MacCready theory in wave

Matt:

The idea: you have a good idea that there is wave lift ahead, either upwind or crosswind. You have a good idea how strong that wave lift will be, or at least you are confident enough that you won't stop for less than x knots of wave lift.

Now, how fast should you fly to that lift? As usual, flying faster gets you there lower, but flying slower gets you there higher.

The general answer is, as always, set the MacCready value to something and speed up through sink and slow down through lift as told. This is easier in wave as the lift and sink generally come on more smoothly and predictably.

But what do you set the MacCready value to? Pretty clearly something higher than the lift you expect to find, i.e. the minimum lift you would stop for.. Since the wave lift does not drift downwind it's better than thermal lift of the same magnitude. But how much better?

The whole point is to work that out. So, for example, if you're flying in a 40 knot crosswind and expect the wave to be 2 knots when you get there (Last weekend, Mendocinos, exactly the situation that caused me to write this), you should set the Mc to 3.5, not 2.0.

John Cochrane BB

Yes, thermals don't drift fully with the wind. The major reason is wind shear. The wind is stronger at altitude than below. That's why we can sometimes slope soar or wave soar cumulus clouds. That's why we do better than expected upwind, and worse than expected downwind. Someday I'll work up a set of tables for that...

John Cochrane

On Thursday, November 10, 2016 at 11:06:12 AM UTC-8, krasw wrote:
torstai 10. marraskuuta 2016 20.48.18 UTC+2 Matt Herron Jr. kirjoitti:
On Thursday, November 10, 2016 at 9:26:31 AM UTC-8, krasw wrote:
On Thursday, 10 November 2016 17:27:32 UTC+2, Matt Herron Jr. wrote:

Hmm,

I think it's the vertical component that distorts the shape, not the horizontal wind. We agree that a parcel of air in the core is rising faster than air on the edge of the thermal. Therefor it's slope is more vertical. It take less time to climb 1000 feet than the parcel next to it. That shorter time means it is blown downwind less than slower vertical parcels and therefor the core tends to pile up on the upwind side of the thermal.

Matt

I was talking about horizontal wind field. What you say makes no sense.

OK, lets take another shot at it. Would you agree that hypothetically if the air in the thermal was not rising then the windspeed in the "not rising thermal" would be the same as the surrounding windspeed? (I neglect the effect of rotational speed of the "thermal" which would increase windspeed on the downwind portion of rotation and reduce windspeed on the upwind portion of rotation). If so, then it is the vertical velocity component of the thermal that changes its shape.

Also, the horizontal windspeed in a thermal cannot be 50% less than the surrounding windspeed. This is a myth. If this were true, then I could extract way more energy out of a thermal by flying upwind inside of it, and downwind outside of it (like dynamic soaring) My velocity would build up and I could use that kinetic energy to climb...

I would say that if you have a thermal without vertical speed you have no thermal. Yes, vortex-like vertical structure of thermal bubble affects vertical shape of thermal, but that has nothing to do with horizontal wind discussed here. And no, it is not a myth, I and many others can see it with out own eyes using modern wind measusing inertial variometer. Usually windspeed difference between thermal center and surrounding is less that 50%, maybe 20-30%, but in well formed cloudstreet it is almost a rule that windspeed halves in best lift. There is not a shadow of doubt that thermal acts as a "semi-transparent" barrier to surrounding windfield. If the wind would not change, we would not need the skill of centering thermal, one single centering correction at the beginning would be enough since thermal would be perfectly round and symmetrical. I'm still waiting to find one that fits the description.

I would like to hear more about the science/physics that support your contention about horizontal wind velocity in thermals and a semi-transparent wind barrier. I can be convinced, but so far not so much.

