Ground effect of the KA-50

[Mud]

Hey all,

 

I notice a huge difference in the effect of ground effect between the UH-1 and the KA-50 and I was wondering if anyone here can explain as to why that is.

 

With a steady low sink rate, the Huey will not touch down due to ground effect, but the Black Shark appears to keep its sink rate and will touch down.

 

Their rotor diameter is more or less the same.

 

My apologies if this has been discussed before. I couldn't find anything by searching.

 

Mud

[chromium]

Different weight, different rotor configuration, different engine power. Not comparable for the same descent speed..

Is it possibile to stop descent with the ka-50, but you would need a slower descent rate.

[sobek]

Their rotor diameter is more or less the same.

 

But rotor area isn't.

 

Also, the Ka-50 weighs between 2-5 times as much.

[Flagrum]

The engine power should not be a factor here, shouldn't it? It is all about the helo just descending ever so slightly - i.e. the engine has in both cases the power to just keep the helo almost in hover.

 

And the rotor area should be rather a factor for a stronger ground effect - as it is larger for the Ka-50?

 

Of the factors mentioned here, only the weight could probably make a difference in my (layman's eyes): more weight = more inertia that has to be "cushioned".

[Isegrim]

Things that i know have effect in very short

 

-Airframe layout

-Rotorblade layout(shape/angles and AOAs)

-Rotor layout (2blades/3blades)

-Rotor RPM

-Downwash speed

-Downwash area

-Weight

 

-Power> but more or less secondary

[Yurgon]

Good question, Mud!

 

I just did some simple tests: In the Instant Action mission "Shooting Range", I went to a low hover, engaged auto hover, disengaged altitude hold, and gave it just enough collective to get a very gentle 1 m/s climb.

 

Now what I'd expect would be for the chopper to settle when leaving the ground effect, at an altitude roughly equal to the the rotor diameter, i.e. circa 14 meters, or at least for the climb rate to decrease at that altitude -- which is more or less what happened in my first attempt: the chopper almost stopped climbing at circa 14 meters, but then when nearing 30 meters the climb rate almost skyrocketed to above 3 m/s.

 

In the following attempts, this happened again most of the time, but sometimes the chopper would settle at roughly 14 meters and just sit there, neither climbing nor descending.

 

When descending down from >100 meters, I would have expected some kind of cushion effect and a decrease in rate of descent when entering ground effect, but did not notice any such effect.

 

From these rather simple tests I would conclude that ground effect seems to be properly modeled when climbing out of it, but I didn't notice any ground effect while descending into it. And then there's the inexplicable rise in climb rate at roughly double the rotor diameter.

 

For comparison, I tried the same with the Huey (Instant Action mission "Ground Attack Easy"), which, both in a climb and in a decent, would settle (or oscillate) between 7 and 10 ft (actually quite a bit lower than I'd have expected), which seems to be proper ground effect modeling both for climbing out of it and descending into it. I also did not notice an unexpected rise in climb rate in the Huey near double the rotor diameter.

[BitMaster]

Very interesting indeed !

 

I have flown, like a few others in here, many small R/C based coaxial helicopters...and they dont double up the lift at twice the rotor diameter distance. Double or tripple lift is noticable if it appears.

 

Need to check that out tonight with zero wind and 20°C etc... empty and max take off weight.

[sobek]

Double or tripple lift is noticable if it appears.

 

Nobody said that lift did double or tripple.

 

Could be that the ground effect is less pronounced in the dual rotor configuration.

Edited April 11, 2016 by sobek

[Flagrum]

Nobody said that lift did double or tripple.

Yurgon did:

(...) gave it just enough collective to get a very gentle 1 m/s climb.

 

(...) when nearing 30 meters the climb rate almost skyrocketed to above 3 m/s.

[heloguy]

Nobody said that lift did double or tripple.

 

Could be that the ground effect is less pronounced in the dual rotor configuration. As

 

Good call. This would make sense, as it would seem like the upper rotor would cause a greater induced flow on the lower rotor. Induced flow is definitely a factor when considering air flow at a hover. Ground effect is supposed to slow induced flow, which contributes to better efficiency. I would be interested to hear about ED's sources on this for the Ka-50.

[sobek]

Yurgon did

 

That would be the climb rate, the vertical component of the airframe velocity. Lift is a force. :smartass::music_whistling:

[Flagrum]

That would be the climb rate, the vertical component of the airframe velocity. Lift is a force. :smartass::music_whistling:

Well, it is always better to be precise ... touche. But does this distinction help us to solve the riddle?

