Wind Gradients and All That

[Guest TOSGcentral]

I am conscious that this particular forum is entitled ‘Training & Student Pilots’ and will therefore attract a lot of low time students seeking greater knowledge and understanding.

 

 

The recent debate about the ‘Turning Downwind Myth’ etc has been stimulating discussion, was handled in a good natured way, but may have got a trifle esoteric for some students and the differing opinions may have caused confusion in their minds.

 

 

I want to tackle the area from a different angle that may give a bit more understanding because the ground, and the aircraft’s ground speed certainly can have a major influence on the energy level (read airspeed) of an aircraft!

 

 

This is in the situation where a Wind Gradient exists that the aircraft is traversing with vertical, as well as forward, motion.

 

 

We first need to lay down some firm foundations. The into wind/out of wind turn situation became confused because insufficient parameters were given. The basic premise is sometimes called the ‘Big Block of Air’ scenario. An aircraft flies in the block of air and (once free of the ground) it does not matter if the block is stationary or travelling across the ground because the aircraft just goes with it. The aircraft is taking its flight energy from the air surrounding the machine and neither knows nor cares if the air happens to be going anywhere!

 

 

However that is only true if the energy of the airmass asround the aircraft is not varying because that will directly affect the energy level of the aircraft. A very simple example is flying through a thermal – as you approach the thermal you traverse the sinking area around the sides, lose a little airspeed as a result then traverse the high energy core that increases your airspeed, then the reverse procedure happens as you fly out the other side. Once back in constant energy air then your airspeed will remain constant in accordance with your nose attitude and the amount of engine power in use.

 

 

So foundation point #1. If the air energy surrounding the aircraft changes then there is a corresponding effect on the aircraft’s energy level!

 

 

This can happen not just in the horizontal (as via the thermal example just given) but in some circumstances if the aircraft is moving vertically in the airmass and gets into different energy airmass levels above or below where it has just left. This is most commonly (but certainly not exclusively) in Wind Gradients (Early students of Meterology please note the term ‘Wind Gradient’ is entirely different from the term Gradient Wind which is something else).

 

 

At this point I need to create a mental image in your mind from which you can understand the mechanics of the wind gradient.

 

 

First take a sliced loaf of bread and remove the wrapper. Liberally butter each slice on both sides and then restack the slices vertically on a table such that the slices are horizontal to the table. We need the butter so the slices will slide across each other more easily than just bare bread which is a bit rough!

 

 

We now apply an even horizontal force to the loaf. The bottom slice in contact with the table experiences friction with the table and is held back and moves slowly. The next slice, despite the butter, experiences some friction and is also held back to a lesser extent – it is beginning to move a little more quickly.

 

 

This slice interacts with the slice above it and holds that back – but less so. Repeat the process vertically through the loaf and you have all the various slices travelling at increasing speed from the horizontal applied force until you are far enough up the loaf that the friction has reduced to the point that the horizontal force is being complied with and all slices above this point are travelling at the same speed.

 

 

Now let us apply this model in real terms. The horizontal force is the air moving across the ground – the wind speed. The higher the wind speed then the greater the force and consequently the greater reaction you will get from it via friction.

 

 

The loaf is a vertical cross section of the moving airmass.

 

 

The table is the ground surface. But unlike your laminex table surface the ground is exceedingly rough. It has trees, slopes, buildings etc and these cause considerable friction to the wind and thus slow the airflow.

 

 

Each slice of bread is an even layer of air, each one stacked on top of each other.

 

 

I want to make the following example simple for mental arithmetic so it is a bit exaggerated but bear with me please.

 

 

We will say that on the day in question the bread slices are 50’ thick and the friction applied to each is worth 5 knots of wind speed against an actual prevailing wind of 35 knots (the horizontal force).

 

 

If we keep subtracting 5 knots per 50’ we arrive with a surface windspeed of 10 knots which will show on the windsock! The top of the wind gradient will therefore be at 350’ (which is quite typical) and the wind speed (or air energy) will be constant above that if there are no other outside energy influences.

 

 

Got that picture OK?

