Wizofoz: I agree with you however I think you will find that I never claimed lift was proportional to groundspeed! For the windshear analogy to apply the momentum must be related to the groundspeed ie. relative to the earth. Why is it therefore that when operating in the "bubble of air" concept then reference to the earth is discounted? Likewise can you help me understand the often observed phenomenon that I cited.

While free flying, the relationship with the ground is purely a navigational one and has nothing to do with the aerodynamics of the plane. Can you really not grasp this?

Vessbot:Turning around inside an airliner does not change your momentum with respect to the earth. It is still related to the vector of the aircraft. The only change you could make is to run from nose to tail down the aisle and then you would only reduce it by your running speed times your mass! A tiny reduction compared to the aircraft. This is the same fallacy about being in a runaway lift(elevator) where the occupants are asked to jump upwards just before impact with the bottom to save thenselves! Won't work.

Jet Fan: I think you are being ironic!!

Winemaker has it exactly right. If you don't believe him/her, or me, ask a friendly mathematician to construct a position vector from a fixed point on the earth to an aircraft performing a constant rate turn in a steadily moving airmass, and differentiate it twice to determine the acceleration of the aircraft. It is always towards the centre of the circle of the constant turn, a point which itself is moving with uniform speed over the ground. To demonstrate it in flight, choose a windy day and ask an instructor (better still, a test pilot) to fly an accurate rate one 360 degree turn under the hood while you look out of the window. The acceleration will feel fine, bank angle constant and ball in the middle, but at low level your track over the ground will scare you stiff and illustrate why trying to turn near the ground in such conditions using visual references is so dangerous.

yep,

exactly that

From negative 499 to positive 501, that's a thousand knot increase. The airspeed is 1 knot before and after, unchanged.

And *I* don't think that there is anything ignored about the momentum, but the windward turn theorists do (like the one I quoted in my reply) and I'm challenging then to apply their reasoning to that situation.

Yes, and is this any different (and why?) from turning around inside a uniformly moving airmass?)

This thread is moving fast and I'm at work so I can't get detailed until later, but yes this is exactly a 1000 knot self-created wind shear according to the premises of the windward turn theorists. Check it again.

No, you do not need to calculate groundspeed to understand winds hear- because the aircraft is not interacting with the ground, so it's momentum relative to it is irrelevant. Windshear happens when the aircraft's momentum relative to the AIR changes.


Say you are turning on a still air day- but on the ground there is a passing truck- you could calculate your momentum relative to IT- it is as relevant a momentum figure as that of the earth, but it doesn't change the dynamics of the situation does it?

As to the increased rate of descent you cite, did it come with an increase in airspeed also? I have often observed older FMCs get caught out but changes in wind and do some radical stuff to maintain programed path.

In case anyone was wondering, pretty sure this is the author of the website. Same ridiculous toss.

Here's a typical flight example of the windward turn that should suffer an airspeed loss according to the windward turn theorists:

Vehicle making the turn: Cessna 172
Wind (uniform airmass over the ground): 10 knots, East to West
Initial heading: East
Initial airspeed: 100 knots
Initial groundspeed: 90 knots (airspeed minus headwind, 100-10)
Groundspeed after 180 degree turn: 110 knots (airspeed plus tailwind, 100+10)
Groundspeed change, aka "self created wind shear:" +20 knots (final groundspeed minus initial groundspeed, 110-90)

And my equivalent example:

Vehicle making the turn: Human body inside an airliner
Wind (uniform airmass inside the airliner): 500 knots, East to West
Initial heading: East
Initial airspeed: 1 knot
Initial groundspeed: negative 499 knots (airspeed minus headwind, 1-500)
Groundspeed after 180 degree turn: 501 knots (airspeed plus tailwind, 1+500)
Groundspeed change, aka "self created wind shear:" +1000 knots (final groundspeed minus initial groundspeed, 501 minus negative 499)

The windward turn theorists say that in the first example there is a tendancy to lose airspeed due to the headwind loss/tailwind gain, but in most situations it's minor enough not to notice since the turn is so slow comapared to the shear amount (20 knots over 1 minute) that the momentum is gradually changed to stay caught up with the airspeed.

