|Feb 03, 2010, 09:30 PM|
B.A.D. 12" Span WWI Bomber FS
Well, I've decided to build a little indoor biplane. Bear with me on this one. I know the Parkzone brick is the coolest thing going on right now, but I've been meaning to do this one for some time.
I just got done with another indoor build, a Sopwith Pup, scratch, that was quite an effort and I needed to relax on something more simple. Don't believe me, check it out here: http://www.rcgroups.com/forums/showthread.php?t=1153096 I started that one as a scale engine mini-how-to, and the rest is history. This info is to give you a heads up on my insanity.....
The B.A.D. Bomber is a one-off, own design, guess we can classify it as a 'Fantasy Scale. My motivation is the Gotha, Vickers Vimy, and DH-4, all rolled into 1. 2 share some common features, one is off track completely, but I like the wing's aspect ratio for what I'm, trying to achieve. Oh, and it has to be German because I want the graphics to be easy and the Britt's got the last build.....
Hope you enjoy this one and maybe we will learn something.
My motivation for my one-off design........
|Feb 03, 2010, 09:52 PM|
Design parameters: Twin pusher (Gotha). After all I want to use an Air Hogs Aero Ace system, thrust vectored, and create something cooler.
The Air Hogs has roughly 40 SqIn of wing, weighs 20 grams fully loaded, about a 3.5:1 aspect ratio and is a little fast and twitchy but flys good. Love the proportional throttle. Lets say we increase the wing area to 48 SqIn, and give it an aspect ratio of 6:1 (DH-4), about all we want to stay efficient.
1/32 balsa, sheet. I know it's strong, light, and this is more a big test on getting smaller and lighter on my builds. The sheet concept keeps it simple, and I have another micro build in mind that will need the strength. OK, so it's a test to see how light the airframe will be....
Gotta have landing gear (heavy), got to look cool (heavy), but I'm going to give it a real long tail moment like a WWI bomber, and a large tail (lifting type and more SqIn!)
First surgery. Second wiegh the 'goods', 10 grams of good 'ol electronics. Put on the thinking cap, and....
So I started crunching numbers, and layed out some templates. (Sorry my CAD days were programing industrial machining programs in the old days and I have not jumped on board, lets do it the 'old fashioned way'.
Hoping to get this bird under 1 ounce, lots more drag I'm sure, but maybe with the longer Aspec Ratio and Tail Moment it won't be so twitchy.
|Feb 03, 2010, 10:22 PM|
A few hours later, we got some wood cut out. I'm using medium 1/32" with some lighter veins for the wings (7.5 grams per 3x36" sheet).
Really light 1/32" for the fuselage (6 grams a sheet). The formers and ribs are very dense, heavy wood, (11 grams per sheet ).
I stab/elevator, and fin/rudder have some cross grain sections to aid in warpage control (see photo). I used the stiff , heavy wood for the cross grain sections, CA'ed on and sanded flush.
I couldn't resist adding some weight and the struts are the 1/8" x 1/32" Bass you see. I know better, but thinking ahead, that is going to be my rigging points too. I also don't trust 2 channel control so much as to slim down this vital area.
And I just had to go ahead and glue up that wonderful nose that the old bombers had. I just couldn't wait! Looks like the Vicker's Vimy, sort of.
The wings have a 1/16" square "leading edge", actually glued to the 'bottom' wing surface, right at the edge. Call it a spar if you will, but it stiffened the wing up and will give some protection in the event the airframe comes in contact with something solid when flying. (That never happens, only to my buddy....)
I am leaving the wings in one piece for now, using a centerline to pre cut the spar slots in both wings. I will also place the ribs before seperating the panels for the dihedral. I rounded the wingtips, ala DH-4 style.
If you noticed that the horizontal tail looks sort of Pup style, and the Fin, 'what's that', since I'm the designer I may change that a little, we'll see....
|Feb 05, 2010, 12:24 AM|
I took the fuselage sides and added 1/32" x 1/32" longerons, CA'ed to the edges, and added a 1/32" thick wing saddle doubler. Next I glued in the uprights in the proper locations where cross pieces will go top & bottom.
I marked these locations lightly with pencil before placing.
I just wicked CA at the edges and let it suck itself into the union.
