Kit: R/C America 27% CAP-232, modified

Engine: BME 2.7; 1100maH Cermark battery pack, JR switch harness

Prop: Mejzlik 20x10; Cermark 3.25" Spin-Right spinner

Fuel: 87 octane Chevron unleaded; Amsoil at 100-1 or Lawn-Boy ashless at 40-1 (break in)

Radio: JR PCM10SX with 950S Rx, 517 on throttle, 4721 on rudder, 4721 on each aileron, 4131 on each elevator, 6.0 volt 1400 maH Cermark battery pack, heavy duty Cermark extensions, JR heavy duty switch, Cermark aluminum servo arms.

Linkages: Dubro pull-pull rudder hardware, stainless 6-32 bolt control horns with Hayes 4-40 clevises (tail), Rocket City 8-32 aileron horns and swivel links (ailerons), Radio South giant scale EZ hinges.

Weight: 14 lbs., 2 oz. dry weight (stock cowl); 13 lbs 15 oz (Stan’s cowl).

Description:

I have always wanted a CAP for IMAC use, but with three 33-36% IMAC planes/projects already in the garage, a big CAP was out of the question. I liked the 27% MW design, but after flying an OS BGX powered 25% Sukhoi for six or seven years, the thought of feeding (and cleaning up after!) a Moki 1.8 held little appeal for me. Enter the BME 2.7….here is a gas engine that looks very promising, with little to no weight penalty (+3 oz.) over the Moki 1.8, and promising more power to boot. With the introduction of the R/C America ARC "clone" kit of the MW CAP, including airfoiled tail surfaces, the urge to get a CAP became irresistible.

Upon examination, my kit appeared to be of very good quality, with only a few areas needing attention (unfilled seams and/or raised joint lines in sheeting), with pre-primed glass cowl and wheel pants. Built by Aeropro in Mexico, wood quality was very good, including "real" 5-layer aircraft ply (unlike the heavy and weak Philippine luan mahogany ply used in so many imported ARC kits). The only problem was that there was TOO much of it – the reports I’d heard about this kit said that it flew great, but commonly came in at 15 to 16 pounds with a Moki, more with a G45 or similar gas motor. Based on observing many MW CAPs, from weights of 14 to 17 lbs, it became apparent that if I really wanted a strong performer, I need to shoot for an all-up dry weight of 14 lbs or less.

After close scrutiny, I decided that I could easily lighten the basic airframe by 1 to 2 lbs by cutting holes in ALL of the A/C ply parts, the fuselage sides, and the control surfaces, along with careful choice of hardware and radio gear. In the meantime, a friend and local pilot (Mike Nauman) had completed his Moki-powered version with similar mods for a final weight of 14 lbs even, so I knew this goal was achievable.

To start out, I took a ruler, circle template, and pencil, and began looking at the basic fuselage structure for areas to lighten. The front of the fuse and engine box got special attention - the firewall is 3/8" thick and the gear plate, front wing bulkhead, and wing bolt mount plate are all 1/4" 5-ply! I ended up cutting six 1" diameter holes in all 4 sides of the engine box ahead of the firewall, a large 2.5" oval hole (between the engine mount bolts) in the firewall, two 2" diameter holes about 1" behind the firewall (these holes go completely through the engine box sides as well), and I opened up the existing opening in the front wing mount former all around by about ½". The wing bolt hold down plate had a very large oval hole cut in the center, basically leaving only about ¼" of material around the bolts and edges (see picture for more detail). I also cut large rectangular lightening holes in the 3/16" thick balsa fuse sides similar in size and location to the Midwest CAP (a surprising coincidence!). The side view photo shows the location of most of the lightening holes (look for the sun shining through the fuse and rudder).

Mike Nauman had saved quite a bit of weight on his CAP by cutting lightening holes in the wing and stab. I am not keen on the "starved horse" look, so I opted not to do this. But I did lighten the tail control surfaces, both to reduce mass (to minimize flutter) and to save some weight in the rear. The rudder was "ribbed" using a diagonal truss pattern (think triangle shaped holes) and I cut 4 large (tapered diameter) holes in each elevator half. I did end up adding back some of this weight savings by counterbalancing the elevators (about 1/2 oz per half), but better safe than sorry! I left the wing essentially stock, except for the joiner – which is HUGE. Made from solid 1/4" aircraft ply, it’s about 16" long and 2" tall, and weighed almost 5 ounces. By drilling a series of ten tapered diameter holes (smaller in the center, larger at the tips) starting about 3" out from the centerline, I was able to get the weight down to 3 ounces. I made a similar effort on the stab joiner, saving about ½ oz. (to paraphrase Ben Franklin – "Watch the ¼ ounces – and the pounds will take care of themselves!). To save more weight, I installed the wing and stab joiners and the glued the stab/wing halves together using Elmer’s polyurethane instead of epoxy – messy but lighter, no doubt.

