Simanaitis Says

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OTTO’S FOUR-STROKE—DIRECTOR’S CUT

TODAY I offer my director’s cut of the previous two items, “The Otto Four-Stroke—Self Taught” (http://wp.me/p2ETap-2hj) and “Otto’s Four-Stroke—The Continuing Tale” (http://wp.me/p2ETap-2hL).

Building a computer model along the lines of Nicolaus Otto’s original Silent Engine is good fun. GMax can be obtained as a free download from http://www.turbosquid.com/gmax. It’s an excellent computer-aided design tool. Microsoft Flight Simulator (or at least its FS2002 or FS2004 iteration) can import a model from GMax; the sim supports features such as selective animation and video capture. Fraps, a free download from http://www.fraps.com, captures this Microsoft Flight Simulator action in other editable formats.

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An Otto Silent Engine, modeled in GMax, exported to Microsoft Flight Simulator, with videos shown here using Fraps.

Indeed, I’ve seen plenty of computer-generated animations greatly superior to the Otto Silent Engine I built. However, they’re typically the product of software tools that are really heavy-duty (spelled $$$$, literally in thousands). That this hobby is accessible for the cost of a single program (an appropriate version of Microsoft Flight Simulator) is amazing.

The following director’s cut shares nuances discovered with my Otto Silent Engine.

• Cam design in GMax. I confess I have no idea how a real camshaft is designed, but here’s a GMax approach for profiling the Otto’s exhaust cam.

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Otto operation in terms of crankshaft rotation, 0 to 720 degrees; the GMax equivalent is from 0 to 200 animation units.

I started by animating the exhaust valve, which is closed during the intake, compression and power strokes, then opening during the last of the four, the exhaust stroke. Next, I animated the exhaust rocker arm so that it appears to be actuating the valve.

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The cam design starts with a circle, its diameter corresponding to the rocker arm’s orientation when the valve is closed.

My exhaust cam began with a circle touching the rocker arm when the exhaust valve is closed. Then, using 10-unit GMax increments (equivalent to 36 degrees of crankshaft rotation), I let GMax perform its animation of the rocker arm while I followed this by distorting the circular shape to match.

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The cam begins to take shape with each 10-unit GMax iteration of rocker arm animation.

The result is an animation of cam and rocker-arm that looks fairly convincing.

• Animations in Microsoft Flight Simulator. Animations appearing in GMax do not translate directly to Microsoft Flight Simulator. However, the latter has a MakeMDL Software Development Kit that lists specific names for many common aircraft animations (e.g., “prop_blurred,” “prop_slow” and “prop_still”).

There is a general “tick18” prefix that endows the part with its animation at 18 Hz (1080 cycles per minute). I suspect its intended purpose is for animating beacon rotation and the like. On my original Otto Silent Engine, as seen yesterday, adding the prefix to appropriate GMax parts (e.g., tick18_conrod) got virtual operation of the engine in Microsoft Flight Simulator.

But today brought a new discovery! The SDK also has an “engine_rocker_arm” part name used with the Microsoft Flight Simulator Curtiss Jenny. By swapping prefixes (e.g., to engine_rocker_arm_conrod), my Otto Silent Engine is no longer silent in the sim. Nor is it confined to animations of 18 Hz.

I’m searching now for a realistic set of whirs, clatters and pops. (Its current sound is left over from many of my vintage aeroplanes.)

Here’s my Otto Silent Engine, Version II.

• General videos. Microsoft Flight Simulator captures video, but in a format that can be viewed only in the sim. Fraps, which captures video in this and other gaming software, yields a format that can be edited, for example, in Windows Movie Maker, yet another free download (http://goo.gl/vrI7ef).

Talk about inexpensive, time-gobbling fun. ds

© Dennis Simanaitis, SimanaitisSays.com, 2014

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This entry was posted on July 14, 2014 by in Sci-Tech and tagged , , , .
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