Yep, it is not like switching aviation fuel. Weight and balance analysis is simple and easy to do.

Nothing puzzling there, you just forgot to underline the most relevant part for this thread and I fixed it for you; the bob weigh was added because the CoG had slipped too far aft causing londitudinal instability just like Quill notes.

Yep, when the CoG slips too far back, instability exist in pretty much every airplane, including Spitfire and Mustang. Bob weighs were used in some models of both fighters exactly for the same reason: CoG shift. Both aircraft were also stable when the CoG was forward enough.

Here is again A&AEE on stability of the early mark Spitfire:

http://www.spitfireperformance.com/k9787-fuel.html

CoG was at 5.8" aft datum line when the airplane was found to be longitudinally stable, that is 2" more forward than Naca tested. Note that Spitfire IX had CoG around 5" aft datum line at service load, no need for bob weigh.

Slow down and tell me where you get the 78.54 MAC on that sheet.

Do you know what percentage MAC is??

The reason the NACA used percentage MAC is because they did their own weight and balance analysis.

The ONLY number that is comparible...is the percentage MAC!!!

MAC as measured by RAE:

19.5+8.4 = 27.9/84 = 33.2%

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG 1.8% MAC FORWARD of the aft CG limit as defined by Supermarine.

If you want to use your 78.54 in MAC with our most narrow aft CG limit...

(19.5 + 5.4) / 78.54 = 31.7% MAC

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG .3% MAC FORWARD of the aft CG limit as defined by Supermarine.

http://www.spitfireperformance.com/k9787-fuel.html

Check it out...That is what the NACA said.....

:rolleyes:

Ok let's not go down the rabbit hole again.

You are using a weight and balance sheet that incorporates the longitudinal stability fix and is from February 1944 to prove the NACA conclusion was not correct.

Yes, the RAE addressed the issue of the longitudinal instability in the Spitfire around 1942. However, the Spitfires used in the Battle of Britain did not benefit from the fix.


This is Spitfire K-9787 and was tested in June, 1939.

http://www.spitfireperformance.com/k9787-fuel.html

If you click on the center of gravity link at the bottom of the page...

The weight and balance diagram is K-9788, the very next Spitfire off the production line.

http://www.spitfireperformance.com/k...cg-diagram.jpg

We can eliminate the February 1944 document from the thread as not applicable and conclude it is the result of the NACA findings.

Which brings us back too:

MAC as measured by RAE:

19.5+8.4 = 27.9/84 = 33.2%

NACA CG as flown = 31.4% MAC

The NACA flew the Spitfire with the CG 1.8% MAC FORWARD of the aft CG limit as defined by Supermarine.

You still have the question that has yet to be addressed. If in theory the Spitfire was so poor in its stability, why did all the pilots who flew it of every nation, sing its praises?

I should make clear that I do not doubt the calculations, but its a basic difference

Report RM2535

http://aerade.cranfield.ac.uk/ara/dl...rc/rm/2535.pdf

and (wing and datum point are the same on the Spitfire IX)

http://www.spitfireperformance.com/ab197datum.gif

They gave exact reference point, wing leading edge at the root and they (NACA) admited that their measurements for MAC maybe somewhat in error.

Well, just like in the other discussion, you ignore the data (USAF handbook) which does not support you and post some irrelevant stuff or accusations.

I don't play that game, let the people see the data and decide themselves.

The only thing that changed was the revised CoG limits for the aircraft with Rotol propeller and bob weighs if the CoG went beyond the normal limits, as often happened in the case of the Spitfire V.

As Quill noted, slight unstability was built in for purpose and prefered by pilots. However, stability margins were narrow and improper loading could easily cause problems.

Which has what to do with the MAC on the weight and balance sheet? You can be they did their own weight and balance analysis too!!!


Negative.

Spitfire K-9788 shows an aft CG that is 33.2%.

The most aft MAC with the Feb 1944 revision is 31.7%.

They closed up the CG limits to address the longitudinal instability.

You have presented the solution to the problem in an effort to claim the problem never existed.

Please start another thread if you feel the need to continue down this rabbit hole claiming the NACA could not perform a weight and balance analysis.