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FSXA Airfoiltools => air file?

Heretic

Heretic

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I've got an aircraft (Aero L-39C) which uses the NACA 64-012A airfoil. Said airfoil can be found on Airfoiltools[1] and convieniently includes plots for different Reynolds numbers.

But which plot am I supposed to use?

Playing with the Reynolds calculator[2] and a typical IAS cruise speed (150 m/s), the L-39's MAC chord (7ft; source [3]) and assuming a kinematic velocity at -10°C, I get a Reynolds number of over 32 million.
Therefor, I suppose that the plots for Re=1000000 are the ones I need if I want to tweak the airfile for cruise?
Also, NCrit is the level of turbulence ranging from 1 (excessive turbulence) to 9 (no turbulence at all), correct? Should I go with NCrit=9 or NCrit=5 which is the other avialable plot?

So once I have the alpha-related curves in the .air file, I have to tweak the velocity-related coefficients. Any tips for a suitable strategy to achieve that?

The engine-related tables are already done if that helps. Getting the right cruise speed out of a given RPM (and maybe the right acceleration after a RPM increase).



[1] http://airfoiltools.com/airfoil/details?airfoil=n64012a-il
[2] http://airfoiltools.com/calculator/reynoldsnumber
[3] http://www.skytamer.com/Aero_Vodochody_L-39C.html
 
As CL vs alpha (table 404) pretty much determines your aircraft's stall speed and AOA at stall I'd start with the plot that gets you closest to that first and work back from there.
 
attention: Cl vs Alpha for a given airfoil is different from what CL vs Alpha for the whole aircraft is. This is what FS TBL404 is. You could use DATCOM+ or AVL and include therefore the effects of tail plane. Alternatively you can superimpose the curves in excel for main wing and tail... Personally I used Beckwiths workbook in the past and manually tweaked stall behavior... it works rather good! What you are concerned with is the slope of the curve most of all. knowing your stall angle you know where it peaks, then you can change the slope to fit for stall speed.... You can use the airfoil curve together with the wing incidence to have an idea where the 0-AoA CL should be....
 
mariopilot said:
You could use DATCOM+ or AVL and include therefore the effects of tail plane.
Click to expand...

I want to give XFLR a try because it's free and easily acessible, but I have yet to figure out how to load predefined airfoils into it.

Jerry's FDWB (for the P-51?) is present as well, but LibreOffice has problems with the macros.
 
I agree with Mariopilot. Most airfoil data comes from wind tunnels and, usually, have the characteristics of an infinite wing with much too low a Reynolds number.
Infinite wings do not exist in the real world. Real wings should take into account taper, sweep angle and tip washout if present. Low Reynolds number usually results in unrealistic post-stall characteristics. Correct Reynolds numbers in a tunnel are unlikely because the model would have to be full size and the velocity actual (you can have a half-size model but the airflow would have to be twice as fast)
Airfoiltools seems to be slanted to model airplanes.
Biggest issue is what Mariopilot pointed out, 404 is for the whole airplane. Since tailplanes typically operate at negative lift to achieve trimmed pitch stability, the total lift is less than the wing alone gives.
All this means that you can probably use the 1 mill Re number lift curve slope, but I'd reduce CL max a small amount. A tool like AirWrench will calculate your stall speed, or you can do it yourself. Just do not save the files from Airwrench.
Beckwith' Spreadsheet is still the best design tool in my opinion and I always use it as a starting point.
I think I read a pilot report on the L-39 recently and will see if I can trace it down. As I recall you needed a strong right arm to fly it
Roy
 
L_39_XFLR.png


i-have-no-idea-what-im-doing-quote-2.jpg




(Well, maybe I do...)


Running NACA 64-blahblah through an Xfoil-like analysis in XFLR yields the C_L_max AOA that's already in the .air file; about 13°.

The analysis seen above for the wings and elevators yields 52° AOA for C_L_max.

The airfoil for the hstab and vstab was made from the GMax model as - of course - there ain't any real data on it out there. Chords, spans and sweep angles were also taken from the GMax model. The wings were then tweaked to reflect known values for area. Weight was tweaked for the known wing loading value.

Not sure if I should include the fuselage as well because it'd be an even greater PITA to set up in XFLR.



P.S: No AirWrench here.

P.P.S:

XFLR can do
- Fixed lift
- Fixed speed
- Fixed AoA
- Beta Range
analysis.

And the available Reynolds ranges are actually pretty large. I could do a constant AoA analysis for, say 250 m/s free stream velocity.

So I guess fixed speed is for table 404 while fixed AoA analysis is for any "due to mach" table?

I need to deactivate any viscuous effects to get a good quota for convergence. Is that bad?
 
Last edited:
Interesting analysis.
Here is a bit from the pilot report at http://www.warbirdalley.com/articles/l39pr.htm
"Stalls are conventional for a jet aircraft of this type, with a stall break that is not sharply defined, and does not result in the nose dropping. Instead, the stall is preceded by an easily identified low-frequency rumble that feels about the same regardless of landing gear or flap position. Wings-level stall speed ranges from 88 to 103 KIAS, depending on flap setting."
Most jets do not have the classic nose drop of a light piston. The "low-frequency rumble" is a sign of airflow separation near the leading edge of the wing. The F-4 had what was better described as "wall to wall" buffeting and that was greatly reduced in the F-4E slatted version.

I also found this report on that site:
"On takeoff, the Hunter lifts off very naturally and in flight, its controls are light and well-balanced through the entire range of airspeeds. Performance-wise, it behaves as well at 600 knots as it does in the traffic pattern, a claim most jets can't make. Even with the smaller Rolls-Royce Mk.120 engine, there is impressive acceleration when you need it. The Hunter is such a thoroughbred, it's almost impossible to fly somewhere straight and level. Aileron rolls are so effortless, they can become hypnotic. The Hunter's stall and spin characteristics are relatively docile and predictable. That's why it's the only swept-wing jet in the world routinely used for spin-recovery training." Happy days!

Anyhow
The analysis seen above for the wings and elevators yields 52° AOA for C_L_max.
Click to expand...
That is obviously garbage, but is what you can get when using a really low Reynolds Number

I need to deactivate any viscuous effects to get a good quota for convergence. Is that bad?
Click to expand...

Well, I'm not a doctor, but I think a couple of stool softeners would cure that.
Roy
 
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