A newbies impression of the 109 and spit

[bongodriver]

Yeah I see the 'static' stability as what nasa like to call 'stick-fixed' stability, to be honest not an issue that causes difficulty in flight, it simply means the aircraft maintains attitude when displaced or simply put its light in pitch due to no resistance, hardly a bad quality in an aircraft you are trying to point around the sky to put a gunsight on target, if it were coupled with vicious characteristics in a stall it might cause problems, but again the Spit was known to be docile.

for an aircraft to be considered stable or unstable it really comes down to the dynamic stability, if the Spitfire was dynamically unstable as I believe the Spit bashers are claiming then it would have been impossible to fly in any form of manouvering due to the increasing amplitude of displacement and the subsequent pilot induced oscilations to correct it, by all accounts there was never an issue with this and further more it's practically inconceivable that a typical monoplane format aircraft of that era with a concentration of weight in it's nose to be dynamically unstable.

[bongodriver]

Kermode may be 'dated' by todays standards but given we are talking 1930/40's aircraft then it covers everything applicable for the time, the new deffinitions of stability you give really have come about since the advent of aircraft capable of all these different states.

[JtD]

Stick fixed is just one version of static stability, you're keeping the elevator angle constant over a speed range and check how the aircraft responds. In case of the Spitfire, no trim change occurred, so the plane would keep the same AoA over the entire speed range at the same elevator deflection. That's neutral stability.

The other version NACA was looking at is stick force stability, if you want to call it that, where you are keeping the stick force constant through the speed range and check how the aircraft responds. In case of the Spitfire, stick force increased with the speed, which in turn leads to smaller elevator deflections which means some sort of positive stability.

The Spitfire was dynamically stable.

All for longitudinal stability.

[Crumpp]

Originally Posted by JtD

Stick fixed is just one version of static stability, you're keeping the elevator angle constant over a speed range and check how the aircraft responds. In case of the Spitfire, no trim change occurred, so the plane would keep the same AoA over the entire speed range at the same elevator deflection. That's neutral stability.

First of all, stick fixed is not a version of static stability. Stick fixed is a control term and just means you are using the maneuver point that the pilot, controls, and the mechanical linkage has friction and mass. Static stability is the aircrafts initial reaction to displacement.

It has absolutely nothing to do with keeping the elevator constant. It is about the oscillations.

The other version NACA was looking at is stick force stability, if you want to call it that, where you are keeping the stick force constant through the speed range and check how the aircraft responds. In case of the Spitfire, stick force increased with the speed, which in turn leads to smaller elevator deflections which means some sort of positive stability.

They are looking for a stable gradiant. They are not keeping the force constant, they are looking for a slope as it moves away from trim speed. They are looking for a smooth increase in stick forces. The stick forces will change as they are based on velocity.

The Spitfire was dynamically stable.

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.
All for longitudinal stability.

[JtD]

I didn't say any of the parts in bold, which you claim to be quoting from me.

[Crumpp]

I'd like to see Crumpp provide some documentary evidence that Spitfires regularly broke up in flight during spin recovery

First of all, let's get what I said correct. Feel free to point out where I make any reference to "regularly". That is your own pointy tin foil hat theory.

I said it could happen to the Spitfire. The Operating Notes clearly warn the pilot of the hazardous longitudinal stability characteristics.

[Crumpp]

I didn't say any of the parts in bold, which you claim to be quoting from me.

Right, I did the bold.

[JtD]

Originally Posted by Crumpp

Right, I did the bold.

In that case, maybe you can next time avoid putting your words in my mouth by using proper formatting.

On the contents you added, I won't disagree with what you've said regarding the stick fixed and stick free stability testing, as it is absolutely right. You should, however, keep in mind that I try to explain things in a way that the concept can be understood by anyone interested, not just those with a suitable education or years of experience in the field. In my opinion, it is easier to understand "no trim change with constant elevator when speed changes" than to understand a description of an initial reaction to displacement.

However, I disagree with

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.

, because NACA says:
regarding dynamic stability:

only the short-period oscillation is dealt with here

and regarding neutral or negative stability which they recorded:

static longitudinal stability

.
To sum it up, NACA did not record long period oscillations for the Spitfire and the assessment of neutral or negative stability was made for static longitudinal stability.

[Crumpp]

To sum it up, NACA did not record long period oscillations for the Spitfire and the assessment of neutral or negative stability was made for static longitudinal stability.

They are talking about static stability. Wow, shows you how much things have changed and how new stability and control was as a science during WWII.

Static instability is horrible in an airplane. Seriously...the FAA and ICAO would send you back to the drawing board if you were seeking certification.

http://www.law.cornell.edu/cfr/text/14/23.173

The longitudinal dynamic stability (Long Period Oscillations) was neutral or negative as recorded by the NACA.

Poor choice of words on my part. Long period Oscillation has specific meaning and it should read:

The longitudinal dynamic stability (Oscillations over time) was neutral or negative as recorded by the NACA.

[JtD]

Static stability is the aircrafts initial reaction to displacement.

Your words. It is my understanding that this is exactly what NACA assessed and what they found to be neutral or unstable, only that they used a different method for testing.
NACA didn't have a problem with (short period) oscillations over time, the Spitfire would dampen any (short period) oscillations within a cycle or two.
NACA did have a problem with the fact that a bit of extra elevator at any speed, if not reduced, would change the AoA for good, meaning the plane would not come back to a more level attitude even when speed was reduced.

Looking at the easy to understand diagrams CaptainDoggles linked, neutral static stability appears to be exactly the problem NACA had with the Spitfire.

I see you deleted your last post, but it might still help if I leave this one up to make sure we all use the same terminology.