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AoA Oddities

S

SmB

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First an introduction: I only recently bought FSX with Acceleration after a long hiatus away from FS. I was quite active in FS from SubLogic days to CFS2/3 (I was a beta tester on CFS3), and I did quite a bit with figuring out .air files (I wrote a paper on helo .air file entries once: "1400-1404 Technical Notes" before MS released any info on this stuff).

I enjoy tinkering with planes as much as flying around, though I am in no sense an engineer, aeronautical or otherwise!

Anyway, I've been playing around with J.E. Narcizo's very nice freeware Piper PA18-150 Super Cub. Mostly I upgraded the engine to the 160 HP version and put on an 82" MacCauley (Borer) climb prop which is used in many Super Cubs in Alaska for short field takeoffs. I can average now 180-200 foot takeoffs and climb at 1100-1300 fpm @ 75mph. Cruise is 100mph reduced from 115mph in Narcizo's flight model. (This was mostly through the aircraft.cfg entries and tables 511-512 in the .air file.)

With the new prop, engine and cruise speed, my PA18-160 shows a pitch and AoA of about -1.4 at cruise (100mph). Narcizo's PA18-150 is about zero at 115mph cruise (but also around -1.2 at 100mph). Interestingly, if I beef up the default J3 Cub engine, it shows a little over -4.00 AoA at 100mph.

Here's my question: Should I worry about this? I know there's a pitch/AoA entry in record 1101 and I suspect record 404 or 473 has some influence on this, but I haven't fooled with any of these yet.

The Super Cub is essentially a J3 Cub airframe. In the real-world, did they do something to adjust this like HStab incidence?

I feel like it would be nice to get AoA at zero at cruise speed. Any tips on how to do that?

Thanks in advance,

Steve
 
SmB said:
I feel like it would be nice to get AoA at zero at cruise speed. Any tips on how to do that?
Click to expand...


Why is that? Does this reflect the real AoA at that q and CL?

Generic Tip #1 CL vs AoA table 404
 
Mr. Baugh?

If so, your papers are very good reading.

This may be overly simplistic, but why not change the angle of incidence if you are just trying to adjust the fuselage angle? How does the real aircraft behave under these conditions?

As stated earlier, adjusting the CL graph may also work but I believe that would cause a lot of other problems. I have one other idea, but it sounds a little silly, so I need to check it out a bit before making a fool of myself.

- Ivan.
 
Hi Guys,

Why change the cruise AoA? Just being picky I guess. Even more, I'm just trying to get re-acquainted with .air files after a long absence and thought I'd ask.

How does the real aircraft behave under these conditions?
Click to expand...

I don't know that. I have collected as much data as I can but I was just guessing that 4-5 degree pitch down at cruise would be odd were I to use the FSX J3 Cub FDE files as a base.

(I'm also learning XML for coding simple gauges, so I have much to play with.)

Thanks for the compliment Ivan. Yes, it is I....

Steve
 
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The simplest approach would be to recalibrate your AoA gauge needle to show the results you want. IOW, -4 AoA would be displayed as 0 AoA.
 
Just a follow up. Table 404 did the trick. This Super Cub is just about done now. A fun little kite to snoop around the hills in Alaska. :)

Thanks,

Steve
 
I have seen videos of some of these old Cubs with monster balloon tyres.
They can take off in about 20 feet with a bit of headwind!
How does your version do?

- Ivan.
 
I have seen videos of some of these old Cubs with monster balloon tyres.
They can take off in about 20 feet with a bit of headwind!
How does your version do?

Although adjusting the CL Graph works, I believe it is not the best way to go because you are probably shifting the entire graph or changing the slope and if the original was correct, then these changes would make the new CL not agree with the drag at a particular AoA. Changing the wing's angle of incidence would not affect the lift / drag relationship.

What I was thinking of earlier was that there are a couple pitch moment parameters in Record 1101 and you can balance their effect with a different angle of incidence on Record 1205 for the elevator. That lets you tune the pitch effect at a particular velocity because Pitch from 1101 doesn't change with velocity while 1205 does. I don't know that this would work any better than what you already have though.

- Ivan.
 
Hi Ivan,

Thanks very much for the ideas! I've seen some of those videos too. Incredible!

I've actually compromised a bit on the take-off length. There's a lot of RW info on the Borer prop mods on the internet (e.g., supercub.org). With this 82" prop at 41-42" pitch, the takeoff is incredible but cruise speed is around 90mph. I wanted a cruise of 100 so I put on the 44" pitch prop (and the 160hp Lycoming). :) With no wind my takeoff is around 180 feet, which is still pretty good.

I followed up a little today on the ideas you gave me above and will keep poking around in the future with your suggestions. My PA18-160 .air file has no record 1205 in it, and a quick check shows about 5-6 of the MS default FSX .air files are also missing this section in them also. But I see the offset in 1205 in the aam and aired .ini files to which you refer. It might be worth adding this and a few other sections to see the effects. Tweaking .air / .cfg files is a neverending story! :)

I did need to adjust the slope in record 404 anyway though. I graphed the J3 Cub, C172, PA28-180, and Maule's and the CL was way lower at the cruise AoA than all of them, then it shot up to higher than them all at stall AoA (about 13deg). Anyway, right now my cruise pitch is about 0.15 and AoA is about 0.1 which I can live with.

My next project is to find a modeller to make a nice Aviat Husky for me to play with. It's got a bigger engine and constant speed prop with higher cruise. Seems like a nice little bush bird....

Best,

Steve

Edited P.S.

Take a look at this PIREP on the Husky sometime by Bud Davisson. He says the 31" tundra tires actually try to fly when using rudder! Now we just HAVE to model that.

http://www.airbum.com/pireps/pirep.Husky.html
 
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