Why is the Spitfire Mk IX still unstable??

Here you go.

CG limits are set by the design. The forward limit by the ability of the elevator to lift the nose at landing speeds and the rear limit is determined by the stability margin.

The stability margin is determined by the distance between the aircraft's aerodynamic center and the CG, in this case when the CG is at the rearward limit. That is usually the point a "stable" airplane becomes a "neutral" airplane as is referred too as the Neutral Point.

In order to change the stability margin, we have to change airplane.

The short answer is we have to change the design of the wing, horizontal stabilizer, or length of the fuselage. Below is an Aerodynamic Center Calculator were the reader can change the parameters and see what causes a movement of the AC of the aircraft:

http://adamone.rchomepage.com/cg2_calc.htm

No design changes that move the AC occurred on the Spitfire. Instead of fixing the stability of the airplane, the designer attempted to increase the pilots ability to control the instability of the aircraft.

What is the characteristics of the Spitfire's stability? It had weak static stability and at normal to rearward CG was dynamically unstable.

That means it oscillated and required double controlling.

Here is an example of what neutral and unstable(divergent) Dynamic longitudinal stability or stability over time looks like:

How do we know the Spitfire Mk I, Mk V, and Mk IX all have the same Aerodynamic Center and Stability Margin?

Well, the Rear CG limit which defines the aircraft's AC relationship remains the same from the Spitfire Mk I thru the Spitfire Mk IX. So without a design change to move the AC and the CG limit is the same....the aircraft's stability characteristics remain unchanged.

Spitfire Mk I:

Spitfire Mk IX:

Both have the same rearward limit of 9in so the stability margin is the same. Interestingly, the 9inches represents a 36.2% MAC for the Spitfire and falls close enough into the 15% to 40% range found in most conventional aircraft.

So, it is a fact, the NACA report on the Flying Qualities of the Spitfire is applicable. Supermarine correctly followed the NACA's advice and did not choose mass balancing for the aircraft but instead choose to aerodynamically balance the elevator. This does represent a design change that moves the aircraft's AC.

The large fluctuations in load factor and inability for the pilot to precisely hold a specific load factor are a function of the aircraft's weak static longitudinal stability. Static stability is the first movement the aircraft makes when the stick is released. It answers the question does the airplane move to return to trim speed or does it move away from trim speed.

So what does this aerodynamically balanced elevator do for the Spitfire pilot? It makes the aircraft more controllable. The area of the Horizontal stabilizer/elevator remains unchanged therefore no AC movement.

It changes the feel of the aircraft to the pilot by increasing his resistance to control movements.

The Spitfire Mk IX, XI, and XVI POH:

The aerodynamic balancing makes the stick force per G have a sharper gradient and increase the forces the pilot feels.

What the airplane feels:

What the pilot feels and how the aerodynamic balancing works:

How did this airplane become such a hot topic in Stability and Control Engineering classes today?

Well, it seems one of Mitchell's contemporaries at the RAE found the answer. It was a common belief at the time among designers that elevator control force requirements changed at high speeds. Therefore, many designers worked to lighten the control loads on the the elevator in an attempt to maintain elevator authority at high velocity. Gates proved that elevator control authority was independent of velocity. In other-words, an elevator that was properly set up at low speeds will be properly set up at high speeds.

The second reason is the RAE lacked any standard for stability and control. Therefore, pilot opinion was not quantifiable and defined. In other-words, the pilots and the engineers did not communicate very well.

Here you can see two reports on Spitfire Mk IX ML186. Both test's are preformed within 10 days of each other to confirm the same things.

It was testing the Spitfire Mk IX with a rear fuselage tank, 7 1/4 inch bob weight, and newly designed metal elevator.

One pilot rates the aircraft as unacceptable for formation flying. Jeffery Quill rates it as acceptable. The other pilot was certainly inferior in his skills to the legendary Jeffery Quill.

Standards were designed to eliminate the Chuck Yeager and Jeffery Quill's factor from advancing a design that required such high levels of piloting skill to be "acceptable".

Spitfire Flying Qualities.pdf

it states clearly the bob weight is not recommended for the redesigned elevators.

Right...that is because they aerodynamically balanced the elevator.

The bobweights are a form of mass balancing and not the ideal solution....quicker and cheaper than aerodynamic balancing though. Mass balancing and aerodynamic balancing do the same thing. They just have different shape to the curve as to what the pilot feels.

Right, and the Mk IX had a redesigned elevator.....so what is your point?

