Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

Prove - with documentation

ok

The Stability and control characteristics of the Early Mark Spitfires:

Now let's look at the Spitfire in an abrupt pull out as measured by the NACA.



First thing to notice is the stick forces. There are light but acceptable in abrupt pull outs. While very steep, the slope of the curve matches our acceleration curve and the controls float without overcoming the inherent stability of the design. The steepness of the curve tells us the pilot is able to very rapidly load the airframe. In fact, the NACA had to make allowance in their stick fixed measurements to prevent damage to the aircraft from acceleration because of the rapid onset the controls allowed.

However, if we look at the acceleration curve we see an abrupt change and not the desirable smooth curve. This points to the stability characteristics contributing to the rapid fluctuations in acceleration that the aircraft exhibits under other conditions.

Next we will get into the unacceptable longitudinal stability characteristics of the design.

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 let's 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.

Taking two point we can compare the slope of the curves of stick input to acceleration over time.

For the intital pull up:

Acceleration over time 3.5G-(-.5G) divided by 4.5s-3.5s = m
m = 4

Stick force over time: (19lbs - 0lbs) divided 5lbs/G all divide by 4.5s-3.5s = m
m = 3.8

*The slopes should match and they are close enough.* +However, our stick force grows at a slower rate than our acceleration.+ This is the initial input of the pilot.

Now let's see the instability.

Stick force over time 15lbs-15lbs divided by 5lbs/G all divided by 6.8s-5.5s = m
m = 0

Of course m = 0, our stick is held fixed by the force measurement equipment

Acceleration over time 4.2G-3.2G divided by 6.8s-5.5s = m
m = .76

So, while our stick remains fixed, the aircraft continues to accelerate on its own. As the nature of instability, there is no correlation stick force input and acceleration.

Now, our pilot in this case only input force to reach 3.5G. In a stable airplane, we should see the aircraft dampen all subsequent accelerations which means the aircraft would not exceed 3.5G without control input.

In this case, the instability or divergent oscillation a 4.2G acceleration with stick fixed slightly below the stick force required to produce a 3.5G 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 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.









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

Just some of the many references to the Longitudinal instability found in all of the early Mark Spitfires.

Spitfire Mk I Operating Notes, July 1940:

[DC338]

Originally Posted by Crumpp

ok

Now let's 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.

Taking two point we can compare the slope of the curves of stick input to acceleration over time.

For the intital pull up:

Acceleration over time 3.5G-(-.5G) divided by 4.5s-3.5s = m
m = 4

Stick force over time: (19lbs - 0lbs) divided 5lbs/G all divide by 4.5s-3.5s = m
m = 3.8

*The slopes should match and they are close enough.* +However, our stick force grows at a slower rate than our acceleration.+ This is the initial input of the pilot.

Now let's see the instability.

Stick force over time 15lbs-15lbs divided by 5lbs/G all divided by 6.8s-5.5s = m
m = 0

Of course m = 0, our stick is held fixed by the force measurement equipment

Acceleration over time 4.2G-3.2G divided by 6.8s-5.5s = m
m = .76

So, while our stick remains fixed, the aircraft continues to accelerate on its own. As the nature of instability, there is no correlation stick force input and acceleration.

Now, our pilot in this case only input force to reach 3.5G. In a stable airplane, we should see the aircraft dampen all subsequent accelerations which means the aircraft would not exceed 3.5G without control input.

In this case, the instability or divergent oscillation a 4.2G acceleration with stick fixed slightly below the stick force required to produce a 3.5G acceleration.

Now I understand that Figure 15 does hint at what you are getting at yet I see no such problem in figures 16, 17 & 18 of the same report and you don't seem to analyse them in your argument? Odd as they are essentially they are same test as figure 15 but in the opposite direction (16) and at higher speeds (17 & 1. Looks like a relatively constant G was held throughout by the pilot. Or am I missing something?

The report alludes to "careful" flying. Does that mean "not careful" flying in the other charts.

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 amount of stick travel as measured by the NACA was not acceptable.

Yet it does not say that is was dangerous flying quality. It just did not meet the Requirements laid out in report 755. It was not built to that standard so should it surprise that it doesn't meet all of them?

Now on the Spit V they did use a inertia weight to combat over sensitive elevators on that Mark. Why did they not demand a retro fit of inertia weights to the MK I & II that would have been in the OTU squadrons at the time if it was such a problem?

[robtek]

I think of the Spit like a Porsche 911, a great car which is a delight to drift around corners, but you really have to work to hold the thin line before it bites you in the a**.

With a regular driver it is still a great sportscar and outperforms many of its competitors, but to have the edge you have to be a pro.

The same will be with the 109, where the pilot has the opposite problem of too high stick forces at high speeds.

Each needs his own tactic to use the quirks of ones plane for optimum efficiency.

[SlipBall]

Originally Posted by robtek

I think of the Spit like a Porsche 911, a great car which is a delight to drift around corners, but you really have to work to hold the thin line before it bites you in the a**.

With a regular driver it is still a great sportscar and outperforms many of its competitors, but to have the edge you have to be a pro.

The same will be with the 109, where the pilot has the opposite problem of too high stick forces at high speeds.

