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Date: Wed, 16 May 2012 15:19:36 -0400
From: Ernest Christley <echristley@att.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Why Apha N??  : [FlyRotary] Re: With great power
 comes...the ability to muck things up
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Ed Anderson wrote:
> Good to hear you are back in  business, Ernest
> 
> I do have one question - why did you switch from Speed Density to Alpha
> N. For sea level applications high power operations I can sort of see
> using Alpha N - but for aircraft application I would have assumed Speed
> Density would be a better choice.  Would be interested in why you went
> with Alpha N
> 

I don't have a mixture control.  The throttle plate is fully opened at 50% of the throttle lever's movement.  The design
is to run the engine as close to lean cutoff as it will smoothly run up to point where the plate is fully open.  Then I
start making it richer, quickly skipping the peak EGT with a retarded spark, until I'm at 12.5 AFR or whatever makes max
power when I have the throttle full forward.

To accomplish all this, I will actually have to use a mixture of Alpha-N (AN) and Speed-Density (SD).  The MegaSquirt
lets you combine a primary and a secondary tune table with various percentages of each.  Backing up one more step, you
have to have a basic tune on each method before you can sanely combine them.

So here is the game plan.

1)  I make AN work as described.  Lean up to 50% throttle travel, then go rich.  I misspoke in the previous email.  AN
is what I have working now.  As long as I stay at sea-level pressure, this will work reasonably well.

2) I create a separate tune using SD.  This tune will run at best power across the throttle control range.  Once the
throttle control hits 50% and the plate is completely open, the throttle will add no more power; the computer will be
injecting the same amount of fuel from 50% to 100% of the throttle travel, because it will be ingesting the same amount
of air.

3) Now I combine the two.  Conceptually, the SD tune will modify the AN tune to compensate for density altitude.  *When
I'm strafing Myrtle Beach at 75% throttle to impress all the bikini clad beauties, the SD tune sees sea-level pressure
and tells the computer to give 100% VE.  The AN tune sees the throttle at 75% and tells the computer to give 75% VE.
The result is the computer give 75% VE.  Hot and flustered from the bikini clad beauties, and having been slapped on the
back of the head from my wife not appreciating them, I climb to the cooler air at 12,000.  SD sees half the pressure
50%, so it tells the computer to give 50% VE.  I still have the throttle at 75%, so AN says 75%.  The result will be the
computer sets the VE at 37.5, and turns the injectors on just long enough to give me 75% of the power available at that
altitude.

TBD are what the SD tune looks like (I can pull numbers from the AN tune to get started), and what the combination looks
like.


*All numbers represented are fictional.  Any resemblance to actual numbers is purely coincidental.