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Torque and Propellers (by Bruce Buksyk) Aircraft engines have high torque ratings, so you can turn a larger prop with wide blades and get higher thrust (with less noise at lower rpm's) than with engines with smaller propellers at higher rpm's. Larger props may also give faster take-off and better climb performance, as well as better cruise. This is because propellers are more efficient at lower rpm's (~1600-2000). For example, when my Coot's rotary ran at 5200 rpm (cruise), my wide-bladed 76" IVO-prop was spinning at ~1825 rpm (5200 divided by the 2.85 reduction ratio). In theory, this is more efficient than the 2500-2700 rpm often used with conventional aircraft engines. Now, let's come to engines. Remember the phrase: “there's no re placement for dis placement"? To be sure, aircraft engines have HUGE displacement. The reason for their low-rated horsepower is their slowly turning rpm's (~2700 max). But aircraft engines have massive torque (the working component). Let's compare engines of the two Bill's (Schmidt's Franklin vs. Coote's Subaru) for discussion. Both have Warp Drive props, and roughly 150/160 hp. at 2700 rpm for the aircraft engine and ~4600 for the Subaru. The O-320 has 320 cu.in. (5.25 liters), but the Subaru has half that at 2.5 liters. The O-320 puts out 311 ft-lbs. of torque vs. the Subaru ~150. Enter Bill Coote's propeller speed reduction unit (PSRU) ratio of 1.9, and X by 150 (the Subaru's torque) to get 285 ft-lbs. The aircraft engine wins for displacement and torque. Therefore, be wary of focusing on hp alone, which is a poor measure of engine output when comparing aviation engines and car engines.
What if we compare the Subaru to the Mazda rotary? This is apples to apples; no comparison with an aviation engine. The difference this time is the mechanical advantage produced by the PSRU. The Subaru and the Mazda are similar in displacement. The rotary produces ~165 ft-lbs. of torque. Since the rotary turns at very high rpm, a higher ratio is used to get the prop speed in line. Thus, a high reduction ratio is chosen. It's the main reason I'm using more reduction (2.85 to 1) as I'm turning the engine faster (rotary's love higher rpm's!) That times my torque of 165 gives over 470 ft-lbs of torque. So let's compare torques (in ft-lbs.): The Subaru puts out (with reduction): 285. The O-320: 311. The O-360: 350. The rotary: 470. Which engine will do the most work and produce maximum thrust? Thus, I worry that Bill Coote won't have enough torque/thrust to get out of the water, but I would love to be taxi-testing, as he is, to find out! (Fortunately, Bill has a back-up plan, so watch for more news!) |
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