For Nick Lappos, re: Contention Concerning H/V Diagram
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For Nick Lappos, re: Contention Concerning H/V Diagram
Hello Mr. L.,
Your help is needed, there is a discussion going on elsewhere in cyberspace concerning the H/V diagram.
One side takes the view that any point of airspeed,altitude that is within the H/V diagram defines a flight condition from which successful autorotation is not possible...regardless of whether the helicopter is climbing/accelerating (takeoff profile), descending/decelerating (powered approach and landing profle), or in level flight (including hover) at the time.
The other side claims that if the helicopter is in a powered approach and landing condition, i.e., descending/decelerating with attendant low angle of attack on the blades conducive to rapid establishment of autorotative upflow through the rotor system, AND at a point within the H/V diagram, that a successful autorotation is in fact possible; in other words, that the H/V diagram is not valid for powered approach and landing, but only for the conditions of takeoff and level flight.
Can you give us your thoughts on this and references to the matter?
Many thanks in advance.
Your help is needed, there is a discussion going on elsewhere in cyberspace concerning the H/V diagram.
One side takes the view that any point of airspeed,altitude that is within the H/V diagram defines a flight condition from which successful autorotation is not possible...regardless of whether the helicopter is climbing/accelerating (takeoff profile), descending/decelerating (powered approach and landing profle), or in level flight (including hover) at the time.
The other side claims that if the helicopter is in a powered approach and landing condition, i.e., descending/decelerating with attendant low angle of attack on the blades conducive to rapid establishment of autorotative upflow through the rotor system, AND at a point within the H/V diagram, that a successful autorotation is in fact possible; in other words, that the H/V diagram is not valid for powered approach and landing, but only for the conditions of takeoff and level flight.
Can you give us your thoughts on this and references to the matter?
Many thanks in advance.
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nowhere in any height velocity diagram description does it say that autorotatios are not possible from whithin that area.
the height velocity diagram depicts an area of flight where an autorotation may not be carried out successfully, meaning without damaging the airframe, etc. etc.
demonstrations of all kinds of autos in this area are not difficult to carry out.
it is possible to do hovering auto's from two hundred feet (when you know they are coming). the extra height allowed in most diagrams takes into account the fright factor.
mostly it comes down to degrees of experience and the environment you are operating in.
the height velocity diagram depicts an area of flight where an autorotation may not be carried out successfully, meaning without damaging the airframe, etc. etc.
demonstrations of all kinds of autos in this area are not difficult to carry out.
it is possible to do hovering auto's from two hundred feet (when you know they are coming). the extra height allowed in most diagrams takes into account the fright factor.
mostly it comes down to degrees of experience and the environment you are operating in.
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I was always told that the 'average pilot' may not succesfully autorotate and walk away from..........
However I'm not sure what you would consider to be and average pilot.
However I'm not sure what you would consider to be and average pilot.
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H/V diagram is based on average skills responding to an unanticipated engine failure from level flight over ideal surfaces. Grey area indicates that a successful landing (saving skin and tin) will require more skill than is expected of that pilot.
If you change from the test conditions (speed, height, density altitude, weight, attitude, etc.) then the chart will no longer be as accurate. How it changes can depend as much on the type as on the change. For example, if you are in a low powered descent then Nr decay should be less, but reaction time may be increased (more subtle indications).
Even though many factors would change that chart, the white area typically offers your best profile in case of engine failure. It is up to the pilot to know when to and when not to operate in the grey area.
If you change from the test conditions (speed, height, density altitude, weight, attitude, etc.) then the chart will no longer be as accurate. How it changes can depend as much on the type as on the change. For example, if you are in a low powered descent then Nr decay should be less, but reaction time may be increased (more subtle indications).
Even though many factors would change that chart, the white area typically offers your best profile in case of engine failure. It is up to the pilot to know when to and when not to operate in the grey area.