If this logic is true, it implies that there could be an example, if we tweak the numbers enough, where the momentum doesn't have a chance to gradually change, and the result would be a noticable airspeed loss. So how about it, if 20 knots over a minute is not enough, how about 1000 knots over a second? Has anyone ever noticed any effects of this while walking up and down the isle of an airliner? Maybe it's still not enough for the effect to rise above the noise floor. Do we have to tweak the numbers further to make the airmass be the inside of a Concorde? Or an Apollo command module on translunar coast?

Of course not, this is bogus and Newtonian relativity holds true, i.e., if you close the window shades all physics occur as if it's sitting still. Or moving uniformly in any direction at any speed.

Well things went dead quickly... surely I couldn't have had the last word, could I? :ooh:

No, THIS is the last word......

Are you sure?

Yes, that was definitely the last post.

I'm not so certain.

Has the thread turned downwind and lost airspeed?

I want to make it clear that nowhere in my website do I say that groundspeed affects an aircraft in flight.

Read the link at post #102 and you will find a rational explanation of the effect of the wind during turning flight.

Stall spin crash burn - beware the downwind turn. But it is mostly pilot error.

No, you will not, you will find complete nonsense which ignores physics, couched in pseudo-scientific terms not understood by the author.

I'll ask you the same question that the other Myth-ers couldn't handle. (either refused to answer or answered incorrectly)

2 identical airplanes with a cruise airspeed of 100 knots. Both are flying east. Airplane A is in still air. Airplane B is flying into a 50 knot wind blowing out of the east. Both turn 180 degrees and fly west. What is the change in velocity from before the turn to after the turn for each plane?

You say wind affects the dynamics of an aircraft in flight- it's the same thing and equally wrong.

You are a patient man Wiz.

I always remember that Mark Twain quote-

“Never argue with stupid people, they will drag you down to their level and then beat you with experience.”

Then I go and do it anyway...…..

Which force is that then?

A plausible question which is irrelevant. You are comparing two states of equilibrium. You are not thinking about what happens during the turn

Seriously? You started this thread and you have not read the website. If you had then you would not have to ask that question.

(I would show you now but Pprune does not allow me attachments)

During the turn the lift vector, which is identical whether the wind blows or not, causes a turn of constant rate and constant (air) radius.

Nope, not irrelevant at all. It is the very crux of the "downwind turn" fallacy and the fact that you are trying to dismiss it as irrelevant just illustrates that you don't have the fundamental grasp of physics required to understand why you are mistaken.

I have looked at that site.
You state (when discussing an aircraft, in a steady wind, doing a constant rate turn)
I don't get what you are saying and it seems to be the crux of your argument.

1. You can only get an acceleration by applying a net force.

The only force, acting on the aircraft, "caused" by the earth, is gravity.

Yet you seem to argue that somehow the earth acts to accelerate the aircraft laterally. How? What force is this, where does it come from and how does it act on the aircraft?

2. That spiral path would appear exactly the same for any circling object if it was viewed from a constantly moving point of reference. No additional force is required to make the spiral path. (eg one of those conical pendulums dropping sand on paper - pull the paper at a constant rate and you get the very same spiral.) Not getting where any additional force is needed to get that pattern.

I am not saying that the Earth accelerates the aircraft laterally. You have just made that up. The aircraft is accelerated by the aerodynamic forces acting on it. Read the link again.The whole thing! There are diagrams!

not trying to make stuff up, apologies if I misunderstand what you are saying but I am not getting what you are saying.

Your argument (as I read it) is that in a constant wind, an aircraft in a constant rate turn, will have (small) changes of airspeed as it moves from a headwind (relative to the ground) to a tailwind (relative to the ground). Is that correct?

Would you agree in this situation that the only forces that are directed laterally on the aircraft are thrust and drag and inclined lift, all are acting due to interaction with the air?

The angle of bank doesn't change so the lateral component of lift is constant.

We do not change thrust.

The aircraft can only generate an acceleration (which would be required to change the airspeed that you say happens) is if we change drag - that is the only force we have left (as far as I can see).

