Papa 51

by Jim "Twitch" Tittle

Article Type: Feature
Article Date: January 09, 2003

Bestill my Beating Heart

The Flying magazine article from May 1998 made my heart race. There was an article about a Mustang. No, not a refurbished North American fighter, but a new, scaled down, build-it-yourself Mustang nonetheless.

Like so many daydreamers I’d seen the Bede BD-5 Micro mini jet that was featured in a James Bond movie and perused the info about the piston-powered model with the pusher prop. That seemed pretty cool in 1974 but this was even better.

Great Idea

Dan Denney had the brainstorm to build the replica Mustang with high performance. Denney had experience with kit planes with the successful Kitfox powered by a small two-stroke engine so this was not a lark. After several years of research, design, engineering and construction, the Thunder Mustang, known as the “Papa 51” originally, made its first public flight before several hundred excited fans at the Nampa Municipal Airport in Idaho in November 1996. Test pilot Dave Morss flies an FM2 Wildcat and P-51A Mustang. He also serves the Confederate Air Force as a check pilot in vintage aircraft.

Friend, Ryan Falconer, cobbled the first engine up from two V-8 auto engines. The engine was build-able from 400 to 641 cubic inches. They built the first engines at 600 cubic inches with a 4.125-inch bore and 3.750-inch stroke, so they could stress the block to see what kinds of problems they could find and didn’t see any. The engine was a 90-degree V-12 using a common pin crank, which made it an odd fire (cylinders fire one bank then the other) but it produced 1,000 HP on the dyno.

With the tuned-port manifold that engine, on premium unleaded pump gas, developed 750 ft. lbs. of torque at 4,500 RPM and 700 HP at 5,200 RPM and still developed over 560 FP torque at 1,500 RPM. Although the V-12 was applied to automotive and marine use, this was the first aircraft application of the engine. That was not the final engine form used, however.

The prop reduction gear was inspired from that used with the Merlin designed by B.J. Schramm. It has but two gears and will withstand force equal to 13.5 Gs while doing snap rolls for 400 hours! The reduction ratio is 2.8 to 1. It is a straight cut spur gear arrangement with a quill shaft between crankshaft and drive gear and is lubricated with engine oil. The stoutness is to make it possible for those customers who want to increase power and go racing. It reduces the 4,500-RPM Falconer V-12 to 1,600 RPM while turning the 8-foot, constant speed MT prop.

3/4THs of a P-51

Ahh! To be there.

Externally the plane, called the Thunder Mustang, is a near perfect ¾ scale of the original. One deviation is the canopy. While the cockpit has to be human-sized—it accommodates two persons—the canopy is not over-scaled to make the Thunder look like a toy. The wing area is not to scale but is virtually undetectable to the eye.

Not laid out like P-51 but has the feel.

The plane has no rivets due to the use of carbon fiber parts that are ultimately glued together like a big model airplane. Well, not really. The carbon fiber parts are anodized, primed aluminum with a honeycomb core. Landing gear, wheels, and prop were custom made to match the original as closely as possible. Both front and back seat have a full compliment of flight controls. There is even a baggage door to stow your two-suiter away.

The wing was based on the famous laminar flow design and the original airfoil. It does deviate to include slow speed improvements like a drooped leading edge and the root is deeper in scale than the original’s to accommodate the landing gear.

From this angle looks like original.

Flying the Thunder is reminiscent of the big one. S-turns must be made to see around the long nose. Checking the dual magnetos is akin to the big WWII bird—shut down one and then the other as the RPM drops accordingly. The Falconer engine runs smoothly even with one shut down. The computer age has not only improved autos but aircraft as well. The dual system automatically set fuel air ratio and timing to counteract for things like hot and high altitude starts. Mixture is controlled with a lever but the computer takes care of small adjustments.

Once off, the Thunder can hold 40-degrees nose up attitude at 5,200 FPM and 90 knots. The control feel is said to be crisp but not too sensitive. The plane stalls much like the original due to the laminar flow wing with a sharp break but nothing a little nose down will not fix. The little Mustang is fully capable of all combat aerobatics from 7.3 Gs to -4.9 Gs. She lands peacefully but fast at 120 knots in the pattern and 90 knots for touchdown. But make no mistake; this is not a beginner’s airplane.

Mini Merlin

The heart of the plane is the latest version of the aluminum block Falconer V-12 used in racing boats, specialty race and street cars, trucks, airboats and now aircraft. It displaces 601 cubic inches from a 4.125-inch bore and 3.750-inch stroke with 10.9 to 1 compression on 100 octane Avgas. It produces 640 HP at 4500 RPM with 700 ft./lb. of torque at 4000 RPM. The cylinder heads are aluminum with splayed valves and dual springs. Those valves are stainless, 2.190-inch intake and 1.610-inch exhaust. Forged J.E. pistons on forged Carrillo rods ride on steel cylinder liners. Main bearings are the same size used on 400 cubic inch Chevrolet auto engines. The cam is a special grind hydraulic roller type for low RPM horsepower and torque with stainless steel rocker arms.

The heart of the beast.

The intake manifold is of Falconer design with a special long-runner electronic sequential tuned-port injection system with end-entry plenums for narrow width. The ignition system is Delco Direct Fire, which has no distributor and dual MOTEC computers.

The engine is 55 inches long and weighs 980 lbs. from the firewall forward with all accessories including the prop and dual batteries. The Merlin alone was 1,690 lbs. And like the original Mustang, the Thunder has a dry sump oil system. There is a supercharger kit still coming that will probably bring peak horsepower to about 1,200.

Thunder Mustangs have qualified at the Reno Air Races at 310 MPH, faster than some full-sized P-51s in the unlimited class. But an engine failure precluded it from actual racing in 1999. In 2000 a Thunder Mustang secured 3 first place and one 2nd place finish in the Sport Class races.

