FTFC20 1939 Popular Aviation Special by Vimana89

[Vimana89]

https://outerzone.co.uk/plan_details.asp?ID=10611 (link to Outerzone original plans)

Design and Aerodynamics
I'm not sure exactly how to refer to the wing, as it's something between a slightly swept wing, a trapezoidal wing, and a mild delta. The original seems to have just a tiny bit of back sweep/raking of the wing tips past the root chord, making it a proper swept wing. My version does not share this feature, and the wing tips line up with the root chord more like on a delta or trapezoidal wing. My version also has a one piece elevator with the vertical stabilizer in front of it rather than a split elevator/tail plane like the original. Other than those two factors, the dimensions and profile are very identical to the original. For weight and simplicity, I went with a one sheet thick wing with no air foil.

I went with a 3ch A.E.T. control setup, although this plane will fly great as R.E.T. too with a bit of dihedral, and I imagine it would work well on 4ch. Wing loading comes in at 13.71, and CG is located 4" back from the leading edge(these figures are also given in the plans). The high mounted swept wing has a lot of inherent "pendulum stability", so I went with no dihedral in the wings with my A.E.T. setup, although the original balsa plans do seem to show a very slight dihedral. The result is a very responsive and maneuverable yet stable plane with gentle stalls and great high speed potential, but not a dedicated "slow flyer", as its responsiveness and maneuverability are not apparent at super low speeds, where the plane is a bit clunky and prone to slight wing rock and gentle stalling. Maneuverability is still good at a modest to moderate cruising speed and outstanding at faster speeds.

 

[Vimana89]

Video Footage

Final Version
Vid 1.)
Finalized prototype/1.0 5x5.45x2 bullnose

Prototypes
Prototype 1, 3ch RET: 2s 650 5x5.45x2 bullnose

Prototype 2, 3ch AET: 3s 850 5x5.45x2 bullnose

 

[Vimana89]

Prototyping/Flight Testing Logs

Prototype 1: 3ch RET
Wing and tail plane close to scale, but nose was lengthened and fuselage length was shorter overall than the original, making a more "short coupled" plane. The dihedral was also made too extreme. Despite all of this, the plane balanced and flew beautify on 2s 650mAh 75c with 5x5.45x2 bullnose props, with a slight bit of wing rock and Dutch roll, but overall, a decent flyer that is easy and forgiving despite the flaws.

Prototype 2: 3ch AET
Control scheme switched to ailerons 3ch, fuselage lengthened, and wing moved up for a shorter nose. Plane could only achieve balance on 3s with an 850mAh 75c battery. 5x5.45x2 bullnose prop retained. A good flyer with very forgiving and recoverable stalls, but overly loaded on weight, making it a bit touchy, more prone to low speed stalls, and sluggish on acceleration and deceleration. Still, a solid performer despite its flaws, but not really where I want it to be.

Prototype 3/ Finalized version 1.0 3ch AET
The slight increase in wing span and chord made the plane less loaded and put some pep back in her step as far as speed and maneuverability. This is still not "slow flyer" material, and handles best at moderate to higher speeds, but is less clunky and more stall resistant at lower speeds than before and can still maneuver well in a fairly lazy cruise. The P.A. Special is sporty, stable, and fast, but also highly aerobatic, doing snap turns, wing overs, and nice axial rolls with ease. Loops take a tiny bit more work but are not bad. Stall behavior is gentle and predictable. Nice stability in both pitch and roll axis. This could make a solid 4ch plane with addition of a rudder.

 

[Vimana89]

Pics

 

[Vimana89]

Build Guide

1.) Wing
Cut out wing. Draw guide lines for gluing the wing to the fuselage as indicated on plans. Cut and bevel ailerons. Install aileron servos and linkages in desired manner. Servo cables/ Y harness can be through-wired or taped along the top of the wing, depending on preference and electronics positioning. Wing is complete, set aside until assembly.

2.) Fuselage
Cut out all fuselage related pieces. Set aside the top/rear fuselage plate until assembly. Cut and remove the material required for the B fold cavities on the main piece and the cavities along the sides of the bottom fuselage plates. Remove the paper from the front/bottom fuselage plate and complete and crack the indicated 50% cut. Complete the B fold on the main fuselage body and let the glue set. Add bottom plates. Mount motor, either with the proprietary inner motor mount (by angling and positioning the pieces as desired for thrust angle), or with a standard FT power pod. Fuselage is complete, set aside until assembly.

3.) Tail Feathers
Cut out tail plane and vertical stabilizer. Draw guide lines for gluing the tail plane to the fuselage as indicated on the plan. Cut and bevel elevator. Position elevator control horn as desired and mount it. Combine tail plane and vertical stabilizer, checking for a flush 90 degree angle. Tail feathers are complete. Set aside until assembly.

4.)Assembly
Start by mounting the tail feathers to the fuselage by gluing along the indicated lines. Next, mount the top/rear fuselage plate, gluing along the indicator lines and also the foam core where it meets the foam core of the front of the tail plane, creating a nice strong hold. Position and install elevator servo and linkages as desired. Once again, the cable can be through-wired or taped along the top according to preference. Install ESC and receiver, and plug in channel three(motor) and two(elevator) cables. Add some Velcro in the front of the fuselage to lock the battery in if this has not been done already while installing the electronics.

Next, take the wing and plug in channel two(ailerons) into the receiver, making sure the cables are in the desired position before mounting the wing. Glue along the indicator lines and along the foam core of the top rear fuselage plate where it will connect to the foam core of the back of the wing between the ailerons for a nice strong hold. A top cover or hatch can be used for the front portion if desired, but leaving it open works great already for venting the ESC and easy access to the battery.

