Flap/aileron span values

Questions & Community Discussions Aircraft
1

It is unclear to me exactly how to determine the values to use for
aileron_span_outboard and the flaps span-outboard. The description for the
aileron span says, "

The outboard aileron span, expressed as a
.

This is the ratio of wing length from the tip to the end of the aileron
surface. A larger aileron will increase the roll moment of aileron deflection,
but it will also increase the local drag generated by aileron deflection." A
ratio is a comparison of two lengths. What are the two lengths this variable
is a ratio of? For the following example, is the numerator A, B, or C (or some
other length)? Is the denominator the wing span, half the wing span, or half
the wing span minus half the fuselage diameter (or some other length)?

549Ă—546 36.9 KB
The description for the flaps
span-outboard variable says, “Outboard span area, as a

This is how far out from the middle the flaps stretch. On most planes this
will be 1.0 - [aileron_span_outboard](https://docs.flightsimulator.com/html/Content_Configuration/SimObjects/Aircraft_SimO/Flight_Model_Definition.htm#aileron_span_outboard).
This value needs to be matching the lift and drag added by flaps. Small flap
systems should add small amounts of lift.” How far out from the middle of
what? The middle of the airplane? Or should this be the percentage of the half
wing span - half fuselage diameter (i.e. distance from the fuselage to the tip
of one wing). Eyeballing the example picture, this would be around 0.6 or so?
Also, what are the default values for these parameters if there is no value
specified in the flight_model.cfg file?

2

AFAIK It it the percent of half-wingspan

3

That would make the most sense to me (or half span minus half fuselage
diameter, so half the span from the fuselage outward), but I can’t understand
the description that was given.

4

No I don’t think you need to remove the half fuse width. Simply length_of_flap
/ halfspan_wing. So if you wing span is 30’, halfspan is 15, and your flaps
are 6’ then the ratio is 6/15 or 0.4

5

Depends on how they coded it. I’d like to get an answer from Asobo.

6

OK So digging into this a little more it seems like it is (half-wingspan -
half-fuselage) if we go back to the original FSX SDK:
[https://docs.microsoft.com/en-us/previous-versions/microsoft-
esp/cc526949(v=msdn.10)](https://docs.microsoft.com/en-us/previous-
versions/microsoft-esp/cc526949%28v=msdn.10%29) span-outboard | The
percentage of half-wing span the flap extends to (from the wing-fuselage
intersection).
—|—
So reading that it indicates “from the wing-fuselage intersection”. Looking at
the P-51 it has a wingspan of 37 ft. Each flap has a span of 112.94" and the
fuselage is described as having a frontal area of 11.54 ft2 which we could
estimate as being ~3.4ft in diameter. (112.94/12) / (37/2)-(3.4/2) 9.41 /
(18.5 - 1.7) 9.41/16.8 0.56

7

Yes, but I was wondering if they had maybe changed that.

8

Hello. The span goes from the wing tip to the wing root. You can use the
AircraftEditor “sim forces” debug panel to see if it matches. Regards, Sylvain

9

Ok cool so my original assumption was correct.

10

@Nocturne FYI

11

This may be the accepted answer, but it doesn’t actually answer the questions
that were asked. How does one determine the aileron_span_outboard and the
flaps_span_outboard?

12

@donstim My bad. We’ll have the doc updated on
this. These are expressed as percentage of the total wing span, from root to
tip. Aileron starts from the tip and flaps start from wing root. Some
important info that is missing from the doc at the moment is that, despite
flaps span being defined in each flap section, the span is common to all and
can’t be less than 40%. Basically, it will take the maximum span value defined
in flap sections and clamp it between 0.4 and 1.0

13

@Nocturne FYI

14

Thanks, that is exactly the answer I was looking for. If possible, could you
identify that as the answer?

15

Would be nice if there was some way to specifically tell where the “wing-
fuselage intersection” is then, too. Or if it at least was documented how that
is estimated. I guess it is the fuselage_diameter parameter, which is assumed
to be constant along the whole length? (So much of this would be much simpler
if the sim would just look at the 3D model for measurements. This would then
require that you put some dummy objects in the 3D model that would follow some
certain naming convention. But that would be trivial to do.)

16

I have to ask why you would put an arbitrary limit on this? Minimum of 40% is
going to mess up the F4 Phantom which we have long-term plans for, the span of
the flap is (off the top of my head) somewhere between. 25% and 30%. I do not
see the need for limits to be imposed by the core flight model which do not
exist in real life?

2 Likes