Temperature and Altitude

Air properties vs helicopter performance

Rotorcraft


A helicopter is a type of aircraft that uses spinning wings called blades to fly. Unlike an airplane or glider, a helicopter has wings that move. Unlike a balloon, a helicopter is heavier than air and uses an engine to fly. 
Wings are curved on top and flatter on the bottom. This shape is called an airfoil. That shape makes air flow over the top faster than under the bottom. As a result, there is less air pressure on top of the wing. This causes suction and makes the wing move up. Rotor blades are like wings and create lift. In simple words, helicopters are types of rotorcrafts in which thrust & lift are generated with the help of rotors.

Lift force equation

L = 1/2ρ*(V)2*S*CL

  • L = Lift.
  • ρ = density of the air.
  • V = velocity of the aerofoil. 
  • S = the wing area of the rotor.
  • CL = Coefficient of lift , which is determined by the type of rotor and angle of attack – AOA.
Lift on an airfoil
Airfoil

Consider the flow of air over a single rotor blade. The air flows faster on the top side of the rotor blade compared to the air flowing on the bottom side. The difference in the speed of flowing air creates a difference in the pressures generated on both the side of the rotor blade. A high pressure region is generated below the blade and a relative low pressure above the blade. This produces an upward force called lift on the rotor blades. When the upward force exceeds the weight of the helicopter, it starts to get off the ground and stays up in the air.

To start dealing with this equation we need to spread it into prime factors. But today we will focus on the density, pressure and temperature of the air. For more information about lift. However as you see density of the air is a part of the equation, so let’s find out first what air actually is.

What is Air?

Air is a mixture of gases – nitrogen, oxygen, carbon(IV) oxide, water vapor and noble gases. The composition of air is not represented by any simple chemical formula, unlike if it were a compound. Composition of Air The constituents of air and their percentage composition:

  • Nitrogen – 78.1% – 4/5 of volume of air
  • Oxygen – 20.9% -1/5 of volume of air.
  • Carbon dioxide – 0.03%.
  • Water vapor – variable.
  • Noble gases – about 1%.

Atmosphere layers

Earth’s atmosphere is divided into five main layers:

  • 5 – Exosphere,
  • 4 – Thermosphere,
  • 3 – Mesosphere,
  • 2 – Stratosphere,
  • 1 – Troposphere.

The atmosphere thins out in each higher layer until the gases dissipate in space. There is no distinct boundary between the atmosphere and space, but an imaginary line about 100 kilometers from the surface, called the Karman line, is usually where scientists say atmosphere meets outer space.

The Troposphere

For our needs we concentrate on the troposphere. This layer is closest to Earth’s surface. It is 7 to 20 km thick and contains half of Earth’s atmosphere. Air is warmer near the ground and gets colder higher up. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here. 

The temperature in atmosphere

Changes are based on how much solar energy is absorbed. The layer closest to the Earth’s surface is the one most important to life on the planet. According to ISA, pressure, temperature, density, viscosity and speed of sound variation can be calculated for a range of altitudes from sea level upward. This is done using an exact solution to the hydrostatic equation for a column of air. The air is assumed to be a perfect gas. In the lower region, the troposphere, the atmosphere has a lapse rate. The temperature in stratosphere remains constant at -57° C.

Temperature and Altitude

ISA

ISA – International Standard Atmosphere. A standard atmosphere has been established by the aviation authorities to enable comparison of aircraft performance, calibration of instruments. There are a number of hypothetical conditions set in this standard atmosphere at 45° latitude.

Example:

  • Sea level pressure – 1013,25 hPa,
  • Sea level temperature – 15°C,
  • Pressure lapse rate – 1 hPa per 9m at lower altitudes,
  • Mass of 1.225 kg/m3,
  • Temperature drops 1.98°C/300m.

Before each fly you need to get fresh value of present altitude, elevation, and QNH.

Environmental Lapse rate

The lapse rate is the rate at which temperature in Earth’s atmosphere decreases with an increase in altitude, or increases with the decrease in altitude. Lapse Rate of 6.5°C/Km. 

Adiabatic expansion

The process of expanding and contracting without exchanging heat is an adiabatic process. The term adiabatic means that no heat transfer occurs into or out of the parcel. Air has low thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small. The adiabatic process for air has a characteristic temperature-pressure curve, so the process determines the lapse rate. When the air contains little water, this lapse rate is known as the dry adiabatic lapse rate. Where the rate of temperature decrease is 9.8°C/km

Logic implemented

Our BP_WeatherManager has control over the sun movement and his properties like temperature over the day and night. Including 24h temperature variations. It will pass information to helicopter dashboard. We starting with 15°C  at sea level. Our troposphere is divided into few horizontal layers. Each layer has same thickness about 1 Km, and also different properties accordingly to above information, and those values also making impact on machine performance. With each layer pilot should experience loss of power. Till the last layer of troposphere where engine may stop working, due to low oxygen ratio in the fuel mixture. Once you manage to get back to lower layer with more oxygen you will be able to start engine again. 

Conclusion

For our small one person helicopter with open cockpit and two stroke engine which is not equipped with any turbo or supercharger. It makes really hard to climb over certain altitude. Less molecules of air with each intaken stroke will produce less power and this may event stop the engine. Automatically this will creates the boundary of area to fly – great! The best place for aviation will be to stay at the lower part of troposphere. For example to keep the racing performance of P01 is the best to stay below 1000 m. However the best experience from fly is when you are very fast, with very low altitude just above the ground and happily avoiding obstacles. Check also the new and more accurate damage system

TODO

Before the next update v.04 arrives to OMH, I would like to touch a few things. This update will generally focus on improving the P01 helicopter. And it will probably be divided into two parts a and b. Where v.04a will be a refinement of logic, and v.04b will focus on the visual aspect of the vehicle. It is planned to separate the main rotor from the rear one and bring few types with different blades types based on different airfoil shapes. This will enable us to introduce interesting innovations in the future, as well as it bring us closer to a greater realism of the situation. 

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