about 10% at 30C, as compared with a freezing day, it gets tough. More rotor speed and angle of attack, and more power, wastefully beating up more intense tip vortices, and imparting rotation to the wake is required.
Then again, with fixed wings, the speed at which lift is adequate for flight also depends on the density, although the leeway between getting adequate lift and compressibility of the flow becoming an issue is likely greater.
While different outcomes might be achieved on different days, straying into territory where the outcome is that marginal is probably not a good idea. I would not encourage stalls to be considered as "chaotic". The airflow over the wing is chaotic post-stall, but when the flow breaks down for a certain set of temperature-pressure-humidity-turbulence conditions is very reproduceable. What about the Elmendorf C17 crash, where to paraphrase "the stall warning always sounds here"? It doesn't sound like a message you want to be giving.
Was that a "rolling take off" for a helicopter on wheels, or choosing/being required to accelerate in ground effect? I guess they're effectively the same thing.
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Thanks all for the clarification of what I witnessed. It all happened years ago just outside London. Probably a Bell Jet Ranger. i haven't always lived in paradise
Lived in Australia during the Vietnam war. Watched the news reports every day. Never bothered me as a kid, it did when I was old enough to understand later.
Lived in Australia during the Vietnam war. Watched the news reports every day. Never bothered me as a kid, it did when I was old enough to understand later.
As above, we used to call them "running takeoff" but I am not sure if that is only on this side of the pond. Likewise, at the bottom of an autorotation with forward GS maintained, we used to call that a "running autorotation" to differentiate it from an auto to a spot.
In practical Terms you are probably right.
In theory it will make a small difference.
Higher DA means lower IAS of the blade. This corresponds aerodynamically already to a lower RPM. AoA at the same RPM will be higher. Stall will occur at somewhat higher RPM.
Since we don't know to an exactness of 10% what that RPM is it won't matter much, though.
Some of the comments and posts on this thread are beyond contempt, especially from supposed seasoned aviators. To insult and demean those who created the Flight Manuals in such a derogatory manner is beyond me.
On my part, this is neither hero worship as implied by HC, nor blind faith. These guys wrote the books that we rely upon to do our job; they come here to share their knowledge in an open discussion and get abused and denigrated.
DB, your post is just appalling: I hope that you are ashamed of yourself and realise that if we never see contributions from the likes of Nick and John ever again then you and HC should go, too.
A simply shocking thing to say.
On my part, this is neither hero worship as implied by HC, nor blind faith. These guys wrote the books that we rely upon to do our job; they come here to share their knowledge in an open discussion and get abused and denigrated.
DB, your post is just appalling: I hope that you are ashamed of yourself and realise that if we never see contributions from the likes of Nick and John ever again then you and HC should go, too.
This chest beating arse was responsible for the demise of the S92 in Newfoundland!
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TC.
I'd like to see you install markerballs while staying out of the HV curve.
These aircraft were made to be hovered. The utility industry is proof of that. There is an increased risk when hovering in the curve, sure, but when looking at that risk along with all of the other risks we are exposed to its a drop in the bucket, hardly the difference between a mission that is overall safe, vs unsafe.
The statement in the RFM is little more than a disclaimer not "sound technical adivce". If you need to be there to work, you need need to be there. I dont know anyone who would spend more time there then is needed to do the job.
Before you say, re-evaluate the job, the aircraft, etc..... that is for a different thread....
I'd like to see you install markerballs while staying out of the HV curve.
These aircraft were made to be hovered. The utility industry is proof of that. There is an increased risk when hovering in the curve, sure, but when looking at that risk along with all of the other risks we are exposed to its a drop in the bucket, hardly the difference between a mission that is overall safe, vs unsafe.
The statement in the RFM is little more than a disclaimer not "sound technical adivce". If you need to be there to work, you need need to be there. I dont know anyone who would spend more time there then is needed to do the job.
Before you say, re-evaluate the job, the aircraft, etc..... that is for a different thread....
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Thomas Coupling
G0ULI (golf zero uniform lima india) happens to be my radio callsign, although I admit it might be deemed unfortunate in the context of this subject. I have fixed wing flying experience and attended the scene of a fatal light aircraft crash in a professional capacity many years ago, which prompted a lifelong interest in flight safety. I was mystified as to what set of circumstances could cause an apparently perfectly functional helicopter to fall out of the sky. It would seem from the answers in this forum and information elsewhere that something such as a vortex ring formation could in theory put a helicopter in such an attitude that the engines would stop from fuel starvation and in that event, the pilot would have very little time to recover the aircraft. I never knew that the main rotor rpm had to be constrained to within such tight limits or fully appreciated that a full main rotor stall would be unrecoverable irrespective of altitude. This thread has been most enlightening, irrespective of some of the irreverent comments. I have had several opportunities over the years to take a flight in a police helicopter, I never took up the offer, although friends thought I was mad not to. I will stick to fixed wing aircraft.
