H-V curve

December 12, 2011 by Tim McAdams

Helicopter manufactures publish a chart in the flight manual that depicts combinations of airspeed and altitude that should be avoided. It is commonly referred to as the H-V curve or, technically, the height-velocity diagram. Typically it is located in the performance section of the flight manual, not the limitations section, so the pilot is not prohibited from flying in these areas. The chart shows shaded areas that should be avoided because in the event of a power failure the helicopter might not be able to perform a successful autorotation.

The instant that a helicopter’s engine quits, it has stored energy in the form of altitude, airspeed and rotor rpm. A successful autorotation is the effective use of that energy to safely land the helicopter. It is worth noting that this same energy, if not used properly, can destroy the helicopter and its occupants. The shaded area on the left side of the chart shows low airspeeds and altitudes where the helicopter does not contain enough stored energy to perform a successful autorotation. The bottom of the graph also shows a shaded area. This area of low altitude, high speed flight should also be avoided because it does not allow the pilot sufficient reaction time to establish a level attitude and may require an aggressive flare that could result in the tail rotor striking the ground.

The chart shown here is from an R44 and depicts a shaded area for sea level and 8,500 feet density altitude. It also shows a recommended take off profile that favors airspeed over altitude until about 50 kts. Other factors such as high power settings (more pitch in the main rotor blades will cause a faster decay of rotor rpm due to drag), high gross weight and pilot experience (the chart is based on the reactions of an experienced pilot) can affect the outcome as well. Due to the nature of helicopter operations like confined area take offs, sometimes pilots need to operate in the shaded area. Knowing the H-V diagram for the model helicopter you are flying is important for understanding when recovering from an engine failure might be difficult or even impossible.

13 Responses to “H-V curve”

  1. Horst Says:

    More than once I had discussion with other pilots about landing. They claim that they perform a landing in a way to be as little as possible in the HV curve, which results in a fast approach with a lot of flare at the end. Certainly not the way I teach it…

  2. Avi Weiss Says:

    Like the often-useless OEM fuel indicators in aircraft, I’m surprised that some of the most critical information a pilot needs continuously is not represented in the cockpit by an active indicator. In this instance, it would be extremely helpful to have an “energy state” indicator in the cockpit, especially in single-engine helicopters that are being used PRIMARILY inside the grey areas of the H-V curve.

    While I realize it probably wasn’t economically feasible in the past, and even arguably “not needed” by very experienced pilots who were intimately familiar with their aircraft, with the ever-increasing automation and computerization of avionics, and the accompanying reduction in cost of those avionics, I feel all new single engine FADEC-controlled turbine aircraft should be equipped with such an “aircraft energy” indicator, much like the AOA indicator in fixed aircraft.

    How it would work: altitude, airspeed, and rotor pitch and speed information would be continuously fed to the flight computer. The computer would determine where on the H-V the aircraft is at, and display a color-coded “energy value” to the pilot: green for “sufficient energy”, red for “insufficient energy”. This coded numeric value would provide an active reminder to the pilot about their energy state, and allow the pilot to determine exactly how much exposure they have in any specific flight mode. Such cognizance might keep the pilot more focused, and help reduce reaction time, which may make the difference between a successful handling of an emergency or not.

  3. Thomas Boyle Says:

    Sounds like a possible application for stored energy, e.g., a compressed air turbine or possibly an electric motor with a modest battery. You only need enough stored energy to fill in the HV area.

  4. Stephen Dines Says:

    I agree with Horst – not the way I taught approaches either.

    The H-V diagram in the flight manual is actually showing how to survive an engine failure during take-off – a requirement during type certification.

    A similar H-V diagram developped using approach power would look a lot different – the ‘avoid area’ would be much smaller.

  5. Dave Lawler Says:

    I was taught that rather than a fast/low landing profile, a landing should be closer to an autorotation profile- so that transition to an auto would be easier if the engine quits.

