Power Required and Power Available
The performance limits of a helicopter are determined primarily by the gross weight of the aircraft and the power available from the engines. Helicopter performance is very sensitive to density altitude (DA). In general, as density altitude goes up, helicopter performance goes down.
Density altitude will increase with an increase in altitude, temperature or humidity, or with a decrease in barometric pressure (QNH). The various points on the power curve highlight important speeds for different performance aspects.
A high density altitude the air is thin (less density) and at low density altitude the air is thicker (more density). A thorough understanding of these is essential knowledge for a pilot to ensure the safe operation of the helicopter under a variety of conditions.
E.g. The VNE IAS (Velocity Never Exceed, Indicated Air Speed) reduces as the density altitude surrounding the aircraft becomes poor (less density).
In the interest of better effectiveness and safety, different flight regimes are performed more efficiently at different forward speeds. The bowl-shape of the power required curve graphically illustrates the reason why.
Optimum speeds are determined by the power curve, they are; Best Rate of Climb, Maximum Endurance, Best Angle of Climb, Minimum Rate of Descent in autorotation and Maximum Range.
Endurance and Rate of Climb Airspeed
Best Rate of Climb airspeed is formed at the point where the difference is a maximum between power required and power available. This rate of speed can be estimated from the change in potential energy. The increase in mass flow from forward flight reduces climb power required as opposed to vertical flight. Induced power is already low in forward flight, so there is little to be gained from a significant increase in mass flow. Also, since a climbing condition produces a significant increase in parasite drag and tail rotor power requirements, excess engine power is concentrated toward those efforts instead of vertical flight.
Best Endurance airspeed is the goal of achieving maximum Time in the air by making the available fuel last as long as possible, and since fuel flow is proportional to engine power, maximum Endurance is at this point.
Also at this speed, minimum rate of descent (ROD) in an autorotation can be achieved, since the power required to keep the aircraft airborne is at a minimum. At this speed, the potential energy corresponding to height above the ground and gross weight can be dissipated at the slowest rate.
Best Angle of Climb Airspeed
Best Angle of Climb airspeed is the ratio between distance travelled over the ground and altitude gained. The angle of climb can be expressed as the angle between a plane horizontal to the surface and the actual flight path followed by the aircraft during its ascent.
This speed is the one a pilot uses when executing a "short field" take-off, because it gains the most altitude in a given distance. Rate of climb is not a critical factor, for the best angle of climb speed is in fact slower than the best rate of climb speed, and clearing an obstacle is not something that is time dependent.
Best angle of climb airspeed for a helicopter is the speed at which maximum excess thrust is available. Excess thrust is the difference between the total drag of the aircraft, and the thrust output of the rotors.
A maximum performance take-off is used to climb at a steep angle to clear barriers in the flight path. It can be used when taking off from small areas surrounded by high obstacles. Before attempting a maximum performance take-off, you must know thoroughly the capabilities and limitations of your equipment. You must also consider the wind velocity, temperature, altitude, gross weight, centre-of-gravity location, and other factors affecting your technique and the performance of the helicopter.
To safely accomplish this type of take-off, there must be enough power to hover, in order to prevent the helicopter from sinking back to the surface after becoming airborne. This hover power check can be used to determine if there is sufficient power available to accomplish this manoeuvre.
The angle of climb for a maximum performance take-off depends on existing conditions. The more critical the conditions, such as high density altitudes, calm winds, and high gross weights, the shallower the angle of climb. In light or no wind conditions, it might be necessary to operate in the crosshatched or shaded areas of the height/velocity diagram during the beginning of this manoeuvre. Therefore, be aware of the calculated risk when operating in these areas. An engine failure at a low altitude and airspeed could place the helicopter in a dangerous position, requiring a high degree of skill in making a safe autorotative landing.
The “best angle-of-climb” (AOC) speed depends upon the power available. If there is a surplus of power available, the helicopter can climb vertically, so the best AOC speed is zero.
Wind direction and speed have an effect on climb performance, but it is often misunderstood. Airspeed is the speed at which the helicopter is moving through the atmosphere and is unaffected by wind. Atmospheric wind affects only the groundspeed, or speed at which the helicopter is moving over the earth’s surface. Thus, the only climb performance affected by atmospheric wind is the angle of climb and not the rate of climb.
The angle of climb on take-off should be normal, or not steeper than necessary to clear any obstacles. Clearing a barrier by a few feet and maintaining normal operating rpm, with perhaps a reserve of power, is better than clearing a barrier by a wide margin but with a dangerously low rpm and no power reserve.
Limited Power Take-off and Climb
The ideal site would be into wind, the surface flat and level with short mown grass. A slight downslope would be an advantage. For this take-off you need enough power available to lift-off into a low hover or light on the skids.
- 1 Add the power available and gently tilt disc forward, movement may begin. (If the skids are 'sticking' to the ground, they can be freed by a little 'waggling' of the tail with the pedals). Carb heat set to cruise, out of yellow (safe if above 18" MP).
- 2 As the helicopter begins to move along the ground it will adopt an accelerative attitude. This will help as friction is reduced by the reduction in skid to ground friction.
- 3 Keep straight with pedals and wait for speed to build.
- 4 When translational lift speed is reached slight aft cyclic will get the helicopter fully airborne.
- 5 An accelerative attitude must be re-selected when the helicopter is airborne by gentle move the stick forward again, putting just enough pressure to tilt disc, but not too much to cause descent.
- 6 When best rate of climb (ROC) speed is reached the nose can be allowed to rise to achieve best angle of climb (AOC) speed to clear obstructions.
- 7 Best angle of climb (AOC) speed should be used to gain height, moving to best rate of climb speed when the obstructions are cleared. Tail rotor will become more efficient around best rate of climb (ROC) speed. The resulting power excess can be added. The transition then becomes a normal one. After, take-off checks should now be done and flight continued as any other take-off.
Best Range Airspeed
Best Range airspeed is stretching the glide distance in an autorotation and is a totally separate situation. Maximum glide range is found at a point tangent to the power required curve on a line drawn from the origin. This gives the highest lift-to-drag ratio to achieve the greatest distance but not the maximum time in the air.
Maximum range speed is found on the fuel flow curve by drawing a line tangent to the curve from the origin. This ratio of speed to fuel flow shows the distance one can travel on a pound of fuel on a no-wind day.
If there is a head wind, the line should be originated at the head wind value, which derives a higher speed and lower range. For a tail wind, the optimum airspeed decreases, but the range increases significantly.