The Windward Turn Theory

[itsnotthatbloodyhard]

Originally Posted by currawong

1/ The downwind turn fallacy should have become extinct with the advent of GPS showing groundspeed. Easily compare actual airspeed with actual groundspeed. Actual numbers are hard to dispute, even for a Flat Earther.

Sadly, I think this thread demonstrates that it’ll never become extinct.

[Bergerie1]

911slf,

You are indeed right about the clever use by birds of the wind gradient with altitude. I have spent hours in the southern ocean watching albatross using this method and also using ridge-lift over huge waves. Birds know more about flying than we ever will!!

https://www.researchgate.net/publica...ication_detail

[hans brinker]

Originally Posted by currawong

1/ The downwind turn fallacy should have become extinct with the advent of GPS showing groundspeed. Easily compare actual airspeed with actual groundspeed. Actual numbers are hard to dispute, even for a Flat Earther.

2/ Our bird theorist needs to get in an aeroplane and fly in ground effect, over varying surfaces, including swells.

That is, if a dreaded downwind turn doesn't get the better of him first....

Sadly, I see the GPS made it worse, they make the downwind turn and see the groundspeed increase, and think they have increased momentum...............

[Wizofoz]

Our friend is doubling down.

He has now directly championed the downwind turn myth.

The Downwind Turn

[zzuf]

Been thinking that it is probably possible to construct a simple analogy for this, though I haven't put any real thought into it.
It involves the idea that, in aerodynamics, it doesn't matter if the aircraft is moving through the air or the air is moving past the aircraft.
Anyway, it involves a large stationary cube of air with a solid floor (the ground) and a solid ceiling.
The floor is moving from (say) left to right and the ceiling is moving from right to left, both at the same speed WRT the air cube.. This being the equivalent of equal speed winds blowing in opposite directions.
Place an aircraft in the stationary air cube, in a constant turn at a fixed bank angle and CAS.
So according to the turning myth, with reference to both the floor and the ceiling, the aircraft is turning both upwind and downwind simultaneously, and according to the theory of the myth it would be both accelerating and decelerating simultaneously.
This cannot possibly be correct.
But as I said I have put no real thought into this.

[currawong]

"The Downwind Turn".

The man is clearly a fool. If the downwind turn was so deadly I would be dead many thousands of times over.

And to some it might be deadly, if susceptible to disorientation.

I would like to see his take on loading it to a knife edge, right to the region of reversed command. Lose a knot and you are going down. Any turn other than into wind would bring you unstuck, if his theory is correct.

[Wizofoz]

Originally Posted by zzuf

Been thinking that it is probably possible to construct a simple analogy for this, though I haven't put any real thought into it.
It involves the idea that, in aerodynamics, it doesn't matter if the aircraft is moving through the air or the air is moving past the aircraft.
Anyway, it involves a large stationary cube of air with a solid floor (the ground) and a solid ceiling.
The floor is moving from (say) left to right and the ceiling is moving from right to left, both at the same speed WRT the air cube.. This being the equivalent of equal speed winds blowing in opposite directions.
Place an aircraft in the stationary air cube, in a constant turn at a fixed bank angle and CAS.
So according to the turning myth, with reference to both the floor and the ceiling, the aircraft is turning both upwind and downwind simultaneously, and according to the theory of the myth it would be both accelerating and decelerating simultaneously.
This cannot possibly be correct.
But as I said I have put no real thought into this.

Yes, similar thought experiments have been put to down-wingians, such as "you are in an empty oil-tanker which is moving at X-knots. Thus the air inside is moving relative to the earth a x-knots. Will a model aeroplane turning downwind inside the tank suffer the "downwind turn" phenomenon.

Tha response is usually "yes.....errr.....no....errr......that's not relevent" followed by a quick subject change.

[Jet_Fan]

I see this thread has been reignited by a non flying theorist and then another non flying theorist stepped up. Then, after being soundly beaten they run away. Did they learn anything?

[autoflight]

• the earth is not flat
• aircraft is actually fly through air even if the air itself is moving
• Newton’s Laws of Motion apply
• the rotational speed of the earth at the equator,
• the speed of light
• the universe is expanding

I believe in the above, but only two apply. A tank full of air in a truck turning a corner with a drone inside or throwing a ball in the air in a speeding train and a few other things previously claimed do not apply.

