Simple windshear answer please!

[A.FLOOR]

An increase in headwind causes an increase in airspeed and vv

Why does an increase in tailwind cause decrease in Airspeed ?
Why does an decrease in tailwind cause increase in Airspeed ?

Thanks in advance

[BOAC]

a) An increase in headwind causes an increase in airspeed and vv

1) Why does an increase in tailwind cause decrease in Airspeed ?
2) Why does an decrease in tailwind cause increase in Airspeed?

1) is the opposite effect to a)
2) is the same effect as a)
??????????

Not sure what 'vv' is without looking it up.

[dolly737]

The reason is the inertia of an airplane.

Simply speaking, if you encounter a sudden tailwind component, the velocity of the air hitting the airspeed probe will be somewhat reduced for a (hopefully) short time, until the airplane returns to the previous thrust to drag equilibrium.

happy landings

[Lump Jockey]

vv = vice versa! It has to do with what's known as IAS/indicated airspeed, the pitot tube, etc.

[mutt]

LumpJockey,

Would you care to expand on your explanation, you have confused me :confused: :confused:

Mutt.

[Tinstaafl]

vv = Vertical Velocity.

[Eckhard]

The aircraft's velocity and inertia is referenced to the earth, not to the air through which it is moving. When the wind velocity changes, its force against the aircraft changes. This force produces an acceleration. If the aircraft's mass is large, it takes a while for this force to effect an acceleration on the aircraft. This is
why airliners are more prone to windshear related incidents than are light aircraft.

Imagine a goldfish swimming around in a large glass bowl. You pick the bowl up and carry it across the room. The goldfish doesn't notice that his 'groundspeed' has changed because his inertia is too tiny and the water just carries him with it. Now imagine a large ball bearing in the same bowl. This time, when you move the bowl, the ball bearing lags behind and tries to maintan its previous groundspeed, because it has greater inertia.

Aircraft are just huge fish swimming in an ocean of air. As the ocean moves across the ground (wind) they get carried along with it. But when the wind changes quickly, they try to maintain their previous groundpeeds.

[A.FLOOR]

Thanks for the answers

But I am more concerned with the relationship between wind, groundspeed and IAS

Yes i understand that an increasing tailwind implies an increasing groundspeed, but why a decrease in IAS
How does the pitot tube figure this out ?

[fritzi]

When flying an airplane, you can completly forget everything about groundspeed. A airplane flies using air, so you have to use airspeed instead.

Ex: A Cessna 182 is parked on a ramp and the groundspeed is obviously 0 kts. But then there is a wind coming head-on to the aircraft at 15 knots. Now the IAS is 15 knots even though the groundspeed is zero knots. This is why you always tie down smaller props. If for some reason a 60 knot headwind comes at a C152 while it is parked on a ramp without being tied down, it would lift off the ground because the airspeed is 60 kts while the groundspeed still is 0 kts and at 60 kts, a C152 would fly.

If you have a tailwind, the Groundspeed will increase even though the IAS stays the same because the engines are still producing the same amount of thrust. If you want to increase the IAS, you have to use more thrust.

How does the pitot tube figure this out ?

The pitot tube doesn´t calulate the groundspeed, it only calculates the IAS.


Fritzi

[terryJones]

Look at it this way.
Ideal day, zero wind. IAS=GS=100kts.
Imagine a SUDDEN wind, on the nose at 20 kts. Effectivly now the air pressure on the pitot is due to a 120 knot INDICATED AS, due to the aircraft not being able to change its speed instantly. We therefore have GS100kts, IAS 120Knts.
Now Imagine the same wind appeared instantly on the tail. The air pressure on the pitot would now be due to an equivelent of 80 Kts, but the ground speed would still be 100 knts. Once the aircraft has accelerated back to an INDICATED 100 Kts the GS would be 120.
Its just a case of rembering how the ASI works.
(At least, that's my interpretation, for what it's worth):D

[Dick Whittingham]

Is this question related to the range problem, by chance? It is true that in a tailwind you maximize the benefit to range by flying an IAS a tad slower than the still air range speed and in a headwind you minimize the loss by flying a tad faster. I ask just to clarify the ground rules.

If this is a pure windshear or microburst question then it has already been answered. Sudden headwind gives transient increase in IAS, more lift etc and sudden tailwind gives a transient drop in IAS, loss of lift etc.

Dick W

[Keith.Williams.]

IAS is derived from the dynamic pressure of the airflow at the inlet to the pitot probe. The dynamic pressure (1/2Rho V squared) is proportional to the true airspeed. The greater the dynamic pressure, the greater will be the airspeed indication.

If an aircraft in flight experiences a sudden tailwind, the inertia of the aircraft will tend to keep its ground speed constant. But the tailwind represents a sudden decrease in the speed of the air relative to the pitot probe. So the pitot probe and airspeed indicator sense a lower value of dynamic pressure. The airspeed indicator therefore indicates a lower IAS.

But the greater problem is the fact that the wings also sense this reduced dynamic pressure. This reduces lift thereby causing the aircraft to sink.

The reduced dynamic pressure also reduces drag. So if the tailwind persists for long enough the aircraft will eventually accelerate until its original IAS is restored, but at a greater ground speed.

So sudden (brief) tailwinds do not affect ground speed, but reduce IAS and increase ROD. Prolonged tailwinds (eventually) increase ground speed.

[MaximumPete]

There's some good stuff on the NASA website on windshear and how they are developing equipement to detect it.