Which height is right?

[SPS]

An interesting (to me, anyhow!) conundrum
has arisen lately. My CFI is to blame.

There was I,happily giving a student the
benefit of some of the books I have read
(the ones that generally agree with each other, and I with them).

"If descending over level ground, ground effect begins to develop from a DISC height equal to one rotor diameter" said I (thinking I had nailed this one long ago).

"Rubbish" said CFI, overhearing me.

"Erm...."I wavered.

"When we're landing on a ridge site, a pinnacle or flying in over tree tops going into a confined area we are not the slightest bit interested in disc height."

"We are VERY interested in SKID height and that's what matters".

My wheels could be heard turning by CFI and student alike......

I deflected.....

".....If descending, ground effect begins to develop from a SKID height equal to 2/3rds of the rotor diameter"......

Silence from CFI, grin from me facing whiteboard.

HA! Phew!

That's sorted....

NOPE.......!

It troubled me all evening. One of the reasons I decided to go with DISC height
was that distance from skid bottom to disc
varies so much and unless the statement
is made type specific (a good point made in the CC thread)it is incorrect.

You cannot even stick to Robinson as the R44
has a much greater skid to disc height than
a 22 (and what of the Mariner?!) I know that the R44 has a greater disc diameter so that rule of thumb should work, but is it good enough for all types? What about a Brantly?

I agree with CFI that disc height is not as relevant. Skid height is what really counts, but with varying skid/disc distances and the need for a disc diameter reference to start with, how can the onset of GE be correctly quoted?

Has anyone got a neat little formula that uses skid height against disc diameter whilst allowing for differing disc/skid distances that is not type specific?
(It would have to suit the tallest to the
Brantly!)

And if anyone throws in varying amounts of induced flow and blade numbers as further variables I shall get all confused!

I'll admit defeat and revert to what I must have said at some point in the past...

"If descending, ground effect begins from somewhere up there......"

Regards

SPS

[212man]

Well, I always understood (various books quote it)OGE to start at about 1.5 times the rotor diameter above the ground, but the curve of hover power required vs height up till then is not linear.

Using this with the 212 and its 48 ft rotor, gives you a OGE height of 72 ft. The graphs quote 60 ft skid height and funny old thing, it's 12 ft from skid to mast top. So I guess that is a reasonable guideline.

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Another day in paradise

[lmlanphere]

sorry, you're right- I needed to check my facts.

[This message has been edited by lmlanphere (edited 19 December 2000).]

[This message has been edited by lmlanphere (edited 19 December 2000).]

[212man]

Er, I think you might be mistaken actually.

My R22 study guide (don't have access to the FM anymore) has the rotor diameter as 25 ft 2 in. It also quotes the height from groundline to rotor hub as 8 ft 11 in. By my calculations that put the 3 ft skid height at just about half the rotor diameter, which would be IN ground effect still. (you would guess this intuitively anyway).

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Another day in paradise

[ShyTorque]

Best thing is to change to a proper helicopter that doesn't rely on it to get airborne in the first place.

[sparecrew]

Since it is the rotor disc that produces the downwash that is retarded by ground friction that increases pressure under the disc that gives you a reduction in induced flow through the disc that gives you more power the lower you hover then it is the height of the disc above the ground that determines the amount of ground effect!
The skids just happen to be the nearest part of the helicopter to the ground and you can't go lower than skids on - in that respect skid height is important. Pilots are better at judging skid height above ground than disc height above ground and in aircraft with a Rad Alt it is the belly of the aircraft height above ground that is indicated.
2/3 of the rotor diameter as a rad alt reading is the generally accepted taeching in the British Military and according to the Robinson figures given above would give a maximum hover height of about 8 feet to retain some useful benefit from ground effect.

[HOGE]

Formula for calculating ground effect:
Widening of Eyes inversely proportional to constricting of Ars*hole times speed of collective being raised.

Especially useful for landing on nil wind days when hot and heavy!!

[212man]

HOGE, I'd concur! Funny how the OGE graphs don't always seem to reflect reality at crucial times, or is that the manifests don't reflect reality. Same result whatever.

Just a quick point re OGE hovers; remember you are invariably nicely entrenched in the dead man's curve, so you need a pretty good reason to be there (that includes twins).

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Another day in paradise

[HOGE]

212man, I need a pretty good reason to be in a helicopter at the best of times!

[212man]

SPS, just curious as to what ground effect your CFI was expecting to encounter in the situations he described (pinacles etc)? Bugger all, I'd have thought, which nullifies any argument about skids or discs.

I've just come accross a bit in FAR-29 (certification of large rotorcraft) which says that OGE is considered to be a minimum of 1.5 times rotor diameter from rotor to ground, so there we go.

PS. Is this the only forum Lu hasn't had something to say on?

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Another day in paradise

[This message has been edited by 212man (edited 20 December 2000).]

[SPS]

Like the formula for GE (eyeballs) etc.!!

Just perfect for me in one of those almost no wind autos to the ground! (No buttons left on seat in eariler times!)

ShyTorque- OK, I get another ribbing over 22's and take it in good spirit but I really was trying to discover (or make up between us) a formula for ALL helis, proper or otherwise!

