--- "John Berger" wrote:

Recent posts on landing have mentioned that the glider must be trimmed properly. What puzzles me is how do you arrive at "proper trim"? I know that moving the hang point forward or back will change trim speed, but when is proper trim achieved? Is it a subjective thing? (The manual for my UltraSport talks about trim in general terms but does not give a speed). Expert opinions would be appreciated.

JVB-Canada

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From: robndi@flytandem.com

Subject: Re: trim speed

Hi John,

The best way to visualize it is imagine a pilot flying in ground effect with lots of extra speed coasting across the LZ at a couple of feet AGL. Lets just pretend he's doing about 30 to 35 mph airspeed. Assuming the trim is set for somewhere well below that speed, he would be exerting some pulling-in effort to keep the glider from ballooning up. At some point as he continues to coast and slow down, he will be gradually letting the bar out and the "pull pressure" will decrease until he reaches reach trim speed, assuming he hasn't stalled first. And as he goes slower than trim more than just a second or so, he must start to push out increasingly to keep from descending.

The really cool thing is that we can use the reference to arriving at "trim" as a reference to when to flare.

How you do this is a personal thing but I can tell you what I do. I set the trim to be somewhere what I will call very slow but not quite stalled. It takes a few flights to get a glider's trim exactly where I want it, and between flights I will tweak a new wing as little as

1/8" fore and aft until it's exactly right... but it's worth the effort. After I get it where I like, I can simply coast to trim, let the glider float at trim at 1' AGL for about 0.956 seconds +- .2 sec and during this time raise my hands to a comfortable ear-height position on the downtubes and then (if wings are level enough to permit) ... flare. (extra digitry in this paragraph is for amusement and illustrative purposes)

I have pretty consistent landings but they are greatly because I am a stickler for where my wing is trimmed. What goes through the brain during coasting in ground effect is... "Coasting, 10 lbs... coasting... 6 lbs, coasting... 3 lbs, coasting... 1 lb,... trim,... thousand.. and.. flare."

Rob

ps: It's also my opinion that the majority of pilots are too wrapped in some form of brain overload to be able to recognize trim to begin with. So for them it doesn't really matter where the wing is trimmed at all. They have deeper problems than simply glider trim. They will push and pull a few times as they coast across the LZ until they eventually go for a combination of flaring running and whacking. But we still love them. They are entertaining. I.M.notso.H.O. :)