Also you claim that the AC is unchanged in the Spitfire therefore it remained (in your opinion) unstable, but you bizarrely forget that the stability is a relationship between AC and CoG, and CoG most certainly does change, especially in the Mk IX with the longer nose and bigger/heavier engine, you also even more bizarrely seem to suggest that a rearward CoG uniquely causes the Spitfire to become unstable when in fact this is a universal phenomenon and in this thread you have simply highlighted how Supermarine overcame an issue arising from the condition in a more sophisticated manner than crude mass balance.

It has taken you months since you got 'stuck in south America' to fabricate this sad and desperate response to perpetuate your famous agenda and you didn't even do a good job, it's still a recycled and irrelevant NACA report on a MkV and huge contradictions.

Right, and the Mk IX had a redesigned elevator.....so what is your point?

Same area so no movement of the AC, just aerodynamically balancing the elevator in place of the bobweight.

rearward CoG uniquely causes the Spitfire to become unstable

It was unique because the instability showed up at normal CG range to rearward. It was even more unique in because despite that instability, the aircraft had a long, distinguished career and was loved by its pilots. That is what makes the aircraft unique and why it is a topic in many engineering departments when teaching stability and control.

The NACA Flying Qualities investigation was only 4.8 inches aft of datum and the normal take off CG is 6.8 inches aft of datum in the Spitfire Mk IX.

Lots of airplanes have been unstable and done just fine.

Lindbergh was a master of understatement. The aircraft is dynamically and statically unstable. "It is one of the worst flying airplanes I've ever flown," said Robert "Hoot" Gibson, former space shuttle commander. "It's a challenge to keep the airplane going straight and make it do what you want."

 

"The Spirit is too unstable to fly well on instruments. It is high-strung, and balanced on a pinpoint. If I relax pressure on stick or rudder for an instant, the nose veers off course," commented Lindbergh.

Lindbergh was "thankful we didn't make the Spirit stable. The very instability which makes it difficult to fly blind or hold an accurate course at night now guards me against excessive errors." He credited the instability with keeping him awake and alive.

http://www.charleslindbergh.com/plane/spirit.asp

The control stick was as long as a baseball bat — too tall, I thought, and I tended to grip it low. As my first flight wore on, however, I noticed that my right hand kept inching higher on the stick to gain the leverage needed to combat large control forces.

See some similarities in what the Supermarine engineers did to the Spitfire and what made the "Spirit of St Louis" a controllable aircraft despite its instability?

No, strangely enough I can find no comparison with a spitfire and the spirit of St. Louis.

You are aware the Mk IX spit was longer in the nose than a MkV aren't you? You are aware this will affect the 'datum'?

You are aware the Mk IX spit was longer in the nose than a MkV aren't you?

Does not effect the stability margin of the aircraft.

You are aware this will affect the 'datum'?

Does not effect the "datum". It effects the CG location. I have already told you the normal take off CG location on the Spitfire Mk IX. It is 29% MAC...burn ~20 gallons of gas and the Spitfire Mk IX is at 31.4% MAC.

Does not effect the stability margin of the aircraft.

 

 

 

Does not effect the "datum". It effects the CG location. I have already told you the normal take off CG location on the Spitfire Mk IX. It is 29% MAC...burn ~20 gallons of gas and the Spitfire Mk IX is at 31.4% MAC.

You said in the opening post that changing the length of the fuselage is an option, so I remind you that Is the case with the MkIX and you contradict your own theory........why am I not surprised.

If the 'datum' is the end of the nose then it will change, but thanks for confirming the simple fact the CoG location is changed.

...In order to change the stability margin, we have to change airplane.

The short answer is we have to change the design of the wing, horizontal stabilizer, or length of the fuselage.

Exactly what happened with the Mk IX cf the Mk I or VA - the wing was redesigned (including introducing extended or clipped wingtips, changing the internal layout and weight distributions, etc), the fuselage was lengthened and the design of the stabilizer was changed.

Interestingly, the 9inches represents a 36.2% MAC for the Spitfire and falls close enough into the 15% to 40% range found in most conventional aircraft.

In other words there was nothing abnormal about the Spitfire's MAC cf other "conventional aircraft" (or is Crumpp saying that the Spitfire wasn't a conventional aircraft design for its time?)

What is the characteristics of the Spitfire's stability? It had weak static stability and at normal to rearward CG was dynamically unstable.

 

That means it oscillated and required double controlling.

