109G-2 vs 109G-6 and parasite drag

[gripen]

Note the small change in difference in CD between the aircraft at sea level (.027-.027=.001 diff) vs at 22k (.032-.030=.002 diff).  That difference though accounts for why we're seeing the slight difference in max airspeed differential between the two aircraft from sea level vs. that of 22k.

I got the Cd0 around 0,027 for the G-2 and 0,029 for the G-6 using real life data at ground level. And Mtt specification gives delta Cd0 +0,001682 for the 2x13mm Mgs and 0,001807 for the FuG 16. The wing bulges would give a bit more so the ballpark is the very same ie delta 0,002 difference in Cd0 between the G-2 and G-6.

[dtango]

Thanks for the info gripen.

Nice to see that the AH variation is in the ballpark with Mtt data.

Tango, XO
412th FS Braunco Mustangs

[gripen]

Yep, that has been my impression as well 

[Anaxogoras]

Forgive me for punting my own topic, but there was something that wasn't explained.

The external drop-tank rack is also controversial because of the drag it causes even after the drop tank is released.  With the rack, I was able to get the G-6 up to 390mph at 22k ft, 5 mph slower than without, before running out of fuel (25% loadout).  At sea level, the G-6 also loses 5mph with the drop-tank rack.

If the drop tank rack slows the 109G-6 down by 5mph at sea level, shouldn't it slow it down by more than 5mph at 22k ft?  This has to be an error because it contradicts everything that you explained above.

[dtango]

Forgive me for punting my own topic, but there was something that wasn't explained.
If the drop tank rack slows the 109G-6 down by 5mph at sea level, shouldn't it slow it down by more than 5mph at 22k ft?  This has to be an error because it contradicts everything that you explained above.

Well firstly I have no idea how this contradicts what I've said .  Whatever the case let's work the question first.  Why would you think the rack slowing down the G-6 by 5mph at sea level would have to slow it down even more than 5mph at 22k?

Tango, XO
412th FS Braunco Mustangs

[Motherland]

Well firstly I have no idea how this contradicts what I've said .  Whatever the case let's work the question first.  Why would you think the rack slowing down the G-6 by 5mph at sea level would have to slow it down even more than 5mph at 22k?

Tango, XO
412th FS Braunco Mustangs

Because the bulges slow it down more at high altitude that low altitude. Why wouldn't it be the same for the drop tank racks?

[dtango]

Because the bulges slow it down more at high altitude that low altitude. Why wouldn't it be the same for the drop tank racks?

That's not what I said.  I said the difference in the slowdown between the G6 and G2 high alt and low alt is due to the lift dependent viscous drag increase with increasing aoa.

Here's another way to understand it:

CD = CD0 + CDi

CD is total drag coeff.  CD0 is the parasite drag coeff.  CDi is the lift dependent drag coefficient. 

CD0 doesn't change from low alt to high alt.  It remains the same no matter the alt.  Adding bulges and racks changes CD0 but this is essentially constant for a given airframe configuration no matter what alt you're at.  Additional racks increase the CD0, but this CD0 is the same at low alt as it is at high alt. 

CDi is more complex than people realize.  CDi includes both the usual induced drag people understand as well as lift dependent viscous drag.  CDi varies with aoa.  Since aoa is a function of air density to produce a certain amount of lift, it therefore also fluctuates with altitude.  This is what is varying between the G2 and G6 low alt vs high alt.

Tango, XO
412th FS Braunco Mustangs

[TimRas]

Why would you think the rack slowing down the G-6 by 5mph at sea level would have to slow it down even more than 5mph at 22k?
412th FS Braunco Mustangs

The ratio of speeds should remain constant, meaning that the difference is not constant.

[dtango]

The ratio of speeds should remain constant, meaning that the difference is not constant.

Agreed if nothing else changed.  However we have no idea how prop efficiency or thrust varies for the G6 in AH with altitude and airspeed.  Secondarily there's also a change in aoa between alts as well and I have no clue how AH defines how CDi changes with aoa.  There's no easy way to evaluate what the difference in speed should be exactly.  Instead I'm trying to address the concept with Anax regarding the nuances of drag, that there's more complexity to it.

Tango, XO
412th FS Braunco Mustangs

[Yossarian]

Sorry if this is a bit of a hijack, but here's a really excellent website I've found for aerodynamics:
http://www.av8n.com/how/

<S>

Yossarian

[gripen]

I got about -18km/h at sea level and -25km/h at FTH 6600m, assuming everything else equal but Cd0 0,027=>0,029, weight 3000kg=>3100kg and about 10% increase in Cd0 from sealevel to FTH due to compressibility.

[gripen]

I got about -18km/h at sea level and -25km/h at FTH 6600m, assuming everything else equal but Cd0 0,027=>0,029, weight 3000kg=>3100kg and about 10% increase in Cd0 from sealevel to FTH due to compressibility.

Ah... the difference appeared to be too high and found an error on power values, after correction I got -13km/h at 0m and -17km/h at 6600m. Otherwise the assumptions being the same.