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Jan 12, 2013, 03:12 PM
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Phil's Thermal Rex-a 2mt hybrid e-sailplane for soaring‏

Phil's Thermal Rex, or Phil's Rex for short, is a hybrid from a fuselage of the scale ASW 28 sold by hobby king. see
and the full size:
the reason why i choose this fuselage is because the looks are like science-fiction but it is very much like the full size. although i did not use the wing shape (especially the wing tips).
(i will be adding data at the bottom of this thread)
the new balsa wing it is a 2mt wingspan, 460 sqin, 543 gr auw w/2x1300 lipos; naca 6409 airfoil.
i used the foam fuselage and tail, and avionics, and designed a woody wing with larger area and an undercamberer airfoil for slower flights.
i got the whole plane and decided to try it as is, before discarding the wing. and boy, was i for a surprise! it falls into a stall and a spin unexpectedly, and i crashed it several times, breaking the nose and reinforcing it, until i throwed away the wing and started my own. fortunately with it the plane is now stable and predictable. originally it comes without dihedral, contrary to the full size, that has some. as the stock was too unstable and prone to snap roll, i added dihedral as per the full size and this helped a little bit; then increased the chord, adding area to the trailing edge, which helped more-but still was a handful. then it was when went into the balsa wing. it has a planked leading edge, polyhedral, and is covered with 21st century microlite. spars are 1/8x1/4" spruce over-under, with 1/64 ply webs, grain vertical. originally it had ailerons, but learned that they are not necessary so i fixed them and removed the servos. now i can throw it at 45 degrees and climbs hands-off very fast. i used planked leading edge, following the trend, but did not raise to expectations, so on subsequent models like the Thermalis i resorted to multi-spars structure, that i have used successfully on many models.
jan 13: another mod that i did while still with the stock wing was to add 1" to the rudder. i kept that afterwards, and the plane has strong response to rudder commands, but not too much. the material was 3mm foam.
doing hybrids presents its challenges, as the shape and dimensions of the fuselage limits the size of the other components, and also coupling them means lots of thinking and tinkering, but is a matter of choice to get into it. i didn't modify the rest of the tail or fuselage, besides reinforcing it around the nose as a consequence of crashes, but learned a lot about how to do it better. as i got a replacement fuselage that was available at the time i got the '28, am working on it with several ideas that will present here.
jan 14: the components: i used the stock parts, and although i crashed it several times due to tip stalls, the motor, prop, and spinner-or the other components-didn't get damaged. just the nose needed some beefing-up. the irony is that it was after i used the balsa wing, that made the plane stable, and flew flawlessly, that i suffered radio failure and went into a spin from high altitude, that the motor shaft was bent and had to replace it. as the 28 does not show what exactly the stock motor is, i had to search and found 1 that seems the same, but as it was oos i looked for a substitute and found 1 that is more powerful-and available, so i got it and installed it: great improvement! faster climb with less throttle. what i found impressive was the performance from the stock prop/hub/spinner-and the new motor-and that with 2 cells-so i intend to use that combo in other lightweight designs.
the supposedly 'stock' motor is this:
the new, more powerful is this:
jan 18.-building hybrids is not easy. i takes lots of thinking how to do all the details, but after all, if you like to tinker, you don't think much about how difficult anything is, but how much fun it is. still, things like the wing attaching was a matter of much thinking and considering all the alternatives. it requires carbon tubing at both halves, and a metal rod with the angle for the dihedral, to join them. and then, how to hold them together so don't come off. and also both have to have the steps to match the ones at the fuselage, so i had to design a relatively complicated set of plywood parts and then plank this section with 1/64 ply. at the end, it worked fine, using screws through the tubings and rod.
jan 22: 2nd fuse.-i decided to use the 2nd fuselage, that i got when i bought the asw 28 (and that has not been available since). i modified it to have 2 belcranks in the fin for the elevator (like the Thermalis). i ran the pushrod through the cf tubing that runs along the boom. will show pics soon. it is finished and awaiting for acceptable weather (meaning not too windy or too cold). the servos remain in the stock position, and the rudder moves with not too much drag, so did not modify it. also added reinforcements to the nose. the stab is solid 1/8" balsa, covered with coverite lite. balances same as the original with same 2x1500 lipo/motor/prop/spinner/esc/rx. i expect same behaviour, perhaps smoother response from the elevator due to the linkage that has no drag.
and am working on the remains of the 1st fuselage, to use 1 servo at the fin for the elevator for comparison, with a new nose.
march 7-after all of the above, at the end it didn't work, as there was too much play at the pushrod that comes out of the upper belcrank, because it has to have a 90 degrees bend, so i went to a servo for the elevator at the top. this time it worked very well, very precise, centering, all. i ran the servo wires through the carbon fiber tubing that is the backbone of the fuse. the rudder servo is still at the front and uses a regular pushrod.
about the rear part of the original fuselage and tail, i ended up developing a new plane. it's called the Lynx. but that is another story.

keep looking at the bottom. more details to follow.
Last edited by phil alvirez; Mar 18, 2013 at 07:19 AM.
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Jan 12, 2013, 08:13 PM
Build straight - Fly twisty
Whiskers's Avatar
The best thing that can be done with that model is to give it a model airplane wing.
The scale wing tips are just plain stupid.
They are not really wing tips. They are Tip Stall Generators.
Jan 13, 2013, 01:04 AM
Registered User
Thread OP
agree. as you see, that's what i did. and from the pic you see that i removed the tips. and by designing a new wing i intend to get a plane that is stable and high performance. i have learned that scale sailplanes have wings that are too narrow and prone to tip stall. that's why i use just the fuselage-for the shape.
Jan 13, 2013, 01:20 AM
Build straight - Fly twisty
Whiskers's Avatar
Yep! some of my friends bought that plane. I took one look at the 1 inch (?) cord near the tips and crossed it off my list.

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