Why is the Spitfire Mk IX still unstable??

[Kurfürst]

I am relieved to finally learn that NACA did not only managed to get us to the Moon, but also suceeded in the greater challange of correctly measuring the MAC and CG of Lady Spitfire entirely on its own. I must confess, I had my doubt for a couple of pages about it, but now all the doubt is cleared and there is but my firm trust in NACA's competence.

[Friedrich-4B]

No issues...

The NACA MAC is not in error, they just did not have the RAE numbers. They did it the conventional way by jacking the tail up and taking out a tape measure.

 

NACA were in error cf the document issued by the A&AEE (not the RAE as Crumpp maintains) - all that's wrong is Crumpp didn't notice the difference.

 

But, that is not the case for Spitfire Mk IX data, it is found undercarriage up just as the NACA MAC is located.

 

 

But let's look if we use RAE Gear Down LEMAC for our 9.5:

 

[(9.5+19.5)/84] * 100 =34.5% MAC

 

.012/.357 = 3.4% error

 

That does not give very good agreement with the RAE Spitfire Mk IX MAC of 35.7%.

 

Why? Length was added to the engine compartment changing our LEMAC.

 

The A&AEE data shown by Crumpp is for a Spitfire IX with 40 imp gallons of fuel in the rear fuel tank; ie: 40 x 7.2 lbs p/imp gal = 288 lbs aft of the normal CG - nothing to do with a longer nose!

 

The note at the bottom of the page says

Consumption of all the rear fuselage fuel moves the c.g. 5.2 ins forward from loading A

 

(7.2+19.5)/84 = .3178 * 100 = 31.8% MAC

 

Besides, undercarriage down isn't a normal flight condition...

 

Simply solving for the NACA MAC gives good agreement with the Spitfire Mk IX data.

 

Using the RAE 84”MAC; the LEMAC is 20.7in vs 21.8in for the NACA LEMAC at 85%. It just does not matter that much and does not change the stability picture of the design at normal to aft CG.

 

20.7/84 = .257*100 = 25.7%

 

21.8/85 = .256*100 = 25.6%

 

So no conspiracy theory required.

 

Does any of this prove the Spitfire IX was longitudinally unstable?

[Friedrich-4B]

I am relieved to finally learn that NACA did not only managed to get us to the Moon, but also suceeded in the greater challange of correctly measuring the MAC and CG of Lady Spitfire entirely on its own...

 

Actually NASA, not NACA got us to the moon with far more sophisticated methods than Crumpp's tape measure.

 

...there seems to be this perception that stability is fixed and does not change in an aircraft.

 

Flying qualities of an aircraft change with velocity and condition of flight. It is not a single characteristics under all conditions. It changes and is linked to loading, configuration, load factor, velocity/Coefficient of lift, drag changes, etc...

 

Which is exactly why making generalizations about the Spitfire IX's so-called longitudinal instability, based on NACA's Spitfire VA and a report on a Spitfire IX with full rear fuel tanks and hand made elevators, is so deeply flawed as to be ridiculous.

 

This thread has run what ever increasingly circular course it was going in - hopefully DCS have far more real-world data available on which to base their model, rather than the somewhat limited material presented here.

[Yo-Yo]

I really can not understand why MAC length difference is discussed. As far as I can see, all sources give CG position referencing to absolute geometric origins - datum point and root leading edge. MAC length can play a role if you are going to compare data in %% of MAC.

[JtD]

NACA gives CoG as % MAC, but yes, the discussion around it is surprising. As Kurfürst said, it's hardly rocket science.

[Crumpp]

Yo-Yo says:

I really can not understand why MAC length difference is discussed. As far as I can see, all sources give CG position referencing to absolute geometric origins - datum point and root leading edge. MAC length can play a role if you are going to compare data in %% of MAC.

 

I tried to explain that. Unfortunately that message was pretty much ignored. :(

 

Now we have to see it for ourselves.

 

 

Fredrich says:

(7.2+19.5)/84 = .3178 * 100 = 31.8% MAC

 

The LEMAC is not 19.5 for the Spitfire Mk IX.

 

Loading condition A Gear Down:

 

84in * .392 = 32.92 - 21.2 = 21.72

 

(7.2in + 21.72) / 84in = 34.4% MAC

 

84in*.314 = 26.37in - 20.7 = 5.67in

 

5.67-.6 for Undercarriage UP = 5.07inches Aft of Datum

 

5.07inches is spot on with the NACA CG location of 4.8inches.

