My 1 Meter EPP Bobtail Plank - RC Groups
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This thread is privately moderated by FeiJi, who may elect to delete unwanted replies.
Oct 04, 2012, 08:08 AM
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My 1 Meter EPP Bobtail Plank

I wanted to make something besides a swept back wing so was looking at Bill Evans Scimitar wings and decided to try one. I can build a wing in 3 or 4 hours but the addition of a fuselage requires another 3 or 4 hours. I like the wings better because they are pretty crash proof but when this plane noses in, something is going to break.
Bobtail Scimitar EPP Plane (4 min 37 sec)

Mine is quite ugly, quite evolved or devolved and took a while for me to figure out why it wouldn't fly.
My first one was finless but had the rear fuselage angled up, not realizing that that would act just like up elevator and make the plane into an infinite looper. Then I cut that off and angled it down which of course had the opposite effect and it would try to fly but as soon as it got level it would make an instant dive. Also since I cut off most of the tail I had more vertical face in front of the CG so the plane would take off then promptly switch ends.
Finally I added a tail but was too tail heavy so it just wallowed around in the air.
I also cut the elevons shorter because someone told me they were too much. I think I will put them back full length now that it flies.
In the end I got enough weight into the nose that it flew and I like the way it flies and looks in the air so I am going to build another one that is lighter, with thinner wings, a higher kv motor and shorter tail. It should really scoot.

(Photo #1) My first attempt probably would have flown fine without the tail if I hadn't angled the fuselage up. I actually like the first plain the most as it looks the coolest, kind of like a retro racer look. I may try the finless again but make the tail horizontal on top and bottom.

1 meter wingspan at about 600 grams including a couple ounces of dead weights. 1500kv motor and 8*6 prop makes for pretty docile performance. Laminated EPP, with a plywood and shower curtain rod holder motor mount.

Update 10/5: (Photo #2) Rebuilt this wing and will flight test tomorrow. No tail, no real fuselage, just a projection out front to mount the motor on. Also much lighter, 450 grams (but may have to add some nose weight). (Tried it, doesnt work, it tries to fly backwards.)
Update 10/6: (photo #3) I never learn. Added a fin and it now flies nicely - quit trying to turn around and fly backward. Going to add back full length elevons. But it does tend to tumble in a wind or on landing.
Update 10/11: (photo #4) Flying this plank is pretty tricky - it flies nice in no wind but the slightest gust will nose it up and lose control. I know the cg is a little back but I extended the tail to make it a little less cg sensitive. Readded full length elevons. Also mounted the motor with zip ties in hopes they will break rather than the airframe. Will test tomorrow morn.
Update 10/12: This plane is a mess, have to add too much dead weight to the nose and now I broke all my props. Will have to put this on hold.
Update 10/12: (photo #5) Thanks to some advice I extended the nose 4 inches and it flies great. Kind of slow and sedate since I broke my 8*6 props and only had 7*6 and a 1500kv motor but thats ok - it flies and I like it. Guess I will quit calling it a Scimitar as mine is probably a disgrace to the name so will just call it a Bobtail Plank. The motor is mounted on a shower curtain rod holder zip tied and tape mounted on 1/8 inch bamboo (got from a clipboard). Very strong stuff, a little heavy but still flexible if it smacks the ground. Will try to make it a little more aerodynamic up front. 10 minute flight on 1100 mah lipo.
Eventually would like to go back to build in photo #1 as that looks the coolest but spending hours making the fuse just doesn't interest me.
As always I use bike spokes for control rods as they never flex and I put bamboo rods the full length of the elevons and then laminate them - they never bend. And I read that the wider taper at the outer of the elevons prevents tipstall - seems to be.
The internet is great for getting ideas and info: I read that one of the reasons for tapered elevons is that before the advent of computer radios you could give more power for rolls and less for elevator by making the ends toward the wingtips wider. Huh! Interesting. I haven't tried any aerobatics on this wing yet, waiting for the rain to go away. Giving more power to outside of wing gives better roll performance, more power to inside gives better loops. Elevons are just elevators and ailerons combined. Here I just thought the taper looked cool. Guess tapering is for making what you want the wing to achieve.
Update 10/27 (Photo #6)Built new fuse, moved tail forward and canopy (looks like half a pop bottle because it is).
Last edited by FeiJi; Oct 26, 2012 at 08:06 PM.
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Oct 11, 2012, 12:17 PM
Registered User
Is your CG at 10% of the root? I'd taper the ailevators to reduce roll twitchiness while keeping enough up/down response. That one simple mod made mine much easier to control.
Also, your nose looks short for a simitar. Leads me to think you may not have the CG correct.
Oct 12, 2012, 04:57 AM
Registered User
Originally Posted by Averan
Is your CG at 10% of the root? I'd taper the ailevators to reduce roll twitchiness while keeping enough up/down response. That one simple mod made mine much easier to control.
Also, your nose looks short for a simitar. Leads me to think you may not have the CG correct.
Ok, I made the fuse 4 inches longer and it flies nicely now. CG is around 10%. Didn't think it had to be that close to the LE but it does. Could have kept the tail short but I like the way it flies. It is really ugly now but I like how the plank part looks in the air. Going to go for a faster motor. This one flies now for 10 minutes on 1100mah lipo and 1500kv motor with 7*6 prop. Broke all my 8*6 props.
Thanks for your input.

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