On Sunday, April 26, 2020 at 10:38:33 AM UTC-7, Kenz Dale wrote:
What's the minimum realistic climb rate that should be acceptable for self-launch? I've heard things like 1:4 ratio of thrust to MTOW (where thrust is approximated as 3x motor/engine power), but planes like the Silent 2 Electro and miniLAK FES seem to exceed this somewhat, with the miniLAK FES having a ratio approaching 1:5.5. The S2E's flight manual publish a climb rate somewhere around 400fpm, and just based on the numbers I would expect the miniLAK FES to be around 300fpm.

The GP 15 Jeta has a ratio of 1:6.3, which is far lower than the rule of thumb for powered flight. Yet, I don't hear anyone complaining.

And gas-powered craft might have 500fpm at sea-level, but they certainly can't attain anywhere near that at higher (density) altitudes.

So while all this sounds slow, when I compare to stall speeds I find that self-launch gliders' overall performance is generally 2-3x better than the FAA's minimum requirement of 200'/1nm for instrument flight (that's the closest I could find to a hard and fast rule for climb rates). So it would suggest that quick climb rates aren't necessary if the forward flight speed is very low.

So what's the right real-world response? Is there a certain absolute minimum for safe glider flight, or is it better to have a great climb angle rather than a great climb rate?

As a practical matter, the climb angle should significantly exceed the glide angle with the engine not operating but deployed, at the density altitude of your field. That way the engine cannot take you to a place you can't get back from. If your dirty glide is 20:1 @ 50 knots, that's 260 ft/min so you'd want a climb rate of 350 or so. The C22 climb criteria (1200 ft in 4 min) suggests a minimum climb rate of 300. That'd be real minimum for me. FES may be slower climbing but also cleaner gliding with a failed motor, so a step back and a step ahead.

I'd be careful of brochure or owner claimed climb rates. Look at the IGC files to be sure. I get about 880 ft/min at sea level and about 530 at 9000 ft density altitude (averaged over many flights, from IGC data - I wrote an app to analyze multiple files automatically).