Prior to every flight, calculating the performance of your helicopter is essential to ensure a safe flight. The main aim of this calculation is to understand how much weight can safely be loaded on board or whether a flight is possible or not for a given weight.
Power produced by helicopter engines is generated by the pressure of expanding gases created with the ignition of fuel and air. The rendement of this process and effectiveness of aerodynamic lift surfaces of an airplane is affected negatively or positively by some environmental factors.
The most significant of those environmental factors are;
- Moisture content of air
One of the most significant factors affecting helicopter performance is the density of air.
As the altitude increases, density decreases. As density decreases performance decreases.
The two reasons for decreasing performance when the air is less dense:
1. Rotor blades create less lift.
2. Engines produce less power.
Think about dense liquids, when they are more dense, they have more particles in a given volume. Now think about air, like liquids, when it is more dense it has more particles in a given area.
Now think about engines and aerodynamic lift surfaces of helicopters; they both need air to create power and lift. As a result of this, we can easily say that we can produce more power and lift when we have dense air.
The main variables affecting air density are;
- Atmospheric pressure
- Air temperature
- Moisture content
Atmosphere is the composition of different gases and each of these gases has its own weight. Not much individually but, when they all come together and form the atmosphere, weight becomes a considerable amount and even though we do not feel it directly, there is the pressure of it on all of us.
Higher atmospheric pressure results from the compression of gas particles in a given area and this increases the air density.
One way to understand this pressure change is thinking about diving in a pool: the more you go deeper, so the more you feel the pressure in your ears. Another way is thinking about the pressure change feeling in your ears during a flight: as you climb, your eardrum has restrain from inner to outer and as you descend from outer to inner.
The reason for this restrain is the pressure difference between your inner and outer ear. As you climb, atmospheric pressure decreases but the inner pressure of your ear remains the same and as you climb it is vice versa.
Atmospheric pressure not only varies vertically, but also varies laterally around the globe. In other words, atmospheric pressure for a specific altitude is variable.
Altimeters sense these pressure variations and show it as a height value. If the pressure change would have been vertical only, altimeters would always show the correct height. But local atmospheric pressure varies both vertically and laterally.
- When local atmospheric pressure is lower than standard, the altimeter shows an altitude higher than real
- When local atmospheric pressure is higher than standard, the altimeter shows an altitude lower than real.
To compensate this error, a window (called Kollsman window) is placed on altimeters. To read the real altitude of our aircraft above sea level, we have to set the local atmospheric pressure value at sea level to this window.
But, to calculate our performance with local atmospheric pressure changes, we still need an uncorrected altitude value. The standard pressure value (1013 hPa or 29.92 In-Hg) has been created for this reason and the altitude corresponding with this pressure value is called pressure altitude.
Pressure altitude helps us to easily calculate the effect of local atmospheric pressure change and read it as an altitude value.
Independent from the real altitude, an aircraft will perform better in lower pressure altitude and perform much worse in higher pressure altitude.
Conditions: You are hovering at an airfield at 3000ft altitude.
Objective: Compare your hover performance for the following two situations:
- Situation 1: Sea level pressure is 1003 hPa.
- Situation 2: Sea level pressure is 1030 hPa.
Calculate the pressure altitude.
Calculate the pressure altitude.
Current QNH is 1030.
The difference between 1030 and 1013 = 17 hPa
17 hPa * 30 = 510ft height variation.
Pressure altitude is 3000-510=2490ft
In this situation our real altitude is 3000ft and pressure altitude is 2490ft which basically means that even though we are at 3000ft the performance of our helicopter during hover will act like at 2490ft.
Result: As lower altitude means better performance due to increasing density of air, in situation 2 our hover performance is far much better than in situation 1.
Air temperature directly effects the air density. As the temperature increases, particles become more separated; as they become more separated, air density and aircraft performance decreases.
Think about a balloon. If you leave a balloon in a place warmer than where it has been inflated, a couple of minutes later, you will find out that it is bigger than it used to be. This happens because with the increasing temperature inside the balloon, the volume of air increases and its density decreases.
The influence of temperature at pressure altitude, compared to what the temperature is assumed to be at pressure altitude in standard conditions (+ 15°C at sea level) is denoted as Density altitude.
Air temperature is assumed to decrease 2°C at each 1000ft (real value is 1.98°C) and the all over temperature deviation from standard conditions can be written as ISA +/- XX.
To find the density altitude, increase or decrease pressure altitude by 120 feet for each 1°C deviation from standart conditions.
Conditions: Actual air temperature at 5,000 feet pressure altitude is +11°C
Objective: Find the ISA deviation from standard conditions.
Conditions: Actual air temperature at 7,500ft pressure altitude is -5°C
Objective: Find the density altitude.
In ISA, temperature at 7500ft should be;
(7 x 2) + 1 = 15
15 - 15 = 0°C
Temperature in given conditions is -5°C,
ISA deviation is ISA -5
Multiply 120ft for each 1°C deviation from ISA.
-5 x 120 = -600ft
Calculate density altitude by adding temperature deviation.
7500-600 = 6900
Density altitude is 6900ft
The moisture content of air decreases the density and thus decreases helicopter performance. The molecules forming moisture have less mass than molecules forming air. When the moisture content of air increases, it has a huge negative impact on helicopter performance.
The humidity factor often has a far greater influence than pressure and temperature factors.
There is no rule-of-thumb formula that allows a pilot to calculate the effect of moisture content in terms of altitude above sea level, as with pressure altitude and density altitude calculations.
Wind has a considerable amount of effect on the helicopter performance. While headwind and tailwind increases or decreases the effectiveness of aerodynamic lift surfaces, cross-wind causes a different handicap for helicopters.
In most helicopters, the tail rotor consumes a portion of the power created by engines to create anti-torque required to maintain desired heading. When there is a cross-wind from the side of which the main rotor is turning, then the required anti-torque and power increases.