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Old Mar 07, 2011, 06:59 PM
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first scratch build a success! help me make it better!

so i made my first foamie... and i figured ide jump in and scratch build one to start.

it has a 250 size 1800 kv skyfun motor, the skyfun stock esc, 2 hxt 9 grams, and a 750 3s lipo. 24'' wingspan, 10'' rc with ailerons, 4'' tc, and a 5'' sweep.

i took it out for a maiden today and to my suprise flew! it didnt really need to be trimmed, but it was flying a little funky.

i know what a nose or tail heavy plane should feel like and do, but when it was flying it was waving through the air only on x axis, like a flag kinda.

im thinking it might be 1 or 2 of 2 things.

1. i think the foam under the motor may be flexing and then snapping back causing the thrust angle to change.

2. i messed up the kfm2 airfoil (wasnt paying attention to my print out) and the step is at 40 of the tip cord along the entire leading edge.

any help would be great. like i said it flys, and i wasnt afraid of crashing it i could fly around no problem, just looked awkward and clumbsy in the air.
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Old Mar 07, 2011, 07:31 PM
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Would you describe it's actions as "porpoising"? If so, that's exactly what my V1 OSW was doing. In my case, I built V2 with more sweep in the LE and got rid of it. Moving cg forward helped the v1, reducing the speed at which it would do the pitchy rise/fall dips. If I ran at over half throttle it almost went completely away, so you may try more nose weight, Looks like battery is all the way fwd already. (same as me) I think i added over an ounce to get it smoother.

Sweep acts like dihedral, and give nice stability.
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Old Mar 07, 2011, 07:37 PM
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yeah that sounds just like what mine was doing... u can see in the one underside picture i had a fair bit of weight to the underside nose to get to the calculated cg, but it was mega nose heavy so i took it all off and then it started dipping. maybe ill add some more and trim up way more?

first flight i had full up trim and was still holding back to keep level

i have another 3 sheets of foamcore peeled and ready (i was assuming it would come home in pieces lol) so maybe ill start a new one. i also have a pair of wings from a airhogs titan thats begging to be turned into a nice delta
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Old Mar 07, 2011, 07:52 PM
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Ninja, nice flying wing! How did you construct your motor mount? That's currently part of my design that I'm working on. Since I'm now flying at a club field I'd hate to have a motor leave the aircraft prematurely!!

I looked at the lite ply tonight at the LHS but I wanted to check first, Springer I'm sure you have a ply motor mount template in your vast arsenal of great drawings.

I've also debated making one out of very stiff thin plastic and using nylon washers on the backside for all through-fasteners.

ps-What happened to your organ donor SkyFun? hehe... I've got one in the box that I need to work on. It was already in pretty sad shape when I opened the box (hinges, canopy, hatch latches, etc.) so I've been putting it off. And hopefully I've learned my lesson about ultra-cheap RxR planes.
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Old Mar 07, 2011, 07:57 PM
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i just used a piece of balsa i picked up from lhs. just cut a square barely bigger then the motor face and made a base of the same size then the supports. hot glued then wrapped in fibre glass tape. i found if i wrapped it the screws can grab real well without splitting the balsa

nothings wrong with it, i upgrade my floater jet motor, which meant skyfun got an upgrade, which meant parts left over for build lol.

on a side note i flew my skyfun at the same time i maidened this delta and my left servo blew midflight... rough landing but no damage.
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Old Mar 07, 2011, 08:34 PM
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My firewall mounts are typically 1/8" ply cut either 1"x1" or 1.25" square and Gorilla glued to the foam layers or monobloc. On the OSW, there are three layers of DS foam at the back of the "fuse", and I glued the ply to them. As a pusher, there's very little loading except in compression.
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Old Mar 07, 2011, 10:47 PM
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Thanks for the info! I'm working on a regular tractor design. So while I'm not much into WOT at all, it's probably still a bit overpowered with the motor I was intending to use and didn't want to rip free on my first scratch-built foamy.

Ninja, that must have been a good piloting job to get the SkyFun down in one piece with only 1 of 2 elevon servos working.
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Old Mar 08, 2011, 04:37 AM
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Looks like a nice wing. A couple of things that might be useful:
Your tip plates are huge, I would cut them down so they are half as tall. Also make sure they are perfectly straight or a bit toe-in ( like /------\ ). It will help with yaw tracking.
As to the porpoising, the elevons should have a slight bit of reflex, maybe 1/4 inch up when lying on a table.
I have found getting the CG to be perfect is a chore on such small wings. You can use this CG calculator to help, but try to add in the elevon surfaces as well. I usually just average them out by adding a bit at the inner and outer chord measurements.
Your elevons also look sort of reversed, they are usually wider at the tips. This will give enhanced pitch and roll control and add some effective sweep.
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Old Mar 08, 2011, 12:00 PM
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hello meshyx, thanks for the info!

so i did a quick search on reflex... and if i understand correctly, just start with my elevons 1/4 inch up, then trim it out from there?

i did have the cg correct using my measurements originally, but it seemed mega nose heavy. maybe ill add some more nose weight, add reflex, and chop the stabs in half.

yeha the elevons were purely just astetics. i figured since they were so large i could get away with having them larger on the inside close to the root.

thanks everyone for the info. i think i might make those mods tonight and head out for another flight.

ill report back
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Old Mar 08, 2011, 04:34 PM
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a quarter inch is about where I ended up with mine. Have you worked out the Mean Aerodynamic chord? if not, google it, and you will find some pictures on how to figure it out graphically. I did it, set the cg at about 25% aft of LE at the MAC and found that to be "just about right". No porpoising, nice flat long glides and neutral handling.
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Old Mar 08, 2011, 05:40 PM
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im reshaping the elevons, after that ill figure out the mac and cg again
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Old Mar 08, 2011, 07:05 PM
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You will certainly need to refigure, as the aileron switch will take it from double taper to true swept wing.
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Old Mar 08, 2011, 09:14 PM
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well a quick redesign in the workshop tonight and this is how it looks.

cut the vert stabs, changed the elevons, and added some reflex (hard to see)

with the new dimensions including the new elevons, i got a new cg of 4.5 from the nose @ 30%, which happens to be right where the cg is currently with no weight on the nose.
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Old Mar 09, 2011, 10:34 AM
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Sounds directionally correct! Post the results!
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Old Mar 09, 2011, 11:53 AM
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I'm curious about your servo geometry, since I'm currently working out those details too.
What prompted you originally to place them at that angle? Your original elevons had enough room for the horn to be more directly behind the servo, but it's fortunate now that you've totally changed the elevons that the pushrod angle still allows plenty of room for mounting the control horn.
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