Height - Velocity Curve

[gyrogreg]

Whenever I see a gyro accident where it appears the gyro may have struck the ground nose first, I think of the Height-Velocity curve. I am often surprised to find there are even experienced gyro pilots who are unaware of this issue at all. There are many others that appear to not respect the H-V curve, if they do know of it. Occasionally it may be an element of a bad gyro crash!

There is a reason that helicopter slang for the H-V curve is "Deadman's Curve". I'm not suggesting this is an element of any recent tragedy - just pointing out it could be some day!

Attached is an article that will be published in the PRA Rotorcraft mag soon. I am providing this as a "sneak preview" as it may have special attention at this time!

Thanks, and fly safe - please!

(The attached is in three pages so it would fit as an attachment and still work on most Adobe Acrobat versions.)

[mark treidel]

Thanks for posting this Greg. I look forward to your articles. They are a sobering reminder of safety concerns that can, and do, fall behind the 'complacency curve'. You bring them upfront & personal for us all. Your efforts are very much appreciated. They make one 'sit up & take heed'.

[dragonflyerthom]

Hi Greg

Must say this is some very interesting reading. It answers some of the questions that I have been asking. Thanks for sharing.

[Leonid]

This one worth a page of it's own in a sort of collection of very important information.

Oh-No... not the "GyroWiki" again.

Leonid

[Hognose]

I am often surprised to find there are even experienced gyro pilots who are unaware of this issue at all.

I recently saw a post on this forum to the effect that gyroplanes have no H-V curve! I think Chuck Beaty would go nonlinear if he saw that.

When I think of all the times I trustingly hung under a hovering helicopter, rappelling, fast-roping, being STABO'd or climbing a rope ladder, back before I knew a lick of rotary-wing aero, I shudder. Most of that was under single-engine Hueys... one flameout away from freefall.

cheers

-=K=-

[birdy]

They certainly leave plenty of room for errer.

but i gess thats better n not enuff.

Wunder if anyone thought of do'n sumthn other than shoven the stick forward to gain AS?

[Al_Hammer]

I recently saw a post on this forum to the effect that gyroplanes have no H-V curve! I think Chuck Beaty would go nonlinear if he saw that.
-Hognose


http://www.gbagyro-sea.com/aerial.htm
Safety - Always in autorotation. No Height/Velocity curve to deal with. In a power-off situation the Hawk 4 will simply land about the speed of a parachute.

A helicopter pilot needs time to get the rotor into autorotation, otherwise a potentially fatal rate of descent can occur. A gyro ,even in a straight vertical descent does not normally reach a terribly high velocity. So, it is somewhat an apples and oranges comparison. True, there are height/speed combinations that put you at risk of a hard landing and pilots should obviously be aware of this.
I would question a couple of comments in Greg's article. For example, slow flight does cause some of the weight to be supported by the prop, therefore tending to unload the rotor a bit, but the rotor is also tilted back more than normal and so must generate more than 1G of thrust in order to maintain 1G of lift in the vertical direction. This amounts to additional loading, possibly offsetting any loss of loading from hanging on the prop.
Also, I can attest from personal experience to the fact that a single place gyro with 25 ft dragon wings will easily recover from a sudden engine stoppage and land safely from as low as 50 ft, even when flying at low speed of 30-40 mph. This actually happened to me and all I did was lower the nose, gain airspeed and flare to a no roll landing. Even Ernie Boyette was doubtful about my altitude claim, but I'm quite certain it wasn't more than 50 ft, based on my observations and those of pilot witnesses on the ground. Autorotation is a wonderful thing.

[EI-GYRO]

If some of the weight is being supported by the prop,
then the rotor has a lower weight to support, and should slow
down.
Does anyone have real-life figures as examples?
Just curious for future reference.
I have tried this in ground effect and got back to 21mph at about 1
foot height. I didnt have the spare capacity to read my RRPM at the
time, and my VW is not powerful enough to push very far behind the
power curve without sink.

