D8 config with BLI flies above min allowed cruise mach number

[mayork]

optimal D8 flies at M 0.82 not 0.72 and the M 0.8 D8 flies at M = 0.9

[mayork]

UPDATE: Only the D8 models see this weird behavior...

[1ozturkbe]

Whaaaa? The only reason I would understand this is if the engines preferred the higher total pressure... Do you see the same trend in D8 with podded engines (no BLI)?

[mayork]

yes same trend with rear podded engines

[1ozturkbe]

D8 with wing engines flies at 0.76 for reference. Which is really odd because the rear podded engine D8 should have similar Mach numbers.

[mayork]

I'm going to note some key differences in the 737 and Mach 0.8 D8 flight profiles here:

• D8 ends cruise at 44k while 737 is 37k
• D8 climb rates are 2500, 1783, 786. 737 is 2500, 500, 500 -D8 climb M is .52, .69, .87 while 73 is .47, .56, .66
• each of the D8 cruise segments is 0.2 hours shorter and climb is shorter so overall flight is like 35 min less
• drag on the D8 is 6000N less despite the higher speed
• Wing profile drag coefficient is .009 for D8 and .006 for the 737 which makes sense
• D8 induced drag in cruise is .013 while 73 is .011...unsure what the discrepancy is
• D8 fuse Cd is .009 73 is .008 -D8 HT Cd is .006 73 is .0059
• VT cd equal to 3 sig figs
• D8 cruise FPR 1.7, 73 is 1.625
• LPC pressure ratio is ~6.2 for cruise D8 and ~4.8 for 73
• HPC pressure ratio is ~4.95 for 737 and 5.68 for D8 -D8 eng sys is 6666 lbf and 73 is 7900 lbf (largely due to different pylon weight fractions)
• 737 wing are is 10m^2 larger at 91.7 m^2
• D8 cruise Re: 25755206, 73 cruise Re: 24406273 (~5% difference)

Note that the input "on design" OPR is equal for the two aircraft so the differences are largely due to internal optimization

[mayork]

@1ozturkbe let me know your thoughts about this? I'm really not sure if this is a bug or a result or what.

[mayork]

are we using the extra fuselage lift to fly higher and then faster due to lower drag? This may be a legit result...

[1ozturkbe]

The D8 fuselage lift fraction is higher, but its drag coefficient is also higher by about the same factor. It is likely that the D8 experiences higher benefit from a smaller wing because of this (because CDi goes with CL^2 whereas the fuselage drag goes with S (linear vs. quadratic). It would surprise me if this was the reason. Could easily test by setting the fuselage lift and drag coefficients equal for the two and rerunning.

[1ozturkbe]

What is equally puzzling to me is that this phenomenon does not occur with D8_eng_wing, whereas it occurs with every other configuration in the D8 family. So odd.

[mayork]

Going to note heee to check the results of rear podded D8 altitude and compare to full D8...one effect could be lower external static pressure helping engine performance with BLI active

[mayork]

@1ozturkbe check this out M08_D8_wing_profile_drag.pdf It looks extremely reasonable. It's a plot of the drag fit I made fixing the Reynolds number, tau, and CL to those of the model for the first cruise segment and then sweeping Mach to see what the polar looks like

[mayork]

rear podded (no BLI) D8 does fly slower (M=0.82) and only goes up to ~40,000'

[mayork]

interesting altitude profile for the M08_D8...

[mayork]

here it is for the 737-800 Screen Shot 2017-05-09 at 10.24.54 AM.PDF

and here it is for the M08 D8 with engines under the wing Screen Shot 2017-05-09 at 10.27.06 AM.PDF

and here it is for the M08 D8 with rear podded engines

[mayork]

So we can force the cruise mach number to be 0.8. If we do that we get this flight profile

cruise reynolds number drops to: 21240186 climb rates become 2500, 1229, 790 total fuel burn: 25034 lbf (vs 24940.14 for the faster case)

[mayork]

@1ozturkbe should we just hard set the speed? It's not a huge fuel savings...

[mayork]

it isn't tail drag.. M08_D8_VT_profile_drag.pdf

I'll now try and implement Woody's suggested fuselage drag model

[1ozturkbe]

K. Just to let you know, it is my goal to remove the fBLI drag correction today/tomorrow in favor of some approximation of fuselage BL defect ingestion.

[1ozturkbe]

This will augment the Mach drag rise that hopefully will come from coupling the wing and fuselage drag models. But my updates pend the nomial_math issues...

