Shear Wind

[tony_hk]

Hi fellow forumers,

I am a pilot wannabe. Here is what I found from the internet about wind shear:

¡§Wind shear can be either an overshoot shear where there is an increase in headwind or decreasing tailwind resulting in an increasing speed of the aircraft. It can also be an undershoot shear where there will be an increasing tailwind or decreasing headwind with consequent loss of speed of the aircraft causing it to sink to the ground if no immediate action is taken by the pilot.¡¨

Could u tell me e.g. why an increase in headwind will result in an increasing speed of aircraft? Does headwind mean the aircraft flies into the wind, right? I think the speed should decrease in that case. I am confused.

Thanks in advance.

[john_tullamarine]

(a) windshear relates to inflight handling and performance .. we are concerned about AIRspeed. The problems generally are transient (ie short-lived) ... and are related to the mass of the aircraft ... bigger aircraft ... greater mass (inertia) .. takes longer for the aircraft to shake its head and adjust to the changing wind field.

(b) navigation relates to speed referenced to the ground ... we are concerned about GROUNDspeed

I think you are focussing on groundspeed .. which results in understandable confusion ?

[tony_hk]

Hi john,

I think I understand what you mean about the difference btw Airspeed and groundspeed, but I miss the linkage between these and my specific question. Could u pls guide me more?

In particular, I need clarification on the following:

(1) Does headwind mean the aircraft flies into the wind?

(2) Does headwind reduce the Airspeed of the aircraft? Just like the effect of jetstream acting on an airplane flying from west to east?

[phnuff]

Tony - as a PPL, I will consider this from a small aircraft perspective, although the basics are I believe the same for big boys.
Airspeed is the important factor when thinking about an aircraft in flight as at a given pitch, it will control the amount of lift generated from the wings and also how much authority the control surfaces have over the aircraft (lower airspeed - sloppier controls because there is less air passing over the surfaces).

So consider a spam can on approach to an runway. The ideal approach air speed is 65kts. If you have a 10kt headwind, this will be equivalent to a ground speed of less than 65 kts simply because the wind is trying to blow the aircraft backwards. Equally, a 10 kts tailwind will mean an airspeed of 65kts but a ground speed of greater 65kts because the plane is being pushed forward. Put simply Airspeed has a component from the engine AND a component from the wind.

Consider a windsheer event. The plane is at 65kts airspeed which may consist of 55kts from the engine and 10kts from the headwind . So, now the 10kt headwind suddenly becomes a 10kt tailwind. Not only do you lose the 10kt wind component, you also now have a negative wind component of 10kt which brings your airspeed to 65 - 10 - 10 = 45kts which is too close to stall speed for comfort, and so you then have to generate more power from the
engine to make up the difference (45kts + 10 + 10 = 65kts) . Of course, if it suddenly turns back to a 10kt headwind, you have to take power out quickly, or else you will suddenly have too much speed (in this case 65 (Engine) + 10 (wind head) = 75kts. Spam Cans dont wanna land at 75kts.

Does this help

[tony_hk]

Hi phnuff,

It does help alot.

Now I am sure I can explain it clearly to other people.

Thanks alot.

Two additional points need some comments:

(1) For an airspeed 65kt appeared on the ASI. Let\'s say 55 kt is contributed by the engine; and 10kt is resulted from the headwind. You mentioned that "this will be equivalent to a ground speed of less than 65 kt".

To be more precise, can I say that the ground speed should be below 55 kt? Because the only forward speed is 55 kt. And the headwind should reduce the ground speed of the aircraft somewhat. Correct me if I am wrong.


(2) I now understand tailwind reduces the airspeed of an aircraft and consequently reduce lift. But why do the airline pilots try to fly with the jetstream? Which one do pilots prefer for the same thrust provided by the engine, a higher groundspeed (tailwind) or a higher airspeed (headwind)?

[Old Smokey]

It all depends on your momentum.

A simple example - You are approaching at 100 Knots airspeed into a 10 knot headwind with power / thrust set for steady flight, so, your groundspeed is 90 knots. Your aerodynamic performance depends on your airspeed (100 Kt), your momentum depends upon your Groundspeed (90 Kt).

Suddenly you encounter a headwind of 20 knots, your airspeed is suddenly 110 Kt because your aircraft remains at 90 Kt groundspeed due to it's momentum, and the 20 Kt headwind gives you 110 Kt airspeed. You have increasing lift, i.e. overshoot shear.

If the aircraft is left alone, after a short time, the airspeed will stabilise at 100 Kt again with a new Groundspeed of 80 Kt because the increased drag at 100 Kt will return the aircraft to the speed for which power / thrust was applied, i.e. 100 Kt.

