Seeking opinions, recommendations, observations, sarcastic remarks, and pictures of your cat.
Airfoil is Clark Y top and GOE 286 on bottom, just like my 36" and 50" prototypes. A bit of washout which I did not measure.
Will be cut from black 1.3# EPP.
Don't steal, the government hates competition.
Here's a recommendation, cut me one!
But of course.
Don't steal, the government hates competition.
More sweep? I always have a hard time getting the CG far enough forward
I was kind of thinking the same. Perhaps about 30-35 degree sweep would be better.
Don't steal, the government hates competition.
How's that?
Don't steal, the government hates competition.
I am a fan of blunt noses... it gives a little extra wing area and a larger area in the front for big heavy things like batteries, cameras, OSD, tea cup kittens, a hatch.
Aerodynamically speaking, you can just give a little up on the ailerons to produce "reflex" but have you looked at the Sipkill1.7/10B airfoil? There is a slew of other good wing airfoils at that link too. The taper to the ailerons is pretty nice, this will create a washout like effect.
You had me at tea cup kittens.
I have considered a blunt nose but blunt section makes the cg move forward, so it's even more difficult to get enough weight in the nose (amirite?)... I have looked at the sipkill airfoils and they aren't symmetrical enough for my taste, they will flutter at high speeds, they are more meant for slow-ish combat flying. I have taken my 36" wing into full throttle straight vertical dives with no flutter.
Don't steal, the government hates competition.
More sweep might only compound that problem. If you can't get your CG far enough forward, you can do a mini-fuselage, longer chord in the center (keeping the TE in the same place), or simply a straight section in the middle. Examples here:
http://www.rcgroups.com/forums/showthread.php?t=1611629&page=6
Sweep looks good to me.
My "Cheap Armyr" is about the same span, looks to be roughly the same wing area. It's AUW ended up at 2.73 lbs, right where I expected. Motor is 275 watts. Not quite 1:1 thrust to weight, but almost. I wanted it to be able to cruise for a while. It is definitely more difficult to launch than Derrick's Tek Sumo as it has a higher wing loading and lower T/W than his. I'm not sure if the frisbee style launch will work with mine, I haven't tried. I don't think I'd want to add too much more weight to it, though it could probably handle some.
If you are going with a hot-wire cut airfoil, I think you should choose one with reflex (trailing edge turned up slightly), like the Sipkill. Reflex will add a lot more longitudinal stability. This can also be added artificially by deflecting the elevons 5-10 degrees up for normal flight (as I have to do with my flat-bottom Armin-style wing), but if you are cutting an airfoil anyway, I'd design reflex into it from the get-go.
Here is a good list of airfoils for flying wings and tailless airplanes: http://www.aerodesign.de/english/profile/profile_s.htm
When you get the wing halves glued together, install your servos & elevons first and do glide tests with a rx and small battery to figure out where your CG should be. Yes, actually throw it. You may have to put the battery on a stick out in front to get it where you want, since you won't have the motor mounted yet. Once you're happy with how it glides, aim for that when you lay out your gear before you cut out slots for all of it. The CG on my wing needed to be significantly aft of where I thought it would be.
Sorry if you know all this already. Maybe someone else can benefit from it otherwise.
edit: I took too long typing, Derrick and you had time to post before I was finished! also mine is 35 degrees.
Yes... I am building a Deep Reaper right now and adding the 6" extension moves the CG from 10.5 inches behind the nose to 9.5".I have considered a blunt nose but blunt section makes the cg move forward, so it's even more difficult to get enough weight in the nose (amirite?)...
You can play with the placement of the motor to help you attain more forward CG. I forgot to mention along with the tea cup kittens, the blunt section allows you to have a larger prop than you normally would. I like this because bigger props are great for constant throttle cruising.
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