Hovering in and out of ground effect

StanFoster

Active Member
Joined
Nov 16, 2003
Messages
17,139
Location
Paxton, Il
Aircraft
Helicycle N360SF
Total Flight Time
1250
I love reading about rotorcraft aerodynamics. One area that used to baffle me was hovering in ground effect...and why less power, or AOA is really needed.

I had been reading several helicopter books...and one put out by the government you would think would be correct.

I initially learned that a helicopter flying IGE needed less power simply because the air was "packed" under the helicopter...more dense...and thus the blades needed less pitch to maintain a hover. This made sense in my newbie mind.....

Further reading in another helicopter book found myself learning that the air was being bottled up so to speak by not getting to exit out as fast from under the helicopter....and this caused the induced flow velocity through the rotor to decrease. Whenever the induced flow velocity decreases....this does indeed cause the AOA of the rotor blades to increase. Then the collective can be lowered to hover with less power required. This also made sense.

Close...but not the cigar. Justin Travis explained to me the best way I have seen...and have not read such a correct discription of this phenomena since.

He clearly explained to me what is going on when a helicopter is IGE. When a helicopter is within one rotor diameter of the ground....roughly.....you are entering ground effect. The rotor tips are generating very strong vortices...just like any wing tip does. These vortices developement however starts getting restricted IGE. The rotor disc area has some of this area consumed by these vortices...leaving the remainder disc area for the induced flow to travel through. When the helicopter is in ground effect...these vortices are restricted...are smaller...and thus consume less rotor disc area. This leaves MORE rotor disc area for the induced flow to travel through...and this automatically lowers the induced flow velocity. Any reduction in induced flow velocity results in the rotor blades seeing an increased AOA. This increased AOA can now be reduced by lowering the collective...and the end result is the helicopter requires less power to hover IGE.

Justin pointed out that it simply is wrong thinking that the air is denser, or packed under the helicopter. His reasoning was backed up by him saying if this was so....his altimeter would show a lower altitude if the air was indeed being packed tighter. He says he never sees any change at all.

I have been reading this topic being discussed right now on another helicopter forum...and I notice that some still believe what I first read.

I find several topics like this amazing ....and I learned you cant always believe what you read.

Stan
 
I wish you had some illustrations to go with your description. Words never are enough for me. I need pictures. Thanks for the info. I keep dreaming of one day adding Helicopter to my ratings. I would want a machine too, but the lottery hasn't come in yet.;)
 
Hi Stan,
I think you're giving the amount of pressure WAY too much credit for the work it's doing.
Off my head quick calculation and Chuckster will correct me if I'm wrong I hope.
A solid disk at 30' diameter would equal approx 11,000 pounds of lift with a pressure delta-pressure of only .027 PSI which would translate to roughly 50ft pressure-altitude to sea-level. Now if you do just 10' it would be 1/10th and roughly 1,100 pounds. That would account that the the compression at the aircraft would be equal, but it wouldn't because we all know the airflow is a torus, generally sparing the helicopter body itself. So playing with the numbers, it's my opinion that it's very possible the toroidal compression under the rotor is the extra life and not little tornadoes at the rotor tips...
It's further possible that the toroidal velocity along with our friend Bernoulli's Principle would reduce the realised pressure at the aircraft proper. However, I'm no aeronautical engineer and my workings with airflow don't generally revolve around moving wings.
I just question the science, that's all.
 
Stan, if you watch a helicopter land in a dusty environment, it is easy to see what is going on with the airflow. As the helicopter gets closer to the ground, the swirling air becomes much smaller in its circular motion. I think what you are talking about will make sense when you watch this phenomenon . See the link below when a large two blade helicopter lands in the dust.

http://www.youtube.com/watch?v=AIQY1O2wXts&NR=1

Scott Heger, Laguna Niguel,Ca N86SH
 
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Sure beats laying down smoke for deplaning.
 
It's actually both!!! (And maybe all three)

It's actually both!!! (And maybe all three)

Stan,

Here is a link to a web form of what the US ARMY Rotary Wing Aerodynamics Book shows:

http://www.dynamicflight.com/aerodynamics/ground_effect/

Bottom Line: Ground effect is essentially felt at 1/2 rotor disk and below, although it has an effect up until 1.25 rotor diameter. The curve is parabolic, so the closer you get to the ground, the more ground effect you feel.