A note on cutting lightening holes: I find that Forstner bits work best for balsa and thin ply, but must be used with a drill press and ideally have a backing plate to prevent tearing in soft woods. I have an inexpensive kit with a selection of sizes, from ¼" up to 3" (in 1/8" increments), purchased via mail order from Harbor Freight for only $70. I highly suggest these over hole saws, as they leave a much sharper edge that requires little sanding or cleaning up. For the thick ply pieces and areas where I could not use the drill press, I used standard hole saws, from 1" to 2.5". Remember to drill slowly and carefully and have a vacuum nearby! Coating the edges of the finished holes (in soft balsa) with thin CA will help prevent tearing/chunking problems during handling; you may also need to add a thin (1/16" thick) reinforcing doubler of cross-grain balsa to prevent lengthwise splitting between holes where the fuse bends just after the wing TE.

The fiberglass parts and landing gear provided in the kit were of good quality (the gear and canopy is sourced from Midwest), but very heavy. I decided to replace the glass parts with new one’s from Stan’s Fiberglass, which would ultimately save about 5 ounces. (the lightweight parts were not available immediately so I decided to temporarily use the stock cowl). A big savings was gained thanks to fellow IMAC pilot and friend Steve Landeau, for making a set of replacement gear using thinner (.15") 2024 aluminum - for a savings of 5.4 oz.! I did add a fairly heavy pilot and MW instrument panel, adding back about 3.5 oz. Oh well – the ¼ lb savings do not offset that 1% score deduction!

The final all-up dry weight is 14 lbs. 2 oz....about 1/4 lb. more than I hoped, but still not too bad, all things considered. Total all-up/uncovered airframe weight is 7 lbs. 1.2 oz. (I did not weigh the empty/covered airframe). I weighed all the parts before and after lightening them (see list below), and it is easy to see where most of the weight savings came from – 1 ¼ lb. from the fuse alone! I did not hollow the turtledeck or front deck; you could save another 2-3 oz. here (for a lot more work). In retrospect, I probably should have lightened the wings and stab. I think an additional 8 to 9 oz. could be saved, for a potential all-up weight of 13 ½ lbs. or less.

Airframe Component Weights

Before After
R Wing (all) 14 oz 14 oz (n/c)
L Wing (all) 15.2 oz 15.2 oz (n/c)
R Aileron 1.4 oz 1 oz
L Aileron 1.4 oz 1 oz
Stab (both halves) 2.8 oz 2.8 oz (n/c)
L Elevator 1.0 oz 0.6 oz
R Elevator 1.0 oz 0.6 oz
Rudder 2.4 oz 1.8 oz
Fin 2 oz 2 oz (n/c)
Fuselage 52.2 oz 36 oz (!!)
Wheel Pants (both) 4.8 oz 3.6 oz
Cowl (with holes) 15.4 oz 13.8 oz
Landing Gear 13.8 oz 8.4 oz. (thanks, Steve!)
Misc wood parts 4.6 oz 4 oz
Wing joiner 4.8 oz 3 oz
Canopy 6.3 oz 5.4 oz (careful trimming)

Total bare weight 143.1 oz. 113.2 oz
Total airframe weight savings 29.9 oz..... almost 2 lbs!

I mounted the BME 2.7 inverted using a Great Planes 1.2-1.5 size adjustable molded nylon beam mount, modified by cutting the center adjuster pieces out and converting it to a true two piece beam mount. Easy and light. I used the older, square shaped Godfrey muffler and it unfortunately required a clearance hole in the cowl; the new oval BME muffler clears with no problem. The nose of the BME is pretty short, so while the head fins clear the cowl (barely) I went ahead and cut a fairly large opening for cooling anyway; the plug cap also required a small clearance hole. Adding a 3/8" spacer to the prop adapter would easily allow the engine to be fully enclosed, which I’ve since done with my new cowl (still requires a cooling hole, of course!). I installed a 4 cell, 1100 maH ignition battery pack immediately behind the firewall, in the space between the gear plate and engine box floor, with the ignition module wrapped in foam and rubber banded to the bottom of the engine box (for vibration isolation).

To my surprise, I found out that with the stock cowl, a 4721 rudder servo and RX mounted on the CG and two 4131’s in the tail, it came out very nose-heavy! I had to cut a hatch into the very rear of the plane - with a 5-cell, 1400 maH battery it now balances at the rearmost suggested CG. I am hoping that the lightweight cowl will allow me to move the battery forward (to reduce inertia in the tail - for better recovery on snaps, etc.). The incidence set up is 0 degrees on wing and tail, 2 degrees up and right thrust (adjustable) and so far this seems ideal, although I do need to carry about 1/16" of down trim – so next time I’ll set the stab incidence at 1 degree positive (i.e. LE up).