 

Without a change in the aircraft's AC, anywhere the stability margin is equal, the airplanes will have the same have the same stability characteristics.

[bongodriver]

No,

the weight and balance table gives 74 gallons rear tank capacity with 7.2lb/gallon and corresponding CoG movement of 5.5". The snippet from the test report has 74 gallons rear tank capacity with 6.8lb/gal and corresponding CoG movement of 5.2". No mix up, just reading.

 

I did not find a reference to 6.8Lbs/Gal in the A&AEE test report only the 7.2lbs in the load sheet, no need to be offended, it's just the 6lbs and 7.2lbs are exact figures relating to US/Imp Gallons so it seemed reasonable to consider a mix up.

[bongodriver]

I tried to explain that. Unfortunately that message was pretty much ignored. :(

 

.

 

No, you are the main culprit for discussing MAC

 

 

Without a change in the aircraft's AC, anywhere the stability margin is equal, the airplanes will have the same have the same stability characteristics.

The NACA MkV does not have the late war elevator which even you have demonstrated makes a difference in behaviour.

[Crumpp]

No, you are the main culprit for discussing MAC

 

No I simply converted to the RAE datum/arm so that the CG location could be compared. Doing that has gotten the others whacking away at their calculators without understanding what they are doing.

 

Consequently we have the LEMAC of the Spitfire MkV being used as the LEMAC for the Spitfire Mk IX.

 

Garbage in = Garbarge out

[Crumpp]

Originally Posted by Crumpp

Simply solving for the NACA MAC gives good agreement with the Spitfire Mk IX data.

 

Using the RAE 84”MAC; the LEMAC is 20.7in vs 21.8in for the NACA LEMAC at 85%. It just does not matter that much and does not change the stability picture of the design at normal to aft CG.

 

20.7/84 = .257*100 = 25.7%

 

21.8/85 = .256*100 = 25.6%

 

So no conspiracy theory required.

 

:thumbup:

[Crumpp]

The NACA MkV does not have the late war elevator which even you have demonstrated makes a difference in behaviour.

 

No it does not have the late war elevator. That modification represents an increase in the stick forces. Check out my first post for the basic explanation of how that works.

 

What that will do is slow the rate at which the pilot can accelerate the stick and make the aircraft more controllable. Instability without control is a bad thing; increase the control and it becomes a good thing for maneuverability. It is still going to have the quirks (double control) but that is the price you pay.

 

That is not an issue and Yo-Yo knows what he is doing.

 

There is a small cadre of members who seem to believe I am on some mission to "destroy" the Spitfire. That is simply a fantasy.

 

The Spitfire was the British F-22 raptor of it's day and represents the pinnacle of British fighter technology for the late 1930's/early 1940's.

 

Stability and Control technology was really in its infancy. There was a common misconception that maneuverability and instability were directly connected. They but only to the extent the pilot can precisely control. This effected the majority of World War II fighters and most of them exhibit some form of instability in their envelope as a result.

 

There was also the erroneous belief that elevator forces had to be reduced at high speed. Some designers broke away from this while others did not.

 

Both of these engineering misconceptions helped shape the personality of the aircraft.

[bongodriver]

That does not give very good agreement with the RAE Spitfire Mk IX MAC of 35.7%.

 

Why? Length was added to the engine compartment changing our LEMAC. .

 

Wow, I just noticed you really did say this.....talk about garbage in = garbage out

 

LEMAC is fixed in to the shape of the wing and has nothing to do with adding length to the nose, you can't argue this point because you were at great pains to emphasise this very fact yourself at the beginning of this thread.

if you wish to maintain this theory then it throws all comparisons between MkVa and a MkIX completely irrelevant.

Even I agree that with the exception of the elevator and some changes to the underside of the wings (radiators) a MkV airframe behind the firewall is the same as a MkIX.

 

 

There is a small cadre of members who seem to believe I am on some mission to "destroy" the Spitfire. That is simply a fantasy.

 

Yes, we have been witnessing the endeavours of your fantasy for a long time now.

 

No it does not have the late war elevator. That modification represents an increase in the stick forces. Check out my first post for the basic explanation of how that works.

 

What that will do is slow the rate at which the pilot can accelerate the stick and make the aircraft more controllable. Instability without control is a bad thing; increase the control and it becomes a good thing for maneuverability. It is still going to have the quirks (double control) but that is the price you pay.

 

That is not an issue and Yo-Yo knows what he is doing.

 

............

