Hi Richard,

I just start learning, reading the book of RPL. I just found this topic of wind shear here, which confused me recently. I hope you could help me out.

Quote
When the headwind drops during the descent, the aircraft will initially lose airspeed and therefore lift. This means the aircraft will descend more steeply - the angle of descent will increase until the aircraft regains equilibrium in the new region of air. After that equilibrium is restored, the rate of descent will return to what it used to be before the wind shear and the descent angle will indeed be shallower.

I want to ask why " After that equilibrium is restored, the rate of descent will return to what it used to be before the wind shear and the descent angle will indeed be shallower."?

Also, you will see the attachment. Why"If the pilot maintains the nose attitude, the IAS will eventually return to 70kt"? I thought that it should be 60 kt because the wind change to 60kt. This IAS really confused me. Does IAS contain wind speed when there is a headwind or tailwind?

Please let me know.

Thanks,

Chao

Let's take away the wind shear and consider what happens if you hold a particular nose attitude. If we consider two different days, just for fun call them Thursday and Friday. On Thursday there is no wind. If I go flying on that day and hold a particular nose attitude, the IAS that results will have a particular value - say 70kt. You can imagine the wind to be an imaginary 'box' of moving air with the aircraft moving along within that 'box' of air. On Friday there is a 10kt steady headwind. If I go flying on that day and hold the same nose attitude, the IAS will be the same - 70kt.

That is because when I'm flying in a steady wind, my aircraft is part and parcel of that 'box' of air so, as far as the aircraft is concerned, there is no wind. For example, if you were in a hot air balloon on a windy day, you would feel no wind on your face because you are moving with the air so as far as you are concerned, there is no wind. Any particular nose attitude will result in the same particular IAS - wind or no wind.

When you pass through a wind shear, you pass from one 'box' of air to a different 'box' of air moving at a different speed and/or direction. Inertia will produce a momentary change in IAS equal to the difference in airspeed between the two 'boxes'. If you hold a constant nose attitude, the aircraft will settle down at the same IAS in the new 'box' of air.

Thanks Bobtait!

Good explanation. But I am still confused about that wind and IAS. What I understand is that because of that wind, there is much more airflow passing through the wings and also going into the pitot tube. Also about that example of hot balloon, it's moving with air so you won't feel any air, but what I confused is that flying in the headwind(different direction )makes you feel the air and, in my opinion, feel more airflow because of that different direction.
Just like take-off, for example,
[ QUOTE ]
If you have a 30 knot headwind, your airspeed will be at 30 knots even if you are not moving. This means that once you start moving and climbing out you can pitch up higher because you already have that 30 knots of extra airspeed from mother nature. That thirty knots combined with what you are generating from forward motion gives you the ability to increase your angle of attack and results in a steeper climb attitude.

So I am not sure if you know where I am confused....

Please let me know.

Thanks

The 30 knot headwind existing before take-off is a different story because you are on the ground and therefore not moving with the wind. To return to the balloon example. If you were in a balloon that was tethered to the ground, you would feel the wind as it flows past you because you are on the ground. However, if someone cut the tether you would immediately begin moving along with the moving air and the wind would no longer exist as far as you and the balloon are concerned. Of course the wind will decide where you end up as it moves you across the ground, but while you are airborne, you would not feel any wind from any direction. The same thing happens in an aircraft.

If you do a series of 360 degree turns on a very windy day, the indicated air speed will drop initially because of the extra drag but once the turn is established, the indicated air speed will remain constant throughout the turns whether you are heading into the wind or downwind it will make no difference as long as the wind does not suddenly change. It's easy enough to prove it - just try it out next time you fly. I made a video of it once because it's not uncommon to hear the old argument that if you turn from upwind to downwind the IAS will drop. It doesn't as long as the wind remains constant..

Hi Chao,

Just another point from your post...

Having a headwind will not allow you to climb with a steeper nose attitude. Don't forget the golden rule of flying:


If you have climb power set, and adopt the "best rate of climb" attitude, the aircraft will climb at best rate of climb speed and get the best rate of climb.
If you have climb power set, and adopt the "best angle of climb" attitude, the aircraft will climb at the best angle of climb speed and get the best angle of climb.

Let's imagine the best angle of climb speed is 62kt. If the wind is blowing straight down the runway at 30kt, as you start your ground roll, you will already have an airspeed of 30kts. All that means is you will reach lift off speed much more quickly than if there was no wind. However, as Bob said, when you lift off, you are now moving inside the body of air. That air is flowing over the runway at 30kts. If your lift off speed is 45kt, you are moving in the air at 45kt but moving over the ground at 15kt.

You can now pitch up to the normal attitude for best angle climb and climb out at the best angle climb speed.

It just so happens that now, with the wind blowing against your direction of travel, your climb angle will be much steeper in relation to the ground. If you were to look at the path of your climb through the air though, the angle would be unchanged. Also, your indicated airspeed would be the same as on a day when there was no wind or a day where the wind was blowing from the opposite direction.

If you were to do what you suggested and climb with a higher nose attitude you would no longer be climbing with the attitude associated with best angle of climb speed and your climb angle would no longer be as steep as it could be.

Remember, with a head wind your climb angle in relation to the ground will be steeper than normal but you will have the same nose attitude as normal.
With a tail wind your climb angle in relation to the ground will be shallower than normal but you will have the same nose attitude as normal.

Also, don't forget, your rate of climb is unchanged no matter what the wind is doing.

Cheers,

Rich

Thanks Rich and Bob. I fully understand now. Appreciate!