The two reasons you mentioned above are the cause:

1) Reduced induced flow
2) Reduced rotor tip vortices

As to the theory that it is "compressed air," that is where you'll get aerodynamic engineers circling back to the argument of how a rotor blade produces lift - purely by Bernoulli's equation, or if indeed the rotor system deflects air, which has mass, which then creates lift. Either way, the end result is reduced induced flow...

From the website:



Ground Effect is a condition of improved performance encountered when operating near (within 1/2 rotor diameter) of the ground. It is due to the interference of the surface with the airflow pattern of the rotor system, and it is more pronounced the nearer the ground is approached.

Increased blade efficiency while operating in ground effect is due to two separate and distinct phenomena.

First and most important is the reduction of the velocity of the induced airflow. Since the ground interrupts the airflow under the helicopter, the entire flow is altered. This reduces downward velocity of the induced flow. The result is less induced drag and a more vertical lift vector.

The second reason is that when operating in ground effect, the downward and outward airflow pattern tends to restrict vortex generation. This makes the outboard portion of the rotor blade more efficient and reduces overall system turbulence caused by ingestion and recirculation of the vortex swirls.

At a rotor height of one-half rotor diameter, the thrust is increased about 7 percent.
At rotor heights above one rotor diameter, the thrust increase is small and decreases to zero at a height of about 1 1/4 rotor diameters.



Hope that helps!
 

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Hi Stan,

All three IGE theories you mentioned are interesting because one can view the evolution of thought as the history of rotorcraft operations and rotorcraft aerodynamic understanding progressed. All three theories are not wrong, however the last one you mentioned can be considered the "most correct" because of the deeper understanding of what is going on.

At "Mother Rucker" (Fortt. Rucker) Army Aviators are taught simply that ground effect of the rotor happens at 50% of the diameter of the rotor above the ground. This is caused by both reduced induced flow back into the rotorblades and reduced or absense of blade tip vortices. The last example you gave just gives a long winded version of this.

There's an interesting video of a small hydrogen peroxide (H2O2) powered helicopter flying around IGE and then into forward flight. H2O steam is produced at the tips of the rotors and the outcome is a visual display of the air flow created by a hovering helicopter, then in translational flight, and then in cruise flight while showing vortices flowing from the edge of the rotor disk.

A great subject to discuss Stan.

Wayne

http://www.peroxidepropulsion.com/files/Intora1-seatLong.wmv
 
Stan,

Regardless of the physics involved, you need to learn how it affects YOUR helicopter.

Yeah, so you have less induced drag IGE and can hover at 55% power, whereas OGE is 75% power (or more).

Where you need to understand ground effect is on approach - at what speed, rate of descent, etc, you can count on ground effect to give you a lift at the bottom. It may save your life on an engine-out autorotation, and it will certainly give you more of an understanding of the benefits of collective application...

Approaches at/slightly ahead of ETL can be performed using only 34% of engine power straight to the ground - much less than a standard hovering landing, which is usually about 55-60%. (The percentages are what I experience, so the absolute numbers shouldn't mean much to you - only the comparison!)

Bottom line is this: you can finesse the landing to use air as a cushion to it's maximum benefit - it's rather neat once you get the feel.
 
Stan,

I'm sure you have already seen it but just in case.

"Chicken Hawk" is a great book that you will not be able to put down.
It has many accounts of the use of ground effect in unusual situations. I found that reading it helped me think about the theory in real life situations.

Probably the best non technical book ever written about helicopters, a must read for all rotorcraft enthusiasts.

I can't remember the authors name, Robert something.
 
Stan,
You have to read Chickenhawk!
 
If you've got somewhere smooth & hard, a run-on landing is a lot easier if there's not much wind - if you've got a 10+ mph headwind you can plop it down with no slide pretty easy.

The catch though is the surface condition - if in doubt, you want zero groundspeed.

Digging in a skid can turn into a rollover - I watched a Navy UH-1N demonstrate that point nicely one day, they were practicing running landings in the grass beside the runway. It'd been pretty wet, ground was soft - dug one skid in & over it went!
 
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