Why is the Spitfire Mk IX still unstable??

[Friedrich-4B]

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.

 

As noted, it would have made things a whole lot easier and less contentious had Crumpp presented the document (or documents) he claims to possess, proving the Mk IX's purported longitudinal instability, rather than rehashing excerpts of material on the Spitfire I or VA, or the IX with a full fuselage fuel tank.

 

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'.

 

As was also pointed out, there were numerous changes on the Mk IX cf the Spitfire I and VA, ie: longer fuselage, redesigned wings, redesigned horizontal stabilizer and elevators - most of which were specified by Crumpp in his OP, as being necessary to alter stability margins.

 

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.

:smilewink: :thumbup:

 

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

 

One wonders what would have happened to the Bf 109 had an extra 300+ pounds (41 imp gall of fuel = 291 lbs, plus tank and equipment) been added well behind the datum, causing a rearward CG shift of over 5 inches for take-off (noting that the total weight of the MW tank plus the usual load of 85 litres = around 160 lbs).

 

In the Spitfire IX's case, Crumpp claims the normal take-off CG in the Mk IX was 6.8" aft of datum, while the position with a full rear tank was 12.2" ("aircraft acceptable for take off and landing and for normal flying...") = 5.4" further aft.

 

What is the characteristics of the Spitfire's stability? Weak static stability and at normal to rearward CG was dynamically unstable. That means it oscillated and required double controlling.

 

The Pilot's Notes state that without a full rear tank and no drop tank ...stability about all axes is satisfactory, and the aircraft is easy and pleasant to fly - no mention of Crumpp's purported longitudinal instability, nor his "oscilla[tion] and double controlling".

 

In other words, Crumpp had all the reasons to show why the Spitfire IX wasn't longitudinally unstable in his OP, only he has chosen not to see them.

Edited October 19, 2015 by Friedrich-4/B
Spelling, grammer

[MiloMorai]

The Spitfire IX/XVI was nose heavy so 20.5 lb of ballast and mount for the ballast had to be added in the rear fuselage (Mom Arm 175").

[Kurfürst]

One wonders what would have happened to the Bf 109 had an extra 300+ pounds (41 imp gall of fuel = 291 lbs, plus tank and equipment) been added well behind the datum, causing a rearward CG shift of over 5 inches for take-off (noting that the total weight of the MW tank plus the usual load of 85 litres = around 160 lbs).

 

There is no need to wonder, since such aircraft existed and was called Bf 109E - and reports also tell of its excellent control-ability near stall. The 109K topped that with another 25 gallons of fuel, or ca. 192 lbs, plus tankage. Of course the 109 had a lot of weight right there to start with -armor, the pilot and the fuel tank beneath him - so its not that much of a relative change in where the weight is placed in the airplane.

 

I guess the crux of the matter is that the CoG change was far also more radical with the installment a extra tank (or any other weight for that matter) in the rear of Spitfire, as it originally had a very much forward placed CoG with a 85 gallons of fuel mounted forward, right behind the engine, than compared to a plane that had it in the wings or below/behind the pilot seat to start with.

 

Perhaps that is very the low margin of stability of the Spitfire originates to. It can be stable at optimum CoG position, but apparently it was very sensitive to any changes in load with quickly occurring instability. Sensitive pitch control characteristics would only add to such troubles.

 

This was noted from the earliest Marks on, see Rechlin's verdict on the issue:

 

"Quick changes of the trajectory along the vertical axis cause especially with the Spitfire

load changes around the cranial axis, coming from high longitudinal thrust momemtum, and

significantly disturb the aiming. In summary, it can be said that all three enemy planes types are inferior to the German planes regarding the flying qualities. Especially the Spitfire has bad rudder and elevator stability on the target approach. In addition the wing-mounted weapons have the known shooting-technique disadvantages."

 

In other words, Crumpp had all the reasons to show why the Spitfire IX wasn't longitudinally unstable in his OP, only he has chosen not to see them.

 

Crumpp has been pretty consistently referring to a narrow margins of stability on the Spitfire, not that it was longitudinally unstable per se, alas it seems the difference in meanings had remained obscure.