 

 

Now, we are trundling down an approach to land with an approach speed of 55 knots in an aircraft that has a stalling speed of 35 knots. At the inception of the approach (at whatever height – it does not really matter – everything will remain constant as you descend and your into wind groundspeed will be 20 knots (55 – 35).

 

 

However once below 350’ you will be in the wind gradient and be losing 5 knots of air energy per each vertical 50’ unless you do something about the nose attitude and/or power setting.

 

 

So your approach speed at the approach attitude you are basing your Aiming Point approach control slope judgement on has now just fallen to 50 knots. 50’ lower and it is down to 45 knots. At 100’ you will be in severe pre-stall sink and at 50’ you will be stalled out! You will now probably be seriously undershooting unless you are landing on a salt flat of huge size!

 

 

Why does this happen? It is primarily because the energy of airmass that the aircraft is flying in is changing and reducing.

 

 

While philosophically it is easy to say that in airmass flying your groundspeed changes into and out of wind even if your airspeed does not – this becomes a bit more drastic in a wind gradient.

 

 

We are now into dynamic physics and if the air energy on the nose falls then something has got to give! Because your groundspeed was 20 knots on entering the top of the wind gradient it does not mean to say that it can instantly rise to 25 knots because the headwind has fallen 5 knots and therefore the calculation of Airspeed plus or minus windspeed = groundspeed does not apply. The aircraft has mass and that cannot instantly accelerate by 5 knots – it takes time even assuming that you do something about it. So you lose 5 knots of airspeed!

 

 

But the key to this is firmly understanding that the aircraft’s energy level has been changed by a change in the energy level of the air surrounding the aircraft that the aircraft is flying in and is drawing it’s own energy from.

 

 

OK – as stated the example given is a bit extreme for flat site flying although I was paid to fly and teach in much worse. But that was on hill sites where other orographic effects may be present such as curl-overs etc or in the lee of a range of hills where there is consisderable air subsidence or lee wave.

 

 

In general terms significant wind gradients do not occur in much below 15 knots of surface wind speed as indicated by the wind sock and can extend generally to 350- 450’ agl. They are anyway easily dealt with via standard procedures that you should have been taught, or hopefully will be taught if you are not quite there yet.

 

 

The basic scenario is approach speed calculation that is commonly 1.5 Vs + 1/3 of wind velocity as shown by the sock. That should not only totally barricade you against general low speed loss of control but will also armour you against a wind gradient although you will probably have to be lowering the nose a bit to maintain airspeed while increasing power to maintain glide slope – but that is just piloting!

 

 

There are a few more wrinkles that you should also understand about wind gradients.

 

 

If, in the example given, you flew level down the runway at 250’ you would not know the wind gradient was there! The airmass energy is not changing and everything will remain stable because you are not moving vertically through it into changing air energy levels.

 

 

While the loss of airspeed is apparent from the example given above, the wind gradient does have a reverse effect where you are actually gaining energy! This can be equally as fraught particularly in low inertia and high drag aircraft.

 

 

On climbing through the wind gradient (eg on take off) you are gaining air energy and therefore airspeed. You keep slowing the aircraft to your normal climb out speed and go up like a homesick angel! You may even choose to impress the onlookers with how fantastic your machine really is by slowing down a bit and climbing even faster. Your groundspeed is slowing and the climb angle (relative to the ground) becomes spectacular.

 

 

Then you climb out of the wind gradient! Your nose is way up and you do not have increasing air energy to sustain that attitude. Your airspeed rapidly falls and you get into pre-stall high sink regimes and start going down – back into the gradient!

 

 

As fast as you lower the nose and dive through the gradient then the more the gradient robs you of the airspeed you HAVE to have to regain control! Heard any stories of otherwise inexplicable dive-ins from relatively low altitude?

 

 

DO NOT DO IT! If you have a gradient then enjoy the extra climb rate but fly a little faster than normal up through it. The last happening of this situation that I had involved me being dragged out of a peaceful afternoon to the airfield, stand around for two hours while the Care Flight helicopter crew tried to resuscitate a dead individual, bollock over enthusiastic police who barged in and crossed runways in front of landing aircraft, immobilise the wreckage for the fire crew who did not know how to, help pick up the body when the Coroner’s crew arrived, meantime keep the media at bay – then eventually go home again and get thoroughly pissed! I have better ways of spending my time other than mourning over needless deaths!

