Questions about geometry on htail_area

[JacobW7364]

Questions about geometry on htail_area:

How to think about geometry on htail_area?

I have noticed that many aircraft developers have at least 30% more area on “htail_area”. Why?

If you look at the A320 original in MSFS, it has 200 square feet in size on this surface, but both FBW A320, Fenix A320 and PMDG have over 300, let’s say 300-400 square feet.
If you look in documents online, they indicate area values for stabilizer, but this includes elevator, I guess.

If I calculate the values I get, it will be the following.

htail_area
Value 1: 213.75 if you add elevator to this value, which is not correct according to the SDK, you get:
213.75+68.464=282.214
Value 2: 219.982+74.354=294.336
I get a value of between 282 and 294 square feet if you add htail_area and elevator together. This is still far from the value stated online, 32.78 square meters or 352.84098, 352 is closer to the value used by PMDG, Fenix and FBW.

20.4 m2 or 219.58377 ft2

Could it be that all of these have miscalculated the geometry? Or is it wrong in the SDK?

The SDK says this:
Area of the static part of the horizontal stabilizer (not counting the elevator area), in sqft.
Notice “NOT COUNTING THE ELEVATOR AREA”

If this is true, the most seasoned and respected aircraft manufacturers to MSFS have miscalculated. Very wrong.

I’ve tried to figure out why but can’t. Are they too front heavy?, is the CG positioned incorrectly, are the forces too small if you make it 30% smaller, have they thought wrong and calculated the total surface of the stabilizer? Is there an error in the SDK?

Why do I think like this?

This is how I calculated:

1: Found as good a picture as I can of the aircraft from above.
2: Obtain a reliable measurement of the aircraft, which I can use as a reference value.
2: Found a program that I can use to calculate the area of an image. The program I use is called: ImageJ
3: Calibrated image by wingspan, Span with Winglets 35.79 m (117 ft 5 in)
4: Plotted the area I want to measure on the picture, then I get, as you can see: 213.75-219.982, i.e. quite close to the value 200 that ASOBO uses on its A320 but far from PMDG, Fenix and FBW.

One thing to note is that if I run CFD the tail stabilizer get to strong with there values.

Another thing I’ve noticed is that if I change this value to what I’ve been getting about 220, the stabilizer seems to be too weak and the plane has to trim much more heavily.

What could be the reason that these aircraft’s developers chose this value? Are the aerodynamics not right in MSFS?

[CharlieFoxtrot00]

I ran into a similar issue with VTAIL area when I was redoing the dynamics for a pretty well-known 3rd party aircraft. I checked several others and found the same issue all around, that the VTAIL area, when set properly, only generates an SDK perimeter of about half the airplane’s tail (irrespective of the rudder). Most devs made the tail area about twice as large as it should have been, in which the SDK displayed the correct perimeter.

I experimented with the VTAIL using both book and visual/SDK values and they absolutely made a difference. However, I set the HTAIL and elevator to the correct area. At first, I had trouble getting enough authority to flare, however, the original CG was well far forward of the actual airplane CG. After I moved that to the correct location, it was much easier and the plane flew pretty much on-book.

Note there were several other things I had to change, including the trim up/down limits, which had only become an independent variable since the sim update prior to my work (this was a while back).

But the VTAIL area issue, despite my requests for clarification, was never resolved.

[JacobW7364]

So it could be that all those devs have calculated wrong?

How do I move CG?
I guess I need to move aero_center_lift to match better.

To get balans correct, trim correct, and pitch correct. What things should I play with then.

[JacobW7364]

How likely is it that e.g. Fenix’s “htail_area” is 333,681 square feet. The value from the internet is that it should be 31 square meters, which if you convert it is 333.68122. But if my calculations are to be believed, it should be between 200-210 square feet. That they would have started tweaking at 200-210 and concluded that it should be 59% larger for the plane to function. Which gives an exact value as 31 square meters converted. I just don’t believe that! Then the aerodynamics in the MSFS would be completely crazy wrong.

