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Converting Giant R/C Plans Into Your Walter Mitty Fighter

By Vince Homer, EAA 162674, vhhomer@hotmail.com

 

Replica fighter

The possibility of building a replica fighter has always fascinated me. As a longtime model builder, it seemed reasonable to use a scaled-up “giant-scale” radio-controlled (R/C) model as a base. After many false starts, I finally landed on a 70 percent P-39. Some readers might be interested in the various aircraft that I considered:

P-51, P-40, etc. – There are lots of giant-scale R/C models of these types of aircraft out there, and since many human-carrying versions are already available from companies such as Loehle Aircraft and WAR Aircraft Replicas, I decided to look at some not-so-well-known aircraft.

Kawasaki Hein, Helldiver, Fokker D-XI – The availability in the last 10 years of round engines in the under-200-hp range has made these aircraft possibilities.
 
F9F, L-39 Albatros, A-10 Thunderbolt (Warthog)  Heinkel He-162, F7U Cutlass – Ducted fans have been written about for a long time, and it appears to be possible. However, other than the ill-fated Saunders JetHawk, no one has managed to build and fly one.

OV-10 Bronco, F7F Tigercat, Widgeon, P-61 Black Widow – These aircraft fascinated me, and to my surprise, most scale well with only slight increases in the fuselage cockpit area to accommodate those of us that don’t have an 18-year-old’s physique anymore. The OV-10 is the most promising in that there are giant-scale plans featuring simple construction and require no dimensional changes to fit a pilot and passenger. A 63 percent scale with a couple of Geo Metro engines seems promising. After all that fun, I have to admit that even if I did find the time and funds to build one of these, I don’t have and probably never will have the piloting skills to fly any of them safely. Besides, the complexity of these projects would be a forever build time! I’m sure these are common problems for anyone considering a scaled warplane project.

Avia B-534 – This Czechoslovakian biplane was used from the 1930s until the Germans overran the country. The German Air Force used captured B-534s, and some escaped to Russia. This biplane features an inline engine and an enclosed canopy and represents a very good candidate for scaling with its large wing area.

Jack Bally’s B-17 project featured in the June ‘09 issue of Experimenter is very encouraging. I really want to be at EAA AirVenture Oshkosh when it shows up for the first time. There’s a Czech-built model engine manufacturer that is building round engines up to 45 hp, and I understand the B-17 builders are looking at using them.

I finally decided on the Bell P-39 Airacobra because of a long-time interest in the design, and I just wanted to build an aircraft that seemed to have been overlooked by the replica crowd.

My experience with homebuilt aircraft is mostly tire-kicking and hangar flying, although I have helped a few with projects and even started a single-place high wing original design in the 1980s. The wing and tail frames are still hanging in my shop in California. As an aerospace engineer who graduated in 1970 (when you couldn’t buy a job in the airplane business), I have spent most of my time working in the oil business and remember just enough to make me dangerous, that is, with the exception of five years as a design engineer with Aero Union Corporation converting ex-military aircraft into fire bombers and supporting aerial refueling equipment.

P-39 cockpit and nosecone mock-up
70 percent P-39 cockpit and nosecone mock-up

The P-39 mock-up was built in a one-car garage while I was living in Saudi Arabia. Saudi isn’t exactly a hotbed of private or homebuilt aviation activity. In fact, there is none. The idea of citizens flying around in their own airplanes doesn’t appeal much to the government there. As a result, my project was a nonflying mock-up. My main purpose was to work out the many details of the conversion from R/C model to homebuilt. The P-39 has the added wrinkle of its aft engine location.

I started with plans for a giant-scale R/C model, and after the cockpit and door dimensions were increased slightly, a quick trip to Kinkosand their enlarging drawing copier produced a set of patterns. The increased cockpit dimensions were necessary because the best size for a WW-II P-39 pilot was 5 feet 8 inches and 160 pounds or less, and I’m a bit taller and wider than that. When I tried getting into the mock-up once the canopy frame was complete, I found my contortionist ability to get in and out of cockpits had decreased with age and that I would also need to hinge the canopy top in a real version, much like some Spitfire models. If it’s difficult to get in and out of the cockpit in normal circumstances, imagine what it would be like in an emergency! This should be a prime concern for anyone designing and building a replica.

