thermalis-a 2mt electric soarer
Thermalis is an idea of an electric sailplane for soaring in calm to moderate wind. it has 80" wingspan and 10" chord almost to the tip, and 764 sq in; weighs 926 grams (32.5 oz) with a 3x1400 mah pack. i just started evaluating it and will provide comments whenever i get enough flights, but so far am flying in cold weather with no thermals. am also playing with cg and trim.
nov 21-the concept: a simple sailplane for rudder and elevator-no ailerons, for relaxed flying and getting into thermals (hence the name), in moderate wind conditions. the wing is 1 piece polyhedral and the fuselage is box type, 1/8" solid balsa, like the horizontal tail and rudder. still, as i wanted to try spoilers and flaps for the 1st time, i decided to add them. the flaps are of the split type (similar to the 1s used on the full size douglas dc-3). another feature that i added was the t tail, that is not as simple as conventional tail, i must admit (i will comment on this later).
>>>will be adding details. check at the bottom.
testing the Thermalis
i started with the cg a little bit forward from the point for which i designed it. the launching went fine, with a strong climb at half throttle. the motor is very powerful and with the 10/6 folder it has lots of power. the climb is at about 70-80 degrees without tendency to tip stall, that was haunting me with the other sailplanes before. the glide is the best i have got compared with the other sailplanes that i have, so am very pleased with it. and it is very sensitive to turbulence, that means that i can read better any thermal around. the 1st 2 days i flew early in the morning, so i had no chance to learn how it could do in thermal weather, but on the 3rd day i flew it around noon, with the cg further back (as it was designed). i did 3 flights to trim and get the feeling of the field. there was turbulence all over, and on the 3rd i saw a hawk that began to turn into a thermal, and i followed it for a while, until both drifted too far away and i had to bring it back. in that flight it specked with 30 seconds motor run, and i was able to get 9:38 for the whole flight. then the wind began to blow hard and it was all for the day.
dec 20, 2012: 1-1/2x6"spoilers and 1x12" split flaps. 1 piece polyhedral wing held with rubber bands. area 764 sqin, airfoil 10%/2% camber, similar to 6409 but with sharper leading edge; 36" flat center, 12" panels, 9" tips (tapered to 6").
spruce or bass used on 1/8x1/4 spars flat over/under and 4 1/16x1/8 turbulators; 1/8"dia cf tubing leading edge, with 1/8x1/4 balsa reinforcement; trailing edge: hard balsa 1/16x1 lower, 1-1/4 upper, v shaped. 1/16 hard balsa ribs, 3" spaced. 10% stab area. 3 cells x 1400 mah pack. 10/6 folding prop. spinner and blades are from Heads Up RC (email headsup@yahoo,com). the parts numbers are: spinner: D-640, 10/6 blades: E-720. the motor is from hobby king: http://www.hobbyking.com/hobbyking/s...ner_Motor.html small, lightweight, but pulls strong at 80 degrees, even with 10 degrees downthrust.
dec 22, 2012: details on the t tail: the belcranks are 1/16 plywood, mounted on 1/16 plywood supports that are mounted to a 1/16 plywood beam and then planked with 1/64 plywood on both sides.
dec 24: 16" bubble canopy is from sig manufacturing co., trimmed at the front to match spinner shape.
dec 28, 2012: decalage: 2 degrees. wing incidence: +2 degrees; stab: 0.
datum line is top of rear fuselage (wing to tail).
motor is at minus 10 degrees (downthrust). a lot, compared to other planes, but it is necessary to compensate for the undercambered wing airfoil, that makes it more prone to rising its nose under power. this way it climbs with little (if any) elevator trim. it pays off, as undercamber is a great feature that makes the airplane float better and more sensitive to thermals, and provides better clues for that, as it wiggles its wings whenever 'feels' 1.