Crumpp has never flown a Spitfire IX in real life, nor do we have accounts from IX pilots stating that the IX "oscillated and required double controlling" under normal loading conditions. Had the Spitfire IX been such a handful at normal, or at it's rearmost CG limits, it would never have been used for low-medium altitude ground attack missions while loaded up with bombs and/or drop tanks

How did this airplane become such a hot topic in Stability and Control Engineering classes today?

We only have Crumpp's word on this, with no evidence to prove that the Spitfire IX was/is a "hot topic". (The only one who has so far made this a "hot topic" is Crumpp, who has pushed this subject many times, both here and in other forums.)

Here you can see two reports on Spitfire Mk IX ML186. Both test's are preformed within 10 days of each other to confirm the same things.

 

It was testing the Spitfire Mk IX with a rear fuselage tank, 7 1/4 inch bob weight, and newly designed metal elevator.

 

One pilot rates the aircraft as unacceptable for formation flying. Jeffery Quill rates it as acceptable. The other pilot was certainly inferior in his skills to the legendary Jeffery Quill.

Standards were designed to eliminate the Chuck Yeager and Jeffery Quill's factor from advancing a design that required such high levels of piloting skill to be "acceptable".

 

No mention of a 7 1/4 inch bob weight - the CG limit was 12.2" aft, cf Crumpp's 6.8" (see below) so it shouldn't be surprising that the stability margin was dicey; Quill did not specify that the IX in this condition was acceptable for formation flying - his comment was "just about acceptable" - hardly a ringing endorsement, but not surprising for an aircraft being tested with its CG 3.2" aft of its normal rearmost limit.

Note that Crumpp maintains that:

...and the normal take off CG is 6.8 inches aft of datum in the Spitfire Mk IX.

So why should it be such a big surprise that an aircraft loaded to 5.4" inches aft of its normal take off datum proves to be more unstable than normal? None of this "proves" anything, other than the Spitfire IX was surprisingly forgiving to fly, even under extreme loading conditions.

Definitely the Spitfires were hard to fly in formation, even taking off in formation.

Spitfires of many MKs.

Exactly what happened with the Mk IX cf the Mk I or VA - the wing was redesigned (including introducing extended or clipped wingtips, changing the internal layout and weight distributions, etc), the fuselage was lengthened and the design of the stabilizer was changed.

Internally redesign of the normal wing without changing the shape/area does nothing for AC movement.

Reducing the wing area tends to move the AC rearward.

In other words there was nothing abnormal about the Spitfire's MAC cf other "conventional aircraft"

The NACA tested the Spitfire at 31.4% MAC and found it to be unsatisfactory. That simply confirms the conclusion the aircraft is unstable to neutral at normal and aft CG ranges.

Crumpp has never flown a Spitfire IX in real life,

Neither have you, what is your point?

We only have Crumpp's word on this, with no evidence to prove that the Spitfire IX was/is a "hot topic".

No, you can run the math and understand the science.

Plus you can look through textbooks on stability and control! :)

No mention of a 7 1/4 inch bob weight - the CG limit was 12.2" aft, cf Crumpp's 6.8" (see below) so it shouldn't be surprising that the stability margin was dicey; Quill did not specify that the IX in this condition was acceptable for formation flying - his comment was "just about acceptable" - hardly a ringing endorsement, but not surprising for an aircraft being tested with its CG 3.2" aft of its normal rearmost limit.

Again, the lack of standard causes two pilots to come to very different conclusions based on their individual skill.

Here is the report, the 7 1/4 bobweight was essential to allowing the aircraft to have a rear fuselage tank. I am sure the stick force gradiant was unstable at this point and the designs CG so far aft so the predictions were not accurate.

http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf

Why should it be such a big surprise that an aircraft loaded to 5.4" inches aft of its normal take off datum proves to be more unstable than normal?

Where is this 5.4 number coming from, btw? Clarify it because your statement does not make sense for the specifics.

However, we might be getting somewhere but not the for reasons' you are thinking.

Think about it.... :smilewink:

The normal take off CG range of the Spitfire Mk IX at 6.8 inches aft of datum has been moved further aft than the normal take off CG was in the Mk V.

Is it any wonder it retains the instability of the MkV? :thumbup:

Definitely the Spitfires were hard to fly in formation, even taking off in formation.

Spitfires of many MKs.

 

 

What does this have to do with the conclusion from the testing of ML186 Spitfire Mk IX being tested with a rear fuselage tank?

Nothing.

