Questions on performance tables in the manual
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Two questions on the performance tables in the manual (pages 68-71):
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It is, of course, expected that the turbonormalized variant will perform better in general than the normally aspirated variant. However, I've noticed that even at altitudes where the normally aspirated variant can still achieve the same manifold pressure as the turbonormalized variant, the TAS given for the turbonormalized variant is often considerably higher -- though also with a higher fuel flow.
For example, take the 55% power setting at 6,000 feet. For the normally aspirated variant, this is given as 21 inHg, 2,100 RPM, 56 PPH, 134 KTAS. For the turbonormalized variant, it is given as 21 inHg, 2,100 RPM, 70 PPH, 151 KTAS. I note that the TAS is significantly higher, though the fuel flow is, too. Is the assumption here that the normally aspirated variant is leaned differently than the turbonormalized variant? (Why?)
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I've noticed one particular number in the tables for the turbonormalized variant that seems to high: This is the TAS for 65% power at 24,000 feet, which is given as 235 KTAS. At the higher power setting of 75%, the TAS at the same altitude is given as only 218 KTAS. Seems like the 235 KTAS is a typo?
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I've already fixed that typo, but thanks for pointing it out. My immediate thought on differences goes to the most obvious difference, which is the propeller. The turbonormalized conversation is equipped with a propeller with a greater coarse pitch angle, which would allow it to perform better at cruising speeds, and especially at lower propeller RPM. Moving a few pounds up front with the addition of the turbocharger would also cause a slight improvement. Tip-tanks would also reduce wingtip vortices, and increase efficiency (not sure how this compares with the drag they introduce). I would have to think long and hard about your leaning question, but I apologize that I'm headed to bed right now. I will no doubt be thinking about it in my sleep. It's also possible that the commercial endeavor that created the turbonormalized conversion introduced some other speed modifications (like gap seals), or tabulated their data in a slightly different way to the original POH. Without making an accusations, obviously, the incentive is there to make the conversion look as good as possible, which is always worth keeping in mind when comparing such things. I'll see if I can find more information tomorrow.
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I've already fixed that typo, but thanks for pointing it out. My immediate thought on differences goes to the most obvious difference, which is the propeller. The turbonormalized conversation is equipped with a propeller with a greater coarse pitch angle, which would allow it to perform better at cruising speeds, and especially at lower propeller RPM. Moving a few pounds up front with the addition of the turbocharger would also cause a slight improvement. Tip-tanks would also reduce wingtip vortices, and increase efficiency (not sure how this compares with the drag they introduce). I would have to think long and hard about your leaning question, but I apologize that I'm headed to bed right now. I will no doubt be thinking about it in my sleep. It's also possible that the commercial endeavor that created the turbonormalized conversion introduced some other speed modifications (like gap seals), or tabulated their data in a slightly different way to the original POH. Without making an accusations, obviously, the incentive is there to make the conversion look as good as possible, which is always worth keeping in mind when comparing such things. I'll see if I can find more information tomorrow.
@Black-Square said in Questions on performance tables in the manual:
I've already fixed that typo, but thanks for pointing it out.
Great, thanks for confirming!
My immediate thought on differences goes to the most obvious difference, which is the propeller. The turbonormalized conversation is equipped with a propeller with a greater coarse pitch angle, which would allow it to perform better at cruising speeds, and especially at lower propeller RPM.
Interesting -- wasn't aware of this difference!
Thanks also for your thoughts on the other possible differences. Whatever the reason, it seems obvious that the two sets of tables were obtained using different leaning strategies. If I find some time I may try and work out how the leaning was done in each case (i.e. how many degrees rich or lean of peak).
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I've done some tests on this, and also looked at the tables more closely.
Interestingly, the only table in which there's a significant discrepancy in fuel flows between the normally aspirated (NA) and the turbonormalized (TN) variant (at altitudes where the NA variant can still achieve the same MAP as the TN variant) is the 55% power table.
For example, at 6,000 feet, 55% power, the fuel flow for the NA variant is 56 PPH, whereas the fuel flow for the TN variant is 70 PPH.
At 65% power, on the other hand, the fuel flow given for both variants is exactly the same: 87 PPH.
I did some testing in the sim. It looks to me as if, in the 55% power table, the fuel flow for the NA variant corresponds to a 100 LOP lean, whereas the fuel flow for the TN variant corresponds to a 50 ROP lean.
The fuel flow in the 65% power table (which, as noted is the same for both variants) seems to correspond to a 50 ROP lean.
So there does seem to be a difference in leaning strategies, but only for the 55% power table, where the NA variant uses LOP, whereas everything else seems to be ROP.