I finished the plane using Mike McConville’s TOC CAP color scheme in yellow, black, and red Ultracote, with trim graphics from Butch at Model Graphics. The "temporary" cowl was finished using Krylon yellow and black, and Perfect True Red (I’ve since installed a lightweight cowl finished with K&B Ultrapoxy). Initial test flights (using an FM receiver) showed a problem with RFI, which was resolved by installing a new JR 950S PCM Rx. After a couple more trim flights, the plane got squirrelly during a slow roll, and after a relatively uneventful landing I found that the right side aileron horn (a stainless 6-32 bolt) had sheared cleanly off at the base. Even though I’d heard or seen no indication of it in flight, I attributed the problem to aileron flutter, based on the MW kit’s reputation, and went home to drill out and replace both horns with 6-32 mild steel bolts (I had been told that stainless was too brittle – but then I’d never had a problem on any previous models using these). Back to the field the next day and on the second flight I lose BOTH ailerons. Fortunately, the CAP has some yaw/roll coupling and I was able to land OK using the rudder. Back home again, swap out the 6-32 horns for Rocket City 8-32 (!) horns and swivel links, repair the damage to each aileron (the reinforcing dowels had begun to pull out), seal the (non-existent) aileron gaps with clear tape, and test fly the next weekend. Midway through the first flight, I begin getting strange aileron response – land again, to find that the left aileron servo gears have stripped. Very frustrated now, I swap the Airtronics 94732Z servos for JR 4721’s…..which last four flights before I again have to make a rudder only landing (stripped both gear sets).

At that point, I began a systematic review of all the potential causes for these failures. The BME, while not the smoothest single in the world, doesn’t vibrate to extremes; and aerodynamically my wing was no different from the other R/C America CAPs flying with no problems. The aileron gaps were sealed and the ailerons themselves were plenty stiff. And with 140 oz+ of servo torque available to each aileron, the servos and linkages were more than strong enough. After doing some static engine run tests, I determined that the problem was due to mechanical excitation (from engine vibration) of the wing at exactly the right frequency to cause my ailerons to mechanically flutter. This was visible by observing the aileron tips (inboard and outboard) at full throttle – they were a blur, with about 1/8" of total movement. I was able to come up with a novel solution - I added static counterbalances to the aileron tips, using a 4" piece of ¼" diameter carbon tube with a ¾ oz. weight on the end, bolted onto the tips of the ailerons. Problem solved – I now have 25+ flights with no sign of flutter…. touching the aileron tips at full throttle, now there is almost no vibration to be felt.

I believe that my peculiar pattern of lightening holes in the nose, along with the hard mounted engine, just happened to hit the right frequency at full throttle to match the resonant frequency of the ailerons. Not something that could be predicted or known beforehand, and a real test of my rusty analytical skills (and patience!). I have since talked to several other pilots who fly the BME 2.7 in R/C America CAPs and Extras, and none have experienced this problem – so don’t be scared off. The BME runs AWESOME and I can only say that the CAP, at 14 lbs, is truly unlimited with this engine. I can take off in 15 feet, pull immediately vertical into point rolls, and keep going until I run out of eyesight (which is pretty good). It starts every time on the 6^th flip and with only a couple of gallons through it turns a Mejzlik 20x10 at 7600 to 7800 RPM. I have a 20 oz. fuel tank and after a few minutes of goofing off and three passes through the Sportsman sequence, still have ½ tank left (and no oily mess to clean up!). Now that I’ve had time to trim the plane, I’ve found a fair amount of pitch coupling (about 12% down elevator mix needed at mid-speed/half rudder in knife-edge flight) with just a touch of roll coupling (4% to mix out).

The rearmost recommended CG is OK for anybody, and could be moved back a bit if you are so inclined. The rudder is WAY effective and needs a fair amount of expo if you don’t like to use dual (or triple) rates – knife edge loops are easy, and you need to be smooth/careful making rudder inputs and corrections – they really show! Snaps are easy (watch that you don’t bury the rudder on entry) and it recovers RIGHT NOW compared to the 25% Extra I’m used to. It can be a bit tricky to land smoothly with power on – you need to fly attitude, not altitude, and is best landed in a 3 (or 1!) point attitude – if you chase the flare with the elevator, it will happily porpoise down the runway. It flies very well and is a great Sportsman and Advanced mount for IMAC use – an ideal step up to a 35% CAP.

In summary, I highly recommend this combination. The BME makes awesome power and is very user friendly. I would also buy another R/C America kit. The quality is good to great, although on my plane there were two problems - the wing and turtledeck skin seams needed some work, and the stab halves were not cut symmetrically (I didn't catch this until I was almost done). The glass quality was only fair (polyester resin) and somewhat heavy, but were pre-primed. For the price, you are getting a great deal - and if you are willing to spend some time with the hole saws, you can get down to a very competitive weight.

If I were to do this again, I'd mount the rudder servo in back, lighten the wings, add a battery hatch about midway back on the fuse, and use the Pro-Lite cowl. I think you could get close to 13, maybe 13 1/4 pounds. A killer set up!

Barry Mattingly