G0ULI (golf zero uniform lima india) happens to be my radio callsign, although I admit it might be deemed unfortunate in the context of this subject. I have fixed wing flying experience and attended the scene of a fatal light aircraft crash in a professional capacity many years ago, which prompted a lifelong interest in flight safety. I was mystified as to what set of circumstances could cause an apparently perfectly functional helicopter to fall out of the sky. It would seem from the answers in this forum and information elsewhere that something such as a vortex ring formation could in theory put a helicopter in such an attitude that the engines would stop from fuel starvation and in that event, the pilot would have very little time to recover the aircraft. I never knew that the main rotor rpm had to be constrained to within such tight limits or fully appreciated that a full main rotor stall would be unrecoverable irrespective of altitude. This thread has been most enlightening, irrespective of some of the irreverent comments. I have had several opportunities over the years to take a flight in a police helicopter, I never took up the offer, although friends thought I was mad not to. I will stick to fixed wing aircraft.
The transfer from donk stop to moving the cyclic to its final position might appear (a) instant and (b) instinctive but I bet you, the pilot subconciously maintained attitude initially.
Thus (500) K.I.S.S. Keep it simple - lower lever, maintain attitude and then we haven't got to worry about Vy, 2 x Vtoss, half Vmin squared for each damn a/c
[SAS: Don't demean others by disassociating yourself from the 'maintain attitude' fraternity...its trite and unbecoming of you].
Thus (500) K.I.S.S. Keep it simple - lower lever, maintain attitude and then we haven't got to worry about Vy, 2 x Vtoss, half Vmin squared for each damn a/c
[SAS: Don't demean others by disassociating yourself from the 'maintain attitude' fraternity...its trite and unbecoming of you].
That's real life for many of us. No cut and dried directives, pearls of wisdom, or checklists of robotic control movements. Just pilotage.
Last edited by Senior Pilot; 16th Dec 2013 at 20:39. Reason: Glasgow Crash comments are for the Glasgow crash thread: not here
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"Relatively Marginal Nature of Helicopter Flight"
I love that phrase! True or not, it represents the content of many of the posts in this thread, helicopters are apparently flying on the edge of the performance envelope quite frequently. If that is the case, then relatively small changes in density altitude, humidity and temperature could have quite large effects on the performance of helicopters and particularly so with relatively low powered, low rotor inertia helicopters.
500: I am not demeaning (thats a word that is getting used a lot lately) of your endeavours - far from it. In fact as I told 170' recently, I salute your bravery. And bravery is the word (not bravado) because you sure as hell need it to get your job done, when it is a known, well tested, well tried fact of life that hovering inside the H/V curve ends in tears.....as it did with your previous post statistics.
All I ask is (a) that operators like you recognise the work done by those test pilots who have laid out the danger zones for us mortals to fly around and (B) you don't try to mitigate your reasons for being there in the first place. Given the choice one would never do what you do ...but you have no choice it seems (as I dont know your industry).
PS: Describing the H/V curve as a "Disclaimer" is atleast disingenuous, at most ignorant.
Fly safely 500..................
Pilot and apprentice (you sure you arent rvdt as well?) IF he had done waht you suggested, the drop from his flare would not justify the impact crater size and depth. That is a very substantial deep crater: I'd say 30+ G?
All I ask is (a) that operators like you recognise the work done by those test pilots who have laid out the danger zones for us mortals to fly around and (B) you don't try to mitigate your reasons for being there in the first place. Given the choice one would never do what you do ...but you have no choice it seems (as I dont know your industry).
PS: Describing the H/V curve as a "Disclaimer" is atleast disingenuous, at most ignorant.
Fly safely 500..................
Pilot and apprentice (you sure you arent rvdt as well?) IF he had done waht you suggested, the drop from his flare would not justify the impact crater size and depth. That is a very substantial deep crater: I'd say 30+ G?
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Thank you TC,
We are not to far apart on this.
Every operator I know reccognizes and respects the performance envelope put out there by the test pilots and engineers. However, staying out of the avoid curve is just not a reality to our industry. Like you said, we do our best to limit the time there, limit weight to increase options in the event something goes wrong, and we all do far more than minimum required maintenance.
I called the avoid curve a disclaimer becuase that is exactly what it is. The manufacturer cannot assure a safe outcome if you are within it during an engine failure. If their objective was to keep you out of it they would have put it in the limitations section of the RFM. They know, as well as we do, that inside the avoid curve is where these machines (particularly utility machines) were ment to operate and they woiuldn't sell many if that were a limitation.
We are getting off topic
We are not to far apart on this.
Every operator I know reccognizes and respects the performance envelope put out there by the test pilots and engineers. However, staying out of the avoid curve is just not a reality to our industry. Like you said, we do our best to limit the time there, limit weight to increase options in the event something goes wrong, and we all do far more than minimum required maintenance.
I called the avoid curve a disclaimer becuase that is exactly what it is. The manufacturer cannot assure a safe outcome if you are within it during an engine failure. If their objective was to keep you out of it they would have put it in the limitations section of the RFM. They know, as well as we do, that inside the avoid curve is where these machines (particularly utility machines) were ment to operate and they woiuldn't sell many if that were a limitation.