The same applies for decreasing tailwind, the opposite applies for increasing tailwind / decreasing headwind.

[Mad (Flt) Scientist]

I now understand tailwind reduces the airspeed of an aircraft and consequently reduce lift. But why do the airline pilots try to fly with the jetstream? Which one do pilots prefer for the same thrust provided by the engine, a higher groundspeed (tailwind) or a higher airspeed (headwind)?

For purposes of increasing range or decreasing time to destination, one would prefer a high groundspeed. Therefore flight with a tailwind is the preferred mode.

Note that you cannot choose to have a higher airspeed for the same power setting in steady-state flight - when you encounter a transient wind condition the airspeed will momentarily rise, but then the increased drag will eventually slow the aircraft back to the trimmed airspeed.

You really need to completely divorce thinking about range/distance type steady state conditions, which is when headwind or tailwind matters - where groundspeed is to be maximised if possible - and short term transient conditions, which is what wind SHEAR is about, and where the important thing is not to have excessive and unsafe variations in airspeed.

There's nothing unsafe about 100kts of tailwind, or of headwind, in cruise; it just gets you there faster or slower. But even a 20kt SHEAR can be bad news.

[phnuff]

I sure ain't gonna argue with old smokey about momentum - other than to say my little plane will have less momentum that his big one and is therefore more suseptable to wind changes. Of course, conversly, my piston engine can add power quicker than his jet because it doesnt suffer from the spool up lag that jets do (am I right in thinking it takes typically around 5 seconds for a jet engine engine to spool up to maximum power?). Of course, the power he can generate makes my piston engine look like a toy, although I need less of it!

As for the jet stream question - (and again standing by to be corrected), part of the job of a commercial pilot is to try to run the aircraft as economically as possible so if he can jump into a jet stream and get the same speed for less engine power (therefore lower fuel burn), he's gonna do it. The thing about windsheer is that while it always has an effect, it only becomes a major problem at times like landing when you are closer to minimum speeds and the ground.

[Mad (Flt) Scientist]

phnuff

Actually, the smaller inertia and lower momentum if a little plane makes it less affected by windshear, since the airspeed more quickly returns to the trim value. (A feather blows with the wind, while a brick experiences the wind changes directly).

If both a heavy and light plane experience a, say, 10kt shear, both will instantaneously experience a 10kt change in airspeed. Even with NO pilot reaction, the lighter aircraft will accelerate back to the trim airspeed faster than the heavy. So the shear affects the lighter aircraft less.

Of course, that 10kts is a bigger percentage of the initial airspeed for the lighter aircraft...

[tony_hk]

I've got it!

Thanks, folks.

[AerocatS2A]

There's been a couple of confusing "non-facts" thrown around in this thread.

If you have an airspeed of 65kts and a 10kt headwind, you don't have a "55kt engine component and a 10kt wind component" that would suggest that your engine does not need to put out the same power to maintain that airspeed, this is false. Your engine is pulling you through the airmass and is giving you 65kts airspeed. The air happens to be travelling at 10kts across the GROUND in the opposite direction. Your 65kt airspeed translates to a 55kt ground speed.

Aircraft need AIRSPEED to fly, groundspeed is irrelevant (except for navigation).

Forget about jetstreams, they are a consistant airmass that your aircraft is travelling through. Where windshear comes into the equation is that it occurs when your aircraft moves from one airmass to another. Each airmass has diferent velocities and there will be a momentary change in the aircraft's AIRSPEED as it makes the transition.

The kind of thinking in some of the above posts is what leads people to believe that you will lose AIRSPEED if you turn from heading into wind, to heading down wind. So long as you are remaining in a stable airmass then your airspeed will remain stable, regardless of what direction you are flying relative to the airmass.

[phnuff]

AerocatS2A
Hmm I've thought long and hard about this and , yep you are right, I am mixing air and ground speed. I stand corrected. As for jetstreams, well since sitting in a jetstream will shorten a flight by increasing groundspeed for a given airspeed, that will equate to a lower fuel burn and therefor more economic flight (or am I wrong about that?)


Mad (Flt) Scientis
I think the mixing of air and ground speed answers explains my confused message too. Again I stand corrected

[AerocatS2A]

As for jetstreams, well since sitting in a jetstream will shorten a flight by increasing groundspeed for a given airspeed, that will equate to a lower fuel burn and therefor more economic flight (or am I wrong about that?)

No, you're quite right on that one. You end up with the same fuel burn per hour but because your ground speed is higher, you will get to your destination quicker and use less fuel in total.

