MSFS Geometry: wing_area , aileron_area , htail_area , elevator_area , confusion

[lamaflieger]

Hello,

i am confused about the definition of the htail_area and the elevator area.
In the ASOBO C152 flight_model.cfg i found this values:

[AIRPLANE_GEOMETRY]
wing_area = 160 ; Wing area S (SQUARE FEET)
htail_area = 15 ; Horizontal tail area (SQUARE FEET)
vtail_area = 14.22 ; Vertical tail area (SQUARE FEET)

elevator_area = 15 ; Elevator area (SQUARE FEET)
aileron_area = 20 ; Elevator area (SQUARE FEET)
rudder_area = 14 ; Elevator area (SQUARE FEET)

What is confusing for me is that the elevator area is the same as the htail_area.
same for the vtail and rudder area.
i would expect that the "elevator_area" is the control surface area, like the aileron for the wing.
i would expect that the "rudder_area" is the control surface area, like the aileron for the wing.
But for that the elevator_area and the rudder Area should be smaller. Like the aileron_area is smaller than the
wing area.

Maybe someone could help. is there a better documentation than the SDK-Docu out there?

best regards
Christoph

 

[rotorhub]

Hi lamaflieger,

I always use some CAD tool to determine areas and distances from the appropriate reference point.
Just guesstimating the 3 view drawings of the 152, the dimensions are not far apart.

htail_area
Area of the static part of the horizontal stabilizer (not counting the elevator area), in sqft.

elevator_area
Area of the moving part of the horizontal stabilizer (not counting the htail area), in sqft.

vtail_area
The fuselage-to-tip area of the static part of the vertical stabilizer (not counting the rudder area), in sqft.

rudder_area
Area of the moving part of the vertical stabilizer (not counting the vtail area),in sqft.

SDK Documentation

docs.flightsimulator.com

Best regards
Rotorhub

 

[B21-soaring]

I think the subtlety is it's not clear whether the aileron area subtracts from the wing area. My guess is it doesn't, but the difference is small enough that no-one's been too worried about it. In other words, we've been designing planes with the TOTAL area for the wing, and also given the area of the aileron, even though that is probably simulated independent of the main wing. If that was consistent with the 'wing-at-the-back' i.e. the htail_area + elevator_area, we would be specifying the wing area excluding the ailerons, and I don't think we've been doing that, unless I missed the memo.

 

[rotorhub]

My understanding is that ailrons are included in the total wing area. I don't think it is any different than prior versions and P3D.

wing_area
Total area of the top surface of the wing from tip-to-tip, in sqft.
There are no comments of exclusions in the SDK. The wing_area also includes the "imaginary" area inside the fuselage body.

aileron_area is most likely related to ailron efficiency/drag calculations. The simulator seem to work out the "pivot point" and dimensions by aileron_area and aileron_span_outboard.
Then you got several parameters to specify the ailron efficiency and elasticity.

aileron_area
The top surface aileron area, in sqft.

aileron_span_outboard
The outboard aileron span, expressed as a Percent Over 100.
This is the ratio of wing length from the tip to the end of the aileron surface.

 

[Roy Holmes]

The ailerons are part of the wing and contribute to lift and drag. exactly like any other parts of the wing. The imaginary part of the wing within the fuselage is also included.
Similarly the elevators affect the lift and drag of the tailplane by essentially altering its shape and effective incidence.
Roy

 

[B21-soaring]

aileron_span_outboard
The outboard aileron span, expressed as a Percent Over 100.
This is the ratio of wing length from the tip to the end of the aileron surface.

I have to admit this weird parameter confuses me - seems it would have been clearer to give the distance from the aircraft center line to the start and finish of the aileron.

So if the PORT aileron was hinged between 50% and 75% of the PORT wing (i.e. from half way out along the wing, to 3/4 of the way out along the wing) , what would the aileron_span_outboard be (e.g. is "wing length" half the wing span ?)

Also it seems FSX/MSFS can't possibly know whether you've specified a short fat aileron (Cessna) or a long thin one (glider), and it only knows where one END of the aileron is.

 

[rotorhub]

If you know the ailron span and the area, you can determine its approximate physical dimension and mean chord (simplified).

As i understand, it aileron_span_outboard is measured from the wing tip and inwards (This is the ratio of wing length from the tip to the end of the aileron surface.) In earlier MS versions, I have always assumed that the flaps span-outboard also defined the remaining trailing edge as ailron surface.

Below is an extract from a handbook that I look up for reference, maybe MS has applied this "rule of thumb" to their calculations;

"Based on this statistics, about 5 to 10 percent of the wing area is devoted to the aileron, the aileron-to-wing-chord ratio is about 15 to 25 percent, aileron-to-wing-span ratio is about 20-30 percent, and the inboard aileron span is about 60 to 80 percent of the wing span."

http://www.aero.us.es/adesign/Slides/Extra/Stability/Design_Control_Surface/Chapter%2012.%20Desig%20of%20Control%20Surfaces%20(Aileron).pdf

Some more stuff here:

http://docsdrive.com/pdfs/medwelljournals/jeasci/2018/3458-3462.pdf

I am still studying this stuff, so I am only making assumptions

 

[B21-soaring]

Re aileron_span_outboard, FYI from my testing a value of 0.9 gives highly effective ailerons, and a value of 0.1 gives ineffective ailerons (all other things being equal, in particular the aileron_area). So the SDK description "This is the ratio of wing length from the tip to the end of the aileron surface" is hard (impossible?) to rationalise.

