Quick Stop (Rapid Deceleration)
This maneuver is used to decelerate from forward flight to a hover. It
is often used to abort takeoffs, stop if something blocks the helicopter
flight path, or simply to terminate an air taxi maneuver.
The maneuver can begin from just about any combination of airspeed and
altitude, but is typically practiced from around 25 feet AGL and 40 knots.
The maneuver can be broken down into 3 parts: the flare, the deceleration,
an the approach to a hover.
To begin the maneuver the pilot flares the helicopter by pitching the
nose up with aft cyclic. The flare would normally cause the helicopter
to gain altitude, but in this case the pilot reduces collective to
prevent a climb from occuring. The power decrease will require right pedal
to be applied as well. The pilot can adjust how quickly the helicopter will
decelerate by how aggresively he flares. The more aggresive the flare, the
faster the helicopter will stop. Too agressive a flare will cause a tach
needle split (the rotor will spin faster than the powerplant), and this
is generally not the way the maneuver is flown, although nothing harmful
will result from this.
The helicopter will decelerate as the pilot holds the flare. Because the
airspeed is decreasing, the flare will immediately start to lose energy,
and the helicopter will try to settle. Instead of allowing the helicopter
to settle, the pilot can increase the flare. This will maintain altitude,
and cause the helicopter to decelerate even faster. There is an upper limit
to how much the pilot should flare the helicopter. The deceleration becomes
so fast it is more difficult to judge, plus the helicopter ends up in an
extremely tail low attitude.
Instead of continueing to pitch the nose up, the pilot can start adding
power to maintain altitude. He does this by raising collective. This not
only maintains altitude, but it maintains rotor thrust and keeps the
helicopter decelerating quickly. The pilot should continue to hold altitude
and attitude until he is about to lose ETL (effective translational lift).
Approach to a Hover
Before ETL is lost, the pilot pitches the nose of
the helicopter back down into an approach attitude using forward cyclic. The
attitude is still slightly nose high, as on a normal approach, but is not
in a flared attitude. The pilot can simply look ahead of the helicopter and
pick a spot aligned with a 10 degree approach angle. He then flys a normal
approach to the spot.
Descending while in the flare
Many pilots actually allow the helicopter to descend during the deceleration
phase of the maneuver. Many instructors teach the maneuver that way. I don't
personally like that method for a couple reasons.
One reason is that I don't think the helicopter should approach the ground
tail low. Although pilots are generally aware of their tail rotor,
occasionally people forget and wipe out the helicopter with a tail rotor
strike. I feel that if the maneuver can be flown in a way to minimize this
hazard, it should be flown that way.
Another reason I don't like descending in the flare is that it means the
helicopter is descending directly into it's downwash as it goes through
ETL. This is a classic way to set up settling with power. Pilots normally
think of high vertical descent rates as causing settling with power, but
a more accurate description is a high velocity aligned with the rotor vortex.
You can settle with power sideways if you try hard enough. If you watch a
pilot performing a quick stop who descends while in the flare, you can see
that he is settling directly into his vortex. Usually the helicopter reaches
ground effect in time to avoid power settling.
My belief is that by maintaining altitude during the desceleration, the
pilot stays above his vortex (similar to an out of ground effect hover)
and eliminates the chance of settling with power, as well as eliminating
the chance of a tail rotor strike. The termination of the maneuver is a
normal approach, so if the helicopter has enough power to fly a normal
approach, it has enough power to terminate this maneuver.
A common mistake is to flare, but not lower enough collective to
compensate for the increased lift. This will cause the helicopter to
balloon, or gain altitude. If the flare is really aggressive, full
down collective will not be enough to prevent altitude gain. Altitude gain
is not desirable because it puts you deeper in the HV curve.
Incorrect anti-torque pedal manipulation
One of the reasons instructors love to have students work on quick stops is
that it really helps them get the hang of the anti-torque pedals. This is
because during the entire maneuver, torque is changing constantly. Failure
to do this will cause the helicopter to yaw during the maneuver.
A pilot who holds the flare so long that effective translational lift is
lost may have a problem if the helicopter does not have enough excess power
available to hover out of ground effect. If the helicopter loses ETL 30
feet in the air, this is equivalent to attempting a 30 foot OGE (out of ground
effect) hover. If the helicopter does not have adequate power for the hover
it will descend and very likely hit the ground hard. It may even enter
settling with power. If it does that it will hit really hard.
Failure to totally stop
A common mistake is to enter the desired hover height with some forward
speed. This often occurs when the pilot does not use sufficient power, and
the helicopter descends to a hover height while it still hasn't had time to
fully stop. The problem with this is that aft cyclic is now required which
is going to cause the tail to drop, and little altitude is available so that
too much aft cyclic could cause a tail strike.
Failure to hold centerline
Another problem that student pilots often have is holding centerline
during a quick stop. The helicopter goes through transverse flow effect while
decelerating in a quick stop, and failure to correct for it with lateral
cyclic will cause the helicopter to veer to the side while in the flare.
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)