Steve:
The engines were 9 cylinder Pratt & Whitneys, which dictated the diameter of the cowl. But, prop shaft to the body was just a little over four feet, which dictated an approximate eight foot diameter propellor. It does look small compared to what the engine could handle. At least they ran a variable pitch.
JHF
Hi guys,
The Twin Beech was designed with certain engine types and propeller parameters in mind. Most engines were in the 350 to 450 hp range and the propeller length was a known. All P&W R-985 engines are equipped with 8 foot props. Some early versions were variable pitch which required the pilot monitor the RPM and adjust it manually. Some were two position props, T/O and cruise. Later WWII versions had constant speed props where the governor kept the rpm at a preselected value set by the pilot, and after WWII the most of the military and civilian airplanes were equipped with full-feathering, constant speed propellers.
The Model 18 design was built for these types of engines, Jacobs, Wrights and Pratt & Whitneys which were all about the same displacement (915, 975 and 985 ci respectively) and turn the same diameter props. There was no shortening of the blade diameter to fit it to the design, the designers drew up the airplane to fit the equipment available.
As for the modelers thinking that the blades are too short compared to the cowling, in racing the blades are shortened to allow RPM overspeed and keep the blade tips from supersonic speeds. Overspeeding allows the engine to make more BMEP, more pitch is used for greater speed per rpm and the diameter reduction allows this. That is where you find Wasp Jr. (P&W R-985) horsepower ratings of 575 hp. This is what is listed in many written accounts for the R-2 Gee Bee. The cowling was about the same diameter of the Staggerwing and Twin Beech, and the prop was several inches shorter than 8 feet.
The NACA cowlings on the Twin Beech and the Staggerwing, AT-6, etc, are very aerodynamic. Inside of the cowling are inter-cylinder and cylinder head baffles that direct air through the fins of the cylinders. This internal aerodynamic flow is as important as it's external shape and the size. The speed ring is very close in diameter of the engine itself for smallest frontal area, and is kept from moving forward by clamping them down around the cylinder mounted anchors. If these weren't in place the speed ring would "fly" forward into the prop. The forward thrust propulsion of the prop is mostly done by the outer half of the blade diameter and the shape of the blade shank at it's root is completely round, it only begins to form an airfoil about 14 inches up from the blades exit of the hub itself. Where the blades are an airfoiled shape, they are about inline with the airfoiled curve of the speed rings external shape. This actually accelerates the air across the speed rings external aerodynamic shape aiding in thrust.
It all works in harmony and provides an efficient aerodynamic package of, power section making hp to turn the prop to blow air across the external and internal shapes to provide thrust to propel the airplane as well as cool the cylinders. This as well, the oil, which has a cooler which intakes it's air between two of the cylinders inside of the cowling. I can assure all that the acceleration and climb performance of the Beech 18 at gross weight outperforms every modern twin engine piston twin. The reason being is that it has the biggest engines and props. Also it's high speed abilities are in line with the more modern piston twins, the airplane will cruise at 165 mph true at low power and 235 mph at high power, this providing the pilot with a huge envelope in which to suit the profile of any flight.
I love the subject of your build, Jim. I've 2000 plus hours in the Twin Beech and still fly one every once in a while, still love it's strength and power.
Chris...