Having used, and given up on, various devices specifically made for flight testing, I've flown with something like this for years for handling testing - purchasable for usually around a tenner from fishing shops and travel luggage shops (and doubtless at the moment online suppliers), a handheld force gauge with a tape measure built into the handle. I define a convenient reference point, usually the front lower edge of an instrument panel for pitch displacement - but as Jamesel says quite rightly, for most handling evaluation work, it's force, not displacement that's critical. A velcro strap is a useful addition for attacking it to the relevant hand control. Remember that you can do all the data reduction you need out of the cockpit, so it's fine if your neutral stick position is 223mm, or whatever - just record the numbers.
Guessing you aren't trained originally as a test pilot, a few pointers...
- Get all of your recording needs pre-defined on your kneeboard, and work from that. The term used for this is "test cards", and it's best to get everything - instructions to you, and place to record data, together on the same cards.
- Don't let any normal airmanship, especially lookout, lapse whilst doing your tests. It's much much too easy to forget to lookout whilst fixated on your experiment in the cockpit.
- Have clear, including time and fuel state based KIO/RTB criteria defined - you do not know how long these tests will take until you fly them, and need to be prepared to land, rest and refuel, and return to the task safely.
- Record all of the "common data" to the test - ensure you have data to record W&CG, engine state, trimmer position(s) for everything you do.
W.R.T. Stalling particularly, a few further thoughts...
- Don't compromise all and any safety practices you'd normally include in stalling. (Safe height, HASELL checks, pre-brief spin recovery, etc.)
- Deceleration rate to the stall can have marked effects on stall characteristics: I usually test ½, 1, 3 kn/s and 5 or max achievable without climbing for aeroplanes in that class
- There's a lot to observe in a stall test - buffet, pitch attitude, airspeed, audible warnings, tendency to wing rock, control response as the aeroplane slows (3 axes!), height loss, response to recovery actions. In my experience, expecting to capture everything in one go is unreasonable. Expect to fly multiple instances of each test, recording 1 or 2 variables each time. This will get you much better data, and at the same give you a further insight into consistency of the stalling characteristics.
- Consider calibrating the ASI against a GPS down to, say, 1.1Vs and assume some consistent extrapolation down to stall of the low speed end IAS/CAS curve. There are multiple ways of doing this - here's a good basic guide
https://bura.brunel.ac.uk/bitstream/...0A%20FINAL.pdf . That said, there should in a certified aeroplane be a published curve, although depending upon how rigorous your assessment needs to be, a cross check on that may have value nonetheless.
- I have seen some light aeroplanes where the stall characteristics are also heavily affected by pitch trimmer position. To be fair, most are not, but consider it.
G