Why does drag change?

The only way I can see that changing is because airspeed changes but that is assuming the effect we are trying to find the cause of - it would appear to me to be the airspeed changes because the drag changes and the drag changes because the airspeed changes therefore the airspeed changes... ?? ie we are assuming the effect we are trying to prove occurs.

I am not getting where an unbalanced force can come from that causes the (small) airspeed changes you claim are occur.

Now if the wind velocity varied (eg a gust or windshear in descent/climb etc) then that would cause a change in airspeed but not in a constant wind. What am I missing here?

I hesitate to get involved in this argument as the positions are so entrenched. However, a little observational input may assist.
I was working as local/tower controller at Valley in Anglesey. It was a quiet Sunday and as was quite common there the wind was gale force although steady and not gusting as it came off the Irish Sea. The surface wind was almost due West and probably 50Kts at least (it was a mbmmble time ago).
A Stampe biplane wished to depart back to England to the East and 'taxied' if that is the right word for driving straight into wind onto the grass at close to full throttle with members of the visiting aircraft flight on both wings and fuselage near the tail plane. The pilot asked for takeoff and waved off the VAF crew and the tail of the Stampe lifted up and then the aircraft effectively took off close to vertically into wind. The aircraft climbed to around 200 ft straight up, then turned left steeply cross wind then downwind onto East. As it did so it lost a significant amount of height as it also gained a significant amount of speed. The last call to me in the tower from the pilot was: "I won't be coming back!".

Yes I know all the arguments about being in a windfield so the ground speed doesn't matter - but I had the feeling that it did. The inertia of the aircraft that was close to stationary had to be overcome for the aircraft to depart at (guess) wind of 60 kts at 200ft plus aircraft eventual flying speed of say 60kts meant that the airframe had to accelerate from stationary to 120kts - agreed helped by the airmass it was in. That inertia has to be overcome regardless and that is due to the frame of reference to the Earth/ ground speed of the airframe. So an extreme example but I assure you that the aircraft lost a lot of height in the turn going past at my eye level in the tower. A more gentle turn may have masked the effect but it would still have been there.
I think it is a case of relative speeds: wind speed to aircraft speed. A military fighter at 400 kts + in a relatively low wind may not notice the effect. An albatross at 20kts may notice the inertial effect of a turn in a wind of equal velocity.

As has already been stated, gravity was the only force being exerted on that aircraft by the Earth. As the pilot turned he would have felt some g-force but as he turned downwind he wouldn't have been pushed into the back of his seat as his groundspeed went from 0 to 120 due to anything other than centrifugal force, which would feel the same as on a calm day.. He could have lost height for any number of reasons but it will always look more dramatic going downwind to a ground based observer.

May also have succumbed to the (potentially deadly) skid/slip illusion. I see it sometimes with students (and sometimes licenced pilots) flying circuits in strong winds, they try and fly their turns using reference to the ground, they try and make things look the way they normally experience their path over the ground by over ruddering or over banking whilst using the same nose attitudes they normally do.

Both actions will reduce climb performance and increase descent rates. Can also set you up for stall/spin at low level :(

It is a powerful illusion when low to the ground at low airspeeds in strong wind.

estimation of distance (height) by the angle subtended at one's eye.
 

Hi Ian W,
Relative to you, the aircraft had to accelerate from 0 to 120 kts (change of 120 kts). Relative to the aircraft it had to accelerate from 60 kts heading North say to 60 kts heading South (i.e. 120 kts change in air speed vector - i.e. the same vector value)

Your estimation of distance is made using the apparent size of a known object (angle subtended at you eye). Your estimation of height is made using the angle between the object and the horizon. As the aircraft appeared to accelerate unusually rapidly down wind, it's distance from you increased unexpectedly rapidly and the angle between the horizon, the aircraft and your eyeball reduced rapidly giving you the illusion of a height loss.

The illusion is similar when observing a "slow" jumbo compared to a faster 737 on the approach when both are flying at say 160 kts. The jumbo appears slower because it appears closer and yet the change the angular position is the same as the "further away" 737.