SonexBuilders.net

One of the tasks an AeroVee engine builder must accomplish it to set the secondary (electronic) ignition timing to match the primary (magnatron) ignition timing. The magnatrons are fixed timing as a result of their mounting positions relative to the flywheel magnet, and this timing has already been determined by the factory as the optimal timing for the engine. The design of the secondary ignition, however, is completely adjustable.

The AeroVee assembly manual explains how the builder adjusts the secondary ignition to match the primary (http://www.aeroconversions.com/support/ ... Manual.pdf pg 45), but this method has a bit of uncertainty to it, even if performed with care. Bottom line, it's hard to set the trigger cap accurately by eye to within a degree or two. You do the best you can when you build the engine, then fine tune the timing once in flight. The procedure to fine tune in-flight is explained on page 46.

The in-flight procedure is straightforward, but has a few caveats. First, you need the engine and plane running well enough to do this test, and smooth air, otherwise the flight will be too "busy" to let things settle down and get good data. Second, make sure the carb is tuned well, or the changes in CHT and EGT won't be as apparent. Lastly, ensure that both ignitions are functioning properly and either one can be turned off in flight - sort out any wiring problems or rough running on one ignition first.

The test procedure is to fly the airplane in cruise at a consistent power setting and both ignitions on for a few minutes to let the RPM and temperatures stabilize. Don't do this immediately after climbing to altitude as the engine will be hot (or at least hotter) after the climb, and you want it stabilized first. Once the temps have stabilized note the RPM, CHT and EGT - this is your baseline. I like to snap a picture of the temps with my phone to simplify the note-taking.

After recording the baseline, turn off the secondary ignition; don't change anything else (throttle, airspeed, mixture, etc). If the engine runs rough at this point, you're not ready for this test anyway. Fly the plane for another few minutes (my experience shows 1-3 minutes is sufficient). Once the temps stabilize again, record the RPM, CHT and EGT once again.

With this data you should see one of 3 scenarios:

1) No (significant) change to RPM, CHT or EGT. You may see a minor reduction in RPM and/or a rise in EGT, but it should be minor (10-20 rpm, 20-50 deg EGT). This is due to the less-effcient burning of the mixture while operating on one set of plugs. The conclusion here is that the CHT didn't increase, so the timing of the secondary matches the timing of the primary. - Good-to-Go.

2) A notable reduction in CHT is observed, followed by a small loss of RPM and a decrease in EGT. If CHT goes DOWN with the secondary off, that's a sure-bet that the secondary timing is too advanced. The secondary is firing first and the engine is making more power as a result of the advanced timing situation. With more power comes more RPM, more heat (increased CHT) and more efficiency (lowered EGT). Advanced timing is a pretty good deal for power and efficiency, but it's dangerous because it places more stress on the engine and increases the chance of detonation at high-throttle (such as on takeoff on a hot day). There's a practical limit to timing advance, and the factory has found the "sweet-spot" over the years. Conclusion, retard the secondary timing slightly (1-2 degrees, if you can manage that fine of an adjustment), and test again.

3) The 3rd scenario is that the secondary timing is actually retarded, but you can't see that unless you re-fly the test and this time do a third piece where you get the baseline, then the secondary-off data, then the primary-off data. If the secondary is retarded you'll notice that the engine is making less power with the overall retarded timing, and the RPM will drop, CHT will drop, and EGT will increase.

The in-flight test is easy to perform, and can give you all the data you need to fine-tune the ignition timing top optimal performance. Give it a try and see for yourself how your engine behaves.

Jeff

And for the rest of us that have no EGT, a single point CHT, a non-digital tachometer, and an Aeroinjector that can't remember the words to the song twice in a row.................... use a timing light.

Area 51% wrote:And for the rest of us ... [snip] use a timing light.

I'm a proponent of using a timing light to set the timing as well , and that's the procedure I've used for years (http://sonex604.com/timing.html). However, flight testing is the ultimate test. Thankfully, it's easy to do, more convenient (requires no timing light, no pulling the cowl or ground running, and no fiddling with a running engine in close proximity to the spinning prop), and most planes have enough instrumentation to make it work readily.

Jeff

Short of having to acquire a timing light to accomplish the task, (seems I remember having to purchase more than one specialized tool to build the plane in the first place) I see nothing more convenient about flying the test subject to get initial readings, removing the cowl, adjusting a fickle trigger magnet, replacing the cowl, take the plane back up to check the adjustment......then rinse and repeat as necessary or until frustration sets in.

