killfile wrote:

"James Cho" wrote in message
om...

Why aren't they more common? It seems like the advantages of them
compared to single main rotor + tailrotor helos are pretty
significant, not great enough to replace traditional designs entirely
but at least sufficient to be more popular than they are now.


It's the classic trade off. Co-Ax helicopters are more stable and don't have
a vunerable tail rotor, but they aren't as manouverable, and require a
vunerable and complex rotor linkage. Manouverability was what saved a lot of
helicopters in Vietnam, so I doubt we're going to be seeing a glut of co-ax
machines.

Matt




A while ago this same subject came up.

I'll copy what I wrote then to this message:

World Air Power Journal vol 31, 1997 includes an article on the Mi-28 in
which a Russian test pilot by the name of Vladimir Yudin is quoted.
According to the article has flown both the Ka-50 and the Mi-28. A
General P. Bazanov also criticises the Ka-50 and a comparison is made
between the Ka-50 and the AH-64.
I'll quote parts of the article:

begin quote
"Coaxial-layout helicopters are great flying cranes, but I wouldn't
dream of going to war in such a helicopter, even the most modern one"
said Yudin.
One reason is that certain horizontal/vertical speed combinations are
unfavourable for this layout, e.g. , descent speeds from 3-4 to 9-10 m/s
and airspeeds around 30 km/h lie within the ring of turbulence zone.

Investigation of the crash of the first prototype Ka-50 ('White 01')
which encountered severe turbulence and crashed out of control, killing
the pilot) showed that critical speed increases as g loads grow.

....

At 2g critical speed is about 70-80 km/h, i.e. a helicopter's speed over
the battle area. In other words, some suggest that coaxial helicopters
are dangerous to fly in combat at 0-80 km/h. They also have directional
control problems at descent speeds of 5 m/s and higher and airpeeds up
to 60 km/h, which are also in the range of a combat helicopter in
action.
....
Coaxial helicopters have been under development in the USSR for the same
50-year period as conventional ones. Why are there so few then and why
have they never been used in real combat? The reason is that while a
conventional helicopter can land successfully after sustaining battle
damage to the main rotor, this is completely impossible for a coaxial
helicopter because a danaged blade can flap 1.5 m up and down and blade
collision is imminent.
General P. Bazanov, who often chaired state commisions during state
acceptance trials of new military aircraft is convinced that the AH-64
would emerge as the winner in a dogfight with the Ka50 because the
Apache can perform complex maneuvres in the vertical plane which the
'Black Shark' cannot.
....
The Ka-50's aerobatic displays at air shows certainly look impressive to
the public. However, General Bazanov points out that the Ka-50 makes
sharp turns only in a sharp climb , and then only left turns because a
right turn would very probably lead to blade collision. Conversly
Russian specialists judge the maneuvres demonstrated by the Apache as
usable in combat.
....
end quotes

Some more problems concerning coaxial rotors are mentioned like high
rotor loading and downwash and difficulties in protecting the rotor
system against wire-strikes.



Regards,
Ralph Savelsberg