Static and Grass Pollen

[Al Smith]

Last summer whilst hovering in an R22 over long grass, whilst still a student, I noticed that to remain in the hover I was requiring more and more power. The manifold pressure was now well into the red. The day was hot and dry. The instructor took over. On landing, after a quck tranisition to the helipads, an engineer was called to have a look. It was subsequently found that the blades were covered in pollen which obviously had upset their aerodynamics. The problem was solved by cleaning the blades. Anyone else had a similar experience? Could this happen flying through dust or ash?

[helicopter-redeye]

You need more power to hover over long grass than over a hard surface.

The pollen may have been incidental but I'm not an expert on palynology.

h-r;)

[Farmer 1]

Yes, Al, it happens.

Why do you think birds spend so much time preening themselves? They're the experts.

To get the best out of your aircraft it has to be squeaky clean, particularly the aerodynamic bits.

[Kolibear]

Several times I've seen fixed wing pilots polishing their props and in discussions, prop-polishing seems like a good practice to follow.

Any ideas on what is a good polish to use - I'm assuming that Mr Sheen is not reccomended? :)

[lartsa]

i was told bees wax for the blades

[hihover]

Al,
Cellular Absorption is the phenomenon that causes a loss of ground cushion benefit, ie, more power required. Long grass will cause this. I am not convinced the pollen would have such a profound effect on its own.

[ShyTorque]

Are you pollen our legs?

As for plank wing pilots "polishing their props" - yes, I'm sure they do - but I've never heard it called that before :E .

"Pollen it through" is something I've heard tell they do before start-up. ;)

[quichemech]

Whatever you do, don't hover over a rape seed field when the pollen is about, especially in a 22, you'll end up sat in it.

[ShyTorque]

Surely, you can't call it rape if you sit on it yourself?

[Heli-kiwi]

I use CRC on a rag once a week to wax the blades (R22) and find that nothing sticks to them, It also makes life very easy when it comes to cleaning the blades and I have also found that erosion levels are dramatically reduced too. On comparing 2 sets of blades with 1200 hours on each having operated in the same environment, It was really noticable how the CRC'd blades had held up.
For the R44 we use a rag taped to a broom and run it along the leading edges with similar results. :ok:

[Al Smith]

Hihover, at thew time we were well below max weight, had been flying for an hour and were doing some hover practice. The problem did not become apparent for about a minute or two, why should we be able to hover with no problem for the first thirty seconds or so and then experience a need for more power? Surely cellular absorption would have occured immediately? Not trying to be pedantic but it seems that it would take a while for the pollen to build up on the blades and hence the need for power would increase as the amount of pollen attracted to the blades increased.

Al.

[hihover]

Al,

You are quite right, Cellular Absorption would be present the moment you came to a hover over the long grass.

I've never heard of pollen making such a significant change.

I don't doubt what you say, I just find it hard to imagine such a build up of pollen that would alter the aerodynamics of a blade so quickly. I would expect such a build up of any foreign matter to be apparent elsewhere and noticeable, eg. on the perspex or fuselage, visible recirculation.

I wonder if bees have the same problem.

[Robbo Jock]

I've certainly seen bees going home on foot because their pollen sacks were too full. Not sure if static had anything to do with it though!

[Al Smith]

OK so you think the idea of pollen on the blades is a joke? surely its just the same as icing on a conventional wing?

[quichemech]

No, as I said earlier. The rapeseed field. One pilot had to get out and walk out, there wasn't enough power to keep them both up. Blades cleaned, problem sorted.

[cyclicmicky]

Had the same problem after flying over rape fields...the pollen was well and truly blathered all over the leading edges of the rotors. Cleaned them off...loads of power again.

[hihover]

Maybe its a small helicopter thing, the combination of tiny blades and lack of power may well require better selection of areas over which to hover.

Good point Al.

Why would anyone hover in a rapeseed field??

[Al Smith]

Thanks guys, problem solved, it does cause problems.

[[email protected]]

Hi hover - I would love to hear your explanation of cellular absorption - I have heard the term before and the explanation offered then was complete bull. I seem to remember it started as a joke at Middle Wallop and then everyone started quoting it seriously as a reason for increased power required hovering over long grass.
Pollen and salt both accumulate on aerodynamic surfaces and reduce their efficiency - keep 'em clean.

[hihover]

Hi Crab how are you pal?

Cellular Absorption - I have not a clue as to the origin of the title, I first heard of it about 30 years ago. So if it was a joke it seems to have developed into an urban myth.

It is nothing more than the inability to build a ground cushion due to the quality of the surface beneath the helicopter. A rocky surface will do it too and I am sure many more, including long grass. The helicopter will require HOGE power, even in a 5 ft hover.

That's as technical as my explanations get.

[[email protected]]

Tam is that you? How is the business going?

Nick Lappos will certainly not be happy with your use of the phrase 'building a ground cushion' as it is one of the urban myths he has debunked on these very pages.

The phrase cellular absorption implies the grass cells absorb the energy from the moving downwash - this would slow the air and produce a higher pressure under the disc - according to the CFS teaching this would work to improve the ground cushion.

Personally I think that long grass gives the beginnings of recirculation but that's as technical as I get on it too.

[hihover]

Yep its me matey, doing fine thanks.

"Building a ground cushion" - I can't think of an easier way to graphically describe something that increases or decreases based on hover height.

As I said, I don't know the origin of the term "cellular absorption" however, I would say that I do not consider it applicable only to long grass. I guess it's a more succinct term than "lack of ground cushion due to surface over which you hover".

I wouldn't like to argue the toss but recirculation says to me more power required due to accelerated air coming back through the disk for a second time, third time etc. I do not see this as the case in cellular absorption, I see only that the lack of a ground cushion means the requirement for OGE power.

I like the simpler explanations nowadays, short words, pictures where possible.

[crispy69]

Pollen will deffinatly effect lift produced.
I have been in 300 where blades have got coated in a layer of pollen and she has been using a bit of power to hover.
By simply cleaning them and with out changing the weight I have gone back out and used 1 to 2 inches less.
Long grass will effect your cushion but it will not suddenly make it dissapear after hovering in the same place for 20 minutes. If you look at the pollen on the leading edge when it builds up it has about the same dimentions as a good hoar frost.
Anything rough on your leading edge will effect lift thereby increasing the power needed.