5.) Bind, program, and systems check the plane. Your 1939 Popular Aviation Special is now ready to fly!

 

[Vimana89]

Plans
hand drawn beta plans for design version 1.0
https://ftforumx2.s3.amazonaws.com/2020/01/278738_83a137208c5c6ca6fd950b7549ccda5b.pdf

I'm not sure it is fair to use plans made by somebody other than myself for the challenge, but it can't hurt to add them here too. Here are higher quality, fully digital plans by @FoamyDM . Plans need a couple slight adjustments and then will be linked here.

 

[Vimana89]

This is a great design, and I have a working prototype already. Video footage to come shortly. My design is mighty mini sized, can be made from one sheet of foam, and uses only the most basic building techniques. At this size, the wing can be a simple one layer affair with no airfoil. I love this plane's classic, conventional look and design, paired with a gentle, high aspect delta wing. Straight wings are nice, but much more finicky. Here's a forgiving, gentle-stalling option for a trainer that requires no airfoil construction at smaller scales, and looks classy to boot? I really can't find anything not to like about this plane, It's like a delta wing mini Scout!

My first prototype is RET, and has a couple bugs. The dihedrals are a little extreme and add a bit of Dutch roll here and there, and I'd like to extend the fuselage/set the tail plane father back. Other than that, it flies like a boss on 2s. Definitely nothing to sneeze at. Before polishing the RET version further, I'll be testing a 3ch "yank and bank" version with a much reduced dihedral and ailerons. My plan is for this to be a modular platform like the TT, and have options for 3ch RET, 3ch AET, and 4ch.

 

[Vimana89]

A small teaser clip of some unedited flight footage of the prototype. The camera cut out because I thought I had recorded this entire flight and landing, which got much better later in as my confidence grew with this plane and its imperfections. This is the shaky launch and rather unimpressive warmup, and then it cuts out before it gets more interesting.

I'll get some better footage of this rough draft plane being flown to its best potential as I work out the more polished version(s).

 

[Vimana89]

I think I'll start by making a nicer RET plane, with extended fuselage, less dihedral, and a more permanent top hatch. If this works well and is an improvement, I can use the electronics from the prototype to build the AET version.

 

[Vimana89]

Much better short clip of the prototype added to the video footage section(post 2).

 

[Vimana89]

Working up AET with elongated fuselage to be closer to original plans and give better balance and elevator leverage. I think I'll try no dihedral first, since this wing is swept and mounted on top. That should give enough stability with no need for dihedral.

 

[Vimana89]

Four parts. Aside from a hatch and a couple straight rectangular plates, this is pretty much the entire plane.

 

[Vimana89]

Proportions look perfect. Going to bevel control surfaces and do linkages and electronics before assembling everything permanently.

 

[Vimana89]

Pic and video clip of the second prototype(AET version) coming shortly. It doesn't matter what control scheme you use on this plane, it flies great! This version is teaching me how to fly with ailerons properly. This is a very capable design, and it has some real high speed potential and responsiveness, but overall has gentle and forgiving flight characteristics. Stalls are gentle, and if you are flying bad, your maneuvers will look goofy and clunky, and you will have a little stall here or there, but this plane will stay in the air just fine.

Once I got a feel for how the plane handles and responds with ailerons, everything started looking smoother and cleaner. I feel this plane is just a bit heavy loaded for my liking with the 850 3s compared to flying the RET with 2s 650. I will experiment with another more polished AET prototype that can fit and balance with an 850 2s.

 

[rockyboy]

Great build! Love it when they fly well!

 

[Vimana89]

Great build! Love it when they fly well!

Thanks, this one is a real gem. The original designer was pretty forward-thinking with this design in 1939. There's nothing not to like about this plane, it's just all around great. With my build slop and lack of experience building many planes with full box fuselages and higher aspect wings, not to mention trouble flying ailerons, this should have given me more trouble, but the '39 just flies like a boss.

 

[Vimana89]

Taking a break from building for a day or two or so, then I will be back at further refining this design. I like it as RET too, but have too many RET planes, so I will strip the imperfect RET prototype's electronics and start the next version soon, which will just be a further polished version of the AET one that can balance just right on a 2s battery.

 

[FoamyDM]

Taking a break from building for a day or two or so, then I will be back at further refining this design. I like it as RET too, but have too many RET planes, so I will strip the imperfect RET prototype's electronics and start the next version soon, which will just be a further polished version of the AET one that can balance just right on a 2s battery.

You've been building like crazy, don't burn out. *says the man with nearly a dozen current active design projects in the air.

 

[Vimana89]

Started drawing up something else to prototype for this challenge, but I'm not sure I want the scale I started designing it at, and I think it would be better to focus on this one for now. I'll be starting the next version of this tonight or tomorrow. This will hopefully be the finalized, 1.0 version on which to base the plans. I am trying to get it to balance on a 2s 850, so I will move the wing back just a tiny bit, and maybe use a 3.7g micro servo for the elevator. I could use those on the ailerons too, but I only have four and they are pricey. I'll most likely trace the pieces as I go for the plans already, and use them if it is successful. I want to try to document the build process a little more in depth, but even so, It will be brief, because this is a simple plane to build.

 

[Vimana89]

Showing this part won't be really necessary for anyone working from a plan in the future, but to make it more interesting and be more detailed and transparent with my builds, I'd like to explain my process for designing and building without software. I draw up the templates on graph paper, and then hard copy them to cardboard, poster board, or spare foam and cut them out. I trace them on the foam and use a ruler and straight edge to flesh out the fuselage and draw out a straight rectangular strip for extra fuselage plates, hatches, spines, etc.