TRACK-AVERAGE SOARING FORECASTS
with
"OPTIMAL FLIGHT" GROUND SPEED AND TIME


Updated:  Sept 9, 2008

OVERVIEW

This program evaluates the importance of thermals and winds along an entire glider flight path, rather than at a single point, by averaging BLIPMAP forecasts of thermal strength and wind along each leg of a flight track defined by user-specified turnpoints.  In addition, wind-adjusted "speed-to-fly" theory for a user-specified glider polar is used to compute "optimal flight" times, speeds, and expected thermalling percentages along the route - this can prove useful for flight planning, though it ignores many factors which can significantly affect actual soaring flights (see caveats below);

The program described here and this webpage are both experimental and under development.  My main intent at this point is to determine the usefulness of such calculations in actual practice.  Changes and updates are expected.  Bugs, problems, omissions, reports favorable or otherwise, & etc. should be reported to the DrJack Forum.


GENERAL NOTES

These predictions are of course dependent upon the accuracy of the meteorological forecasts!  Thus they should be examined critically and evaluated using one's knowledge of forecast model limitations.  Model forecasts are inherently noisy - not every wiggle can be believed!  Relative comparisons will be more accurate and useful than the specific numbers provided. 

An advantage to these forecasts is that they are tailored to a specific area and path.  Possibly their most useful feature is to provide single numbers, such as the spatial average climb rate or the "optimal flight" flight time, to evaluate conditions over a specified area. 

Comparison between different models, RAP and NAM, for the same path can be used to evaluate the uncertainty in the day's predictions (such comparison can easily be made for a prescribed flight track by creating two different Bookmarks, one using NAM forecasts and the other RAP - see the "Bookmark" section below). 

The current computations do not allow for the variation of conditions with time.  Such could be added (for RAP forecasts in particular), but I first want to evaluate the usefulness of the present tool before considering adding more complexity, which might not actually make the computations significantly more useful. 

I urge any user of these "Track Average" forecasts to simultaneously view the Thermal Updraft and BL Wind BLIPMAPs upon which they are based!  BLIPMAP graphics have the advantage of utilizing the mind's powerful pattern analysis capability to better evaluate a complex forecast, such as the spatial "noisiness" of the meteorological forecasts or possible advantages to deviations from a perfectly straight flight path. 


"OPTIMAL FLIGHT" NOTES

The accuracy of the "optimal flight" results depends upon the many assumptions made by "speed-to-fly" theory, which can be overly simplistic (though the foremost factor affecting accuracy is, of course, often the accuracy of the meteorological forecasts! )  "Speed-to-fly" theory assumes that a flight consists entirely of simple circling climbs in thermals followed by optimal speed glides to the next thermal (and thus omits the initial tow and final glide).  Note that the only forecast parameters used are thermal strength and wind speed+direction - there is no dependence upon thermal depth or terrain or clouds (or anything else).

Limitations of this approach include:

Thus these predictions should be examined critically and evaluated using one's knowledge of factors not included in these simplistic calculations (including forecast model limitations).  Again, relative comparisons will be more accurate and useful than the specific numbers provided.  BTW, interpretation is greatly enhanced if one also views the Thermal Updraft and BL Wind BLIPMAPs upon which the optimal forecasts are based! 

Note that an optimal flight solution does not always exist for a leg !  If the forecast thermal strength is not large enough to keep the glider aloft or to allow forward progress for the given wind conditions at a single grid cell along the leg, then no optimal flight exists for that entire leg.  In such a case dashes will be displayed in the columns giving optimal flight results for that leg (and for the total).  Note that while unfavorable thermal/wind conditions at only a single grid cell can render the entire leg unfeasible mathematically, in actuality the flight may often be possible.  In reality such a problem area might actually be passible, through a long glide or circumvention for example, particularly if the path through that grid cell is shorter than a full grid width - but the simple mathematical assumptions used do not allow that possibility. The along-track plot will indicate the location of such problematic conditions, as the optimal ground speed will be 0 and the thermalling percentage 100%. 

If you wish, the optimal flight forecasts can be altered or "tuned" by changing the tmult input parameter, which multiplies the predicted "Thermal Updraft Velocity" values.  Ideally that factor would be 1, but the method of predicting thermal velocity has some slop in it due to questions about the applicable averaging time so in fact the "best" theoretical value may not be precisely 1.  However, determining a true "best" value would not be a simple task since it would require excluding all factors which can bias the results - one would need a day on which meteorological predictions proved reasonably accurate with many flights which approximate the simple "speed-to-fly" model being flown by skilled pilots.  Simply altering the predicted optimal flight time using this factor assumes that any error in the optimal flight prediction results from thermal strength forecast errors, which may not be the case!  You may find a value differing from 1 which better predicts enroute flight times, but that does not mean that that number is applicable to everyone and all circumstances. 

While the optimal flight numerical values can be used flight planning, this is useful only when its simplistic assumptions reasonably match the actual gliding situation, as when simply moving from one thermal to the next over flat terrain directly toward a goal.  I have strong doubts about the exactness of the precise numerical values, given the methods simplicities plus the inaccuracies and uncertainties inherent in meteorological modelling.  And in addition there are pilot variations - everyone knows that two different people can have two quite different times over the same track on the same day!  My expectation is that these forecasts will be most useful in indicating relative differences between days over a given area, not for telling exactly how long it will take you to fly a given route!  Still, I don't fly in flat terrain where this theory is most applicable so am not in a position to truly evaluate it - that I must leave to others. 