My conclusion is that they all missed the SDK words: “Area of the static part of the horizontal stabilizer (not counting the elevator area), in sqft.”. PMDG, Fenix and FBW must have had to make very drastic changes for the plane to fly well. Because they do, no doubt that they fly well.

If I were a flight model developer, I would be very grateful that a super nerdy Swede comes up with something like this. But they can also pretend that they did it on purpose. Or ignore that they made a mistake. They don’t really need to be ashamed considering that all the big ones have made the same mistake.

Fenix cfg

FlyByWire cfg

PMDG 737-800 cfg

AirBus Data from internet!

Boeing Data from internet!

[MariopilotPD808]

To add further to the confusion, the standard way of defining wing surfaces for flight physics/dynamics purposes usually includes the “extended” area that connects the wings together through the fuselage as if they continued until the centre point.

Quite possibly, removing the elevator area and adding up the “middle portion” could yield a number similar to the effective physical area …

[JacobW7364]

This sounds likely!

I have also read this somewhere.

Not for Fenix though! Because they have exactly 31 square meters and converted it is 333.68122 square feet, and they have in there cfg 333.681.

But for PMDG I buy that.

But apparently MSFS doesn’t count geometry like what is standard. This the developers must take in to account. I think they should go over all geometry. For example wing span that is 117.5 feet or something and in PMDG is 112. Not a big thing but should affect roll speed.

I still se it as a quite big error, with elevator.

Would be interesting to se how the plane would behave if they used the correct geometry and then adjusted from that.

I don’t know how to change cg and lift and that is probably why I can’t adjust htail_area to 200-220.

Because then trim values become wrong.

But if I adjust to correct 220 square feet and turn on CFD and trim the plane to a stable sate, I feel that flaire is much better.

Better in the cense that you need more realistic 50% elevator to flaire instead of 10-20% like it is now.

[JacobW7364]

Would be good if Asobo could clarify geometry a bit, also with pictures. Please? I don’t want it to grow to a big mystery?

[JacobW7364]

I realized after reading my post that it was quite confusing. That’s why I’m redoing the whole thing.

First of all where do I get my data from.
Eg. from the picture below which shows 117.45 feet. It is basically the same as what is written in QRH. Boeing 737-800 with Winglets. it is also the same as on Boeing’s own website. It was then concluded that the wingspan is very close to 117.4-117.5 feet. This figure is not much to question.

I want to be transparent so everyone can review and tell me if I’m wrong. I also understand that a wing surface is curved and not straight, this can naturally give a certain margin of error. I don’t know how big it is.

Then again to how I measured.
I used the program ImageJ

1. Draw a line between the wine tips, out on the winglets.
   I used this line to bring the scale in the picture to 117.45 feet. (This means that when I draw a surface on the image, the program can calculate the area of this surface)

I drew the edges of different parts of the stabilizer and elevator. Then I chose Analyze/Measure.

These are the results I got on a Norwegian 737-800. There are guaranteed better ways to measure in 3D or get metrics directly from Boeing that PMDG should get.

The first picture shows the entire stabilizer with elevator and fuselage. It shows 329,447 square feet. PMDG 737-800 has = 352.84098 square feet on htail_area.
352.84098/329.447=1.071
This means that PMDG already now with incorrect measurement has approx. 7.1% too large htail_area.

The second picture shows the entire stabilizer without elevator and with fuselage. It shows 238.423 square feet. PMDG 737-800 has = 352.84098 square feet on htail_area.
352.84098/238.423=1.479
This means that PMDG has approx. 48% too large htail_area.

The third picture shows the stabilizer without the elevator and the fuselage. It shows 204,188 square feet. PMDG 737-800 has = 352.84098 square feet on htail_area.
352.84098/204.188=1.728
This means that PMDG has approx. 73% too large htail_area.

This is the Elevator with trim tab, which is 81,709 square feet.

This is the Elevator with trim tab, which is 81,709 square feet. PMDG has elevator_area = 70.503613 square feet.

So what value of htail_area is most correct?

I think the most correct is either 238,423 square feet, or 204,188 square feet, which is also pretty close to ASOBO’s default A320 which has 200 square feet of htail_area. This means that the PMDG must be between 40-70% wrong.

