Power & Torque
Richard Weber
There is a widespread uneasiness among modelers about the difference between torque and power. Most of us have come across the school definitions: torque is a force applied about a lever arm, and power is the time rate of doing work. These definitions are correct, but they are inadequate to clarify the practical meaning of how the terms apply to model aircraft propulsion.
Torque, power, and a driving example
The relation between torque and power was illustrated vividly for me some time ago. While driving uphill in the Blue Ridge Mountains my weak clutch began to slip when the accelerator was pressed hard enough to maintain speed in fourth gear. But in third gear, the car could maintain speed and even accelerate slowly. Thinking about this, I realized that the clutch slipped when too much twisting force, torque, was applied. Yet in third gear the car could accelerate using less torque from the engine. A certain power level was needed to go uphill at the given speed. Hence, it was evident that enough power could be delivered to the wheels.
In fact, torque and power are related by the following simple equation: Power = 0.000019 × Torque × rpm (power in horsepower, hp; torque in pound-feet, lb·ft)
Look closely at the equation. Power is the product of torque and rpm. At a fixed engine speed, an increase in torque gives a proportional increase in power. Considered the other way, at fixed torque an increase in rpm means an increase in power.
When comparing two model engines you may hear, "This one has torque, the other has power." The apparent contradiction is explained by the fact that the rpm at which the two engines develop their output will be different. Remember, when torque is specified it is always at a particular rpm.
Comparing engines: OS FS-60 vs K&B 3.5
The first figure shows power and torque curves for the OS FS-60 4-cycle engine and the K&B 3.5. Notice the K&B 3.5 can produce more power than the OS FS-60 when both engines operate above about 12,000 rpm; the K&B 3.5 operates above 16,000 rpm. At low speed, however, the OS FS-60 has more power and torque. So when you read "need torque to turn large props," what the writer means is "need good power at low rpm," as illustrated by the OS FS-60.
Still, if both engines in the example run at their power peaks, the K&B 3.5 will fly a model faster, provided the propeller can be matched correctly. In some cases, such planes with large cowls and high drag would require gearing down the prop. Gearing gains torque at the expense of rpm. Power delivered to the prop is unchanged except for small losses in the gears. In practical terms, the OS FS-60 would outperform the K&B 3.5 with large props because the OS FS-60 has more power and torque at low engine speed.
Propeller load and engine speed
The second figure shows the power and torque required to turn a typical 11‑7 propeller at different speeds on the test stand of a stationary airplane. It can be seen that increasing rpm raises power more rapidly than torque. This is because, for a prop, power again is the product of torque and rpm.
At 13,800 rpm — about top speed for an 11‑7 with a Schnuerle 60 — the engine is supplying a power of 1.6 hp and a torque of 0.61 lb·ft. After the plane is flying, the prop unloads and the engine speeds up until engine power available once again equals the power absorbed by the prop.
Summary
A propeller is turned by torque. Power is the measure of both the torque and rpm supplied to the prop by the engine. Power is the fundamental drive that moves the airplane. More power flies it faster, if that power is transferred efficiently to the air as thrust.
Transcribed from original scans by AI. Minor OCR errors may remain.
