Sunday, April 11, 2010

Why don't helicopters go straight up and down vertically?

In Chickenhawk by Robert Mason starting on page 169 Mr. Mason brings up a scenario where he and Riker fly an approach to a forward command post on the top of an isolated hill covered with tall brown grass and a few small trees. They circled once and then made the approach. As they came in over the edge of the sharp drop off a pup tent flew away in their rotor wash. After shut down Mason went back to see how the kid who lost his tent was doing. “Sorry about that, we didn’t see you when we came in.” “Oh, that’s okay, sir.” The kid smiled and threw the tent down near a bush and said he’d build it stronger next time. “Well that’s a bad spot for you” Mason said. “You’re downwind and you’ll be on our approach path every time we land.” “I never thought of that, how come you don’t come in like this?” The kid made his hand stop in midair and drop straight down. Mason said “Vertically?” “Yea vertically, can’t a helicopter do that?” Mason said “We can in a pinch, but it’s dangerous. We like to keep some forward motion so we’ll be able to autorotate in case we lose power."

And that my friends is a question I get asked all the time by my non-flying friends when we discuss helicopters.

If you watch helicopters you will notice that they usually take off and land very similar to airplanes, not vertically straight up and down. Why you ask, aren’t helicopters meant to go straight up and down? Well yes they can but only when we have to and we only stay in that flight regime for a minimum amount of time because it’s not safe and within the dead man’s curve (shaded area of the height/velocity diagram); in other words the helicopter won't be able to autorotate and land safely in case of engine failure. Not sure I like the term “dead man’s curve” but it is what it is.

I think now is a good time to bring up the 5th commandment of the 10 commandments of helicopter flying:

V. Thou shalt maintain thy speed between ten and four hundred feet, lest the earth rise and smite thee. (Complete recovery is doubtful in case of power failure at slow speed within the dead man's curve of the height/velocity diagram.)

So what does this mean? The ability to autorotate is handy but unfortunately there are some configurations of flight, airspeed and altitude, that won't give you time to autorotate safely and avoid a crash.

Here's a typical height/velocity (H/V) diagram that we can take a look at, and talk about what the chart means. Every manufacturer of helicopters is required to provide an H/V diagram for each model.

                     Bell JetRanger 206B HV curve

Like the 5th commandment of helicopters states, airspeeds between 10 and 400 feet AGL (above ground level) should be at 45 MPH (indicated airspeed does not mean ground speed) for the chart above. In the R-22 we stay approximately 10 feet above the ground until we hit 40 knots then let the nose come up and climb out at 60 knots; we are basically staying below the shaded area and flying out and up. Between 10 and 400 feet AGL we don't like getting slower than 53 knots in the R22. Again you can go slower it's just not a good place to be and should be kept to a minimum.

So, at 300 feet with zero airspeed you'd be in the shaded area and in an unsafe configuration. Also medium to high zero airspeed hovers will put you between a rock and hard place.

AND REMEMBER when the engine fails LOWER THE COLLECTIVE...

And that my friends is why we normally avoid vertical up and down zero airspeed flight configurations.


  1. That was a good post.I agree that avoiding operations in the shaded portion of the height velocity curve is a good idea. At some point in your career you will encounter certain types of work that will require you to spend considerable time flying in the shaded area.
    Operations such as long line vertical reference work for seismic,firefighting and many others will require you to spend hours in the "gray area"
    Just remember that the decision to fly inside the curve is yours and not your passengers. Passengers, like your soldier in the story don't understand how helicopters work. If you spend, as I have, several thousand hours within the shaded area of the height velocity curve you can never get too comfortable. Always think, could I defend my actions if something went wrong?

  2. Thanks Keith, I appreciate your comment. I hope someday down the road to work in the gray area more often and survive to talk about it. Stuff like fighting fires and long line vertical reference work sounds rewarding if I ever make it to that stage in my career. I just paid a visit to your site "Helicopter Pilot, will travel" and I must say it looks interesting. I’ll be sure to check back regularly. I just read the post about the firefighting with an Erickson Air Crane based in Athens, what an awesome machine. I guess to be a helicopter pilot in the firefighting industry means lots of traveling. Again thanks for the comment and advice.

  3. I just paid a visit to your site "Helicopter Pilot, will travel" and I must say it looks interesting. I’ll be sure to check back regularly.