Ground Effect

[Corporate Yank]

While hovering around the coffee pot the other day, several of our local drivers got into a rather spirited discussion regarding "diminished ground effect when hovering over tall grass or water". Anybody care to offer opinions?--CY

[Skycop]

RAF CFS teaches that the downwash causes a "bowl" shape in grass resulting in increased recirculation. This increases the induced airflow and therefore reduces angle of attack for a given blade pitch angle (similar for water surface). Also, the taller the grass then the higher the aircraft actually is from the true surface (I've seen 8' grass!) and there's less ground effect to be had anyway. The more practical problems are that if you hover too low over long grass then you are likely to "ding" a tail rotor blade. If too low over water the practical problem is self-induced IMC due to spray. The best answer is not to hover over either if the aircraft is so power limited as to be badly affected!

[matador]

OK guys, since I can see Nick is back with us and Lu might be pretty soon I would like the gurus to enlight us about this matter.
I've read all thetheory and previous discussions but one thing I would like to know is if there are different values for G.E. depending on tipe of rotors, I mean if it's the same for a single main rotor ( 212 and so on ) two intermeshing rotors tilted in some angle ( KMAX ) or horizontally suited ( CHINOOK ) or one on top of the other ( KAMOV ), or even in downwash values, how does that affect?
My curiosity comes out to understand the effect of such rotor distribution on fire fighting jobs.
Ready to learn as much as posible.
Buen vuelo

[NickLappos]

matador,
I am not sure the rotor geometry causes changes in ground effect, since ground effect is almost purely the proximity of the blade to the ground, regardless of the type of rotor the blade is attached to. Than being said, there are big differences to how various rotor configurations handle in the hover, not due to ground effect, but to the hover itself.

Off the top of my head:

1) Downwash drag, or vertical drag - When the downwash hits the machine itself, it spends energy and also causes a down force that must be added to the total rotor lift. The vertical drag directly robs payload, and is different for different geometries. Tilt rotors and winged helos have the most vertical drag, with those big flat areas under the rotors. Most helos have 3 to 5% vertical drag penalties (a 10,000 lb machine spends 500 pounds) but a winged helo or tilt rotor often spends 10%.

A Chinook has higher vertical drag loses than a single rotor helo, mostly because the broad fuselage is under the tips of both rotors, where the downwash is highest, so the drag is more fierce. Tilt rotors are also burdened this way.

The ideal low-vertical drag machine has its fuselage tucked under the hub, and no broad parts near the rotor tips (look at the round, thin tail cones of the Mil products to see how they were influenced by minimizing vertical drag).

2) Rotor interference - When one rotor dumps its flow into another, the turbulent air is a problem for the lower rotor, and robs power/lift. A Chinook might lose 5% lift that way (enough to almost wipe out its gain due to having no tail rotor to rob power.) That is the reason for the big step-up of the aft rotor on a Chinook - in forward flight, the rotor wakes are seperated so the forward disk downwash passes under the wake of the aft rotor, and they never actually meet. A synchropter or coax might lose 10% power in a hover because almost all the lower rotor operates in air that was disturbed by the upper rotor.

3) tail rotor loses - Having no tail rotor is better for power, since the tail rotor eats about 5% to 7% of the total power.

[TheFlyingSquirrel]

On the subject of TR's - what are biggest influences of either port or starboard positioning of the TR in heli design? ( Whether US of French design )

[NickLappos]

TFS,

The two biggest effects of TR position are:

Pusher or tractor - If the rotor blows high speed air on the fin (tractor), it wastes a bit more thrust (eats more power).

Spins with the downwash (bottom toward the aircraft) or against it (spins bottom away). The bottom away works against the natural downwash flow, and has more thrust, and less chance of LTE.

[Matthew Parsons]

matador,

What Nick said (it depends) is bang on and unfortunately the only right answer. Fortunately, what matters is not what the theory predicts but how the helicopter actually performs. Even more fortunate is that the effects of downwash are quite simple to measure, and may already have been for each type you're considering.

Something to consider, above two rotor diameters, ground effect doesn't alter the performance much. Near fires, wind patterns and local air temperatures would probably have a much more significant impact on performance than ground effect would.