[gyrogreg]

Heron, Sorry but I don't agree with several of your statements here. The H-V curve is not solely because you have to put the collective down in a helicopter - although that may explain why helicopter H-V curves start out at more height. The H-V curve is because, anytime you want to make a safe landing - not counting the vertical crash you suggest is OK? - you need an adequate amount of airspeed when you reach the ground, mostly to be able to raise the nose in the flare. If you arrive at the ground without enough airspeed to flare, you can't raise the nose. If you strike the ground in a nose down rapid descent, this may not be desireable or survivable.

Your Hawk 4 example - you must be planning on using the collective when you get to the ground. In most gyros, there is no collective, so you need some forward airspeed in order to raise the nose in the flare to stop the descent rate. Not valid to reference a collective pitch rotor gyro to support your arguments.

The numbers you report for your single place gyro - 50 ft at 30 mph are probably outside of "deadman's curve" on most light single place gyros! - maybe right on the edge at least. Try this (no don't) at 50 ft at 10 mph!

EI-Gyro already pointed out above that you are thinking backwards in regards to rotor loading (and RRPM) in the condition when some of the weight is supported by the prop - prop is carrying part of the load, so the rotor actually has less load - and less RRPM. Verify this to yourself by watching your RRPM as you do this - it will be less when the prop is supporting some of your weight - but please do this at altitude!

Heron, you are giving too much credit to autorotation. I agree it is wonderful in many respects. But if to recover or maintain a good airspeed upon the engine quitting, or for some other reason, you quickly lower the nose - the sudden (but brief) reduction in G-Load spikes the RRPM lower immediately. This can be severe if the pilot is excited to lower the nose fairly quickly - less than 1 G - such as climbing at a very high nose-up attitude when the engine quits! Soon after the nose is lowered - and the rotor RPM drops, the rotor is very suddenly reloaded back to 1 G after getting the over the hump with the nose down. At this sudden full loading again, the lower RRPM rotor is not as efficient at processing the airflow until it speeds back up to normal again - so it takes longer to get back to flying RRPM. This is analogeous to over-running the rotor on takeoff - too much air for the RRPM. This might all seem negligible to you, but just go try this - at high altitude, please:

Descend at idle power at zero airspeed - vertical descent. At a specific altitude, lower your nose fairly quickly to recover your normal landing low approach airspeed. You may be surprised how much altitude you lose before you establish both a normal glade angle and normal approach speed. If you agressively lower the nose - excitedly because when you are close to the ground you can't help it - you will note how it "drops like a rock" at first. This is where the RRPM is trying to catch back up! In this "drop like a rock" stage, you probably would not be able to raise the nose in a flare if you had too. And, this "drop like a rock" stage, is nothing like the parachute descent you might get if you did not initially lower the nose to regain airspeed - but then you would not be able to flare either.

It may be better, when trying to recover airspeed to not so quickly or agressively lower the nose to do so. This might be hard to resist when close to the ground and the engine quits. Lowering the nose more gradually, subjects the rotor to less G load drop, and the rotor does not slow so radically. This slower RRPM makes it easier for the rotor RPM to catch back up and may help to regain normal glide and airspeed a bit more quickly. Try this at altitude also. But, when you don't have a lot of airspeed to regain, and not much height, how quickly you get the nose down is a compromise when there isn't much height to work with either. Thus, the reason for, and why to respect the H-V curve for gyros too!

Believe me, Heron. H-V curve is real and real important for gyros too! I am speaking from a bit of experience! One day a student I had - commercial helicopter pilot - seemed to insist to climb out at a steep nose-high altitude within the H-V curve. I would explain to him why we can't do that, but I think he also thought there was no H-V curve in a gyro!!! I guess to prove his point, one of the next takeoffs he decided to chop power at 50 ft in a steep nose-high slow airspeed climbout!!!! The result, an almost immediate near zero airspeed! I immediately jammed the throttle forward and lowered the nose to try to keep and regain some airspeed. The engine coughed for it seemed like an eternity, but then came back full power! Even with power restored, it dropped like a rock for too long and I was barely able to raise the nose in time to impact hard - but at least level - drove away but the prop hit the keel! I'm sure his intent was to demonstrate there was no H-V curve for gyros upon losing power - but I know I would not be here now to tell you how it worked if he had managed to keep the power off through the full drop!

