Height / velocity curves
 

Hi - question regarding HV curves. How is the pilots judgement regarding the ability to make a safe landing validated?

predetermined criteria
 

The pass/fail criteria for a landing will be defined before flight, normally to respect a limit such as minimum rotor speed or maximum landing velocity. Pilot skill is generally a second order issue

Any technique must be capable of being flown without exceptional piloting skill (ie an average pilot must be capable of carrying out the procedure) so if a qualified TP thinks it is difficult to fly repeatedly then it's a 'fail'.

DM

Is the landing velocity or minimum rotor speeds consistant across all types?

No ; that will depend on pilot skill ;any pilot having an engine/transmission failure within the H/V envelope will/should attempt to land with minimum vertical velocity and as little fwd/sideways/yaw velocity as possible,and to that end will use all the stored rotor energy available.(in the same way anyone in a fixed-wing a/c should endeavour to dissipate the energy during a forced landing..ie if you are going to crash ,do it under control).

as above
 

The landing velocity after engine failure will be limited by the U/C design criteria, an aircraft designed for shipborne operations will be allowed to land with a higher velocity than a pure civil type. Wheels or skids are another factor.

Minimum rotor speed may be determined by considerations such a head stresses, controllability, tail rotor authority or electrical generator rotational limits and will vary aircraft to aircraft

Remember that HV curves also apply to flyaways so a succesful landing may be after a flyaway and circuit.

Remember seeing a video in 1970 during a US Army flight safety course of the OH-6 doing hovering auto from altitude during its certification. They progressively got lower and lower until the pilot rolled it up in a ball. That plus a bit was then set as the top altitude/zero speed on the H-V graph. The comic relief was the pilot climbing out of the helo, which was lying on its side, and throwing his helmet on the ground.

The H-V curve will be vetted by the certification authority (for civil helicopters, anyway), so the test pilot has to be mindful of that. Also, you never want to find the real end point!

Dug this out of a book titled "Helicopter test and evaluation":-

Avoid area testing

The final stage of a power-o

ff assessment programme is to define the avoid curve, avoid

area or height/velocity diagram. An example of the type of presentation commonly

seen in aircrew manuals is shown in Fig. 7.13. An avoid area diagram shows the

operational pilot the combinations of airspeed and height above the surface from

which a total loss of power is unlikely to be survivable. Armed with this information

the pilot can plan the aircraft’s flight path to minimize the time spent at risk inside

the area. A number of assumptions are normally made in the construction of height/

velocity diagrams:

Ω

The aircraft is in level flight or the hover at the moment of power loss.

Ω

There is a total intervention delay time of 2 seconds.

Ω

There is no wind and the surface is suitable for a landing.

Shameless plug - The Little Book of Autorotations has several chapters on the HV curve.
So says the author.-*

Shawn is the 2 second delay standard for the industry?

I hope not - an r22 would have it's blades bunny flapping by then!

2 seconds is what the military uses for the intervention on all controls...
Civil requirement is 1 second delay on the collective only.

The difference is odd because it suggests that a mil pilot is slower to react than someone who may fly only a few hours per year..

How did that come about?

The civil logic is actually quite reasonable - one could argue that the time to react should be longer, but if you're properly trained, and in the right frame of mind, one second is almost long enough. (The reason for saying it's almost long enough is that everyone I know, and me as well, who's had an engine failure has been surprised, and would have like more time to react).
But the main point about the civil logic is that you have to have your right hand on the cyclic pretty much all the time, and feet on the pedals pretty much all the time, and that you'd probably react to uncommanded changes in those axis within one second. Collective? Not so much - you might have your hand off the collective when the engine failure happens, so you have 1 second to react to get it down.
Hope that helps...

Thanks Shawn