A cushion of air

March 30, 2009 by Tim McAdams

Many times you will hear helicopter pilots refer to hovering in ground effect as resting on a cushion of air. Technically speaking, what they are referring to is the extra performance that hovering in-ground-effect (HIGE) provides versus hovering out-of-ground-effect (HOGE).

Ground effect is defined as a condition of improved performance that results from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.

A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.

The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.

The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.

When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.

Helicopter pilots need to consider this when making very steep approaches as it has caused accidents. Typically, what happens is while a pilot is attempting to land, they allow their airspeed to get too slow and their approach too steep. They then realize they do not have enough power to slow the descent rate. In this case, the helicopter begins settling from a lack of available power. This is not the same as an aerodynamic condition called “settling with power,” which involves the generation of a vortex ring state (subject of future blog).

The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance. It is very important for pilots performing some missions such as ENG (Electronic News Gathering) or external lift operations to know if their helicopter can hover out of ground effect. Safe helicopter operations depend on good performance planning.

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9 Responses to “A cushion of air”

  1. Kris Says:

    I think it could be a bit more precise concerning steep approaches. It’s not slow airspeed per se that can cause problems with steep approaches. Rather it’s too high a rate of descent that causes problems.

    I’ve had several instructors tell me that they don’t mind a slow airspeed as long as the rate of descent is under control. On the other hand, a slow airspeed combined with a high rate of descent are asking for trouble. Kris, Mercer Island, WA (10 year recreational helicopter pilot with commercial rating)

  2. Tim McAdams Says:

    Kris,
    Thanks for the comment. You’re absolutely correct when you say that a slow airspeed coupled with a high rate of descent can cause problems. What you are referring to is the potential for “settling with power” and putting a helicopter in that situation must be avoided. That is a subject I intend to address in a future blog.

    While a slow controlled rate of descent is a much better technique, pilots still need to make sure they have sufficient power available. Attempting this type of approach at a high density altitude with a heavy helicopter can, and has, caused problems as well. Early in my career I was attempting to land an R22 (without a governor) in a field at about 8000 ft DA. At about 50 feet and with a 100 to 150 fpm descent rate, the low rotor rpm horn came on. I went to add power and the throttle was wide open. The only thing I could do was reduce the collective control a little, maintain airspeed just above ETL, and do a run on landing. I was very lucky that the field was level and I had enough space. It was quite the learning experience and I quickly understood the importance of the HOGE chart.

  3. Chris Says:

    Hi Tim,
    One way that I use the HIGE and HOGE is to determine where I can take off and where I can land. Your HIGE tells you what altitude you can maintain a hover and can conduct a normal takeoff. Your HOGE tells you what altitude you can conduct a landing into a confined area. Neither tell you whether you’ll have the power to slow your rate of descent: for that, you need to “keep the rotor loaded” at the termination of your approach. As you slow to ETL, a gradual application of power slows your downward inertia and tells you what your power margin is. This produces that slow, controlled rate of descent that you are aiming for.
    Thanks for bringing rotors to AOPA!
    –c

  4. J. Trigg Adams Says:

    Tim;

    From an old Marine H-34 driver, may I offer a bit of insight before you confuse the terminology: “settling with power” refers to not having enough power to maintain altitude, and generally refers to slow or hovering flight. “power settling” refers to the condition gotten into when you set up the vortex ring state, and as you know can only be gotten out of by acquiring forward speed. (I have a theory that one could enter autorotation, but I don’t know of anyone who has tried it.)

    For credentials, I hold a Sikorsky S for a rescue picking up three survivors in a blind canyon in zero-zero conditions, well above HOGE. If you’re curious how I figured it out, email me. And I have a graphic set of navy 8×10′s depicting the results of power settling right in front of me on a carrier. It was rather interesting to watch. On the same cruise I inadvertently let a student get me into settling with power, resulting in a busted off tailwheel.

    But, for Igor’s sake, don’t screw up the terminology. l have enough trouble explaining helicopters as it is.

    Trigg Adams

  5. George Smith Says:

    Hi Tim – This is George from Leesburg -Your old student. Remember our flight back from CA.? Im still flying kids. -George

  6. Spencer Smith Says:

    Hey TIm,

    Let me start off by saying you have written some great articles here on AOPA.com. I finally have read them all. I was starting to waiver on keeping up my AOPA membership . I know some fellow helicopter pilots who do not hold an AOPA membership because they feel the AOPA does not support the helicopter industry. When I found out you were writing a blog on AOPA.com I knew I would renew my membership without hesitation. THANKS for being a supporter of the industry and I look forward to more stories. STRONG WORK!!!!

    Hope all is well,

    Spencer

  7. MarkRight Says:

    Great article as for me. It would be great to read something more about this topic.

  8. Steve Says:

    Hello all:
    I work for a Helicopter manufacturer in the Marketing and Sales department. My background is aircraft maintenance with no piloting experience. I help develop compelling collateral material for my company and often get tasked with developing HIGE/HOGE charts. Can anyone direct me to a course and or links to help in developing such charts or does that ave to come with piloting experience? I tend to get lost in the meetings when discussion about critcal wind azimuths, generator loads, engine charts m(min spec), etc…are talked about. Pleas provide input to any related course for better development of charts.

    Thanks in advance,

    Steve

  9. Bill Says:

    Steve, I’m not a test pilot an engineer. I only fly the helos. From my standpoint, I rely on the data you give me to ensure the aircraft is performing as advertised. I thought the engineers developed the charts with anticipated results based on aerodynamics. Then Test Pilots confirmed the results and modifications were adjusted, if needed.
    Am I way off?

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