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A Souped UP P-51 replica
By Dick Cavin



There is no denying that the P-51 has a special mystique that almost defies rational description. Here is an almost fifty year old fighter whose charm remains untainted and undiminished. Its only real rival momentarily captivated the flight line at Oshkosh a few years back when the first BD-5 jet made its debut. Yes, the crowds oooh and ahhh when the latest fighter jets blast off with afterburners blazing and go through their amazing routine of rocket like climbs and almost supersonic fly-bys amid a torrent of ear splitting sound. Thrilling, yes, but when the crowds hear the P-51 singing its own inimitable Merlin song on a fly-by or take-off roll, it's instant Big Drool time and out come the invisible crying towels. The staccato cracking of the short stacks as a P-51 taxis by is reason enough for pulse rates to jump and fans to enter into a glazed eye trance.

Who knows of all the Walter Mitty fantasies that a nearby P-51 conjures up among the faithful, but Merlin Magic is undeniably a powerful potion. Everyone passionately desires one of their very own, even though they well know that some millionaires would be hard pressed to own one on today's market. No matter, though. A lot of P-51 fans are willing to settle for the next best thing to a real live one…a scaled replica. Over the years there have been several designs appear in the 5/8 to 3/4 scale range and their powerplants were the 200 hp Ranger engine from WW-II trainers up to large liquid cooled automotive engines. Construction materials have run the gamut of all that is available - wood, metal, tubing and fiberglass.

For whatever reason, these designs have not been an overwhelming success in terms of numbers built. Perhaps the performance on the Ranger powered version wasn't up to the expectations of potential builders. One of the later kit efforts (now defunct) that used a large liquid cooled automotive engine for the needed power and authenticity, eventually bore a price tag of close to $50,000. This obviously shrinks the available market. Structural complexity of these designs may also have been a deterrent factor. It was close to 20 years ago that the late Ken Rand fired the imagination of thousands with his low cost KR-1, an overgrown model airplane in essence, and the first truly composite airframe. His method of building a basic wooden box like structure to carry the loads and supplementing it with plastic foam and a fiberglass skin to give it shape was controversial for a time. Its continuing success eventually silenced its critics.

In1974 the success of the KR-1 led to the formation of a new company with a bright new marketing idea - War Aircraft Replicas, Inc. (W.A.R.) of Santa Paula, CA. Their research found it feasible to build approximately 1/2 scale airframes of all Allied and enemy fighters of WW-II, using one basic wood fuselage and wing and sculpturing the individual contours with easily worked polyurethane form, covering it with thin fiberglass. Originally the airplane was to be powered with the economical VW engine, as the KR-1 had used so successfully, but most builders opted for regular aircraft engines in the 85-100 hp range instead, possibly because they wanted a little zippier performance.

The original replica design was the FW-190 and it naturally was received quite well in Europe, where a number are known to be flying. Of all the 1500 sets of plans and kits sold, the F4-U replica has been the most popular in the U. S., Canada and Australia with over 30 having flown. All told there are over 100 of the W.A.R. replica fighters that have flown. The Japanese Zero is the latest. The 1/2 scale P-51 has been a back burner project until a relatively short time ago, even though the contouring of a realistic looking air frame presented no real problem. The real fly in the ointment has been the lack of an inline engine in the size and horsepower range needed. They could put a dummy radial engine cowl around a horizontally opposed engine and make it look pretty authentic, but they refused to stick horizontal cylinder "cheeks" out of an otherwise smooth inline cowl.

Although Rex Taylor didn't know it at the time, the NASAD engine forum at Oshkosh '87 triggered the beginning of a new engine for the W.A.R. P-51. He had long wanted to do a replica Ryan STA and on the way home he thought about the Honda Prelude engine as a likely power plant. The table of engine stats in Chilton's Repair and Tuneup Guide for 1973 to 1986 cinched it for him. A few days later he and an old friend, Roger Ring, went to a Carefree, AZ flyin. There they met Jim McKeehan of War Aircraft Replicas, who had a big empty hole in their new P-51 replica

cowl. When Rex told him of the Prelude specs, Jim became very excited. He realized that this might really be the end of their long search for a suitable engine for the P-51. They had given up on the Saab engine and they also fell victim to the Mazda rotary fizzle of an Oklahoma promoter. The project now began in earnest. Rex went to a Phoenix dealer in used engines and found several used ones from Preludes - all in the $600-$850 range. There was one that suited him an '83 1829cc. This model develops 100 hp and 100 pounds/feet of torque at 5500 rpm. It also had 104 pounds/ feet of torque available at 4000 rpm, indicating that it develops a lot of power (torque) in the 75% rpm range (where the cruise rpm would be). Rex calculated that a 2:1 gear reduction ratio would be about optimum, allowing it to turn 5500 rpm on take-off with 2750 propeller rpm. A cruise engine rpm of 4500 would give 2250 prop rpm, allowing them to marry it to a large diameter, slow turning propeller, exactly what was wanted for the P-51 replica.