In the early days of windshear understanding, we used a technique of increasing approach speed with the expectation of loss of IAS due to known loss of headwind closer to touchdown. Loss of headwind and then increasing tailwind is an approach that many experienced pilots would tend to avoid. My modern airliner might even warn of the the danger with a wind display or a warning. Classic horizontal windshear on approach is flight between areas with changing winds with a particularly bad case being loss of the entire headwind and increasing tailwind, with urgent action required to survive.

If the above can be reasonably accepted, we are almost there.

So what has windshear got to do with a simple downwind turn? In both cases the aircraft is flying into a reducing headwind and in the case of the downwind turn, we have set the circumstances to be eventually an equal tailwind.

In the downwind turn we are deliberately flying from an area with a headwind to an area with a tailwind. We are effectively creating our own windshear.

On approach, windshear might develop over maybe 5 - 10 seconds with urgent action required, but our downwind turn might take a minute. Depending on the actual wind for our downwind turn, little or no pilot response may be required. If the wind was 50K, and the aircraft was already low and slow, the loss of IAS would be more noticeable.

Fortunately for aviation, turns would normally be around rate 1. The minute taken to more gently progress from a 50K headwind to a 50K tailwind gives the aircraft auto flight systems the opportunity to recover some of the IAS loss. If our 200K IAS aircraft could make that same 180 degree downwind turn in 30 seconds, recovery of lost IAS would take more effort.

My downwind turn is simply a 180 degree turn from a headwind to a tailwind. It could be at any altitude and does not necessarily refer to a light aircraft turning from take-off heading to downwind in a circuit pattern.

[A Squared]

Originally Posted by autoflight

.

So what has windshear got to do with a simple downwind turn? In both cases the aircraft is flying into a reducing headwind and in the case of the downwind turn, we have set the circumstances to be eventually an equal tailwind.

In the downwind turn we are deliberately flying from an area with a headwind to an area with a tailwind. We are effectively creating our own windshear.
.

No you are not. In the real winshear examples you are crossing wind gradients. In a turn to downwind in a constant wind you are not crossing any wind gradient. A wind gradinent is the fundamental element in windshear and it doesn’t exist in a turn in constant wind. To be more specific, if you are flying into a 20 knot wind out of the east and you turn west, you have not crossed any wind gradient, because gradient means change and the wind hasn’t changed one iota. The wind is still blowing the same 20 knots in the same direction, out of the east.

So, no, turning downwind is nothing like windshear

[Vessbot]

Originally Posted by autoflight

So what has windshear got to do with a simple downwind turn? In both cases the aircraft is flying into a reducing headwind and in the case of the downwind turn, we have set the circumstances to be eventually an equal tailwind.

In the downwind turn we are deliberately flying from an area with a headwind to an area with a tailwind. We are effectively creating our own windshear.

No, we're not. A Squared beat me to it. In wind shear, you're transitioning from one air mass to another, in a small enough timespan that the airplane's inertia temporarily preserves groundspeed, and thereby changing your airspeed. If given enough time after the transition to reach steady state, the plane would stabilize at the original airspeed.

In a turn within the same airmass, there is nothing affecting the airspeed other than drag (the same way that drag would affect your airspeed if the airmass were not moving, i.e., zero wind). Rate of turn does not matter for the ability to recover lost airspeed, because there is no airspeed loss to recover. If it was a function of a gradual enough rate of turn, as you claim, that implies that there is some rate of turn/wind combo for which the "graduality" safety effect would not work, and there would be a significant airspeed loss. How about when you are walking upwind at 1 knot at knots in an airliner, toward the tail, with a groundspeed of negative 499 knots. You turn around to face the nose (downwind) in one second, and keep walking at 1 knot airspeed, but now your groundspeed is 501 knots. You just walked through a one thousand knot self-created windshear in one second, according to your theory, which predicts that there should be some effect. But there is none. When should this effect start kicking in? Remember that your theory also has to account for the "groundspeeds" around the center of the Earth, around the Sun, around the center of the galaxy, etc.

[Winemaker]

I'm a winemaker, not a pilot, but have a huge interest in aviation and really enjoy this site. Reading this thread I have only one thought: If you're flying in a constant wind it doesn't matter how you turn, upwind or downwind, as you are moving in the frame of reference of the wind. The wind velocity can be 10 knots or 150 knots relative to the ground, it just doesn't matter. You are flying in a volume of air and the airplane is only reacting to forces generated by its movement though that volume. Ignore the ground speed, that's not what generates lift. Turn left, turn right, turn downwind, turn upwind, it's all good as you are effectively turning in still air as you are swept along over the ground. You are going to move across the ground pretty fast flying downwind in a 150 knot breeze and you might be standing still in a 150 knot headwind, but it just doesn't matter. Thanks for all the great comments on these threads, it's pretty educational for me.