[mayork]

if we can do that, cool...but I think the main concern here was the link between fuselage drag and wing area not the way we were handling D8 drag reduction

[1ozturkbe]

yeah for sure, but it is also super sketchy that we have a BLI drag reduction that in no way depends on the engine parameters or the flow around the body. I think I can make all of these come together in a way that makes sense.

[mayork]

the simple drag model Woody suggested won't work because of how we take into account the wing sweep mach reduction...working on a new solution now

[mayork]

UPDATE: after https://github.com/hoburg/d8/commit/5f1b370f2743efff07aa2c32292e9ac3f662e178 we still fly at slightly over M=0.84 for the Mmin = 0.8 D8 case

[mayork]

interesting note, after the commit mentioned above only the D8 with BLI flies above the min mach number, everything else flies at the Mcruisemin.

The M=0.72 D8 wants to fly at M=0.78.

Here are plots for the M=0.8 D8 M08_D8_altitude_profile.pdf M08_D8_fuse_drag.pdf M08_D8_HT_profile_drag.pdf M08_D8_VT_profile_drag.pdf M08_D8_wing_profile_drag.pdf This all looks legit to me. Next thing i'm going to do is dig into the engine

[1ozturkbe]

Just as a clarification, now Cdfuse is based on Rfuse? Wouldn't it have been better to have based it on Afuse?

[mayork]

Didn't realize Afuse is a variable...right now it is based on l_fuse * R_fuse, which i suspect is equivalent to Afuse

[1ozturkbe]

Afuse is a cross-sectional parameter, which would simulate a kind of form factor in a drag model. Potentially useful.

[mayork]

oh true I didn't consider that...I scaled off of L and R in an attempt to account for increased BL growth along a long fuselage...I suspect it is not overly relevant which option is used?

[1ozturkbe]

Actually, I think if you divide the coefficient by a factor of Rfuse, and put Rfuse^2 into the drag model, it would improve it already.

[1ozturkbe]

But the principle is this model here, where l/d is the fineness ratio, and F is the form factor:

[mayork]

where is that from?

[1ozturkbe]

http://www.ae.metu.edu.tr/~ae451/lecture11_aerodynamics.pdf I think they took the material from Raymer, or Shevell's 1983 book.

[mayork]

@1ozturkbe pull engine, fixed a small typo in a fit. Although it appears to change little.

[mayork]

I did some digging in the engine, didn't see anything that looked off except for that typo i fixed in one spot (which i why I said to pull)...

[mayork]

I think i might have solved this...getting deep into the engine code right now

[mayork]

@1ozturkbe pull engine. the M 0.8 case now flies at the min and 0.72 case flies at 0.725. I'm good closing this if you are.

[1ozturkbe]

What's the update in the engine, I'm curious?

On Wed, May 10, 2017, 21:33 mayork notifications@github.com wrote:

@1ozturkbe https://github.com/1ozturkbe pull engine. the M 0.8 case now flies at the min and 0.72 case flies at 0.725. I'm good closing this if you are.

— You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub https://github.com/hoburg/d8/issues/64#issuecomment-300657673, or mute the thread https://github.com/notifications/unsubscribe-auth/AO9SX0j7AgMGq8laWJs8g_-zTIyltaMYks5r4mVagaJpZM4NUMe2 .

[mayork]

this

            fanmap = [
                self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}']) == (1.05*self.engineP['N_f']**.0871)**10,
                (self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}']))**.1 <= 1.1*(1.06 * (self.engineP['m_{tild_f}'])**0.137),
                (self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}']))*.1 >= .9*(1.06 * (self.engineP['m_{tild_f}'])**0.137),

became this

            fanmap = [
                self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}']) == (1.05*self.engineP['N_f']**.0871)**10,
                (self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}'])) <= 1.1*(1.06 * (self.engineP['m_{tild_f}'])**0.137)**10,
                (self.engineP['\pi_f']*(1.7/self.fanmap['\pi_{f_D}'])) >= .9*(1.06 * (self.engineP['m_{tild_f}'])**0.137)**10,

I had put the fit exponent on the wrong side but only in the fan map which allowed like a 2.5x variation in FPR instead of a plus minus 10% variation in FPR

Very minor, hard to catch. Took me a while to hunt it down.

[1ozturkbe]

Noice. At least you got it!

[mayork]

yep. The fuselage drag change also contributed to fixing this...scaling off of fuselage area drop mach by .02, scaling the drag with mach another .02, and the final .02 was from the engine bug. Curious that only the D8 seemed to take advantage of the extra FPR variation. Anyways, closing this now.