For Nick Lappos, re: Contention Concerning H/V Diagram

[arismount]

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.

[imabell]

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.

[IntheTin]

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. :confused:

[widgeon]

I guess the average plot would have 1.6 testicles .

[Matthew Parsons]

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.

[paco]

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

[[email protected]]

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.

[Steve76]

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.

[oldbeefer]

CRAB - so you've never done a towering take-off in a single engine machine, then?

[[email protected]]

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.

[arismount]

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.

[Capt Hollywood]

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

[Bravo73]

Hello,
Have tried an advanced search feature with "H/V Diagram" and only end up getting this thread.

Arismount,

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

[NickLappos]

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!

[Shawn Coyle]

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....

[212man]

I never understand why it is such a cause of confusion. Clearly, it cannot be valid for all conditions :ugh: . 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 :D .

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 :ok: .

[Shawn Coyle]

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.

[arismount]

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.

[imabell]

arismount,

now combine the height velocity diagram with the height versus gross weight diagram and you can have a lot more fun.

[paco]

"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

[[email protected]]

Nick and Paco, is 'heliops' an online publication? I haven't heard of it before and if you are both writing in it I want to read it.

[paco]

It's a magazine.

Phil

[topendtorque]

If you change from the test conditions (speed, height, density altitude, weight, attitude, etc.) then the chart will no longer be as accurate

Hmmmm.

Need to be a bit careful where newbies are exposed to normal beer talk chaps.
Is not 'speed' , 'attitude'?

Density altitude, yes does change things, have a look at the rotor rigging graphs in the robinson maintenance manual.
Weight chnages nothing except the time interval, presuming y'all got your auto RPM set up as per the book and not a bit high.

Probably not a good idea to a quote time interval of collective down at any greater than 1 second, that's the rule stick to it.

One very useful rule of thumb for EOL recovery is always try to operate with your collective DOWN as much as possible at all times. Looks after the engine too.

As far as climbing etc, check the airspeed on the graph, how many of you are at 250 feet say and in cruise configuration at 25 knots?

On one of the other threads referred to one dude went into a long discussion about 1929fpm climb being the same as 9.8mpersec - as in gravity - therefore it would take 2 seconds to be established in auto with no power. one to slow down a'goin up and one to speed up a'goin down.

Gee whiz who says you have to be going downhill before you can be in auto?

All that has to happen is the airflow has to be reversed.

saw a super puma on pprune the other day flat out at ground level, so i'll be game and say that I guess all helicopters must be able to do just that. ??:ugh:

So - what happens when it suddenly goes quiet in that configuration? any one ever teach you don't drop the collective at that point, why you might be a whole lot bettter off giving the collective a bit of a flick UP, so as to get the tail and everything away from the ground for when you want to start a bit of aft cyclic to change the speed,- errr-- attitude to dress yourself up for a zero speed touchdown, of course remembering to start bringing the colllective back down again.

The A/C will have performed - during autorotation - a climb then level flight for a bit then descent profile as the A/S comes back before the final touchdown sequence.

[Shawn Coyle]

topendtorque:
Weight does change a lot of things on the HV curve - more than one helicopter has a 'family' of HV curves for different weights and density altitudes.
The intervention time of one second is there to cater for the pilot who doesn't have hands on the collective - and even that is a very short time. All the pilots I know who had real engine failures were quite surprised and said it probably took longer than one second to react. The US military requires a two second intervention time.
You mentioned climbing...
Another one of the things not published in flight manuals for most light helicopters is the takeoff profile. The profile is supposed to keep you out of the HV curve by about 10 feet and 5knots - anyone who is climbing out at 40 knots in a Bell 206 for example is going to be unpleasantly surprised at the results should the engine fail.

[[email protected]]

Topendtorque, most people in the military will have been taught a low level EOL from high speed - you do not raise the lever! Gentle aft cyclic will zoom climb the aircraft and keep the Nr up (flare effect) but you will need to lower the lever shortly after starting the climb to conserve the Nr. The advantage of the manoeuvre is that it gains you height and therefore time to select a sutiable area for your EOL. Note that you must be gentle nosing over to select your normal auto speed as the slight reduction in G will decay the Nr.

The amount of height gained depends on the entry speed, the rate of cyclic application and the aircrafty type and weight bvut the Gazelle used to gain 2 - 300 feet in this manoeuvre from 120 kts.

