Learning Project Piper Cherokee - RC Groups
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Sep 27, 2017, 03:20 PM
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Learning Project Piper Cherokee


Iíve been building with balsa since the 60ís (Started in foam-core wings in the late 70ís). But, aside from some small foam blocks here and there, I had very little experience in making fuselages and tail feathers out of foam.

It appears that round fuselages (i.e, the Lockheed Vega/Orion/Sirius designs) etc., are much easier with foam than with balsa, hence my motivation to try foam. I'm tired of trying to plank large, round fuselages!

So, I did a lot of studying from threads on this and the Flite-Test forum. I read Keith Sparksí book on foam construction, then sat down and designed a simple, stand-very-far-off scale Piper Cherokee for a standard foam core/balsa sheet wing, a foam/balsa tail, and a foam slab fuse. I plan to glass the whole plane with a mix of Epoxy on the balsa sheeted wing and Polycrylic on the rest.

I really simplified a lot of the design as this model is mainly a testbed for me to learn new building techniques. So, please don't be critical of its scale shortcomings; I could've drawn up a very non-scale model, but I've long wanted a Cherokee, so..

This Cherokee is the same size as most of my balsa birds, 432 sq. inches, and is to have similar power (Power 15 to 25 on a 3S pack). I know that with conventional wood construction, the model would come out around 48 oz., RTF. With the Cherokee, I hope to establish a baseline reference point thatíll help me with designing and building larger and more complex models.

I am open to constructive comments and suggestions!

CD
Last edited by Captain Dunsel; Oct 03, 2017 at 06:47 PM.
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Sep 28, 2017, 04:39 PM
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A day's progress...tail feathers sanded and ready for glassing, motor and nose gear mounted to firewall, and main gear blocks glued in place. I was happily surprised by how little Gorilla Glue I used in sheeting the wing, but not happily surprised when I found a warp in the right wing panel. So far, though, I'm happy with using polyurethane glue for wing sheeting instead of epoxy (considering that I learn to sheet cores with (then new) HobbyPoxy Formula II, it is a big change!).

CD
Sep 29, 2017, 04:24 PM
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Busy day today, so I only got a small amount done. Tomorrow is busy (animal shelter volunteering, followed by lunch and evening Mass), then Sunday is our club's participation in a full-scale airshow (both my wife and I are hoping to fly in the demo). So, there may not be a lot more done before Monday!

It never ceases to amaze us how busy we both are, even though we both retired in 2011. We volunteer at (and for) the local animal shelter, do R/C club stuff for two clubs, exercise, and do 'ordinary' retiree stuff (i.e., go to the Doctor). How'd we ever have time to earn a living?

CD
Oct 02, 2017, 03:06 PM
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Got a little done Sunday afternoon (after being at an air show...yes, I'm a gluten for punishment!). Did much more, today, after our morning exercise (not the fun kind!) then grocery shopping. Retirement can be very busy!

Specifically, the wing panels are now sanded, the tips band-sawn, and filler block glued on. The fuselage servo tray is in place, the fuse sides glued together at the tail, and most of the fuse top is glued in.

Some notes:
1. I used a Foam-Tac knockoff (HobbyKing/Bob Smith?) for much of the fuselage foam gluing. "Fake-Tac", as we call it, works and sands like Foam-Tac, with it's big drawback being that it's worthless for glue hinges -- a must for many of the EPP foamies we fly.

2. At the airshow, I got a close look at a few Cherokee wingtips and decided that the classic "H-Ray" style tip would be a pretty good simulation of a Cherokee's tips. So, I got brave () and used a bandsaw to cut the wingtips. Worrying that the blade might splinter the 1/16" balsa skin, I had my fingers crossed (which does make it hard to push stuff through the saw!).

3. I was toying with putting a clear windshield and cockpit windows on the model, but I don't want that to pull me away from the primary reason I'm building the Cherokee -- Foam, not clear plastic. So, I decided to add foam blocks to the front of the cabin, then sand it down. The 'windows' will be painted on, later. Not as pretty, but lighter and simpler (especially if finishing and painting the rest of the model gets heavier than I anticipated).

Tomorrow, I hope to get the wing panels joined, with the balsa stock TE's and torque rods, the fuse flipped over and the wing bolt plate mounted, and the music wire LG cut & bent.

CD
Oct 02, 2017, 08:59 PM
treefinder
springer's Avatar
so you hot wired foam cores from the thick blue foam board, then sheeted them with balsa and are gonna glass over that? If so, that sound like overkill to me. I'd skip the balsa sheeting altogether. In fact I would probably use packing tape instead of glass/epoxy. The glass will be more durable, of course, but for a foamie, the tape works really well to provide a dent resistant and glossy skin, and adds stiffness to the sections as well.

I'm sure now that you are dabbling in the "foam side" of the hobby, you will find a lot more things different from what you are used to. I pretty much use foam board (truth in advertising - I sell Model Plane Foam) and have made elliptical and round fuselages with minimal interior bulkheads compared to what I would have used in a balsa construction.

Overall weight is another thing you will find can be greatly reduced with foam and it's peculiar techniques. The wing in the picture is 7' span, and weighs two pounds auw with 2200mah 3s lipo. The fuselage in the second pic is one piece of MPF rolled to an elliptical section tube then cut at aft end and tapered.

