Third vector says thermal is thataway, but how far? - RC Groups
May 05, 2012, 09:51 AM
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# Third vector says thermal is thataway, but how far?

I'm hoping that I learned a bit about air reading at the IHLGF last weekend. I've been reviewing JW's third vector diagram, and trying to put it into practice, but now I'm hung up on the question of how far to look for a thermal once I've gotten an idea of what direction to search.

I assume the velocity of the thermal inflow is a function of both the size of the thermal and its distance.

I can see how spatially separated signs (trees, streamers, etc) would help triangulate, but what if don't have those?
 May 05, 2012, 10:33 AM launch low, fly high Rate of change of the wind provides a strong indication of size and distance.
 May 05, 2012, 04:01 PM Come out swinging Joe, Can you explain why you want to fly along a straight path and how to interpret model response and what it means in terms of the thermal's position? Sean
 May 05, 2012, 05:32 PM Throw it like you hate it weird double post
 May 05, 2012, 05:33 PM Throw it like you hate it if i assume the thermal is tracking downwind with the prevailing wind, and the prevailing wind is constant in both velocity and direction, then can i tease apart both the size AND distance of the thermal? for instance, if the thermal is close to me the third vector will rotate quickly, while if the thermal is very far off, the third vector will rotate slowly. this is like looking at a commercial jet high in the sky, and it looks like its going very slowly, but in reality it is moving fast. only because we are far from it, does it look to move slowly. is this a correct way to determine distance? magnitude and direction of the third vector gives information on both size and distance, but from this alone i cannot determine both the size AND location independently. correct? we need look at how the third vector evolves? paul
 May 05, 2012, 07:08 PM up from the skies What's a vector-sorry I don't know that stuff Tom
May 05, 2012, 07:42 PM
American Exile
Quote:
 Originally Posted by Tmfury What's a vector-sorry I don't know that stuff Tom
A vector is magnitude and direction.
May 05, 2012, 07:55 PM
Throw it like you hate it
Quote:
 Originally Posted by Tmfury What's a vector-sorry I don't know that stuff Tom
this is the diagram to which Frank was referring. this should help relieve your confusion.
paul

### Images

 May 06, 2012, 12:33 AM launch low, fly high With sufficient integration time, one "should" be able to sort out both the relative distance and strength of a passing thermal. A close but very strong thermal results in a very large change in wind direction and wind velocity. A distant but very strong thermal results in a gradual and weak change in the wind direction and wind velocity. A close but very weak thermal results in a quick but weak change in the wind direction and velocity. A distant and weak thermal results in almost no change (what little change that happens, happens slowly). The mathematician looks at the inputs/outputs, and concludes that there are two independent variables, and two dependent variables. Therefore there is a closed form solution. The independent variables are thermal strength and thermal distance. The dependent variables are wind change, and rate of wind change. The art is in the determination of the closed form solution! Last edited by Joe W; May 06, 2012 at 10:07 AM.
 May 06, 2012, 08:31 AM Registered User Joe Could you please check the last two sentences of your last post. It looks like a typo, where you wrote independent when you meant dependent for one of them. On another topic; what confuses me the most is when there are multiple thermals. I have no idea how to work that situation out. All I observe is that the vectors are lying because they don't really indicate where the lift is anymore.
 May 06, 2012, 08:50 AM Registered User Translation for JW's last post: I go where the squirrel in my pocket tells me to ;-)
 May 06, 2012, 10:08 AM Will fly for Bourbon Amen
 May 06, 2012, 10:13 AM launch low, fly high Phil, Yes, a "typo", or just sloppiness... The multiple thermal scenario is a bit more challenging. Poway has lotsa multiple thermal cases. And Poway can be a bit more challenging than average for reading due to the occasional very small but very strong thermal that doesn't really register very well on the vector radar.
 May 06, 2012, 10:26 AM Come out swinging Joe, Can you explain why you want to fly along a straight path and how to interpret model response and what it means in terms of the thermal's position? It seems the solution involves third vector interpretation coupled with model response. Sean
 May 06, 2012, 12:54 PM launch low, fly high The rationale for flying in a straight line is so that I can better evaluate the wind that the plane is flying through. If I fly a meandering course, I have poor luck with seeing local wind changes. For example, if I am flying a direct crosswind heading, I can see fairly easily if the crosswind is decreasing (thermal upwind), or increasing (thermal downwind). If my flying is more of a meandering course, it is more difficult to determine the wind change. You will see this in my flying style. I'll make a pre-launch air read, then fly directly towards the initial call. En route, I'll evaluate the relative wind as based on the airplanes response. If my initial vector was a bit off on the heading, I'll fly along until I notice a change in the crosswind, then turn downwind as determined by the crosswind change. Ideally, you wait until the rate of change of the crosswind drops to zero, then turn 90 degrees. This is somewhat difficult to evaluate in the real world! If you get nothing else from this thread, remember these words, "listen to your plane"... Your plane is trying to tell you far more things than just "I'm going up (or down)". The art of dlg IMO is in learning to listen to your plane.