You will note that the nose assembly has a former, it was 1/16" thick and I wrapped the skin around the top and bottom circular formers. By only letting the wrap extend 1/32" of the front formers edge, it allows the fuselage sides to be pulled against it as a glue joint.
The fuselage sides are already felling much stronger.
|Feb 05, 2010, 12:31 AM|
Added the formers to one side using a small square to keep them straight. Glued the other side to thes same formers whils holding it flat on the bottom.
Added the nose block/former. Pulled the rear together and glued. Next added the cross pieces top and bottom, cutting them to length as I went. Boy does a good set of tweezers help at this scale!
Everything is glued with thin CyA to this point.
|Feb 05, 2010, 12:43 AM|
Well, remember I was most interested in weight using all sheet balsa for an airframe. This is stout already and the weight? I'm amazed. 1 gram (+) so far.
My scale only goes by one gram increments.
A couple more views, on the scale.
|Feb 05, 2010, 06:24 PM|
|Feb 06, 2010, 09:20 AM|
Pensacola, FL, USA
Joined Dec 2000
I'm following along with you.
You may be interested in this video.
|Feb 06, 2010, 04:54 PM|
Joined Nov 2001
Better Built in Balsa
Fantastic build. I love balsa and was always mystified why the Depron builds seem to be lighter. Now I see that it (as I thought) is attention to the details of the build, proper selection of material and good craftmanship. This motivates me to try for a Vapor sized and weight J3 or L4. I will be following this thread closely as you obviously have the skills and the knowlege to bring this project off with a reasonable weight. My goal would be to achieve wing loading of about .25 grams per sq inch. Do you think this is achievable ?
|Feb 06, 2010, 05:53 PM|
On mine because I always do scale and my gear is already 10 grams, I might be lucky to be in the .56-.59 grams Sq/In. Sort of funny using metric weight and Imperial measurements! Pretty heavy, but I'm testing weight for strong for another project. Done enough foam to know it would be heavier than this bird.
However a spectrum AR6400 is 3.9 grams, guessing a battery 3 grams. The Vapor brick is lighter I think? Vapor battery might be that 70 mah one, probably lighter? So bear with this madness:
AR6400: 3.9 gram
Battery: 3.0 gram
pushrods/etc: 1.0 gram
Total: 8 grams
I know you could get a balsa airframe, single top wing, struts, & yes landing gear (home made wheels) to 8-10 grams in lets say 48 SqIn wingspan like mine, using sheet. That would be 18 grams total AUW / 48 SqIn = .375 grams per SqIn.
So at 18 grams you would need a 72 SqIn wingspan (72 SqIn * .25 grams per SqIn= 18 grams)
Or if you keep 48 SqIn as a design, get your weight down to 12 grams. At 12 grams your airframe could only be 4 grams!
So let us think, If strength was not an issue I would cut tiny little strip wood with 1/32" balsa, and build an airframe like a rubber freeflight using stick and covering. It does work, just need a light hand, tweezes, and good reading glasses. Once covered would be strong enough for your gear, and you have better control with the vapor radio gear. Also as light as it would be, if you hit anything probably no damage. If something hits you, say goodbye.
Last option is I bought some bulk balsa, and some sheets are so thin you can see through them, weigh 2-3 grams each. I thought you could sheet with these but there will be no sanding!
1/32" balsa is cheap, a light piece in the 6 gram area might work too if you get your whole design on a single sheet, just might be a tiny bit heavier. The airframe would be fairly large, but the motor might just fly it, only real slow.
I wish you would try it and share the build with us. Sheet construction goes very fast!
Thanks for the kind words on my little build.
|Feb 06, 2010, 06:04 PM|
Finish the Fuselage.
Didn't take long at all to sheet the top and bottom of the fuselage. I've added the tail hook at this time before sheeting. I used .014 music wire with a spring loop bent in. I've done this before and the airframe will never bend it out of shape at this weight.
Only problem I know I over did the mount for the gear and added too much weight right where I do not want it, the very back! I've designed a very long tail moment for looks and stability, but this will cost me. I have no fear! Going on with it, we'll see what happens as I can move my main gear forrward if I want.
The fuselage is so strong I can really mis-handle it at this stage, just have to be careful not to dent it with a fingernail or something dumb like that. Almost no twisting effect noticed when I stress test it either. Overbuilt already, probably.
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