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"?

The figure as such is meaningless when it comes to stability issues, unless you can also come up with a figure of how much they added in front. After all, between the say B and K, there's like what, a good ton difference in weight? So like, 20% went to the rear, the other 80% to the front? I guess you are aware that stability was just as much an issue with the 109 as with the Spitfire or any other aircraft that wants to be flown, and while they might have had "no trouble" to add weight in the rear, they certainly didn't have the CoG move about the aircraft randomly and unchecked. The rearward limit for the Bf109G was at about 30% MAC, with certain loadouts approaching this figure, offering marginal stability.

Now that this got quoted again, I had to :doh:. The NACA report says the mean aerodynamic chord of 85 inches starts 4.8 inches behind the chord at the wing root. How this means "the CoG is 4.8 behind the datum", oh well. It was tested at 31.4% MAC, which corresponds to 7 inches behind the datum. Also according to NACA, this corresponds well to the fully loaded state (31.1% without ammo). Fully loaded the CoG position is most rearward, as most consumables are located a little bit behind this point. For what it's worth, 7.0 is also the rearward limit for the VIII.

You named the topic "why is the Spit IX still unstable" and the only instability it had are the long period oscillations. Like most other fighter aircraft of that time. Low to neutral stability OTOH isn't unstable, so what's your point? To me it looks as if you were trying to redefine the term "unstable" so that it covers "unstable and the Spitfire". From my perspective, we're either speaking about Spitfire instability and then about phugoids or we don't speak about Spitfire instability at all.

I'm just referring to the first chart in your opening post. Of course I know you're subject to changing the subject whenever facts don't suit your advertised fantasies, so no problem if you're doing it again.

Wow, the Spitfire developed phugoids. I really can't think of any other WW2 fighter aircraft that did the same. Except for all of them.

At the full throttle altitude of the BMW801D, the Jumo213A produced 200 extra hp, roughly 1500@5500 compared to 1300@5300, combat/climb power. Or the Jumo213A still produced the 1300hp the BMW801D was capable of at 5300m at 6300m, basically gaining 1000m altitude performance.

Yeah, buffet would be nice. But not as important for the IX as for the early marks, considering the heavier engine.

Kurfürst, I've already said that I don't care enough about the issue to do actual research. I go with the numbers I know, which happen to be the ones you keep posting plus a figure of 642 km/h you keep ignoring, and if you were to include the VIII, a figure of 405 mph. But all that's there does not sufficiently support your point of view. Anyway - have you ever considered to look at speeds not at full throttle altitude, say 5000 feet? With the biggest effect of intake, injection and supercharger differences eliminated, this would give a much better view at the picture then comparing speeds at 5500m with speeds at 6500m.

Which would lead to the rejection of most Mk.IX built, since contract obligations were not met. Well, maybe they would have been taken into service, if provided for free. 404mph +/- 3%. Legal obligation. You gave a single VVS test, some estimated performance of a testbed and a Mk. VIII test. On the bottom line, one figure. I gave another one, same source. Reminder: 642 km/h / 399 mph. I don't even see how that averages out to 390, but even if it did, I wouldn't be so stupid to accept broad generalisation based on two figures.

Impossible, since it doesn't meet the minimum requirements. It actually goes against the limited facts available. Yeah, and lets take RAF tests to accurately describe Fw and Bf performance, after all, the Russian tests back them up.