Wrong default Fuselage_lateral_cx value + suggestion

Bug Reporting Hub Aircraft & Systems
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SDK Fuselage Lateral CX ‘’…The default value is 0.4 - which is about the perpendicular drag of a cylinder - and the value should usually fall between 0.2 and 0.8 for most aircraft.‘’

This sentence is maybe the cause of many aerodynamic problems, if developers are following it.

0.2 - 0.4 doesn’t equal the drag of a cylinder, it equals the drag of a sphere, which results in unrealistic low drag for 98% of all aircraft.

0.8 actually equals the drag of a cylinder and should be used as a starting point / default value,

with a suggested maximum range of 1.3.

Testing values above 0.8 with the result of much more realistic aerodynamic behaviour additionaly proves that 0.2 - 0.4 shouldn’t be suggested anywhere.

Suggestion for: fuselage_lateral_cx

Range : 0.7 - 1.3

Default: 0.8

SDK Fine Tuning ‘’…Here you should also take a moment to set the Fuselage Lateral CX value: Fuselage CX has a d efault value of 4.0 …‘’

4.0 is probably a typo?

Additionally: The Cessna172 G1000 is currently using a fuselage_lateral_cx value of 4.3, which is most likely a mistake.

Answer from the SDK page:

The documentation will be updated with a new range, from 0.2 to 1.2, keep in mind that this is a recommended range and the value could be way above/under it but it will lead to unrealistic behavior.

The C172sp value will also change, 4.3 is indeed a mistake and it will be corrected in a future update.

Best regards

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That’s a good find. When I get home I’ll doctor my script to spit out the values of this attribute for all the planes I have.

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Thanks, I already checked a few,
so far developers are using the suggested small typo values 0.2 - 0.8 through out…
or they didn’t changed it and using the incorrect default 0.4.

Same thing with base aircraft content.

The only exception is the Cessna 172 G1000 variant.
There they are using 4.3
(which makes sense if the correct default is 4.0 and not the current 0.4).

That’s how this typo was found.

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Unless the 4.3 is the typo. :wink:

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Not possible, because the Cessna 172 G1000 would behave very weird if a maximum of 0.8 would have been intended. But instead it flys better then the other Cessna variants, which are using the wrong default values.

Additionaly, they themselves wrote at one point 4.0 as default value in the SDK.

But then somebody must have mixed up the numbers and was typing 0.4 instead of 4.0 (which can quickly happen) and because of that continued to write 0.2 - 0.8 instead of 2.0 - 8.0 :joy:

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It’s probably not going to help that nose wiggling effect you get in turbulence, where it feels like something has grabbed the nose, and is shaking it about that point, but it may have other benefits.

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fuselage_lateral_cx

Defines the perpendicular drag coefficient of the fuselage, which occurs when the airflow is going perpendicular to the front axis (ie: sideways - left to right or right to left) but also going up and down.
This coefficient has an impact on drag when side slipping, as well as a general impact on yaw stability and pitch stability.

It’s possible that it has an effect on the nose wiggling, but not tested yet.

If developer are using 0.4 as default value
and 4.3 is equivalent to a Cessna172

Then it’s no wonder why all aircraft are wiggleing like RC-Planes through the air,
because 0.4 would actually match a little bit bigger RC-Plane.

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Should probably be in the SDK section.

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Ah thanks, I only read this:

The SDK Bugs & Issues section is no longer used to create topics.

I thought they want us to write here, but looking further there is actually a complete own website for the SDK…

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I’m copying the post in there,
if the case is true it’s actually a very big thing for all developers :sweat_smile:
so maybe it’s not that bad having it here and there.

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If it is a drag coefficient as typically used, 4.0 is probably the typo. See Drag coefficient - Wikipedia

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I looked it up:

CX = Drag coefficient

I didn’t know what these values even mean, thanks for sharing the wikipedia link, very interesting :slight_smile:

Then they did two typos, one time in the SDK
and the second time in the Cessna 172 G1000 flightmodel.cfg, where they used the number 4.3
for the fuselage drag coef.

How is it even possible that the Cessna can fly with this high number? :joy:

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That being said, I noticed something peculiar when checking out a document cited in the Wikipedia article. The SDK says 0.4 is the “about the perpendicular drag of a cylinder.” If they are using the typical drag coefficient, 0.4 is about equal to the drag coefficient of hemisphere and sphere, but not a cylinder.

The circular cylinder is more like between .75 and 1.2 (see Fluid-Dynamic Drag : Sighard F. Hoerner : Free Download, Borrow, and Streaming : Internet Archive page 3-16, figure 28). The drag coef. never goes below about .7, even for a cylinder with an aspect ratio of 1 (e.g. 1" span x 1" diameter). Logic would dictate the coefficient would get bigger the more squared the cylinder is, and lower the more elongated its cross-section in relation to the direction of airflow. So perhaps fuselage_lateral_cx should be sitting more in the range of like 0.7 to 1.3 rather than 0.2 to 0.8.

It almost makes you wonder if the folks writing the SDK saw the picture of the sphere and assumed it was a cylinder. A sphere has much less drag than a cylinder. Since I have little knowledge on the subject and no engineering background, it’s pretty much speculation on my part. But if there’s something to it, that would still be pretty significant for developers to know.

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That was exactly what I was about to ask

14ilf1l

0.4 = sphere
and 0.8 = Cylinder

And assuming most of the airplanes don’t have a ‘‘perfect’’ Cylinder shape and more edges,
the value should be higher most of the times.

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Those cylinders are positioned axially (in line) with the airflow, but they did have perpendicular ones in the book that came from too linked above. Either way, 0.4 - 0.8 seems way too low.

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I said it elsewhere, but I’ve found 0.8 to be a good start for the average aircraft. I’ll crank that up to 1.0 for something that’s a tad more draggy, 1.2 for a monoplane on floats, and 1.4 for something super draggy like the AN-2 on floats. Regardless, 0.4 is too low for anything besides the tiniest fuselage such as on a glider

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I found another picture where they showcase the cylinder with 0.3 - 0.6.

However suggesting the values starting with 0.2 (a sphere) is ridiculous.
As the fuselage is a mixture of a cylinder and cuboids starting with 0.8 makes totally sense.

The problem is they are setting 0.4 as default, which is way to low for 90% of the aircraft.
And then suggesting 0.8 as maximum, which should actually be the starting point.

F1616.01 Bluff body drag coeffs.rs_590_600_f

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Not when you look at that streamlined body they don’t. Admittedly these 2D sketches don’t help at all.

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I posted this to the WB-Sim Discord. This is no official answer, but the WBSim seems to use the value of 1.0 (have not verified yet). The Value of 4.3 for the G1000 therefore seems to be way too high.
Guy is saying it can drop between 3000-5000fpm in a slip, which for sure doesnt seem right.

Again, i have not doublechecked this, just parroting at this point.
The mistake seems to be the value of 4.3.