The Arnold AR-5

On August 30, 1992, at 8:15 a.m., on a warm Sunday morning, I flew the AR-5 through the three kilometer traps north of Davis, CA at 213.18 mph, setting a new world speed record in FAI Class C1aO (under 661 pounds gross). What a thrill!

It wasn't so much the actual flying...I was so worried that something might go wrong I didn't really get a chance to enjoy that part very much, but more the slow realization later that I had actually done it! After all these years I had finally learned enough to design and build an airplane from scratch...and it was good enough to set a world record. Very heady stuff, indeed! I recommend it to all you folks who've ever thought about doing something similar.

I've dreamed of doing this since I was a little kid. I built countless models and imagined myself sitting in those tiny cockpits pulling high G turns and doing victory rolls. It was always a little fighter, never a bomber, and I wanted to design it.

Over the years I've pondered and meditated for hundreds of hours over photos of Me. 109s and Spitfires, Bearcats and KI-84s, P-51s and C-205s...all of those beautiful, expensive, hot rod airplanes that I loved as a boy. Their lines have been burned into my subconscious for nearly half a century.

I also collected all the technical information I could find (and understand) that was related in any way to the design and construction of an efficient low wing, single engined airplane, and then cogitated on all that stuff for years. It's a wonder I can still talk.

By the late '70s, I couldn't stand it any longer. I'd been in the Army, graduated from college, and was gainfully employed in my chosen profession, but I was doodling airplanes all the time. In 1978 I left a perfectly good job as a filmmaker and drove to the Mojave Desert to learn about composites from Fred Jiran at his glider repair station. Fred has been credited for having introduced Burt Rutan to the foam and fiberglass techniques developed by German sailplane designers for building prototypes without molds. Fred was then building all the prefabricated parts for the new VariEze. I brought a wing test section I had built and was having problems with and Fred, too busy to answer my many questions, hired me to build a VariEze fuselage and some molded, prefab wings.

I learned a lot, and got to poke around in all the latest gliders, but it only lasted six months or so...too much hot sand and wind, and too many stickery things out there for me. But I've been designing and building airplane parts ever since. Thanks, Fred. And thanks, Burt, for showing us all those simple ways to build composite airplanes. Most of my work has been on other people's development prototypes, and most of them have not been very successful to date. Ah, well, it's been fun.

In late 1981 I started drawing the AR-5 with the idea that if it were successful I'd sell plans like Burt Rutan was doing and, although I didn't see a great market for a light weight single seater, it might help pay for the prototype. Back then, product liability looked like it was becoming a problem, but I was sure reason would prevail and the courts would soon straighten themselves out. I built the fuselage and tail group in 1982 and put it aside for a while to wait on engine developments. When I started the design, rumors were floating around about turbocharged Onans putting out 35 horsepower, and I drew the forward fuselage for something that size, hoping that a better engine would come along later. Years passed, and many promising engines came and went. I drew cowls for Aero Motion Twins, Pong Dragons, sawed-in-half Volkswagens, and finally, TWO STROKES. It took a long time for me to come around to the two strokes. I'm a dyed-in-the-wool, four stroke kind of guy, and I'd watched the ultralights struggle with the early engines. They didn't look too good. Someone was falling out of the sky at least once in a while. But after a bit, they seemed to get better, and when the Rotax emerged as the most reliable of the bunch, I drew a cowl for it. Didn't look half bad. The more I looked at the engine, the more interested I got. It appeared that, with some work, I could squeeze the exhaust system under the cowl, the rest of the package seemed about the right size, and the weight was just perfect. And the water cooled engine was pumping out 65 hp! I was giddy. I realized that I would have to beef up the airframe to handle the extra speed, and that would add weight, so I'd have to beef it up to carry that extra weight, and so on, but the performance numbers I was coming up with were too attractive to ignore. It was going to be a real fighter!

About the time the airplane was on the gear, Mark Brown introduced his lovely Star-Lite with a Rotax 503 in it. Now we're getting somewhere. Here was something that looked like a real airplane, and Mark was saying that the engine was staying together, and maybe I'm not so crazy after all. And the Avid Flyers and Kitfoxes seemed to be having good success with the water cooled 532. I bought my 532 in late '87. The AR-5 first flew in early '92. Flight testing and development went fairly smoothly. No problems with the airframe. Cooling and propellers took most of the time. It's now a reliable weekend friend, and it does great victory rolls. I've finally got my fighter!