[bongodriver]

Crumpp has been pretty consistently referring to a narrow margins of stability on the Spitfire, not that it was longitudinally unstable per se, alas it seems the difference in meanings had remained obscure.

 

You're having a laugh surely? in a thread titled 'why is the Spitfire Mk IX still unstable', where he even makes a specific note that the Spitfire stability margin is 36.2% MAC and therefore in fact quite ordinary. making the point that changing the AC requires redesigning the wing, fuselage and horizontal stabiliser and immediately contradicting himself by denying any of those factors had an effect on the Spitfire, relying on a theory that the Spitfires light controls were the problem and then contradicting himself by denying the redesigned elevator solved it.

The only thing the OP does consistently is to have his crackpot theories shot down and then skulk off to hide and leave you to try and clear up the mess.

 

There is nothing obscure in the meaning of stability margin, if you are inside the margin the aircraft is stable.....simple as that.

[NineLine]

The only thing the OP does consistently is to have his crackpot theories shot down and then skulk off to hide and leave you to try and clear up the mess.

 

This is my last warning on this thread, if you cant discuss in a mature manner without taking shots at one another, I will start handing out warning points, reminder, 30% warning level is a week off, 40% is a month off, 100% is a year off. Just keep that in mind.

[Cripple]

I think the clarity of this "discussion" would be helped considerably if the main participants could take a step back and agree a mutual acceptable definition of Stability (and, by association, Instability).

 

It's refreshing to see that Ad Hominem arguments are not tolerated. At least not on this thread...

[bongodriver]

I don't believe anybody is talking about anything other than the standard definition of stability.

[Friedrich-4B]

There is no need to wonder, since such aircraft existed and was called Bf 109E - and reports also tell of its excellent control-ability near stall. The 109K topped that with another 25 gallons of fuel, or ca. 192 lbs, plus tankage. Of course the 109 had a lot of weight right there to start with -armor, the pilot and the fuel tank beneath him - so its not that much of a relative change in where the weight is placed in the airplane.

 

What I asked is what would happen to a 109 if an extra 300+ pounds been added well behind the datum, causing a rearward shift in the CG of 5.4"; no doubt 109s of any species would have reacted in a similar way to the Spitfire, or P-51B/C/D for that matter.

 

I guess the crux of the matter is that the CoG change was far also more radical with the installment a extra tank (or any other weight for that matter) in the rear of Spitfire, as it originally had a very much forward placed CoG with a 85 gallons of fuel mounted forward, right behind the engine, than compared to a plane that had it in the wings or below/behind the pilot seat to start with.

 

There's no reason to believe the Spitfire's weight distribution was any better or any worse than the 109, or any other single-engine, single seat fighter of its time; if the Spitfire had been designed to have its fuel tanks under and behind the pilot, its datum would have changed accordingly. An extra 300+ pounds distributed well behind the CG would affect the stability margins of any relatively small, powerful aircraft, no matter where its fuel tanks were placed.

 

Crumpp has been pretty consistently referring to a narrow margins of stability on the Spitfire, not that it was longitudinally unstable per se, alas it seems the difference in meanings had remained obscure.

 

Crumpp has been pretty consistently referring to the Spitfire IX's purported longitudinal instability - including the claim that it "oscillated" and needed "double handling" in flight. The title of this thread Why is the Spitfire IX still unstable?? shows where this is all leading - for about the fourth or fifth time in this forum alone.

Edited October 19, 2015 by Friedrich-4/B

[MiloMorai]

I guess the crux of the matter is that the CoG change was far also more radical with the installment a extra tank (or any other weight for that matter) in the rear of Spitfire, as it originally had a very much forward placed CoG with a 85 gallons of fuel mounted forward, right behind the engine, than compared to a plane that had it in the wings or below/behind the pilot seat to start with.

 

 

Please notice the location of the fuel tank is right above the wing in a location that would not require much trim change as fuel is burned off unlike your Bf109 with the fuel tank well behind the CoG.

