# Height/Velocity Diagram

#### Roundwing

##### Member
It was mentioned in another thread about a helicopter operating in the grey area of the height/velocity diagram and I would like to point out that in the conditions described, the H/V diagram does not apply. He was at high speed and if the engine had failed all he would needs to do is a quick stop with an autoroation at the end.
In fact, not all helicopters have the grey area that he was refering to. This is the area from 0 to about 20 feet above the ground.

When I was going for my helicopter rating I expressed concern about operating in any of the grey area while on approach and it was pointed out that the diagram is for the Takeoff portion of the flight. That is because you are pulling the most power during takeoff and it is when most engine failures occur.
I avoid the grey area, and do not advocate using it on a regular basis. The diagram is food for thought, and consideration must be made when operating in it.
Each one of us must make those decision for ourselves.

Rick

#### WaspAir

##### Supreme Allied Gyro CFI
It was mentioned in another thread about a helicopter operating in the grey area of the height/velocity diagram and I would like to point out that in the conditions described, the H/V diagram does not apply. He was at high speed and if the engine had failed all he would needs to do is a quick stop with an autoroation at the end.
In fact, not all helicopters have the grey area that he was refering to. This is the area from 0 to about 20 feet above the ground.

When I was going for my helicopter rating I expressed concern about operating in any of the grey area while on approach and it was pointed out that the diagram is for the Takeoff portion of the flight. That is because you are pulling the most power during takeoff and it is when most engine failures occur
I wholeheartedly disagree with the suggestion that the H-V avoid region only applies to takeoff. The risk is for any sustained period. I have watched people cruise for miles just off the deck at high speed, and the risk there is real.

I don't know which models you might have in mind, but I don't recall ever flying a single engine helicopter that didn't have a high speed - low altitude band on the diagram.

The suggestion of a quick stop if the engine fails in that regime denies the very reason for that band. When the engine quits there, you are likely to hit the deck with speed before you have time to react effectively. At the very least, putting your tail rotor into the dirt at speed as you try to quick stop is not likely to help.

One of the reasons that gyros typically do not have such a region on their H-V diagrams is that they are already (and always) in autorotative flight and no transition to that state is required. Transitions take time that you might not have.

#### wolfy

##### Gold Supporter
I was taught that in that area of the HV curve the deciding factor is whether you can get it slowed down enough before running out of alt.
You can only quick stop so aggressively whithout over speeding the rotor.

wolfy

#### Mayfield

##### Gold Supporter
Copied from the OSH Midair thread because this is an important subject for rotorcraft drivers and should be discussed.

In the OSH Midair thread, Dave Storey made a post that demonstrated a significant misunderstanding of the HV diagram. Dave is a low time helicopter pilot, so his misunderstanding is not surprising, and this post is not a criticism of his skill. In an attempt to use his post as a teachable moment, I made the comments below. It is more appropriate in this thread and hopefully will be discussed.

It is understood that the HV diagram is actually a performance chart and not a limitation in most cases, but operation within the shaded portion of the chart is in fact dangerous and should be, in most cases, discouraged.

Hi Dave,

I certainly understand your preference for helicopter flight. Most pilots develop a preference during their flying lifetime. I'm fortunate that I love anything that will get me off the ground.

However, please study up on the HV diagram a little. Vance is describing the high-speed portion of the HV diagram. Generally, in a small helicopter, it is under 20 feet AGL and from just above ETL to about 65-80 knots. These numbers of course vary with disc loading, gross weight, etc.

And I assure you, if you have a catastrophic engine failure below about 10 feet and over about 45-50 knots, you will not be able to exchange energy for altitude and enter autorotation. You will, almost certainly, hit the ground while you are still in the recognition stage of the recognition response cycle.

Please try not to push back on this advice until you get a little more experience. Practicing this with a throttle chop is not the same thing Dave. Even if you are in the 95th percentile for recognition response time (0.35 seconds or so) you will be too slow. Actually, unless really ready, most of us have recognition response times of almost 2 seconds.

I think I can put this in perspective. Get a safe altitude at 60 knots. Chop the throttle and count 1 second (one thousand one). Note the altitude loss. It will probably be less than 30-50 feet. no big deal unless you are at 20 feet or below.

This is the strength and value of this forum Dave. There is a tremendous reservoir of knowledge here for the taking. Helicopters, gyroplanes, pretty much everything that flies. Sometimes I get a little full of myself, a little smug, even jaded. But then I read a post by one of the participants here that opens my eyes to facets of a problem I had not considered.

Jim

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#### Roundwing

##### Member
I wholeheartedly disagree with the suggestion that the H-V avoid region only applies to takeoff. The risk is for any sustained period. I have watched people cruise for miles just off the deck at high speed, and the risk there is real.
I never said "only". My concern was the grey area to the left on the diagram. You have to fly through this area each time that you approach to land.

I would be more concerned with hitting a tower or power lines at low level than an engine failure. Both are risk that I personally will not take.

As I said before each of us has to make the decision for ourselves on what risk we are willing to take.
I don't know which models you might have in mind, but I don't recall ever flying a single engine helicopter that didn't have a high speed - low altitude band on the diagram.

Guimbal Cabri G-2.

Rick

#### Mayfield

##### Gold Supporter
Hi Rick,

Thanks. I just looked at the POH of the aircraft you referenced and there is no high-speed portion of the HV diagram. I am amazed, but grateful for learning something new. Is there any discussion on why this helicopter has this characteristic?

Jim

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#### bryancobb

##### Junior Member
This is a 269 H/V. The important points on this chart that need to be committed to memory are...

450 feet above the ground is probably the lowest you can be in a hover and succeed at an auto if your engine quits. Perfect technique would be required.

Between 30-40 knots, you need to be climbing to at least 8 ft AGL and then you SHOULD be able to succeed at an auto if your engine quits.
Flying all the way up to VNE should be safe at any altitude above 8 feet. At 55 knots, you can probably auto from as low as 150 ft. AGL.

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#### Roundwing

##### Member
Jim,

Sorry but I do not recall discussing the lack of the low altitude grey area on this machine.
I am very impressed with it. Well built. Handles very well. Good performance.
No bad habits. Good amount of inertia in the rotoer blades when dealing with unscheduled gravity events.
Very well thought out.
The blades do turn the wrong way though.

Rick

#### bryancobb

##### Junior Member
Jim,

Sorry but I do not recall discussing the lack of the low altitude grey area on this machine.
I am very impressed with it. Well built. Handles very well. Good performance.
No bad habits. Good amount of inertia in the rotoer blades when dealing with unscheduled gravity events.
Very well thought out.
The blades do turn the wrong way though.

Rick
There WAS discussion about the low altitude gray going all the way up tp 50 feet AGL, which is very unusual for a helicopter. I remember someone said "threading the needle was required."

Note: When a helicopter (Guimbal) has a small diameter rotor, it can sometimes be OGE in a 3 foot hover. That probably explains the absence of low level shaded area. It never gets the benefit of ground effect when an engine failure happens at low level at any speed.

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#### Roundwing

##### Member
There WAS discussion about the low altitude gray going all the way up tp 50 feet AGL, which is very unusual for a helicopter. I remember someone said "threading the needle was required."
Brian,

My response was directed to Jim and his question to me.
If you reread his question you will understand why I said what I did.

Rick

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