 

The Spitfire was the British F-22 raptor of it's day and represents the pinnacle of British fighter technology for the late 1930's/early 1940's.

 

Stability and Control technology was really in its infancy. There was a common misconception that maneuverability and instability were directly connected. They but only to the extent the pilot can precisely control. This effected the majority of World War II fighters and most of them exhibit some form of instability in their envelope as a result.

 

There was also the erroneous belief that elevator forces had to be reduced at high speed. Some designers broke away from this while others did not.

Both of these engineering misconceptions helped shape the personality of the aircraft.

 

So which is it? did they redesign the elevator to reduce or increase stick forces?

[Crumpp]

Bongodriver says:

LEMAC is fixed in to the shape of the wing and has nothing to do with adding length to the nose

 

NO, the MAC is fixed to the design of the wing.

 

It is a function of the wing's chord.

 

http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm

 

LEMAC is measured from a point to the MAC, usually the nose or front of the aircraft.

 

You have to know the LEMAC and the MAC for the system to work properly. Mixing up the values like Fredrick and JtD did will produce an error.

 

Originally Posted by Crumpp

Simply solving for the NACA MAC gives good agreement with the Spitfire Mk IX data.

 

Using the RAE 84”MAC; the LEMAC is 20.7in vs 21.8in for the NACA LEMAC at 85%. It just does not matter that much and does not change the stability picture of the design at normal to aft CG.

 

20.7/84 = .257*100 = 25.7%

 

21.8/85 = .256*100 = 25.6%

 

So no conspiracy theory required.

 

In the case of the Spitfire, it is measured from the intake.

 

 

Just think, by the time this thread is over, you will learned how to actually calculate this stuff, LOL.

 

So which is it? did they redesign the elevator to reduce or increase stick forces?

 

It does both for different reasons, bongodriver.

 

When the RAE tested that elevator design they tested three devices to increase the stick force gradient.

 

1. Inertial weight

 

2. Control cables as tight as possible

 

3. Friction device at the elevator cross shaft

 

All three of those changes increase the hinge moments to increase the stick force gradient.

 

You can think of the Rear CG limit as the minimum stick force gradient the designer will accept.

[bongodriver]

NO, the MAC is fixed to the design of the wing.

 

It is a function of the wing's chord.

 

 

Shape/design....same thing in this case, the MkV and MkIX have the same datum point and the same airframe/wing so there will be no change in LEMAC.

 

 

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

 

So, same datum and same wing no? or are you about to change the story again?

 

In the case of the Spitfire, it is measured from the intake.

 

Which intake? Spitfires had a few different intakes of various length.

 

Just think, by the time this thread is over, you will learned how to actually calculate this stuff, LOL.

 

I'd hold off on acting like a condescending smartass, you really haven't got a grasp on anything yourself.

 

It does both for different reasons, bongodriver.

 

When the RAE tested that elevator design they tested three devices to increase the stick force gradient.

 

1. Inertial weight

 

2. Control cables as tight as possible

 

3. Friction device at the elevator cross shaft

 

All three of those changes increase the hinge moments to increase the stick force gradient.

 

You can think of the Rear CG limit as the minimum stick force gradient the designer will accept.

 

1. never fitted to production MkIX's

 

2. if you say so

 

3. if you say so

 

none answer the actual question, was the redesigned elevator to increase or decrease stick forces?

[Crumpp]

bongodriver says:

none answer the actual question, was the redesigned elevator to increase or decrease stick forces?

 

 

Crumpp says:

It does both for different reasons, bongodriver.

 

You must have missed reading the answer to your question.

 

bogodriver says:

Which intake? Spitfires had a few different intakes of various length.

 

bogodriver says:

Shape/design....same thing in this case, the MkV and MkIX have the same datum point and the same airframe/wing so there will be no change in LEMAC.

 

 

You are confusing methods, bongo.

 

The RAE used the same datum point for their Arm/moment methodology and MAC methodology for expressing the CG location.

 

That does not mean they used the same LEMAC.

 

The LEMAC will then change to maintain the non-dimensional ratio % MAC is expressed in.

 

Originally Posted by Crumpp

Simply solving for the NACA MAC gives good agreement with the Spitfire Mk IX data.

 

Using the RAE 84”MAC; the LEMAC is 20.7in vs 21.8in for the NACA LEMAC at 85%. It just does not matter that much and does not change the stability picture of the design at normal to aft CG.