[phnuff]

AerocatS2A

I am still pondering the issue of airspeed/groundspeed and windsheer. May I PM you for clarification of something

[Tinstaafl]

An easily demonstrated analogy is with those moving slidewalks at airports.

Walking along the floor, your walking speed exactly matches your speed over the stationary ground. This is the equivalent of airspeed matching groundspeed when flying in nil wind.

Continue to walk & step onto a moving slidewalk. For a brief moment you're off balance because your body hasn't yet accelerated to match the surface over which it rests (or moves, same thing). This the shear.

After a moment, walking along the moving surface will be no different to walking along the floor. Your walking speed over the slidewalk is exactly the same as when walking along the floor, however your walking speed over the *ground* is faster (or slower if you go in the other direction).

At the end of the slidewalk there'll be another brief moment of imbalance from the shear as you transition from moving with respect to one environment (the slidewalk) to moving w.r.t. a different environment (the floor).

This effect would be the same no matter which way the slidewalk was moving as you stepped on or off it. It would also be the same if you stepped from one slidewalk onto another moving in a different direction or speed.

[tony_hk]

If you have an airspeed of 65kts and a 10kt headwind, you don't have a "55kt engine component and a 10kt wind component" that would suggest that your engine does not need to put out the same power to maintain that airspeed, this is false. Your engine is pulling you through the airmass and is giving you 65kts airspeed. The air happens to be travelling at 10kts across the GROUND in the opposite direction. Your 65kt airspeed translates to a 55kt ground speed.

AerocatS2A: thanks for your correction. But I am unclear about the above.
Do you mean if a 65kts airspeed should be maintained, the engine output should be the same nomatter how strong is the wind? if this is right, am I correct to say that the airspeed is totally unrelated to the wind speed (I think this is not true, right)? I am wondering if we can "suspended" an airplane airborne without thrust (maybe done by a expelling an airmass downward, like a heli?), while the headwind is 10kts, what will the airspeed be?


I am still pondering the issue of airspeed/groundspeed and windsheer. May I PM you for clarification of something

phnuff: can you also forward the messages to me?

Tinstaafl: I like your idea of slidewalk. That helps my understanding. So the change of momentum/inertia when you first stepped on the slidewalk can be treated as the change of airspeed once you experience the head/tailwind of a windshear, right?

[phnuff]

Aerocat/Tinstall
This analogy of the moving walkway is perfect (for me anyway). I could understand the comments about the aircraft moving with respect to the airmass, however made a logical jump to thinking that in that case there can be no problem associated with moving from one air mass to another because you continue to move with respect to the new airmass - and that is something I knew to be rubbish. The walkway analogy makes it very clear.

Making a fool of myself in public is not something I like doing, but this time, I am actually glad I did because I now understand the matter much more. Thanks for the correction guys.

[AerocatS2A]

tony_hk,

Airspeed is entirely unrelated to windspeed.

Airspeed is the speed of the aircraft through the air.

Wind is the movement of that air over the ground.

In ideal conditions, what the air does over the ground has no bearing on what the aircraft does through the air.

I can't really do any better than the moving sidewalk analogy other than to reiterate the distinction between moving from one airmass to another (windshear), to flying within the one airmass (a steady non-turbulent wind).

In the former case airspeed changes momentarily and is the subject of this thread. In the latter, the airspeed stays constant for a given power setting and aircraft attitude (climbing or descending) regardless of whether it is a tail wind or headwind, and is often confused with the subject of this thread.

Anyone's free to PM me, though I'm no more qualified than the others around here.

[Tinstaafl]

Airspeed vs groundspeed is a common confusion. Another analogy:

Imagine a power boat moving along a lake ie still water. The boat's speed through the water matches its speed past the shore/ground and is constant for a given power setting.

Now put the same boat at the same power setting moving upstream in a river. The boat's speed through the water will be unchanged. It's speed past the ground, however will now be slower due to the body of water - & the boat imersed in it - moving in the opposite direction. This is exactly the same situation as walking the wrong way along the slidewalk.

If this seems odd, think of something floating in the lake or floating with the current in the river. You don't wonder why it should have a groundspeed even though it's speed through the water is zero (floating with the current, remember).

Similarly to this 'headwind' example, if the boat turns around & cruises downstream then its groundspeed will be faster than its waterspeed.

If it cruises across the current then its ground speed will be the vector sum of its waterspeed *and* the current ie a triangle of velocities.

NOTE: The direct head- or tail-current plus waterspeed is also a vector addition. It's just a special case where the all the directions happen to be aligned so simple addition or subtraction works.