Fly Your Own Racer at Reno - Part Two... Speed Secrets
by Chris McMillin -
Photos by Scott Germain, via Chris McMillin and with Permission from Sean D. Tucker 

Home   Photo Galleries   Message Board

In my last installment we found an airplane and learned how to fly it. With the airplane basics taken care of, the speed secrets await our attention. There are many and varied ways to prepare a biplane racer for the pylons; some do not cost a cent. Others will cost a bundle, so we will discuss our options and get down to the business of going fast with two wings.
The four cylinder Lycoming O-360 horizontally opposed air cooled 360 cubic inch parallel valve engine is the engine of choice in the Biplane Class. It is the largest - and we all know 'cubes are king.' The engine is rated at 180 HP at 2700 RPM and 28 inches of mercury. We can squeeze more horsepower out of the engine and airframe  by doing a few basic modifications.
Removal of the Brackett air filter.
Advance the ignition timing.
Use of 100/130 octane gasoline.
Enrichen mixture.
Use of thinner oils like SAE 40, multigrade or synthetic.
Removal of alternator, generator, vacuum pump, etc.
With these modifications you will see an increase in RPM, and in turn, speed. You are now able to convert the additional horsepower into speed by modifying your propeller. The idea is to reduce the prop's diameter - thereby increasing RPM. But we don't want to just spin the prop faster, we want to convert it to an increase in thrust. So we increase the 'pitch' as well. This will increase your propeller's efficiency in the high speed regime and increase top speed at full throttle. On my first (O-320 powered) racer, my friends and I cut a perfectly good Sensenich 74DM60P down to 70 inches in diameter. We noted an increase in speed of 9 MPH IAS and figured we should leave well enough alone. In retrospect there could have been more speed available in re-pitching the prop, but time ran out. The airplane was flown as high as 169 MPH IAS at Reno. This was on a 69 degree F day, zero wind and the engine was turning 3350 RPM (!) on the end of the back straight. 
A racing prop was not available for Reno with my new O-360 racer, so the little 70X60 Sensenich was hung on the crankshaft. We found it would make about 3250 RPM, but with none of the free wheeling smoothness of the O-320. I took the prop to the prop shop and had it measured to see what pitch it was and increased it by two inches. Now the engine would hardly pull past 3200 but indicated airspeed was up to 180 MPH. This is a little synopsis of the black art of fixed pitch props. A word of caution here, metal props have a limit to how short they can be and still have protection from harmonic fatigue failure. Find a shop with knowledge in racing prop modification. Most homebuilders will know who to put you in touch with.
The rules allow a lot more to be done internally to the engine. These are typically done in the shop when the engine is apart for overhaul, but if you need speed you can split the case of your engine and get busy with these mods yourself.
Balancing of the rotating assembly.
Shot peening parts for increased reliability.
Ceramic or lubricating coatings, reducing internal heat.
Reprofiling camshafts, any lift or duration is allowed.
Enlarging or adding oil system paths.
Porting and polishing of cylinder heads, optimizing air charge flow.
Using pistons of 10.5 to 1 compression ratio.

Inside the 4-into-1 exhaust. The middle hole is a oil breather outlet.

There are a few things on the outside of the engine that can increase performance, too. In terms of the exhaust system, a crossover is a fine set-up. Even the four-into-one is a popular option on the course. Few things have a more dramatic effect than a good exhaust. Dave Rose’s tuned exhaust is said to make several hundred RPM due to its tuned extraction effects. Right: exterior of the engine and the 4-into-1 exhaust manifold.
To minimize exhaust heat decreasing the efficiency of the intake charge, insulating tape can be wrapped around the intake tubes.
Providing the proper fuel/air mixture within the carburetor or the fuel injection unit is also key. I chose to go with a Bendix fuel injection system, feeling it gives a much more controllable mixture with its port injection system. Mine was custom flowed by Lycon in Visalia,CA for a richer mixture than normal because of the higher engine speeds necessitating more fuel for the higher air flow rates. Some feel a ram air intake can easily cause a lean condition with carburetor or fuel injection system , so a balanced fuel/air ratio must be kept.
A new change in rules in the Biplane Class allows electronic ignition systems. This aligns the class with one of the latest innovations in the homebuilt industry. Electronic ignition allows easier starting and more power than with the magnetos. The fixed ignition timing on a magneto has always limited the advance to somewhere around 30 degrees BTDC (Before Top Dead Center). Now advance can be variable: retarded for start and advanced for racing. Some systems can run more than 50 degrees of advance for increased performance.
With these innovations, the O-360 is predicted to make more than 250 HP.
Now we turn to the most artistic place for speed - aerodynamic drag reduction. On a biplane, one could be forgiven for thinking, “why bother?” But because a biplane has so much drag, cleaning up a few things will make a big difference. 
Consider the most drag inducing part of any air-cooled engine airplane first. It is the cooling system. How the air gets in the cowl, through the cylinder fins, and then out of the cowl has a dramatic effect on the overall drag of the airplane. Cooling drag has been a problem addressed to a science by the Formula One crowd. Plenum cooling systems are the most popular due to the control of air, and thus efficiency. Small intake ducts and balanced outlets allow the least drag, so the bipe's copy these designs often. (Chris' cowl will be the same that Sean D. Tucker uses on the Oracle Challenger aerobatic biplane shown here.)
Reduction in flat-plate drag is done by reducing frontal area. On my ship, a two piece fiberglass pressure cowling with the smallest frontal area possible is fitted. Along with this, a rod type gear similar to RV homebuilts replaces the original bungee three strut set-up. Small wheel pants are very important speed equipment, Get an especially a tight fit around the wheels.

Right: Chris' modified RV-4 landing gear setup is much more efftcient than the stock Pitts design.


Also important is;

Reduction of trim drag: realignment of the horizontal stabilizer for high speed flight with no elevator deflection.

Slipstream spillage: check your airframe for low pressure areas that cause airflow from inside to leak outside, or vice versa.

Fuselage to lower wing fairing: this prevents high pressure air below the wing from traveling straight up the sides of the fuse behind the wing.

Cabane struts to upper wing fairngs: you’ll see every iteration of these at the Biplane/F-1 hangar, It is important.

Reducing and eliminating intersection drag: fair the intersections you can’t eliminate.

Wing area reduction and airfoil optimization.


This last item is the most expensive... It requires you build new wings. The best way to go is a swept cantilever set with no exposed wires. The minimum wing area is 75 square feet, and the aircraft must have a minimum empty weight of 600 pounds. Figure what airfoil you'd like to use based on your aircraft's weight. Lets see... One mile straights... Half mile turns... I wonder what the G load at 250 MPH is…?
After these things there is still a nearly endless list of items like, CG location, aircraft weight, induced drag reduction, pilot technique, pilot weight, and racing luck. With all of these fun factors out there, how can you resist? Flying, formation, low level, pylon racing, hot rodding and engineering - this class has it all. And you can do it. Come out and join the fun!


Home   Photo Galleries   Message Board