flatland thermals vs slope thermals

[Botterii]

I was wondering if thermals behave differently on a slope vs. flat land. I frequently fly at NCAR (see: http://www.houseofthud.com/Denver_RC_soaring_sites_reviewed.html#ncar) and I could swear at times the lift in a thermal seems better the further into the wind and away from the slope you fly. That’s not to say I haven’t followed a thermal up the slope and back behind it as the thermal drifts down wind because I have. sometimes it just seems to do the opposite of what you’d expect a ‘flat land’ thermal to do—drift down wind. I thought someone had talked about this before…? Any ideas?

[Sparky Paul]

Depnds on the day. I've almost spec'd out my Paragon...114" thermaller on the small slope I usually fly at, in the summer heat.
Today with my newest the lift there wasn't that great, as the wind was close to 50 mph!
The Paragon wouldn't have lasted 30 seconds there with the mainly horizontal "thermal".
The wing I was flying couldn't penetrate the increased wind speed as it spilled over the top of the slope.. Thrown out thru it, and kept away it went up nicely, but if got behind, then it was forced down.

[Daemon]

Funny that you should mention my slope site review page in reference to this phenomenon because I was just talking about it not long ago. Thermals most definitely behave differently when they drift into a hill, and we see it all the time here in Colorado. I posted my views on this in this "Thermals in high winds" thread a few weeks ago.. Check it out
http://www.rcgroups.com/forums/showthread.php?s=&threadid=101098

The main oddity is the bit about the wind blowing through the thermal, where you can keep the nose pointed straight into the wind in front of the slope, and ride it up to speck altitude while moving away from the hill, where if you make turns and drift with the wind you'll fall off. I know this phenomenon occurs frequently, and I'm pretty sure I know how it works, but I've never heard a theory other than mine, to explain it.

ian

[Daveairway]

ian,

Great Read !! Your theory really explains my slope well.

Thanks
Dave

PS: The link again http://www.rcgroups.com/forums/showthread.php?s=&threadid=101098

[Sparky Paul]

The slope I fly at a lot is at the bottom of a long valley oriented across the valley. Maybe 70 feet all up from the valley floor, but the wind comes down probably 500 feet in about a mile.
I've had a thermaller up to spec height there in lightish wind.
In strong winds the lighter planes can't penetrate the accelerated air at the slope lip, and get blown back and into the ground. A loaded plane can get out, and then go up well away from the slope. Turns must be shallow and always into the wind, or it comes back downwind really fast! And has to be turned back upwind before it gets to the lip.
And even the heavy planes can't make it back to the slope lip if they go back behind it, when they get close to the ground. Hovering to a landing is usual then.
There's another at the incoming open end of the same valley off to the side in a bowl. This one is best used from partway up. You can fly along the edge and get up in the lift or fly straight out and go up also.

[John Webb]

Somthing I've noticed on the slope I fly at is that there tends to be a wind void at the bottom of the slope. This has the effect of allowing the air to heat up more in the void area and as a result the thermals tend to pop more often, and with more intensity, from the bottom of the slope. The thermal then gets combined with the slope lift, which makes for big lift. I guess that would be the biggest difference between flat field thermals and slope thermals.

John Webb