- Thanks, Greg

[ferranrosello]

Very good post, Greg. I've performed landings from a vertical descend frequently, and at the beginning the gyro is plumbeting to the ground.

My gyro is an Ela 07. The rate of descend at gross weight in a vertical power off autorrotation is about 1300 fpm. In my gyro takes about 200 feet to recover this condition, reaching a final air speed of 60 mph, which lets a very smooth touch down. Getting 50 mph requires few less altitude, but the touch down is hard if the maneover is not done perfect.

The nose must be lowered quickly but smoothly. If the nose is lowered too slow you will need much altitude to get a safe AS. If it is done too agressively you will fall faster without getting as much airspeed as possible.

How to train for this kind of maneouver?

Firstly you need to know what speed is required to land your gyro safely power off. And you must train this particular kind of landing.(60 mph in an Ela, I guess about fifty in a single gyro).

Once you are proficient in this landings is time to go to the next step: knowing how many feet you need to accelerate to that AS. Try this test plenty of altitude and you will need a good altimeter.

.- Firstly establish a powered 0 ground speed flight into wind.

.- Secondly chop the power to idle. Don't try to get a 0 AS indication (the anemometer is not reliable at such speeds). Don't change your attitude while chopping power. Hold this attitude until stabilizing a rate of decend. Then look at your altimeter. When passing by a good altimeter reference low the nose quickly but smoothly. Dont move the stick, just increase the forward pressure until getting about 15 to 20º of pitch down attitude.

.- Thirdly hold this attitude. You will see the sinking, but not a low g feeling. Afer a few seconds of plumbeting you will see like your glide path is shallowing. You will need to increase the forward stick pressure in order to maintain the pitch down attitude.

.- Then look at the ASI. You will see the AS increasing rapidly. With an anticipation over your desired AS of about 10% start a gently nose up in order to maintain your desired AS (60 mph in an Ela). And look at the altimeter to see how may feet have you descended since starting the pitch down.

Try this maneover several times until getting a confident value. Add to this value 100’ and you will be ready to try it in a real approach. I recomend to use a long runway at the beginning of this training). You can fly a final keeping more than 500' over the runway (I prefer about 700'). When you are very close to the threshold start the vertical descend. You must get your approach air speed about 100’ over the runway. You can do a go around if you are not confident enought at any time before starting the flare.

This kind of manouver is good for getting good knowledge and judgement of what to do when you want to reach a landing spot that is to close to our position.

Regards. Ferran.

[Mayfield]

I think many of you guys are misunderstanding the H/V curve.

In the helicopter, it is a takeoff profile.

It is NOT "a chart that describes combinations of altitude and airspeed from which a safe landing can be made in the event of an engine failure." This is a very common misconception. The chart only depicts the behavior of the helicopter in a high blade pitch angle mode of flight....such as a climb.

Sorry Justin. I disagree with the above statement. It "IS" a chart that delineates combinations of altitude and airspeed from which a safe landing might not be made in case of an engine failure while operating within the shaded area.

If what you state above were true, the chart would say something like: "Avoid operation in shaded area during T/O or climbs."

Instead it says: "Avoid operation in shaded areas."

It clearly is more critical during takeoff with high pitch settings. A T/O profile is provided to ensure adequate A/S and altitude in case of an engine failure. It is dangerous, however, to operate within the shaded area.
An engine failure in a helicopter, within the shaded area, at most loadings, in non-descending flight, will result in a really hard thump at the bottom.

There are, of course, two exceptions:

1. Once steady state autorotation is established the H/V diagram is no longer valid.

2. In power descents the H/V diagram is very much "safe sided."

R/S

Jim Mayfield

[docrob]

This probably only has historical significance, but......

