Lecture X: Wind Factors

AIRCRAFT WEIGHT & PERFORMANCE

Wind

Understanding Wind

• Wind is defined as the horizontal movement of air relative to the earth's surface.

• Airplane speeds up or slows down according to the speed and direction of the wind.

• Since a flying airplane is suspended in the air, the air can be considered to be still and the plane moving, but since the aircraft has to go front one ground location to another the wind speed and direction effect how the airplane is flown

• Thus, changes on wind speed and direction has a marked impact on aircraft performance.

Understanding Wind

• In terms of direction, three types of wind are Head Wind, Tail Wind & Cross Wind.

• Other types are wind also included Wind Shear & Jet Stream.

Wind Direction

Headwind• Head wind refers to the wind blowing opposite to

aircraft motion.

Tailwind• Tail wind refers to wind blowing along aircraft

motion.

Crosswind• "Crosswind" is the wind component at right angle

to the flight path, causing an aircraft to drift either to the left or right of the intended path.

Wind

Wind Shear• Wind shear is a sudden change in wind speed and

direction will cause the plane to lose or gain airspeed and ground speed instantly.

Jet Stream• The term "jet stream" is often used to describe the general

flow of upper atmospheric winds. But it has a specific meaning of upper atmospheric winds blowing faster than 57 mph. Jet streams play a key role in the weather by steering storms and also helping determine where storms form. Usually, when meteorologists talk about "jet stream" winds, they're talking of winds more than 20,000 feet above the ground

Wind Speed and Aircraft’s Speed

• Wind speed is the speed of wind, the movement of air or other gases in an atmosphere.

• It is the average velocity at which the air travels over a one-minute period and is measured in nautical miles per hour (NM/H or knots).

• Aircraft’s speed is the speed of an aircraft. It can be measured based on ground or air around it.

• The speed of the aircraft relative to ground over which it is flying is referred as GROUND SPEED.

• The speed of the aircraft relative to the air around it is referred as AIRSPEED.

relationship between ground speed, airspeed, and wind speed

• Based on above equation, obviously seen that the changes of wind speed and direction will affect the aircraft’s ground speed.

• For example, if the wind blows in the opposite direction as the plane's motion, Vwind will be a negative number.

• Thus, to calculate ground speed just subtracts the wind speed from the airspeed.

• Ground speed (Vground), is air speed (Vairspeed), plus or minus wind speed (Vwind) relative to the direction the aircraft is traveling

relationship between ground speed, airspeed, and wind speed

• Flying directly into the wind (head wind) will cause the ground speed to be slower than the airspeed. If you are flying 100 mph into a 20 mph headwind, your ground speed will be 80 mph.

• Flying with a wind from behind (tailwind) will cause the ground speed to be faster than the airspeed. If you are flying 100 mph with a 20 mph tailwind, your ground speed will be 120 mph.

How does wind affect aircraft performance?

Wind Effect• Referred to as wind effect, the speed and direction of the wind will alter the progress

of any aircraft in flight.• Before each flight the pilot gets forecasted wind speed and direction to determine the

estimated compass heading and ground speed of the aircraft.• Although an aircraft has its own engine power, the pilot must compensate for the wind

speed and direction, in order for an aircraft to maintain the performance & desired course.

• This is because the changes of wind has a direct effect on : • LIFT Produced By The Wings• The SPEED of an airplane.

• Changes on wind speed and direction has a different effects on aircraft performance during the phases of flight.

Wind Effect on LIFT Produced By The

Wings• Wind speed and direction can affect the lift produced by a wing.

• Let say, for take-off, airplane need to generate excess lift to become airborne.

• Note that, airplane’s wing can only create lift if air is moving over it (headwind).

• Referring to lift equation L=Cl½ V²S , ρ clearly seen that the Lift force depends on the square of the airplane speed relative to the air (Vairspeed) .

• Remind again, take-off with headwind will reduce ground speed, which in turns reduce the take-off distance but still move with required take-off speed. This is reason why airplane require headwind for take-off.

• Head wind conditions also important during climb, descent and landing.

• This is because headwind will increase climb angle/descent angle and cause shorter take-off/landing distance while reduce ground speed.

Wind effect during take-off

• Headwind will increase the airplane performance by shortening the takeoff distance and increasing the angle of climb.

• However, a tailwind will decrease performance by increasing the takeoff distance and reducing the angle of climb.

Wind effect during landing

• A headwind will increase airplane performance by steepening the approach angle and reducing the landing distance.

• A tailwind will decrease performance by decreasing the approach angle and increasing the landing distance.

• Again, the pilot must take the wind into consideration prior to landing.

Wind detector

Automated Weather Observing Systems

(AWOS)Wind Sock

Wind sock and AWOS: Indicate wind direction & speed to guide pilot determine the correct runway to use for landing and take off

Wind Effect on Airplane’s Speed

• Wind speed and direction also can affect the speed of the airplane especially during cruise.

• Let's assume that the aircraft's cruise speed, or its speed relative to the local air, is 500 mph.

• Let us also assume that the plane is traveling in the same direction as the wind such that it experiences a tailwind of 100 mph.

• Since the wind and vehicle both move in the same direction, their velocities add together, and the ground speed becomes 600 mph.

Effect of tailwind on ground speed

Wind effect during Cruise

• A tailwind will increase performance by increasing the ground speed, which in turn reduces the fuel requirement for the flight.

• A headwind will decrease performance by reducing ground speed, which in turn increases the fuel requirement for the flight.

Effect of headwind on ground speed