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Nov 28, 2020, 04:22 PM
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Captain Dunsel's Avatar
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Build Log

Pursuit: A low-winged foamie with KFM3 airfoil

I'm trying a few new (to me) ideas in my latest build. To start with, I took an all-foam, paper-covered design (Novice) and made a low-winged version (tentatively named the Pursuit, as I plan to paint it in pre-WWII USAAF or USN colors).

The new ideas (again, for me) are:
1. KfM3 airfoil (Thanks to Springer for his tip on how to make the bent MPF wing LE).
2. Mixed MPF and 1/4" pink foam (hot-wired from large sheets) construction
3. No paper cover (eek! -- will it be strong enough?).
4. Where strong filler is needed, I mixed microballoons with clear Gorilla Glue; where filler is just 'for show', I used cheap spackle (Dollar Store).

The plans aren't completely up to date. I WILL update them, after I verify the model will fly! I did have to print them on two Arch E pages, so I've included the .DXF file (zipped) for anyone who wants to use them in a CAD program (they were draw in LibreCAD). I will note that the sewn-on tailskid is gone, to be replaced by a steerable wheel as we now fly off pavement.

I do have a question for you guys:

1. How do I get paint to stick to moving tape? Buff it with 400 grit sandpaper?


Added: I realized I should explain few more bits.

First, the '360' is because the model is 360 sq. inches, that is, 2.5 sq. ft. Using proportions like that makes it easier to think things through.

Second, the Novice was covered with brown paper and Titebond on the wings, with the rest Titebond and newsprint. Since the Novice came out around 28 oz., and since the paper & glue is pretty heavy, I'm hoping to keep the Pursuit under 20 oz. That would give me a wing loading around 8 oz/ft. sq., making it a floaty model, which is what I like (Vulnerable to the wind, yes, but also maneuverable).

Third, my planned-on motor is a 2200 Kv Turnigy, to be run on two cells (the UBEC is for if I need to go to three cells).

Last edited by Captain Dunsel; Nov 28, 2020 at 04:46 PM.
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Nov 30, 2020, 03:58 PM
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Captain Dunsel's Avatar
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Got a bit done today, in between cleaning for Christmas decorations and watching severe weather in the area. Installed the rudder & elevator pushrods & servos, glued plates under where the ailerons servos will rest, and sheeted the aft bottom fuse.

Taking things slowly, as there's a lot of new stuff I need to figure out, despite my having read through a multitude of build threads.

Dec 01, 2020, 09:06 AM
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rotagen's Avatar
Nice looking plane that will be quite stable I think.

I'm curious as to what you will use for power plant (esc and motor)?

The fuselage and wing look sturdy but heavy compared to what I tend to build.
I've had bad luck with laminated-wing or double-layer DTF wings due to their weight - but I'm pretty sure my power system wasn't up to par.
Dec 01, 2020, 09:54 AM
springer's Avatar
Coming along nicely!
Dec 01, 2020, 10:20 AM
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Two things:
Without paper, you will likely need a spar of some sort for the wing (disappointing when the wing folds in an unintentional high-G maneuver ).
The elevator (moving portion) looks really small, so you might have pitch authority issues - better to have a large elevator move a little than a small one move a lot - the large one gives you the authority you need when you need to get to the edges of the flight envelope.
Dec 01, 2020, 12:15 PM
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Captain Dunsel's Avatar
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In order of how you guys commented:

Rotagen, I plan on using a Turnigy 2200Kv motor on two cells. I've used the same motor on a 3S1500 pack, flying models around 30 ounces, with plenty of power for Cuban 8's, loops, etc. The reason I'm looking at a 2cell pack is because I'm using 4 servos on a Turnigy ESC. That's more than the ESC can handle, whilst still providing enough BEC power to keep the receiver paying attention to me.

As to the weight, this is a low-winged version of the Novice I built a few years ago. That was covered with Titebond & brown paper on the wing, with the rest covered with newsprint. The Novice (same power system, on 2 and 3S packs) weighed about 28 oz, but was very acrobatic. In my tests, papered airframes came in nearly twice as heavy as non-papered ones, so I'm hoping to keep the model in the low 20 ounce range.

To be totally honest, I'm still not sold on KFM wings this large. As much as I dislike cutting cores (despite having been doing it for a long time), I may return to cut cores and use MPF for the rest of the structure (MPF is firmer than the cut pink foam).

Springer, thank you! The only problems I'm looking at right now are the fragility of the bare foam (not that papered foam is that much sturdier -- it chips so easily ) and I'm not sure if the bandage tape (3M Nexcare) is going to work for hinging the control surfaces. Not sure if I should put a coat of Polycrylic, etc., over the Nexcare 'netting'.

Nuteman, there is a 1/4" sq. bass spar under the KFM top, plus a 2" strip of strapping tape across the wing bottom. The wing seems fairly stiff. I think the elevator area should be fine. Again, it's the same as on the Novice (and on the Budgie, my biplane version of the Novice).