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Here is my understanding of the situation re H/V curves - would welcome any comments:
Otherwise (wrongly) known as the Dead Man's Curve, this is a chart for helicopters that compares speeds against heights for areas of increased vigilance, or where not to be if you want to maximise your chances of successful recovery from an engine failure; that is, you don't want to be at high altitudes with low speeds, or low ones with high speeds, so the best place to be is in the gap between the shaded areas.
In other words, the graph shows combinations of speed and height that the average pilot would find it difficult to land safely from in emergency (one exam answer says from which it is not possible to make a safe autorotative landing). More precisely, it is where continuous operations should be avoided, as engine failure is likely to result in damage to the helicopter. It should still be observed when flying a twin-engined helicopter, in case an engine fails, but some machines, like the AS 355, often don't have a height/velocity curve at all for some flight regimes.
Takeoffs and performance calculations should take account of the curve, which is constructed at maximum weight, with no wind at a density altitude of at least 7,000 feet. Those of lesser quality must be verified (e.g. actually flown) by the Authorities.
A couple of points to note: one is that you should check if the chart is in the Limitations or Performance section of the Flight Manual (the BK 117's moves to the limitations section once you go into high density seating). If it's in the latter, its requirements are recommended, not mandatory. In some circumstances, it is more dangerous to try to avoid the curve, especially if you might only be in it for a few seconds (as when getting out of a confined area, for example). Another is that it is not valid for climbing out or approaches, being calculated for level flight conditions (i.e. a steady state constant airspeed & attitude), but lawyers and juries don’t appreciate the niceties, so it would be prudent to take note of its requirements, since engine failure while climbing through any of the shaded areas will result in airframe damage, as you are using higher power settings and angles of attack. You also have a fair amount of inertia, so RRPM will decay nicely while you wait for airflow to start going up through the disc after you have continued going up a short way before descending. On approach, your hands are on the controls and you are using less than cruise power, so the figures don't work the same way.
Anyhow, the vertical shaded area in the diagram (Area A) is called the low speed section, which takes account of:
The rate of descent required to drive the rotor
Rotor inertia characteristics or RRPM decay rate from the time of engine failure until the pilot wakes up, plus reaction time (to give the average pilot a chance, a one-second delay is factored in for minimum skill levels)
Landing gear design limitations and hard landing risk to the occupants
Translational lift values and sink rates
Area A is actually split in two parts at the knee of the curve, although it's never shown (the knee is the furthest point at which the curve extends). The lower portion is for takeoff power (no intervention), and the upper is for level flight (cruise power, hand not necessarily near the collective, so one second allowed for intervention time), and the whole area will expand with Density Altitude.
As mentioned above with regard to climbing, if the engine fails in the low speed area, your collective pitch is higher, so rotor RPM will decay earlier. The helicopter will also continue to climb under inertia before stopping and descending, which delays the onset of autorotation (meaning that the RRPM will be even lower).
The other shaded areas (B and C) are collectively known as the high speed section, and the clear area between them is the takeoff corridor. Their size is based on recognition time, rotation time, and altitude loss and groundspeed. You could use Area C over open ground where obstacle avoidance is not required and you can use a short ground run.
Phil
Otherwise (wrongly) known as the Dead Man's Curve, this is a chart for helicopters that compares speeds against heights for areas of increased vigilance, or where not to be if you want to maximise your chances of successful recovery from an engine failure; that is, you don't want to be at high altitudes with low speeds, or low ones with high speeds, so the best place to be is in the gap between the shaded areas.
In other words, the graph shows combinations of speed and height that the average pilot would find it difficult to land safely from in emergency (one exam answer says from which it is not possible to make a safe autorotative landing). More precisely, it is where continuous operations should be avoided, as engine failure is likely to result in damage to the helicopter. It should still be observed when flying a twin-engined helicopter, in case an engine fails, but some machines, like the AS 355, often don't have a height/velocity curve at all for some flight regimes.
Takeoffs and performance calculations should take account of the curve, which is constructed at maximum weight, with no wind at a density altitude of at least 7,000 feet. Those of lesser quality must be verified (e.g. actually flown) by the Authorities.
