Let's talk about ground effect...

[JaredYng]

I think we can all agree that nobody really has it figured out exactly but I have a question regarding my knowledge about it.

My understanding is that we get increased performance because of reduced induced flow. We can go into what causes that and a "cushion" as some people call if you want but for now I'll spare you.

So assuming its due to a reduction induced flow, someone explain to me why "tall" grass decreases ground effect.

I would think that it would further reduce induce flow thus increasing ground effect. The only think I can think of as to how it decreases it is lets say you are hovering over 3 ft. tall grass. You're new ground is the top of the grass because theoretically you don't know how tall it is if you're hovering above it. This results in you being 3 ft. higher than the ground thus reducing ground effect.

Is this the case or am I way off? Please enlighten me!

[jayteeto]

I don't think that we all agree that!! Tall grass 'leans' outwards and can start a small amount of recirculation.

[A.Agincourt]

The only relevance of tall grass in this scenario is that it disrupts formation of the pressure 'bubble'.

In ZERO wind and a perfect hover - the down-wash of the rotor creates a slightly higher area of pressure beneath the disc than the surrounding environment. The value of this slight increase in pressure should be seen in comparison to a single rotor blade at the specific point in its orbit that it achieves its design maximum potential performance. At that point the inflow angle reduces by comparison to that which would be present in an OGE hover. The slightly higher pressure causes an off-set whereas in an OGE hover, the pressure above and below the disc is similar. Reduced inflow angle results in .............................I'll leave the rest to you. However, regarding long grass as opposed to short grass or concrete [similar hard flat surface] the long grass with its many tendril facets, disrupts the build of the pressure area by attenuation and therefore reduces its dimension in ration to length of grass. In effect, long grass in this instance reduces the effective maximum height that this effect extends to. It is still there, but lower. On windy days, the effect is also diminished but 'teased' out downwind and assuming you hover into wind, over long grass, you will see the effects diameter. The tall grass absorbs/deflects because it is not rigid.

:mad:

Best Wishes

[212man]

The only relevance of tall grass in this scenario is that it disrupts formation of the pressure 'bubble'

Didn't take long for the magic 'bubble' to appear:ok:

It's very true that the movement of the grass absorbs energy, thereby increasing the power required. The moving, uneven, surface also disrupts the tip vortices, which increases the drag.

If you have very tall grass, and you are down low in it - effectively sitting inside a tube - then Jateeto's explanation could come into effect too.

[paco]

The grass reduces the reduction in induced flow.

phil

[Pilot DAR]

The grass reduces the reduction in induced flow

So, that means it's getting less worse more slowly?

Though the pure theory slightly escapes me, my practical experience seems to have been that while hovering over tall grass, or other uneven surfaces, turbulence is created in the downwash. The additional control inputs required to maintain a precise hover, probably consume whatever benefit in lift which might have resulted from the ground effect.

I stand to be corrected by better informed pilots...

[Um... lifting...]

I thought the tips of the grass poked holes in the bubble and all the Bernoullis leaked out, making the bubble squishier.

[RVDT]

someone explain to me why "tall" grass decreases ground effect. Sez who?

If measured from the impermeable surface what is the exact difference and how was it measured?

Answers on the back of a postage stamp please.

[lelebebbel]

it is measured by trying to hover an R22 at max GW over a field in the summer.
The difference is that the MAP is off the chart and the passenger has to get out when the grass is high, vs. the machine hovering just fine one week later after the grass has been cut.

[rotorfossil]

Another explanation amongst many. Ground effect is a balance between the reduced induced flow (good news) and the increased tip vortices (bad news). Long grass etc inhibits the outflow such that it is more easily swept up into the tip vortices which then affect a greater proportion of the blades.

[ab33t]

Moving surface causes disruption in induced flow

[Nf stable]

....I'm with rotorfossil.:ok:

[Flyting]

...or you could just read your basic Aero D notes from when you did your PPL

:sad:

[ShyTorque]

But what about those of us who have never held a PPL? :oh:

[Shawn Coyle]

Used to demonstrate this quite effectively by hovering over a concrete runway and then slowly edging over a sparsely grassed area at same collective setting. Always resulted in a settling towards the ground.

[Bertie Thruster]

is this tall grass?

http://i70.photobucket.com/albums/i97/nmhsu/inwego/3faaaade.jpg

[212man]

It's definitely ground effect!

[Whirlygig]

is this tall grass?
yes, and look how it affected your hover capabilities.

Cheers

Whirls

[sycamore]

Quick,call the RAC, or perhaps the Green one !

[Bertie Thruster]

I don't think this tall plant is grass?

http://i70.photobucket.com/albums/i97/nmhsu/inwego/598a1757.jpg


...............Ahhh. Notar.

[Bertie Thruster]

This one is definitely a grass;

http://i70.photobucket.com/albums/i97/nmhsu/A614.jpg

[Flyting]

http://www.ecofriendlymag.com/wp-content/plugins/wp-o-matic/cache/21848_legalize-marijuana-california-environment.jpg

what effect will this have on your hover performance...

[Bertie Thruster]

.........had an effect on this man....................

http://i70.photobucket.com/albums/i97/nmhsu/WHEREAREYOUSHORTY.jpg

[RVDT]

At last! A use for the Notar technology..........................taken a while tho'.

[Bertie Thruster]

Have you tried it, RVDT? It's a breath of fresh air!

[ShyTorque]

Bertie, if that's the landing I think it was, just south of the golf course, the farmer was very impressed that you managed to land the skids in his tractor's wheel tracks, avoiding damage to his crop. ;)

[RVDT]

BT,

Flown them and fixed them (Notar). First in 1997. Sorry, still don't get it.

[topendtorque]

But what about those of us who have never held a PPL? :oh:

Well, That is possibly where you missed out old son, all that frollicking around in the long grass?

Bertie's two posts: that looks like a weed to me, but I should know it, (brainflash; is it the oilseed that we call Rape out here?) Bertie's other post is a cereal (perhaps maize) but;

Flyting, the little men with their large overcoats, bulging pockets and mono eyebrow will be around to see you shortly:mad:
They will wish to speak quietly with you in a dark corner.

[Whirlygig]

is it the oilseed that we call Rape out here?Yes ... and we call it rape here too!

Cheers

Whirls