Additionally, it is the test pilots that came to that conclusion. Is your post an attempt to undermine their credibility or something? What is your point?

Wow, the Spitfire developed phugoids. I really can't think of any other WW2 fighter aircraft that did the same. Except for all of them.

Yo-Yo says:

You think that Mk IX will make you an absolute winner?

I have bad news for those who consider P-51 too stick sensitive and thus prone to stall...

As Spitfire has neutral stability there is only 3/4" of stick travel to stall as it was reported by NACA. Really silk hands or full scale joystick required...

It will be no mercy, hardcore only - all will be as Mitchell designed.

MMMM......:smilewink:

Looks like I am not the only one who understands the issue.

JtD

Dynamic stability is not even the subject, JtD.

I'm just referring to the first chart in your opening post. Of course I know you're subject to changing the subject whenever facts don't suit your advertised fantasies, so no problem if you're doing it again.

JtD says:

I'm just referring to the first chart in your opening post.

Without reading the post. Yes, the Spitfire was dynamically neutral to unstable and that does increase the pilot work load to hold altitude, formation, or any specific attitude of flight but none of the design changes to increase control where done to specifically address the long period oscillation.

You do understand that, right?

Crumpp says:

The large fluctuations in load factor and inability for the pilot to precisely hold a specific load factor are a function of the aircraft's weak static longitudinal stability.

With the exception of one chart, the rest all show the static stability of the aircraft and simply cannot represent ANYTHING else.

So, outside of using the word phugiod in a sentence, please help me to understand your point.

Thank you!

You named the topic "why is the Spit IX still unstable" and the only instability it had are the long period oscillations. Like most other fighter aircraft of that time.

Low to neutral stability OTOH isn't unstable, so what's your point? To me it looks as if you were trying to redefine the term "unstable" so that it covers "unstable and the Spitfire".

From my perspective, we're either speaking about Spitfire instability and then about phugoids or we don't speak about Spitfire instability at all.

This again.

Since you seem to have forgotten:

Anybody who claims to be presenting any information regarding Spitfires would do well to read cover to cover, and then re-read Jeffrey Quill's book 'Spitfire' ISBN 0-09-937020-4. He covers the whole gamut of stability, bob-weights and the redesigned elevator thoroughly and to those approaching this issue without agenda would find some of the apparent issues here raised by our renowned Spitfire bashers a little stretched.

 

Quill regarding stability in the MkI:

 

page 231: "I remarked, however, that the stability margin was tight"

page 231: "The aeroplane was on the borderline of instability..."

 

Quill regarding the Mk. V Issue:

 

Page 232: "...a fair amount of additional of additional operational equipment had gradually crept into the aircraft."

Page 233: "Therefore for each sub-variant of the Mk.V (and there were Va, Vb and Vc)detailed instructions for the correct loading of the aircraft were issued to squadrons to ensure that the centre of gravity remained within the limits set down."

Page 233: "However, the importance of these loading instructions was not generally appreciated in squadrons in the daily round of operational activities they tended to be disregarded."

 

He goes on to report that he and another production test-pilot who had both been flying correctly loaded MkVs had the chance to fly some line aircraft and found that those flying on the squadrons to be almost dangerously unstable - so to summarise more accurately, the bob-weights were introduced as an emergency measure more to counter the effects of poor following of procedure at squadron level than some inherent design flaw of an unstable aircraft.

 

As for the revised elevator:

 

"The 'Westland elevator' [an experimental type not fitted operationally - DD_Fenrir], as it became known , demonstrated the way forward; but our aerodynamicists at Hursley Park thought that an even more effective answer could be obtained by enlarging the horn balance of the standard elevator and this we did by stages. The effect was astonishing. At last a way had been found to improve the basic static stability margins of the aeroplane..."

 

So we have an aircraft that is not inherently unstable; it just has a small range in which the CoG can reside before the aircraft becomes unstable. We have bob-weights introduced as an overarching policy to Mk.V aircraft only (certainly not required on aircraft whose loading instructions have been correctly followed) to mitigate the repercussions of incorrect loading, and an elevator design that renders the bob-weights superfluous - all before the Mk.IX variant we are reputed to be having in game saw service.

 

You know, with the slightly longer nose and the increased weight of the supercharger, this increased moment forward of the CoG could well hint that the Mk.IX was a more stable aircraft than a Mk.V anyway, even before the improved elevator.