At this point Rex and Roger Ring, his machinist buddy from San Diego, began brainstorming the pros and cons of various speed reduction methods. They looked at Lou Ross Engineering's planetary gear drive system, the Hi-Vo chain drive and the HTD cog belt method, as used by Dave Blanton. One problem they had to fight right down to the finish wire was keeping the physical size of the complete engine and redux unit tightly in hand, so that it would fit the P-51 cowl with no protrusions or enlargement. The Ross planetary gear system made it too long, as did the Hi-Vo chain method. Mocking up Dave Blanton's HTD cog belt drive set up showed it would do the job.

The cast iron cranks in automotive engines are subject to breakage from resonance induced by sometimes alarmingly high gyroscopic loads at the propeller, in addition to reciprocating pulses from the combustion process. The cog belt system is a superb damping device (shock absorber) for these loads when the prop shaft is adequately supported, so this was the one chosen. The 1829cc engine uses the new lightweight cast iron block technology and after they stripped off 60 Ibs. of all the smog control junk, intake and exhaust manifolds, etc., the bare engine weight was only 160 Ibs. At this point they estimated the complete engine weight would be 220 Ibs., excluding radiator and coolant. The final weight with the complete cooling system, engine mount and manifolds came out to less than 260 Ibs. - a few pounds less than an 0-200 or 0*290 with baffles, manifolds, accessories and mount would weight!

They were really cooking now with these figures in hand, but the road ahead was still a rocky one, as they fought the space problem. Another problem was to keep everything accessible for service. They agonized over how they could re-arrange accessory items such as starter, alternator, water pump and other movable items, to gain a little more space. They even laid the engine over on its side by 20 degrees and this meant they had to use a beveled base for the carbs. They also kept bumping up against having to re-contour the oil pan to make room at the front of the cowl. By this time they were getting frustrated with the shoe homing process that seemed to defy solutions that didn't involve significant compromises. Then one day Rex, Ring and Rex's son, Pat, started the old "What if" brainstorming session, when the real solution presented itself. Pat suggested they turn the engine around, with the water pump end next to the firewall. This gave plenty of room for the oil sump at the other end.

Now they could put the standard water pump pulley back on, use the stock 60 amp alternator, and even use the side draft carburetors that are standard on many Preludes. The only real negative of this method was that the prop would turn backwards to the American way of thinking. Now that the flywheel and ring gear were on the pointy end of the airplane the only remaining problem was the actual design of the redux unit. Basically they used the same system that Dave Blanton used, except that they chose to use an idler pulley as a belt tensioner, which can exert 21 Ibs. of force on the back side of the belt. Blanton chose to use a method that would utilize a bolt operated arrangement to raise or tilt one of the pulleys.

It was now early spring in '88 and Peter Nieber, the new president of War Aircraft Replicas, arrived in Eloy one night with the P-51 airframe on a trailer. Now came the problem of building the engine mount, hooking up engine controls, plumbing the fuel system, installing the electrical system, mounting the 11" x 19" radiator in the belly scoop and running the large aluminum tubes up to the water pump, etc. The days remaining before Oshkosh '88 were now getting short, so things shifted to high gear. In a last minute change, they put on twin Ultracarb carburetors in place of the stock carbs. By doing this and fabricating their own intake manifold they were able to save about 10 pounds, but they are sacrificing the water heated manifold that might protect against carb ice. They reverted to a standard sheet metal collector around the exhaust manifold for carb heat.