As far as correcting for downwind drift in thermals, the reason is this; In the presence of wind, (and in particular, wind shear as John mentions below) the thermal rises at an angle. if the glider were going up at the same rate as the air in the thermal we would not need to correct for this as you state. However, we have a sink rate. Because of this we are constantly and slowly falling out the bottom of this tilted thermal. We must make upwind corrections in our circles to stay in the lift.

torstai 10. marraskuuta 2016 21.28.25 UTC+2 John Cochrane kirjoitti:
Yes, thermals don't drift fully with the wind. The major reason is wind shear. The wind is stronger at altitude than below. That's why we can sometimes slope soar or wave soar cumulus clouds. That's why we do better than expected upwind, and worse than expected downwind. Someday I'll work up a set of tables for that...

John Cochrane

Uhhh... No. There is practically no wind shear in convective layer, apart from surface friction layer, unless you put a mountain to disturb flow. The convection transfers momentum of airflow vertically up and down, resulting uniform *average* wind field vertically. Please look at flatland soundings of any convective day, shear layers under cloudbase do not exist.

What you are describing with slope soaring is windshear across inversion layer. Wave can exist only in stable layer above inversion (where hydrostatic stability acts as a "spring" force), whereas convection requires layer of constant potential temperature, ie. neutral stability. Sometimes inversion layer undulates and this makes it possible to contact wave close to cumulus base (cumulus penetrates to inversion). It is not slope or ridge soaring, there is no heated ridge creating slope wind.

On Thursday, November 10, 2016 at 3:06:19 PM UTC-5, krasw wrote:
torstai 10. marraskuuta 2016 21.28.25 UTC+2 John Cochrane kirjoitti:
Yes, thermals don't drift fully with the wind. The major reason is wind shear. The wind is stronger at altitude than below. That's why we can sometimes slope soar or wave soar cumulus clouds. That's why we do better than expected upwind, and worse than expected downwind. Someday I'll work up a set of tables for that...

John Cochrane

Uhhh... No. There is practically no wind shear in convective layer, apart from surface friction layer, unless you put a mountain to disturb flow. The convection transfers momentum of airflow vertically up and down, resulting uniform *average* wind field vertically. Please look at flatland soundings of any convective day, shear layers under cloudbase do not exist.

What you are describing with slope soaring is windshear across inversion layer. Wave can exist only in stable layer above inversion (where hydrostatic stability acts as a "spring" force), whereas convection requires layer of constant potential temperature, ie. neutral stability. Sometimes inversion layer undulates and this makes it possible to contact wave close to cumulus base (cumulus penetrates to inversion). It is not slope or ridge soaring, there is no heated ridge creating slope wind.

You are quite simply incorrect. Wind direction and strength changes happen all the time when we soar. Shears are very common when these happen.
UH

John,

I haven't yet read your paper (I will as soon as I get on the plane), but before the discussion on this topic goes any further I would like to point out that I published a "Generalized Speed-to-Fly Theory" back in 1991, which covers both thermal and wave lift, and anything in between, for example thermals that drift downwind at a speed that is less than the speed of the prevailing wind.

You can download a copy of my paper he
http://trimill.com/CuSoft/PolarExplo...STF_Theory.pdf

At the time I also published a PC program called Polar Explorer which uses this theory to calculate the correct speed-to-fly in any conditions, among many other useful things it can do.

Polar Explorer program is a bit difficult to use because of the old DOS based interface. In order to run it on a Windows or Mac OS X it requires a DOS-emulator program (I use DOSBox).

The program, is now freeware and it can be downloaded from
http://trimill.com/CuSoft/PolarExplorer/index.htm

Branko Stojkovic
XYU

It's all that I derived and more -- I didn't do angles other than 0 or 90. Well, maybe the tables are of some practical use!
John Cochrane BB

On Friday, 11 November 2016 00:25:06 UTC+2, wrote:

You are quite simply incorrect. Wind direction and strength changes happen all the time when we soar. Shears are very common when these happen.
UH

No, it is not common, it is simply impossible since shear layers in free atmosphere always come with stable layer and by definition we cannot soar trough these. Please tell me where and when was the last time you did soar trough vertical shear layer. I will find sounding data to prove you wrong. Or you can post sounding yourself (even RASP forecast sounding will do). I've been looking at soundings (real and forecast) of good gliding days for 20 years and never, not even once, have I seen wind shear in convective layer (between top of surface friction layer and inversion, to be exact). Insisting opposite is like saying apple falls upwards.