Back in olden times, just after the earth's crust cooled, I went through the rotary wing "Q" course at Ft. Rucker, AL. The occasional "ballsy" instructor would demonstrate -- though it was strictly forbidden -- a no-pitch-pull autorotation. We were flying OH-13Es and 'Gs (early model Bell 47s,) which were pretty light, and had very low inertia wooden blades. The technique was to fly a normal autorotation with maybe a little extra airspeed (45 knots plus a little, if I remember correctly) and flare the helicopter on to the ground without pulling collective at the bottom, much as we land gyros. The maneuver required a fine hand and a sharp eye, and if the airspeed got a little low before the flare was started, a VERY hard touchdown resulted, sometimes with bent skids or a cut off tailboom. Note that in this maneuver, the rotor was already in autorotation long before the flare/landing, so the argument about time required to establish autorotation following a power failure gets a little weak. Also, we routinely practiced hovering autorotations, and from only three or four feet altitude, (so there wasn't time to build up a lot of vertical speed,) and the pitch pull had to be pretty precisely timed to avoid a hard landing.

It may be correct to say that the helicopter H/V diagram is developed from take off data, but from the above experiences, I devoutly believe it also applies to landings.

Maybe this applies to this thread. On the other hand......

Dr. Rob

[Timchick]

Justin, Did you look at the PDF pages posted by Greg? It's not the same graph you're using. His article is called "HEIGHT VELOCITY CURVE for GYROPLANES" and it's based on the flight characteristics of gyroplanes, not helicopters.

[Mayfield]

Justin,

I guess we'll just disagree.

To others:

I recommend you heed the notice on the H/V diagram. Avoid Ops in the shaded area.


http://rgl.faa.gov/REGULATORY_AND_GUIDANCE_LIBRARY/RGFAR.NSF/0/07168a9e3cf60b7f852565f600658df4!OpenDocument&ExpandSection=-3,2#_Section3


Jim

[gyrogreg]

I don't know how much credibility you might put in the FAA manuals, but check page 19-3 in the gyroplane section of the FAA Rotorcraft Flying Handbook (http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf).

This shows a gyroplane H-V chart. The discussion also compares the differences between helicopter and gyroplane H-V curves. The first paragraph says:

"Like helicopters, gyroplanes have a H/V diagram that defines what speed and altitude combinations allow for a safe landing in the event of an engine failure."

Thanks, and safe flying - outside the H-V curve! - Greg

[Hognose]

All Hammer quoted GBA as saying:

Safety - Always in autorotation. No Height/Velocity curve to deal with. In a power-off situation the Hawk 4 will simply land about the speed of a parachute.


I think, as Al suggests, they're talking about the lack of a need to make the transition from powered to autorotative flight. Having to do this while in the shaded area has brought many a helicopter pilot to grief.

Al further says:

A gyro ,even in a straight vertical descent does not normally reach a terribly high velocity. So, it is somewhat an apples and oranges comparison. True, there are height/speed combinations that put you at risk of a hard landing and pilots should obviously be aware of this.

One of the other posters cited an 800 fpm descent and notes that it will probably cause damage if you touch down at that rate. For the record, GBA's "parachute" quote is in line with this. The military parachute I used to jump the most descended from 18-22 feet per second depending on load and density altitude so we're in the same ballpark. (The current smoke-jumper-developed chute gives a softer landing. These are round, steerable chutes, not squares that you flare for landing. Sometimes we can't see what we're landing on so we have to have this type of non-sight-dependent chute).


Also, I can attest from personal experience to the fact that a single place gyro with 25 ft dragon wings will easily recover from a sudden engine stoppage and land safely from as low as 50 ft, even when flying at low speed of 30-40 mph.

Al, I don't see what you're disagreeing with here. You haven't said there's no HV curve, only that it takes a different shape and size in your monoposto. The numbers Greg posts are based, I'm sure, on his own machine which is going to have a much larger shaded area by dint of its weight and disc loading.

cheers

-=K=-

[Mayfield]

Justin,

I haven't corresponded with Nick for a couple of years. How is he?

In the excerpts you provided I see nothing that indicates the H/V diagram is primarily for T/O and climb. The climb portion of the flight envelope is clearly most critical. You and I do not disagree there.

