Craft Drift

Help

How to use Craft Drift
Pilot your craft at a constant speed (power) in a variety of directions. Try to hold a constant heading for 10 seconds in each of 4 directions. Eg Pilot left 045 degrees to your course, then right 045 degrees to your course and perhaps pause half-way between these (eg 020 degrees left and right). The steadier your speed and the more spread the directions you travel in, the better the estimate of drift (due to wind or tide).

For clarity of text
Whilst Craft Drift works for all kinds of craft and can be used to estimate drift for different reasons, in this help text we will write regarding Flying and calculating the Wind, but this should be read to also include piloting a boat or other craft (such as a hovercraft or jet-ski) and calculating tidal effects, or other water currents.

How does Craft Drift work?
1) Calculating
Craft Drift monitors your speed and direction and plots these as points on a kind-of graph: The graph is like a big compass with a dot on the direction you are going in, but the faster you are going, the further the dot is from the centre. If you kept going the same speed and went in all directions, these dots would make a circle. The faster you went, the bigger the circle. Now if you were doing that in moving air or water, all your plotted points would drift the same amount which would just have the effect of moving the circle of dots. So Craft Drift does its best to draw a circle through the dots it's drawn for all your velocities (the Mathematical word that encapsulates speed AND direction). It then analyses how much this circle has moved from a centralised position and reports this as the estimated Wind Speed. In addition, by measuring the radius of this circle, a good estimate for your air (or water) speed can be made too.

The theory is good, and if you pilot your craft (air or water) with a perfectly constant speed in conditions that present a steady wind (or current or tide), then you will get excellent results, even if you only have a part-circle of dots, Craft Drift will fit a good circle. Setting a constant power, or engaging an autopilot that fixes your speed may help here. In practice however, your travelling won't generate a perfect circle of dots and Craft Drift will get confused.

2) Measuring quality
This is where the second key feature of Craft Drift comes in. As well as analysing and presenting the bigger picture generated over the last 10 minutes, Craft Drift also constantly monitors your recent motion over the last few minutes. It effectively measures how consistent your speed is and how close to a circle all the dots are. Once it notices a period of steady piloting that produces consistent results over a wide enough set of compass points, it will grab that consistent set and report it as a good reading to report. This is shown as the top reading immediately under the visual display. If you have sound enabled on your phone it will also read out the Wind direction and speed. It will continue to monitor the situation, and if a better set of readings is found (dots closer to a greater part of a circle) then the calculation is updated. Eventually the reading will be updated because fresher data (provided it has passed the minimum quality threshold) is considered more useful. A pretty good estimate of the current wind is more useful to you than a super accurate reading from an hour ago, perhaps at a different Altitude many miles away.

3) Reporting the best reading
In practice, this means you don't have to keep looking at Craft Drift. If you are about to Orbit an area for a landing, no need to press any buttons, just fly a steady speed for a couple of minutes of accurate flying in a variety of directions and Craft Drift will automatically notice a set of accurate, useful readings. The best data will be automatically selected and read out to you.

Craft Drift controls and options
a) The written information beneath the main display
The 4 lines underneath the main Wind reading (so lines 2-5) give details for the total picture shown on the display. Lines 2 and 3 give the estimated wind and air speed as determined by the yellow circle drawn to join the dots on the diagram. Beneath that (on line 4), n= gives the number of data points used. Readings are taken approximately 1 per second, but some (or many) are filtered out (see below). The percentage is how much of a full circle your dots have made up. If you just fly between North and East, with some NE NNE ENE and bits between, this would be reported as 25% (a quarter of the compass covered). After that is the standard deviation that the points are from the drawn circle, so the smaller the better. After that, s/sd is speed divided by standard deviation. This turns out to be a good measure of 'quality'. It's really the reciprocal of sd/s which gives the standard deviation as a proportion to the speed that you're flying. The bigger the number, the closer the dots are to the circle and the higher the quality of the estimate. As a starting point, 30 seems to relate to readings of good quality provided readings also cover at least 25% of a full circle. These threshold values can be set to your preference though (see below). The final line simply gives any status messages; errors, or whether the data is Live or simulated.

