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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Reno from the Pitts 
from  Graham White


'� when the rpms come up and the manifold pressure comes up your brains go out the exhaust' (Quote from well-known race pilot.)

Thank goodness we did not have a repeat of last year when the Reno air races were shut down due to the despicable acts of the terrorists. This report will focus on the Unlimited Class aimed at the techies and gear heads. Its spotlight will be the 'behind-the-scenes' goings-on that keep these remarkable racers in top form. The format of this report will be a day-to-day description of the scuttlebutt and overview of mechanical issues and tweaks made to the racers. Up until the final Sunday, weather conditions were simply perfect; dry, no wind although quite hot, average temperatures were in the low 90s each day. By Saturday afternoon, winds had started to pick up. Sunday morning dawned with the wind still blowing and by Sunday afternoon wind gusts were in the 40 mph range. Not only did this make the already difficult job of flying these ultimate racers even more difficult, the sand storm kicked up by the winds made visibility a serious issue. And goodness knows how much damage was inflicted upon the airframes and engines after being sand blasted at speeds of over 500mph.

Although taken for granted in the auto racing world, wireless telemetry is now taking hold. This over due innovation now allows the pilots perform their primary job � that of flying the airplane without worrying about temperatures and pressures etc. Instead, the ground crew can now monitor all the instruments in the cockpit. In this way, if anything should start to go away, the pit crew can alert the pilot. It also keeps the pilot 'honest' by not over boosting or over revving the engine. Top teams typically have a plan as to how much power is required to win a race. The pilot dials in this power and hopefully does not have to exceed it.

Sunday September 8:

The great thing about arriving several days before the 'official' event starts is the fact you can drive your car right into the pit area. After Monday, the line of demarcation kept getting pushed back until by Wednesday, September 11, I had to park in the 'official' spectator's parking lot which meant a long walk. Ah well, I could use the exercise any way.

One of my first stopping-off places was the Dago Red pit. Prior to being shut down on 9/11, 2001 Dago Red suffered a cracked cylinder head during a practice run which necessitated the replacement of the 'B' bank and head assembly. In the off season, the original bank was mated to the replacement head and at the same time six new pistons were fitted.

Upon arrival at the Dago Red pit, it was apparent that this highly modified P-51D was in serious trouble. The day before disaster struck when it fell off a jack (don't ask..!!). Considerable airframe damage was wreaked upon the rear fuselage, coolant exit door and perhaps most significantly, the elevator. Dago Red's team jumped into high gear. First order of business was to put in an urgent call to Dago Red's brilliant and charismatic crew chief; Bill Kerchenfaut. Bill has been in the racing game seemingly forever and commands the respect of every team. If anyone could pull off a miracle, Bill would be the guy. After driving all night, he set to and started hammering away on the airplane � literally and figuratively. When I arrived at the pit Bill was under the dog house pounding away at the coolant door with a ball peen hammer. Work continued at a furious pace throughout Sunday.

Work progresses at a furious pace on Dago Red. In this shot the left elevator is seen removed.


Two masters at work. David Fagoaga on the left and Bill Kerchenfaut on the right repairing Dago Red's elevator. The devices sticking up through the skin are called Clecos and are used as a temporary expedient to hold the skins together until permanent rivets can be installed.

Voodoo is another highly modified P-51D. Sporting what is arguably the most attractive paint scheme it was nice to see this formidable aircraft in the pits. Last year Jack Hovey had built up a race engine for it. Unfortunately, during a test run at a relatively modest 90 in.Hg. manifold pressure, two connecting rods let go. Cause of the failure was a back fire in the induction system. At the time no one could figure out the cause of the back fire. In the ensuing year, it is thought that stale fuel may have been the culprit. Jack rebuilt the blown-up engine � but not to full race standards. However at 90 in.Hg. and 3200 rpm Voodoo was no slouch either. A P-63 propeller hub assembly drove T-28 blades and an AD Skyraider regulator accounted for some of Voodoo's impressive performance.

Power behind Voodoo is Jack Hovey, one of the most experienced Merlin re-builders in the business. Jack has built the winning engine for a number of unlimited Gold Racers. For this year, Voodoo's engine was mildly modified running 80 in.Hg. and 3200rpm. Low induction temperatures and high oil temperatures were just two of the challenges for this team.

Miss America went through an engine change to replace the stock ferry Merlin with a full blown Rick Shanholtzer race engine. This race engine featured the usual go-fast modifications including G6 Allison connecting rods and a Pete Law ADI system modified from a Pratt Whitney R-2800 CA3 unit. The question is often asked; why use the ADI regulator from an R-2800 when an ADI regulator was developed in the 1940s for the Merlin by Bendix. The simple answer is; no parts. However, parts, i.e., diaphragms for R-2800 CA3 ADI regulators are plentiful. A badly cracked propeller extension shaft didn't help the engine change go smoothly. But, another one was soon scrounged up to replace the cracked one.

Comparison of the two styles of blade & fork connecting rods used by Allison with the V-1710 (top) and Rolls-Royce with the Merlin (bottom). The more complex 'marine block' style Rolls-Royce rods are replaced by the stouter Allison rods in some race prepared Merlins. (Allison drawing courtesy of Dan Whitney).


While undergoing an engine change to replace the ferry engine with a Rick Shanholter race engine, Miss America's crew found this distressing sight � a badly cracked propeller extension shaft.

