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I use the Eagletree V3 speed sensor at about $35 on Hobby King. This measures air pressure as model moves forward relative to static pressure. The difference gives speed.
You have to be careful about various items when talking speed :
Ground speed - this is by radar, doppler, GPS, timed run
Airspeed - this is by Pitot sensor as the V3 above
The two are not same and by definition cannot be compared. The true speed of an airframe is Airspeed ..as an airframe regards itself as moving through air and 50mph is 50mph whatever ... ie in air moving at 20mph ... ground speed will be 50 + 20 = 70mph with wind, 50 - 20 = 30mph against wind .. but airframe will still be flying at 50mph in the moving air.
Second that when measuring speed - if you dive into the run - the speed will be higher .. what generally you are looking for is straight and level speed ... This can be difficult as the PKJ is a small model and distance flying is not so easy .. so a low-power shallow dive into the run - full power at level gives reasonable results.
Note with on-board speed sensor as the V3 - you do not need to average out with / against wind speed runs. But if you use GPS on board - then yes yopu do need to as the GPS is measuring ground speed.