Hovering–Stable or not?

February 27, 2009 by Tim McAdams

In a recent blog (“Standing on a Basketball”) when discussing hovering, I stated that a helicopter is dynamically unstable. A reader commented, “Helicopters have neutral dynamic stability. They are not unstable.” This made me think that perhaps I should dig a little deeper into the subject. Sometimes a mathematical model or an engineer’s definition can be perceived differently when used in a practical application.

I’ll start with a basic definition of stability. An object is unstable if, displaced from its position, it continues to oscillate with increasing amplitude. It would be considered stable if it oscillated with decreasing amplitude, eventually returning to its original position. In the case of neutral stability, it does neither of these. That is, the amplitude does not increase nor does it decrease.

To get an engineering perspective on helicopter stability, I reviewed what Ray Prouty has written. Prouty has contributed to the helicopter industry for more than 50 years. He has done work ranging from preliminary design to performance and flight testing. He has also been honored for his contributions to the industry by the prestigious American Helicopter Society (a group that emphasizes engineering excellence in rotorcraft design www.vtol.org ), which named him an Honorary Fellow in 1983. He has written three books on helicopter aerodynamics and his writing is some of the best in terms of taking a complex engineering concept and explaining it in easy to understand language.

Chapter 8 of his book titled Helicopter Aerodynamics addresses dynamic stability. Here he provides a great explanation of what happens when a hovering helicopter is displaced by a gust of wind. He states, “A typical helicopter will go back and forth across its starting point with an ever increasing swinging motion until the pilot (or someone else) stops it.” According to Prouty, the rate of growth from one cycle to the next is a measure of the degree of instability. Finally he concludes that a hovering helicopter is unstable. Based on my experience teaching students to hover, I agree.

However, in a quest to produce a more stable helicopter, engineers designing early rotor systems developed devices that acted like gyros. While this made hovering much more stable, it reduced controllability. It was later determined that with practice a pilot could learn to hover without these stability enhancing devices. Today, systems like electronic forced trim and Stability Augmentation Systems (SAS) provide increased stability without sacrificing controllability. Helicopters equipped with these systems would behave more dynamically stable or neutral in a hover.

More information about SAS and other topics is available in Prouty’s books and I would recommend them to anyone who is interested in learning more about helicopter aerodynamics. He is a long time columnist for Rotor & Wing magazine and still writes for them occasionally. For more information on how to obtain his books or read his past columns, visit their website at www.aviationtoday.com/rw/

  • Jeff Bishop

    When I was in engineering school, we modeled a helicopter platform mathematically using laplace transforms and did a study on stability. The mathematical conclusion was that helicopters are inherantly unstable. But I guess the proof is in the pudding. So I would siggest that you ask any chopper pilot to stabilize his bird in a hover in an environment with no wind or any other input forces and then request that he let go of the stick for 60 seconds. You could do that in a fixed wing plane no problem, right? Do it with a chopper pilot and see what kind of answer you get and that will be the answer to your stability question.


  • Gregory Beck

    I must agree with Jeff, as an aerospace engineer (although not a helo designer) and a helicopter pilot. That would be a very long 60 seconds (or very short) and would not only require no wind, but literally zero control loads, consistent air density, perfectly controlled engine RPM, ideal bearings and machined elements, a center of gravity that was perfectly accounted for, and probably a complete absence of insects flying by. I think the term “neutral dynamic stability” simply translates into “unstable” in absolutely any real world situation. Interesting and thought provoking discussion.

  • Gary

    As a young aviator I was asked by an IP to define a fugoid oscellation. I think the explanation of the wind effect in the above passage on a hovering helicopter may be a good explanation of a fugoid oscellation. One problem I have found with defining these processes rests with the pilot at the controls. Some of the oscellation is due to the pilot holding the control and although not all voluntary it still occurs due in some fashon to the physical connection to the controls. As the helicopter moves one way the pilot checks the movement with control input, or just the movement of the aircraft causes a physical movement of the control because the pilots body moves. These involuntary movements should be taken into account in stability and as a pilot develops a feel for the aircraft, he checks those movements instinctly and almost as involuntarily.
    Other discussions about stability I have heard seem to center on the system’s ability to return to a stable state. Some airplane pilots talk about the ability of small planes to right themself when a stall is entered and a new pilot can’t seem to regain control. Letting go of the controls will allow the aircraft to correct its unstable condition naturally. As pointed out earler helicopters don’t return to a stable state on their own. Viewed in this manner the helicopter is unstable. Adding things like force trim and SAS may help to remove some of the involuntary control input, but generally won’t correct an unstable platform. In helicopters I think the stability control is in the pilots seat.

