Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

that story about the NACA not having any drawing is true... but they neither had any for the 109 they tested

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

[MiG-3U]

Originally Posted by Crumpp

Why are you regurgitating the same stuff I just posted. Puzzling...

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.

Originally Posted by Crumpp

The fact Supermarine recognized that longitudinal instability and took measures to fix it invalidates any pointy tin foil hat theory the instability did not exist.

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

(v) Stability - The aircraft is laterally stable at all speeds except in the immediate vicinity of the stall when it is unstable. The aircraft is directionally stable engine 'OFF' and 'ON' at all speeds, but on the climb this is difficult to assess owing to insufficient rudder bias. Longitudinally, the aircraft is stable with centre of gravity forward, but is unstable with centre of gravity normal and aft with engine 'OFF' and 'ON'.

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.

[Crumpp]

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...

CoG range 5.4"-7.9" aft of datum point, no inertia device needed

(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.

[Crumpp]

Longitudinally, the aircraft is stable with centre of gravity forward, but is unstable with centre of gravity normal and aft with engine 'OFF' and 'ON'.

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

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

[Crumpp]

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.

[Glider]

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

[MiG-3U]

Originally Posted by Crumpp

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

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

Originally Posted by Crumpp

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!!!

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

Originally Posted by Crumpp

WTF, are we going down the rabbit hole like your "superchargers see pressure altitude"???

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.

[MiG-3U]

Originally Posted by Crumpp

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.

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.

[Crumpp]

Report RM2535

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!!!

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.

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.

[Crumpp]

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.