Have you thought about how easy GAs we have?

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AKar
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Have you thought about how easy GAs we have?

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In terms of engine management, that is. :)

For entirely unrelated reasons, I did some statistics of the Lycomings. While at it, just for the sake of it, I checked how the A2A GA fleet falls into the distribution. I did that by plotting the BMEP (Brake Mean Effective Pressure, or engine's torque per displacement) against engines as per their output horsepower. This makes up a pretty good "stiffness factor", or how hard the engine is pushing in terms of internal pressures to produce its horsepower. (The maximum takeoff power is used here.)

Image

What we note is that our high performance fleet is actually pretty lame - in this measure, the highest-tuned engine is in our tiny 172 (and in Cherokee 180 which equals 172 with 180 hp rating)! The engines operate at below-average torque figures. Given the nature of these GA engines, with certain assumptions this implies wide detonation margins, and some tolerance against mishandling in what comes to "engine settings", or steady-state lever positions.

The newcomer, the Comanche 400, does not actually follow this rule quoted as "there is no replacement for displacement" solely. Not only there is some comparatively high amount of power available from those eight cylinders, it is also running those cylinders at highest average pressure of any Lycoming naturally aspirated engine. In terms of Lycomings, it is a true high-performance naturally aspirated engine. Also, noting how the Aerostar relates to the Comanche, it sits significantly higher on the chart while sharing the displacement of 541.5 cubic inches.

For conversion to common gauge units in planes having BMEP instruments, 1 MPa is just about 145 psi.

-Esa

Roadburner426
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Re: Have you thought about how easy GAs we have?

Post by Roadburner426 »

I see you forgot the civilian Mustang so I did the calculations for you. You need to extend the HP line out to 1,315, and then the BMEP in MPa is 1.45 based on my calculations. So well above the entire chart. :mrgreen: On a side note it is interesting to see that the Aerostar 600 is just a tad bit above the line as I figured it would have been higher than that. I guess there is a reason they decided to put the 350hp engines in it. Maybe now that I figured out how to calculate this stuff I will take a look in to it for fun. So is the curved line in the middle of the chart like a "safe" line where engines that fall below that are pretty much bullet proof. Then anything above that the reliability starts falling off the further up you get? I would think a lot of these engines are still fairly reliable even with their higher internal pressures as a lot of the engines still maintain the same TBO and inspection cycles of the lower power engines.
S. Jordan
AM; United States Navy
FSX/P3Dc4 Hours: 3100 and counting! All A2A birds in the hangar except the 172.

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AKar
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Re: Have you thought about how easy GAs we have?

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Roadburner426 wrote:I see you forgot the civilian Mustang so I did the calculations for you.
I only had Lycomings in my Excel. :mrgreen: I'll work in the Conties and so on, but it is manual work so it will take some time. :mrgreen: (I put in some other stuff as well, such as ignition timing and compression ratio, for some more...in detail studies.)

You push the point well: note how the horsepower from the Mustang is well off-the-chart, but yet, its BMEP figure is somewhat comparable. :) With liquid cooling, intercooler, and high-octane fuel. Indeed, the amount of torque produced per displacement is a figure that immediately gives some insight if something must be going on! Here, pushing the tech immediately shows whereas a mere power-per-displacement figure can mislead some.

About the line: it is just a trend-line, there is no significance about it other than showing that the higher horsepower engines tend to be in general just a little bit higher-pushed in BMEP than the lower-rated ones. If an engine sits above another, it produces more torque-per-displacement regardless of how they sit along the line. :)

-Esa

Roadburner426
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Re: Have you thought about how easy GAs we have?

Post by Roadburner426 »

The Mustang definitely has tons of power. After putting a bunch of hours on it over the last month, and then flying the Comanche the other day the take off power is a night and day difference. It is funny though how the piston engines produce more torque than horsepower. However the turboshaft engine in the H-60's I work on produces 1,680hp, but only 500ft-lb of torque. I guess the improvement comes in the fact the engines usually run trouble free for thousands of hours, and don't require any sort of overhaul until 6,000 hours has been accumulated on the gas generator rotors and they reach their life limit. A piston engine would die long before that at similar power levels.
S. Jordan
AM; United States Navy
FSX/P3Dc4 Hours: 3100 and counting! All A2A birds in the hangar except the 172.

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AKar
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Re: Have you thought about how easy GAs we have?

Post by AKar »

Roadburner426 wrote:It is funny though how the piston engines produce more torque than horsepower. However the turboshaft engine in the H-60's I work on produces 1,680hp, but only 500ft-lb of torque.
The same applies to pistons. I share the Excel when I'm finished with it, but what you'll note is that many helicopter engines (HO and HIO series) lie pretty low on the graph. They make good power per displacement, but they do that by running fast whereas the RPM of the rotor is reduced significantly.

For any rotational engine, the torque is the first order of an output - exactly analogous to the thrust of the jet engine. The maximum torque available from a given displacement is physically more constrained than the maximum power you can get out from a given engine. By downsizing, a small, fast-turning engine can give out respectable power - this is the idea behind using reduction gearboxes in airplane engines as well - I think the Mustang had one as well. :)

-Esa

n421nj
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Re: Have you thought about how easy GAs we have?

Post by n421nj »

I guess that's why the c172 and Cherokee are considered trainers
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AKar
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Re: Have you thought about how easy GAs we have?

Post by AKar »

n421nj wrote:I guess that's why the c172 and Cherokee are considered trainers
Not quite. :) The A2A Comanche would be the-most-trainer-of-them-all. :)

-Esa

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