But only ASOBO can answer how to calculate or someone who is an expert in aerodynamics. I don’t know why it is so quiet from them? If it’s to, not offend PMDG, Fenix and FlyByWire, or if it’s because they don’t know. Or if there is some other reason that I don’t understand.

But it is clear that htail_area cannot be as large as it is on PMDG, Fenix and FlyByWire.

[JacobW7364]

Hi @FlyingRaccoon

Can someone from Microsoft / ASOBO please clarify geometry. Preferably with images in the SDK.

How should we count, measure?

[JacobW7364]

Naturally, I became curious and started calculating the MSFS Boeing 737-800. Was quite hard to find any pictures from the top view, so I hope the stabilizer is correct on the sketched picture.

First of all where do I get my data from.
Eg. from the picture below which shows 68.4 meters. On Boeing’s own website it says NEW - Wing
68.4 m (224 ft 5 in) span. In CFG it said 224.57 feet, so I used that. It was then concluded that the wingspan is very close to 224.4-224.6 feet. This figure is not much to question.

I want to be transparent so everyone can review and tell me if I’m wrong. I also understand that a wing surface is curved and not straight, this can naturally give a certain margin of error. I don’t know how big it is?

Then again to how I measured.
I used the program ImageJ

1. Draw a line between the wine tips, out on the winglets.
   I used this line to bring the scale in the picture to 224.57 feet. (This means that when I draw a surface on the image, the program can calculate the area of this surface)

I drew the edges of different parts of the stabilizer and elevator. Then I chose Analyze/Measure.

These are the results I got on a sketched picture from a 747-800. There are guaranteed better ways to measure in 3D or get metrics directly from Boeing that ASOBO could get.

The first picture shows the entire stabilizer with elevator and fuselage. It shows 1518.387 square feet. MSFS 747-800 has = 800 square feet on htail_area.
1518.387/800=1.897
If 1518 square feet is correct means that MSFS has approx. 90% too small htail_area.

The second picture shows the entire stabilizer without elevator and with fuselage. It shows 1044.806 square feet. MSFS 747-800 has = 800 square feet on htail_area.
1044.806/800=1.306
This means that MSFS has approx. 30% too small htail_area.

The third picture shows the stabilizer without the elevator and the fuselage. It shows 858.367 square feet. MSFS 747-800 has = 800 square feet on htail_area.
858.367/800=1.0729
This means that MSFS has approx. 7.3% too small htail_area.

So what value of htail_area is most correct?

I think the most correct is the 858 square feet

This somewhat answers my question, which I asked in the previous post!

How to calculate?

It seems that you should only include the non-moving part, you SHOULD EXCLUDE THE FUSELAGE AND ELEVATOR.

Otherwise, Asobo has done wrong! ?
I find it highly unlikely that Working Title who have updated the flight models would have missed simple geometry. Which even I can find?

If Asobo has not made a mistake, then PMDG has made a mistake. We can also state that 100% Fenix has made a mistake. They should at least remove the elevator area from htail_area.

They can’t all be right at the same time!

[MariopilotPD808]

Hmm, curvature (by dihedral, airfoil shape) shall not matter. The planform area is what counts.
I do not see the MSFS DevMode displays of wing geometry (in-sim) has a way to account for how much area is shaded by the fuselage, as no such data is configurable in the CFG files. Hence, it will display the leading edge and chord lines correctly only if the “fuselage” area is included. Wether or not that is the correct value for realistic behaviour, that must be answered by ASOBO …

[BusPeeGee]

I think it’s linked with how trim works.

If you create an acft you will realize the trim shares same surface than elevator. It’s valid for a GA where trim compensates aero effort on deflected surface but not on an airliner where elevator and THS are 2 different things and their effect are cumulating. On old P3D they were splitted.

On airliners such as the one mentioned earlier, it seems they oversized stab to get enough room for both components.

In the end it’s a moment calculation. If it’s flying level then they must have play on another parameter to get it right.

Experience shows that if you set all
Geometry as per the book in MSFS you don’t get perf figures of the same book during the tests.

Am not surprised fenix folks had to twists a few official number to get desired results