Milviz FG-1D CG vertical location

K5083

K5083

SOH-CM-2024
If you fly the Milviz FG-1D, it might have struck you that the aircraft is excessively prone to tipping onto one wheel, and even scraping the wing tip, when taking off and landing, and it's too easy to tip onto its nose when braking. This is especially obvious compared with the FlyingIron F6F. Yes, the Corsair was a twitchier plane than the Hellcat, but the difference seems excessive.

I decided that the vertical position of the CG of the Corsair might be set too high. In the flight_model.cfg, Milviz has it set to 0, right on the datum line. That seems wrong to me. Most of the Corsair's mass, including the entire wing because of its cranked layout, is below that line. I haven't found a drawing with the exact correct position of the CG, but I decided it's likely as much as 2 feet below the datum line. So I changed empty_weight_CG_position to 0.15, 0, -2.0. Preliminarily, it seems to make the plane behave more realistically on the ground.

In addition, I noticed that the position of the fuel when you fill the drop tanks also has a vertical position of 0. Now that's just lazy and wrong. I reckon the fuel in the drop tanks is hanging an average of 4 feet below the datum line. That happens to be the value that Aircraft Factory used in their FSX Corsair. So I changed the vertical position of the three drop tank fuel entries to -4. That should make the plane feel more realistically sluggish when the drop tanks are full.

Most people, and it turns out this developer, only pay attention to the fore/aft position of the CG. But the height of the CG matters too!

Just some ideas in case you want to tweak yours.

August
 
K5083 said:
If you fly the Milviz FG-1D, it might have struck you that the aircraft is excessively prone to tipping onto one wheel, and even scraping the wing tip, when taking off and landing, and it's too easy to tip onto its nose when braking. This is especially obvious compared with the FlyingIron F6F. Yes, the Corsair was a twitchier plane than the Hellcat, but the difference seems excessive.

I decided that the vertical position of the CG of the Corsair might be set too high. In the flight_model.cfg, Milviz has it set to 0, right on the datum line. That seems wrong to me. Most of the Corsair's mass, including the entire wing because of its cranked layout, is below that line. I haven't found a drawing with the exact correct position of the CG, but I decided it's likely as much as 2 feet below the datum line. So I changed empty_weight_CG_position to 0.15, 0, -2.0. Preliminarily, it seems to make the plane behave more realistically on the ground.

In addition, I noticed that the position of the fuel when you fill the drop tanks also has a vertical position of 0. Now that's just lazy and wrong. I reckon the fuel in the drop tanks is hanging an average of 4 feet below the datum line. That happens to be the value that Aircraft Factory used in their FSX Corsair. So I changed the vertical position of the three drop tank fuel entries to -4. That should make the plane feel more realistically sluggish when the drop tanks are full.

Most people, and it turns out this developer, only pay attention to the fore/aft position of the CG. But the height of the CG matters too!

Just some ideas in case you want to tweak yours.

August
Click to expand...

Hi August,

Thanks for this. Would you min sharing the entries for the external fuel tanks please? By that I mean which stations should be changed. Thanks!

Priller
 
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I've left town for a few days since starting this thread but I don't want to ghost you until I get back, so I'll explain what I can from memory and post a more detailed response later this week.

In the [Fuel] section of the flight_model.cfg there is a list of entries for different tanks named Center1, Center2, etc., each with 5 values, which are the Z, X, and Y positions and the usable and unusable fuel capacity. Most of the entries are all zeroes including zero fuel capacity, so they are not real tanks, I don't even know why they bothered putting them there. Four of them have non-zero fuel capacity values. The first, Center1, is the one internal tank on a Corsair, it sits right about on the CG ahead of the pilot, you can leave that alone. The other three with values in them, I think Center2, External1, and External2, are the three drop tank stations - center, left, right pylons.

The first three values are the longitudinal, lateral, and vertical positions in feet from the datum reference point for each tank and they are all 0, 0, 0. Change the third value to -4 to move it 4 feet down from the datum line. The lateral positions of the left and right tanks probably should be corrected to move them off the center line as well, but this isn't crucial because they drain together so they are never imbalanced and on the Corsair they are clustered pretty close to the center, unlike many fighters where they are outboard of the landing gear and can have a meaningful impact on the rolling moment.

Hope this helps!

August
 
The lift slope of the wing is much too high, at ~7.5, which is much higher than the max theoretical of 6.28 for 2D flow. A wing of aspect ratio 5.35 would be around 4.5. Basically, there's too much lift per degree AoA. The Clmax needs to occur at a higher AoA. The vertical position of the CG is given for an empty airplane at 9.94" below the thrust line. Not sure if they're using the thrust line as the datum or not, I'd have to look into it.

Screenshot (441).png
 
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I suppose Milviz' datum line isn't more than a few inches from the thrust line, so my guess at the CG location was off by at least a foot. I'll try out a more accurate location, but I like the way it handles now, so I'll go back to 2 feet if I think it's still too jumpy at the 10-inch location. It isn't as if I'm messing with some otherwise perfect gem of a flight model.

With full external tanks at 4 feet below the datum and spread 2.5 feet to the left and right, she really plants! It is nice when the loadout makes an appropriate difference to the handling.

August
 
I for one, August am having great fun with the changes you proposed. She flies brilliantly now!

Priller
 
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