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Old Sep 14, 2011, 09:57 AM
planepainter
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Mt. Juliet, TN
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Question
Where did all the thermals go?....

I took my newly converted E Aspire out to Peeler Park for what I thought was going to be a good day of soaring. But after time went by all I seemed to find was sink- and BIG sink at that.
The forecast was for temps at about 90 degrees with light winds out of the south west. Looked good to me! There were even some die-hard buzzards around that were taunting me to come up and play.
But all through the day all I managed to catch was a moderate thermal that only lasted a few minutes. Now, I will admit that I am still relatively new to soaring and my skills still leave a lot to be desired. But I am getting to where I can read the signs fairly well and can manage to exploit lift when the plane signals. I have tried to learn from the best, Larry Vincek and Bruce Breidenbach for getting me thus far. But yesterday even the buzzards were buying airline tickets...
In light of the dismal soaring day I have to ask: what were some of the factors (besides a doof commanding the plane) that led to very little lift? Even Bruce, after having no luck himself, said that it could be that the temps at altitude and near the ground were close and would not allow much convection...
Besides that, what else could have been some factors?


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Old Sep 14, 2011, 10:29 AM
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There's a difference between very little lift and big sink. If you got big sink, then somewhere around there was lift. Otherwise the air would all be pooled around your ankles! If the air is going down someplace, it's going up someplace else. And vice versa.
----
On some days the air just doesn't go up and down as much as others. I am weak on meteorology and couldn't tell you why. However, I bet on a day like that, Joe Wurtz could still stay up. Using very small control movements and great subtlety.
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Old Sep 14, 2011, 11:22 AM
planepainter
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Lincoln, I am sure there are fellows out there that can find lift anywhere, I have seen them in action. But I guess my question is not so much why I could not find lift, I am still learning to read the signs, but what would cause so little of it on what would otherwise seem to be a great lift day?



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Old Sep 14, 2011, 11:44 AM
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I don't have all the science to explain, but in over 40 years of flying RC sailplanes, I have noticed a few things

High relative humidity tends to reduce the thermal activity ... is this due to the water vapor in the air absorbing the heat, I don't know.

Days with little change in temperature often are less active thermal days than those days with a 10-20+ degree variance between night and mid-day temperatures.

One of our club's members used to talk about the dry vs wet adiabatic lapse rate and why one day was superior in thermal conditions from another ... it sounded good but

I would suggest some reading of full scale soaring texts that get into the science and meteorology of thermal soaring ...

Have fun, learning has always been an enjoyable pasttime to me

tk
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Old Sep 14, 2011, 03:15 PM
jjc
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If you had big sink, there were area's of lift. If there are no thermals, there won't be any sink, just flat air conditions. John
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Old Sep 14, 2011, 05:13 PM
planepainter
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I think we experienced big sink once at the far east end of the flying range. I could not exploit the lift associated with it as I was too low and too far down wind. Even with an electric I did not want to chase that one as my eyes are just not that good. But for the most part the rest of the day had almost nothing either way. There was some small stuff I was able to join but was not gifted with any "boomers" that I could find. I was, however, very pleased with how my Aspire E flew. It was in every way comparable with my non-electric New Spirit 2 meter. It flew extremely stable with no bad habits of any kind. I compared the wings of my New Spirit and the Aspire. The Spirit wings (as I have them) weigh 2 ounces more than those of the Aspire. The Aspire is 1 inch wider in span and the wings have a thinner profile. The Aspire is 1 inch longer in the fuse from the trailing edge of the wings to the stab.I am going to do an AUW of the two planes to compare the difference. Should be interesting.....