 

 

I trust this little account may have informed you and perhaps even have entertained you.

 

 

Aye

 

 

Tony

 

 

[Yenn]

Tony

 

Two points. Could you explain why your airspeed decreases when you enter sinking ait and vice versa.

 

Would you also agree that the layers of different speed air are not hard edged, that is there is a gradual change of wind speed and therefore a gradual change of airspeed.

 

 

[slartibartfast]

Tony.

 

Love your work. That is truly inspired and inspirational. I can have no respect for anyone else because you have it all.

 

However, to use bread to illustrate a point on a forum run by a Mr. Baker? I think you are - to put it politely - currying favour, if not outright raising dough.

 

Surely another less politically motivated metaphor could be employed?

 

And (to start a sentence in exactly the way it shouldn't be (I have a cold)), as a moderator could I ask you to never type "pissed" again?

 

Thanks,

 

Ross

 

 

[vk3auu]

Would it be reasonable to infer from the above example, that the term "sink" is actually a misnomer, although the effect produced is sink? In the example "sink" is actually caused by "wind shear".

 

David

 

 

[Guest TOSGcentral]

Some Answers

 

Ian (Yenn) To take your last point first - you are quite correct! The wind gradient is just that, a gradient not a series of steps. So you in fact generally get a reasonably even fall off of airspeed when descending through one. However using the sliced bread analogy is a simple and clear manner of getting the windspeed fall off, and the reason for it, into people's heads that you are hitting cold with something they were not aware of.

 

The sink/airspeed question you posed is not quite so clear cut. In the thermal example I used (primarily because virtually everyone has direct experience of flying through one and I like to build new subjects by expanding on what people already know) then generally the sink band surrounding a thermal is lower energy air than the airmass surrounding it and certainly of a much different energy level to the thermal core. There is to some extent also the factor of some change in relative airflow which impacts on the aircraft's angle of attack and hence on its lift. As we are aware lift is a combination of A of A and airspeed so change the A of A and you will get a corresponding change in airspeed.

 

On the other hand if you fly horizontally through a lee wave system and into the down-going portion then there is generally little or no warning at all - you just start going down maybe alarmingly so and your airspeed probably will not be affected. The air now happens to be going downwards and taking you with it.

 

The essential difference between the two scenarios is that the thermal situation involves changes of air energy at a given altitude the aircraft is attempting to maintain. Change the energy of the air and it impacts on the aircraft's energy level and therefore its airspeed.

 

In the lee wave situation there will probably be no change of air energy at all, only direction of air travel. This is not to say that you do not get substantial energy changes in wave systems. Try flying through a wave rotor and you will get the definitive example of airmass energy changes to the extent that your eyes will be sticking out like chapel hat pegs!

 

So your question is difficult to answer concisely other than by saying if the 'sink' is caused by airmass energy change then it will impact on your airspeed but if there is no airmass energy change then it will not.

 

David, No, not really wind shear in the examples given. In the thermal example the sink is actually sinking or subsiding air and I distinguish between that and actual wind shear for other training reasons so confusion is not created - but that is just me.

 

For example I never use the term rotor except in connenction with lee wave systems. What a lot of people refer to as 'rotors' are just grotty bits of turblence behind buildings or lines of trees and I dismiss them for what they are - just mechanical turbulence. I took a while to get used to the Australian penchant for using the term rotor very broadly as I have flown frequently in the real thing and have the utmost respect for them.

 

The other point I used the term sink was in the context of pre-stall sink. This is simply a function of the aerodynamic consequences of entering the low speed end of the Total Drag Curve when moving towards the stall. The drag begins increasing very greatly and requires substantial power to combat. If you do not use power then you will most certainly sink!

 

For further example - when teaching stalls I concentate very heavily on this aspect. A Thruster immeadiately pre stall will be sinking at over 500' per minute but still be in a flat attitude. This is the actual danger of the stall - not the stall itself but the situation where all that sink may take you. Most students do not relate going down quickly to normal looking attitudes. Going down to them is the nose pointing at the ground to some extent!