A couple of points to note: one is that you should check if the chart is in the Limitations or Performance section of the Flight Manual (the BK 117's moves to the limitations section once you go into high density seating). If it's in the latter, its requirements are recommended, not mandatory. In some circumstances, it is more dangerous to try to avoid the curve, especially if you might only be in it for a few seconds (as when getting out of a confined area, for example). Another is that it is not valid for climbing out or approaches, being calculated for level flight conditions (i.e. a steady state constant airspeed & attitude), but lawyers and juries don’t appreciate the niceties, so it would be prudent to take note of its requirements, since engine failure while climbing through any of the shaded areas will result in airframe damage, as you are using higher power settings and angles of attack. You also have a fair amount of inertia, so RRPM will decay nicely while you wait for airflow to start going up through the disc after you have continued going up a short way before descending. On approach, your hands are on the controls and you are using less than cruise power, so the figures don't work the same way.
Anyhow, the vertical shaded area in the diagram (Area A) is called the low speed section, which takes account of:
The rate of descent required to drive the rotor
Rotor inertia characteristics or RRPM decay rate from the time of engine failure until the pilot wakes up, plus reaction time (to give the average pilot a chance, a one-second delay is factored in for minimum skill levels)
Landing gear design limitations and hard landing risk to the occupants
Translational lift values and sink rates
Area A is actually split in two parts at the knee of the curve, although it's never shown (the knee is the furthest point at which the curve extends). The lower portion is for takeoff power (no intervention), and the upper is for level flight (cruise power, hand not necessarily near the collective, so one second allowed for intervention time), and the whole area will expand with Density Altitude.
As mentioned above with regard to climbing, if the engine fails in the low speed area, your collective pitch is higher, so rotor RPM will decay earlier. The helicopter will also continue to climb under inertia before stopping and descending, which delays the onset of autorotation (meaning that the RRPM will be even lower).
The other shaded areas (B and C) are collectively known as the high speed section, and the clear area between them is the takeoff corridor. Their size is based on recognition time, rotation time, and altitude loss and groundspeed. You could use Area C over open ground where obstacle avoidance is not required and you can use a short ground run.
Phil
Arismount - you will always get pilots who are convinced they know better than the test pilots/manufacturers etc and believe their superior skill will allow them to get away with operating just outside the flight envelope or, in this case, just inside the HV curve shaded areas.
They might be right or they might just be kidding themselves - anyone can show a successful auto from inside the HV curve because it is premeditated and therefore the reaction time is very low - the HV curve assumes a normal reaction time (2 seconds if memory serves) before the lever is lowered. If reaction time were removed from the HV curve it would be much smaller but completely unrealistic.
The HV curve is valid for level flight only as the amount of data and the number of graphs required to cover climbs and descents at various airspeeds would be nonsensical and confusing.
So we have one graph and rather than accepting the data on it and complying with it, 'superior' pilots will start finding excuses as to why they can operate inside it and still be safe (right up to the point where the engine fails and someone sues you) and come out with all sorts of grandiose explanations as to why they are right to do this.
Arrogance breeds overconfidence which causes accidents. Flight safety message over.
They might be right or they might just be kidding themselves - anyone can show a successful auto from inside the HV curve because it is premeditated and therefore the reaction time is very low - the HV curve assumes a normal reaction time (2 seconds if memory serves) before the lever is lowered. If reaction time were removed from the HV curve it would be much smaller but completely unrealistic.
The HV curve is valid for level flight only as the amount of data and the number of graphs required to cover climbs and descents at various airspeeds would be nonsensical and confusing.
So we have one graph and rather than accepting the data on it and complying with it, 'superior' pilots will start finding excuses as to why they can operate inside it and still be safe (right up to the point where the engine fails and someone sues you) and come out with all sorts of grandiose explanations as to why they are right to do this.
Arrogance breeds overconfidence which causes accidents. Flight safety message over.