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Old Sep 14, 2011, 05:47 PM
planepainter
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Aspire E AUW: 35.87 ounces

New Spirit AUW: 31.00 ounces

( I admit, though, that in the weight of the Spirit I did not factor in
the weight of the hi-start.... )




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Old Sep 14, 2011, 09:07 PM
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Here in New Jersey, it's been raining a lot the last 6 weeks (In August we had more rain than in any month in recorded history). The best lift occurs during seasons of drought. Normally the first few weeks of September are the best of the whole year. Not this year.
If you also have gotten a lot of rain, you will probably be in the same boat or should I say ark.
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Old Sep 14, 2011, 11:39 PM
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Combine your flying with a barbecue. Then you will have a thermal no matter what. I figure a barbecue should be enough. I know that the heat going up the stack from a furnace (for a house) is enough to stay up in, if not very high.
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Old Sep 14, 2011, 11:53 PM
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The laws of therodynamics requires that there be an equal amount of lift and sink. Murphy's law says that the sink will be easier to find and will be between you and the lift.
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Old Sep 15, 2011, 06:06 AM
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I have two non-E Aspires (2M). On any given day, I'll fly one then the other. At my skill level, one of them will consistently stay in the lift I find and the other is hit is miss. While there is a small weight difference between them, after much study I believe the bad one just isn't setup properly,, be it AUW, CG, or Decalage setting; the later is an easy problem to create and being off just a skosh makes all the difference..

Part of this hobby to me is starring down the challenge here to learn why my problem. One thing you need to do is be subtle with the control inputs on such days...
but if not setup right, the plane won't process light conditions, period. My good bird initially flew poorly as well, but I mod'ed the stab to lighten and was extremely careful how I put it back on and spent a lot of time establishing the new CG.

I also have two E-Spectras with a similar problem.

I'm suggesting your modification may need scrutiny.. You just can't spend enough time with the setup of any plane IMO.
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Old Sep 15, 2011, 07:30 AM
greg
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somerset, nj
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Soaring Forecasts

Quote:
Originally Posted by planepainter View Post
But I guess my question is not so much why I could not find lift, I am still learning to read the signs, but what would cause so little of it on what would otherwise seem to be a great lift day?
you may be interested in an article by Lee Murray in the Jan 2001 RCSD, pg 20.

The Soaring Forecast web page calculates thermal indices and generates a crude plot indicating altitude.

Teaching Soaring Weather puts most of this together and describes how the Soaring Forecast works.
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Old Sep 15, 2011, 10:45 AM
MMR
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Could also be that you were out at the 'wrong' time. We usually talk about lift cycles - the length of the cycles varies for each day. I have spent plenty of time at our field fighting sink and weak thermals, only to find that a strong lift cycle is starting when I'm forced to recharge or have to leave.
I am now working on my lift-cycle reading skills...

Martin
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Old Sep 15, 2011, 11:50 AM
planepainter
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Mt. Juliet, TN
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Hi Martin. I know what you mean about lift cycles. To the south of our field is a tree line and the trees are the kind that are pretty dark. Usually on a day like the one I spoke about I can launch and try to ambush some lift coming off of the trees and follow the thermal to the north across the field gaining altitude all of the time. I can then fly back and surf a spot close to the last one and ride another one. But on that day there just didn't seem to be much going on...


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Old Sep 18, 2011, 08:20 AM
Marc PUJOL
Joined Feb 2010
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Finding a thermal is like finding mushrooms. It is a matter of field.

Yes of course, it is a matter of temperature, mousture...

But where are they? Think "accumulator" and "springboard". You have to find where the energy can be accumulated and where the air that was heated in such accumulator can detach from the groud (the springboard). A sring bord can be a line of tree, but also a ditch between two field, a difference of colour.

So think accumulator upfront (thanks to the wind direction) a springboard

Then when you have made your prediction, think deviation. The air in going up but is also deviated by the wind.

Also think that there is a minimum altitude to catch the thermal. With few wind and with good accumulator, of course thermals can be catch from 20m altitude. But this is quiet accasional. Most of the time, thermal are small, wind is more than 20 km/h and then, you can only catch thermal starting at 100m to 200m...

And do not try to go in the direction of cloud. They are two high, and to fare away (eaven you think they are at few 100m away, to be catch with a good probability of success.

So observe the field, walk around in the country side, look at google earth, and make your thermal picking flight stratégy to optimise your chance. A good thermal flyer is someone who have a flight strategy every time.

Marc
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