T-28 "reference datum"

[PRB]

In FSEdit, there are several items whose location is specified in terms of some offset from a “reference datum”. I know what that is. It’s used a lot during weight and balance calculations. And on the configuration tab of the flight dynamics section, you can tell FSEdit exactly where this reference datum is.
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My problem is: where should I place this reference datum? It’s often some arbitrary point a few feet in front of the nose, but is it a specification set by the manufacturer? For all the data I have on the T-28, this does not appear to be included!

[fliger747]

What you need to tell your airfile programs is the reference point that the aircfaft model designer used. Then you can figure out where the aircraft data reference is and convert as necessary. Often (not always) the model designer will locate the zero zero point of the plane near the final desired CG, which in FS is usually at 25% MAC.

The real plane will choose some arbitrary point, wing leading edge, firewall, nose, who knows....

Unfortunatly I do not have this plane or I could tell you. I may be able to sort it out in a couple of days for you if you can wait.

[PRB]

Ahh, thanks Tom. That makes perfect sense. I do have Tim Conrad's T-28C, which I am using for the aircraft model for my "homework assignment." I was able to open the air file for that plane. I will use the reference datum values in that air file for mine. Beyond that, I will derive all my numbers myself, so I can learn how this is done!

[errmail]

Ahh, thanks Tom. That makes perfect sense. I do have Tim Conrad's T-28C, which I am using for the aircraft model for my "homework assignment." I was able to open the air file for that plane. I will use the reference datum values in that air file for mine. Beyond that, I will derive all my numbers myself, so I can learn how this is done!

What you really need to do is to find out the precise location of the visual model's "origin" rather than a reference datum. As Tom said, in a real plane the RD can be any location and it's usually some convenient spot that makes it easy for a pilot or crew member to do weight and balance calculations. In FS we don't need to set the reference datum on the plane's nose, it can be in the same location as the aircraft's empty weight center of gravity, which conveniently for us is always exactly the same location as the visual model origin. For purposes of flight modeling, those two terms are the same. The visual model origin = aircraft empty weight center of gravity, always and without exception. It is possible to offset everything in the flight model to the actual aircraft's RD and several of the default aircraft do this; you can easily see if this is the case because if there is any value other than "0" entered in the RD location field that is what has been done. Also, flight models are often less than satisfactory if this method is use. So you can quickly tell what the plane's reference datum value is by looking in the aircraft.cfg file, but you cannot know what the model origin is without the right tools to view it. And if you don't know the second value the first value is going to be useless.

There is only one way to find out where the model origin is, and that is to use a program that can display the visual model in such a way that the origin can be seen. The program used to create the model can do this (for example, gmax or FSDS) as can a third-party development tool called Aircraft Container Manager. Unfortunately ACM cannot be purchased any longer so if you don't have it you may be out of luck as it's not freeware. But you could always try to contact Tim and get him to send you a screenshot of the model in the program he uses, which would show you the precise location of the origin.

You aren't going to be able to get any information on this from the .air file or aircraft.cfg; the values in those files don't reveal the actual location of the origin although you might be able to make a pretty close guess based on the location of the contact points, fuel tanks, and so on.

[fliger747]

Paul:

I am downloading the plane and will try to post a drawing as suggested above.

Might be a while.

T.

[fliger747]

Paul:

Here is the broadside view, you can see the CG and point of origin are closely co-located. Squares are 10'.

T.

[PRB]

Thanks Tom, that helps a lot. errmail, the RD values in the aircraft.cfg are indeed 0,0,0, so it's likely the model's origin is also the CG, as you said.

[errmail]

Thanks Tom, that helps a lot. errmail, the RD values in the aircraft.cfg are indeed 0,0,0, so it's likely the model's origin is also the CG, as you said.

Yes, that's standard procedure for any modeler who's been doing it a while, and Tim usually gets it real close.

[Piglet]

I set up my models so CG is right about 25% Mean Aerodynamic Chord, or MAC. On a almost square wing such as on a T-28, it's pretty much simply a quarter of the way aft of the wing leading edge. Normally on nose gear planes the CG is just before the mainwheels, and on taildraggers, it's just behind the mains.
In real world the reference is often set up by the manufacturer's own whims and needs. Some are logical and use the spinner tip or firewall as 0,0,0, while others will use a point a few feet in space in front of the plane's nose. usually they do this if they think future versions will have extended noses or radomes, or what not.

[pfflyers]

Normally kibbitzers annoy me, but having some real world experiance in this field I think most designers use the forward of the nose datum so all station values can be positive. This really tidies up your arithmatec when doing weight and balance computations.

[fliger747]

This is often true in most real world aircraft, however in the majority of FS planes the datum is set near the CG, if not at it in the final event.

[Real Old Salt]

Could it be that modern designers have cleaned up their act? Older aircraft were all over the place. For instance the one I'm involved with now,the A-26 has it's zero line at the point were the interchangeable nose section separates.