I'd rather keep the elevator smaller, so pushrod slop, etc., makes unwanted control surface movements less of a bother.

Dec 01, 2020, 02:52 PM
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FlyingW's Avatar
Hey Captain Dunsel,

Another nice design, good luck with it.

I agree that KFM wings may not be good for larger foamies. I just tried a couple of techniques to make airfoil wings from MPF, but could not get consistent results. I wonder if one could make a sandwich of two layers of MPF, wide enough to wire cut a thin airfoil...

In the end I reckon I have to go back to the building supply store for another block of 2"x24"x96" blue foam.

Take care,

Dec 01, 2020, 02:57 PM
springer's Avatar
I missed seeing the span, but I have done high 30's to 47" span kfm3 wings with a leading edge spar/ protective nose that are as acrobatic as I like (loops within a couple plane lengths, for instance) and not broken a wing yet. They are flexible as opposed to rigid, and really severs maneuvers generate variable dihedral, but no breaks.
Dec 01, 2020, 03:32 PM
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eflightray's Avatar
I'm perhaps jumping the gun as the model is not complete.

I assume you will fit an undercarriage ?

If not, and you intend to belly land, those aileron horns could do a lot of damage, possibly ripping out.

Dec 01, 2020, 08:40 PM
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Captain Dunsel's Avatar
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Didn't get anything done today, as it was time to put up the Christmas tree...we all know that means fixing ornaments, rewiring lights, etc., so no time to build.

A bit more info: The motor I plan to use is a TurnigyD2826/6, 2200 Kv. I'd like to keep the model lighter and go with a low pitch/low speed flight profile.

Flying W, at least you can get blue foam! All we have around here is the pink stuff Home Despot carries. Lowes advertises green foam, which would be easier to paint than the pink, but our local Lowes is always 'out of stock'. On the other hand, I did find a 4' X 8' sheet of 1" green foam along the highway last's now cut into 2' X 4' billets and is awaiting use. Micki is used to my stopping along the highway to check out foam sheets along the road!

I know folks have laminated and cut thicker wings from home insulation foam, but there's always been an issue with using a glue that melts with the foam. I saw your Ugly Stik type wing under development. Moving tape does wonders for keeping the foam from splitting when bending (one thing MPF is much better than pink foam for -- the pink stuff snaps very easily when bent too much).

Springer, the span is just under 45". With the filament tape and 1/4" sq. bass spars, it's pretty stiff. Not as stiff as some wooden wings I've built, but I expect it'll be fine at lower airspeeds.

Eflightray, yes, I bent the gear yesterday. The gear will mount to the fuse, angled back under the wing LE. Sadly, I can't find K&S wire anymore. All I have is some Chinese wire that I can bend with my bare hands.

Dec 02, 2020, 10:33 AM
springer's Avatar
Next time you order foam, remember the wire. (Of course, with two boxes already, you won't need more too soon. Though you really are getting into foamie building!)

With that size and Kv motor on a 45" plane she should scoot! For my planes in that size and roughly the same construction I have used a 50gm 1400kv with 8x6 prop and 3s 1500mah lipo. Probably just a different method to the same end.
Dec 03, 2020, 05:25 PM
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Captain Dunsel's Avatar
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The wings sat on the shelf for a few days, before I could take them down to look at them. Kind of wish I hadn't

The tape I used for aileron and elevator hinges is, evidently, not viable. The ailerons were pushed aft and all were very stiff. I guess I'll have to gin up new control surfaces and use CA hinges
Dec 03, 2020, 06:33 PM
springer's Avatar
How did you tape them? Single strip?

My fave is the over/under, where I take two small (1"x1/2") pieces and stick the sticky sides together overlapping 1/8" . Then alternate sticking one end to top surface other to bottom. Gives a durable, very low force hinge. Depending on plane intended use I may lay a strip over the top centered on hinge line for extra security. Tedious, but once I get in the groove goes fast. Today on a zoom call a FB showed how he slits the two foam edges and applies GG, then slips monokote in the slots.i may try that. He uses sticky note paper strips on monokote to stiffen the edges which gets glued into the joint.
Last edited by springer; Dec 03, 2020 at 06:39 PM.
Dec 03, 2020, 08:39 PM
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Captain Dunsel's Avatar
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One strip on the top and one strip on the bottom. In the attached drawing, the red is the tape and the black is the foam.

I did the taping two days ago. In the interim, the control surfaces pushed away from the flying surfaces nearly 1/4" and the controls stiffened noticeably. So, I've trashed the control surfaces and cut out new ones. I intend to hinge them with "V" shaped LE's and use CA hinges.

Dec 03, 2020, 10:19 PM
springer's Avatar
Ah, yes, I used that early on as well, but found that it eventually separated and was difficult to get a tight joint line. That led me to the over/under method, still tape, but way better. I'm not sure about CA hinges, haven't used them, but be careful with CA, it sometimes melts the foam, and is always stiffer than the foam and becomes a stress riser that sets it up to break right next to the joint. Foam tac, GG clear or similar would be better.

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