I would be interested in hearing of any useful comparisons between the optimal flight forecasts and actual flights.  By useful I mean results which provide scientifically appropriate comparisons, that is:  (1) omit flights which do not roughly match the simple "speed-to-fly" model (e.g. not over flat terrain),   (2) omit days on which there were obvious failures of the meteorological forecasts (e.g. an unforecast intrusion of high-level clouds),  (3) cover multiple days (to average out the inherent "noisiness" of any single day/flight).  Ideally they would even be for more than one pilot! 


PROGRAM OUTPUT AND INTERPRETATION

Sample output for the "Bookmark" example given below is (but results shown here differ from those obtained if you run that example, since you are obtaining results for a different day)

            FLIGHT TRACK AVERAGES             
 -------------------------------------------- 
 ("optimal" uses wind-adjusted speed-to-fly) 
     Valid:  NAM  TUE 15 Aug 2006  21z       
     PolarInfo:  LS-3 (L/D=40)                       
     WeightRatio:  1   DryWeight:  383 kg    
     ThermallingSinkrate:  1 m/s 
     CANV Lat,Lons:  39.00 -117.00
                     41.00 -116.00
                     39.00 -117.00
 -------------------------------------------- 
 L   -Spatial-Avg- ----Optimal-Flight-Avg---- 
 E       Tail Clmb Tail Clmb      Gnd Air Thm 
 G  Dist Wind Rate Wind Rate Time Spd Spd Pct 
      km   kt  m/s   kt  m/s  min  kt  kt   % 
 1   247   13  2.0   12  1.9  140  57  78  26 
 2   247  -13  2.0  -13  1.9  190  42  83  49 
TOT  494    0  2.0   -2  1.9  330  49  81  40 

The text summary indicates that for NAM model forecasts for the current day at 21z, the simple spatial average along the specified route (an out-and-return) is for a climb rate (Thermal Updraft Velocity minus Thermalling Sinkrate) of 2.0 m/s.  An optimal flight by a LS-3 would experience an average climb rate of 1.9 m/s on both legs, indicating less time spent in regions of stronger updraft.  The outbound leg has a tailwind, on average, which becomes a headwind on return.  The distance of each leg is shown with the time, ground speed, air speed, and percentage of time spent thermalling for each leg.  On the tailwind leg, a lower airspeed when gliding is required for optimal flight, yet the actual ground speed is higher and less time will need to be spent thermalling.  The plots indicate how conditions vary along each leg - though these details need to be taken with a grain of salt, realizing that predictions at a single grid point, as these are based upon, can be very noisy!  For this case, they show that forecast variations of thermal strength and tailwind result in thermalling percentage and optimal groundspeed varying significantly along the outbound leg.  Note that if conditions at one grid point produce a groundspeed of 0 and thermalling percentage of 100%, the leg as a whole has no optimal solution! 

Interpretation Notes:


RUNNING THE PROGRAM

The program can be run in two ways: 

(1)  Interactively, using the regional viewers: 

A flight track is selected using mouse clicks on a forecast map displayed in a regional viewer with user selection of glider-specific data, after which a popup window displays the corresponding optimal flight time and speed and average meteorological conditions.  To use this viewer feature (as also documented in the viewer's own popup instructions), 

(2)  Using a browser URL or Bookmark: 

Flight track and glider-specific data can be specified in a browser URL, with output being displayed in the browser. Since the required URL is a long one, it is usually most convenient to store a specific path as a bookmark, which when loaded into the browser will display the optimal flight time and speed and average meteorological conditions for that path based on the latest model forecast data.  Alternatively, browser-mimicking software can be used to send the desired URL and receive the output. 

http://www.drjack.info/cgi-bin/get_bliptrackavg.cgi?latlons=39.00,-117.00,41.00,-116.00,39.00,-117.00&region=CANV&model=NAM&day=0&time=21&polar=LS-3&wgt=1&tsink=1&tmult=1 

APPENDIX

Available Pre-Loaded Glider Polar Data
The following polar names can be input to utilize pre-loaded Polar and Ballast information (based on WinPilot Carl Herold data): 
--1-Person-Sailplanes--
1-26A
1-26E
1-34
1-35A
1-36_Sprite
604
ASW-12
ASW-15
ASW-17
ASW-19
ASW-20
ASW-24
ASW-27_Wnglts
DG-400_15m
DG-400_17m
DG-800_15m
DG-800_18m
DiscusA
GenesisII
H-201_StdLibelle
H-301_Libelle
IS-29D2_Lark
Jantar2_SZD-42A
Ka-6CR
L-33_Solo
LS-1C
LS-3
LS-4a
Nimbus2_20m
Nimbus3_25m
Nimbus3T
Nimbus4_26m
PIK-20B
PIK-20D
PIK-20E
PIK-30M
PW-5_Smyk
RussiaAC-4_13m
StdCirrus
SZD-55-1
VentusA/B_17m
VentusB_15m
ZuniII
--2-Person-Sailplanes--
ASH-25_25m_Pil
ASH-25_25m_Pas
ASH-25M_Pil
ASH-25M_Pas
DG-500_Pil
DG-500_Pas
DG-500M_Pil
DG-500M_Pas
DuoDiscus_Pil
DuoDiscus_Pas
Grob103_TwinII_Pil
Grob103_TwinII_Pas
JanusB_18m_Pil
JanusB_18m_Pas
Nimbus3D_25m_Pil
Nimbus3D_25m_Pas
Nimbus3DM_25m_Pil
Nimbus3DM_25m_Pas
Nimbus4DM_26m_Pil
Nimbus4DM_26m_Pas
Nimbus4D_Pil
Nimbus4D_Pas
StemmeS10_Pil
StemmeS10_Pas