OGE Power in Helicopters

Welcome back! I'm Jacob and in this video we go over Out of Ground Effect (OGE) Power. That is, the power required to hover a helicopter OGE as opposed to In Ground Effect (IGE). If you remember from my Airflow at a Hover (   • Airflow at a Hover in Helicopters  ) video, it takes more power to hover OGE than IGE. So how can you tell if you have enough power? This video goes over just that.

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OGE is commonly associated with a height of 1 rotor diameter above the ground. That is, if your rotor system is 30’ wide, at about 30’ above the ground the helicopter leaves ground effect and operates OGE. Simply put, this height and anything above it requires more power because the rotor vortices can build much more drastically than if the ground affects them. It’s important to know if you have enough power for this condition prior to getting into to a point where you may need it. Failure to do so frequently results in aircraft accidents / incidents.

Let’s say you want to fly your helicopter from sea level to a helipad in a mountain city at an elevation of 5,000’. You want to pick up 500 lbs. of cargo and bring it back. Prior to departing, every good pilot should do some performance planning to find out power required and power available. Doing this you’ll use weather information to gather temperature and pressure altitude to determine engine power or Max Torque Available (MTA). Then, you’ll compare aircraft weight to determine how much power is required to hover. All of this information will be unique to your aircraft, the mission weight, and the environmental conditions. All of this information is calculated sing your aircraft’s performance charts in the operator’s manual.

For this example I’m making up figures for the sake of easy math and teaching. Let’s say I calculate at sea level with +20 degrees Celsius my aircraft can weigh as much as 7500 lbs. and have OGE power. My aircraft takeoff weight is planned to be 5,000 lbs. MTA for the environmental conditions is 100%. OGE calculates to be 75% and IGE is 65%. Now lets run the numbers for our destination. Max weight for OGE power comes to say 6000 lbs. We plan on weighing 5500 lbs. after refuel and picking up 500 lbs. of cargo. MTA is now 90%, OGE is 85%, and IGE is 75%. The margin between power available and power required shrinks (from 25% difference to 5% difference) as weight and altitude goes up. The power available comes down because there is less oxygen as you climb and engines hit their limiters sooner.

So now I check the numbers. There are 3 easy ways to validate that you have OGE power. 1st: OGE power required should be less than MTA. 2nd: Actual weight is less than max allowed OGE weight. 3rd: IGE is less than the OGE go/nogo weight. By this I mean there’s a 5% margin between OGE and MTA. So if I add that 5% margin to my IGE number I come up with 80%. Thats my go/nogo number for OGE power. If IGE is less than 80%, I have OGE power. If its greater than 80% I won’t.

One last reason this is useful is that you can adjust based on contingencies. Let’s say at arrival they ask if you can carry 800 lbs. of cargo instead of 500 lbs. Looking at the numbers I can bring up to 1000 lbs. of cargo (Max OGE weight minus non-cargo takeoff weight) and still have OGE power. But if they want me to pick up 1300 lbs. I would have to say no or come up with a takeoff than didn’t require OGE power.

So that’s how you pre-plan power. What if you’re flying right at the edge of OGE power and need a little more lift to just get up and going? That is, OGE power is right at MTA. Here are a few tips:
1. In a neutral wind condition, you can do a spiral takeoff by applying right pedal for counter-clockwise rotating rotor systems (left pedal for CW). This reduces tail rotor anti torque and in essence gives power back to the main rotor by reducing pitch/drag in the tail rotor. Warning: Never do this in high winds in your Loss of Tail Rotor Effectiveness (LTE) regions or you could have an uncontrolled spin (My LTE video here (   • Loss of Tail Rotor Effectiveness in H...  ).
2. Hunt for the winds. Relax on the pedals and let the aircraft weathervane into the wind. Once you find it, translational lift slowly takes effect and the helicopter climbs more efficiently.
3. Springboard takeoff. Starting from a hover, slightly reduced collective then rapidly pull up collective to MTA/OGE power. The aircraft will momentarily descend and create a high pressure cushion underneath which acts like a springboard to push the helicopter up. The springboard is gives the aircraft extra inertia and is most effective in confined areas where the air cannot escape as easily under the rotor system.

By using one or more of these techniques you can squeak out just a little bit ore lift on your OGE takeoffs. As always, I’m Jacob. Safe flying.