 

 

[Crumpp]

Cripple says:

think the clarity of this "discussion" would be helped considerably if the main participants could take a step back and agree a mutual acceptable definition of Stability (and, by association, Instability).

 

You are correct. The terminology is not understood very well.

 

Referring to the Spitfire Mk IX, I said

 

It had weak static stability and at normal to rearward CG was dynamically unstable.

 

The weak positive static stability is more of an issue for a combat aircraft than the dynamic instability. It most common trait is the inability of the pilot to precisely apply a given load factor and hold it steady. In modern aircraft, this trait is tested by having the pilot hold a load factor such as 3G's for specific maneuvering profile.

 

 

This is a common stability and control characteristic of several World War II designs.

 

Here is the FW-190A:

 

 

Once you understand the characteristics, it is easy to see why the FW-190A was great low altitude fighter and so good at high altitude. It was much more than simply the BMW 801 engine. Increasing the stability margin in the FW-190D9 series would have help the stability and control at altitude.

 

In the Spitfire, the aerodynamic balancing creates the feel to the pilot as if the aircraft had a larger static stability margin but it does not actually change the aircraft's static stability margin.

 

The Spitfire pilot only needed to apply 3/4 inch stick travel and 5 degrees of elevator movement at cruise speed to exceed stall AoA.

 

Here is a great example of not understanding what the terminology means:

 

well behind the datum,

 

The datum is just a reference point. On many aircraft all CG locations are aft of datum because the datum point is the tip of the spinner.

 

In most cases it is located on the nose of the aircraft

 

http://avstop.com/ac/apgeneral/terminology.html

 

I think he means well past the rear CG limit. On my aircraft, the CG better never be forward of the datum because it is no longer in the airplane. :cry:

 

But, Hey, what do I know?! :music_whistling:

 

Kurfurst says:

This was noted from the earliest Marks on, see Rechlin's verdict on the issue:

 

"Quick changes of the trajectory along the vertical axis cause especially with the Spitfire

load changes around the cranial axis, coming from high longitudinal thrust momentum, and

significantly disturb the aiming. In summary, it can be said that all three enemy planes types are inferior to the German planes regarding the flying qualities. Especially the Spitfire has bad rudder and elevator stability on the target approach. In addition the wing-mounted weapons have the known shooting-technique disadvantages."

 

Once a design is finalized, it is not easy to change the fixed parameters that determine the CG limits.

 

Most aircraft were longitudinally unstable with a rear fuselage tank full of fuel. Both the P-51 and FW-190 experienced the same thing but not severe enough to preclude service entry. The issue with the Spitfire was that even empty, the tank was past the rearward CG limit.

 

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

[Crumpp]

What I asked is what would happen to a 109 if an extra 300+ pounds been added well behind the datum, causing a rearward shift in the CG of 5.4"; no doubt 109s of any species would have reacted in a similar way to the Spitfire, or P-51B/C/D for that matter.

 

The location of the 300lbs to the datum is irrelevant. The location of the CG to the datum is relevant to determine if the CG is in limits.

[Friedrich-4B]

Here is a great example of not understanding what the terminology means:

 

well behind the datum,

 

The datum is just a reference point. On many aircraft all CG locations are aft of datum because the datum point is the tip of the spinner.

 

http://avstop.com/ac/apgeneral/terminology.html

I think he means well past the rear CG limit. On my aircraft, the CG better never be forward of the datum because it is no longer in the airplane.

 

But, Hey, what do I know?! :music_whistling:

 

Crumpp's own definition of the Spitfire's datum (Note: it wasn't set at the tip of the spinner [see below]):

 

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.

 

and what was written in the report that he cited, http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf :

 

It was concluded that the aftmost acceptable c.g position for full combat manoeuvers was 9.9 ins. aft of datum....fitted with a 75 gallon rear fuselage fuel tank...The centre of gravity of the aircraft was 12.6 ins. aft of datum...