212 - Well, to be fair I may have over generalised to get the idea over quickly.
It wasn't so much the landing site or the size of it that was the point but to bring out the significance of skid height in preference to disc height.

I agree that skid height is more relevant to a Pilot and this will be changed in my teachings and my book. Not 'because', but out of a desire to give the best and most accurate tuition I possibly can, for my own
satisfaction and future pilot safety.

I also think there must be some GE wherever you land, (depending on the surface, of course) although I agree that it could be minimal and uneven (which brings its own set of problems).

Its one of those times when you have to teach something specific so a student can answer questions in exams and quite another when you are out in the hills. I come up against that every day in one form or another.

I wonder if this is why I seldom meet another Lecturer/Instructor, the contrast being a problem? I've met many of both but
I can't remember the last person I met that did the two jobs at the same time.

The two threads, this one and the 'CC' underline some of the difficulty that arises
when an exam has questions that are not type specific - And now I am revealing my train of thought - That it just isn't good enough, too much generalistion by someone who may not
even be a Heli pilot can and does cause lots of ambiguety.

Enough of my ranting ! Thanks all.

BFN SPS

[ShyTorque]

SPS,

RAF CFS always used to quote 2/3rds rotor span.

Long blades are always better. Very noticeable on the whirlwind, not so much on many more modern types.

I too can't imagine what your CFI meant about ground effect over ridges, pinnacles or trees. Ground effect in those circumstances is negligible.

Oops, spelling.

[This message has been edited by ShyTorque (edited 22 December 2000).]

[SPS]

Well it looks like I'll run with a skid height of 2/3 rotor diameter from now on.

Some of the landing areas we have here are quite small but enough to give some GE.
At least that formila will be generally more accurate than the disc height/ one rotor diameter version. 2/3 works well for the R22, 25'2" giving GE beginning approx 16 ft skid height over a hard level surface in no wind conditions.

They are also at 6-8000ft and that makes us pretty interested in any bonus we can get!

Thanks all for the input.

[teeteringhead]

Skid height has got to be irrelevant to the GE question (although v, relevant to other landing considerations).
Consider:
Same helo with alternative skid gear, eg Bell 206 with hi- or lo- skids. Same engine, rotor, airframe and drive train. Changing the kids ain't gonna change the DISC HEIGHT (or eyebeall height)at which GE becomes useful, 'cos the rest of the helo (to be anthropomorphic) doesn't know what skids you're wearing today!

[This message has been edited by teeteringhead (edited 27 December 2000).]

[A109]

I agree with what appears to be the final consensus of the thread. The disk is the only thing that has any relevance to ground effect although obviously if your skids were 20 feet high themselves then you would have to hover oge to land.

[4dogs]

Folks,

This always happens when you try to simplify a complex subject.

Power required to hover at a particular weight, altitude and temperature is a function of the height of the rotor disc above the surface. The typical graphs shown in Bramwell, Prouty and other learned texts are usually non-dimensionalised (I will come back to that) but they show you the relationship.

Practicality (ease of teaching and relative ease of measurement) means that we use a proxy for disc height.

The proxy, skid/wheel height, is not even what we usually use in practice: most of us actually hover at an eye height that we learn will keep the skids/wheels safely away from the ground. Nonetheless, we talk about skid/wheel height because it has the most tangible outcomes.

In reality, experience tells us that the optimum skid/wheel height varies between machines as a function of rotor configuration, installed power, etc. The typical non-dimensionalised graph in a textbook does not show the effects of changing those variables. Often these variations only becomes evident when operating at maximum weight, but are the crux of operating a particular machine safely by maximising the available power margin to take-off or terminate.

The onset of ground effect commences at a greater height with greater weight due to the more energetic downwash. This can be demonstrated with machines that reasonable weight ranges. However, the demonstrations are static and can be misleading, simply because when you need the assistance of GE you are usually in a dynamic situation - accelerating or decelerating. The very weak GE at 2/3 RD is of little practical value, particularly when you are heavy.

It is the power-height relationship that tells you the important bit - because it is a square function, you get the most benefit closest to the ground. If you cannot get close to the ground (essentially level, unobstructed and of about 2xRD radius) then do not plan on assistance from GE: there are just too many thing in the real world that steal it from you when you most need it.

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Stay Alive,

[email protected]

[sparecrew]

Strangely enough 4 dogs - that is what I posted earlier only without the bull...

[4dogs]

Sparecrew,

Please forgive me for failing to recognise your incisive, no-bull**** contribution as the final word on the subject. I happily admit to being misled by the continuing debate.

I apologise for misappropriating some of your oxygen.

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Stay Alive,

[email protected]

[SPS]

I agree that disc height is relevant, I was taught that way and have used it until now.

I also agree that Pilots actually judge height (above the ground) by 'eye height'.

I further agree that if we are to teach eveything else (hovering, level skids before engine off etc.) by skid height then it would be sensible to have a common 'currency'
for the student and use skid height as a guide for the development of GE.

I agree with everyone...?!!

As has been said, this happens every time someone tries to make a simple rule.

Maybe I'll go with the alternative I outlined earlier....

"GE begins from a skid/eye/disc height
of somewhere up there...!"