|Dec 31, 2013, 12:49 PM|
My new small field motorglider
For some time I've wanted to build a light small field motorglider for my home field, hemmed in by trees. Also something I could pack up and put in the front of my pickup truck.
For a design, I'd always admired, the Bradley "Termite" -- an all balsa design. I used the plans downloaded from Parmodels to work from, but modified the design for foam construction. I altered the fuselage formers and side panels to suit blue fanfold thickness, and lengthened the nose to make balancing with the light gear I intended to use less of a problem. I'm making a cowl of paper mache right now, and will have a spinner and folding prop on her soon. Currently swinging a 5x3 fixed prop.
The final model, maidened this morning (in 15F temps!) weighs 4.5 oz, and has a wingspan of 36".
It's way overpowered even with a 10 gram motor and 350 mah two cell. I'll probably reduce prop size further -- it will climb to altitude very quickly at 50% throttle.
It looks like it will be a good soarer when we get to thermal weather. Unfortunately a long way off for us here. Nice flat glide so far, as is. Anyway, thought you might like a look.
Happy New Year to all!
|Dec 31, 2013, 03:42 PM|
Thanks Balsa, ttersu, Whiskers!
I'll try to get some video ASAP, but we're in for an Arctic Express for the next week -- getting into the negative numbers here.
Whiskers, I put a 2" strip of printer paper set in diluted yellow carpenter's glue around the leading edge of the first panel of the wing -- making a D-cell. (Well to be more accurate, foam filled C-cell.)
The bottom of the wing is the fanfold plastic skin. Cut away an inch at the LE so the paper could be glued directly to foam. The bottom is very slick -- painted very dark blue for contrast. This plane has basically reverse USN camouflage! It shows up well in the sky overhead, and low against trees.
The top of the wing is sanded to conventional foil curve, though since it is a single thickness of fanfold, it's a very thin section. The top is not covered -- just filled and painted. Every effort was made not to add weight, where possible. I was shooting for something in the 4-5 oz range, and made that @ 4.5 oz AUW. The plane has 135 sq in. So wing loading is quite respectable!
The wing joints are reinforced on top with a 1" strip of paper set in glue.
Wing hold down aft is a nylon screw and foam/paper blind nut I experimented with awhile ago
(post 259 on)
in a breakaway cross-member.
The wing was as you can imagine like a floppy noodle when first built, despite the bottom having the plastic on. I could see it was a flexibility problem on the top surface (compression) in the inner panels and I knew the paper/glue was very good in compression, so did the C-cell thing. I figured I'd gradually add paper strips or spars until things were rght. Well no need for more, instant cure with the single LE strip. The wing is plenty stiff with that in the center panels even without reinforcing the tip panels. I left them as is, again, to keep weight at a minimum.
Guess that's all I can think of construction wise for now.
|Dec 31, 2013, 03:53 PM|
FYI just checked and the full 36" wing, painted both sides, with plastic sheet on bottom and paper reinforcement weighs 34 grams.
|Jan 01, 2014, 02:44 PM|
I was able to do a short hop today -- a bit breezy and 18 degrees F out, but the hat cam was working.
This is really an up and down run. I cut the motor at 32 seconds into the video -- just when it comes into view again in the clouds -- and glided the rest of the time.
Climb throttle was only 50%, and I was holding down elevator on the way up. Definitely will switch to a smaller prop then the 5x3 since I don't need the power
|Jan 01, 2014, 07:19 PM|
Joined Mar 2004
Thanks for the video! It flies very well. I just enlarged the plans to 1.34X for a 48 in WS. I wonder id the 10gm motor would be enough to power that, as I have a spare?
|Jan 01, 2014, 08:47 PM|
It glides well and when it gets its folding prop it will be even better.
When the thermals come back to play it'll never want to come down.
A nice reclining chair may be needed.
|Jan 01, 2014, 08:57 PM|
I'm not sure which 10 gram motor you have so it's hard to say. If you are careful you should be able to build it at about 8 oz.
Mine is an EMP 2020 2500 kV with 5x3 prop on 2S 350 mah
Assuming an AUW under 8 ounces you should be okay. If you go over that you might need something bigger.
But I think under 8 oz is do-able.:
I used an orange receiver with case removed
I used two 4 gram servos
I used a HobbyKing 10 amp 7 gram ESC
I removed skin from all blue fanfold except wing bottom
I would suggest a little wider paper strip over LE of center panels than I used -- say a strip 4" wide (2" top and bottom of wing). And also a 2" total wide strip over the tip panel LE's. (Which I didn't have.)
Also, a 1" strip on top of wing over all wing joints.
The nose needs to be extended -- or else you will need ballast, increasing the AUW.
Don't assume a bigger battery will compensate -- you shouldn't need a bigger battery. It will just add weight, and not as concentrated or as far forward as real metal ballast. I think the practice of putting big batteries in (for ballast) is a bad one.
Your plane if sub-8 oz will yield no more power through the same motor with the same prop than mine. Duration for both is probably a half hour easily with gliding and occasional motor runs -- possibly longer -- I haven't tested that fully yet.
Ideally avoid any ballast at all, though that may be hard to achieve.
A folding prop and spinner will add some weight but on the good side of the CG.
Use minimal paint.
If you go ahead with construction and post here, I'll be happy to help along the way if I can.
|Jan 01, 2014, 09:01 PM|
Hey Whiskers! ....and Springer! Thank youse guys, to two long time pals and plane nutz!!
you two kinda got me back into this after I'd been away...... now I'm hooked again.
|Jan 01, 2014, 09:23 PM|
United States, UT, Garden City
Joined Aug 2011
Looks like another winner Steve. I haven't flown a motor glider in years, must be relaxing! So what became of your shoestring?
You got me on the micro bug with that little bugger so I have started a 24" jr. falcon foamie. It would be done but I got the flu a week ago and haven't got any further than the hotwired foam wings done and the plans. I'm keeping good pics and will put a build log out shortly.
Awesome glider bud!
|Jan 01, 2014, 09:48 PM|
well, sweet, I'm glad you took the bait (we hang it out for anyone that passes by...) and I can see that you are back full bore! I too, wonder if you maidened the Shoestring. This guy will be interesting to see once the thermals catch it (small field? hmmmm, We'll see....)
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