[currawong]

How about the boat crossing the river, for those that prefer to communicate in grunts.

Turn 180 degrees. Downstream. Rate one.Turn takes up lots of room as getting washed downstream.

Turn 180 degrees. Upstream. Rate one. Turn takes up less room as getting washed downstream.

Boat speed and rate of turn relative to water is constant.

Boat speed relative to bank varies due current.

Closest I can get to using monosyllables.

[itsnotthatbloodyhard]

Might as well give up, folks, there’s no point. It’s like a sort of religious zealotry. Doesn’t matter how simply you explain it or how much you appeal to logic and reason, you’re fighting an unshakable belief (with occasional abuse and misunderstanding of terms like ‘frame of reference’ and ‘Newton’ thrown in for a bit of pseudoscientific gravitas). Nice try though.

[Meikleour]

Most people seem to agree that airspeed fluctuations due to windshear are the result of the aircraft having an inertial groundspeed which causes the changes to be apparent as it experiences differing air masses and takes a finite time to regain its momentum ......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)

I have observed many times descending into NRT with a tailwind of 200kts (not uncommon at certain times of year) with the autopilot in IAS hold and smooth wind conditions. The arrival called for a 90 degree heading change and whilst the IAS remained constant the rate of descent increased dramatically once the turn was complete (ie. much lower groundspeed) before stabilising back to the normal rate. This behaviour always seemed at odds with the theory.

[Wizofoz]

Originally Posted by Meikleour

Most people seem to agree that airspeed fluctuations due to windshear are the result of the aircraft having an inertial groundspeed which causes the changes to be apparent as it experiences differing air masses and takes a finite time to regain its momentum ......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)

It's not ignored, just often misunderstood.

An aircraft turns by tilting it's lift vector. Lift is proportional to airspeed, not ground speed. Momentum is relative to velocity which is relative to a frame of reference. It is not exclusively proportional to ground speed.

[Vessbot]

Originally Posted by Meikleour

......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)

Can you describe how the effect of momentum should be taken into account in what should happen to the human body when turning around inside a moving airliner and experiencing a near-instantaneous thousand knot reversal in groundspeed?

[Brercrow]

Originally Posted by Meikleour

Most people seem to agree that airspeed fluctuations due to windshear are the result of the aircraft having an inertial groundspeed which causes the changes to be apparent as it experiences differing air masses and takes a finite time to regain its momentum ......................now, when flying in a stable but moving air mass where does the effect of aircraft momentum go? (why is it always ignored?)

I have observed many times descending into NRT with a tailwind of 200kts (not uncommon at certain times of year) with the autopilot in IAS hold and smooth wind conditions. The arrival called for a 90 degree heading change and whilst the IAS remained constant the rate of descent increased dramatically once the turn was complete (ie. much lower groundspeed) before stabilising back to the normal rate. This behaviour always seemed at odds with the theory.

This is explained as follows: The aircraft turns due to a horizontal component of the lift force. Lift force is the equal and opposite effect of momentum given to the air. In a turn, part of that momentum is horizontal. As the aircraft turns from downwind to crosswind it loses momentum as it loses groundspeed. This acceleration is caused by the aero-forces acting on the aircraft modified by the angle of drift. The air gains horizontal momentum and part of that is in the same direction as the wind. So aircraft loses momentum and wind gains momentum.
At the same time, due to inertia, the airspeed is affected by the reducing tailwind component and there is a slight tendency for airspeed to increase. (Same as an increasing headwind)
If the aircraft is descending in IAS hold and constant thrust, the effect is that the auto pilot will hold the airspeed and the rate of descent will decrease slightly but only during the turn.
When the aircraft rolls out on the crosswind heading, the thrust is insufficient to maintain the reduced rate of descent and the IAS hold pitches the nose down to maintain the airspeed. The rate of descent increases rapidly but settles back once equilibrium is restored at the original rate of descent.

[Goldenrivett]

experiencing a near-instantaneous thousand knot reversal in groundspeed

Errrr.....by my maths, that is going from 499 kts to 501 kts ground speed i.e 2 kts change in ground speed with respect to both earth and to the aircraft.

What momentum is being ignored?

[Jet_Fan]

Originally Posted by Goldenrivett

Errrr.....by my maths, that is going from 499 kts to 501 kts ground speed i.e 2 kts change in ground speed with respect to both earth and to the aircraft.

What momentum is being ignored?

You turn to face the other direction so your ground speed becomes negative INSTANTLY!