As far as "fiddling with a running engine", the only "fiddling" I'm aware of while the prop is spinning is pulling the trigger on the timing light. To be fair, it should be considered a two-man operation, with someone you don't particularly like holding the timing light, and the timing marks should be on the flywheel and not on the prop hub.
I doubt there is a single case in which the engine was started for the first time at the end of the runway and flown during the initial start-up. Point being, there are multiple opportunities to check and adjust before any first flight of an engine. Ground running needs to be only long enough to verify the pointer and mark line up (or don't).

Since one of the things we're trying to accomplish here is to match the secondary ignition to the primary to keep advanced timing from damaging the engine during high power output, it seems problematic that full throttle operation during takeoff and initial climb are required to get to a safe altitude for the CHT/EGT experiment. A timing check with a light can be accomplished at an idle and well in advance of WOT.

Here's an observation I've made. Watch a number of Aerovee "first start" videos. In a majority of those starts, the engine "kicks back" on the starter. Unless you recognize that behavior for what it means, (excessively advanced timing), you could be flying your engine to it's demise the first time out.

Finally, setting the advance with a light can work with ALL Aerovees.......not just "most".

Hi Guys,

Question: Has anyone tied their airplane down and checked the secondary ignition timing, with a timing light, as RPM was varied over a wide range? I seem to remember that there was a significant shift with RPM.

Wes

Wes,

That's an interesting question. I'll have to run that experiment again (it's been years since the last time) and find out. One of the difficulties we have in assessing the secondary timing is that at low RPM (under 2000 rpm, maybe as high as 2500 rpm) there is very little change in how the engine behaves or sounds when running on one ignition or the other, even if the timing is pretty different between the two. It's not until you get into the full-power range that the differences become apparent.

Jeff

Area 51% wrote:Short of having to acquire a timing light to accomplish the task, (seems I remember having to purchase more than one specialized tool to build the plane in the first place) I see nothing more convenient about flying the test subject to get initial readings, removing the cowl, adjusting a fickle trigger magnet, replacing the cowl, take the plane back up to check the adjustment......then rinse and repeat as necessary or until frustration sets in.

As far as "fiddling with a running engine", the only "fiddling" I'm aware of while the prop is spinning is pulling the trigger on the timing light. To be fair, it should be considered a two-man operation, with someone you don't particularly like holding the timing light, and the timing marks should be on the flywheel and not on the prop hub. I doubt there is a single case in which the engine was started for the first time at the end of the runway and flown during the initial start-up. Point being, there are multiple opportunities to check and adjust before any first flight of an engine. Ground running needs to be only long enough to verify the pointer and mark line up (or don't).

Since one of the things we're trying to accomplish here is to match the secondary ignition to the primary to keep advanced timing from damaging the engine during high power output, it seems problematic that full throttle operation during takeoff and initial climb are required to get to a safe altitude for the CHT/EGT experiment. A timing check with a light can be accomplished at an idle and well in advance of WOT.

Area 51% brings up some good points about using a timing light. You can use the light even before the first flight to fine-tune the secondary timing (or at least get it pretty close). You will need to fabricate and install a pointer, and add the appropriate marks on the prop hub or flywheel for the light to reflect. You'll need a helper, and you'll need to observe some basic safety practices when working around a spinning prop, unless you have a friend you don't like, as he suggests. ;)

Lastly, like Area 51% points out don't fly your plane with the timing too far advanced or you can potentially damage the engine. Take your time setting up the secondary timing, follow the manual, and consider using a timing light to verify the process used in the manual got you close. The real value of the timing light is early in the process when you're looking to get it close enough for flight.

Jeff

Any guides on how to setup and use a timing light on the Aerovee?

Its pretty straight forward. Put a mark on the flywheel and case corresponding to 28 BTDC. The engine manual shows you how to locate this. I used a silver sharpie. I put mine around the 2 o'clock position looking in from the rear. I'll post a picture tomorrow when I pull the cowl for the oil change. Clamp the light to the battery and put it on #1 primary spark plug wire. I have done it several times in the past. I recently bought this one to do my Onex.

https://www.harborfreight.com/timing-li ... 40963.html

I like it because if you miss a little on the initial timing marks, you can turn the dial to get them lined up. Once you get it working on the #1 Primary plug, shut it down and move it to the lower plug wire and try again. You should instantly see the secondary ignition mark. Anything above an idle is fine since its fixed timing. I usually go to around 1400 to not blast the person running the light. Once you get it set, you can try revving it up a bit but mine never moves.