  • http://Yahoo Tom Reesor

    Helicaopters are not stable. Even hovering one the pilot is always making constant corrections. Hovering is much like sitting on top a bowl of jelly during an earthquake inside a tornado.
    Tom Reesor, AOPA 158444 (50-year member)
    Airline Transport Pilot: Rotorcraft helicopter and airplanes. Commercial pilot: Rotorcraft gyroplane, gliders, & free balloons.
    Gold seal CFI: Airplane single & multiengine, rotorcraft helicopter & gyroplane, glider, instruments airplane & helicopter.
    FAA Wright Brothers memorial award

  • James Doyle

    Even if one could theorhetically prove the “stability” of a helicopter he or she would find in actuality that indeed a helicopter is not stable. As a newly minted helicopter pilot I can assure you that I would be quite pleased if stability was the natural state of the machine. Heck, isn’t that what makes flying them so much fun! The challenge in controlling the machine rests in the skill of “balancing” all that motion(oscillation); My verdict, unstable, in any state except of course resting in the hangar!

  • John Joseph Sr.

    I always enjoy anything that flies. The most descrptive and simple approach to describe the helicopter controlability in a hover is to visualize the rotor as a convex lifting disk above you and we are the pendulum weight underneath it. Torque pedals will compensate for the opposite rotation of the pendulum to allow for a heading to be established and held while hovering and moving slowly. Collective will allow you to control the lifting effect of the rotor disk. The cyclic stick will tip the disk the direction that you would like to have the pendulum ( with us in the cabin area} to move, the more the disk is tipped the more quickly we move in that direction. It is like jumping on a pogo stick ( never look at your feet or you will become unsteady}. By explaining the operation of the helicopter like this I have had several teenagers hold in a hover (within a time period of 15 minutes of assistance} steady for 3 to 5 minutes. The smile that you get from them after their 20 minute first flight in a helicopter cannot be descibed in words. Like it was their first accomplishment in life. Yes Helicopters do require input to make it respond and does not lend it self to return to its original position after being displaced.

  • JD Thomas

    In response to the first comment, I have flown helicopters for years and was a Maintenance Pilot in the Army. Believe it or not a UH-1 will hover itself if it is rigged correctly and the environmental conditions are right. I didn’t believe it myself until an “old guy” showed me that trick. Of course it won’t do it for 60seconds but I bet we did it for at least 20-30 sec. The “Huey” was and is a fine helicopter regardless of the opinions of the ARMY.

  • http://www.finnlife-cabins-uk.co.uk Jane

    Vry interesting to get some good information on this subject 😛 😀

  • HRPufnstuf

    I did most of my training in a Robinson R44, and I wouldn’t let go of the controls for a second. However, I had an opportunity to train in a Bell 47, and the instructor had me put the helicopter in a hover at 200′, then put my hands in my lap. He lowered the collective ever so slightly to start a very slow descent, and then put his hands in his lap. We settled very slowly (probably at least 60 seconds), until the helicopter stopped descending when we reached ground effect. He then looked over at me, smiled as he took hold of the controls again, and said “do that with your Robinson.” I was impressed.

  • Silverexpress

    I am a bit confused by this. Yes, I understand the physics of helicopters, but my confusion lies in the use of the word “stable” when describing a helicopter. I see the words “stable helicopter” together plastered everywhere on the internet. Would the following be more acceptable?

    1. A helicopter with dampened control surfaces.
    2. A helicopter that tries to return to a state of equilibrium
    3. A helicopter with a faster rate of return to a near state of equilibrium

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