Turns out, though, I was a bit too optimistic about the courts being able to straighten themselves out. Liability is more a problem now than ever. I've been listening to the horror stories for ten years, and I'm scared to sell plans. Maybe someday things will change and I'll get my nerve up to take the plunge, but until then I'm limited to selling some video tapes about the unique design and construction process. Four videos are available now. We plan to continue to make them as long as people continue to be interested.

About the AR-5

The AR-5 is a sport plane (not a racer) roughly the size of a Midget Mustang...a little bit shorter fuselage and a foot and a half more span on the end of each wing. It looks bigger, sitting next to Kit Sodergren's on the flight line. It stands taller.

With starter, alternator, battery, coolant, and radio gear, but without pilot or fuel it weighed 488 pounds on the certified scales we used for the NAA officials. With Craig Cato's excellent 50x70 record setting prop, in normal, everyday flying conditions, it will cruise between 165 mph TAS (3 gph) and 175 mph (4 gph - two strokes are thirsty).

It feels quite solid and stable in the air. Control forces are light to moderate ad well harmonized, and the roll rate is high. The stall comes at around 56 mph clean, and 53 mph with full flaps. Descent on final is fairly steep, and landings have all been uneventful. It's a pussycat on the ground. The wide, telescoping spring main gear and large rudder work well. I fly in all kinds of wind and have rarely had to use brakes to stay out of trouble. It feels like my old Aeronca 7AC on take off and landing. Flaps come down at 100 mph and I fly final at around 80. Visibility in the air is excellent, but, as in any fighter, the pilot can't see directly over the nose when the tail is on down, so I do wheel landings and try to get the tail up quickly on the takeoff roll. Because the cowl is so narrow (no cylinders sticking out the sides), I don't have to do much S turning on the taxiway to see where I'm going. Fuel capacity is 12 gallons which gives me a range of 500 miles with 45 minutes reserve. Better than an Me. 109!

The airplane climbs 1000 to 1200 fpm at 115 mph with Craig's speed prop, but at less than that speed the rpm falls below the Rotax's power band and the rate of climb diminishes. The takeoff roll is not very exciting...especially on these 90 degree days we get around here. Static rpm with the race prop is only 4400 which, according to the manual, is down around 40 hp, at best. I can still get in and out of 2000' strips, but I miss the 1500 fpm climb I got with the first prop I tried. It would only do 175 flat out but I sure did like the acceleration. The sound and feeling reminded me of the outboard boats I drove as a kid. Nothing violent about it...just a surprisingly strong, smooth push. I want that back again. It needs a constant speed prop.

The airplane is exceedingly quiet as it takes off or passes overhead. The stock Rotax muffler works very well, and wrapping it with insulation eliminates all that tinny dinging I've always associated with two strokes.

Inside, it's a different story, however. Below about 5800 it only buzzes, but above 5800 it gets downright noisy in there. Because 5800 rpm equals 175 mph or better and marks the top of the recommended cruise range, I spend most of my time at lower settings. The cockpit is very comfortable. I'm almost six feet tall and rarely bump my head in turbulence and have never felt cramped or confined. Inside dimensions are similar to a VariEze's.

The engine is a box stock 582 Rotax. I burn Chevron Super Unleaded and Red Line Kart Oil (synthetic). I've never had the head off, but we did have to modify the exhaust system to get it inside the cowl and, although we tied to use as many of the stock system's components and dimensions as possible, we found it necessary to make part of the system oval-section instead of round and I suspect that we're not developing the full 65 hp. The engine seems to run well enough, though. I've been flying for a year and a half now, first with the 532 while we worked out the installation and cooling problems, and now with the 582 for the last 40 hours or so. I had about l75 hours on the airframe as of October (1992).

So why does this almost-full-sized, blunt nosed, fixed gear airplane perform so well on such low horsepower? Low drag, of course. It has a coefficient of drag of around .016. Very low. It has an equivalent flat plate drag area of .88 square feet. Very small. But how come?

