JtD

NACA gives CoG as % MAC, but yes, the discussion around it is surprising. As Kurfürst said, it's hardly rocket science.

Also to do a proper short conversation between the rather insignificant differences between 85" and 84" MAC: The 85" MAC is naturally situated a little bit more inward, and if you look at the Spitfire wing, you'll see that the wing edges in that area are swept at a ratio of about 1:2 (conservatively), so that the 84" chords starts about 0.3" behind the 85" chord and ends 0.7" before it does. So NACAMAC%*85 - 0.3 = RAFMAC% * 84 So where the Spitfire V was tested at NACA at 31.4% MAC, this translates to 26.7" behind the trailing edge at 85" which is 26.4" behind the trailing edge of the 84" RAF MAC and corresponds to 31.4% RAF MAC. With datum point 19.5" behind the LE of the 84" chord, we're at a CoG of 6.9" aft the datum. Pretty much the same as we have for a fully loaded IX with an empty rear tank and about 1" behind a fully loaded VIII or IX without a rear tank.

No, the weight and balance table gives 74 gallons rear tank capacity with 7.2lb/gallon and corresponding CoG movement of 5.5". The snippet from the test report has 74 gallons rear tank capacity with 6.8lb/gal and corresponding CoG movement of 5.2". No mix up, just reading.

Actually they say 7 for that particular plane, and their fuel isn't even at 6lb/gallon.

Thanks a lot. So, with a fuel tank in the rear that's empty you'll have a CoG of about 6.8" (31.2% MAC) back and without the empty fuel tank it's somewhere around 6" (30.2% MAC) . That makes sense, given that the a near identical VIII is at 5.8". NACA btw. still tested at ~7"(31.4% MAC).

Please show or at least name the manual you got these figures from. Thanks.

Luckily enough they simply added the wing tip modification to the weight and balance table and left the CoG range untouched. For instance, the Spit VIII went with the 3.5 to 7 range no matter what, but changing wingtips (here extended vs. elliptical) changed weight 20lb and moment by 270"lb.

I don't think the rearward limit of the permissible CoG range is really equivalent to the neutral point of a design or something like that. Modifications that effect the rearward limit may or may not effect the neutral point. They might as well just effect the controllability in the rearward range of CoG positions, but not stability as such. So I think there's little point in using permitted CoG ranges to discuss small margins of stability. Might be a rough indicator, but that's it.

If you have forgotten what I'm talking about I suggest you read this topic. It's not that long. And there's no innuendo. Just fact. The Spitfire at NACA was not tested with CoG 4.8 behind the datum as you've stated and 22.5 cm are not 11.3 inch as you've stated. The correct figures are ~7 and ~9. Why do you keep discussing this? It's not up to opinion, you've made a mistake and could be happy that you've learned something. Feel free to rant on if it helps you, but that's really all I'll say about this.

It's absolutely a sliding scale.

That's true, and yet you failed.

To what - that you can't tell the difference between the position of the CoG relative to the datum and the position of the LE of the MAC relative to the LE of the chord at the root or that you don't know how much an inch is in cm?

Kurfürst was speaking about the range, not the most rearward position. Spitfire is like 3.5 to 7 or 5 to 8.5 or something depending on the model and details, so about 3.5. It's 40.4-62.9cm for the 109G-2trop in the sheet Kurfürst showed, so about 9.

Rgr on the first part. On the quoted second part I wonder - how was the forward limit on the Spitfire determined? In terms of MAC, the rearward limits on the 109 and on the Spitfire were close, so a difference in permissible range comes mainly from the forward limit. I don't recall having ever seen anything illustrating that for the Spitfire, in fact, 99% of tests I've seen dealing with permissible CoG ranges deal with the rearward limit.

It were Messerschmitt guys who said it was "indifferent" in certain conditions. Maybe they were silly, I don't know. I take it the bottom line is you're saying that the 109 had a ~9" range, the Spitfire ~3"?