Basic Engine and Propeller Performance
From Bondline
Contents |
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 to the right shows the power developed by the O-320 A/E (150HP, low compression) and O-320 B/D (160HP, high compression) engines. Note that the percent difference in maximum rated power is 6.7%. The impact of this improvement will be discussed shortly. As it turns out, this 6.7% increase in maximum rated brake horsepower does not sufficiently explain all of the performance benefits obtained by upgrading the 150HP O-320 to 160HP.
Propellers Used by Grummans
| AA-1/A/B | McCauley 1A105/SCM-7153 (climb, 71" dia, 53" pitch), McCauley 1A105/SCM-7154 (climb, 71" dia, 54" pitch), McCauley 1A105/SCM-7157 (cruise, 71" dia, 57" pitch) |
| AA-1C | Sensenich 72CK-0-52 (climb, 72" dia, 52" pitch), Sensenich 72CK-0-56 (cruise, 72" dia, 56" pitch) |
| AA-5 | McCauley 1C172/SBTM-7357 (climb, 73" dia, 57" pitch), McCauley 1C172/SBTM-7359 (cruise, 73" dia, 59" pitch), (By STC) Sensenich 74DM7-0-59 (low compression, 74" dia clipped to 72.5" and tips beveled, 59" pitch), Sensenich 74DM7-0-61 (w/high compression STC, 74" dia clipped to 72.5" and tips beveled, 61" pitch) |
| AA-5A | McCauley 1C172/SBTM-7357 (climb, 73" dia, 57" pitch), McCauley 1C172/SBTM-7359 (cruise, 73" dia, 59" pitch), (By STC) Sensenich 74DM7-0-59 (low compression, 74" dia clipped to 72.5" and tips beveled, 59" pitch), Sensenich 74DM7-0-61 (w/high compression STC, 74" dia clipped to 72.5" and tips beveled, 61" pitch) |
| AA-5B | McCauley 1A170/FFA-7563 (cruise, 75" dia, 63" pitch), (By STC) Sensenich 76EM8S10-0-61 (climb, 76" dia, 61" pitch), Sensenich 76EM8S10-0-63 (cruise, 76" dia, 63" pitch), Sensenich 76EM8S10-0-65 (cruise, 76" dia, 65" pitch) |
| AG-5B | Sensenich 76EM8S10-0-61 (climb, 76" dia, 61" pitch), Sensenich 76EM8S10-0-63 (cruise, 76" dia, 63" pitch), Sensenich 76EM8S10-0-65 (cruise, 76" dia, 65" pitch) |
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 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 not how much thrust power it develops. 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.
Engine and Propeller System Performance
In the Lycoming O-320 section above, it was stated that the maximum rated power increase of 6.7% does not explain all of the performance benefits seen when upgrading to high compression cylinders. The reason is because of where the propeller power absorbed curves intersect the engine power curves. These points of intersection are the equilibrium points of the engine-propeller combination.
The plot to the right shows the low and high compression O-320 brake power curves but also the propeller power absorbed curves for a generic (from the propeller curves above) McCauley 7359 at various airspeeds. Because points were picked off of the graph, errors exists most notably in the static RPM case. However, the trends are correct. Although the high compression STC adds about 6.7% more maximum brake power, it effects a much higher improvement in propulsive power and thrust. At lower airspeeds, the thrust power improves by 14% or about double what you would expect from the engine alone. This effect decreases greatly at the higher speeds to about 8% compared to the engine-alone 7%. But this is still about 14% more power than you would expect!
For takeoff performance, the reduction in ground roll is approximately the improvement in thrust power. According to the chart to the right, this is about a 10% reduction in takeoff distance. Because climb depends on excess power, performance benefits from the high compression STC are discussed there.