The carburetor change to two 34mm Ultracarbs came about as a result of Honda discarding the stratified charge engine in '85, going to a regular combustion chamber, thus eliminating the need for special carbs. Rex switched the project to the later engine in midstream and this is the engine he ended up with and the one to be supplied. The final ignition system became quite simple. The ignition sensor pack is driven off the camshaft, utilizing the "Hall effect" sensors used in HAPI engines for eight years. Tiny magnets imbedded in a rotating wheel trigger the sensors to fire individual coils for each cylinder. Each coil fires two different cylinders simultaneously. Separate power sources are the battery and the alternator, providing '1all safe" ignition. An alternator failure would still give 3 hours of engine run time before battery power was expended. The ignition system is actually more dependable than conventional magneto systems, even though only one plug per cylinder is used.

With time fast running out to make Oshkosh '88, this meant longer hours and hard work in order to reach the $64 question of "How will it run?" The engine fits in the P-51 cowl with room to put your hand between it and the cowl anyplace. Everything is now accessible for inspection and service. The redux gear driver pulley has been mated to the crankshaft via the eight threaded holes in the crank that Honda uses to hold the flywheel. Two husky 3/8" aluminum plates attached to the engine block support both pulleys and stub shaft bearings, just like Blanton does. Sprockets are hard anodized for almost indefinite wear resistance and the Gates HTD cog belt is in place and properly tensioned.

Now comes the big moment. Hit the starter and it fires. After a couple of carb and timing adjustments they open the throttle slowly. At two-thirds throttle they knew they had a winner, an engine with more than adequate power to fly the P-51 replica. Swinging a Ioaner prop from Props, Inc. that would turn 2250 rpm static on an 0-200, they found the gutsy little "Mini-Merlin" would turn the same prop 2420 static rpm. Obviously it was cranking out a lot more torque at the hub than the 0-200, giving it a thrust equivalent of an engine of about 125 hp. Even more impressive was the fact that it was a 107 degree day, at a density altitude of 5000 ft.

The next question was "Would it cool?" At that same 107 degree temperature they ran it full throttle for 15 minutes and it maintained a steady coolant temperature of 180 degrees F. with 9 Ibs. of pressure in the cooling system. Yes, the belly scoop radiator really did the job. At present they don't think they will need an oil cooler, but if so they'll plumb it so that the cooled water will be used. The big 19-1/2" dia. fiberglass spinner had been made on tooling fabricated by Joel Thorns, the designer of the P-51 and was trued up on a lathe and then installed. They were then ready for the final weight and balance. Empty weight was 798 Ibs., with the CG coming out perfectly in the middle of the allowable range.

Now there was no more time left. Oshkosh was only a week away and there was no possibility of flying jt there. There was only time for a dozen or so lift offs/landings, with everything going great, and then it was put on the trailer for the long ride. They had hardly gotten the little P-51 settled among the others in the Scaled Warbirds area when the crowds swarmed it. P-51 Magic was at work again. If they could have just flown it and let the fans hear that Mini-Merlin run it would have been mobbed.

Even though it drew a crowd constantly I doubt that most of those who saw it really realized they were looking at homebuilt history in advance. This little jewel of an engine has the potential to be the power plant of choice for any aircraft that can use 125-130 hp in the nose. War Aircraft Replicas already has plans for a scale P-40 and it’s a certainty scale replicas of Spitfires, Hurricanes, P-38s, Me-109s and, who knows... maybe a P-82 twin fuselage Mustang.. . will be showing up, too. The very real possibility that these scale replicas can top out in the 180-200 mph bracket will bring new builders out of the woodwork in numbers. Don't be surprised if you see LongEZs, RV-3s, Tailwinds, etc. with a MiniMerlin in the nose. It will be interesting, too, to see who will be the first to come up with a one time STC for a T-Craft, Luscombe, or such.

The Mini-Merlin will be offered in two options. The builder can buy his own engine and use a kit of all the bolt on accessories that Rex will supply, along with an explanatory video, or he can purchase a complete engine that has been test run. In the case of P-51 builders, the $5495 will include everything between the firewall and prop for the first 10 to sign up. The P-51 is now in a complete test program for both the airplane and engine. Everythin9 is going so smoothly that they are now concentrating on finding the optimum propeller or propellers.

Final teat results on the aircraft and engine as well as production specifics will be made sometime in the future. In the meantime we know EAAers everywhere are saying, "Thanks, Rex, and all the others involved in the project, for your skill and know how in adding an exciting new dimension to homebuilding." (For additional information, contact HAPI Engines, Inc., 602/466-9244.) Sport Aviation January 1989
Website Copyright January 01, 2003 by Grant Haywood