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Author Topic: O.S. 25FP trainer design build  (Read 449 times)

Offline Branko Ljubetic

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O.S. 25FP trainer design build
« on: February 19, 2020, 06:06:03 AM »
Hi everyone. First, im the guy who asked on facebook if this is the right forum for me, simple control liner fan of walt musciano and small planes, or if this place is only for super fancy competitive stunters. This generated an unwanted debate here, with some offended folks, so if you are one of those, i will say sorry again. I just dont wanted to be offensive with no one, in fact i like to use forums since theres lot of experience from old masters.

That said, i opened a thread some months ago on RCG about  designing and building a .19 stunt trainer for a spare torpedo .19 i have. So i want to start a thread here also, but things changed from then.

Things in my country (Chile) are really bad now, from decades of sustained growing and development, now politically deeply polarized, making friends and family fight. We thought we were over it, but no, its the @#%##@ history of latin america. What old mans told us about sixties is exact same thing as now. Really sad. So then i stopped doing airmodelling for awhile.

Anyway, from now i will continue doing my stuff. Moved in the same city after hollydays (i live in Iquique, northern Chile) , now organizing my new shop, wich is in a basement, but big enough.

I usually fly first in the morning 1/2A planes and an Akromaster i built. The akro is fine flying with a Torpedo  15, but i want it wilder, so i put the torp .19 on it. That leaves me without a .19 engine, so i have to choose to build a trainer for the Torpedo .15 or a spare O.S. 25FP wich are closest one. Due to the fact i want to build a flapped stunter, the .25 seems to be the best idea.

With that in mind, i have to enlarge the project.  In my experience a .25FP is good for pulling profile old .35 designs, so must have near 500 sq/in of wing area. So i used an old own design of a plane for a McCoy .35 for making the wing, and need to draw the fuse, wich still be a profile one, maintaing main nose and tail momentums defined on this plane, not the old one, wich is 1/3 of the length venturi to CG and 2/3 length CG to elev hinge line. Luckily, i saw the templates of the ribs while organizing new shop, so i cutted the ribs.

Next week i will draw the fuse and start building this plane, but need to finish organizing everything. Right now the pleace looks like someone throw a frag grenade inside.

Branko


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Offline Leester

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Re: O.S. 25FP trainer design build
« Reply #1 on: February 19, 2020, 04:19:12 PM »
Welcome Branko, I look forward to seeing your designs and build. Good luck at home, politics is a nasty game !!
Leester
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Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #2 on: February 19, 2020, 08:28:21 PM »
Thanks for your kind words. Of course i dont want to change the course of the thread to the filthy politics, but yes, it certainly sucks. Seems like we learned nothing, sadly.

Anyway, im trying to do my stuff to change my mood, and airmodeling is one of the things i love to do.

Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #3 on: March 29, 2020, 06:09:07 PM »
Back on track after awhile. Cutted main spar and checked fit. Then need to cut lightning/lead outs and T.E. openings on ribs. Need to find a place to upload pics!

Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #4 on: April 12, 2020, 02:16:34 PM »
Well, cutted ipenings in ribs, checked alignment, and glued everything. I used the cord method to align, wich basically is glue the root and tips ribs on the exact same physical plan (having the main spar aligned and nailed to the board, and the putting a cord from root rib L.E. and T.E. to tip ribs in order to check every rib alignment. Since this is a tappered L.E. and T.E. design, i found this to be really important. I used old burnt dacron lines to do that, so maybe is not easy too see in pics. Now working on bellcrank and mechanism to move both flaps independently





Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #5 on: April 23, 2020, 11:53:08 AM »
Built the wing. Solid main spar inside a D tube with cap strips. All ribs enlightened. Dont have a wing jig, so i used a thread method using old burnt dacron lines: Glued root and tip spar in the exact same plan, and then used dacron lines in both LE and TE to align remanent ribs




Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #6 on: April 23, 2020, 12:03:06 PM »
Then worked on bellcrank
 Since is a tappered wing (just for the looks) with tappered LE and TE, i thought on different methods to operate flaps, including flotable double piece or installing a 90 bellcranks on both flaps. The i remembered Seagull RC planes uses a small 3 way part to conect one servo to 2 rods (used tonconnect flaps or elevator halves. The part dont allow movement in any axis, so it is pretty like one solid piece.


