-------------- Original message from Tracy Crook <tracy@rotaryaviation.com>: --------------

I doubt anodizing is a factor.  Keep in mind that the starter we use is a light duty one but its rating is 1.2 KW.   With a typical cranking voltage of 10 volts, that's 120 amps.  With that kind of current, the only logical routing of the ground cable is as directly to the load as possible. 

Do the math.  At that current level, it only takes about .008 ohms  to drop your working voltage a full 1 volt.  That's 10% of your cranking power lost.  Rotaries are especially sensitive to cranking speed because they lose more compression than piston engines at low speed.  Low compression makes starting more difficult.  Losses in this circuit must be minimized for many reasons.   Also note that the ground connection point that many have said they are using goes very near the crank sensor and its wiring.  This proximity makes inductive coupling into the CAS circuit more likely.  

Add all these factors up and it is not surprising that many builders have starting problems.

Tracy  

On Tue, Nov 24, 2009 at 8:52 AM, Jeff Whaley <jwhaley@datacast.com> wrote:

I’m curious about the engine mount style you guys are using with this voltage drop situation.

I can’t imagine this problem occurring with the bed-mount/aluminum plate between the oil pan and engine block configuration.

Surely a 3/8” or ½” plate of aluminum and 20 bolts will conduct from housing to housing?  - unless maybe it was anodized?

Jeff

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Mark Steitle
Sent: Monday, November 23, 2009 4:41 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: No start

 

Steve,

 

Glad you got things figured out (we were running out of suggestions).  Sure gives one a good feeling after battling a problem for so long.  BTW, I have two grounds on my engine.  One is to the front cover, one to the front side housing.  I'll check into moving one up to the starter.

Mark

On Mon, Nov 23, 2009 at 3:33 PM, Steve Brooks <cozy4pilot@gmail.com> wrote:

Al & Mark,
I would say that many people may have their ground to the engine block. Mine has been that way since I built the plane, and worked fine for three or four years before I started having an issue.

I had to order a couple of new crimp connectors, so that I can make a longer ground cable. Hopefully they will arrive before Thanksgiving, so that I can get it all hooked up over the long weekend.

Steve Brooks


Al Gietzen wrote:

Steve,

I read your other post on what you discovered regarding your bad ground. I wonder how many of us have our engine grounds tied to a bolt near the front cover or end housing? (Mine does...)

Mark S.

Mine does as well – bolts to the front cover. I have never had a starting issue, but I have followed this discussion with interest because a couple of years ago I did a starting spark check and found the sparks considerably weaker than the mode 8 (or whatever) check. I just assumed it was due to the voltage drawdown when the starter was running. Now it’s worth another look.

I recall giving it some thought when doing the wiring (My EC/EM manuals apparently predated Tracy recommended wiring diagram). I concluded that the various things bolted to the engine, like the manifolds, would provide good conduction paths regardless of the sealing between layers – and the 20B has a couple more layers. Both my intake and exhaust flanges are continuous and connect all the layers, and there are other things making the connection to the front cover, so this would be less of an issue compared to an installation having separate flanges to each housing.

Anyway, Steve; I’m glad you found the problem. And thanks; it’s a help to the rotary community, and again proves the value of a list such as this.

Al G

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