Each needs his own tactic to use the quirks of ones plane for optimum efficiency.

This is very true

[NZtyphoon]

Originally Posted by DC338

Now I understand that Figure 15 does hint at what you are getting at yet I see no such problem in figures 16, 17 & 18 of the same report and you don't seem to analyse them in your argument? Odd as they are essentially they are same test as figure 15 but in the opposite direction (16) and at higher speeds (17 & 1. Looks like a relatively constant G was held throughout by the pilot. Or am I missing something?

Yet it does not say that is was dangerous flying quality. It just did not meet the Requirements laid out in report 755. It was not built to that standard so should it surprise that it doesn't meet all of them?

Now on the Spit V they did use a inertia weight to combat over sensitive elevators on that Mark. Why did they not demand a retro fit of inertia weights to the MK I & II that would have been in the OTU squadrons at the time if it was such a problem?

Slight correction on the Mk V - the reason the inertia weights were added was to help overcome a problem with poor cg loading at a squadron level, plus the added weight of new equipment not used in Spitfire Is and IIs (see Quill http://forum.1cpublishing.eu/showpos...&postcount=781 )

[Crumpp]

The report alludes to "careful" flying. Does that mean "not careful" flying in the other charts.

Good questions.

No, these where done with "careful flying". If you read the report, they were done with force measurement equipment hooked to the controls.

The pilot could move the stick but when he let go, the equipment held it fast so the forces could be measured.

He could move the stick if needed to keep the accelerations within safe limits.

So, the controls in the test are done with about as careful flying as you can get. Most Spitfire pilot did not have a force gauge holding their controls fixed.

Yet it does not say that is was dangerous flying quality

It is dangerous under conditions the pilot can overload the airframe. That is why you see the incidence of in-flight structural failure's and the warnings in the Operating Notes.

It is dangerous when you need to shoot accurately and it is dangerous when you need to make an abrupt maneuver to avoid and enemy attack.

Yes, it can be controlled by the pilot and mitigated by his skill level. It requires such input.

You have hit upon the entire reason the Air Ministry did not have stability and control standards outside of pilot opinion.

The British were major pioneers in stability and control in the beginning but kind of floundered after World War I.

[Crumpp]

the reason the inertia weights were added was to help overcome a problem with poor cg loading at a squadron level

Again, the RAE may have blamed it on that but they were also behind in Stability and Control research. The NPL pretty much stagnated until the efforts of Gates and Lyons came to fruition post war moving AWAY from the conclusion stability and control could not be defined without pilot input.





The opinion of the NACA was much different and their test aircraft was NOT overloaded and at a normal CG.

[bongodriver]

Notice how the problem was 'tails' breaking and not 'wings' bending that were the main case for structural failure, of course Crumpps highlight there mentions nothing about early Spits and in fact probably is refering to the MkV.

[Al Schlageter]

It is dangerous when you need to shoot accurately and it is dangerous when you need to make an abrupt maneuver to avoid and enemy attack.

Spitfire pilots didn't seem to have that much trouble shooting down German a/c during the BoB despite being out numbered in the air over south-east England.

That is why you see the incidence of in-flight structural failure's and the warnings in the Operating Notes.

There are such warnings for American a/c.

[Crumpp]

Why did they not demand a retro fit of inertia weights to the MK I & II that would have been in the OTU squadrons at the time if it was such a problem?

I know it was later modified with an inertial elevator. They did something to correct it, you can bet.

The fact remains, the RAE skirted around the problem because they had no real estabilished foundation for what to do with longitudinal instability.

Especially when the pilot's opinion ran contrary.

It is really interesting if you like the history of technological development. There was a guy in England who laid down all the math just before World War I. It was in center of pressure and metacenter so his mechanics were not completely correct but all his principles were as well as the use of polynomial co-efficients to describe motion. Professor GH Bryan's really cracked the nut on stability and control.

Some of his conclusion's are used today. The problem was when he tried to explain it, it was so complicated that most engineer's eye glazed over, mouths came open, and the drooling begain. Then, some pilot would hop in the same plane his big complicated set of equations had predicted was unstable and fly off in it.

You can control an airplane that is unstable, especially the long period oscillation the NPL became focused on. The 1903 Wright Flyer was so unstable, the techique used to land it was to fly close to the ground at low velocity and let the skids touch on the downward oscillation.

You could not estabilish a stabilized approach that is common in todays airplanes.

They flew extremely unstable aircraft all the time in the early days of aviation. The velocity and forces were low enough that stability and control just was not that important.

the major flight characteristic ever present is the feeling that if you took your hands off the stick or your feet off of the rudders, the Eindecker would turn itself inside out or literally swap ends." He also indicates that the all-moving surfaces continually hunted back and forth with an attendant feedback into the pilot's hands and feet. These characteristics describe an aircraft that by modern standards would be considered unpleasant to fly, would be unlicensable, and certainly would inspire little confidence in the mind of the pilot.

http://www.hq.nasa.gov/pao/History/SP-468/ch2-2.htm

That all changed with the advent in the powerful monoplane fighters of World War II. The speed and forces involved pushed the science of stability and control to the forefront.