 

Ross. LOL slarti You have sprung me and made my nefarious plans public. However I will restrain myself and not become a hot, cross bun!

 

Equally I will promise to be a good boy and be less free with the use of common Oz vernacular!

 

Aye

 

Tony

 

 

[vk3auu]

Tony et al,

 

Do you think we concentrate too much on the stall, particularly with the lighter aircraft, and not enough attention is given to the rate of descent before the stall? If I get my CH701 on the wrong side of the power curve, I can easily get a rate of decent above 1000 feet per minute and still be above the stalling speed.

 

I get the impression that a lot of heavy landings happen because of this high rate of descent just before the flare, and in the actual flare the wing is not able to generate enough lift to arrest the descent before the ground is encountered. This seems to happen, particularly in short field landings, where a fairly slow approach is required.

 

David

 

 

[Guest TOSGcentral]

David,

 

 

I do not think there is over-concentration on the stall per se. What I feel much more strongly about is the tendency for instructors to just work off the flying training syllabus in terms of individual exercises that can be ticked off and the student progresses, but, not relate those exercises into a broader and growing practical skill that has validity to them becoming an Airman rather than just a certified pilot.

 

 

I do not really want to get into a philosophical discourse on instructor training and syllabus management – or even syllabus content, but I will say just a few words.

 

 

In the flying training that I conducted I covered everything required in the stated flying training syllabus but I changed the order of exercises (in some areas quite drastically for some tastes) and also introduced additional exercises and could get away with this because the training speed gained from re-arrangement could absorb the additional material and in fact it was picking up a few spare hours against what the students generally hoped would happen.

 

 

Those hours were of huge value as I only chose to instruct on the most difficult of trainers (I do get a bit jaded and like some stimulation in my work) so I had to be efficient.

 

 

To give you an example (which is pertinent to your query) one major inclusion in my syllabus was formal treating of the Total Drag Curve. Now stick this on a board and the graph, all the bells and whistles etc – causes glazed eyes! Built it gradually on a board and make the student involved with building it and suddenly they find that a lot of that ‘theory’ they are supposed to know does in fact have very practical application and they suddenly begin understanding the practical significance of what you are teaching them – such as Safe Speed Near the Ground for example – it is not some arbitary figure it is something they can work out for themselves even at their beginning level.

 

 

The situation does later underline very powerfully that if they are in the short field landing, max angle climb out, or dealing with wind gradients etc scenarios then their piloting management needs to be both aware and careful than at higher energy regimes. They can relate the theory very positively with the practical.

 

 

You have just read an example! In my opening post I also related the wind gradient to the approach speed calculation – which most students will hopefully be familiar with – so the wind gradient did not become some strange dragon but that what we ask the student to do, what airspeeds we wish them to do it at, do have relevance and are countering situations that in the early days they have no knowledge exists.

 

 

Techniques such as those begin building situational awareness at a very early stage.

 

 

I hope that answered some of your question David – even if obliquely. My parting thoughts are that there is wealth of advancement available in recreational flying training instruction if only controllers would cast training for the common denominator of student and understand that we have to build a very firm and interactive bridge between principles and practice that is of real use to those students.

 

 

Aye

 

 

Tony

 

 

[farri]

Tony,

 

Excellent work,you are correct,I hope everyone understood it and I`m sorry about your experience.

 

I had to become involved in a rescue once when a very experienced pilot crashed from about a 1000 feet on what appeared to be the perfect morning, the gods were with him though and a couple of us were able to revive him but he was quite badly injured and required a fair time in hospital.

 

Don`t wory too much about political correctness.

 

farri.

 

 

[Yenn]

Tony.

 

What you are seeming to say is that when a plane encounters sinking air it slows down because the pilot uses back stick to reduce the descent.

 

As far as high rates of descent, how do we recognise them when we are at altitude. I don't have a VSI.

 

I know that when near the ground that if the speed is too low the rate of descent can reach high figures and the only way to control it is to use forward stick, and if too close to the ground. throttle.

 

It is easily possible to get into a situation where the sink rate is high and cannot be arrested by flaring, even though you are above the stall speed.

 

 

[Guest TOSGcentral]

Ian what gave you the impression, from what I wrote that...

