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Maximum Performance Takeoff

This maneuver is used to transition from a hover into forward flight when obstacles prevent the use of a normal takeoff.

Maneuver Description

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 speed.

Common Mistakes

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 gaining altitude.

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 Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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