Any which way the wind blows (physics brainteaser)

[ThinAirDesigns]

If by "Unistrumented" you mean no Vario, altimeter, and airspeed gages, and no peeking at the ground and horizon, I don't think it's possible.

I stated "uninstrumented" just to simplify the elimination of GPSs and other more high tech solutions. -- but you've intrigued me so I'll be more specific.

Given 2 of the 3 instruments and a means to maintain wings-level, yes

Here's what I'll give you:

> viewing the horizon for wings level is just fine (or even an artificial horizon if you so wish) but the ground is featureless and bland -- you can derive no directional information from it.

> you may have vario, altimeter and airspeed.


If you can do with with the above, it has potential of being quite usable.

JB

 

[Tumbleweed]

This fails for all sorts of reasons and I'll pick only one for now:

The spot you marked will only remain under that plumb bob as long as the pitch angle (think nose to tail) remains the same. Once the aircraft leaves the ground, changes in pitch from speed and lift changes will alter the planes orientation relative to vertical ... meaning that the bob can hang in many different spots, even in still air.

An example of this would be: if I'm flying in a straight line in perfectly still air @ 60knots and I mark the plumb bob spot and then I speed up to 70knots, the bob will now hang *forward* of it's previously marked location. This happens simply because to speed up I had to reduce the angle of attack of the wing and I did that by lowering the nose. This alters the aircraft orientation relative to vertical and is reflected in the new bob position.

The position you marked on the ground will reflect all sorts of things unrelated to it's positions in flight including something as simple as how much air is in the aircraft's tire(s).

JB

Okay, I forgot that you are talking about a glider, so pitch and yaw are allowed. But I thought you said the speeds were constant, not changing. Now you've added forward speed as well as uplift, sink rate and wind speed. There is no forward speed in the original set of conditions. Is calculating one necessary?

 

[ThinAirDesigns]

Okay, I forgot that you are talking about a glider, so pitch and yaw are allowed.

Pitch and yaw are allowed on any aircraft I've ever flown -- powered or not.

But I thought you said the speeds were constant, not changing.

Read the OP -- the windspeed is fixed and constant ... The pilot may fly the aircraft any which way he/she pleases.

Now you've added forward speed as well as uplift, sink rate and wind speed.

I've added no speeds. I've taken no speeds away. Read the OP

There is no forward speed in the original set of conditions.

It's pretty hard to imagine a soaring aircraft in flight without "forward speed". Read the OP.

Is calculating one necessary?

The pilot may do as many calculations as he/she pleases. Read the OP.

JB

 

[Clive]

Follow the arrows that point north...

Assuming the source of each thermal is fixed on the ground (instead of say, a fire that moved downwind exactly as fast as the wind ) then as the column of hot air rises, it is also being moved horizontally by the wind. The column ends up looking like the leaning Tower of Pisa because the amount of horizontal translation from the source is proportional to altitude. A good pilot can therefore find a thermal in the usual way, and then effectively "slide up and down it" to determine the direction of the wind. (This came to me as I returned from helping put out a scrub fire!)

 

[Dan O.]

I suspect experienced pilots do this all the time. Relying on ground clues can be deceptive when working thermals.

ETA: And what Clive said.


PS: Last I heard, the fire wasn't out yet and the wind is picking up again.

 

[RossFW]

Clive,

Yes, but the glider is also part of the airmass. It will equally drift with the wind. The only way to tell which way the thermal is "Leaning" is by comparing the gliders position over the ground with its altitude as it encounters lift, and this is not allowed by the OP.

The only way I can see is that a series of thermals is usually produced by the same source and drift downwind. If, by trial and error you determine which dirrection the NEXT thermal is coming from by flying into it, you know you are flying upwind.

 

[RossFW]

hp,

A quick attempt at an explanation of Dynamic soaring:-

Under certain circumstances, there will be a distinct difference in the winds direction and speed between two layers and over a relatively short vertical distance. This can be an Aviation hazard when it is close to the ground and near an airport(Wind shear).

One of the most reliable sources of this is found on the lea side of a hill on a windy day.

The effect of flying through a shear layer is that, because of momentum, the aircraft will have a short-term change in it's speed relative to the air (Known as True Air Speed). this would be modeled (though not according to Humber!!) by walking on to a moving walk way wearing roller skates. Until friction can accelerate you to the speed of the walkway, the wheels of the skates would turn and you would initially maintain your previous velocity, and thus you would have a temporary velocity relative to the walkway.

