Stability and Control characteristics of the Early Mark Spitfires

[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

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.

[Crumpp]

Now let's get back to the NACA report so there is a better understanding of the issue.

We will look at a condition of flight essential to a dogfighter. The ability to make abrupt turns.

The pilot must be able to precisely control the amount of acceleration he loads on the aircraft. All aircraft performance depends on velocity. In order to get maximum performance out of the aircraft above maneuvering speed, Va, he needs to be able to make a 6 G turn and not exceed that load factor to prevent damage to the airframe. Below Va, the pilot needs to control the acceleration so that he does not stall the aircraft making the abrupt maneuver as well being able to maintain a maximum performance turn.

Doing that in an early Mark Spitfire was difficult and something only a skillful pilot could perform.

First the NACA report. Abrupt 180 degree turns were conducted at various entry speeds to gauge the level of control the pilot had in maintaining steady accelerations. The turns were also done to the stall point in order to gauge the behavior and amount of control.

"In turns at speeds high enough to prevent reaching maximum lift co-efficient" means turns above Va.





"By careful flying" a pilot can hold a steady acceleration. That agrees with the Operating Notes warning for the pilot to brace himself against the cockpit to get better control when making turns.

Now lets look at the measured results.



Here we see in a rapid left turn performed at 223 mph the test pilot is unable to hold constant acceleration on the airframe. Very small variations in stick movement and stick force changes of 1-3lbs results in large fluctuations in acceleration.

Next let's look at the pilots ability to control the accelerations in the pre-stall buffet.



Here we see the pilot was able to load the airframe to 5G's in 1 second to reach the pre-stall buffet 3 times. The smooth positive sloped portion of the curve represents the aircraft flying while accelerations are increasing. The top of the acceleration curve represents the pre-stall buffet. The bottom of the curve represents the stall point.

The take away is:

1. The large accelerations change for very little elevator movement.
2. The very rapid rate at which the pilot was able to load the airframe to 5G's.
3. The equally rapid rate at which the airframe unloaded down to 2G's when the pre-stall buffet was encountered. In 1 second, the aircraft went from 5G's to 2G's due to buffet losses. This means a rapid decay in turn rate resulted.
4. The violence of the pre-stall buffet combined with the longitudinal stability and control caused large fluctuations in the accelerations on the aircraft.


Last part of the NACA we will cover for today is the stick force travel. The amount of stick travel as measured by the NACA was not acceptable.




Next let's look at the opinion of Stability and Control Engineers on the Early Mark Spitfires.









Tomorrow I will post some of the plethora of references to this same issue of longitudinal instability as found in the Spitfire Mk I Operating Notes from July 1940. You will see the same references or similar to the same issue the NACA measured in the Spitfire Mk II Operating Notes.

There is no doubt that the Air Ministry was aware of the longitudinal instability of the early mark Spitfires.

[Glider]

A lot of words and a lot of effort into defend a position or a view but nothing to address the basic question:

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?

There is of course another inconvenient point that should be considered and that is have you done these calculations on the Me109E?

I say this because if you believe that the Spitfire to be dangerous and the German pilots considered the Spitfire to be much easier to fly than the Me109E, How dangerous do you think the Me109 was?

[MiG-3U]

Originally Posted by Crumpp

Which has what to do with the MAC on the weight and balance sheet?

It gives exact location of the leading edge needed for calculation (p.7).

Originally Posted by Crumpp

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

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

I claimed normal service load:

7.7" at normal service load 1939
7.9" aft limit for DeHavilland without inertia device
7.5" aft limit for Rotol without inertia device

This means that on normal combat load there was only change in the case of the Rotol prop assuming that aircraft was properly loaded.

Besides, the revised limits were originally issued sometime around 41/42.

Otherwise you seem to have chosen same tactics as in the FTH discussion so my part end here now.

Cpt Doggles, you see my point now.