Maximum Performance Takeoff
This maneuver is used to transition from a hover into forward flight when
obstacles prevent the use of a normal takeoff.
Position the helicopter
First of all, we want to use the shallowest takeoff angle which gives us
safe obstacle clearance. The shallower the takeoff, the less we get into
the shaded area of the HV curve.
The helicopter should be placed light on the skids if the terrain allows.
This maximizes ground effect
which will give us some extra forward and upward momentum. If the terrain
does not allow touchdown, maintain the lowest safe hover you can.
Begin the takeoff
With the helicopter light on the skids, increase power to maximum. For
training, we teach approximately a 40 knot pitch attitude, but in actual
confined area operations the attitude will be determined by the angle
required to clear obstacles.
A word about Rotor RPM and MP/Torque
One critical part of a maximum performance takeoff is that rotor RPM must
be at maximum allowable RPM. There are two reasons for this. The first
is that the rotor system is more efficient at higher RPM than at lower
RPM because of the reduction in induced
drag. The other reason is that the powerplant can typically put out
more horsepower at higher RPM versus lower RPM.
In our Robinson R22 helicopters (without a governor) I tell people to
calculate their limit manifold pressure setting, while sitting on the
ground. Then during the takeoff I advise pilots to pull collective until
they reach a power setting 1" less than maximum. A quick glance at their
rotor tachometer will tell them whether the rotor RPM has drooped. If it
is slightly low, the last 1" of manifold pressure can be used by rolling
on throttle, thus increasing RPM to the top of the green. On the other
hand, if the rotor tach indicates maximum rotor RPM, the last inch of
manifold pressure can be used by simply raising collective.
In our non-correlated Enstrom, it's even easier. Coordinate throttle and
collective until throttle hits the stop (!). If the rotor RPM droops below
the top of the green, you've pulled too much collective and you simply need
to lower it slightly to allow rotor RPM to recover.
In our JetRanger, I increase power until I reach my limit of torque or
temperature. I usually leave myself a little leeway so that a quick jab
of left pedal won't spike me past redline (even though there is a margin,
I don't like to use it). If N1 topping causes an RPM decrease, lower the
collective to maintain maximum rotor RPM.
Clearing the obstacle
Continue to accelerate and climb until the obstacle is cleared, and then
adjust the attitude of the aircraft to accelerate to normal climb out
Not starting from within maximum ground effect
If the helicopter has plenty of power to spare, a maximum performance
takeoff can be started from a normal hover. When power is marginal and
we need to get every bit of performance from the helicopter, starting
from the lowest hover possible will give us extra performance during
the first few seconds of the maneuver.
Rushing, and using too much power
People seem to think they have to increase the power rapidly. This tends
to make them overshoot their target, and pull more than maximum allowable
power. While the power increase shouldn't be too slow, it should
be slow enough for the pilot to be smooth and precise.
Dragging down the rotor RPM
This is probably the most common mistake pilots make. Especially when that
tree is getting too close! However a pilot needs to fight the urge to raise
collective, and instead maintain maximum power on RPM and concentrate on
smooth cyclic work to get up and over the obstacle.
Using too shallow a takeoff angle
Some pilots have a tendance to keep the angle shallow, try to gain speed,
and then use aft cyclic to "zoom" over the obstacle. Most of us find this
technique a little frightening. I'd rather be gaining airspeed in
the climb than trying a zoom where I'm losing airspeed as I try to
make it over the obstacle. That could cause me to mush into the obstacle
if airspeed got too low.
There is one technique I use which looks a little like this, but isn't
really the same thing. On some hot days, the helicopter simply may not have
the power to hover higher than a foot or two. This simply isn't going to get
you over a 40 foot tree! One technique is to accelerate forward into
translational lift and then
bring the nose up into a 40 knot attitude. The idea is that with translational
lift, the helicopter has enough excess power to gain altitude. However, we
are not gaining airspeed and then bleeding it back off for altitude.
Instead, as soon as we reach ETL, we begin a climb, still accelerating and
Pitching the nose up when an obstacle gets near
As you climb toward your obstacle, a natural reaction is to pull back on
the cyclic to try to get some extra height above the obstacle. You don't
normally want to do this. Remember that as you accelerate toward your
best climb speed drag is decreasing, giving you more power availble. Pulling
back on the cyclic slows you, and gives you less power available.
You want to continue to hold enough forward cyclic to be in an attitude
which gains (or at least maintains) airspeed as you climb out. If it becomes
obvious that you are not going to clear the obstacle, abort the takeoff and
land where you took off from. Until then, resist the urge to pull back on
cyclic because you could easilly lose enough airspeed to actually start
to sink into the obstacle you are trying to clear.
A word about aborting maximum performance takeoffs
Any pilot should always have a few options. One of the really nice things
about helicopters is that you can abort almost any takeoff, land where you
took off from, and then try the takeoff again. This takes proper planning,
and recognizing that the abort is required before it is too late. If you
have been taught maximum performance takeoffs, but not how to abort the
takeoff, you should get some more training. Under normal circumstances, you
should never have to commit yourself to making it over the obstacle.
paul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)