[Rich Lee]

There are many disagreements, often between knowledgeable experts, in the interpretation of performance charts. It happens at times that experts have different opinions but each is correct because of their individual experiences. Each test pilot who responds to certification questions does so from their individual experience. The Limiting Height – Speed Envelope (H-V or ‘Dead man’s Curve) has generated more than its fair share of controversy over the years.

My H-V testing experience has been in the testing of helicopters for US Military and FAA certification on land and sea. Most of my comments will be based on the FAA certification basis for single engine helicopters below six thousand pounds.

Certification regulations change over time and the methods and procedures to show compliance with these regulations also change. CAR6, FAR 27 or FAR 29 is the various regulations that have been the certification basis of most US manufactured helicopter s. Each individual regulation has been amended so to speak of the individual requirements of one regulation you must be specific as to time. Strange as it may seem there are different and acceptable methods of testing to show compliance with the regulations as long is it meets provides for an equivilent level of safety. Each manufacturer has individual methods of showing compliance. Sikorsky is different than Bell which is different than Hughes which was different than Robinson or McDonnell Douglas or Hiller or Boeing and every other company in the business.

The FAA published an advisory circular 27-1 to assist certification applicants by providing a history of the certification process and to provide at least one acceptable method to demonstrate compliance with Part 27 and to some extent the other parts.

The above is written so that it is understood that my experience is different than that of Nick or Shawn or other test pilots, and what follows will reflect that bias.

Nick writes:
“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.”

I respond:
My experience is that there is a build up process during Limiting Height – Speed Envelope testing. Tests are conducted and test data generated at several different combinations of weight, altitude, wind and temperature and that data is accurate. The Limiting Height –Speed Envelope information presented in the Performance Section of the Rotorcraft Flight Manual is that required by regulation. The guidance is that the flight manual should list any procedures which may apply to specific points (e.g., high speed points) and test conditions, such as runway surface, wave height for amphibious tests, marginal areas of controllability or landing gear response, etc. The HV curve should be presented in the RFM using actual altitude above ground level and indicated airspeed. Manufacturers may include data for other tested conditions but it will be in the unapproved section of the flight manual if not validated by the FAA. The curve is not ‘inaccurate’ at other weight, winds, altitude, temperatures, landing surfaces or any other specific condition not tested; it is just not applicable to those conditions and can be used to predict performance.

Nick writes:
“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.”

I respond:
Item 2 is the reason I have chosen to post on this matter. The FAA has two primary methods of entry for H-V testing. A one second delay or normal pilot reaction time are the ‘programmed delays’ allowed by the FAA and which technique is used depends on the point tested. The technique applies to collective and the interpretation concerning allowable movement of the other controls such as cyclic and pedals, as well as the definition of normal pilot reaction time are the subject of considerable debate. Of one thing I have no doubt. A test pilot, who has completed a buildup program, is concentrating on only one task, and who initiates a simulated engine failure will be faster than a pilot who is task saturated, not practiced, and unaware of an engine failure event. No matter how fast a ‘normal pilot’ - the bunny pilot - he or she will not be faster than the test pilot who established the point on the curve. When I hear someone say that a good, experienced pilot can somehow beat the curve my blood boils. Do not count on it! As one example of the reason I am so confident in this opinion consider that scientific testing in controlled circumstances has concluded that recognition and response times can be as much as 4.4 to 5.7 seconds; contrast that with the 1 second (at most) delay in testing.

Nick writes:
3) If you are descending, the H-V curve is wrong, the real one would be very much smaller.”

I respond:
The H-V curve is not wrong. It is accurate for the stated conditions. The tested data presented in the H-V curve does not represent a specific condition of descent. You may consider the values presented for hover or climb or level flight to be conservative compared to descending flight, but not ‘wrong’.

Nick writes:
“4) If you are climbing, the H-V curve would be very much bigger.”

I respond:
This depends on the phase of flight where the climb occurs. In the take-off corridor the data reflects climb. A climbing OGE hover would however require a larger H-V curve. It is not the climb or climb rate that is the significant variable but the power used (main rotor blade pitch) because that increases rotor decay. There is also the type of engine failure that occurs. As an example a test pilot who retards throttle or power lever on a FADEC equipped engine will see a programmed rate of engine decay that would be much slower than that experienced in an actual engine failure. That equates to less time for the pilot response.

