First let me point out the big error in the procedure I wrote:

 

-The center of the combustion chamber is 60^o on the rotor from the tip seal; or 120^o (should read 180) on the E-shaft.

- Count the total number of teeth on the ring gear, and divide by 3 (should read 2). Now rotate the engine by that number of teeth (either direction) and rotor 1 will be very close to TDC.

 

I wouldn’t argue the precision of the method; but for the housings you show, the offset appears to be much less than 10 degrees. The e-shaft TDC offset would then be 1 ½ times that as you split the difference between the plugs and multiply by 3:1 ratio. And being a few degrees off on the rotary is not of much consequence.

 

Ultimately the timing at which we operate is determined by the timing curve built in to the engine controller.  I’m pretty sure Tracy has modified that some over the years.  And then there in-flight tuning of the timing at cruise condition that you may like to look at.

 

When I got my 20B with a 12A front housing from Atkins, the relationship of the pointer to the marks on the pulley made no sense. I used my simple method to determine a TDC mark, and worked from there.  On the dyno we put a pressure transducer into a trailing plug hole and adjusted timing a bit while operating (I don’t recall conditions – I think 22” MAP) to get the pressure trace to look right. We chose a point 2 steps retarded on the EC2, - 3.75^o.

 

Subsequently I did some variation at cruise conditions to see effect on performance, and again ended up at two steps of retarding, although I found performance change almost undetectable over a 3-4 step range (~7^o) to either side. So that setting is pretty consistent with Lynn’s data that the method leads to a TDC that was a bit BTDC; but is of no consequence.

 

That’s my story, and I’ sticking to itJ.

 

Al

 

 

 

 

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Steven W. Boese
Sent: Sunday, February 27, 2011 10:38 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Steve Boese TDC Finder

 

Al,

 

Depending on exactly which housings you have, that method may be off by more than 10 degrees.  See the attached photo.

 

Steve Boese

 

 

Of course there is always the way that doesn’t require any parts – well; maybe a piece of wire, or something to use as a pointer near the teeth on the flywheel ring gear.

 

-Remove the rotor 1 spark plugs (and one from rotor 2 to relieve the compression).

-Turn the engine until the tip seal on rotor 1 is in the center of the leading plug hole.  Mark the gear tooth at the pointer.

-Turn the engine until the same tip seal is visible in the trailing plug hole. Mark the tooth at the pointer.

-Now turn the engine back to the gear tooth at the pointer that is half way between the 2 marked.  The tip seal is now at TDC.

-The center of the combustion chamber is 60^o on the rotor from the tip seal; or 120^o on the E-shaft.

- Count the total number of teeth on the ring gear, and divide by 3. Now rotate the engine by that number of teeth (either direction) and rotor 1 will be at TDC.

 

Al G

 

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Steven W. Boese
Sent: Sunday, February 27, 2011 4:10 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Steve Boese TDC Finder

 

I like this better than what I came up with.  It will be easier to find the parts and assembly will also be easier since none of the initial parts need to be modified.  As long as one is careful not to draw the water or oil into the engine, it will also be easier to use.  Although there always seems to be a better way to do something, this seems as good and simple as it can get.  No more excuses for not knowing where TDC is.

 

Steve Boese