If the shear is a sudden increase in tailwind, or reduction in headwind, the result is a loss of airspeed, and therefore a loss of the ability of the wings to produce lift (L=cl 1/2 RHO v^2 where V is speed through the air) This is the aviation hazard as, in severe cases, it can exceed the aircrafts ability to accelerate whilst producing enough lift not to crash.

If the opposite is true, however, and the aircraft is flown into an increasing headwind, there is a temporary INCREASE in airspeed. A highly efficient aircraft, like a glider, can store this increased energy for some time, as it will only produce a modest increase in drag. If done properly, this increase can be enough for the aircraft to reverse it's direction either through a banked turn or a half vertical loop.

In the idealized situation, therefore, the aircraft went from having a tailwind, and pulled up into a headwind. It now reverses direction. I t now has a tailwind, and the layer BELOW it is a headwind. It dives down, receives ANOTHER increase in airspeed, reverses and repeats indefinitely.

It still works if both air streams are in the same direction, just as long as there is a sufficiently
large difference in their velocities.

Hope that helped!!

 

[Clive]

Clive,

Yes, but the glider is also part of the airmass. It will equally drift with the wind. The only way to tell which way the thermal is "Leaning" is by comparing the gliders position over the ground with its altitude as it encounters lift, and this is not allowed by the OP.

Imagine the pilot finds a thermal and quickly gets the aircraft into a stable orbit inside it while also maintaining some fixed altitude. He maintains this orbit (a perfect circle from above, relative to the air). But the aircraft is also being carried in the direction of the wind (relative to the ground). As it does this it must eventually start to "fall out" the bottom side of the "leaning tower" which is fixed relative to the ground. That is detectable by the pilot because he then needs to use varying amounts of aileron input to maintain the required turn rate and angle of bank as the aircraft flies partially in and out of the thermal. From that he can infer the direction of the "lean".

Also, I see now that Rehn has already given essentially given the same answer earlier.

 

[Uncayimmy]

Can I use a helium balloon?

 

[rehn]

The server was so slow yesterday so I did not have time to write a longer answer ( wife was nagging me “tea is ready” )
Check this image:
http://www.aerospaceweb.org/question/nature/thermals/thermals3.jpg
The thermals will give you a “fix point” to the ground. This is more easy to understand if you image that you stay at the same altitude ( same sink rate for the soaring craft as for the strength of thermals ). If you just moving in circles with the constant speed you will soon “drop out” from the thermal on the lee side.
This will of cause not work if you are soaring in a deserts area where there are no fixed areas generating thermals. In deserts the thermals are often seen as “ dust devils” on the ground and they are moving with the wind.

 

[fredriks]

Like a balloon, thermals are considered to drift *with* the airmass -- and even without addressing the finer points of that argument, certainly to an aircraft circling in a thermal, the aircraft and the thermal are moving laterally at the same speed.

Why to the pilot (or anyone else for that matter), would they appear to be "moving north (in relation to the air mass)."?

JB

The standard answer to this is given by clive and others (the thermals is sometimes tilted and you rise slower than the thermal and fall out from it).

I am a little surprised about your answer because I believe this explanation is given in all hg and pg books.

I am not sure that it is always true though as I sometimes have to do the opposite and fly downwind to no not loose the thermals.

I believe the air motions around and in a thermal are extremely complicated and differ from thermal to thermal but that the standard explanation might still be one of the most common cases.

 

[RossFW]

As it does this it must eventually start to "fall out" the bottom side of the "leaning tower" which is fixed relative to the ground.

Thermals actually drift at wind speed. I used to have an ASW19 glider. The nifty thing was the GPS/VARIO used to be able to calculate a pretty accurate wind by recording your drift whilst circling in (and drifting with) thermals.

ETA:- Actually, I see were you're headed. As the heated air is blown down wind it is also rising, so the thermal DOES have a fixed relationship with the ground.

Yep, I think what you say would work- when you fall out of lift, you are downwind of the thermal.

Nice

 

[fredriks]

Thermals actually drift at wind speed. I used to have an ASW19 glider. The nifty thing was the GPS/VARIO used to be able to calculate a pretty accurate wind by recording your drift whilst circling in (and drifting with) thermals.