Please note everything was assembled with tape and horns are note attached yet. Following is a video of the mechanism tested. After the test i changed right wing rod for a straighter and stiffer one, to avoid maximum the flex:


 

Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #7 on: April 23, 2020, 12:10:08 PM »
Then i sheeted and sanded the wing, wich is ready to cover now.


Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #8 on: April 23, 2020, 12:21:04 PM »
Then i went with the stab/elev. At first i wanted a simple sheet stab/elev, of abouth 6mm thickness. Then i changed my mind, since i have just regular experinces with sheet surfaces twisting over time. So i planned a 3mm frame inside a pair of 1/16" caps. First cuted the caps (of both elev and stab, since i planned to cut them when assembled) then draw and build the frame structure over one cap, and then glued the remament cap, finishing with sanding, hinging and putting the conector "C" rod. Nothing si glued yet, but it is ready to cover:



Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #9 on: April 23, 2020, 12:26:17 PM »
Now i will design and build a profile fuselage. I want to build it with a lenght (from elev hinges to venturi) of 75% of the wingspan, using 2/3 of the lenght as tail momentum and 1/3 third of the lenght as nose momentum. I obtained measures and airfoils by comparing nice planes from many designers, ranging from Aldrich and Palmer to Kaz Minato.

Now im in doubt of building the fuse with a simple sheet (y believe like 10mm thick or building a structure inside caps, any advice will be apreciated. Thanks.

Offline Tim Wescott

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Re: O.S. 25FP trainer design build
« Reply #10 on: April 24, 2020, 10:13:44 AM »
Please note everything was assembled with tape and horns are note attached yet. Following is a video of the mechanism tested. After the test i changed right wing rod for a straighter and stiffer one, to avoid maximum the flex:

Even though you've got the wing buttoned up, I'd advise that you go to a fixed flap joiner in the wing, and lucky boxes in the flaps.  The only way to make that sort of geometry work is if both pushrods have a straight shot back to the bellcrank -- any kink, even in thick wire, is going to make that side flex more.

(And I wouldn't advise flaps on a trainer -- but I learned that with my own-design flapped trainer with a 20FP in it, so you may need to learn the lesson the same way I did).
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Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #11 on: April 24, 2020, 01:28:58 PM »
Thanks for your advice. What problem did you have with flaps? Im in search of a long tail moment, so i cant see any issue with that, i do like flaped planes i really enjoy flying the nobler, as example. Dad nobler uses the old tricky Fox .35 that he loves (i do have a diferent opinion, but respect the giant influence of this powerhouse in C/L). This Fox .35 power is comparable with an O.S. 25FP i believe, so my plane is with a little less area than the nobler, but aiming to have lesser wing loading, and of course less cross section being a profile.

About the flap config, if i believe i must rebuild in order to improve, there is no problem, since i enjoy the workshop as much as flying. But honestly, i dont know. I tested the system, and tested the rods, moving consistenly. The movement of the rod tends to be linear, since the middle distance between the flap horns can not vary to one side. Of course the lateral movement of the bellcrank affects the amount of travel of both flaps, but in the test it is minimum until 25 or 30 of deflection, so in working range both flaps operate almost the same. I modified the right rod of the pic, by putting a larger diameter rod, and with a straighter shape, in order to reduce to the minimum the flex. In any case, the diferent amount of movement is really low, way lesser than a diferential flap setup, like the thunderbird (i do like the plane, but i can not see any improvement with these setups).

Thanks for your input!

Branko

Offline Tim Wescott

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Re: O.S. 25FP trainer design build
« Reply #12 on: April 24, 2020, 02:02:59 PM »
It depends on your level of flying ability -- if you're starting out with the pattern, then a flapless plane is loads easier to trim.  But really, if you can't do square squares and round rounds all on top of each other, then unless you have someone to help you trim out your flapped plane you should probably go flapless.
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Offline Branko Ljubetic

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Re: O.S. 25FP trainer design build
« Reply #13 on: April 24, 2020, 02:57:41 PM »
Thabks for your advice. I think ill go flapped, thinking on building a fancherized Twister or a banshee, i choose to design my own. Im not a good pilot, but will improve, thats the point. Thanks.

Branko


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