Sometimes, in these internet forums, people talk past each other because we don't really have a good handle on the other person's experience level.

Jim

[Mayfield]

Hi Justin,

I'm pretty low time in helicopters also. and worse, the time I have managed to get is spread over 40 years and 10 types. I don't have enough time in any one helicopter to be "good" in it.

I also agree that Ray Prouty and Gordon Leishman are very impressive. I have been fortunate to hire both of them as consultants in the past.

AC 27.1 is a great document. I am very familiar with it. It was pretty much my "go to" document when I was working on the certification of the Hawk 4 gyroplane. FAR part 27 tells you what to comply with, the AC tells you how.

I hope you continue to share your thoughts and experiences here.

Jim

[ferranrosello]

Jtravis, the quote in your last message is absolutely wrong. I see it has been written in a government publication, but... it is wrong.

What Nick Lappos says is absolutely right. When producing the H/V curve the helicopter is always in level flight, with no climbs and with no descends.

So the H/V curve is not telling how to do an approach or a climb after take off. If you are descending your shaded area should be bigger. On the contrary, if you are climbing it should be smaller. But there is still another question. If you are climbing you will lose more AS after the engine failure that if you are at level flight. And the opposite occurs if you are descending. So the H/V curve, together with other information available in a serious flight manual, is considered good for normal approaches and climbs (rates lower than 500 fpm).

Secondly, the H/V curve should change in size according to flight conditions like weight and density altitude. If only one curve is exhibit in the flight manual, it will match with the hardest certification conditions only, and the curve will be false in any other condition. This particular question is not mentioned in many flight manuals, what is very convenient for manufacturers ...and very bad for pilots.

Ferràn

[Harry_S.]

Back to gyros if I may, with this scenario:

An experienced pilot in a "lead sled" RAF 2000 for example, approaches to land into the prevailing wind. Let's say the wind is a calm 5 mph. The pilot, for whatever reason, flares too early and is literally 10 ft. off the surface and practically "stationary" off the ground.

What's gonna happen next? Any options? Gyro damaged or destroyed?


Cheers :)

[EI-GYRO]

Harry, has he got his engine running or is it 'dead stick' ?.

[ferranrosello]

Another question. What the knee of the curve means.

While you are descending after an engine failure the aircraft will be accelerating its rate of descend, until getting an steady rate. The vertical height lost in this vertical acceleration is the height of the H/V curve's knee. At this particular height you will need the fastest airspeed to be able to safely land the helicopter.

If you are lower than this height the helicopter will not reach the maximum descend rate. So the airspeed needed for the landing will be slower. Ii you are higher you will reach the maximum rate of descend, but you will be able to accelerate to a faster air speed without increasing the rate of descend, so the initial airspeed needed will be lower too.

Ferran

[Harry_S.]

Harry, has he got his engine running or is it 'dead stick' ?.



Fergus, yes power is available.

If no power...I believe we know the inevitable. :eek:


Cheers :)

[EI-GYRO]

Well, my guess would be, firewall the throttle, keep pitch level, aim for flattest
landing possible.
Logic: Any improvement in airspeed will reduce the descent rate.
Flat landing more likely to stay upright.

Do I win the flat banana.?

[GyroDoug]

I am glad this has been brought up and is being discussed. I am sure we all need to be more aware of where we are flying and how that may affect a recovery from an engine out or other circumstances. As you were describing people that come in for a landing in a vertical descent, it made me think of all the times I have seen Larry Neal land his butterfly in what seemed like a vertical descent with no problem and I wondered what his thoughts were on that issue so I called him and talked to him.

He is very aware of the H-V curve and always flies with it in mind, but because that curve is much smaller on his Butterfly it may not seem like he is following the rules that everyone else has to follow. He said he has repeatedly tested his Butterfly with the G-Force Landing gear in what he calls a Helicopter type landing configuration. He comes in in at a forward airspeed of 20 mph (which if he has any headwind can appear to to be almost zero airspeed) and without any flair at all the landing gear will absorb the full landing impact with no harm to machine or man. With the G-Force landing gear the mains hang down lower (once released) than the front wheel because of the 18 inches of travel built into the system so even if you were to hit the ground in a slightly nose down attitude you would still take the bulk of the force on the mains that are designed to absorb the impact. Without the need to pick up speed to be able to flair and pick up the nose there is a huge margin of safety just because of the design of the Gyro. While he doesn't recommend coming straight down at zero airspeed for the Butterfly it will handle a landing with a very low airspeed and not even need the flair to land safely so it greatly increases the chances of successfully handling an engine out at most speed and altitude combinations.