  6. Alan D. Resnicke Says:

    Mr. Dines -

    I flew USAF helos for 2500 hrs. I don’t understand how the H-V diagram will change based on power – either takeoff or landing. Please explain further…

  7. John H Says:

    Avi’s “energy gauge” is an interesting idea, but you’d have to call it something else, like a “danger gauge”. You’d have the same energy at any power setting, all else being equal, but a much higher potential rate of decay in rotor system energy at high power (due to the high pitch). That would be mitigated by a quick reaction time, but there’s no way for the gauge to know what your reaction time is going to be.

    That should answer Alan’s question, too. High power (such as during takeoff and climb) equals high pitch angle, which equals a shorter required reaction time to get the collective down. And I was taught that the H/V diagram is not for landings for that very reason: you’re already close to an auto, anyway.

  8. Alan D. Resnicke Says:

    John,

    Given “normal” loading and conditions, perhaps your collective setting is closer to full-down (auto) than on takeoff… but may not apply when coming in heavy, in hot weather, as Chalk 4, trying not to run over the guys in front of you and avoiding vortex ring state at the same time… The H-V is still there and you’re at high power/high pitch setting. I was taught (and experienced) that some things can’t be avoided – only managed. (Similar situation applies when coming out of a tight LZ with high trees or buildings… you know you’re in the “deadman’s curve” and pray nothing goes wonky or try to react accordingly.)

  9. Matthew J. Domnarski Says:

    One technique in a maximum performance take-off is to start from minimum hover (less power required) and then a smooth pull on the collective without delay to maximum torque to provide momentum during take-off and minimum time at risk in the H/V curve. It sounds rushed and dangerous (don’t over-torque!) but the theory is that the maximum momentum will also provide you with maximum reaction time should the engine quit on take-off.

    There are ever-changing factors with every flight so its MOST important to be familiar with the risks and weigh the odds, thus minimizing as many risks as possible.

  10. Avi Weiss Says:

    John H;

    Yes, reaction time can’t be judged, and therefore the indicator would be ostensibly meant to first provide an exact measure of “available energy”, and secondarily provide an “assumed” safety state based on “typical” reaction time of an “average” pilot (3-5 seconds). The indicator can be made to be more conservative by assuming a slower reaction time, but the idea would be to provide at least some guidance as to where in the envelop one is operating.

    As to Alan’s question about H-V curve applicability for landing, yes, if one is coming in “heavy” on a hot day, the descent will likely have a significant amount of collective to keep things from getting out of control, and thus the same “high pitch / reaction time” combination that necessitates having an H-V curve WOULD also apply during landing… and another strong argument for having a real-time display of energy in the cockpit would be more helpful than a couple of static charts based on some combination of weight and temperature.

  11. Alex Kovnat Says:

    I looked at the “dead man’s curve” shown above for the Robinson R-44. One notes that the safe procedure in takeoff, is to fly close to the ground until the airspeed increases to ~45 knots, then climb. How many feet of runway would be required to accelerate to this airspeed? How does this runway requirement compare to an STOL airplane like the Helio Courier?

  12. EF5Twister Says:

    All helicopters operate as a result of the ability to differentially compress air to create lift. i don’t understand some of the nuances above but i do understand that much.

    I just saw a video of a helicopter with two rotors that spin in opposite directions which counterbalances the torque that one rotor by iself would generate. What do you think of the practicality of such an idea? Would it substantially increase maintenance?

  13. Daniel Lee Says:

    Alex, I’m a student pilot in the R22. In a normal takeoff it seems 45kts climb speed is reached in about 1200ft. We lifoff from the midpoint of short runways, the midpoint of taxiways, and from small concrete pads over a grass field. At least in training we do a lot of “obstacle” takeoffs, which means an almost vertical climb profile and only a few hundred feet of forward movement, and yes that’s inside the Deadman’s curve the whole time. When licensed I’ll almost always use the standard takeoff and landing profiles, and stay well out of Deadman’s curve.

    EF5, the idea of two opposite spinning rotors for a heli is great in theory, and works equally great on the toy choppers that are available now. However with helis that carry humans the increased complexity, results in a corresponding higer cost and lessened reliability. The lowest priced choppers already cost over $250 an hour to operate, and with twice as many moving parts, it will have a catastrophic and fatal failure probably 16 times as often. And if you’ve flown the toy helos with dual rotors, they have a hard crash about once a minute…

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