A newbies impression of the 109 and spit

[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.

[Crumpp]

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

Good diagrams and I have some others that will make it clearer for you all.

That diagram would make you think the elevator is held constant but it is not.

Keep in mind when that report was written there were not any standards of the day. It is not like testing processes or airworthiness. It was a very new science that was not covered in convention. In the 1980's there was even a "counter-revolution" in stability and control engineering.

[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.

[NZtyphoon]

Putting the Pilot's Notes in context:
Also issued along with the Pilot's Notes were Pilot's Notes General A.P. 2095 which explain the limitations in the Pilot's Notes, and the reasons for them: (2nd ed, 1943):

Part I Note A Flying Limitations
1. Introductory.
(i.) The Pilot's Notes for each type of aircraft lay down certain flying limitations. They state, broadly speaking, the demands which it is safe to make of the airframe. Non-observance of the flying limitations may lead to increased maintenance work, or, in extreme cases, to structural failure in flight.

(ii.) In the fixing of these limitations there is of course a margin or factor of safety allowed. This factor for airframes varies according to the degree of confidence with which their strength and likely stresses can be predicted, but is commonly around 2. This means, for instance, that a wing which is intended to withstand 4g should not break until 8g is imposed, but there is increasing risk of strain and failure as g rises above 4.

(iii.) The flying limitations also involve questions of safe handling from the aspect of controllability.

(iv.) In combat and emergencies pilots must take risks with their aircraft, balancing one risk against another; limitations must be strictly observed in so far as there is no sufficient reason to exceed them.

First, the Pilot's Notes were deliberately conservative to ensure that most pilots flew well within the limitations of the airframe thus avoiding too many overstressed, high maintenance aircraft on operational service.

However, the RAF acknowledged that in combat it was up to the pilot to choose what risks needed to be taken; whatever legal status the pilot's notes had in peacetime that legality could be overruled under combat conditions because if pilots were expected to fly by the rules all the time they were easy meat. One reason so many pilots were shot during their first combat was because they hadn't yet learned how to fly their aircraft at or beyond the limits set down in the pilot's notes.

4. Manœuvres not Permitted
(i.) Intended spinning of operational aircraft is permitted only in the case of certain approved single-engine fighters within the limitations stated in the Pilot's Notes. (Normal Methods will usually effect recovery - A.P. 129 Ch. III)

(iv.) The reasons underlying these prohibitions are partly considerations of aircraft strength and partly of control. Aircraft are designed to fulfill their operational role and not to perform manœvres of no operational value...

Why spin when you don't have to?

RAF Pilot's Notes for operational single-engine fighters which permit spinning:
Spitfire VII and VIII, IX & XVI, XIV & XIX: Spinning permitted but with a height limit of 10,000 feet.
Typhoon: permitted, without bombs or drop tanks
Tempest V: not permitted until proper tests had been carried out
Mustang III: only when rear fuel tank was empty
Corsair: Spinning not permitted