The fact that the Gyro can handle a 20 mph landing with no flair also means it is safe for him to take off at a slower airspeed and fly behind the power curve because if he does loose his engine the Gyro will simply come down and the gear will absorb the impact and it turns into a non event. Through design and landing gear capability he has greatly reduced the size of the H-V curve for his Gyros. Just another safety feature built in by design. Isn't it a great time to be into Gyros?

Gyro Doug

[Mayfield]

The power required Vs power available curves that most of us use look something like the attached.

This particular curve is for a ~500# gross weight machine. for the "Dreadnought class" machines MPRS (Minimum Power Required Speed) is ~60MPH or so.

Note: Chart provided via courtesy of Chuck Beaty

Jim

[helipaddy]

Hi
I don't quite understand the dotted line on the the power required curve or drag curve that corresponds with the engine power available.

Surely the engine power available would remain relatively constant, only varying with propeller load (variation of RPM due to Propeller induced flow).

It appears from the graph that the best rate of climb airspeed is faster than the minimum power required airspeed.

Im familiar with helicopter drag curves and power required. the power available is costant, the power required at a given airspeed varies and looks like the drag curve as drawn. the best ROC is when the least power is required to keep the helicopter in the air i.e. the helicopter is most efficient, the rest of the power available is used for climb..

Why is the engine power available varying on the graph like it does??

Paddy

[iconnary]

Paddy, I believe a more descriptive label for the chart would be "Power required to maintain straight and level flight", or perhaps "to maintain altitude"

At the left end of the shaded area you are making use of all available power in high angle-of-attack slow flight. At the right end you are at full power in normal flight, and would have to enter a dive to gain further airspeed. In the range in between you have available power for acceleration or climb.

[Passin' Thru]

Yep, what Iven said. Power available is your max power minus what you are already using. Therefore the power available cannot be constant at all airspeeds.
ROC; It seems you are confusing RATE (Vy)and ANGLE (Vx). The ANGLE of climb (Vx) would infact be about the minimum power required for s&l flight.

.

[C. Beaty]

Power available is propeller thrust x velocity. At zero velocity, available power is also zero.

[Mayfield]

Here is a neat reference.

http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page8.html

[StanFoster]

I love these kind of posts....thanks..


Stan

[helipaddy]

So The "Engine power available at full throttle" line on the graph is in effect the "Thrust horsepower available with the engine/ propeller combination at a given airspeed"

the Max point on the curve is the thing that threw me, this can be moved with propeller pitch, so that a climb prop would have the "Engine power available" peak at the airspeed of the lowest point on the power required curve. the graph as drawn looks like a graph for a cruise prop.

So a gyro power curve/ power available graph has more in common with a fixed wing than a helicopter.

Chuck, If power available = Prop thrust X Velocity the line on the graph would be straight. Or is it Power available= propeller effective thrust at a particular airspeed X velocity?


Also, Back to thread, I think the knee of the H/V diagram as drawn by greg may need adjustment. the reason I think is that its too "pointed".

The energy that can be used in a heli or gyro to to perform a successful engine out landing is kept in 3 "buckets" airspeed, altitude, and rotor rpm.

Airspeed energy is kinetic = 1/2Mass of the aircraft times the Velocity squared.

Rotor rpm energy is kinetic= 1/2 Mass of the blades times the rpm squared

Altitude energy is Potential energy= aircraft weight times the height.

Therefore there will be a larger increase in available energy in a given airspeed increase than a given altitude increase because the velocity is squared.