Here's what I think:

First, it's smooth. I've used the same finishing techniques Burt used on his VariEze/Long--EZ designs and, like him, I've used laminar flow airfoils and paid attention to contours and gaps.

But the AR-5 is almost as big as a VariEze, and it goes as fast as the fastest O-235 powered one I know of, and it does it on half the horsepower. "Smooth" is good, but it doesn't fully explain it.

Second, it's light. That's mainly due to the light engine. Regardless of what you've heard, foam and fiberglass airframes are heavier than wood or metal ones, especially if you put a pretty finish on them. The AR-5 has probably got 30 pounds of paint and filler on it. It's, maybe, 150 pounds lighter than a VariEze. Significant, for sure, but still not a complete answer. If you could take 150 pounds off a VariEze, could you get it through the traps at 213 mph on 65 hp? Maybe, but I think there's something else at work here. I took the record away from a gentleman in Austria who flew a clipped winged BD-5 with a 582 Rotax in it. He was supported by the Rotax factory, so I'm sure his engine was putting out at least full rated power, and the BD-5 is a very small, retractable geared, streamlined little bullet that is probably as light as the AR-5. I beat him by 13 mph without resorting to any speed tricks beyond washing the bugs off. "Light" helps, but it isn't everything. So what gives?

I think low interference drag "gives".

Interference drag reduction, as I've applied it, is more than just reducing the interference between boundary layers in the corners. What I've tried to do is arrange the wing and fuselage and canopy so the combination produces a sort of "Poor Man's Area Rule", as the late John Thorp is said to have called it. I spent a lot of time shoving things around on the general layout drawings, paying attention to the wing root fairings, and carefully positioning those things that have to be there anyway, like the canopy (and even the wheelpants), and reshaping the fuselage itself. If you get it right, you can reduce the drag of the wing/body combination, and that's what I think happened on the AR-5. I think I got it approximately right. The exhaust streak that forms on the fuselage side above the wing root is remarkably straight. I love that exhaust streak.

I noticed, after all this manipulation at the drawing board, that the shape that resulted happened to lend itself to a unique kind of construction that allowed me to build the fuselage by hot wiring it the same way Burt hot wired the wings on the VariEze. No fancy compound curves to carve. In fact, very little foam carving at all. The shape of the fuselage comes out almost automatically. The .4" thick sandwiches that form the majority of the primary structure are all cut to shape and thickness with the good old hot wire. Pretty darned neat!

It is this aerodynamic and structural concept that I intend to address in the videos. This principal of interference drag reduction (and the simple method of fuselage construction that goes with it) can be applied to any design.

Like many of you, I've been frustrated by the difficulty of finding accurate performance data on homebuilt designs. We homebuilders are sort of like fishermen. We tend to exaggerate a bit None of us likes to admit we've built our airplane too heavy, or that it doesn't go as fast we think it should, so we fudge a little when we talk about it. I've tried to resist those "fish story" impulses in myself while compiling the data in this article, and I believe my numbers are accurate, but I invite challenges. Help keep me honest. Ask the guys who fly with me. Check the weight and speed with the National Aeronautic Association (NAA). Or come out and fly alongside. And speaking of challenges...the AR-5 turned out to be a fine little dog fighter, just as I'd hoped as a kid. I've already got a couple of Midget Mustang credits. Like to try turning with me?

Comparisons

I took this information from the VariEze Owner's Manual. I assumed 105 hp from the O-200 Continental because everyone overwinds them. Most VariEzes weigh 630 to 660 pounds. The fast ones will do 200 mph. The VariEze has slightly more frontal area due to the thick strake tanks. Both airplanes have fixed main landing gear. The VariEze has a retractable nose wheel, while the AR-5 has a small, fixed tailwheel.

The drag of the VariEze is approximately 158 lbs. The drag of the AR-5 is 97 lbs. It's hard to attribute all that difference to cooling drag, although I'm sure the AR-5 has somewhat less. I think part of the difference is interference drag. It's a major player on an already clean airplane. My ballpark guess is that reducing it, alone, might have added 10 mph to my top speed.

Mike Arnold, Nov, 1992 (reprinted from Sport Aviation Magazine, Jan 1993)