 

Supermarine's Datum, as set by the company:

 

The manufacturer has the choice of locating the datum where it is most convenient for measurement, locating equipment, and weight and balance computation. http://avstop.com/ac/apgeneral/terminology.html

 

 

So, we have now established that the Datum set by Supermarine wasn't at the tip of the spinner; But, Hey, :pilotfly:?! :music_whistling:

 

What I asked is what would happen to a 109 if an extra 300+ pounds been added well behind the datum, causing a rearward shift in the CG of 5.4"; no doubt 109s of any species would have reacted in a similar way to the Spitfire, or P-51B/C/D for that matter.

The location of the 300lbs to the datum is irrelevant. The location of the CG to the datum is relevant to determine if the CG is in limits.

 

Here, Crumpp conveniently ignored the comment ...causing a rearward shift in the CG of 5.4"... ;)

 

What the A&AEE report does show is that the calculation of 300+ pounds well aft of the Spitfire's datum was off; with a full 75 imp. gal rear fuselage fuel tank, the extra weight was about 540-550 lbs pounds = a CG of 12.2" aft of datum (the original rough calculation was assuming a 41 gallon tank).

 

For a proper assessment of the P-51D's flight characteristics with the rear fuselage fuel tank, one need look no further than here:

 

Edited October 21, 2015 by Friedrich-4/B
Spelling

[JtD]

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.

Now that this got quoted again, I had to :doh:.

The NACA report says the mean aerodynamic chord of 85 inches starts 4.8 inches behind the chord at the wing root. How this means "the CoG is 4.8 behind the datum", oh well. It was tested at 31.4% MAC, which corresponds to 7 inches behind the datum. Also according to NACA, this corresponds well to the fully loaded state (31.1% without ammo).

 

Fully loaded the CoG position is most rearward, as most consumables are located a little bit behind this point.

 

For what it's worth, 7.0 is also the rearward limit for the VIII.

[Kurfürst]

What I asked is what would happen to a 109 if an extra 300+ pounds been added well behind the datum, causing a rearward shift in the CG of 5.4"; no doubt 109s of any species would have reacted in a similar way to the Spitfire, or P-51B/C/D for that matter.

 

Yet the claim remains unproven and an opinion piece that remains at odd with the facts, namely that it was done on the 109E and it did not have stability issues at all.

 

There were no stability issues or deterioration of flight qualities or restrictions when 187 lbs worth of MW-50 liquid were added to the rear fuselage of the late 109G/K either, as opposed to the Mark IX which even with half empty rear fuselage tanks (cc 33 gallons remaining = cc. 250) lbs and hand-made elevators showed had serious stability issues and a tendency to tighten up in turns.

 

There's no reason to believe the Spitfire's weight distribution was any better or any worse than the 109, or any other single-engine, single seat fighter of its time

 

Well, no reason at all, except physics and looking at loading sheets and actual stability margins.

 

Apparently, aircraft with forward mounted fuel tanks (Spitfire, Hurricane, some Italian fighters like the Macchi 202) exhibited the narrowest longitudinal stability margins.

 

if the Spitfire had been designed to have its fuel tanks under and behind the pilot, its datum would have changed accordingly.

 

A change in the... datum point... when the fuel tanks move. Would you kindly expand on this further, as I cannot fathom why that would be so?

 

An extra 300+ pounds distributed well behind the CG would affect the stability margins of any relatively small, powerful aircraft, no matter where its fuel tanks were placed.

 

You do realize that this doesn't make much sense as the CG of an aircraft is, in fact, constantly changing during flight and depends on load condition and that there is not just one fixed point? That is why they bother to issue loading plans.

 

It depends entirely on the allowed CG margins and not all fighter designs had such meager stability margins as the Spitfire, therefore, not all designs would react the same way to 300+ pounds in the rear. The 109, which you seem to be very fond to make a comparison with, added 485 pounds of fuel in the rear fuselage during its development without any trouble.

 

Crumpp has been pretty consistently referring to the Spitfire IX's purported longitudinal instability - including the claim that it "oscillated" and needed "double handling" in flight. The title of this thread Why is the Spitfire IX still unstable?? shows where this is all leading - for about the fourth or fifth time in this forum alone.