On the graph as drawn in Gregs post the line going from the Knee to the 200ft point shows an increase in altitude with a reduction in airspeed that may cause a lesser amount of total energy to play with in the event of the donkey goin quiet.

If the H/V Curve extends upwards from the 45mph point as Ive drawn it this sorts this problem, this is why the H/V diagrams for helis are rounded at the Knee as opposed to pointed.

Paddy

[helipaddy]

another H/V diagram

[gyrogreg]

Paddy,

You are certainly correct, all performance numbers and charts can be technically calculated to better precision than most practical published charts and numbers provided.

But, there are so many variables that just cannot be quantiifed, so most specs are rough estimates - hopefully with a good safety margin added in for the worst cases. In my experience, I feel it is more important for the pilots to understand and appreciate the concepts and a rough idea of numbers. For instance, considering all the variable involved, the theoretical difference between best rate of climb and best angle of climb, often is indistinguishable in practice - the experienced pilot has to search for optimum during the actual climb! For H-V curves, power curves, takeoff distances, etc., some of the wide tolerance variables involved are: wind, temperature, GW, rotor specifics, pilot skill, prerotation, ground conditions, obstacles, anxiety, reaction times, climb attitude and angle, engine power, power loss transient profile - quick or gradual. A huge and unquantifiable variable in the H-V curve is how aggressively the pilot lowers the nose from what attitude and airspeed when the engine might quit - too fast, lose a lot of altitude initially, too slow and don't attain good glide speed quickly - pilot skill in that specific situation is a very big variable.

For instance - the true top of the H-V curve for a fully loaded Magni M16 is about 200 ft. To allow for all the possible variables, including my piloting performance, my personal top of the H-V curve is 300 ft. When you start adding in fuzzy variables and safety margins, straight lines and precise numbers start getting real fuzzy.

For instance 2: Short field takeoff numbers are usually published. But, with all the variables involved, I recommend that if you felt you had to consider the published numbers to clear an obstacle on takeoff, don't even try! A lot of variables apply here - prerotation and rotor speed proficiency, ground conditions, wind, wind rotors around obstacles, etc. Etc. Etc. Consider the likely pilot anxiety also, it would be very hard to perform the prerotation and takeoff and climb exactly optimally anyway!

The shape of the "knee" of the H-V curve could technically perhaps be much more precise, but all the other variables would need to be precisely identified also - and that would be a lot of charts and numbers to commit to memory for rapid recall upon need! If an engine quits suddenly at 10 or 20 ft, no matter what speed, you don't have much time to consider much about the chart! It is just best to know the approximate numbers and stay well on the good side of them!

- Thanks, Greg

[helipaddy]

Yep Greg, You are right.
With my heli flying I follow the recomended takeoff profile when I can be it in a single and or a twin. For most helis the best angle of climb and best rate airspeeds are pretty obvious when you get the hang of the particular type. The experience I have on Gyros are pretty limited (magni) but I think that Vx and Vy are very close.
One of the greatest degraders of performance is humidity and this is not mentioned on any perf chart.

I find it pretty hard to get sufficient airspeed for an effective flare on entering an auto from a 400ft oge hover in a R22. below about 40kts the heli will just fall through the flare.

With regard to the knee portion of the graph I was just trying to add some safety buffer to the graph to give a bit of margin for the conditions You mentioned above.
Thanks
Paddy

[ferranrosello]

I fly an Ela and the numbers I use are exactly the same than Greg, 200' is the height where shaded area starts, but I use 100' more for safety reasons.

An interesting thing in twin gyros, is that Vy and Vx are slightly different depending on the Take Off weight. In the Ela Vy is about 60 mph and Vx about 50 mph.

Helipaddy, I think that the shadded area of a gyro has the knee much higher than the single engine helicopter. The only reason is that the gyro does not need enter an autorrotation...

Your numbers about energy are perfect. But I think that the vertical speed developed after an engine failure has big influence in the energy needed to stop the descend. So when you have the engine failure at a lower altitude than the knee the air speed needed for a safe landing will be lower.

Anyway it is difficult to say what is the real shape of the shadded area without doing a set of trials.

Regards.Ferran.