A newbies impression of the 109 and spit

[bongodriver]

Go look up a reference that proves me wrong, showing the spitfire had positive static stability.

I don't claim the spitfire had dynamic stability, I say it is neutral, which simply means it does not try to return to it's original condition, as opposed to dynamic instability where the aircraft would increase it's amplitude of error, enough proof against dynamic instability is the fact the aircraft was not notorious for PIO (pilot induced oscilation) incidents, neutral stability is a desireable quality for an aircraft built purpousely to be agile/manouverable......seems Mitchel designed it just right.

I attached a scan from Kermodes 'Mechanics of flight' and have highlighted in red the 3 conditions, perhaps there are differences in our understanding but I feel it explains where I'm coming from.

[CaptainDoggles]

Kermode's terminology is a bit dated, I think. What's the publication date?

Most textbooks and the industry people I know prefer the following breakdown:

• Dynamic Stability:
  • Positive
  • Negative
  • Neutral
• Static Stability:
  • Positive
  • Negative
  • Neutral

[bongodriver]

Originally Posted by CaptainDoggles

Kermode's terminology is a bit dated, I think. What's the publication date?

Most textbooks and the industry people I know prefer the following breakdown:

• Dynamic Stability:
  • Positive
  • Negative
  • Neutral
• Static Stability:
  • Positive
  • Negative
  • Neutral

can you scan or point me to a source for this, seems a bit odd th have these distinctions......what's the difference between neutral dynamic stability and neutral static stability?

[CaptainDoggles]

I'll see if I can provide you a scan later today, but in the interim have a look at the Centennial of Flight website which is run by NASA, so I'd say it's a reasonably authoritative source.

http://www.centennialofflight.gov/es...ility/TH26.htm

Edit: You can also have a look at this textbook, an earlier edition of which I used in school. Stengel's Flight Dynamics

[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.

[CaptainDoggles]

Originally Posted by 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.

The laws of physics haven't changed since the war. Stability isn't something that an aircraft is "capable of".

Originally Posted by bongodriver

if the Spitfire was dynamically unstable as I believe the Spit bashers are claiming

I don't think anyone here is claiming the spit was dynamically unstable, nor would I say anyone here is a "Spit basher".

It's not "bashing" the spitfire to say it was not statically stable. Facts are facts. If you're going to get emotionally invested in this discussion then I'd rather not engage with you.

[bongodriver]

Originally Posted by CaptainDoggles

I don't think anyone here is claiming the spit was dynamically unstable, nor would I say anyone here is a "Spit basher".

Well a 'spit basher' would say that, no point admitting to it if you hope to keep under the bias radar and hide the emotional investment in it's counterpart, ok glad we cleared the dynamic stability issue though as thats the only real stability issue which would cause significant problems.

Originally Posted by CaptainDoggles

It's not "bashing" the spitfire to say it was not statically stable. Facts are facts. If you're going to get emotionally invested in this discussion then I'd rather not engage with you.

it is 'spit bashing' if that characteristic is being used as an example of a stability problem, I don't mind if you don't wish to engage with me, please don't be under the impression I need it.

Originally Posted by CaptainDoggles

The laws of physics haven't changed since the war. Stability isn't something that an aircraft is "capable of".

understanding of physics has however, stability is an inherrent capability of an aircraft, you must realise that a Cof G shift can cause a change in stability.

[CaptainDoggles]

Originally Posted by bongodriver

Well a 'spit basher' would say that, no point admitting to it if you hope to keep under the bias radar and hide the emotional investment in it's counterpart, ok glad we cleared the dynamic stability issue though as thats the only real stability issue which would cause significant problems.

Is it your favorite aircraft or something? I've stated nothing but facts and you're calling me biased.

it is 'spit bashing' if that characteristic is being used as an example of a stability problem, I don't mind if you don't wish to engage with me, please don't be under the impression I need it.

I never said it was a problem. I just said the spitfire is not statically stable.

understanding of physics has however, stability is an inherrent capability of an aircraft, you must realise that a Cof G shift can cause a change in stability.

CG shift has nothing to do with the topic at hand.