 

20.7/84 = .257*100 = 25.7%

 

21.8/85 = .256*100 = 25.6%

 

So no conspiracy theory required.

[bongodriver]

You must have missed reading the answer to your question.

 

Nope, nothing missed, you simply haven't answered the question.

it's not surprising to hear your theory it does both and in each case it's just wrong, it is after all one of the main themes of your agenda.

 

 

You are confusing methods, bongo.

 

The RAE used the same datum point for their Arm/moment methodology and MAC methodology for expressing the CG location.

 

That does not mean they used the same LEMAC.

 

The LEMAC will then change to maintain the non-dimensional ratio % MAC is expressed in.

 

Actually I'm not confusing anything, I really am keeping it as simple as it should be, the Spitfire loading diagram from A&AEE distinctly shows that there is 19.5" from the datum to the LEMAC for a MAC of 84", you can muddy the waters with the theorised NACA MAC of 85" and a different datum point all you like, you are not producing valid results to compare between a MkV and MkIX unless you use the same dimensions, you are creating a diversionary roadshow of your amazing math skills trying to dazzle the audience with your brilliance, the fact is you are just producing garbage.

MAC is fixed in the design of the wing, if Supermarine calculate a MAC of 84" then that is the MAC and there must be 19.5" between it and the datum....end of story, 85" is a MAC for some other design of aircraft with different dimensions and therefore not a comparison in this case, you simply cannot arbitrtarily shift the position of LEMAC from the datum, it is fixed in the design.

[Crumpp]

Here bongodriver....

 

This may help you to understand.

 

 

The RAE used 84inches for their MAC.

 

Load condition A:

 

39.2% of an 84" MAC = 84in * .392 = 32.92in

 

We have a distance of 32.9in from our LEMAC to a point that equals 12.2in aft of the RAE datum.

 

32.9in - 12.2in = 20.7inches for the LEMAC

 

Substituting that LEMAC into the MAC forumla gives us our ability to convert CG LOCATION.

 

Originally Posted by Crumpp

Simply solving for the NACA MAC gives good agreement with the Spitfire Mk IX data.

 

Using the RAE 84”MAC; the LEMAC is 20.7in vs 21.8in for the NACA LEMAC at 85%. It just does not matter that much and does not change the stability picture of the design at normal to aft CG.

 

20.7/84 = .257*100 = 25.7%

 

21.8/85 = .256*100 = 25.6%

 

So no conspiracy theory required.

 

I am very patiently trying to explain this to you. Maybe you will understand it and it will not be such a personal event.

[Crumpp]

Nope, nothing missed, you simply haven't answered the question.

it's not surprising to hear your theory it does both and in each case it's just wrong, it is after all one of the main themes of your agenda.

 

 

 

Actually I'm not confusing anything, I really am keeping it as simple as it should be, the Spitfire loading diagram from A&AEE distinctly shows that there is 19.5" from the datum to the LEMAC for a MAC of 84", you can muddy the waters with the theorised NACA MAC of 85" and a different datum point all you like, you are not producing valid results to compare between a MkV and MkIX unless you use the same dimensions, you are creating a diversionary roadshow of your amazing math skills trying to dazzle the audience with your brilliance, the fact is you are just producing garbage.

MAC is fixed in the design of the wing, if Supermarine calculate a MAC of 84" then that is the MAC and there must be 19.5" between it and the datum....end of story, 85" is a MAC for some other design of aircraft with different dimensions and therefore not a comparison in this case, you simply cannot arbitrtarily shift the position of LEMAC from the datum, it is fixed in the design.

 

There is no agenda. This is not personal to me so do not make it so. Get that nonsense out of your head, please.

 

Yes, you can find the %MAC for any aircraft design. That is the beauty of it and why the NACA choose that method in the absence of RAE data.

 

They simply had to load the aircraft normally, pull out a tape measure and some scales to get all the weight and balance information required to do their testing.

[bongodriver]

Here bongodriver....

 

This may help you to understand.

 

.

 

Stop questioning everybody's understanding when they put you on the spot, I understand perfectly well you don't know what you are talking about.

I certainly don't need you to explain anything to me.

[bongodriver]

There is no agenda. This is not personal to me so do not make it so. Get that nonsense out of your head, please.

 

Really, you spend almost every waking hour on forums trying to convince people about the Spitfire, it is very personal to you.

[NineLine]

ok, this has turned into a discussion between two people, I suggest you two work this out over PMs, in the end, we will see what the FM is like when the Spit is released. Nothing great is coming from this discussion anymore.