Icidence..don't want to find it by coincidence, want to be incedent FREE!(Help!)

[Demon-Leather]

Hi there,.. Bob here. :D I'm redesigning a small rubber job for electric r/c from an old vintage plan I had blown up a while back to around 175%.(it was an all-sheet rubber job designed by Walt Moony circa 1965, and I think the origional span was around 16") Since this will be a pseudo-scale plane there is an awsome "fudge-factor" I can play with, and I'm using the plan as only a "basic" guide for measurments, shapes etc.. The problem is, that the incidence on the wing looks waay too positive to Me (since I don't have an incidence meter, I'll have to guess but this looks like around 5-6degrees, if not more) The wing will be a low-wing, flat bottomed Clark-ish-"Y" or perhaps even something like a Goldberg Jentle Lady glider airfoil. Normally, when I design a plane,I usually use a 1 degree ("fudged-in".. what looks like 1 degree) Now,.. on a biplane I'll try to keep the top at zero,and the bottom at 1 degree..USUALLY, everything turns out ok with me & my projects,and they seem to fly,at least adequately. :rolleyes: The thing is,.. I'm designing this one for a "build thread" here, and it will be geared for novices to build..No actual plans, 1/16 all-sheet balsa drawn on 4" X 36" sheets. I don't want to give any newbees a "bad-taste" in their mouth with the very first plane stalling & crashing them repeatedly,and sending them scrambling for a foam ARF. So.. If You can help-out, by all means.. PLEASE do! I will post some photos to help demostrate what I'm talking about... Thaks guys! :D Bob

[Demon-Leather]

This will be the project, a Piper PA-25 Pawnee.. or as it will be known in the build thread " The Pseudo Pa-Nee"

[Demon-Leather]

I've shaved nearly 3.5" off the front for a molded plastic cowl.. and laid the side out pretty much to the actual enlarged plan,.. and check it 3 times...

[Demon-Leather]

Now.. it looks like an awful lot to me... as You can see I've dropped it a full 1/8th of an inch,.. and it still seems excessive to me. Even dropped a full 1/8th of an inch, the difference between the leading edge & trailing edge is a full 1/4" higher than the trailing edge from the baseline, which is level with the stabilizer/elevator assy (top of 1/16" sheet)..The plane was origionally designed with a single sheet wing with a couple ribs to give it shape,and is an undercambered airfoil, as You can see in the plan,.which might be the problem,.
I designed another all-sheet job called the "Pseudo Pa" and even though it came out incredibly heavy(10 oz on the money, RTF) for the chosen power system (GWS geared IPS, running old Qualcomm Li-Ion pack) It DID fly. I "fudged-in" about 1 degee positive incedence in that one.
This one will have much more wing area than it's predacessor (Pa was 36" and a 4.5" chord,with 2.5" no-lift fuselage. The Pa-Nee will be around 42" with a 6" chord, but about 4" of that span is fuselage) She came out VERY heavy, due to My poor wieght-management,and grossly "over-building" her(old gas habits are HARD to break! :( ) This one, the wood has been carefully selected, and I'm expexting (hoping) for around 8.5-9oz realistically....

[Demon-Leather]

This, is the "Pseudo-Pa" flying her maiden.. and giving me a bit of a fit due to a slight twist in the right wing. She had to be drilled full of lightening holes just to get her to that whopping 10oz.. this one I'm hoping "No-Holes"...
She did her job,and proved the concept would fly, now we can build them better,...lighter.

[vintage1]

There is a real issue here that always crops up with FF and especailly rubber conversions.

The models were designed to fly hands off - and that generally meant a very forward CG, lots of dihedral, lots of decalage (incidence to you) and massive amounts of down and side thrust. They never flew striaght and level: In general a climbing left turn transitioned through a brief period of level and straight flight into a gliding right turn.

Its not clear whether this is free flight or RC. If free flight stck to what is there: If RC of course you have an elevator, rudder, and possibly ailerons, and you can knock out nearly all of the dihedral and decalage, and downthrust, and move the CG more aft to compensate.

In which case go by what your instinct tells you is normal for a model like this.

[Sparky Paul]

What Vint says... and..
1/4" in 5 inches is 2.862°, not a whole lot. Probably be just fine. (Side opposite over side adjacent)

[BMatthews]

Paul and you are on the right track. But even at 2.8 I think it's a little much. I'd drop it to 3/16 and call it good. Flight testing will show you if it is OK or needs to go down to 1/8 or so.

I made two of the Pawnee sheet rubber models in my youth. One was heavy and the other very light as I learned to select wood properley. The second flew but suffered greatly from my novice trimming abilities. I notice that the paper on your original plans seems to be as yellowed as my copy is.... :D

Selected contest weight like wood will be imperative to the success of this design but it should be fun as long as you can convince folks to pick their wood carefully and they are ABLE to pick it.

PS: For the size of model you are intending to make I can't help thinking that an all sheet fuselage and tail mated to a built up wing may offer more chance of good performance. But of course the effort required will climb. What about a Ken Willard like built up wing of light 1/32 upper and lower skins with 1/32 ribs? A "proper" flat bottomed airfoil and better stiffness than the curved sheet offering. The lower and upper sheeting could be cut to the tip shape and the lower sheeting bent up with wedges to mate to the upper sheeting and then glue them together. The resulting upward curved tips would provide a measure of tip dihedral to sharpen up the control response.

Mind you it would be harder to build I must admit.

[Demon-Leather]

I was thinkin about 1/8th.. close enough! Great to have a second opinion(and 3rd) :D The construction IS based on Ken's designs (My favorite designer)... just minus the bottom sheet. and only Lite Film as a skin. Being a Glow & diesel plane builder, I over-built the Pa's wing, using much too hard (and heavy) sheet, along with a rock-hard leading edge.( after constructing I had to go through the backside and hollow it out,.. to show hoe heavy it was, I lost a complete oz of the wing just doing that! :eek:) Then had to cut large rectangular holes in the 1/16" sheet.. I learned a LOT about weight on that wing! :( There are only 4 ribs in wing construction. strength comes from a 1/16 x 3/16" strip in front of the rib and a carbon fiber laminate in between the strip & the leading edge material. The Pa wing survived a 4 revolution cartwheel without even a rip in the covering... if it had been a built-up,.. I'd have brought it home in a baggie I bet,.. (which, by the way I fly, I took one with me for the test! :D )
here is the story, if You are interested..
http://www.rcgroups.com/forums/showthread.php?t=236459
and here's what the wing looks like...

[HELModels]

I find the discussion of FF conversion interesting as the clunker I tried to design flew better hands off than with any input at all. Any attempts to control her via radio usually resulted in a violent snap roll into the ground.

I spent lots of effort building out of foam and fiberglass so she survived long enough for me to put a polyhedral wing on her which tamed her somewhat.
The problem was busted props and bent shafts. She had lots of decalage, huge control surfaces, and loved trees. Her progeny is expected soon but will be a pusher to protect the heart, hers and mine.

Incidentally, the polyhedral FF wing was salvaged from an ancient freeflight model with the same span and chord. This old FF had about 10 degrees of decalage and a Flying horizontal stabilizer, and very rearward CG. I remembered how great that plane flew and figured the wing had some good mojo thanks to Eppler.

Good Designing, Good Luck :)