Another Allison rodded engine for this year's races was the ultimate sheep in wolf's clothing. Stu Eberhard's P-51D is deceptively stock looking and yet lurking under that innocuous looking cowl is a race Merlin with Allison rods. Bill Rheinschild's P-51D was another wolf in sheep's clothing. He too had a race Merlin that ran 105 in.Hg and 3500 rpm. These two aircraft were well matched having relatively few airframe modifications and yet sporting race Merlins.

One of the big surprises this year was #232, Michael Brown's constantly improving Sea Fury 'September Fury' powered by a Wright R-3350 PRT (sans PRTs) engine with direct fuel injection. He did a 462mph lap � on 17 cylinders..!!! The dropped cylinder was caused by a failed ignition coil. Remember this engine, like all the other R-3350 powered racers, uses low tension ignition. That means each cylinder has its own ignition coil so if this coil fails then the cylinder goes dead. Clearly, this was one airplane to keep an eye on. A major contributor to September Fury's dramatically improved performance was a revised ram air induction system. Stock Sea Fury's powered by the original Bristol Centaurus featured a pair of induction scoops in the wing leading edge at the wing root. Each ram air scoop fed induction air to the twin entry, two speed supercharger. Each entry was on the side of the blower housing so locating the ram air scoops in the wing leading edge made perfect sense. Not so with the R-3350 with its down draft Bendix PR-58 carburetor. Getting ram air to the carburetor involves some complex ducting with multiple 90 degree bends when taking induction air from the leading edge. All these bends and manipulations of the air are not good for pressure recovery. Last year September Fury showed up with a short ram scoop located on top of the cowling. For this year, the ram scoop extended to the leading edge of the nose bowl. The scuttlebutt in the pits claimed an additional 10 in.Hg. was available at speed with the newly configured ram scoop. Optimized for high speed, the scoop probably did not supply full pressure recovery at low air speeds. But this aircraft still had more than enough power for take-off chores.

New ram air induction scoop installed on 911 September Fury. Reportedly good for 10in.Hg., it had a remarkable effect on the performance of this aircraft.

Check Mate, the Yak powered by the R-2800 'bitsa' engine, was another aircraft that underwent considerable tweaking and improvement during the off season. Its R-2800 is made up from a CA3 power section and CB17 supercharger � among other things. Another significant improvement was the installation of an R-2800-44 nose case. Dash 44s had the lowest reduction ratio of any R-2800; .350:1. This nose case is extremely rare as its only application was the North American AJ Savage and relatively few of these aircraft were built. With its cut-down Douglas AD Skyraider prop, the R-2800 could now turn it at a more efficient speed due in no small part to the 350 nose.
Stability about the longitudinal axis was always of concern. A larger vertical stabilizer fixed that issue.

The Pratt & Whitney R-2800 CA3 powered Yak � Check Mate. Most noticeable change for this year was the new vertical stabilizer of larger area.


Pete Law, on the left, and the owner of Check Mate, John Moore on the right, discuss a carburetion issue.

The perennial Grumman F8F Bearcat, Rare Bear, has fallen on hard times. A no show last year, this year it was hoped it would return to its former glory as the fastest unlimited racer. Alas, that was not to be. A newly rebuilt R-3350 had been installed shortly before Reno but persistent problems plagued this aircraft and its team. Initial attempts at running resulted in no oil pressure and no scavenging. Before the oil problems were figured out, over 20 gallons of oil were pumped into the engine. The primary culprit for low oil pressure was the installation of an incorrect oil ferule that acts as an oil transfer tube between the power section and the blower section. Once this gremlin was figured out, attempts at starting resulted in sever back-firing. Normally one would not be over concerned with engine back firing. But not with engines fitted with Bendix injection carburetors. This remarkable fuel metering device measures mass air flow and based on this meters the appropriate amount of fuel into the engine. The mass air flow measuring and fuel metering is accomplished via sensitive rubber diagrams. Bendix carburetors depend on a carefully calibrated fuel pressure; too much and the fuel diaphragm will be damaged, too little and the engine will run lean � if at all. It turned out that the fuel pressure had been cranked up to the point where the fuel diaphragm had indeed been damaged. The resulting overly rich mixture caused the back fires, further damaging the diaphragms. A recently flow checked spare carburetor was installed. Now the engine responded far more favorably to the extent high power runs were made on the ramp. But the Bear was not out of the woods. A lengthy squawk list needed to be attended to.

The Devil is in the details. An incorrect oil ferrule, on the right, was installed on Rare Bear's R-3350. When the correct one was fitted, shown on the left, oil pressure came up and the engine scavenged as it should.

Critical Mass is another 'top-of-the-heap' 3350 powered racers. It has all the right modifications; PRT engine and the hard to find EA2 nose case with .355:1 reduction gearing. With a potential 4,000 hp Critical Mass was a racer to be reckoned with. That is until disaster struck. After a good practice run, Critical Mass was taxiing to the pits when the gear folded up on pilot Tom Dwelle. Serious airframe and engine damage was inflicted on this superb racer.

Early Sunday morning Critical Mass sits proudly in the pits. Just few hours later, this magnificent airplane was seriously damaged.


Critical Mass prior to the gear collapse calamity.


Distressing sight of a mega-buck Aeroproducts Skyraider prop destroyed


Sad sight of Critical Mass after the gear fold incident. Serious damage was also inflicted upon the outer wing panels.

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