If you switch the switch 'On' next to the best (top) Wind reading, then the display will show the sequenced data used for this accurate reading (rather than the total data). The lines below giving further information also switch to show the readings and measures of quality relating to this best reading. Switch the switch off to return to the 'Live' display and readings. It's worth saying explicitly that the data used for the 'best reading' will have been from a continuous section of flight, it's not just picking points at will to make a nice circle.

b) The controls at the bottom of the main screen
From left to right: The 'bin' button clears the display and resets all calculations to be from now. The (-) and (+) buttons control the number (from 1 to 8). This is gives quick access to 'Duration' slider on the settings screen which is used to filter unstable velocities. For example, if set to 6, then it will require 6 consistent readings of the same speed and direction for them to be included in the calculation. Filtered (not included) speed readings are small red dots. Consistent (included) readings are bigger, white dots. The Far right 'gear' icon opens the settings screen.

c) The settings screen
The top 'preset settings' just set the other settings (lower down on the page) to some good values for the average of these situations. You may wish to use this as a starting (or returning) point, from which you can make your own adjustments if you wish.
The 2nd section allows you to filter out stationary readings, such as when taxiing. If a GPS speed is less than the minimum OR less than the given percentage of maximum recorded GPS speed, then it is filtered to not be included in any calculations.
The 3rd section allows you to only use readings when in stable flight (consistent speed and direction) The speed change includes direction as it relates to the magnitude of a velocity change; on the display it relates to how close one dot is to another. Again this can be given in absolute or relative terms. If the speed change is within the absolute change specified here in knots on the top setting OR if it is within the given percentage of the speed of the reading concerned then that is considered consistent. If n adjacent readings are all within this tolerance of a given reading then that reading is deemed stable. n is specified by the Duration slider which goes from 1 to 8. This value can also be changed with the (-) (+) buttons on the main screen.
The 4th section filters out (to not include) readings that are higher or lower in altitude by this amount than the current altitude that you are flying at.
- Note: Sections 3 and 4, above can for example filter out take-off runs and clime-out which typically are not useful to include.
The 5th section defines what constitutes a high enough quality to be worth logging as a 'Best' Wind reading and to read out. The s/sd and percentage of a circle were covered above. The 3rd item (points in a sector > ...) is used to help calculate the circle percentage: The circle is split into 40 sectors (each of 9 degrees). If the number of velocity readings in a sector is more than this parameter, then that part of the circle is considered to be covered (2.5%) Hence the percentage readout is only accurate to the nearest 2.5%.
The 6th section allows a different way to calculate the yellow 'best fit' circle. Instead of Mathematically finding the circle that best goes through the dots (minimising the standard deviation from the circle), switching the 'Outer circle method' on forces the circle to enclose all velocity readings, so it will be an outer ring. The circle will be selected to maximise the number of velocity readings (dots) within the tolerance given. Situations with very variable speeds, but a definite capped maximum speed, and in many directions, fits this method of calculation well. An example, might be a jet-ski. This might work better for you in other situations, but research has shown this can give some odd readings and is harder to measure for quality.
The 7th section is a tick option to delete all data. As well as ticking this, you need to click the 'Save' button below.
The 8th section is primarily for development, but is left in as it can be useful for your own improved understanding to get more out of Craft Drift. Basically, if you click 'Save velocity data' it saves all your speed recordings since you last cleared them with section 7 above. Note that the 'bin' button on the main display does not reset the recorded data that this will save, nor does the automatic binning of data more than 10 minutes old on the main display. Once you have saved a 'session', you can then 'load' it (pick from those that you have saved, identified by date and time) and the main display will then simulate using this data set. You can change the above settings to get the most out of your data which obviously better reflects your flying. This allows you to safely analyse and tweak Craft Drift once back at home, removing the temptation to be distracted whilst actually flying. Once you've 'loaded' a data-set and it is running this as a simulation, return to this section and tap 'Stop Simulation' to return to using live GPS data. Your data files are accessible through the Apple iOS Files App installed by default on iPhones and iPads, so you can delete them in the Files App or send them to other people for analysis, or to yourself for backup or analysis on a different device (eg copying them from your iPhone to your iPad).
The 9th section is required to save any changes you made above. Without pressing 'Save', none of your changes will be made.

Craft Drift familiarisation
The best way to safely get a feel for how Craft Drift works is probably to open settings, and select the 'Walking (experimental)' preset. Then go for a walk, ensuring you maintain a constant pace. You will see how the dots reflect the direction in which you walk and how a circle is fitted to this. This provides a good bench-mark and feel for accuracy as the recorded Wind should be 0 knots, as you are not being drifted by Wind or Water currents as you walk on land.

Send feedback
If you find settings that work particularly well for specific situation; perhaps a certain style of flying, or a particular aircraft then please share these in the Feedback page. Thank you. Equally if you have problems to report, or ideas for improvements then please also use the Feedback page.