We are getting off topic
SAS: Don't demean others by disassociating yourself from the 'maintain attitude' fraternity...its trite and unbecoming of you].
If you are offended by that comment....perhaps some Self Reflection is in order.
Am I walking on your Corns or something?
I prefer to stick with Aviators and that leaves out anyone who embraces Cook Book Methods of Helicopter Flying.
Sadly, for some reason, we seem to be plagued with an over abundance of them lately.
As Cowboy Logic tells us.....lead a Horse to Water but you will have to wait for him to make up his own Mind to drink.
Pilot and apprentice (you sure you arent rvdt as well?
Cant think of anyone on here that is a worthy match for you though TC.
I see absolutely no reason why a TP should be treated any differently from anyone else on here, though I am all in favour of courteous interaction rather than personal abuse of course. TPs have been trained and are just doing their job like the rest of us (well not me, I'm retired!). There are some good TPs and some incredibly useless and stupid ones. The good ones are not perfect, they have good ideas and attitudes, but also some bad ones at times. They are after all just human, and not gods, even if you would wish it were not so.
Last edited by HeliComparator; 17th Dec 2013 at 09:07. Reason: In the hope of an outbreak of peace.
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After the AAIB Special Bulletin was released I stated that in order for the aircraft to meet the CAA's description of a high speed vertical impact with rotors stopped, it was likely that the aircraft suffered a complete loss of power and that, for whatever reason, pitch was maintained on the main rotor. (This being the only probable explanation for such an impact, albeit speculative).
From the examples provided in this thread, and from experience, we know that in single engine flying there is an attentiveness (or at least there is supposed to be) towards the potential for a loss of power. This attentiveness is geared towards rapid response in the event of an engine failure with the certain knowledge that it will be necessary to conserve and manage Nr. The lever is going to have to do down in nearly all cases (apart from a low hover etc.) meaning this action can be taken (along with the necessary cyclic inputs) without delay.
With twins there are additional considerations, principally the second engine, and specifically determining which one has failed. The multi-engine pilot response to a power failure is therefore slightly delayed (in principal) to that of a single engine pilot. This delay is compensated for to some extent by the relatively low risk of double engine failures in twins.
However, on those rare occasions that a double engine failure does happen, it is possible to see how a multi-engine pilot may not instantly take the required action in the same way as a single engine pilot would, and for the right reasons, because the multi-engine pilot must assess the nature of the power failure, even if it is to determine that both engines have failed. In a single, any sort of interruption to powered flight is not only obvious but it is equally obvious that there is no recourse. This is not the "normal" mindset of a multi-engine pilot who will be trained to recover from a power failure with partially powered flight from the remaining engine.
Now that Eurocopter have officially admitted that there is a problem with the EC135 (and related types) fuel indication system and that this problem includes the possibility of over reading, are we seriously looking at the potential of a double engine failure due to a shortage of fuel? A prospect which I had initially discounted as being improbable.
If the ill-fated G-SPAO suffered a double engine failure below 1000ft, at night, I am starting to see how it may have been possible with a low inertia rotor to maintain pitch for some moments after both engines failed - even though a part of me still struggles to accept this.
What I am still unable to envisage is how the rotors stopped so completely prior to impact in order to achieve the unscored and unmarked blades which were apparent at the crash site. We are talking about a few seconds from a loss of power to "blades still"!
From the examples provided in this thread, and from experience, we know that in single engine flying there is an attentiveness (or at least there is supposed to be) towards the potential for a loss of power. This attentiveness is geared towards rapid response in the event of an engine failure with the certain knowledge that it will be necessary to conserve and manage Nr. The lever is going to have to do down in nearly all cases (apart from a low hover etc.) meaning this action can be taken (along with the necessary cyclic inputs) without delay.
With twins there are additional considerations, principally the second engine, and specifically determining which one has failed. The multi-engine pilot response to a power failure is therefore slightly delayed (in principal) to that of a single engine pilot. This delay is compensated for to some extent by the relatively low risk of double engine failures in twins.
However, on those rare occasions that a double engine failure does happen, it is possible to see how a multi-engine pilot may not instantly take the required action in the same way as a single engine pilot would, and for the right reasons, because the multi-engine pilot must assess the nature of the power failure, even if it is to determine that both engines have failed. In a single, any sort of interruption to powered flight is not only obvious but it is equally obvious that there is no recourse. This is not the "normal" mindset of a multi-engine pilot who will be trained to recover from a power failure with partially powered flight from the remaining engine.
Now that Eurocopter have officially admitted that there is a problem with the EC135 (and related types) fuel indication system and that this problem includes the possibility of over reading, are we seriously looking at the potential of a double engine failure due to a shortage of fuel? A prospect which I had initially discounted as being improbable.
If the ill-fated G-SPAO suffered a double engine failure below 1000ft, at night, I am starting to see how it may have been possible with a low inertia rotor to maintain pitch for some moments after both engines failed - even though a part of me still struggles to accept this.
What I am still unable to envisage is how the rotors stopped so completely prior to impact in order to achieve the unscored and unmarked blades which were apparent at the crash site. We are talking about a few seconds from a loss of power to "blades still"!