I understand that your project is an experiment, (aren't all the projects with foam just experiments??), so you will learn much from it, but my suggestions are just that: food for thought!
Oct 03, 2017, 07:19 AM
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Springer, I cut the cores from white foam, then sheeted them with 1/16" balsa. My plan is to glass them with .5 oz. glass cloth, then paint them. For the fuse, I plan on 3/4 oz. cloth as a base, then .5 oz. as a finish coat (likewise on the tail feathers).

I'd love to have gone just with a pink (no blue in our area!) core, then glass covering and (maybe) a spar, but I've been unable to get a feel for what is sufficiently strong. It's like building a wooden wing; I know 1/4" sq. spruce (or bass) spars with shear webbing works well for the 3 lb. acrobatic planes I like to fly. I just don't know what the equivalent is in foam (except for the 'classic' balsa sheeted foam wing system).

I put out a question several months ago, trying to get some advice on how many layers of glass, and of what thickness, would work on a model, but didn't get much back in the way of response. You're the first one to give me something tangible, so "Thank You"!

CD
Oct 03, 2017, 10:14 AM
treefinder
springer's Avatar
You're certainly welcome! For what its worth, both of those planes i posted (and 40 or so others) have been cartwheeled and crashed during development flights and dumb thumb incidents. Damage is frequently minimal and easily repaired. As you get more into foam, you will get a better feel for how much structure to add and where (the fun of trying new stuff!). Things i have found and now do as a matter of course include doubling the foam wall thickness in area from wing mount to motor mount really helps survivability in nose-ins. Thin aft fuses (like my spitfire's) may need reinforcing with cf or bbq skewers to keep them from snapping in cartwheels. Stuff like that is counter intuitive perhaps to a balsa mindset. But a fundamental difference is that foam as a skin is different than monokote and stringers. There is tremendous strength and stiffness in 6 mm of foam skin.

Enjoy!
Oct 03, 2017, 01:28 PM
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What is the wingspan on this?
Oct 03, 2017, 03:36 PM
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Dewey, the span is 47.5" The design has evolved since I started building, including the wingtips -- they're simple wedges, with light 3/16" balsa skin on the outside of the tip.
Last edited by Captain Dunsel; Oct 03, 2017 at 03:46 PM.
Oct 03, 2017, 03:45 PM
Registered User
Springer, I agree, it's amazing how strong (yet light) some structures can be. Some years ago, I built a rolled fuse Ki-43, using 1/16" balsa over some very skimpy bulkheads & minimal stringers. That sucker was strong enough to endure some pretty bad landings!

When I started designing balsa R/C planes, I kept a binder full of 8.5 * 11" copies of plane plans from different magazines. When I wanted to solve a particular design problem, I referenced that binder and swiped, uh, adopted other folk's solutions. Problem is, I haven't been able to find that large a database of foam projects, plus the the materials used have changed so much in just a few decades that a solution from ten years ago is totally obsolete today.

Tuesday's Progress:

1. Trimmed wing tips and loosely glued wing panels together, then added torque rods and wing center-section TE, plus TE protective plate.

2. Since I'm not putting in a full cockpit, I'd rather move the servos upwards and give more room for the pushrods. So, I pulled the bolt plate from the fuse and removed the servo tray. Then, I cut new braces, a new slot in the wing TE bulkhead, and glued everything back in place.

3. Added more pink foam to the front of the cockpit, to be sanded down for the windshield.

I weighed the fuse (with the motor installed), together with the remaining foam blocks. I want to weigh it, again, when I'm done carving and hollowing the fuse, to see how much weight that removes. So, it's starting at 15.25 oz.

Tomorrow and Thursday should be busy (chore) days, so I probably won't get much done. I hope to bend the main gear, fiberglass the wing centersection, and start glassing the tail feathers.

CD
Last edited by Captain Dunsel; Oct 03, 2017 at 03:57 PM.
Oct 03, 2017, 04:10 PM
Registered User
This is interesting to me because I'm thinking about building a 48" foam Cherokee right now myself.
Oct 03, 2017, 05:05 PM
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Dewey34, please feel free to use anything I've done or drawn as a starting point ('cause I know better can be done!).

CD
Oct 03, 2017, 06:29 PM
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I was thinking of doing a folded box type fuselage and folded armin style wing. I'm trying to draw it up so I can cut the fuselage out of one piece of foam board.
Oct 04, 2017, 12:46 PM
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Quote:
I hope to bend the main gear, fiberglass the wing centersection, and start glassing the tail feathers.
Got two out of three done -- wing CS is glassed and main gear is cut & bent. Had to do more filling on the tail feathers, so glassing them is on hold.

(Gee, I'm quoting myself. Makes me sound like a politician!).

CD
Oct 04, 2017, 01:54 PM
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Extreme Sports's Avatar
If it's of any interest, I've also just made my first foam core wing. But I came at it from the complete opposite side of the spectrum. It is for a 44" span cub, has a 5mm x 1mm CF spar and is covered in 1.3mil document laminating film (plus paint). Weight is a mighty 80g. Admittedly the cub has struts, which give a huge structural advantage over a spar, but even without the struts, this wing is acceptably strong.

Foam core plus balsa plus glass sounds like lots of work and lots of weight. May be necessary for a ~70' wing, but sounds like a huge overkill for 47.5". The thing about building light is you soon find out if you have built too weak, or can try lighter still. But if you start at the heavy end, you have no idea how 'unnecessarily strong' you build is.

Nice build though!


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