I opted to buy one of Vince Hallam's surplus airspeed indicators and am glad I did. I collected the package from the Sorting Office this morning and tried it out with my arm stuck out the car window as I drove home.
You can see that it is an analogue system (no batteries to run out). Essentially it is a Newtonmeter with a cup, not unlike a milk powder scoop, on the end of its lever....that's what I will use as a substitute if this one blows off! There is a strong clip for fixing it to an upright to the pilot's right (on the A-Frame). And a safety lanyard is also provided. I might possibly remove the clip where it bolts to the unit and make a more stable clamping system, possibly using velcro, if this isn't as solid as it looks.
It flicked around a bit, but then it will have encountered loads of turbulence from the car and I wouldn't expect it to affected by my trike anything like as much. I have only glanced at the dial and haven't yet glanced back at my odometer, but so far I am very pleased with my purchase, which was posted to me in a plastic food jar, which struck me as ingenious; I think I shall keep it in there for safety's sake.
Vince says if I stuck it out his kitchen door right now it would read 30! At Sywell it is around 20, which is why I am not flying now.
Can't wait to try it out on the Dragonfly.