From: Nick Lappos <nlapposNOSPAM@miami.gdi.net>
Subject: Re: Definition of "Translational Lift"?
Date: 17 Jul 1999
Newsgroups: rec.aviation.rotorcraft

Chris Morrison wrote:

"Could someone give me some input as to whether my
understanding of the term "Translational
Lift" is correct.

Translational Lift is the increase in rotor lift caused by
relative airflow horizontal to the main rotor
disk. The effects of Translational Lift become noticeable as
the helicopter accelerates through
about 12 to 14 knots and maximize around 50 knots.

Translational Lift is the main reason a helicopter requires
the least amount of power for level flight at speeds of
about 50 knots."

Chris, Your definition is a good working one, but just drop
the 50 knot top end from the discussion.  The change in lift
occurs around 15 knots, and does not ever change back to the
less efficient state.

One way to understand translational lift is to see what the
rotor undergoes at speeds below 15 knots, and what changes.

In a hover, the flow through the rotor is affected by some
inefficient reingestion of the downward air at the tips of
the rotor.  Because the tip flow is captured by the air
entering the rotor from above, it is forced to recircle
upward around the edges of the disk and is reinjected into
the rotor.  This tip loss becomes wasted effort for the
rotor in two ways.

1) The flow of air at the tip does not join the main flow
downward, and so does not impart its momentum to the
aircraft.  This might be pictured as if you were under
water, pulling with your arms to swim back up to the
surface.  Imagine that we encased your arms in boxes that
were attached to your body.  Your arms can swing freely
inside the boxes, but the water in these boxes never
circulates outside them at all.  No matter how hard you
swing your arms, you never rise in the water, all you do is
churn the trapped water.

The main reason why this lost tip flow is a power hog is
that it effectively makes your rotor smaller.  Because the
last few feet of the rotor are loafing, the inner segment
must work harder.  This makes the rotor behave as a if it
had a much smaller diameter, which consumes much more power.


2) the tip air still creates some drag as it passes through
the rotor, and the blades must work to push it.  This
consumes power, and produces no lift, a secondary waste of
power.

When the aircraft is moved forward (or wind is passing
through the rotor as it stands still) the extra energy of
this velocity overcomes the natural attraction (suction is
one way of describing it) that is trying to draw the tip air
upward. At about 15 knots forward flow wins, and the whole
rotor including the tips contributes to the job of lifting
the machine.  This tip flow is now no longer recirculated,
and we describe it as translational flow.  The gain in rotor
efficiency is seen as more lift for the same power, and so
is seen as translational lift.

One way to kill tip loss is to encase the rotor in a shrowd
or ring (remember the ring tail?)  These block the tip
losses, and increase the efficiency of the rotor.  A
fenestron fan shroud does this as well.  A main rotor shroud
would be a monster weight and drag penalty, and so is
impractical.

The same tip loss effects work on a propeller, which is a
rotor working in hover mode all the time.  Shrouded props
are more efficient.  In fact, almost all airliners are
propellor driven now (ask yourself what a "high bypass
fanjet" really is!), with the props shrouded in large,
efficient ducts to help kill the tip losses.

The hover tip losses are magnified if we move the rotor
downward in descent as we hover.  At a certain rate of
descent, the tip recirculation begins to be the dominent
rotor flow, and almost no part of the rotor is producing
lift.  This becomes the dreaded "Vortex Ring state" that
consumes lots of power, and makes bad approaches into
smoking holes.  Really, it is not some massive lift loss, it
is really a large increase in the power required at that
condition, which might consume all the excess power you
have, and make it impossible for you to stop the descent at
the bottom. The cure is to drop your nose, and accelerate a
bit to get your speed up and wash away the recirculation.
If you drop collective, you can also clean up the flow, but
down collective makes the "houses bigger", as the fast
movers say, and you might not have much altitude to play
with.  If your helicopter has gobs of power, you can fly
straight up to recover from vortex ring state, in direct
contradiction to what your instructor told you.  ("This is
done by professionals, don't try this at home" as the TV car
ads say while the SUV is jumping the Grand Canyon.)

Sorry, I got carried away agreeing with your answer, Chris!

Nick



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