So, would it be fair to say that increasing fuel flow in the full rich but not-quite-rich-enough case would only retard thetaPP maybe two degrees at most?
Dan, unless I am misunderstanding your statement, I suggest that in the case of where a mixture is
ideal full rich at takeoff power and reduced back say 10%, this has a more significant effect than the next 10% getting leaner. As for two degrees at the most, well that is not insignificant if that is what you are suggesting.
In other words, the last extra dose is what matters the most. To start splitting the numbers up it was something like this, roughly, from full rich to
As luck would have it, so you are pretty lucky, George Braly sent me some data from fuels testing this morning. And what do you know?
x100
The exact question you are asking has its answer hidden within! Now I am under NDA's so I cant publish it here but the data I can take from this confirms what I have said previously. This is from certification testing and can be relied on. The data collection was for completely different and unrelated purposes and the answers yielded are a serendipity of this.
The recent Avstar and Precision FCU's have had under-flows around 15%, and the data points I have of ThetaPP are tabled below which reflect this range. These are averaged over a few runs and cylinder to cylinder variation is to be expected and also from one sample to the next can be a bit of variation, so please anyone reading tis, these are fuzzy numbers that vary by the milliseconds (about 20 times a second) so be careful how you read this.
This is comparing full rich to FR-10% and FR-20% of flow and the % of ThetaPP compared to the previous position.
CYLINDER Delta TPP-10% Delta TPP-20%
C1 >> 88.45 >> 92.50
C2 >> 85.00 >> ----
C3 >> 89.40 >> 95.25
C4 >> 90.55 >> 97.30
C5* >> 97.85* >> 99.10*
C6 >> 92.70 >> 95.50
* this cylinder has an anomaly that could easily be explained by the pressure sensors and the fact this one has been deliberately subjected to a lot of abuse. In any case it shows a similar trend, but beware the raw data.
Interesting parameters here are the EGT values at each point represent a full rich, 1250-1320dF range using a standard 8.5:1 compression ratio engine. Assume this to be about 250-300dF ROP mark. The -10% flow figure was yielding around 70-75dF higher, so about 225-175dF ROP, and the -20% flow was about 150dF higher or about 150-100dF ROP. The ThetaPP shift overall was around the 2.5 to 3 degree range, a little less than the observations I made during less than ideal examples to extract data. Remember this stuff bounces around all the time so no hard numbers, fuzzy averages. Mid you these are not leaned off as much either, so lower changes of TPP are to be expected.
Interesting that if you compare this fresh data from the dyno today, the indicators I have been using to determine who has enough flow and who does not is validated. The concerning thing is that for as long as any of us can remember, Lycomings have flowed well, and TCM not. Now TCM publically declare a bit more would be nice, and Lycoming are shipping variations. Who the heck would know??
I hope that has been as fun for everyone as it was me in collating and delivering the data.
Last point, the thermal transfer is affected by the boundary layer, the higher the pressure, the smaller the layer, the more transfer.