My first “sit-in-the-cockpit-and-make-airplane-noises” experience really sums this up; with the cockpit frame, seat, and panel in place, I tried to gracefully get into my creation. The combination of not having a wing to stand on and the small door opening proved daunting. I could get in, but getting out was another matter. I was halfway out when I managed to tip over the whole fuselage assembly landing on my left lower buttocks. The next day, it looked like I had been hit with a Gretzky slap shot, or maybe my wallet had exploded. The lesson: Bigger doors, or better yet, hinge the top section of the canopy.

The construction is a straightforward wood stringer/bulkhead approach with a glassed plywood skin, very much like some have done with Loehle aircraft. Originally, I was looking at a Mazda 13B engine, but lately, after looking at the partially built fuselage, I’ve been thinking that a Chevy V-6 or Buick/Olds/Rover 215 V-8 with a lower crankshaft centerline might fit better. The Mazda engine (also a wooden mock-up) is currently installed in the fuselage with a cog belt propeller speed reduction unit (PSRU) on the engine. This was needed to get the driveshaft below the pilot’s seat.

Mazda with PSRU and driveshaft
Mazda with PSRU and driveshaft

driveshaft
The driveshaft is seen running from behind the seat, through a carrier, and to the nose.

Aft engine weight is certainly a problem as the original aircraft had heavy guns in the nose. Some of my friends from the Sports Car Club of America (SCCA) have suggested using Porsche 935 Turbo constant velocity (CV) joints for the drive shaft. If a V-6 or V-8 was used with the lower crankshaft line, a planetary PSRU would fit nicely in the nose, which also helps with the weight and balance. To check and play with the weight and balance, I made a small-scale profile fuselage with scaled weights. Weight scales to the third power, scaling is a subject for another article.

1/6 scale weight and balance fuselage
1/6 scale weight and balance fuselage

In order to test the cockpit layout, I have installed the pilot’s seat, drive shaft, instrument panel, control stick, rudder pedals, and throttle quadrant. It all fits quite comfortably, and the visibility seems good. The instrument panel uses some red-tagged instruments I got (for a reasonable donation) from the fine people at the Oakland Aviation Museum in California.

Replica fighter

Replica fighter

The windscreen is a bent flat sheet, but the canopy top skylights are compound curves. To make those, I needed a mold. I found a 30-gallon plastic barrel with just the right curvature on the top portion of it. Using the curved barrel plastic as the outer surface of the skylights, the canopy frame skylight sections were filled in with plaster of Paris. The plastic was removed and a mold was made of the canopy/skylight area. This mold will be used to vacuum-form the skylight sections.

The door blanks were laminated with five layers of 1/8-inch plywood using the cockpit frame as an armature. The window areas were then cut out, and a pocket was left in the lower portion of the door so the window could be lowered. The window Plexiglas fits in through a slot in the top of the door frame. In a real version, I think I would make composite doors using the mock-up ones as plugs to make molds.

Replica fighter

The landing gear was made of PVC pipe and wheelbarrow wheels. The prop is flat-bladed from plywood with a foam spinner I turned on my wood lathe. In fact, the entire mock-up is made of wood and PVC pipe fittings with the exception of the alternator, spark plugs, and a few other bits.

Mazda rotary engine
Attention to detail is over-the-top for a mock-up. The wood Mazda rotary engine even has real spark plugs, spark plug wires, and a real alternator.

At this point the P-39 is hanging from the ceiling of my shop awaiting its turn in the project queue right after the Himax and a couple other inevitable diversions. I would like to install a recently acquired 215 V-8 along with a drive line, PSRU, and propeller. A running version would make a nice display at Oshkosh. Anyone know where I can get a red-tagged, three-blade prop?

Replica fighter

Throughout the photos in this article, one might be intrigued with the other projects seen hanging from Vince’s ceiling. Each one is a work of art and fodder for future articles. But for those truly fascinated, I’d recommend contacting Vince directly. -Pat

 
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