Trimming to Catch Thermals?

[Slowmatch]

Hello All,

I'm interested in trim strategies that help a model to tighten up its turning circle and core a thermal when it encounters lift?

E20 in particlular but any pylon power duration or glide info (without VIT) is of interest.

This is the sort of question I would have asked Hepcat back in the day and I think there were some old threads on it before the Great Reset.

Thanks,
Jon

[JohnOSullivan]

The easiest way is to use tail tilt (raised tip on the inside of the circle). This works well for models with rearward CG (>80). This will have little effect on power phase but works on the glide.

The technique I use is to have a small amount of washin on the centre panel of the wing on the inside of the glide circle (for right turn, the right centre panel). This trim is fairly usual for UK power models.
This provides the left roll in the climb and balanced with a little right rudder tab gives a nice spiral climb. 
In trimming close to the stall in the glide, on encountering turbulence or entering a thermal, the washin can cause a minor stall situation on the inner wing and cause it to drop and tighten the glide turn. As the model speeds up after this “mini stall” it will resume normal flight with a tighter radiius.

[Slowmatch]

Thank you John.

I have my model set up in exactly the same way for a R-R pattern: tail tilt, wash-in tab on right inner wing for L roll, slight R rudder, 2 degrees L thrust. It's trimmed to glide close to the stall only I don't seem to be getting a wing drop and tighter turn. It has a constant chord wing with slight wash out on the tips so it is perhaps resistant to asymmetric stall.

My initial experience seems to be that it hits lift and speeds up a little which causes the wash in tab to roll it a bit straighter. I could try more L thrust and more R rudder but I quite like the climb as is.

https://www.youtube.com/watch?v=nrokwSBJtV8


Jon

[OZPAF]

That looks pretty good to me Jon. Just as well the DT went off there - not much space with that performance and breeze at "Fairy meadows?"

slight wash out on the tips so it is perhaps resistant to asymmetric stall.

Yes I think this could be the reason.

I have a modified 14" span Blue Ridge Special rubber model - also with a right/right pattern, with washin on the right wing at about 2/3 span and no washout on either tip of the rectangular wing. It appears to stall on the inboard wing when hit by a thermal.

https://www.youtube.com/watch?v=g6OjiNWfQzY    It appears to stall slightly and tighten the right turn at about 40-43 secs.

I remember Tony Mathews describing an unusual warp arangement used by some Wakefield fliers to assist the model to find and centre in thermals, but can't remember the details.

Good luck.

John

[USch]

Hi Jon,

I think wash-out on a constant cord wing is detrimental. The tip cord is already "too big to fail".

But there's another point I dont like in the YT film. A few meters after the launch the model seems to veering off the straight line with the fuselage not in line with the flight path (side slip). Maybe too much wash-in? Also having to use left thrust on the motor has me wondering.

Urs

[OZPAF]

That is a interesting comment Urs! Could it also indicate a fin on the small size?

John

[Slowmatch]

Thanks for the comments guys, and apologies for the late reply - it's been a bit manic here.

I remember Tony Mathews describing an unusual warp arangement used by some Wakefield fliers to assist the model to find and centre in thermals, but can't remember the details.

Yes, this is what I vaguely remember, thanks John. Be good if we can find it.

Urs, the L thrust is to open out the turn - it still has enough prop effect to turn R under power, and minimal rudder. I had already noted the strong right yaw on launch and tried to analyse what's happening. I suspected thrustline but actually I think it is just a bit of a poor launch attitude due to me holding the camera. It's launched a bit too far off the wind direction and so has strong sideslip/yaw which it corrects as it climbs away.

The wash out is for two reasons, one is that the model is designed for beginners so maybe belt and braces in terms of wing drop. But also the wash out on a constant chord tweaks the lift distribution to reduce drag a bit. You may well be right of course - but that was my reasoning.

[USch]

About what Ozpaf writes is the so-called Lindner trim. Lindner was probably the most successful glider (A2) flyer in the '50. In the morning and weak thermal condition, he used to give the outer wing some slight wash-in to center better the thermals. It is a very tricky adjustment because such a layout can lead to spiral instability in strong thermals. Today it is used mainly on Wakefields (F1B) and F1A glider models. Never on any typ of power models. And you are talking about beginners' models. Forget it in your case! It's nothing for low aspect ratio wings and you cannot fit a wing-wiggler.

Wing tip wash-out is very popular in GB as far as I can judge. But believe me, even without you will never get tip stalling on a parallel cord wing. On this argument I be willing to risk a bottle of good wine, say an old Chianti or something like  
 
I noticed that after some second your model got rid of the side slip and climbed very nice to the right. Try to fit a piece of TE at the dihedral brace to add more wash-in and reduce the left thrust line?

Urs

[JohnOSullivan]

In following up on this thread, I came across the following BFMA article which, although an old one addresses the inner panel washin situation quite well. Elton’s description is from the original Aeromodeller article from june 1970. Although I had not recalled  Elton Drew’s Lively Lady with washin on the inner centre panel of its wing, it bears a close relationship with the situation I referred to in my first post.  He was flying left turns and had 3/32” washin on the left  centre panel ( on the inside of the turn).  Although this was on an A/2 (F1A) glider, his description of the tightening of the turn into the thermal matches with what I have found.

https://freeflight.bmfa.org/wp-content/uploads/2016/07/Thermals-and-Free-Flight.pdf


Second article by William L. Baker describes Lindner’s 1954/55 outer turn centre panel washin and contrasts it with the more usual inner turn panel washin which seems to be more logical. (“hold the inside wing up”).

Third article by Hans Thomann does not address trim but gives information on possible thermal location.

The remaining articles are by practiced and well acclaimed  model fliers and deal with thermal finding, well worth the read.

[Slowmatch]

Thanks John, some really good stuff there. I need to read and digest a bit more to get my head around it though.

I suspect that in making the model stall resistant, I have also made it thermal resistant 

The explanation given by Zaic of increased speed is how I had understood a response to a thermal from my RC flying. Maybe the model would benefit from more R rudder and adjust thrustline to compensate.

Urs, the model has always had a wash-in wedge on the R inner wing (giving L roll.) I increased the size of this on subsequent flights (to give a better climb spiral) but it seems to have made the model go L (ie straighten up) in turbulence/lift rather than tighter the circle.

Perhaps fin sizing is another variable to play around with.