 

"What you are seeming to say is that when a plane encounters sinking air it slows down because the pilot uses back stick to reduce the descent."

 

That is not what I meant nor inferred. My stance is quite plain - if there are energy changes of the air that the aircraft is traversing then these will have a corresponding influence on the aircraft's energy levels and result in either a loss or gain of airspeed.

 

What anyone may do with the controls is consequential to this - or in the case of wind gradients - has be pre-predicted and the aircraft set up to a higher energy level such that it does not go bankrupt from the rapid withdrawals that the gradient can make.

 

Look - rather than debate this just go out and do it! Simply take a convective day, choose a nice plump cumulus with a nice dark bottom and fly across the base - holding your attitude constant. Watch what happens to your energy level as expressed by your indicated airspeed!

 

Regarding VSIs - Personally I have them in trainers exactly for the reason you have correctly given. In wave conditions there is not much problem because your altimeter will be acting as a VSI - it will be moving that fast!

 

For anything much above about 800' agl you are too remote from the ground to rapidly pick up sink in benign attitudes until it is really staring you in the face via ground proximity. This is why it is essential (in initial training) to have students understand some of the mechanics of the Total Drag Curve and adopt sensible barricading procedures as a matter of course.

 

Tony

 

 

[Vorticity]

This explains something that has been driving me nuts! The only way I could describe it was like I couldnt keep the drifter up. But then all of a sudden, in a different area or after a break on the ground it would be fine again. I could maintain alt and get a good IAS. Thanks for the explaination.

 

I am thinking of ditching the VSI from my plane is it depresses me with the 503 and I could use the space for something else. The drifter I learnt in didnt have one and I didnt miss it.

 

 

[farri]

Wind energy.

 

Tony,

 

I think what some pilots don`t properly understand is that if any of the forces change we have a new situation.

 

Lets take straight and level flight,all the forces must be equal and as the wind changes energy,as you put it,or the pilot moves the controls at all, a new situation will occure and it is no longer,straight and level flight, for that moment.

 

As the wind increases or decreases it`s energy,(speed),the airflow over the aircraft increases or decreases,a new situation has occured, this in turn increases or decreases the lift of the aircraft for that moment and the aircraft will go,up,down or sideways,accordingly.

 

I read some discussion in another section about the constant rate of climb in a climbing turn,regardles of wind direction, it appeared to me that some didn`t understand that for this to occur all the forces must remain constant.

 

Tony,I havn`t given this explanition for you, as I know,you well and truely understand this.

 

Cheers.

 

farri.

 

 

[Guest Juliette Lima]

Hi Vorticity,

 

The VSI can be very handy for precision flying exercises.... eg. slow flying, perhaps you might want to reconsider??

 

Cheers

 

JL

 

 

[Guest Juliette Lima]

Hi Tony,

 

Your articles as always make great and informative reading. Thank you.

 

What about a tutorial on 'Nailing the Aiming Point'.....would this help overcome the wind gradient issue on final ....acknowledging that things can happen rather quickly on final in a high drag aircraft ?

 

Thanks again

 

JL

 

 

[farri]

Low and slow and in ya go.

 

Frank.

 

 

[Guest TOSGcentral]

Hi JL,

 

Yup - that would be a good subject to cover. I will give it some thought as it will be a bit cute to write simply and clearly.

 

But it would be good to have something down about it - particularly for high drag & low inertia vis a vis visual approach control using an aiming point as the prime aid.

 

The issue is that if you go into the top of the gradient at too low an aerodynamic energy level then you could run yourself out of engine energy as a replacement, find the aiming point steadily going up and the aerodynamic energy steadily going down and there is precious little that you can then do about it - particularly if you have a line of trees between you and the runway with probable curl-over on your side of them.

 

May be a few days before I can come up with anything as I am a bit busy with engineering re-design of Thrusters at the moment.

 

Aye

 

Tony

 

 

[facthunter]

Terminology.

 

I know that I am a bit late with this contribution, and I'm not trying to be pedantic, but I am more comfortable with the term "windshear". Any takers? Nev..

 

 

[farri]

Windshear,Definition.

 

Windshear: Any change in wind speed or direction.