OUCH!

The rub comes with the price of the kit, of course, folks. The first ten made were $175,000 with $195,000 thereafter and in 2001 it was up to $285,000. You get the airframe components, the RFI V-12, and mount, cooling system, gearbox, prop, spinner two seats with dual flight controls, landing gear, wheels, tires, brakes, fuel and electrical systems. A few instruments aren’t included along with upholstery and paint. 2,000 hours are optimistically quoted as build time though I’m sure you could figure it’s really longer. But if you have the price of the kit for $30 an hour you could hire a good mechanic and get the thing built for $60,000.

This would still be far lower than purchasing a real warbird for somewhere in the $2 million range. Plus parts and maintenance would be tiny in comparison to real P-51s where some things have to be custom replicated.

So this is one of those things I’d get into if I won only $5-6 million in the lottery after flying lessons to work up to it. If I hit $25 million I might go for the real thing. Look, a restored Shelby 427 C.I. Cobra is going for $2.25 million these days!

In comparison to the original Mustang, lets look at specs of both with “NA” for the North American P-51D and “TM” for the Papa 51 Thunder Mustang. You’ll see that performance is not just ¾ scale of the original either.

• ENGINE:
  - Ryan Falconer Industries 640 HP@ 4,500 RPM 601 C.I. liquid-cooled V-12 TM
  - Rolls-Royce Merlin 1,695 HP@ 3,000 RPM 1,649 C.I. liquid-cooled V-12 NA
  
• LENGTH:
  - 24.2’ TM
  - 37.0’ NA
  
• WINGSPSAN:
  - 23.8’ TM
  - 32.25’ NA
  
• WING AREA:
  - 104 sq. ft. TM
  - 233 Sq. ft. NA
  
• WEIGHT:
  - 3,200 lbs. loaded TM
  - 10,100 lbs. loaded NA
  
• POWER TO WEIGHT RATIO:
  - 5.0 lb/hp TM
  - 5.95 lb/hp NA
  
• RANGE:
  - 1,495 miles on internal fuel @ 322 MPH @21 gal/hr TM
  - 950 miles on internal fuel @395 MPH/1,300 miles @ 260 MPH @ 32 gal/hr NA
  
• FUEL CAPACITY:
  - 103 gallons TM
  - 269 gallons NA
  
• DESIGN LIMIT LOAD:
  - 7.3 Gs TM
  - 9.0 Gs NA
  
• RATE OF CLIMB MAX:
  - 5,200 FPM TM
  - 2,941 FPM NA
  
• CRUISE @ 75% POWER:
  - 345 MPH TM
  - 395 MPH NA
  
• MAX. LEVEL SPEED:
  - 385 MPH
  - 437 MPH
  
• NEVER EXCEED:
  - 504 MPH TM
  - 505 MPH NA
  
• CEILING:
  - 25,000 ft. TM
  - 41,900 ft. NA
  
• STALL CLEAN/GEAR-FLAPS DOWN:
  - 88/78 MPH TM
  - 101/86 MPH NA

Just for fun check out the site and hear sounds of the Thunder and see more images at http://www.thundermustang.com/

BD-5 Micro

Compact speedster

Ok, you want to know something about the tiny jet mentioned earlier, the Bede BD-5 Micro. Well, like all things in hindsight, (Doh! I shoulda bought one back then!) the staggering price of the piston powered BD-5 kit was $2,600 in 1974! Already built it was only $4,400! (I bought a new 1973 Z-28 for $4,195) It featured a 70 HP Hirth 2-cycle engine displacing 720 cc with a cruise speed of 229 MPH.

BD-5 Micro cockpit

But the Micro was the same airframe with a French-made, 66-pound TRS-18 200-pound thrust jet engine. It could output 283 lbs. thrust maximum. It measured just 2-feet long with a 1-foot diameter. All this power on a 910-pound loaded airplane.

The Micro was smaller than the Me 163 dimension-wise. In fact it had a similar wingspan to the V-1 but was shorter by half. Its wings measured 17.0’ and it was 12.5’ long. The top of the tail stood 6 feet. The 2-foot wide cockpit was a snug fit. Wing area was only 37.8 sq. ft. Empty it weighed 425 lbs. sitting on its tricycle landing gear.

The Micro was certified for an ultimate positive or negative 9 Gs. It held 50 gallons of fuel good for 520 miles with a 30-minute reserve. At cruise of 102 lbs. thrust it used 17 gallons per hour and at 283 lbs. you’d go through 47 gallons in an hour.

A very slick plane

The tiny jet took a 1,100-foot takeoff roll and landed in 800 ft. Climb was 2,400 FPM at sea level where top speed was 332 MPH. At 15,000 feet climb was still good at 1,150 FPM with top speed at 325 MPH. At 25,000 ft. 294 MPH could be attained and a 30,000-foot was possible. It stalled clean at 81 MPH and “dirty” at 71 MPH at gross weight with a 60 MPH landing speed at landing weight. A dive could see 385 MPH. All accounts of flight characteristics were favorable with no vices.

And there you have it. Two cool “kit” planes for the ultimate model airplane building skill test and 3-dimensional flight sim experience! A guy can daydream, can’t he?

Sources:

• Goyer, Robert
  “Thunder Mustang”
  Flying, May 1998
  
  
• Green, William
  Fighters Vol. 4
  Doubleday & Co., 1960
  
  
• Headquarters AAF Training Command
  “Pilot Training Manual for the Mustang (AAF Manual 51-127-5)”
  Office of Flying Safety, Winston-Salem, NC, 1945
  
  
• Weegham, Richard, B.
  “Micro”
  Air Progress, February 1974

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