 

Its really unfair to fault Crumpp if you cannot see the connection between these shape of these curves and instability, and "double handling" and the correct conclusion that it had rather narrow margins of stability.

[Kurfürst]

The location of the 300lbs to the datum is irrelevant. The location of the CG to the datum is relevant to determine if the CG is in limits.

 

Exactly.

 

For what its worth, the Mk XIV's allowable CG margin:

 

"Permissible C.G. range for all forms of flying...........from 3.5 inches to 6.5 inches aft of datum point."

 

I don't think 3 inches (7.5 cm) can serve you well. ;)

[JtD]

... The 109, which you seem to be very fond to make a comparison with, added 485 pounds of fuel in the rear fuselage during its development without any trouble...

 

The figure as such is meaningless when it comes to stability issues, unless you can also come up with a figure of how much they added in front. After all, between the say B and K, there's like what, a good ton difference in weight? So like, 20% went to the rear, the other 80% to the front?

I guess you are aware that stability was just as much an issue with the 109 as with the Spitfire or any other aircraft that wants to be flown, and while they might have had "no trouble" to add weight in the rear, they certainly didn't have the CoG move about the aircraft randomly and unchecked.

The rearward limit for the Bf109G was at about 30% MAC, with certain loadouts approaching this figure, offering marginal stability.

[bongodriver]

You are correct. The terminology is not understood very well.

 

Referring to the Spitfire Mk IX, I said

 

It had weak static stability and at normal to rearward CG was dynamically unstable.

 

Yes, you did say that, many many times over many many forums for a long time now, and we are still waiting for the evidence, I mean actual evidence, not a NACA report on the wrong aircraft and a bunch of hypothesis.

 

The weak positive static stability is more of an issue for a combat aircraft than the dynamic instability. It most common trait is the inability of the pilot to precisely apply a given load factor and hold it steady. In modern aircraft, this trait is tested by having the pilot hold a load factor such as 3G's for specific maneuvering profile.

 

This is a common stability and control characteristic of several World War II designs.

 

Here is the FW-190A:

 

 

Got anything on a Spitfire MkIX?

 

Once you understand the characteristics, it is easy to see why the FW-190A was great low altitude fighter and so good at high altitude. It was much more than simply the BMW 801 engine. Increasing the stability margin in the FW-190D9 series would have help the stability and control at altitude.

 

Yet the MkIX Spitfire was the key to countering this very aircraft and did it very effectively.

 

In the Spitfire, the aerodynamic balancing creates the feel to the pilot as if the aircraft had a larger static stability margin but it does not actually change the aircraft's static stability margin.

 

I've almost heard it all now, a stability 'margin' is not 'felt' the stability of the aircraft is what is felt, if the aircraft 'feels' stable then it actually is.

 

The Spitfire pilot only needed to apply 3/4 inch stick travel and 5 degrees of elevator movement at cruise speed to exceed stall AoA.

 

in a single isolated test on a completely different mark know for its loading issues.

 

Here is a great example of not understanding what the terminology means:

 

This caveat should be the opener to your posts.

 

 

Once a design is finalized, it is not easy to change the fixed parameters that determine the CG limits.

 

Other than putting bigger heavier engines, lengthening fuselages, redesigning internal structures, redesigning wings, tails and control surfaces you are absolutely correct, if you don't change a MKV it is still a MkV.

 

Most aircraft were longitudinally unstable with a rear fuselage tank full of fuel. Both the P-51 and FW-190 experienced the same thing but not severe enough to preclude service entry. The issue with the Spitfire was that even empty, the tank was past the rearward CG limit.

 

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

 

You may need to read that again, it states that once 34 gals had been used (approx. half capacity) from the rear tanks the CG was 9.9 inches aft of datum, that is within range, it states clearly that 34 gallons burned was sufficient for the aircraft to engage in combat manoeuvring.

[MiloMorai]

Why all this yapping about the rear fuselage fuel tank in the Spitfire?

 

How many gamers, err simmers, are going to fly with a maximum fuel load?

