Stability and Control characteristics of the Early Mark Spitfires

[Crumpp]

The worst case scenario means here a Rotol propeller and CoG behind the aft limit for that configuration, like in the NACA tested Spitfire.

Fixed Pitch Wooden Airscrew

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'. Longitudinal stability records are attached.

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

[Crumpp]

Note that typical pre war or BoB service CoG for a Spitfire with DeHavilland propeller ok even for the Spitfires flying today.

Not with the longitudinal instability......

[NZtyphoon]

The worst case scenario means here a Rotol propeller and CoG behind the aft limit for that configuration, like in the NACA tested Spitfire.

Originally Posted by Crumpp

Instead of just being smart and making rolleyes how about proving, with documentation, that this statement is incorrect. Prove that you are not making a worst-case scenario out of just two documents: Prove that the Spitfire had such bad longitudinal stability characteristics that it affected its abilities in general flight and in combat and, above all PROVE that this can be replicated in a flight sim made for PCs.

[Crumpp]

Instead of just being smart and making rolleyes how about proving, with documentation,

This is the second or third time in this same thread the same argument has arisen.

The instability existed in all early mark Spitfires at normal and aft CG until it was fixed with the inertial weights.

It is a function of the tail design and elevator, static margin, and fuselage length.

The Operating Notes are full of warnings about it. It was not limited to one propeller or a specific load.

It was at NORMAL and AFT cg.

NORMAL....

Noun: The usual, average, or typical state or condition.

Your whole premise of the Constant Speed Propellers being the "most adverse condition" is just plain wrong.

Which do you think is heavier? A three bladed CSP or a two blade fixed pitch wooden propeller? The correct answer is the CSP.

What do you think happens to the CG when you add weight to the front of the aircraft? Do you think it shifts forward or back?

[NZtyphoon]

Originally Posted by Crumpp

This is the second or third time in this same thread the same argument has arisen.

The instability existed in all early mark Spitfires at normal and aft CG until it was fixed with the inertial weights.

It is a function of the tail design and elevator, static margin, and fuselage length.

The Operating Notes are full of warnings about it. It was not limited to one propeller or a specific load.

Prove that you are not making a worst-case scenario out of just two documents: Prove - with documentation - that the Spitfire had such bad longitudinal stability characteristics that it affected its abilities in general flight and in combat and, above all PROVE that this can be replicated in a flight sim made for PCs.

[Crumpp]

Here is a little experiment you can do at home, NzTyphoon.

Make a paper airplane. Toss it.....

See how stable it flys.

Now add a paperclip to the nose and throw it again.

Which is more stable?

[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.

[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 )