Simanaitis Says

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TODAY I offer my director’s cut of the previous two items, “The Otto Four-Stroke—Self Taught” ( and “Otto’s Four-Stroke—The Continuing Tale” (

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 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, captures this Microsoft Flight Simulator action in other editable formats.


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.


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.


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.


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 (

Talk about inexpensive, time-gobbling fun. ds

© Dennis Simanaitis,, 2014

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