Still, the major question is why Crumpp you have such a hard on for the stability reports of a Mk.V airframe which given the known issues as thoroughly covered by Quill above had narrow stability margins, when we have the Mk.IX coming, a related but substantially different aircraft in weight, distribution of said weight and also design of aerodynamic flying surfaces which impact the stability of the a/c?

Why are you pushing this?

And yes, Yo-yo's right - as he is for most aircraft in this sim! Virtual flying with a stick that has only 5-6cm of horizontal travel will be an issue with any sim aircraft when that range of movement is equivalent to 3 or 4 times that much in a real aircraft - it is always going to cause issues, particularly in aircraft with sensitive response to elevator movement.

So what's your point?

Internally redesign of the normal wing without changing the shape/area does nothing for AC movement.:

if the redesign changes the weight then it will have an impact on the CoG, and therefore will change the AC/CoG relationship.

Reducing the wing area tends to move the AC rearward.

Tends to? or does, tends suggests it could go either way and is purely circumstantial.

The NACA tested the Spitfire at 31.4% MAC and found it to be unsatisfactory. That simply confirms the conclusion the aircraft is unstable to neutral at normal and aft CG ranges.

NACA tested a MkV, not a MkIX, maybe we should base 109G data on 109E data?

Neither have you, what is your point?

His point still stands, it is you who claims with outright conviction of Spitfire MkIX instability without a shred of credible let alone official evidence.

No, you can run the math and understand the science.

 

Plus you can look through textbooks on stability and control! :)

Look at the mess you make of it :music_whistling:

Again, the lack of standard causes two pilots to come to very different conclusions based on their individual skill.

2 pilots of different skill levels would have different conclusions on a standardised aircraft, what a daft thing to say.

Here is the report, the 7 1/4 bobweight was essential to allowing the aircraft to have a rear fuselage tank. I am sure the stick force gradiant was unstable at this point and the designs CG so far aft so the predictions were not accurate.:

Yes, a bob weight was fitted as a precaution while exploring the limits of the rear CoG range, sounds like a sensible precaution, it also mentions the weight only being fitted for 'some' tests, but it certainly never became a production feature.

The normal take off CG range of the Spitfire Mk IX at 6.8 inches aft of datum has been moved further aft than the normal take off CG was in the Mk V.

 

Is it any wonder it retains the instability of the MkV? :thumbup:

the normal CoG 'range' has moved back because the MkIX has more weight in the nose compared to a MkV, CoG range and CoG are not the same thing, this simply means more weight can be distributed further aft because it has more built in weight in the front. effectively meaning the MkIX is less critical to a rearward CoG.

This makes sense as the MkIX could be fitted with an aft fuel tank and post war be converted to a dual seat trainer.

it doesn't retain the instability, there is no evidence of it, it is purely an agenda led claim made by you, it's really quite shocking to see how someone can be so bare faced.

Crumpp has already claimed that he has a definitive report on the Spitfire IX - not the Mk I or VA - proving that it was longitudinally unstable.

Surely Crumpp would make things easier on himself and everyone else by posting this purported information on the Mk IX, instead of regurgitating the same old material on a very early Spitfire I, and a VA tested by NACA, thus forcing himself and others to "do the math" using incomplete information?

As it is, it is far more likely that DCS has ready access to far better, more accurate information on the Spitfire IXs flight qualities, making this thread nothing more than an exercise in futile speculation, based on a random sampling of material gathered from other websites and Morgan & Shacklady.

Ahh so refreshing to see the subject alive again !

What would be refreshing is if some (might I say - usual) of participants of this discussion would finally be able to present something, anything that would at least resamble like objective evidence, rather than just their limited pool of snide remarks.

For example, I would like to see some evidence to the notion of the Mark IX had ample stability margins and that they managed to cure the failings of the previous Marks in this regard.

So far the most 'serious' arguement put forth to this end is that 'hey, something was changed, so it must have been cured, completely'.

But then I read the flying instructions for the later rear fuel tanks and I wonder... ;)

The OP's claim is a pure fantasy, there is no need for evidence beyond the materials he includes, one has to simply look at the documents in their entirety and not the cherry picked extracts.

There is evidence already provided how the relatively mild instability issues of previous marks were solved, the onus to prove MkIX instability is with the accuser, it would be refreshing to finally see someone admit this whole topic is dead in the water.

Aircraft instability with rearward CoG shift is a universal phenomenon, but 'someone' has a weird habit of putting Spitfire in place of the word aircraft..

Phrases like 'pool of snide remarks' are simply return fire from another 'usual' participant.