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Old Nov 04, 2012, 01:12 AM
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Question
3 ch foam plane rolls hard when turning

Hi iv just built this foam plane from dollar tree foam its a 3ch with a 40'' span and the fuse is 27''

Im having a prob with it wanting to roll pretty hard and nose down when i turn is this a lack of dihedral or something else any help would help
Thanks
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Old Nov 04, 2012, 08:06 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
12,996 Posts
I would go with lack of dihedral for a start..

Also your wing tips may have the equivalent of washin, because of the radiused leading edge. Allowing the tips to stall before the center section. Washout is required.

The wing looks quite flexible, so any dynamic twist in flight during a turn could exaggerate things. Are those rubber bands ?, used for stiffening ?.

The vertical stab area, (fin), is also quite small and could do with being twice the area shown.
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Old Nov 04, 2012, 09:37 AM
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there are rubber bands holding on the wing but under the wing is bamboo stick
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Old Nov 04, 2012, 08:12 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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Certainly a lack of dihedral for starters. Also not having played with dollar store foam I am wondering if it is thick enough and thus STIFF enough when bent into this sort of wing to avoid flexing and causing a speed related warping of the wing.

One way to counteract any speed related warping would be to add supportive wing struts similar to the V shaped ones on a Piper Cub that extend from the base of the fuselage out to the last rib on the wing. The struts need to be some sort of wood or other material that will not buckle or flex under the flight loads so that they hold the wing from warping.

I know that your bamboo sticks are intended to try to avoid any such warping but they are small and used in less than the best way to provide proper stiffening.

And finally your fin and rudder are WAY too small. Increase the height and chord of the entire vertical tail area and rudder by 1.5 to 1.7 of the present size to get them up to a size which is more in harmoney with the wing and horizontal areas.
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Old Nov 05, 2012, 02:17 AM
Fit to CFIT
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Thousand Oaks, CA
Joined Mar 2004
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That type of airfoil is limited to very slow speeds at very high angles of attack. If the plane is flying faster than you can walk, on crutches, backwards, then it's way too fast for that airfoil. You'll have a massive separation bubble under the leading edge that will move around erratically resulting in severe pitch and roll instability. I highly suggest closing the bottom of the wing with a flat sheet of foam which should completely cure the problems whilst increasing the strength of the wing 10-fold. Alternately, you could just use covering film (or even saran wrap for a test). Even just some film would vastly increase the strength. Remove the bamboo and make a larger fin once you have it sorted.
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Old Nov 05, 2012, 05:11 PM
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well Im trying to keep it slow but want it to be more stable to, and if i close the bottom of the wing would that allow it to fly pretty slow
and with that airfoil would that make it pull up when i give it more throttle cause it seems to do that also
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Old Nov 05, 2012, 05:22 PM
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I just want to say thanks for all the help for everything so far but im getting some blue FFF foam tonite i think its better than the dollar tree stuff and im going to rebuilt the plane with all the new information i know now ill take some more pictures and show you what i get next lol
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Old Nov 05, 2012, 05:52 PM
Fit to CFIT
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Thousand Oaks, CA
Joined Mar 2004
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Wrap some sandwich film around your wing to quickly test the concept before building another.
If you want to keep the undercamber for ultra-low speed flight at the expense of everything else I suggest very mild camber and/or a Jedelsky-style structure to minimize flow separation at the lower LE.
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Old Nov 06, 2012, 11:43 AM
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The 'Wack, BC, Canada
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There's actually not that much wrong with a simple arc shape. Just use an amount which is more reasonable. If you make the lower side arc about 4% of the chord the resulting airfoil will fly more or less normally at moderate speeds and still slow down for slow flying really nicely. It still won't do high speed with grace but it won't cause the model to tip around.

To keep things simple so you don't need to really "build" the wing I'd suggest using functional wing struts as per the J3 cub and so many other full size planes such as I suggested before. Struts would both hold the wing against the G load flexing as well as lock the wings to a flat non twisted shape.
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Old Nov 06, 2012, 07:14 PM
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United States, UT, Salt Lake City
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I know it sounds too simple but on smaller models - just join two sheets to create a simple camber.
sure it is tent shaped but on small slow models it works just fine
Beauty is in the mind of the beholder--
an if'n you be holding this thing and be giv'n it a nice slow launch - it will likely be fly'n nicely
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Old Nov 09, 2012, 08:16 PM
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thanks

I'm working on my new one now i think im going build the wing the same way but put less camber and does someone know about the cg would be the wing is 8'' from front to back ill put some pics up as soon as im done Thanks again
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Old Nov 10, 2012, 03:40 PM
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2.25"
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Old Nov 10, 2012, 04:36 PM
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Quote:
Originally Posted by richard hanson View Post
2.25"
Rough estimate assuming no lift contribution from the tail and fuselage and ignoring the rounded tips, I imagine?
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Old Nov 10, 2012, 04:43 PM
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If all is well- somewhere around 25-30 % of MAC is close enough to get you going. over 30%- tricky unless you are into flailing n flipping
Under 25%- I seldom use unless I am doing some speed thing where stability is very important
There is no magic number -you just want a workable setup which you can adjust minimally.
Tips?
forget em - they make little if no difference in these sizes
Just make it look nice and try for exact mirror -from side to side .
Making the wing sturdy enough and light enough is a task that takes practice .
You can never make one too light - but too flimsy is too easy.
Pick an airfoil which is easy to build n replicate side to side
The absolute shape (some ideal wind tunnel number) really isn't important -
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Old Nov 19, 2012, 09:50 PM
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springer's Avatar
SE MI
Joined Oct 2004
9,545 Posts
Your fanfold version will be a lot stiffer, but the one you have will fly fine if you just 1. add washout to the wingtips (curl the trailing edge at tip up about 1/4" - make sure it's the same amount on each side); 2. add a piece of DS foam to the top of your rudder forward over the vstab to increase rudder height (maybe 2" tall and inline with the stab LE and rudder TE) and 3. break the wing mid span and add 2" of dihedral per side. It'll still want to roll into a dive, so you will need to add some up elevator in the turn to keep the tail down/nose up - it's typical of that genre. Also typical of the light weight undercamber wing is to nose up and climb when power is applied - just compensate with some down elevator.

All my onesheeters are DS foam so far and they use similar techniques and fly just fine.

Don't forget the foamies - scratchbuilt forum higher up on first page of rcg. Tons of help there!
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