Interesting HT behavior.

[1ozturkbe]

Just wanted to post this as an issue, because I have not observed this behavior before, going from a M072 737 to a wing-engine D8. The discussion follows from the comments under commit 70e5e92d.

[1ozturkbe]

Main observation: m_ratio higher in 737 (1.41 vs 1.11), which seems to help SM constraint satisfaction. This seems counter-intuitive, because lower AR tails have lower effectiveness, but higher m_ratio? As a note, the SM constraint is:

TCS([aircraft['SM_{min}'] + aircraft['\\Delta x_{CG}']/aircraft.wing['mac'] \
                 + self.wingP['c_{m_{w}}']/aircraft.wing['C_{L_{wmax}}'] <= \
                                            aircraft.HT['V_{ht}']*aircraft.HT['m_{ratio}'] +\
                                            aircraft.HT['V_{ht}']*aircraft.HT['C_{L_{hmax}}']/aircraft.wing['C_{L_{wmax}}']]), # [SP]

[1ozturkbe]

Btw Martin, I am almost convinced that the m_{ratio} on the RHS of the equation should be a division, not a multiplication. Am I mistaken? Lmk. Might be significantly influencing our results.

[mayork]

i don't think so. I just checked and it aligns with Drela's boom flex write up

[mayork]

@1ozturkbe can you get VSP shots of optimal 737 w/fixed BPR at a min SM of 0.05 and 0.15? The behavior is not what I expect

[1ozturkbe]

Saw Drela's email about the different SMs between the M0.8 and 0.72 aircraft (0.15 vs 0.05 respectively). Makes sense. I guess we should just change all of the subs to reflect this. I will post a VSP figure here for the 73s with the different SMs, and see what the difference is.

[mayork]

well run it first and look at what happens....unless i'm missing something the higher static margin made fuel burn better?

[1ozturkbe]

I will post this incrementally. For a M0.8 73 with SM = 0.05, the Wf is 38700lbs.

[1ozturkbe]

For M0.8 737 with SM = 0.15, the fuel weight goes down to 35236lbs. Vht grows from 0.5 to 0.6

[1ozturkbe]

So yes, a higher static margin makes the fuel burn better. Is this a weird finding? I'm not sure... Truly odd that pushing an already tight constraint actually improves the performance though.

[1ozturkbe]

I will generate the standard killer chart and its VSPs, and then analyse what is going on.

[1ozturkbe]

I am having so much trouble actually generating these figures. Do you occasionally get UNKNOWNs in your solutions? I fail to understand why there should be any difference between our solve performance.

[1ozturkbe]

Anyways, finally managed to generate new killer chart with the new SM. the results are... interesting... D8_standard_killer_chart.pdf

[1ozturkbe]

Slowing down now (and reducing the SM) actually increases fuel burn? Wowowow. The rest of the findings follow TASOPT/NASA roughly, other than the 2010 engines bit. what is the difference in this step @mayork?

[1ozturkbe]

I am looking at the results now: TSFC: 0.62 -> 0.59 Pretty much identical fuselages, vertical tails, horizontal tails L/D drop from 22.1 to 21.5, identical drags during the cruise phase. utterly puzzling why the L/D doesn't improve with the slowdown.

[1ozturkbe]

Fuselage drag doesn't scale well... Fuselage drag 10% higher for the slower aircraft! Needs to be fixed.

[1ozturkbe]

Why are the fuselage drag coefficients higher for the M072 D8 vs. the M08 D8, especially since we have the Mach scaling? I think something wonky is going on. But I have to get some sleep now.

[mayork]

Damn just realized I think I normalized the TASOPT value by RfuseLfuse not DfuseLfuse. Will check this morning.

[mayork]

As far as UNKOWN what environment are you running in?

[1ozturkbe]

@mayork I still think the fuselage drag coefficient is the reason why we are getting a wonky result from slowing down. Why is the drag coefficient of the M0.72 737 16% higher than the drag coefficient of the M0.8 737? The higher drag results in only a 6% drag reduction for the fuselage for flying 10% slower, which would explain why we are getting a net penalty.

[mayork]

I don't think it's wrong. those values are directly from TASOPT....it's still probably less drag even tho the coefficient is higher

[1ozturkbe]

It's still funny to me that as SMmin goes up from 0.05 to 0.15, the fuel weight goes down... I just replicated the results above for a second time. I will be digging into this...

[mayork]

@1ozturkbe is this still active or can it be closed?

[1ozturkbe]

This issue needs to be addressed, still active.

[1ozturkbe]

no longer able to replicate this. I'm not sure exactly why, but it seems well-behaved (tail grows, fuel weight increases). Closing.