This is purely conjecture, but if we were able to eliminate the fuselage effects, then I'd expect that the tandem configuration would have the greatest ground effect advantage while the intermeshing would have the least. My thinking on this is based on the direction of the downwash. With a zero wind condition it's (theoretically) completley vertical for a tandem, tilted slightly laterally for a conventional tail rotor due to the lateral cyclic required to counter the tail rotor side force, and showing relatively tremendous horizontal velocities for the intermeshing configuration due to the fixed lateral tilt of each rotor.

To be fair, that impossible comparison is looking at disk height, and due to the forward flight requirement of the aft pylon to be higher than the forward pylon in a tandem, the disk height of the tandem will typically be higher than that of others.

In the end it's far too complicated to deduce and fairly straight forward to measure.

[Dave_Jackson]

[Edit] Post revised and moved to seperate thread.

[TheFlyingSquirrel]

So Nick, the S92 and the Blackhawk - was there any specific reasoning for putting the TR on the Starboard side ?

[NickLappos]

TFS,

The TR on Hawks was designed to allow fast fold and packaging into a C-130, so the absolute best aerodynamic solution gave way to the best mechanical solution. That being said, the Hawk family has a very powerful tail rotor. I have demonstrated 125 Km/Hr on the doppler in either direction!

Dave, your helo looks most impressive! I wish you all the best of luck!

[slowrotor]

Matador,
Is your question about ground effect efficiency?
Or are you asking about downwash problems near or above a fire?

[matador]

Actually I was thinking about both, I'd like to find out how much groung effect (if any) could be considered for any help lifting up the load of water and afterwards the effect of the downwash during the drop, all that to find out best distance to hover while picking up the water (you know, belly hooking, ventral tanks, long/short lines ...) and best height for the drop not feeding the flames with more oxigen.
Now I'm going to need some time to undrestand all that has been said so far
Thanks a lot and buen vuelo

[JerryG]

Although I've never been fire-fighting I have been amazed at just how far down the downwash will travel in an almost perfect column if you stay in the hover for any length of time in light winds.

At 400 feet in the hover (A109 for a film job - probably not a very clever idea in the first place!) I remember seeing a tight pool of downwash in the corn off to one side. It was so clearly defined that it was almost as if there was an invisible helicopter hovering there at ten feet.

I'd assumed up until then that the effect would be largely disipated over that distance but, man, those are powerful hair dryers (and fire fanners) we drive!

Cheers
JerryG

[slowrotor]

Fire Fighting helo

Just my opinion after thinking about the downdraft from a helicopter to some detail.
The heavier the helo the more downdraft. The trend in helo design is toward higher disc loading for transport of people. For fire fighting perhaps a lighter disc loading would be appropriate. The K-max is probably designed with low disc loading and fire fighting in mind.
Also the intermeshing rotor downwash may not be exactly straight down. I would be interested to know more about the K-max downwash.
Helos make good crop dusters because the downwash helps blow under the leaves. Does the downwash help or hurt over a fire? I dont know.

I was burning yesterday, a big fire clearing land. I noticed a little rain actually made the fire burn better. My theory is a little rain helps strip the ash so the fire burns better. To put the fire out is not easy and required large gulps of water, not mist.

[Dave_Jackson]

A fire draws fresh air in from the sides and 'discharges' the spent air upward. A low velocity downwash from a helicopter would oppose this action. Perhaps it helps keep the de-oxygenated air at the point of the fire.

Just a guess.

[Matthew Parsons]

A video went around a while ago of a helicopter fire fighting display. The pilot had difficulty putting it out, but the video clearly showed the downwash making things worse.

That's just one study. I agree that what Dave said is possible. I've never done any fire fighting, but have spoken briefly to someone who teaches it and there's definately a lot to know about fires, fuels and techniques.

As far as disk loading, if a higher downwash velocity is a problem and transit speeds aren't a concern go with low disk loading. It won't change the total energy in the air (that's mass dependant only), but it will slow it down.

[helmet fire]

matador,
I think you have asked two very seperate questions here, and the answer to one is irrelevant to the other.
Firstly you ask about rotor designs and the resultant downwash issues. To oversimplify the issue, higher disk loading, higher downwash (or more intense) and I think this is the thrust of what you are really trying to ask. But as Nick and others have said, other factors are at play, and all design is aimed at finding the best compromise to achieve the desired mission profiles, thus the question you actually asked is extremely complex.