[Kurfürst]

The figure as such is meaningless when it comes to stability issues, unless you can also come up with a figure of how much they added in front. After all, between the say B and K, there's like what, a good ton difference in weight? So like, 20% went to the rear, the other 80% to the front?

 

Same is true to the Spitfire, they added a lot of weight elsewhere, after all the first Spitfire weighted something 5300 lbs and the XIV some 8500, the 21 some 9200 lbs. That's something like 3200 lbs difference, and about 10-15% of that went in the rear with the aux. tanks.

 

So how is that all that weight going to the front in the form of heavier and heavier engines just couldn't balance out the things added to the rear on the Spitfire?

 

The point is that if the aircraft has wide stability margins to start with, its easier to add things to the rear. A lot of things. Its really not all that complicated, really.

 

I guess you are aware that stability was just as much an issue with the 109 as with the Spitfire or any other aircraft that wants to be flown,

 

No, I am not aware and it wasn't just as much of an 'issue'. Of course CoG limits have to be observed on all aircraft, but that's not necessarily an 'issue' if its easy to do in all load conditions.

 

The issue is that some here want to put forth an arguement that CoG limits are just as much of an 'issue' on a plane that has 8.9 inches of margin for CG movement as the one which has a mere 3 inches...

 

and while they might have had "no trouble" to add weight in the rear, they certainly didn't have the CoG move about the aircraft randomly and unchecked.

 

Certainly, it cannot go unchecked. It is true that the Messerschmitt guys were evidently a lot more successful in not having the CoG move about the aircraft randomly and unchecked compared to the Supermarine guys, that much is true. They still had to observe the CG limits of course but its a lot easier job to do when you have 2-3 times the CG limit to play with.

 

The rearward limit for the Bf109G was at about 30% MAC, with certain loadouts approaching this figure, offering marginal stability.

 

That thing about the marginal stability on the 109G is just silly, JtD, and you know it.

[Crumpp]

Now that this got quoted again, I had to :doh:.

The NACA report says the mean aerodynamic chord of 85 inches starts 4.8 inches behind the chord at the wing root. How this means "the CoG is 4.8 behind the datum", oh well. It was tested at 31.4% MAC, which corresponds to 7 inches behind the datum. Also according to NACA, this corresponds well to the fully loaded state (31.1% without ammo).

 

Fully loaded the CoG position is most rearward, as most consumables are located a little bit behind this point.

 

For what it's worth, 7.0 is also the rearward limit for the VIII.

 

You do realize that the LEMAC is always the reference point for expression of the CG location in MAC??

 

LEMAC is Leading Edge Mean Aerodynamic Chord.

 

Not sure the point of your big revelation.

 

The T-45’s leading edge of the mean aerodynamic chord (LEMAC) is located 240.9" aft of datum (which isthe tip of the nose boom). The MAC extends aft of the LEMAC 74.03" and the CG location will bemeasured as a percentage of the entire MAC that the CG is aft of LEMAC

 

http://navyflightmanuals.tpub.com/P-1231/P-12310069.htm

[JtD]

Kurfürst wrote...

 

It were Messerschmitt guys who said it was "indifferent" in certain conditions. Maybe they were silly, I don't know.

 

I take it the bottom line is you're saying that the 109 had a ~9" range, the Spitfire ~3"?

[Crumpp]

No response JtD?

[Kurfürst]

It were Messerschmitt guys who said it was "indifferent" in certain conditions. Maybe they were silly, I don't know.

 

The only close the rear permissable cg position (roughly 60-62 cm) on the 109 I could find was when it was mounting 4x50 kg bombs, but that was still within limits. All other conditions I checked (gondolas, 250 kger) well within the limits at 56-57. In other words, the loaded plane (clean fighter, gondies, 250 kg - the most common configurations) had as much rearward CG "reserve" as the whole allowable CG load margin of the Spit.

 

I take it the bottom line is you're saying that the 109 had a ~9" range, the Spitfire ~3"?

 

From what I have seen, yes.

Edited October 21, 2015 by Kurfürst

[MiloMorai]

You can backup that with official Mtt documentation Kurfurst?

[Kurfürst]