A rather complex indication of the maneuverability of a military helicopter is the return-to-target maneuver. In this maneuver, the helicopter passes over a spot (target) in forward flight and then tries to return to that same spot in the shortest possible time. The most critical ground rule is that the whole maneuver must be done at constant altitude. This simulates a nap-of-the-earth maneuver in which the "target” might be hostile and capable of destroying the helicopter if given the opportunity. Shorter times could be achieved if the maneuver were done with a zoom and a dive.
The optimum return-to-target maneuver at constant altitude consists of a combined deceleration and banked turn until the flight path is pointed back at the target followed by a level flight acceleration. A discussion of the maneuver in reference 5.18 points out that the deceleration could be increased by sideslipping to increase the parasite drag, but that flight tests using this technique revealed the danger of the pilot becoming disoriented and losing the target. For this reason, calculations should be done assuming a zero sideslip turn during the banked deceleration phase.
The maximum rotor thrust for the deceleration phase may be taken from the isolated rotor charts of Chapter 3 as the zero torque value at the upper stall limit (ACq/g = 0.008). The calculated ground path for the example helicopter doing this maneuver is shown in Figure 5.17. The procedure for the step-by-step calculation was as follows:
• At the initial tip speed ratio, find CT/amix from Chapter 3 rotor chart at Cq/g = 0, and ACq/g = 0.008.
• Find dxpp from charts.
• Calculate load factor:
Cl/®mix COS ClTpp
П — ———– ———-
• Example Helicopter
• Initial Speed = 115 к
• G. W. = 20,000 lb
• Sea Level, Std. Day
• Acceleration Done at Takeoff Power Rating
• Deceleration Done in Autorotation at 100% rpm
• Calculate radius of turn:
• Calculate deceleration:
• At end of time increment, А/, calculate:
— R——— ,deg
Ал – = VAt cos |/, ft Ay = V А/ sin |/, ft
• Repeat the procedure using the appropriate rotor chart as speed decreases until |/ is in the third quadrant and
tan |/ =
• If the speed drops below the autorotative limit as defined in Figure 5.12, a powered, steady turn at this speed should be used with the corresponding load factor taken as the ratio of maximum gross weight to actual gross weight from a power required curve such as Figure 4.24 at the appropriate engine power rating.
• Use an acceleration curve such as Figure 5.14 to continue the flight path until the origin is reached.
Helicopters are occasionally used for towing in special situations such as minesweeping, rescue, or salvage operations. The maximum towline tension that can be maintained is a function of the maximum rotor thrust and the angle the towline makes with the horizon. For most towing operations, the speed will be slow enough that hover conditions can be assumed to apply. The equation for the towline tension can be derived from the balance of forces acting on the helicopter as shown in Figure 5.18. When the equations for the vertical and the horizontal
components of the forces are solved simultaneously, the ratio of towline tension to gross weight is:
Figure 5.18 shows this ratio as a function of у for several ratios of maximum thrust to gross weight. The example helicopter at sea-level conditions can develop a maximum net rotor thrust of 27,800 lb out of ground effect, as shown in Figure
4.35. If it is flown at a gross weight of 17,000 lb and the towline is kept as flat as possible, it can maintain a towline tension of 22,000 lb.
Although aerobatic maneuvers are not considered to be normal helicopter flight conditions, loops and rolls are quite possible and have been done by a variety of designs.
A well-done loop does not put excessive loads on any of the aircraft components. Figure 5.19 shows a loop being done by an airplane, which, having a separate
T=D C. F. = W
FIGURE 5.19 An Ideal Loop
propulsion device, is easier to visualize than a helicopter. The loop shown is idealized in that a constant speed is being maintained and the flight path is a perfect circle. The highest load factor is at the bottom of the loop, where it is only 2 g. Although no airplane or helicopter can do such a perfect loop, nevertheless any loop can be considered to be a relatively mild maneuver as far as loads go.
There is, however, a problem with respect to control. At the top of the loop, where the rotor thrust is zero or at least very low, all helicopters have reduced control power in pitch and roll; some—those with teetering rotors—may have none at all. If the pilot wants to make a cyclic correction in this situation, he might be surprised by how far he has to move the stick in order to get a response. The rotor will respond readily to the cyclic pitch and may tilt further than the designers made provision for. This is the classic setup for mast bumping on teetering rotors and for droop stop pounding on fully articulated rotors.
Reference 5.19 describes the piloting technique required to loop the Sikorsky S-67:
The loop is initiated from a slight dive at approximately 175 knots. The cyclic is pulled aft and collective lowered slightly to limit control loads. As the aircraft passes the 90° point (straight up), collective is added to maintain positive g. Airspeed at the inverted point in the maneuver averages 50 knots. The average time to execute a loop is 21 seconds. The load factor range for the maneuver runs from 2.5 plus g at the entry to 0.7 g inverted to 2.5 plus g during the recovery.
A roll is similar to a loop, as shown in Figure 5.20. Most aerobatic airplanes rely on substantial fuselage sideforce to support them when the wings are straight up and down. The Sikorsky description of the maneuver in the S-67 is:
The roll maneuver is conducted to the right only to eliminate the problem of interference between the collective stick, pilot’s left leg, and the cyclic stick. Generally the maneuver is started from 150 knots in level flight. The aircraft is pulled to 20° nose up and the pitch rate is reduced to a minimum. As the airspeed reaches 130 knots, full right and a slight amount of aft cyclic are introduced. As the aircraft reaches the 270° point (three-fourths of the way through the roll) lateral cyclic is returned to neutral and additional aft cyclic is introduced to counteract the nose tucking that initiates at approximately the 270° point. The roll takes an average of 6 seconds to complete and the load factor ranges from 0.8 g to 1.7 g for the maneuver.