As to fire fighting and downwash, I believe it is largely but not totally, a pilot issue. Firstly, IMHO, to oversimplify my first oversimplification about disk loading, the heavier the machine, the higher the disk loading and thus more intense the downwash (exceptions here are plentiful, like Kmax, and machines like Bell 205/212 has 17% slower downwash than BK117 even though it is heavier). BUT, the heavier the aircraft, the less they need to be hovering over a fire, thus if the pilot is on the ball, downwash over and active fire is not really an issue.

Where downwash does become an issue over the fire is where pilots do not understand optimal drop profiles for the machines they are flying. Because most of us learnt to fight fires on smaller machines, it is hard to break the small machine required habit of coming to a hover or very low speeds before you drop when you move up to the medium and heavy machines. Hover drops in mediums/heavies are GENERALLY a waste of water unless doing mop up (in which case it is likely to be a waste of money!).

Each peice of dropping equipment has it's own profiles dependant upon foam densities, foliage penetration requirements, and fuel loadings. Thus a bambi bucket has different profiles than a belly tank, and Isolair will have different profiles than Simplex. It may suprise some of the fire pilots out there, but medium bambi bucket machines (say 1000L to 3000L ) and above generally have an optimal drop profile at around about 30kts/30ft! (That's 30 ft bucket hieght). Even in dense foliage the speed recommendation is not below 15kts, and in the grasslands, it may be as high as 45kts. But time and time again, you see mediums come to the hover or less than 15 kts at a hundred plus feet to drop. At the recommended speeds, downwash is not a significant issue on the fire, the issue only really arises when you have the incorrect techniques and excessively low speed drops from larger aircraft. So you can see that for dropping on active fires, downwash is a pilot issue rather than an aircraft design issue.

Mop up, winching, and water pick up are all hover required (or low speed required) fire tasks, and the design of the aircraft is important here in terms of downwash. The higher the downwash intensity, the more risky each of these manoeuvres becomes.

High downwash during the mop up has the potential to strip fire weakened branches down on to personnel, generate ash to destroy viz, kick up coals and rekindle the fire. Thus low downwash machines are more suitable for this phase of the operation (B206 series and Kmax are ideal - Kamov is silly).

Winching ops is similar: the more intense the downwash, the more risk to people on the ground with debris, srtipping branches, spin, and viz issues. I have also witnessed winch aircraft start a perfect ring of fire around there winch point! But winch aircraft have a compromise here, and thus choice of the machine should be balanced with other considerations such as twin, or single, winch type fitted, crewman station, number of ground crew capable of being lifted, etc. These other considerations rule out machines like Kmax, and reduce the economical effectiveness of machines like the Bell 206 series. I would suggest that downwash becomes the over riding issue when we start to talk Kamov, Chinook, etc.

As for the water, all the machines have to get water, so downwash is just one of those things. Some times in Australia it can be an issue due to the dust around dams, etc, but even this can be overcome with a quick drop around the dam at kick off.

[Shawn Coyle]

Downwash and ground effect are two different things.
You can have downwash hitting the ground, but not affecting the performance (this is the more than about 1.5 rotor diameters part where ground effect is considered to be non-existant).
You can be less than 1.5 rotor above the terrain, but if it's something like trees, you may also have no ground effect.
Ground effect will depend on the disk loading, so for the same helicopter lightly loaded ground effect will disappear at a lower height above the ground than when you are heavily loaded (unless you have variable diameter rotors, something I haven't noticed recently)

[matador]

Fisrt of all I would like to thank you all for your responses, they all hold a nice piece of advice.
Of course G.E and down wash are two differnt aspects of the air blown downwards and also agree 100% with HF on fire fighting issues and technics.
The simple question I was trying to ask (but very difficult to answer in few words ) is:
Supposedly G.E. has some values up to 1.5 rotor diameters, but if you have a tandem rotor, Which rotor diameter do we use to calculate, one of them? or the addition of the two? or some value in between..?? and what if you have one on top of the other??
In downwash values same thing applies. If you have to maintain in the air a 5 tons helicopter, which would have the more downwash ? the one with a main rotor ? or other tipe of rotors (intermeshing, one above the other..??)
Have you heard about that saying ??... A fool is able to ask questions that a wise man will not be able to answer .... or something like that. (Not that I consider myself one of them - fool or wise )
P.D. It all has to do with some russian experienced pilots talking about downwash on KAMOVS that I would like to check.
Buen vuelo

[Dave_Jackson]

matador,

There are no universally agreed upon algorithims for determining the disk area of twin main-rotor configurations.
This web page will provide an overview. The linked page will provide additional information.

Dave J.