TMD, the jewel in AFPD's crown?

TMD, the jewel in AFPD's crown?
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TMD, the jewel in AFPD's crown? - 6/10/2009 12:46 PM

mikehoulder

Posts: 87
Score: 100
Joined: 6/26/2008
Last Login: 11/6/2009
From: Coronel SuarezPcia Buenos Aires , ARGENTINA
Status: offline

Sadly, I don't have enough time to study other flight simulators. But I welcome comments from people who have had time to build models both in AFPD and, for instance, MS FS or RealFlight.

I have build some aircraft models for AFPD using Metasequoia as a construction package. As, I'm sure, in common with most other people who have done the same, I find the difficult part to be the creation of the TMD file. This file contains an enormous number of parameters or values which set the physics of the model: how it flies, how it balances, what strengths the components have etc.

I have groaned and complained often over the lack of documentation of the parameters and their effects; even getting really angry over the issue and making accusations of a lack of professional standards in the creation of AFPD. Now retired, I was a senior software engineer for a very well-known international company.

But it is this that may be my downfall. Computer programs in themselves are not real; they are abstract creations of the mind. The programmer can pick and choose a partial reality where individual inputs from the user have clear and well-defined effects and so can be clearly documented. But good simulators and, particularly, AFPD are different. A change in one small part can bring a whole chain of unforeseen consequences throughout the model. This is the justification for saying that AFPD supports the physics of flight.

And here is the difficulty with documentation. A good documentation of the TMD file might well be a three volume work of higher level mathematics which would need three years to study. I'd love to do that; but at my age I don't have the years to spare.

Most people, when they create an AFPD model. use a copy of a TMD file from a similar aircraft model and make some small and very limited adjustments. That's fine if you stick to a limited range of aircraft models. But if you go outside the box: Oh, brother!

Just recently I have been doing that: trying to create a model at a 1:1 scale with a wingspan of 17 metres. Yes, there are limitations in AFPD. It has to be usable. If the calculations and numbers could be sufficiently large to cope with wingspans from 17 metres down to 10 centimetres, the program would be too slow to use. This must be the case for any and all simulators, not just AFPD.

So with this model, I went outside the box, using numbers in the TMD file which were very much larger than the normal. And here I found the problem that has lead to this post. Incredibly frustrated, I tried to strengthen just one wing joint and found that it affected the whole aircraft and the way it flew. How on earth can that be!!!!

Then it struck me. You can't strengthen a wing joint in the abstract. To do so in the real means increasing the size of the joint using stronger and heavier materials. So, I believe, by strengthening the wing joint, I added weight to that point and changed the centre of gravity of the aircraft and, of course, the way it flew.

Like a good programmer, I thought that changing a single Kf parameter thatin the TMD file which according to the sparse documentation available changes the tensile strength of a joint would do just that. Wrong, wrong!
To obtain the desired effect, I had to change a whole host of other parameters. This indicates the problem the AFPD writers have with producing good documentation. It really isn't possible. So, I beg pardon for my previous irritation and anger.

It also increases very greatly my respect for AFPD. AFPD really does support the physics of flight. I believe the TMD file is really a jewel beyond price.

Mike

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