Minimum Touchdown Speed

The touchdown occurs at the end of the autorotative landing flare in which the helicopter has been brought from steady autorotation with moderate forward and vertical velocities to a condition with little or no velocity in either directiourAn idealized flare maneuver is illustrated in Figure 5.11. It starts with a cyclic flare at

constant collective pitch in which increased rotor thrust and its aft tilt are used to decrease both the vertical and the horizontal velocity components. At the end of this cyclic flare, the aircraft should be near the ground with its vertical component zero—or within the design sink speed of the landing gear—and with its horizontal velocity corresponding to autorotation at the angle of attack to which the rotor has been pitched. The maximum safe flare angle is the highest angle from which

the helicopter can be subsequently rotated nose down to a level attitude during the time that the rotor energy can be used to develop hovering thrust. (Two qualifications should be noted: The flare angle may be limited in an actual case if the pilot fears that he may lose sight of the ground in a machine without good downward visibility; and some helicopters do not have to flare to a level attitude if

they have a tail wheel or skid structurally designed to take high loads.) The final nose-down rotation and collective flare are done during the time required to use up the rotor energy and should result in both the vertical and horizontal velocity components ending up as low as possible.

A method for estimating the final touchdown velocity is as follows:

• Calculate the maximum allowable tip path plane angle of attack at the end of the cyclic flare as a function of the maximum nose-down pitch rate, @m2X, as limited by the longitudinal control power of the helicopter and the time available for the maneuver:

axppm„ = 0m„ А/, deg

where the maximum pitch rate is given by the equation:

• yQ

= —ABv deg/sec

where Bx is the forward cyclic pitch available. See Chapter 7 for the derivation of this equation.

The time for the maneuver is:

At =

In this equation, the maximum value of CT/o at the end of the maneuver can be taken from hover charts such as those in Chapter 1. If the resultant flare angle is more than 45°, use 45° as the value.

• Use Figure 5.12 to find the tip speed ratio, jilut0, at which autorotation can be sustained at the maximum flare angle while still developing a vertical component of rotor thrust equal to the gross weight. Figure 5.12 was produced from the isolated rotor charts of Chapter 3 by letting

Find the minimum touchdown velocity in knots as:

where the term (g/2) tan aTPPA/ is the decrease of forward speed during the nose-

FIGURE 5.12 Conditions for Autorotation at End of Cyclic Flare

down rotation to the horizontal. Applying this process to the example helicopter gives:

This method assumes an ideal flare maneuver in which a skilled pilot does the right thing at the right time. It is actually a difficult maneuver in which the pilot must simultaneously satisfy the equations of motion for vertical forces, horizontal forces, and pitching moments with only his cyclic and collective controls in order to end the maneuver within narrow limits of height above the ground, rate of descent, and forward speed. Reference 5.13 states the problem thus: "Pilot apprehension is a factor because of ground proximity and rate of closure.”

Although the initial rate of descent does not enter into the calculation directly, it does affect the pilot’s chances of achieving the ideal flare. The higher the initial rate of descent, the less time he has to correct mistakes in control inputs. A study made with a B-25 airplane showed that satisfactory deadstick landings could be made up to rates of descent of 2,500 ft/min. Above that, the quality of the landings decreased. Presumably, helicopters have a similar limit.

Even though the military have had a requirement for many years that the minimum touchdown speed be 15 knots or less, there is a lack of actual test data on the maneuver, so the method has not yet been checked against an actual flare.