I do not blame anyone believing in strange things as most of the advanced soaring textbooks have been written by persons who have obviously little background in meteorology and have not used sounding diagram. That does not prevent them writing chapters of nonsense about interpreting them.

At 05:20 10 November 2016, John Cochrane wrote:
Following some fun discussions with Kempton Izuno I worked out
MacCready
theory in wave. It turns out we should fly faster.

http://faculty.chicagobooth.edu/john.../docs/wave_mac
redy.pdf

Enjoy! Let me know if you see bugs.

John Cochrane


The speed to fly for wave lift is simple.
In lift, fly fast.
In Sink, fly faster.

_never_ fly crosswind in sink.

When flying upwind (to a primary for example) your speed needs to be
set by your gps not your Asi.
in zero lift, fly upwind at your best glide speed (as read by your gps)
over the ground (assuming a fairly flat polar on your glider)
To clarify that further, best glide + windspeed = airspeed to cross upwind

in zero lift.
In sink, keep ramping that ground speed up as the sink increases, the
only limit being Vne on your Asi (corrected for Alt, back to Tas)

Mcready will not work in wave as it doesn't know what the windspeed is
and that is the critical thing for setting your into wind ground speed.

What flatland thermal pilots need to do first, is stop pointing the glider

where they want to go, Crab, almost always (except when flying directly
into or down wind, which is almost never)

And picking where to fly is more important than how fast you fly.
if you need to cross wave bars, do it at a Bridge or where the lift is weak

(as the sink will be weak too)

So, things to do in wave.
1) Ditch the McReady ring, it doesn't apply as it doesn't know what the
windspeed is.
2) Crab, almost everywhere.
3) in lift, fly fast, in sink, fly ridiculously fast. (up or downwind,
never
cross wind, never)

Unless you are pole squatting for badge purposes or just don't like the
gel coat on your glider, Va is minimum speed on a halfway decent wave
day, Vne is a good indicator that you probably shouldn't go too much
faster when crossing sink.

Said slightly tounge in cheek.
But really, crab ! everywhere.
And fly faster (much faster than you think)

Can you look at the sounding for Winslow, AZ for June 5, 1995 and tell
me what happened, or is that too old?

My partner and I were on safari and, on that day I ground launched from
Holbrook, AZ intending to fly to Flagstaff, then north across the Grand
Canyon at least to Heber City, UT and back south to Parowan. It was a
grand plan that didn't work out. Flying west in great lift, passing the
Meteor Crater, the lift suddenly turned off and the air went dead
smooth. I retreated and tried at least three more times before calling
my crew and sending him back to Winslow, where landed (and stood on the
corner).

On 11/11/2016 1:51 AM, krasw wrote:
On Friday, 11 November 2016 00:25:06 UTC+2, wrote:
You are quite simply incorrect. Wind direction and strength changes happen all the time when we soar. Shears are very common when these happen.
UH
No, it is not common, it is simply impossible since shear layers in free atmosphere always come with stable layer and by definition we cannot soar trough these. Please tell me where and when was the last time you did soar trough vertical shear layer. I will find sounding data to prove you wrong. Or you can post sounding yourself (even RASP forecast sounding will do). I've been looking at soundings (real and forecast) of good gliding days for 20 years and never, not even once, have I seen wind shear in convective layer (between top of surface friction layer and inversion, to be exact). Insisting opposite is like saying apple falls upwards.

I do not blame anyone believing in strange things as most of the advanced soaring textbooks have been written by persons who have obviously little background in meteorology and have not used sounding diagram. That does not prevent them writing chapters of nonsense about interpreting them.

--
Dan, 5J