From: nlapposNOSPAM@miami.gdi.net (Nick Lappos)
Subject: Re: Avoiding Vortex Ring on Approach
Date: 22 Nov 1999
Newsgroups: rec.aviation.rotorcraft

Good stuff snipped, then 180walt said:
>
>        Respect the h/v and crosswind diagrams.
>
>        If you're worried about LTE on approach, try this-  Crab down to losing
>translational.  As you start losing trans lift, bend approach path dead
>into wind.  It's not a straight track, but it's a safe one.  If that
>last change is more than a little, you're doing something wrong.   MOre
>than a little is about 45 deg.


Great advice for avoiding LTE.  I'd also add that you should try to sneak the
machine into a hover.  Any need for a largish collective pull at the bottom
invites LTE, especially if the pull takes you much above hover power.  Since
torque is torque, any main rotor torque squandered in an excessive pull could
swamp your tail rotor's ability to keep things balanced.  All this was learned
the hard way in a overloaded Cobras on some PSP ramps back in the Two Way
Rifle Range.

>        The danger with *vortex rings* is not having room (altitude) to
>recover.  At altitude, it's exciting.  Down low, it bends stuff.  My old
>body won't take a lot of bending.

Vortex ring is the fancy term for the rotor eating its own downwash.  Simply
said, when you descend near vertically at the same speed as your downwash, you
invite the rotor to simply push around the same old air, instead of making
mayhem on fresh stuff. If the rotor eats its own downwash, the net effect is
on the power you must use to prevent rapid descent.  Basically, the rotor
wastes so much power just churning air that there isn't any left over to break
your fall  ( we could describe powered flight as the clever use of fuel and
noise to temporarily prevent falling).  The magic descent rate depends on the
disk loading (pounds of gross weight per square foot of rotor disk).  The
lighter the disk loading (like in typical recip trainers) the easier it is to
find Vortex Ring state in a normal approach.  Typical downwash (induced)
velocities for light recips might be about 10 knots (17 ft per second, 1000 ft
per minute).  If you steeply descend near that rate, you will nibble at Vortex
Ring.

Many cases of falling thru on short final might simply be having insufficient
power to hover OGE, and not full blown Vortex Ring.

Tailwinds on  takeoff are bad news mostly because to go from trimmed -10 knots
to forward flight, you must pass through 0 knots, where power needs are
highest.  If you can barely clear the trees as it is, that sinking feeling at
to 0 knot point can produce real depression, the non-prozac kind.  If you have
lots of power, and know what you're doing, downwinds are not that big a deal.

Nick Lappos
back after all these weeks

From: Nick Lappos <nlapposNOSPAM@miami.gdi.net>
Subject: Re: Definition of "Translational Lift"?
Date: 18 Jul 1999
Newsgroups: rec.aviation.rotorcraft

180 Walt wrote:

"Nick, I've been trying to get at this exact question. Gobs
of power translating into sufficient thrust to...
Break the ring state returning to normal hover flow? Another
flow mode?

Seems to non theoretician that "settled with power" mishaps
mostly a problem in marginal power a/c- Because they're
already pulling max avail and attempting more leads to loss
of turns which leads to the interview with the accident
board. Yes, I understand normal helos with "adequate power"
do this also.

In theoretical-land (That place engineers live) could I say
there's a magic figure that tells me I could go vertical out
of the vortex ring state?"

Walt, you grabbed the inconsistancy exactly. The Theory
would say that the power required at the Vortex Ring state
is nearly infinate, but in practice, if you have enough
power, you can increase pitch and reestablish normal flow.
This was experienced by our Skycrane pilots, who had an
aircraft that could hover at over 50,000 pounds, but when
they dropped the load, they only weighed about 22,000
pounds.  Thats like hovering at 44% torque. In that state,
the Crane could be flown up with a collective pull, I am
told.

This is good usenet jaw flapping, but as you correctly
surmise, when in an aircraft with almost no power margin
(read that as one that's earning a living) VR state must be
strongly respected and avoided.

I found VR state in one of our machines doing single engine
landbacks.  With just the right rate of descent and only
about 10 knots forward speed, the aircraft suddenly fell
like a ton of bricks.  I was at 50 feet, coming down at
about 50 feet per second.  Waited till the bottom, and
pulled the collective up to the stops just as we touched
(yea, maybe it was a bit harder than "touched"). It bounced
twice, exceeded the landing gear maximum allowable loads by
a factor of two, and then rolled to a stop.  The aircraft
was flying again in a few days (iron works, you bet).  I
personally thanked the airframe structural designer, Jim
McVicar, who is responsible for eventually getting me to
this keyboard.

Nick

From: Nick Lappos <nlapposNOnlSPAM@miami.gdi.net.invalid>
Subject: Re: v-22 Vortex ring state
Date: 05 May 2000
Newsgroups: rec.aviation.rotorcraft

Vincent,
The overaconfusion about vortex ring state is almost
institutional.  Actual events of VRS are rare, and quite
difficult to define.  Actual cases where insufficient power to
recover from a low speed descent case are common, and many times
confused with VRS.

VRS is restricted to those cases where wake reingestion is
overriding, ie, where induced velocity matches freestream
velocity.  For the V-22, that might be a vertical descent of 90
feet per second (5400 ft/min).  This is quite unlikely in the
accident scenario.

What is more likely is that the power required to perform the
desired maneuver was very much more than the power available.  If
the pilots were very fast, and then initiated a rapid quick stop,
the power needed could be very much more than that needed to
simply hover, and I'll bet there was little power margin above
hover power.  We have all been there, and the collective pull,
with a rotor droop and a sinking feeling is something you
remember for a long time.

Where it can be even more of a problem in the V-22 is that those
little rotors are very highly loaded relative to stall.  The load
factor that a loaded V-22 can pull in helo mode is probably only
about 1.5 G's. A hairy flare that builds load factor could
actually raise the power required to even higher amounts, due to
the very high power consumed at stall.

Don Hollenbaugh's concern about control loss, especially pitch
down, could be a product of one rotor stalling and one not,
perhaps due to roll control inputs.  Remember that roll on a tilt
rotor is controlled by increasing the collective pitch on one
rotor, and decreasing it on the other.  Increasing collective
could deepen the stall.  A roll-tuck could be the result of such
assymetric problems.  I know of at least one tilt rotor manned
simulation exercise that showed "roll reversal" due to this stall
problem.

In a nutshell, I think it unlikely that vortex ring state is to
blame, but that power limitations or stall or both (as rotors are
drooped at power limits, stall can quickly develop) can possibly
explain the problem.

A possible scenario:  The pilots are hot at the bottom of the
approach, flare sharply, and fall through as they settle to a
hover.  The rotors droop with the power pull and the pilot
corrects some roll. The roll control request causes an increase
in the stall on one rotor, and the aircraft rolls and tucks
sharply.


Nick