How You Can -- More or Less -- Get More with Less

Many people are irritated by my claim that the climb rate and cruise performance of a KFM107-powered Moni (supposedly 25 hp) can be matched with a 4A032-powered Mosi (supposedly 16 hp). Until the Mosi is flying and generating test data, opinions and beliefs are likely to remain unchanged... hell, even I won't believe it 'til I see it .. but for now I can at least estimate, calculate and predict, so here's a plot of my performance predictions for these two aircraft. The data for this plot is from my spreadsheet-based performance model, reported in pounds of thrust or drag as a function of TAS (true airspeed) reported in miles per hour assuming a 5,000 ft. elevation, which is about what most of the airfields are around here (Salt Lake City area).

Total drag force (see solid lines on the plot) is the sum of induced drag and parasitic drag, and each vary greatly with airspeed. Basically, induced drag is dominant at low speed, while parasitic drag is dominant at high speed. Note that at the lower end of the speed range, the Mosi's greater wing span of 33 feet results in significantly lower induced drag than the Moni's 27.5 ft. wing span. This drag advantage diminishes with increasing airspeed, and above 105 mph, where parasitic drag is greater than induced drag, the Moni's overall drag is lower due to its slightly smaller wing area (75 vs. 84 sq.ft.).

The Moni's thrust was calculated for a standard Moni propeller (33" diameter, 18" pitch) spinning at 6,000 RPM, while Mosi's thrust was figured at 3,600 RPM with a 40" diam., 30" pitch prop. Note that the Moni's thrust is indeed higher over the entire speed range (see large dashes on plot)... but before you jump to conclusions, note that maximum climb rate is determined by thrust minus drag (T-D) at the best climb airspeed. After taking-off and clearing the runway, you attain your maximum climb rate at about the same airspeed used for engine-off minimum sink gliding flight... which is also the same airspeed for maximum endurance or minimum required power to maintain level flight.. they're all the same. Because the Mosi has slightly larger wing area and wing span, it has a best climb airspeed of 48 mph, while the Moni's best climb airspeed is 55 mph. Comparing the T-D curves (see small dashes on plot) of the Moni vs. the Mosi ... note that the Mosi T-D peaks at 48 pounds, while the Moni T-D reaches a maximum of 45 pounds. The result is that the maximum climb rate of the Mosi should be at least as high as the maximum climb rate of the Moni in its standard configuration... That's an encouraging result!

Not shown on the plot is the power delivered by both engines at this best-climb condition.. it's 17.2 bhp for the 4A032 and 19.8 bhp for the KFM107... you see? Getting more with less... more or less...and if you assume the 4A032 can only crank out 16hp max.. using the same 40x30 prop, the Mosi has a maximum T-D of 45 pounds, exactly the same as the KFM107-Moni.

The performance model also predicts maximum airspeed (where T-D equals zero). The KFM107-Moni is shown to reach a top speed of 99 mph with an engine power output of 13.1 bhp, while the 4A032-Mosi is expected to top out at 94 mph, needing 10.4 bhp from the engine... but wait... let's review the assumption of constant propeller RPM. At increasing airspeeds, propellers naturally unload the engine and loose thrust because the angle of attack on the spinning blade airfoils is decreasing. If you keep increasing airspeed without increasing the RPM, the prop will eventually provide zero and then negative thrust as it starts to "pinwheel" while your aircraft goes into a steepening dive. If you want to get to higher airspeed in level flight, it becomes necessary to increase the pitch of the propeller and/or increase the propeller RPMs. The problem is that the Moni's propeller blade tip speed is already at the sonic limit of Mach 0.80 due to the 33" diam. direct-drive prop on the KFM107 engine spinning at a nominal 6,000 RPM. It's generally considered good practice to have propeller blade tip speed at Mach 0.75 or less. Beyond Mach 0.8 tip speed, higher RPMs will result in little or no increase in thrust, but it will result in rapidly increasing fuel consumption and prop noise decibel levels, so there's no good reason for it.

In contrast, the Mosi's 40" diameter prop at 3,600 RPM has a blade tip speed of only Mach 0.59 at 100 mph forward airspeed.. consequently, it should be very quiet. You would have to spin a 40" prop at 5,000 RPM to reach the Mach 0.8 limit. If the 4A032 engine with a 40x30 fixed-pitch prop is spun up from 3,600 to 4,000 RPM, as it should be able to do with a low load on the prop, the Mosi can attain a maximum level speed of 105 mph, which is 6 mph faster than the Moni... This will require 14.1 bhp from the engine and will maintain the blade tip speed at a benign Mach 0.65 .. and with a higher pitched prop, CRUISE EFFICIENCY CAN GET EVEN BETTER!!

But you usually aren't flying around with the prop at maximum RPM. After getting to a desired altitude, you probably want to level off and get somewhere as efficiently as possible. It would be typical to cruise at 60-80 mph to increase your range and fuel economy. To do that on the Mosi requires throttling the engine down to perhaps 2,400-3,200 RPM. Fortunately, 4-stroke engines generally have a wide RPM range for smooth operation, but 2-stroke engines have been known to run rough when they are throttled down below their relatively narrow power band (ironically, 2-stroke engine guys call it "4-stroking" when their engines run rough at lower RPMs). But ignoring that, and just comparing engine power requirements in level cruise at 60-80 mph, the 4A032-powered Mosi requires 5.2 to 8.0 bhp, while the KFM107-powered Moni needs 7.1 to 8.9 bhp.

Therefore, it is expected that with its more efficient wings and propeller, and the flexibility of being able to vary RPM over a broad range without exceeding engine or tip speed Mach number limitations, the 4A032-powered Mosi should be able to match or exceed the climb rate and cruise performance of the KFM107-powered Moni in spite of engine maximum horsepower ratings that would suggest otherwise.

But what about fuel consumption? The 4-stroke 4A032 is expected sip fuel at about one-third to one-half the rate of the KFM107 2-stroke at normal cruise power settings. That ought to keep Al Gore & his cronies off my back.. And what about pollution levels? Not even in the same league! 2-stroke engine exhaust is notoriously dirty, as your sense of smell can easily confirm. With all the increasing emphasis on environmental issues nowadays, it just makes sense to use a quieter, cleaner and more fuel efficient power source... why irritate your friends & neighbors?... it's best to keep them on your side!

I've studied this alternative engine option pretty thoroughly and am satisfied with the engine & propeller plans I've made for the Mosi, at least, so far. But watch out.. if it can go wrong, it will go wrong!.. Murphy and his gremlins are always ready to spoil your best laid plans without warning. If it turns out that in static tests the 4A032 doesn't crank out nearly as much power as I thought ... I'll have to decide whether it's worth it to keep tweaking the engine to squeeze more power out of it, or shift to a different 4-stroke engine option... in any case, I ain't goin' back to no stinkin' 2-stroke!


As always, your comments are welcome.