You should do a search on this topic. This has been covered extensively years ago and the end result from people who know (Nick and Shawn) is that NO - the HV/Deadmans Curve is predicated only for departure.
Can you auto in the HV? Of course...! Its just a matter of dropping the lever and reversing the flow. You can auto from anywhere in the HV curve but unless you know its coming you may not walk away from the landing.
Can you auto in the HV? Of course...! Its just a matter of dropping the lever and reversing the flow. You can auto from anywhere in the HV curve but unless you know its coming you may not walk away from the landing.
Oldbeefer - yes plenty of times but I always knew it was going to hurt if the donk stopped. I haven't said that you shouldn't ever be in the avoid curve - that is plainly impossible - it is operating in it and pretending it doesn't apply to you that is the problem.
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Help in search
Hello,
Have tried an advanced search feature with "H/V Diagram" and only end up getting this thread.
If anyone has the link to the thread that Steve mentioned where N.L. & Shawn weighed in on the subject, please post it.
Thanks in advance.
Have tried an advanced search feature with "H/V Diagram" and only end up getting this thread.
If anyone has the link to the thread that Steve mentioned where N.L. & Shawn weighed in on the subject, please post it.
Thanks in advance.
I'll get me coat......
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Here's a couple of links.
Bell 47 crash video
http://www.pprune.org/forums/showthread.php?t=212241
Engine failure in the climb question.
http://www.pprune.org/forums/showthread.php?t=202016
Bell 47 crash video
http://www.pprune.org/forums/showthread.php?t=212241
Engine failure in the climb question.
http://www.pprune.org/forums/showthread.php?t=202016
Originally Posted by arismount
Hello,
Have tried an advanced search feature with "H/V Diagram" and only end up getting this thread.
Have tried an advanced search feature with "H/V Diagram" and only end up getting this thread.
When using the advanced search, try changing the 'Find Posts from' feature to 'Any Date'.
Using your search term above, this should give you 133 hits (if you restrict the search to 'Rotorheads' only).
HTH,
B73
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Having made a living producing H-V charts and the like, let me put my thoughts in a nutshell (it is coincident that my next Flight Dynamics column in "Heliops" is on the H-V curve, look there for more details!):
1) The H-V Curve is only precise at the one tested condition of weight, altitude, wind and temperature, it can be very very inaccurate if you are at a much lower weight, higher wind or much lower altitude/temp.
2) Is based upon programmed delays in lowering the collective. If you are quick as a bunny, your personal H-V ciurve would be much smaller, if you are slower, you cannot trust the H-V curve because it is too small for you.
3) If you are descending, the H-V curve is wrong, the real one would be very much smaller.
4) If you are climbing, the H-V curve would be very much bigger.
5) If you have 5 knots of steady wind, the H-V curve would be very very much smaller. With 20 knots of wind, there is practically no H-V curve.
It is my opinion that the H-V curve is a single point check of the engine-out performance, useful as only a general guide. It is not a bibical statement of assurance, but it is also not to be ignored. In a world where many people call for absolute, "thinkingless" solutions to problems, the H-V curve serves as a testament that you just can't put complex performance issues into one chart. To those who want absolutes, go into the toy business, helicopters are not for you!
1) The H-V Curve is only precise at the one tested condition of weight, altitude, wind and temperature, it can be very very inaccurate if you are at a much lower weight, higher wind or much lower altitude/temp.
2) Is based upon programmed delays in lowering the collective. If you are quick as a bunny, your personal H-V ciurve would be much smaller, if you are slower, you cannot trust the H-V curve because it is too small for you.
3) If you are descending, the H-V curve is wrong, the real one would be very much smaller.
4) If you are climbing, the H-V curve would be very much bigger.
5) If you have 5 knots of steady wind, the H-V curve would be very very much smaller. With 20 knots of wind, there is practically no H-V curve.
