lift vs. altitude. what do folks in the mtns do?

[shnoakes]

i recently took my spirit 2m sailplane on vacation with me to ruidoso, nm (elev ~6900 ft.) i had a very bad day at the field while i was there, the glider just didn't act "the way it should have." i was wondering how much of that could be attributed to being more than 6000 feet higher in elevation than where i usually fly, in dallas, tx (elev ~460 ft.) i did some poking around on the web and did some math that showed i was enjoying about 25% less lift in ruidoso than i would have in dallas.

which brings me to my question: what do y'all in the higher elevations do when you want to go for a quick fly? do standard kits work for you? does anybody modify/upscale their wings to accomodate for elevation? or was i maybe just on dope that day?


many thanks,
scott

[Ollie]

The air is less dense at altitude so that a bigger volume of air has to be deflected downward by the wing in order to produce sufficient lift. The only way the wing can do so is to fly faster in the less dense air. Also, the higher the temperature the less the air density. Because the plane has to fly faster, the cirdles will be larger for a given angle of bank and sinking speed. This will make working small low level thermals more difficult. Planes with lighter wing loading will do better at high altitudes. If an 8 ounce per square foot wing loading is what you like at Dallas, then try a plane with a 6 ounce per square foot wing loading in the mountains.

[shnoakes]

many thanks for the feedback, ollie. btw, you're at about 20 feet asl!!

i've modified the ol' spirit a bit and turned it into something of a floater here in dallas. but in ruidoso i experienced it as something of a sinker!!

if i understand your point correctly, to get performance in ruidoso similar to what i've become accustomed to in dallas, i can basically forget the spirit - i need to choose a different kit? and this kit should be selected by having a proportionally reduced wing loading?

best regards,
scott

[Ollie]

Yes, try an Olympic II or a Paragon and build it light. Use light radio gear too.
Replace any heavy wood with contest balsa. Use Ultracoat light on the tail but the wings need Monocoat for torsional stiffness. Use carbon pushrods in teflon tubing from CST to save weight. Use nylon clevices instead of metal to save weight. If you save weight where ever you can, you can get the wing loading down to 6 ounces per square foot or even a little less.

The Mirage from Isthmus models will build very light too.

[Sail 'n Soar]

Actually, I ran the numbers last night but decided against sending them. Ollie mentions the approach of going to a lighter wing loading. Relative to the behavior of your existing bird, going from 500 to 6,900 feet you are talking about flying ~8% faster (square root of the density ratio) for the same trim condition. That also translates to sinking 8% faster at the same L/D. There will be a slight Reynolds number change, with the higher speed partially compensating for the lower density, but it shouldn't impact things much.

None of the above indicates the big change in performance you were indicating. Big thing is that you do need to fly faster and your turns will be wider while thermalling at the same bank angle.

[MikeC]

When flying in places like the New Mexico mountains, be careful not to fly down wind on a down slope or near one. Also notice that in such terrain there can be bad air for fairly long periods of time- if you are flying in a down cycle it can be ugly. I have flown the same floater type sailplane( a flying wing no less) in New Mexico at 5200 ft asl and in upstate New York at 800 ft asl and did not experience what you have mentioned.

Mike