Basic Engine and Propeller Performance
Introduction
The purpose of this page is to discuss performance-increasing modifications to the engine and propeller system of the Grummans. Because performance can mean different things, actual increases in performance are discussed in the appropriate Performance sections.
The Lycoming O-320
The O-320 on Grummans is a horizontally-opposed, carbureted, 4-cylinder engine with 320 cubic inches of displacement. This engine typically comes in low compression (7:1) and high compression (8.5:1) varieties. The low compression version was rated by Lycoming at 150HP at 2700RPM, and the high compression engine was rated at 160HP at 2700RPM. The Cheetahs came from the factory with the low compression engine. Bill Scott of Precision Engine developed an STC to convert the low compression cylinders to high compression cylinders with an RPM restriction of 2650RPM. The RPM restriction allowed Bill Scott to avoid testing since the restriction placed the high compression engine within the tolerances allowed by the FAA for the low compression engine. The FAA will no longer approve changes of this type without testing. The maximum horsepower, with the RPM restriction, is roughly 157 HP.
The plot shows the power developed by the low-compression O-320 engine (lower curve), and the higher curve is the 150 HP engine adjusted by a simple ratio to make 160 HP at 2700 RPM. Because the high-compression engine has different characteristics, this is not strictly correct. Power developed by an engine is the product of the torque output of the engine and the engine RPM adjusted by some factor to make the units work out. Since the high-compression engine has different torque characteristics, the power curve will necessarily be different.
Propellers Used by Grummans
| AA-1 | |
| AA-5 | |
| AA-5A | McCauley 7357 (climb, 73" dia, 57" pitch), McCauley 7359 (cruise, 73" dia, 59" pitch), Sensenich 74DM6-0-59 (low compression STC, 74" dia clipped to 73" and tips beveled, 59" pitch), Sensenich 74DM6-0-61 (high compression STC, 74" dia clipped to 73" and tips beveled, 61" pitch) |
| AA-5B |
Propeller Pitch
Propeller pitch is the theoretical distance the propeller would move forward, without slippage, in one revolution. Matching a propeller, i.e. diameter and pitch, to an engine is very important. Propeller pitch is like gears on a car. If the gear is too low, the engine over revs. If the gear is too high, the engine bogs down. Propeller pitch is usually measured 3/4 of the distance from the propeller hub to the tip. Propeller pitch is can be specified in two different, but equivalent, ways. The first is the blade angle, and the second is the pitch. The two are related as follows: beta = atan( pitch / (2*pi*0.75*d/2)), where beta is the blade angle, pi is 3.14159, and d is the propeller diameter. Note that the pitch and diameter must be specified in the same units.
Propeller Performance
Propeller performance is usually characterized by thrust and power coefficients and propeller efficiency. The thrust coefficient is related to how much thrust the propeller is developing. Simple? The power coefficient is related to how much power is takes to turn the propeller. Sometimes this is usually stated as the amount of power the propeller 'absorbs'.
The thrust coefficient is defined as CT = Thrust / (density * n^2 * d^4), where density is the air density, n is the revolutions per second, and d is the propeller diameter. The power coefficient is defined as CP = Power Absorbed / (density * n^3 * d^5). Since the propulsive power is, by definition, the product of the thrust and flight speed, and efficiency is the ratio of propulsive power to absorbed power, the efficiency of the propeller is eta = J * CT/CP, where J is the advance ratio. The advance ratio is a measure of the angle-of-attack on the propeller blades and is given by J = V / (n * d). Typical efficiency, thrust, and power coefficients are shown to the right. For the Cheetah 59" and 61" pitch propellers with 73" diameters, the blade angles are 18.9 deg and 19.5 deg, respectively.
