Which height is right?
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