 

Frank.

 

 

[Guest TOSGcentral]

Yup Nev,

 

 

I am happy to add to debate on this aspect.

 

 

You are (of course) technically correct. But for my tastes (particularly in training applications) the term ‘wind shear’ is too generic. Attention may not be initially focussed enough and subsequently may cause confusion and diminishes initial training objectives.

 

 

There are of course anyway two distinct forms of wind shear – horizontal shear and vertical shear – so you could get the seeds of immediate confusion if just the generic term is used?

 

 

A wind gradient is one aspect of vertical shear that is primarily about wind speed changes, but at the other end of the spectrum also includes micro-bursts – which we are not about to be flying students in or near if we can avoid it.

 

 

So I far prefer to stick a distinct label via the term ‘wind gradient’ and students have no illusions about what I am talking about – a speed change in wind over a height band that is specifically being addressed in respect to approach and take-off considerations. The condition is also common unlike other more extreme vertical wind shears.

 

 

For other training uses I use the term ‘wind shear’ to specifically address horizontal wind shear where you have two adjacent airmasses one on top of the other that are travelling in different directions (possible at different speeds) and you may get a pronounced turbulence layer at the contact point between them.

 

 

Even then confusion can occur from observed conditions. A very similar sensation to horizontal wind shear boundary layers is exhibited in a reasonably strong temperature inversion. Here we can have the same thin vertical band of turbulence but caused for other reasons.

 

 

As an example: Just a few days ago I was subjected to the definitive (and not common) example of what I use the term ‘wind shear’ as.

 

 

It was a lovely early Queensland morning and 8/8ths blue with 3 knots of NW wind on the surface. Ideal conditions to take a girl for a trip around the local dams that she had not seen before.

 

 

The forecast predicted increasing SE winds during the day but there was no cloud to determine how early that would come in. As it turned out it was already there!

 

 

As we climbed out and turned onto a SE track. It became obvious that the NW wind speed was increasing and I estimate got to around 15 knots. That was OK as it gave a good ground speed and would give us a leisurely ground speed from which to admire the view when I turned the corner into wind on the elongated oval track that I had planned – then a quick downwind trip home.

 

 

We turned the corner and were still in a slow climb transiting 2000’ agl. Then we abruptly hit quite positive turbulence that was enough to alarm my passenger. My first thoughts were time and the clear blue sky – probably a temperature inversion and the band of turbulence so associated (but a bit early in the day for that). So I continued climbing and it did not get any better.

 

 

So thinking about orographic effects from the rather tall hills around us I decided that retreat would be the better part of disturbing my companion further and we would get out and back to the flat plains.

 

 

So I turned around and we just about stopped in the face of a 30 knot SE wind!

 

 

Next step was to go down through the turbulence band again and into the NW tailwind – then around the corner and into the NW headwind – so it all got pedestrian. After that I gave it away, landed and we settled down to doing some airframe geometry checks while the steadily increasing wind came down to the surface and really shook the hangar!

 

 

What we had actually been facing was about a 160 degree horizontal wind shear plus the two airmasses at about 15-20 knots difference – so we got a helluva turbulence boundary layer that was tipping us up to 45 degrees in roll. Not quite an early morning scenic flying jaunt. After we got down there was still absolutely no sign of it from what you could see in the sky!

 

 

OK that was really just a bit of a low key flying story. But what I was trying to convey is that interpreting the airmass that you are flying in can be a bit demanding, even when you are experienced. So in training, pertinent descriptive ‘labels’ can be important to equip future pilots with a foundation on which to work out what is actually going on. That going up or down just a bit can get rid of the turbulence but which way do you go so you maintain headway in low speed aircraft?

 

 

You are exactly correct Nev but I feel that my approach is more practically correct.

 

 

Aye

 

 

Tony

 

 

[facthunter]

effect.

 

Tony, you have put a lot of effort into explaining this, and it is most important to realise the effect, and therefore cope better with the way the aeroplane behaves especially near the ground where, in the approach phase, there is not a lot of excess energy to play with. Regards, Nev..

 

 

[blueline]

Hi Tony - Love your work BUT could you trim just a couple of hundred words off each post!!:thumb_up: