Myth busting! I MIGHT STALL IF I TURN DOWNWIND!

[Ultralights]

MYTH: I MIGHT STALL IF I TURN DOWNWIND!

 

 

 

 

 

 

WRONG! The myth is that if you are flying into a headwind (of say 20 knots, and you are indicating 100 knots airspeed, thus covering ground at 80 knots) then if you turn 180 degrees, the headwind will become a tailwind and you will find yourself with only 60 knots of indicated airspeed after the turn! (80 knots groundspeed with a 20 knot tailwinds leaves you with only 60 knots) WRONG!! MYTH! Of course you have a headwind before the turn, and a tailwind after, but this has no aerodynamic impact on your whatsoever. You can forget about the ground when doing (non ground-effect) aerodynamics. It means nothing. It is nothing but dirt underneath you that is not touching you, and that you tend to use as a reference since most places you go are conveniently located on it. You might as well use the moon as your reference, or the sun, or maybe the center of the Milky Way Galaxy. These are nothing but reference points, and have no impact on the nature of the flying machine. The aircraft is borne by the wind, and reference to GROUND SPEED has no impact at all on aerodynamics.

 

So, you might be saying: Wait a minute! If you have 100 knots of airspeed for the whole turn, then you must have come into the turn with 80 knots of ground speed, and come out of it with 120 knots of ground speed! You just got 40 knots of speed for free! IMPOSSIBLE!

 

No, you did NOT get 40 knots of speed for free. You simply aimed your plane in a different direction where its true reference-system (the moving air) HAPPENS to be going in the same direction as the reference-system you keep using (the ground). Just think of the air as being still and the earth moving underneath it at 20 mph. Now imagine the airplane making a 180-degree turn through the nonmoving air. Nothing special, right? But your speed over the EARTH was different before and after the turn, because of the movement of the EARTH happening to be in the direction as the plane or not. Now you should see that the downwind turn has no impact on speed at all since you are moving with the AIR. The Earth can rotate underneath you however it likes... it makes no difference to you. You only THINK it does because the millions of years of evolution that caused our brains to turn out the way they did has us thinking about things in reference to the GROUND because that is how we evolved to move: With our LEGS over the GROUND. It is not natural for you to quit thinking about the EARTH reference and use the AIR as a reference instead, so we (sometimes) INCORRECTLY think of the ground as a reference... even when flying!

 

Someone emailed me in response to this, respectfully pointing out that if flying North into a 20 kt headwind and then turning to the South, it still seems dangerous to him since how could the airplane pick up that extra 20 knots in the downwind turn to the South to hold his precious airspeed? The answer: IT DID NOT HAVE TO PICK UP 20 KNOTS OF SOUTHERLY SPEED AS IT TURNED... THAT 20 KNOTS OF SOUTHERLY SPEED WAS THERE BEFORE THE TURN EVEN BEGAN, IN THE FORM OF A 20-KNOT LOWER GROUNDSPEED AS HE FLEW NORTH! Again, it is nothing but a change in your reference system.

 

 

[Guest pelorus32]

Lovely one Ultralights.

 

I agree with you one hundred percent.

 

So now...use the same frame of reference to explain why aircraft fall out of the air when they encounter wind shear on final.....;)

 

Regards

 

Mike

 

 

[Guest Andys@coffs]

Let me try an analogy that will cover both.

 

A pest fly is in a traincabin that is zipping along the nullabor plain. to him his points of reference are those things inside the cabin and cares little that as he buzzes to the forward facing wall (this of course is the opposite of our flying point of reference being the ground) he is travelling at the speed relative to the cabin plus the speed of the cabin across the nullabor. The train in this case is an artificial wind.

 

Now lets assume that the train track is able at a particular point on the plain to execute a 90 degree bend and that the carriage is able to stay on the tracks and remain at speed as it executes the turn (of course in reality it cant for the resons of inertia) . If the fly were in the middle of the cabin airspace when the turn occured it is highly likely that he would deviate from his intended path, but would after the turn correct and think nothing more of it.

 

However if he was only centimeters from the side wall as the train executed that immediate turn he may well find himself smeared across the wall wondering why. While the fly has a small mass it still has inertia and you cant change inertia instantaneously (practically that is).

 

Where a change in wind occurs with rapidity, an aircraft has to overcome its own inertia before getting back to what it was doing before the sudden change. In some ocasions it is that innertia that will bite us(perhaps causing us to exceed VNE for a small period, or to stall for a small period) , in other cases the inertia helps us absorb turbulance. Flying my Trike which is slow with low weight when compared to my J230 the inertia in both means that a turbulent ride in the trike will most likely be less in the same airspace in the J230. A rapid and powerful change in wind direction (vertical or horizontal) can probably more easily be accomodated in the trike than the J230 for the same reason of lower inertia.

 

In fact a practical example of that is that where I learnt to fly the trike there was a 20kg(ish) "passeger" called Sally (A school bag full of concrete) that was taken flying in 2 seater trikes to smooth the ride for those a bit skitish of turbulence. fundamentally sally raised the inertia by increasing the overall mass.

 

 

[Guest smenkhare]

i saw this in either flying or aopa flight training. I think is was rod machado that was the author, wrote a page on why you don't have a higher chance of stalling.

 

 

[Tony]

Newton Laws and aerodynamics is a deadly natural mix

 

In my opinion it all depends on the mass of the aircraft, velocity of the wind, velocity of the aircraft and the rate of the turn from upwind to downwind. See this makes four parameters already !

 

Nothing to do with ground mass of the Earth, it does not provide the frame work, Newton knew that !

 

To make my input shorter, because the light aircrafts have much less inertia they can faster accomodate the speed CHANGE needed when air changes than the heavy ones.

 

So the Cesna is NOT better off than the ultralight in this particular excercise.

 

However, the ultralights do fall from the sky just BECAUSE they do have very little inertia indeed.

 

Paradox ? No. Explanation:

 

Having little inertia they do slow down so quickly therefore they often strife to MAINTAIN the propper air speed needed.

 

The indicated speed is everything !

 

 

[Flyer]

Try and convince aeromodellers of this fact.

 

Within the aeromodelling fraternity, of which I belong to, reference is made between ground speed and the aircraft. An aeromodeller will see the plane moving slowly upwind and increase power accordingly and then see it behaving like a rocket on down wind, consequently throttling back.

 

I would think that this is where some of the myth comes from as there are still fellas in my club that subscribe to this theory. Regardless of how hard you explain that the plane doesn't know its windy, you get the "oh you belong to that theory do you?"

 

They still haven't worked out how myself and a couple of others manage to fly backwards in a decent headwind.

 

as already noted... its all about IAS and not GS.

 

Windshear on the other hand is a different kettle of fish. A block of air moving in one direction versus another block of air in a different direction, often the opposite way. Often found around inversion layers (can vouch for this), mountain ranges (along with lots of other nasty stuff), faces of thunderstorms etc.

 

If you are moving in a parcel of air and that parcel of air reverses direction by 20 knots, this will be an indicated loss of airspeed by 20 knots. If you're close to the stall speed, increase speed or make sure you've got enough height to recover from a stall.

 

Regards

 

Phil

 

 

[Guest rolfeja]

Hi guys,

 

Sorry i dont agree with this and I think that the aircraft could definetly have a higher risk of stalling. Lets say for example you a flying an aircraft at 50 knots into 40 knot headwind. now lets assume you turn 180 degrees so that you have a 40 knot tail wind. The aircraft has to pick up speed by 40 knots. the aircraft takes time to pick up speed and if the turn was fast enough so as to not give the aircraft the time needed to pick up the speed it would stall. Sure if you turned slowly the aircraft would accelerate and by the time the turn is completed it would have accelerated by 40 knots and you would probably not even notice it. but if the turn was fast ie a high rate turn you could definetly stall. Thats how i see it anyway.

 

 

[Flyer]

Hi Rolfeja

 

Dont mistake ground speed with air speed. In your example you will have a GS of 10 knot upwind and a GS of 90 knots downwind. Your indicated airspeed will still be 50 knots regardless of upwind, crosswind or down wind. If you are flying in a stable pocket of air that is moving across the ground at 50 knots, the only way you know that you are in wind is by your ground speed. For all intensive purposes the plane couldn't give a hoot if there is wind.

 

The time your example holds true is in a wind shear and you are quite right, the plane will have trouble.

 

regards

 

Phil

 

 

[Guest rolfeja]

but the aircraft has mass and when the aircraft turns to fly down wind it will have to accellerate. it takes time to accellerate and if the aircraft cannot accelerate quickly enough to catch up with the relative change in the air direction it will lose some air speed.

 

regards

 

Jamie

 

 

[Guest rolfeja]

One example I can think of is if you are flying backwards with your model plane in the strong wind and you turned it around really fast so that it is flying downwind and you did not increase the power wouldnt the aircraft decend unless you increase the angle of attack?

 

 

[Flyer]

what change in air direction? The wind is just a pocket of air moving across the ground. If you are in that pocket of air you will feel no wind because you are moving with it. You will only know you are moving with reference to the ground.

 

This rule applies to a pocket of air moving at 1 knot and a pocket of air moving at 1000 knots....

 

What you are referring to is wind shear....

 

regards

 

Phil

 

 

[Guest rolfeja]

true but theire is a relative change of wind direction to the aircraft

 

 

[hihosland]

true but theire is a relative change of wind direction to the aircraft

Nope!!

 

If the aircraft stays in balance the air flow relative to the aircraft is consistently on the nose of the aircraft.

 

Davidh

 

 

[Guest rolfeja]

hmm i will think about it a bit more and try it out next time im flying by watching my asi and see what happens:)

 

 

[Guest pelorus32]

Hi Rolfeja,

 

This is all to do with your point of reference. To start with imagine that you are in a hot air balloon. This "aircraft" has no capacity to generate speed on its own account so it moves with respect, only to the movement of the airmass. It is STILL within that airmass. Its movement relative to the ground is as a result of it being still in a moving airmass. Indeed if you could "see" the airmass and not "see" the ground all you would see is the balloon still within the airmass.

 

Any other aircraft is just the same, except that it moves at its AIRSPEED (say 80 knots) with respect to the airmass that it is in. The simplest reference is still air. The aircraft will move at 80 knots - its airspeed - through the airmass in any direction and move with respect to the ground at the same speed in any direction as its airspeed - 80 knots.

 

Now if we induce a "speed" to the whole airmass of 20 knots in a northerly direction what happens? The aircraft will still move at its airspeed of 80 knots in any direction with respect to the airmass. Nothing has changed from the still air situation (indeed as far as the aircraft "knows" it is in still air. It's just that this still air is moving across the ground, but it's still for the aircraft). But it will move 20 knots slower with respect to the ground when travelling south (60 knots) and 20 knots faster with respect to the ground when moving north (100 knots). But these are ground speeds not airspeeds.

 

But ground speed has nothing to do with whether an aircraft stalls or not. Only airspeed matters as far as the aerodynamic performance of the aircraft is concerned. The airspeed never changes unless you change the aircraft configuration in some way so unless you change something to increase your angle of attack beyond the critical you can never stall. A simple turn does not constitute such a change in configuration.

 

Regards

 

Mike

 

 

[Guest rolfeja]

Hi Mike thanks for your response. There is one thing though that has not been taken into account in this explanation. The inertia of the aircraft. how about we talk about turning from downwind into a headwind. . If the aircraft was to suddenly turn into the wind wouldnt it pick up some speed for a moment because it would take time for the inertia of the aircraft to slow down. Or we can think of it in another way. Lets first say there is a 50 knot westerly. Now lets imagine an aircraft is climbing verticly up in the air until it almost stalls at this point the pilot kicks in full rudder and the nose comes around pointing straight down at the ground. When the pilot has some speed he pulls sharply out fo the dive into the tailwind. now with that sudden change in direction wouldnt the aircraft loose some speed until it ha time to catch back up with the wind. Or what about if he pulled out of the dive into the wind. wouldnt he suddenly have a 50 knots of extra airspeed that may last for 3 seconds or so until the inertial of the plane has been slowed back to the oringinal airspeed.

 

 

[Guest TOSGcentral]

Dear God – this one again!

 

 

Rolfeja you are entire right and entirely wrong! Pelorous has it by the nuts!

 

 

Where you are right Rolfeja is if you are dealing with a change in the energy of the air that you are flying through.

 

 

This, within the closest context to what you are espousing (and I am talking of an airmass removed from ground orographic effects, is only in upper air wind shear or flying in and out of thermals – i.e. in the horizontal and then it is the energy of the surrounding air that is changing not the energy of the aircraft. The aircraft is just trundling through a stationary block of air (relative to itself) that happens to be also trundling across the ground and taking the aircraft with it.

 

 

The majority of where you are correct is in vertical changes in airmass energy. These are normally, but not always, close to the ground and are caused by air movement over the ground.

 

 

Many instructors have grave difficulty getting through to students why an aircraft will start losing airspeed when descending through a wind gradient. But that is a vertical change in airmass energy dictated by ground friction with the moving airmass (wind).

 

 

The confusion in the student is because by this stage they have normally been thoroughly drilled on airspeed/ground speed combinations. So given a transition from a 20 knot headwind when flying at 60 knots to a 15 knot headwind then reasonably your groundspeed should increase by 5 knots – Right? Wrong!

 

 

We are talking here primarily about time and this is where your comments on intertia come in. The aircraft cannot instantly pick up 5 knots of airspeed over the ground! Something has to give and that is the depleted air energy that depletes the airspeed of the aircraft and actually slows it by those 5 knots – unless you do something about it by lowering the nose!

 

 

But that is near the ground – possibly up to about 300’ agl on flat sites but much higher on hill sites where curl-over or wave effects may be present.

 

 

Frankly I despair over this one because the text books do not arm students well enough, or broadly enough, for actual hands-on flying and some of those students become flying instructors.

 

 

Suffice to say that when I was training flying instructors they were faced with a threat (or some of them were anyway). If they once more mentioned ‘wind’ when referring to airmass basic flying dynamics then they would cease training immediately until they learnt a bit more about meteorology and physics! It never actually came to that – I did the teaching myself (extra to the course).

 

 

Pelorous is entirely right and in one important part he is most right – perceptions of the observer.

 

 

I am a bit tired right now because it has been a busy few days for a number of reasons. But when I have time I will put a brain teaser on this thread that will demonstrate that an ordinary car can be travelling at 60 mph and be standing still at exactly the same time!

 

 

Watch this space!

 

 

Aye

 

 

Tony

 

 

[Ben Longden]

Is it me, or have we forgotten the angle of attack thingy with relation to stalls?

 

Something about airflow at an angle of greater than 16.5 degrees to the base line of a wing will result in loss of lift.. in other words, a stall?

 

Movement of an AC in a parcel of air will relate as Mike said to the ground speed / Indicated airspeed concept.

 

Remember the basic training we had, and why we do not conduct a climbing turn with the wings greater than 20 degrees from the horozontal? It does not matter what the wind strength or direction is... its got everything to do with stalling the outer wing.. or rather preventing the outer wing from stalling during the manouvre.

 

Angle of Attack.

 

Sounds like a George Bushism....

 

Ben

 

 

[Guest rolfeja]

but surely the aircraft must have to accelerate when turning downwind?

 

 

[Guest rolfeja]

what i mean is if there is a 40 knots wind and the aircraft turns down wind wouldnt it have to accelerate by 40 knots to have the same airspeed?

 

 

[farri]

Down wind turns.

 

This is an easy one.

 

An aircraft will only stall," WHEN THE CRITICAL ANGLE OF ATTACK HAS BEEN EXCEEDED"

 

Definition.

 

Angle of attack is, "THE ANGLE OF THE CHORDLINE TO THE RELATIVE AIRFLOW"

 

Chordline is,"THE LINE RUNNING FROM THE LEADING EDGE TO THE TRAILING EDGE OF THE AEROFOIL".

 

Relative airflow is,"THE AIRFLOW, FLOWING DIRECTLY OPPOSITE THE FLIGHT PATH OF THE AIRCRAFT".

 

Full Stop.

 

farri

 

 

[Guest TOSGcentral]

Rolfjea,

 

Accelleration is a relative term as is decelleration and both require a medium to measure the relative change of speed.

 

It may help readers if you explained what you are using as a reference point!

 

For examples:

 

1. An aircraft turns downwind relative to a moving airmass. Relative to the ground it has gained speed over the ground.

 

2. An aircraft is flying in a huge hangar and makes a turn. The air in the hangar is trapped and so there is no 'wind'. However the huge hangar happens to be on wheels and is trundling along at whatever knots over the ground.

 

What the aircraft experiences is nothing other than the aerodynamic influences of the turn. But relative to the ground the hangar is trundling over - certainly it will gain or lose speed. So what? It has not changed its airspeed. It has not changed its angle of attack other than to provide energy for the turn. So where does a stall come into your equation?

 

Tony

 

 

[Yenn]

Forget aircraft and get yourself a fish tank. Take it for a train trip and watch how the fish swim. Do they go faster when they go towards the rear of the train or towards the front.

 

Really this topic has been done to death here and in other places and I am amazed that it still comes up for discussion, unless there are a few stirrers who want to keep the cat in with the pigeons. Thinking of which. Why not take a few pigeons on the train as well. I am sure they will stall every time they turn at the end of the carriage.

 

 

[Guest rolfeja]

lol yeah i give up I am in 2 minds now. If i think of it like the plane in a hanger i think i am wrong but if i think of it like a plane pulling from a into a down wind I cant help thing that it will lose airpseed. in anycase I have turned down wind almost everytime I fly and havent had any problems:) I am going to speak to a very smart physics teach I know who also is a pilot and ask him to explain.

 

 

[Guest rolfeja]

thats suppose to say a plane pulling out from a dive to a tailwind