 

I mean, no parameters changed, all outside forces(?) remained the same. And still, the climb rate increased significantly ... does that not mean, that the lift force "magically" increased? ;o)

[sobek]

does that not mean, that the lift force "magically" increased? ;o)

 

Probably, yes. But saying that, coming from a hover, it trippled you'd be left with a rocket, not a helicopter... ;)

 

 

What would be interesting from the point of testing:

 

What happens when the sink rate is arrested with additional collective? Does it take the additional collective to remain in a hover? When you start climbing, do you suddenly need less collective to remain in a hover?

 

Suppose the sink rate is low enough for touch down without damage, does the helicopter become airborne once again after touching down? Does it remain in a hover?

[BitMaster]

I didnt have an A when i graduated physics but a change from 1m/s to 3m/s is a threefold in power, regardless.

 

Where the increased efficiency comes from is a mystery as no one applied more collective or higher rotor rpm.

 

And yes, i hovered in all configs, heavy and empty, wind and no wind, yes, i had the same miracle up-boost.

 

 

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[ShuRugal]

I didnt have an A when i graduated physics but a change from 1m/s to 3m/s is a threefold in power, regardless.

 

erm, not quite. it's three times the velocity, but that does not mean 3x the power. At loaded weight of 21,000 lbs, the rotors are producing 21,000 lbf of thrust to hover. If thee power output tripled, they would be producing 62,000 lbs, which would accelerate the helicopter upwards at 3G (29.4 m/s/s)

[sobek]

that does not mean 3x the power

 

With a lot of simplification, power is force times distance divided by time.

 

So since for a higher speed, force has to increase, you need even more than 3x the power to achieve 3x the vertical velocity. Of course this is highly theoretical because IRL you need power to stay in a hover. The increase in power needed to start climbing a little is rather small compared to what is already needed to stay in a hover.

 

However, 3 times the force would mean a lot more terminal vertical velocity (which is too complicated to calculate on a coaster, a little out of scope for now), so that would mean a *much* greater increase in power.

 

Still, this is all starting to get pretty OT...

[Mud]

How does one summon Yo-Yo around here?

He'd probably be the right person to explain or check it.

[Bushmanni]

Collective controls vertical force ie. acceleration not vertical speed. You will have very little drag in vertical axis opposing the vertical movement when you are climbing in hover as your climbing speed is very small. So if your climbing speed stabilizes at 3 m/s vs 1 m/s it means very small additional force is responsible for that. You could get that from small breeze higher up if there was even a hint of wind in the test scenario, or small horizontal velocity component if the chopper didn't stay in perfect hover.

[Flagrum]

This almost looks like that the IGE just causes some dampening. Which would probably be just fine when descending, but also is in effect when ascending. And once out of IGE, the dampening is gone and the helo reacts more agile.

[msalama]

but also is in effect when ascending.

 

And what's wrong with that?

[Flagrum]

And what's wrong with that?

My understanding is, IGE means that the rotor creates an air cushion (higher air density) where the rotor blades are more effective. Therefore, the power needed to keep the helo in a hover is less than when hovering out of ground effect.

 

So when I apply just so much power that I sloooowly ascend, once I get out of ground effect, that power should probably not be enough to continue with the same ascend rate (or stop ascending at all). But what was found out in this thread was the opposite: when the helo gets out of ground effect, seemingly the rotor blads, now in thinner air, suddenly become _more_ effective ... and the ascend rate rises.

[msalama]

Right. Got you. Makes sense.

[ShuRugal]

My understanding is, IGE means that the rotor creates an air cushion (higher air density) where the rotor blades are more effective. Therefore, the power needed to keep the helo in a hover is less than when hovering out of ground effect.

 

This is a good simplification, and is the one I always use when explaining to non-aviation-minded folks. A more technically correct explanation is that the ground also interferes with the formation of wingtip vorticies, drastically reducing their size as clearance from the ground decreases, and therefore reducing induced drag.

[Xxx]

Interesting thread!

 

Perhaps a way to test if there is sufficient ground effect in the model, is to see if there is any Vortex Ring State reproducible when in ground effect? With the dampening of the tip vortices, when in ground effect, is the Vortex Ring eliminated?:huh:

 

Whatever the answer, this simulation is certainly a fascinating and addictive one for me. The "art" of flying this ship in combat is truly amazing!

 

I think this is the oldest sim from ED. I take my hat off to them, its a great one!:)

[sobek]

But what was found out in this thread was the opposite: when the helo gets out of ground effect, seemingly the rotor blads, now in thinner air, suddenly become _more_ effective ... and the ascend rate rises.

 

Did you check if the governor interfered in any way with your power setting, i.e. did the EGT change when you transitioned out of ground effect?