 

[WarpD]

I don't think it's a ratio, I think it's a percentage.

 

[rotorhub]

I did a rough CAD drawing of the MSFS 2020 Cessna 152 main wing and ailerons. The numbers match up with the config.
The ratio is set to 0.6, which corresponds well with a visual estimation of the wing. 60% of the semi span (trailing edge) is aileron. The rest is the flap.

60% of the semi span span is about 10 feet. Knowing that the single aileron area is 10 squared feet, the aileron average chord should be 1 foot.

 

[B21-soaring]

Ah, nice analysis. So the aileron_span_outboard is measured from the tip of the wing to the INBOARD end of the aileron, and the sim assumes the aileron is broadly from there to the wingtip. Makes sense. I started with the assumption the aileron position really needs TWO measurements, one for each end of the aileron, and when FSX collapsed that to one lateral measurement plus the area of the aileron I guessed (wrongly) that was the outboard end of the aileron plus some assumption of typical aspect ratio of the aileron. I'm so used to every assumption in FSX/MSFS not working for gliders. I can see it would still be difficult to accurately model some power aircraft, e.g.:

 

[Roy Holmes]

Have a look at a plan view of the English Electric Lightning and its ailerons. The one size fits all approach really does not fit that airplane.

 

[rotorhub]

Can odd placed or multiple ailerons be realistically simulated? Or is it more a matter of getting the roll rate tweaked to match the performance of the plane?

I know that the SDK have some spoileron variables that might be useable, but I have never looked at it. "Hershey bar" wings are the best

 

[Roy Holmes]

From the pilot's viewpoint it matters not what the aileron looks like, just what the roll rate is when you apply aileron. Incidentally in MSFS very high roll rates are possible and will often lead to pitch up and loss of control. This is most likely due to the cross coupling of yaw rate from aileron induced adverse yaw or side force from roll rate. The yaw during roll creates high pitch rate, stall AOA is exceeded and MSFS loses control. You can reduce the roll rate by entering some aileron "elasticity" in the [AIRPLANE_GEOMETRY] section of the flight_model.cfg. Something like: aileron_elasticity_table = 0:1, 300:1, 400:0.8, 600:0.7, 1000:0.15 reduces the effectiveness of the ailerons with increase in airspeed. You could also change the aileron up and down limits. Really you should operate on the appropriate entries of the aerodynamics section, but they are legacy parameters and do nothing in the "modern" flight model. As in so many areas the modern flight model compromises performance when anything is complicated by the shape of the vehicle. Or put another way, instead of fixing the problem it has to be "tweaked".

 

[B21-soaring]

60% of the semi span (trailing edge) is aileron.

Hmmm, on reflection, counter-intuitively given the theory the 0.6 is measured from the wingtip to the inboard end of the aileron, reducing that 60% to 10% (i.e. 0.1) in MSFS should INCREASE the effectiveness, but I definitely got the opposite when I tested that. I.e. in MSFS a defined-area aileron with aileron_span_outboard = 0.9 is much more effective than the same area aileron with aileron_span_outboard = 0.1. So after 20 years of this parameter we're still not sure what it means.

 

[WarpD]

It is percentage of the wing from outer tip inward.

 

[Roy Holmes]

I am at a loss to see where there is uncertainty. The SDK says:
The outboard aileron span, expressed as a Percent Over 100.
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.
Roy

 

[rotorhub]

I agree with Roy ans WarpD. You can not compare the efficiency in this manner. Your 0.9 span with the same area, is more or less a full span trimtab on steroids. I prefer to use real data in the aerodynamic cfg, and then tweak other parameters/coefficients to achieve the desired behaviour.

 

[B21-soaring]

You can not compare the efficiency in this manner

I beg to differ - the effect of the aileron_span_outboard parameter is clearly inverted within MSFS compared to what conventional wisdom would suggest its effect should be. I don't think that matters if your value is around 0.5.

I prefer to use real data in the aerodynamic cfg, and then tweak other parameters/coefficients to achieve the desired behaviour.

So do I.

 

[Roy Holmes]

I know I'm getting old but the aileron_span_outboard parameter is larger if the aileron is larger by being longer. For the same deflection of the aileron the roll created will be larger if the aileron is larger. You said that your 0.9 aileron gave more roll than your 0.1 aileron ie the bigger one gave more roll. The SDK says the same thing.
Please explain what you mean by "the effect of the aileron_span_outboard parameter is clearly inverted within MSFS compared to what conventional wisdom would suggest its effect should be" I'm particularly interested in the "conventional wisdom" part of your argument.
Roy