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Academic subject requests
#31
Hey Mark I have a question for you on helicopter flight dynamics I was hoping you could help me out with.

The TLDR version is: Can helicopters hover over water? Not like 100’ over water but down in what would be considered “in ground effect” over land?

The long version:

So I was flying with my instructor over the weekend, and there’s a video that had been on my mind. Not sure if you’ve seen the one of that Sweitzer that was waiting on his friends to clear him a landing spot on the shoreline of a lake they were staying at so instead of just orbiting the area until the LZ was clear he descended and brought it to a hover over the lake. He didn’t have enough power, got into LTE did a few spins and ended up in the lake.

The discussions on Facebook seemed to conclude that he should’ve performed a proper power check before attempting the maneuver, so I decided to ask my instructor about how he would’ve approached that situation. Well he pretty much ended the discussion before it began by saying that the pilot was a complete idiot because helicopters cannot hover over water. Period. End of story.

Now I was always under the impression that hovering over water is NEVER a good idea and it should never be done except when it’s absolutely necessary, because the inherent instability of the surface of water negates some of the ground effect helicopters receive while hovering over land, therefor much more power is required to maintain the hover. So if a helicopter has the power to hover out of ground effect, it should in turn, have the power to hover over water. 

However, my instructor told me that when you see coast guard helos rescuing people from the ocean that they’re hovering at like 100’ and using a long line because even those helicopters are incapable of hovering directly over water.

Now I know in the Enstrom hovering over water would be an extremely bad idea because we can barely hover over the ground in ground effect, so hovering over water would be a great way to take a swim and lose a helicopter, which I’m sure is why he tells me why there is absolutely no way to do a power check in flight, it can only be done while hovering in ground effect.

So was just hoping you could help me understand what exactly I’m missing here lol
"Into the fires of forever we will fly through the heavens
With the power of the universe we stand strong together
Through the force in our power, it will soon reach the hour
For victory we ride, Fury of the Storm!" 
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#32
Found this on YouTube. He's in "ground" effect (or water effect if you want to call it that) but still has a little forward speed. Looks dangerous just due to the water rotor wash obscuring your vision and windscreen. Maybe that's what caused the accident you mentioned, water causing the pilot to lose control.


https://youtu.be/UHM4M1ssiFU

Here's another link to a fire heli hovering while it fills its water tank

https://youtu.be/hzyRQTJjFNw

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#33
(10-22-2019, 04:21 PM)Eblezien Wrote: Found this on YouTube.  He's in "ground" effect (or water effect if you want to call it that)  but still has a little forward speed.  Looks dangerous just due to the water rotor wash obscuring your vision and windscreen.  Maybe that's what caused the accident you mentioned,  water causing the pilot to lose control. 


https://youtu.be/UHM4M1ssiFU

Here's another link to a fire heli hovering while it fills its water tank

https://youtu.be/hzyRQTJjFNw


Yeah as you said they still seem to be maintaining a bit of forward airspeed, maybe because if they notice rotor RPM starting to drop it’s easier to push on through ETL and fly out of there.

But as far as the original video, I also don’t know at what elevation they were at, higher elevations would’ve compounded the effect that the water would have on the helicopter and the pilot was not prepared for that. But the helicopter acts exactly as you would expect it to in a low power available LTE situation, but I’m hardly an expert, hence why I’m here lol
"Into the fires of forever we will fly through the heavens
With the power of the universe we stand strong together
Through the force in our power, it will soon reach the hour
For victory we ride, Fury of the Storm!" 
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#34
I think there is no ground effect over water, just as there is no ground effect over grass, but I would think you could hover over water using OGE power requirement. I think I saw that video and heard he was over pitched due to lack of power, which caused main rotor rpm to drop, reducing tail rotor rpm and tail rotor effectiveness.
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#35
Casey, maybe ask your instructor how helicopters with floats can operate from water, see his response to that one.
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#36
Here, this throughly debunks it! R44 with floats, hovering, landing, and at the end a water takeoff.

https://youtu.be/BdnuxAyy8V4
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#37
(10-22-2019, 07:37 PM)Eblezien Wrote: Here, this throughly debunks it! R44 with floats,  hovering, landing, and at the end a water takeoff.

https://youtu.be/BdnuxAyy8V4

Yeah okay so that is pretty convincing there! Okay so next question...what would you do if your instructor told you something like this? Like do y’all think something like this is a problem or what lol, I’m not sure what to do with this information lol
"Into the fires of forever we will fly through the heavens
With the power of the universe we stand strong together
Through the force in our power, it will soon reach the hour
For victory we ride, Fury of the Storm!" 
Reply
#38
Casey,

Forgive me for not answering your questions regarding hovering over water sooner than this. I was checking on here fairly regularly and seeing little or no activity and I just checked today (after many months) to find there were some questions directed at me. So, once again, please forgive me.

Now, first things first: Ground effect does exist over water and grass and any other surface. Ground effect is really just the action of the rotor downwash being decelerated by an underlying surface. The closer to the surface the greater the decelerating effect or ground effect. A good example of ground effect is if you stand a 4'x8' piece of plywood up so that it is 8' tall and then let it tip over to hit the floor. It will not slap the floor and make a loud bang instead, it will swoosh and have very little sound as it settles. This is because the air is being compressed between the plywood and the floor as the plywood falls. The compressed air between the floor and the plywood ultimately cushions the plywood from hitting the floor hard. This is a simple demonstration of ground effect.

The decelerating effect or ground effect is less and less effective the farther the rotor system gets from the surface. At 1.25 times the span of the rotor there is no more appreciable ground effect. These are simple terms and analogies.

When hovering out of ground effect (HOGE) there is no resistance on the downward flow created by the rotor system. Because there is no resistance the airflow is both high velocity and high volume and will require more blade angle and power to keep up with the airflow demand to keep the helicopter in the air. That is as simple as I can make it.

So, helicopters can hover over water and land on water. The danger there is that when close to the water the spray can make it very difficult to see hover references and can cause a kind of vertigo because of all the swirling mist and spray without any hover references to contrast with that movement. That's why over water hoists are done at 100'. I've done a lot of over water hoist rescues and rescue training both in the day and at night and hovering low over the water without references is a great way to put one in the drink.

As for the Schweizer that landed in the water due to overpitching with insuficient power available. I believe it took off from a much lower altitude where the density altitude was low then flew to a higher density altitude due to elevation or temperature increase or both. This could have happened due to a lack of proper performance planning or not doing a current conditions and power check once arrived in the landing area or all of the above.

Hope this helps clear some things up.
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