It is my opinion that the H-V curve is a single point check of the engine-out performance, useful as only a general guide. It is not a bibical statement of assurance, but it is also not to be ignored. In a world where many people call for absolute, "thinkingless" solutions to problems, the H-V curve serves as a testament that you just can't put complex performance issues into one chart. To those who want absolutes, go into the toy business, helicopters are not for you!
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To add my two cents worth -
Beware of comparing the military and civil HV charts. Military charts call for two seconds intervention delay between failure and lowering the collective, and all test points are done in level flight. Civil charts call for one second intervention only on the collective (cyclic and pedals can be moved immediately), but requires that the portion below the knee be demonstrated at takeoff power with no intervention necessary on the collective.
Many years ago, a senior military office in Canada decided that since the military chart for the OH-58 was much larger than the chart in the Bell 206 that the military chart was much safer....
Beware of comparing the military and civil HV charts. Military charts call for two seconds intervention delay between failure and lowering the collective, and all test points are done in level flight. Civil charts call for one second intervention only on the collective (cyclic and pedals can be moved immediately), but requires that the portion below the knee be demonstrated at takeoff power with no intervention necessary on the collective.
Many years ago, a senior military office in Canada decided that since the military chart for the OH-58 was much larger than the chart in the Bell 206 that the military chart was much safer....
I never understand why it is such a cause of confusion. Clearly, it cannot be valid for all conditions 
. It is generally valid for an adverse weight and DA combination, and usually relies on there being a smooth hard surface for the touchdown. It is only valid for stablilised level flight (well, the curves published are: you can create one for alternative situations.) Some manufacturers state these conditions 
.
The proof of the pudding that it is not fixed, is that some helicopters have helipad take off and landing procedures that would "appear" to be contradicting the published HV curve. Clearly, there is NO HV curve, under those sets of criteria associated with operating those particular procedures
.
. It is generally valid for an adverse weight and DA combination, and usually relies on there being a smooth hard surface for the touchdown. It is only valid for stablilised level flight (well, the curves published are: you can create one for alternative situations.) Some manufacturers state these conditions .The proof of the pudding that it is not fixed, is that some helicopters have helipad take off and landing procedures that would "appear" to be contradicting the published HV curve. Clearly, there is NO HV curve, under those sets of criteria associated with operating those particular procedures
.
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212man:
You're right about some procedures appearing to contradict the HV curve. For those with Category A procedures as an additional performance section (i.e. <20,000lb and pax restrictions - which are automatically Category A to begin with), it will generally appear in the limitations section of the flight manual supplement that the HV curve is no longer relevant as long as the procedures and weights in the Category A manual are observed.
The Category A manual will have Weight Altitude Temperature limitations (yes limitations) to ensure that the performance is capable of being met. About the only time you'll see WAT charts in helicopters in my experience.
As with everything else that's got more than one engine and weighs more than 7,000lbs, life becomes quite complicated, and we don't do any justice in our training to this level of complexity.
You're right about some procedures appearing to contradict the HV curve. For those with Category A procedures as an additional performance section (i.e. <20,000lb and pax restrictions - which are automatically Category A to begin with), it will generally appear in the limitations section of the flight manual supplement that the HV curve is no longer relevant as long as the procedures and weights in the Category A manual are observed.
The Category A manual will have Weight Altitude Temperature limitations (yes limitations) to ensure that the performance is capable of being met. About the only time you'll see WAT charts in helicopters in my experience.
As with everything else that's got more than one engine and weighs more than 7,000lbs, life becomes quite complicated, and we don't do any justice in our training to this level of complexity.
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Well, well
Thanks gents for the information, I now realize I have been wrong in my thinking about this subject for many years. I wonder what else I have been getting wrong, isn't that scary. At least an old dog can learn new tricks, thanks again.
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"I now realize I have been wrong in my thinking about this subject for many years. I wonder what else I have been getting wrong"
Arismount - another coincidence! Like Nick, see my article in the next heliops!
Phil
Arismount - another coincidence! Like Nick, see my article in the next heliops!
Phil