spitfire mk xiv (14)

[RTHolmes]

Am I being obtuse or wouldn't the centre of gravity have to be in approximately the same position as any other Spitfire, i.e. pretty well cock on the lift centre of the wings?

OK, I may be way off here but this is how I see it. C_G should be in front of C_L for stability, the further apart they are, the more stable (less twitchy/responsive).

The VIII and XIV are essentially the same airframe so we can assume that C_L is in the same location relative to the datum point (D), which is also in the same place.

Despite the extra weight, the XIV has a lower static moment (relative to D) than the VIII, which means that the C_G is further forward.

This should make it more stable in level flight, although requiring more elevator trim to keep it level, and heavier elevator input to deflect it (as reported in tests.)

The other factor is the distribution of the masses as I mentioned earlier. putting all the extra mass as far as possible from C_G means that more force is required to pitch the aircraft (heavier elevators again.)

Spit VIII:

+----D----C_G----C_L----------------------

Spit XIV:

+----D--C_G------C_L----------------------



The net result should be an aircraft which is more stable, but which requires greater forces to deflect it.

I think ...

[FLS]

In the case of the Mark XIV how much farther forward? Also if they fitted a counterbalance surely they could put the GofG accurately where they wanted it?

I don't know the details for the Spitfire. One thing you want to keep in mind is that with a cambered wing the lift center moves forward as AOA increases so they are unlikely to be co-located in level flight where a pitch up would put the CG behind the CL. If the CG was within an acceptable range you likely wouldn't add weight and reduce performance if you didn't have to.

[RTHolmes]

Spit VIII: 120gal, 4x .303, 2x 20mm.
Takeoff weight 7,780lb, moment +41,553 lb in
=> C_G is 41,533/7,780 = 5.3" aft of D

Spit XIV: 110gal, 4x .303, 2x 20mm.
Takeoff weight 8,475lb, moment +30,699 lb in
=> C_G is 30,699/8,475 = 3.6" aft of D


so C_G is 1.7" further forward in the XIV. I cant imagine that this is enough to explain the vastly different handling characteristics, which is why I'm sticking with the idea of the location of the extra mass and the increased moment of inertia explaining it.

[nrshida]

Wow, great explanations, much appreciated. This makes perfect sense as I understand the centre or pressure relative to the CofG business (que Bert's Cigar explanation in The World's Fastest Indian), however: assuming this could be adjusted through the use of tail mounted counterbalances did they simply get the position of the CofG wrong to cause the bouncing business? Again, sorry to bang on about this but the late model Bf109s seem much more longditudinally stable than the Mark XIV.

[RTHolmes]

a 1.7" shift is not very much, I'm pretty sure you'd get that much just between full and empty fuel tanks. it must be down to the distribution of mass.

Thought experiment:

1. imagine you have a 1m long alloy rod with a 2kg weight in the middle. now hold it in the middle and rotate it.

2. now remove the 2kg weight from the middle, and put a 1kg weight on either end of the rod. again hold it in the middle and rotate it.

same mass, same C_G, but it takes much more effort to rotate it, and when it gets going, lots more effort to stop it. this is what I reckon causes the bouncy nose.

[nrshida]

Aha! But see that thinking experiment implies to me a less bouncy nose. Porsche 944!

[Tupac]

Where is the datum on the spitfire? It's different for every plane.

[Chalenge]

Not much fighting going on above 40K of course.

Of course not... except when the 91st is in business. I guess you miss those nights. 

[Babalonian]

Am I being obtuse or wouldn't the centre of gravity have to be in approximately the same position as any other Spitfire, i.e. pretty well cock on the lift centre of the wings?

I thought the Mark XIV was based on the fuselage of the Mark VIII which was already structurally superior to the earlier Marks? I know that old Willy was quite clever with the design of the Bf109, specifically with the machined bulkheads which carried the stresses of the engine, wings and landing gear, but clearly Joseph Smith was similarly competent so I think it's a safe assumption that the Mark XIV could handle the structural loads associated with the additional power and weight.

What interests and has always confused me was the stability issue, especially using the Bf109 as a comparison:

I thought a larger polar moment of inertia translates to greater inertial stability, not less. I'm not knowledgeable enough about the minutiae of aircraft design to foresee a fundamental difference, but in other vehicles a larger, or in this case a longer, distribution of the mass about its centre of gravity tends to make a vehicle more docile and less inclined to change its direction quickly. Thus I would have assumed the the Mark XIV would bounce around less not more. Must be missing a vital piece of understanding  

How can essentially the same change: i.e. adding a heavier and more powerful engine to an airframe designed for a lighter less powerful one, in the case of the Bf109 make its handling more docile, more controllable in a vertical stall and a more docile gun platform do the inverse in the Spitfire equivalent?

Did the MkI even have half the junk (by weight or count) behind the pilot than... oh lets just toss a IX in for comparison?  I'm thinking oxygen, long-range radios, etc..

I would think the CoG could change as frequently as there were modifications.

[RTHolmes]

Where is the datum on the spitfire? It's different for every plane.

also from http://www.spitfireperformance.com/rb141weights.html:

"Maximum permissible all-up (i.e. take-off) weight for all forms of flying...........8500 lb.
Permissible C.G. range for all forms of flying...........from 3.5 inches to 6.5 inches aft of datum point."

[RTHolmes]

Aha! But see that thinking experiment implies to me a less bouncy nose. Porsche 944!

take your 944 and add 100kg of lead to the front and rear bumpers (and make the tyres 1/2" wider all round so it has the same grip). it will be more reluctant to turn in, and when you apply the power mid-corner and the back end steps out, it will be harder to catch

[FLS]

FWIW I'm not seeing any issues with the Spit 14 pitch or yaw stability. Any difference with the 109K4 is likely the difference in wing loading.

[RTHolmes]

really? its definitely rather bouncy with small elevator inputs. I found the trick to stable gunnery is to keep it loaded, its solid as a rock then. that does restrict your options though.

[FLS]

It's a fast aircraft with a big wing. It's not bouncy it's responsive.    

Trimming a little nose down is always a good idea for precise pitch control but I'm not seeing any restrictions other than letting off the pressure flying on the deck.

[Amsoil21]

yea,, definitely bouncy though and not just sensitive.. i have flown it quite a bit now and have got into it a little better but its still throwing surprises at me.