Nick writes:
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.

I respond:
This should be self evident by the curve itself and general pilot knowledge of aerodynamics. The reverse is true of tail winds and cross winds which is why the conditions on which the curve is predicated must be understood by the professional pilot. I have longed believed that CAT A performance should be predicated on 17 knots of down wind (hover controllability) because so few pilots understand what a tail wind can do to single engine fly away capability.

I do not disagree that the Limiting Height – Speed Envelope is a performance chart that should be used as a guide from which to make predictions based on specific conditions but; it is a useful guide nonetheless.

There are many who have long advocated a change to the certification basis for engine out performance and the Limiting Height – Speed Envelope such as more realistic pilot reaction times or additional performance testing such as a take-off power climb at VY, and actual engine out testing (Hiller did it once).

[Brian Abraham]

Certainly a subject about which we could all learn (when you going to write that book Nick?) as its always a topic of conversation in crewrooms and you are left wondering as to the "real" story. Enlightened by some expert commentry here. Recall a high houred Cat A twin captain saying the graph represented the aircrafts ability to fly away after an engine failure. Certainly not what it said on the graph (S-76).

[Shawn Coyle]

Rick:
A very thorough post.
I think what Nick was meaning about 'quick as a bunny' was those who, in training, can safely execute an engine failure from inside the published curve. What they don't realize is the other conditions relevant.
I've been accosted by experienced instructors on this subject.

[topendtorque]

Thankyou Rich an’ all, yes very good posts. It would be superflous to go into serious detail of particular procedures. It must be remembered that there is only one power source during autorotation – gravity – from that premise NEVER practice judgemental techniques or procedures for the real thing with the needles joined.

At that point – disregard your sight picture and FLY away. This is because with the needles joined you are transmitting power to the drive train and changing your flight path and - stuffing up your sight picture judgement.

I think it is great that the previous posts have got a good broad canvas approach out on the board so that it can be seen that there are more ways to kill a cat than choke it with butter.

The old dog instructor that set me up for operational check and endorsement training used the analogy of autorotations being like playing a fighting fish on a light line, he runs with power, let him, he goes limp, reel him in, etc.

The underlying message of all posts is that when you finish ab-initio stage, be cool until you can hunt around and find an–old dog- to help you straight on issues that worry you. His pointers will be good, so will others have good points.

It is still a worry to suggest that those who enjoy the privileges of a rotary license may have reaction times way beyond one second. I would have to say that it is an extreme worry to me that those that even enjoy the privileges of a motor car or motorbike license would be of like – inept - disposition.

If the military, which has been oft quoted as training for a two second delay, ever trained students in a Robinson then they would soon promulgate a more mature point of view. Just spend five seconds talking with (ex military) Tim Tucker about how difficult it was for him to adhere to the mandatory one second delay for Robinson certification.

For me anyone who is at all slower than one second doesn’t even get a second chance.
I would have much preferred to hear about sharp military pilots being trained to an even sharper degree than their civil cousins, without putting too fine a point on it, please do not take offence.

For those that fly around single engine with their hand off the collective, change your career, your first emergency WILL be your last. At low level your collective is your PRIMARY control and your LIFE, look after it.

Depending on the pilot or his job I often spend quite a bit of time at low level, real low level, doing EOL’s with differing procedures, involving both collective (coriolis effect) and cyclic (flare and coriolis effect). Remember, when you have a sudden and TOTAL power loss, like - not a pansy turbine fadec run down, then the A/C bloody well sinks.

If you are an idiot and fly at six inches above the ground for no reason you will crash if you rely on the cyclic. Get someone who knows to show you why. No disrespect to Rich’s or others post. In the meantime fly a bit higher.

All operational check and training requires the check pilot to use his experience to blend as much as possible their calculated assessment of just what may be needed to suit the particular pilots’ job profile and temperament.

A super important point is that not one person who has been checked by an ‘old dog’ need think themselves smart in any way just because ‘old dog’ checked them out, quite the reverse in fact.

[Banjo]

Hey Paco,

will look for your article, hope all is well with you.

Another 5 hours or so today and only 4 and a half of them were in the curve...just got to hope that CWP stays dark.

Banjo

[paco]

Hi Banj - you on the twin yet? I'm still counting insulators over here - force of habit! :)

Best to all over there!

Phil