jan 4, 2013: the wing.- an airplane is a wing with things. so the most important part is the wing. and the most important-and neglected-part of the wing is the airfoil. most sailplanes use a flat bottom airfoil, and the most sophisticated use flaps to modify the airfoil for thermaling, by adding a little undercamber, but this makes the wing more complicated, and requires a way to seal the gap to avoid loses. and this variable camber is used for hi performance competition. i opted for a simple 1 piece wing with fixed undercamber. after all, the plane is intended for soaring, just for fun. the airfoil has a sharp leading edge that i have learned improves the performance (see Frederick Schmitz small Reynolds study at the end of page 5 here). the structure: i use multi-spar for improving the efficiency and having a lighter, more rigid and stronger structure. in airplanes this size it makes the airflow stick to the surface (again, see Schmitz data). i have learned this by making wings with smooth surface and planked leading edge, and then with this multi-spar, and the efficiency is improved noticeably. not only that, the stability improves too.
jan 6: holding the wing: even if the instal with rubber bands could be done with 2 dowels parallel to leading and trailing edges, i opted for going along the fuselage so in case of a crash the wing will slide easier and have less damage. i could resort to same way as the gentle lady plane with a dowel along, but eventually got carried away and decided to test the idea of using 3/32" piano wire and 'j' bolts attached to formers that are located at fuse at leading and trailing edge. it works the same in case of a crash, is more complicated to do it, but it is easier to place the rubber bands at the rear.
jan 7: the wing structure: the main spar is of 2 pieces of 1/8x1/4 bass, flat (1/8 vertical, 1/4 horizontal) , 1 on top and the other at the bottom, joined by a web of 1/64 ply, grain vertical, from rib to rib all the way. it makes a 'c' shaped spar that is very strong and rigid, with very low weight. there are also 4 pieces of 1/16x1/8 bass (vertical) on top that, besides adding to the structure, work also as turbulators.
aug 3: m i a.: today my thermalis disappeared in fog to never be seen again. i have never seen fog developing so fast. it was at the moment the sun rised. o well, i was planning to make a 2nd thermalis anyway. i will show the changes here (servos at the tail like on the maverick, and a more powerful motor).
sep 12: i finished the 2nd thermalis and flew it several times. it flies even better than the original. posted details and pics on separate post above.
keep posted. more details to follow.
oct 12: back home! when flying at the field, i saw a tractor pulling into the field's road and delivered to me the thermalis 1. it was in a sorry state, after 2 months sitting in the rain and high humidity. the motor and firewall came apart, but otherwise all was in relative good shape-except for being soaking wet, even inside of the wing. i am in the process of restoring it, and had to remove the covering of the fuselage and the bottom of the wing to let the wood dry up. it will take some time, but then i will cover it and fix the nose. it dove in the ground and hit hard, but as the wing is held in place with rubber bands, it came out without damage. the farmer told me that he found it at the south at about 1 km. when i last saw it, it was flying at the west field, and then i decided to put it into a spin so it would fall nearby, but never imagined that it would drift so far before hitting the ground. i guess these planes glide so well that even with the spin it went far before touching ground. at the impact the wing and canopy fell and left the fuselage open and got all wet inside, and i was surprised to learn that the esc and receiver seem to work fine-but not the servos, they vibrate and don't get back to neutral. and the lipo was 0.000 volts on each cell. very dead, but that's the way lipos are, so i expected that. the motor runs ok, after removing the mud from the spinner and blades, that don't not show any damage or even scratches. the stab has to be built again, as it is all warped, but after all, the damage is not too bad. so the end of the story is a happy 1. just to let you know.
really am doing this restoration as a matter of pride, to see the thing again in good shape, as the mk 2 is flying fine. but i can't let it die in that state. by the way, i removed the flaps and spoilers, as i could not find them much effective.
oct 21: ready and waiting for a window in the weather to try it.
will provide details on the restoration.
Last edited by phil alvirez; Oct 21, 2013 at 11:44 PM.