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Running Takeoff

This maneuver is used to transition from the surface into forward flight when there is not sufficient power available to sustain a hover. This might occur if the helicopter is underpowered, is at high gross weight, or high density altitude.

This maneuver can be performed with either wheeled or skid equipped landing gear. In the case of skid equipped helicopters, the skids are equipped with skid "shoes" which are made of a very hard material. The shoe can be replaced when it is worn through, preventing any wear of the landing gear itself, which would typically be made of soft aluminium.

This description assumes skid gear, but is generally the same for wheels.

Maneuver Description

The helicopter is typically surface taxied into position. The skids are alighned with the direction of takeoff, and power is increased until the aircraft is light on the skids. Slight forward cyclic is used to start moving the helicopter forward.

During the slide, lateral cyclic is used to hold the centerline, and the anti-torque pedals are used to keep the skid gear aligned with the ground track. As the helicopter gains airspeed, the rotor system becomes more efficient, which makes the helicopter lighter, which reduces friction with the runway and allows the helicopter to gain airspeed even faster.

As the helicopter approaches translational lift, the pilot can use some aft cyclic to bring the helicopter off the ground. The reduction in friction will allow the helicopter to accelerate forward. If the pilot pulls back too far on the cyclic, airspeed will be lost and the helicopter will settle back onto the runway.

Eventually translational lift is attained, and the helicopter has enough excess lift to gain altitude and airspeed. The helicopter is kept low to the ground while it accelerates to best rate of climb airspeed. This ensures that if the helicopter loses airspeed and settles back onto the surface, that it will be a gentle touchdown, typically followed by a short slide and then a climb back into the air.

Once best rate of climb airspeed is achieved, the pilot can climb to altitude.

Common Mistakes

Not using maximum allowable rotor RPM

Helicopter rotor systems are more efficient at higher RPM, so using anything less than maximum allowable rotor RPM means there is less performance available. Since we are in a performance critical maneuver, it makes sense to maximize power available by bringing RPM up to the highest allowable value.

Piston helicopters also benefit because power available is directly proportional to RPM. If the engine is run at 95% of maximum allowable RPM, power available is 5% lower than if the engine is run at 100% of allowable RPM. This combined with the rotor system efficiency issue means that it is critical to maintain maximum RPM during any maximum performance maneuver.

If the helicopter has no excess power to spare, the pilot needs to be careful to avoid raising collective during the slide. Doing so may cause RPM to decay, reducing total power available. The pilot should not be in the habit of using collective during this maneuver. Instead, the power should be set to maximum, and all control inputs should use the cyclic. If the helicopter settles back toward the surface, minor collective inputs can be used to cushion the touchdown, but the pilot should be careful not to bleed down his rotor RPM with these inputs.

Failure to keep the skids alighned

It is critical, especially on turf or other soft and rough surfaces, to keep the skid gear alighned with the direction the helicopter is sliding. Allowing the helicopter to yaw can cause a sideways rollover.

Early Climbout

If altitude is gained too early, the pilot loses ground effect and will be unable to accelerate as rapidly as if he stays low. Also, if he attempts to climb out at too low an airspeed, the helicopter might not have enough power available, and could settle back onto the runway, possibly resulting in a hard landing.

We generally train people to remain below 10 feet until they have at least 45 knots of airspeed.

Too much forward cyclic

Many pilots will use a lot of forward cyclic in an attempt to get the helicopter to slide. This has two negative effects. One is that less vertical lift is available, because so much of the main rotor thrust is tilted forward. This increases the weight on the skids, and therefore the friction.

An even worse problem is that as the helicopter hits translational lift, the sudden increase in thrust will tend to nose the helicopter over forward. With the skids in contact with the ground, this pitch motion combined with the drag of the landing gear can cause the tips of the landing gear to dig in and flip the helicopter over forward.

The proper technique uses only a very little bit of forward cyclic, keeping most of the lift vertical, such that when translational lift occurs, the helicopter lifts off the ground in a skids level attitude. If the pilot can't get the helicopter to slide forward without excessive cyclic inputs, either he isn't using enough power, or the helicopter simply does not have enough power available for the conditions.

Failure to maintain runway centerline

A common mistake is to not correct for lateral drift during the slide. Failure to correct for lateral drift could potentially cause a dynamic rollover, but in any case it imposes side loads on the landing gear and is not good pilot technique.

The cyclic must be used to maintain centerline during the slide. If the helicopter is sliding to the right of centerline, left cyclic is required to bring it back. Generally you would not want to yaw the helicopter with pedal to regain centerline. Exceptions to this would be when gross lateral corrections are required at low speed. That shouldn't be required if the pilot uses good technique in the first place.

Trying to take off with too little power

If the helicopter is ridiculously underpowered, or the pilot does not use nearly enough power, the slide is going to be very long. Indeed it is possible to run out of runway in such a situation. If the slide is really going to be that long, the pilot should be examining whether the helicopter is safe for flight under the current circumstances. Possibilities are to offload some weight or wait until density altitudes are lower (ie. lower temperatures).

The pilot needs to keep in mind that if the helicopter can't hover, autorotational performance is going to be marginal.

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

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