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What is a safe minimum rate of climb for ultralights? I understand most ultralights climb at least 300-800 ft/min and in fact the Canadian ultralight specs recommend a minimum rate of climb of 300 ft/min. Even the Lazair's specs note a rate of climb of 400 ft/min.

Why does Canada recommend 300 ft/min specifically? Why not 120 ft/min? 60 ft/min? Even at 60 ft/min, you could clear a 4' fence in 4 seconds. Of course you would want some clearance, so say 20 seconds. Seems pretty good in my book. Need to clear 50' trees? Just kite around in a circle.

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    $\begingroup$ IANAP, but circling at the end of the runway just to gain enough altitude to clear a tree sounds A) really dangerous, and B) really annoying for the pilot behind you waiting to take off. $\endgroup$ – FreeMan Sep 16 at 18:12
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    $\begingroup$ That assumes you're taking off from an airport, and not your own "back 40". lol It also depends what speed you are flying at. If Lazair's stall speed is 18mph, and Vc is 1.3Vs, that makes it's minimum cruise speed about 24mph. $\endgroup$ – Fred Sep 16 at 18:16
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    $\begingroup$ "Ultralights" is a very, very broad category when it comes to performance. What really bothers me is, why on earth would you want to climb as slow as possible? As quickly as possible might not be safe either, but there is a good reason why Vx and Vy are established and used! $\endgroup$ – Jpe61 Sep 16 at 18:18
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    $\begingroup$ @FreeMan Most airports wouldn't allow you to kite around to gain altitude as you would most likely be skimming the tops of the hangers.... $\endgroup$ – Fred Sep 16 at 18:28
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    $\begingroup$ Mentat answer : You decide how many fatalities you can happily live with and set regulations that are unlikely to make you too too unhappy in a bad year. | People are always going to die. If your assigned task includes limiting death rate to an acceptable level then limiting how deathly a death trap you are going to permit seems wise. $\endgroup$ – Russell McMahon Sep 17 at 9:34
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When you fly gliders you discover it's quite common to run into air that's descending at 1-200 fpm, or "sink" in soaring-talk. Descending air next to a thermal, or air descending due to downsloping terrain. It's a lot more than that at times, but a couple hundred fpm is typical.

On a day where there's any convection (with rising air, there is always equivalent descending air adjacent to it) you are in sink quite a lot. A 300 fpm ROC provides a reasonable assurance that you will be still able to climb while in most (but certainly not all) conditions of descending air, barely.

If you have an ultralight that can only climb 100 fpm, this might be fine on a smooth day with stable air, if you're patient. If there is any vertical motion in the air however, you have a problem. The minimum climb rate requirement is an attempt by the regulator to ensure that that average ultralight buyer will have a machine that won't kill him because it couldn't even out-climb a bit of subsiding air at the end of the runway.

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    $\begingroup$ Upvote but I think 100-200 fpm is an understatement. In my experience localized 500 fpm sink is extremely common - would expect to see that at some point on most flights - and I've run into 1500+ fpm several times. $\endgroup$ – pericynthion Sep 16 at 23:40
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    $\begingroup$ Well yes but you generally don't fly ultralights on those sorts of days with the low mass and low wing loading. It's quite unpleasant. I'm talking about the mild thermic or subsidence conditions that someone flying an ultralight in the morning or evening, or a quiet day, might encounter. $\endgroup$ – John K Sep 17 at 0:04
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    $\begingroup$ @pericynthion : but that 500 fpm sink doesn't reach all the way to the ground, just as thermals don't reach all the way to the ground. From my gliding experience, it was extremely uncommon, in a flat area, to have any thermals whatsoever under 200 meters (strong enough to at least cancel the sinking rate of the glider). On hilly terrain it's different, but you should have ample height reserve when crossing over ridges in an ultralight anyway. $\endgroup$ – vsz Sep 17 at 4:10

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