Did you fly in the flatlands or in the mountains?

Most thermals in the mountains has definitely a base fixed to a feature on the ground (spine, ridge, top etc). You still drift with a speed close to the wind speed when you thermal in the thermal though.

But one thing I have realized when flying (at least in mountains) is that the wind speed are changing quite a lot between the thermals and that the drift in the thermals also might be quite different between thermals.

 

[Clive]

Thermals actually drift at wind speed. I used to have an ASW19 glider. The nifty thing was the GPS/VARIO used to be able to calculate a pretty accurate wind by recording your drift whilst circling in (and drifting with) thermals.

I know the "real world" is far less regular. You could have no wind for a while, so a vertical thermal forms, then the wind could pick up and the previously formed vertical column of rising air (or some part of it) might then drift at the same velocity for some time. That could explain your observation.

However, from the original description I took it that a thermal in this problem's "cartoon world" was a column of air rising (vertically) at 500fps relative to the ground, and the wind was a constant northerly at 6 mph at all altitudes and that these conditions had basically also existed since some time long before we took to the skies in a brave attempt to find the direction of the wind.

If each thermal has a fixed source, then I'm confident the answer that I gave (for cartoon world) is essentially correct. Sure, the air in the thermal is drifting sideways at the speed of the wind (or that is how I read it), but it is also rising at 500 fps. That means the column of rising air must be "leaning" at some constant angle, and is also "fixed to the ground" at it's source. Even if the source of the thermal no longer exists, what was formed earlier must still be a "leaning tower", albeit with some lower portion missing.

On the other hand, if the source of each thermal can move then perhaps we need to know how fast, and in which direction. Could the source be a large black wind-powered cart moving directly downwind faster than the wind?!

Perhaps JB can clear the air?

 

[rehn]

I am not sure that it is always true though as I sometimes have to do the opposite and fly downwind to no not loose the thermals.

Yes, if you are climbing you have to fly downwind in relation to the ground if you want to stay in the thermal.

 

[fredriks]

Yes, if you are climbing you have to fly downwind in relation to the ground if you want to stay in the thermal.

Yes, but what I meant was that sometimes had to fly downwind relative to perfect circles flown in the thermal. I have to extent the downwind leg of my circle sometimes.

The standard advice given in all books are that you sometimes need to fly a little upwind relative to the circles, you have to extent the upwind leg of the circle.

 

[ThinAirDesigns]

One working solution has been offered so far. rwguinn has hinted that he may have another involving "2 of 3" primary instruments and a method to keep wings level. tumbleweed appears to be working on one that involves principles unknown.

I will address the one currently on the table that is demonstrably usable:


*******************

Imagine a bubbler in the bottom of a canal. It sends up a continuous stream of small bubbles -- continuous enough to casually appear to be one stream of air. To an observer viewing the canal and bubbles through a clear port in the canal wall, the bubbles rise at an angle as they drift downcurrent. One thing to remember however is that at any particular level in the stream, the bubbles are moving *with* the current at the same speed as the current.

The above is the idealized thermal model used in the OP problem.

The key to the pilot's ability to gain usable information regarding wind direction lies in the difference in *vertical velocities" between the aircraft and thermal. All three objects (aircraft, thermal, airmass) have the same horizontal velocities but each differs in the vertical.

As the pilot makes perfect circles in the thermal they are moving *up* relative to the airmass, but *down* relative to the thermal. One can easily see that it's impossible to hold perfect circles while descending through a tilting column and remained centered. Each circle (or thereabouts) one must correct to the upwind side to remain centered in the column. If one does not do this, you fall out the low side (downwind side) of the tilting column more and more with each circle.

The change in climb rate at different points of the circle is generally noticable enough to experienced pilots that there isn't even a need for a vario to recognize it -- you feel it in the seat of your pants.

If to stay centered in the lift, you have to repeatedly 'unbank' and lengthen one side of the circle, it's the point you're aiming at when the bank angle is the least where the wind is coming from.

*********************************

Rhen was the first to allude to the answer, and I believe he understands it enough to be able to use is productively. The wording of his answer however contained a flaw that kept me from unconditionally accepting it:

rhen:

Since the thermals are generated from a fixed position on the ground, they will appear to the pilot that the are moving north( in relation to the air mass ).

Since the thermals drift *with* the wind, they don't move relative to the airmass (in the idealized model at least) and thus cannot "appear" to *anyone* to be moving relative to the airmass. In the working solution, they *do* appear to the pilot to be moving relative to the pilot/aircraft and thus he/she must go chase it back upwind.

Rhen, in #14 does get credit for being the first to show up with a working solution if not perfectly explained.

Clive in #24 got us to the 80% mark with his "tilting" visual. Right on Clive.

in #15, RossFW takes issue with Clive:

Yes, but the glider is also part of the airmass. It will equally drift with the wind. The only way to tell which way the thermal is "Leaning" is by comparing the gliders position over the ground with its altitude as it encounters lift, and this is not allowed by the OP.

I will point out to RossFW that since the thermals vertical velocity is different that that of the airmass, one does not need to compare to anything on the ground to tell if the thermal is tilting -- simply make perfect circles and if you remain perfectly centered, the thermal isn't tilting. If you must make consistent corrections in one direction, there's your clue.

In #28 Clive also responds to RossFW with a perfectly good explanation.

In #30, rhen comes back with an addition to his original post demonstrating whan I figured all along -- he gets it.

In #31, fredriks essentially said that rhen was right originally and was surprised that I didn't accept his answer. Fredricks, notice that in my response I didn't say rhen was wrong in principle, but rather *asked a question* of rhen trying to draw out a better explanation from him.

I'm calling this for rhen AND Clive -- nice work guys.

Just as an interesting aside, are you a pilot of any sort Clive?

JB

 

[ThinAirDesigns]

I know the "real world" is far less regular. You could have no wind for a while, so a vertical thermal forms, then the wind could pick up and the previously formed vertical column of rising air (or some part of it) might then drift at the same velocity for some time.

Yeah, things can vary a bunch in the real world. I was pretty careful with my wording not to say "can a pilot know for sure which way the winds is blowing" and instead went with the "useful" term.

Can a pilot of an uninstrumented and unpowered soaring aircraft make any useful determination regarding which direction the wind is blowing at flight level *without* using clues seen outside the cockpit?

It's not a 'sure thing', but it 'sure helps'.

However, from the original description I took it that a thermal in this problem's "cartoon world" was a column of air rising (vertically) at 500fps relative to the ground, and the wind was a constant northerly at 6 mph at all altitudes and that these conditions had basically also existed since some time long before we took to the skies in a brave attempt to find the direction of the wind.

You are correct, however if you notice in my above statement, I include the words "at flight level" .. that's because this method really does give you information about wind direction at your level. If the wind was to have switched 90degrees from ground level that will be reflected in the current slope of the thermal.

If each thermal has a fixed source, then I'm confident the answer that I gave (for cartoon world) is essentially correct. Sure, the air in the thermal is drifting sideways at the speed of the wind (or that is how I read it), but it is also rising at 500 fps. That means the column of rising air must be "leaning" at some constant angle, and is also "fixed to the ground" at it's source. Even if the source of the thermal no longer exists, what was formed earlier must still be a "leaning tower", albeit with some lower portion missing.

All true.

It's also true as RossFW points out that there are many variations within each thermal --- often times you *do* make adjustments to the downwind side. These adjustments tend to have less of a pattern however.

On the other hand, if the source of each thermal can move then perhaps we need to know how fast, and in which direction. Could the source be a large black wind-powered cart moving directly downwind faster than the wind?!

Perhaps JB can clear the air?

It wasnt me I swear!!

JB

 

[Clive]

Just as an interesting aside, are you a pilot of any sort Clive?JB

Okay - I guess I blew my cover. I was once a pilot, many many moons ago (in the military, various powered aircraft), but before that my very first taste of flying was a handful of glider flights when I was still at school. I guess I must still remember some of the stuff I learned back then, even though I've done no "real world" gliding since (or for that matter, any "real world" piloting of any kind since leaving the military). MS Flight Simulator is as close as I get these days, but at least that lets me play at being an "old, bold pilot" without the usual consequences.

 

[ThinAirDesigns]

Just a general comment to all participants in this thread, some of which I have shared other threads with: This forum has a very high percentage of bright, reasoned people as compared to many other forums I have participated in.

This speaks very well of skeptics IMO.

Thanks to all.

JB