Crumpp

Yes and those reports and proof is posted. This is all measured and quantifiable Fredrich.

By taking away area from the Horizontal Stabilizer.... The horizontal portion of the tail area remains unchanged result in NO movement of the aircraft's AC and no increase in the stability margin required to move the Rear CG limit. This would allow the forward CG limit to move. That does not effect the aircraft at normal to aft CG. The angle of incidence change in the horizontal stabilizer fills the function of the bobweights by increasing the stick force per G and the pilot's perception.

The elevator modification allows exactly what Yo-Yo says. It has nothing to do with the rear CG limit. It has to do with moving the forward CG limit allowing the elevator to raise the nose on landing with the longer/heavier engine.

I know you think that but it is ok, really.

Some airplanes are like that. A Citabra for example the trim is so powerful, pilots have killed themselves doing exactly what you did in the DCS P-51. You can also land many aircraft just with the trim.

When an design is changed or reconfigured, adding ballast simply corrects the CG back to its original location to prevent possible adverse loaded condition. It has nothing to do with changing the CG to new limits and simply maintains the stability and control characteristics of the original aircraft. For example: Airplanes New Empty weight moment = 1150 New empty weight CG is 2 inches forward Arm for our ballast attachment point is 100 inches [1150*(+2)]/100 = 23lbs of weight attached to our ballast point to move the empty weight CG back to its original location.

Good points Cripple in placing the comments in context, I agree that is important. However I think understanding what Yo-Yo wrote is the key to understanding the general comments found in the Spitfire Operating Instructions: Flying qualities is Coefficient of Lift dependent. As speed increases, our coefficient of lift decreases. Lets look at the slope of the static longitudinal stability curve of the Spitfire and see what happens. When the curve is moved up or down, the point it intersects the 0 line is our trim speed. In this case the red line approximates the curve shifted up to represent the aircraft trimmed for a speed of 250mph IAS. Forgive my not so good line shifting....it is not meant to make specifics only to show the concepts. The slope of the Spitfires stability curve gets larger the lower the coefficient of lift. Our stability increases and at 250mph IAS, the aircraft has good static longitudinal stability and with the light stick forces, I would certainly agree it is not only satisfactory at this speed but it must have been quite pleasant to fly around. That does not have anything to do with the weak static longitudinal stability experienced elsewhere in the envelope or the dynamic longitudinal neutral to unstable found at normal to aft CG.

Good observation but if you fly taildraggers, it is actually pretty smart. http://advancedtailwheeltraining.com/tailwheel_basics If you have a free castering tail-wheel on take off or landing, it can turn into a disaster real quick. Most tail-wheel locks on free castering designs are a lever in the cockpit that you have to engage. Pilots can forget to engage the lock and it is an extra step to unlock it to gain maneuverability on the ground when needed. The German system allows you to just move the stick forward to unlock the tail-wheel. There is nothing to forget on take off or landing and it becomes natural once you are used to it to gain the extra bit of maneuverability when taxiing.

Thank you Yo-Yo! I am amazed at the quality and depth of FM's you are able to produce on a home PC. You should call Flight Safety, I think they could learn a thing or two from you! :thumbup:

And it is listed in the manufacturer's list of design changes that have to be done to convert a Spitfire to a TR9. :smilewink:

Already been discussed in my very first post starting this thread. It does not increase the area of the tailplane and therefore does not shift the stability margin. It simply adds another form of balancing to the elevator. Why is an airplanes elevator balanced? To shift the control forces to something more controllable under unstable conditions.... Let's see: 1. A million different bobweights of various sizes tried and experimented with... 2. Multiple attempts at other forms of balancing... 3. Multiple elevator redesigns and an obsession with elevator manufacturing tolerances on test aircraft..... 4. Post war area added to shift the stability margin (the correct fix and what should have been done in the first place) 5. Post war aircraft all have with restricted CG limits compared to wartime variants; even have specific longitudinal instability behaviors noted on some the type certificates! And most damning... No change in the relationship between the CG limits and no design changes to move the Aircraft's Aerodynamic Center from the Mk I to the Mk IX. And yet we have pages and pages of staunch defense that the NACA conclusion was not correct and the aircraft was not longitudinally neutral to unstable at normal to aft CG. :P

It does not have the standard elevator so the propeller type is irrelevant. I has the modified horn balance elevator so the Aft CG limit in wartime would be 9.0in. Same limit as a wartime Spitfire Mk IX. Notice the note above the propeller types...."with standard elevator". In otherwords, not horn balanced. The tandem Spitfire front cockpit is moved forward and the aircraft has a 9 1/2 lb bobweight. I cannot find the fuselage length specifications but I would venture it has a longer fuselage as well. What you should take away is the fact the wartime Rear CG limit was too far aft under stability and control standards. The NACA assessment was correct and applicable to the Mk IX series since the stability margin remained unchanged in the design. The aircraft was neutral to unstable at normal CG to the rear CG limit.

There is a post war modification increasing the horizontal stabilizer/elevator area. It goes from the wartime area of 31.46m^2 to 33.83m^2.

Welcome to the discussion. The front cockpit is moved forward in the tandem Spitfires otherwise the conversion would not be possible. Solo, it is a different animal from a Spitfire Mk IX and I have no doubt it falls in the forward of normal CG range for wartime Spitfire Mk IX. By definition, the rear limit is determined by the onset of longitudinal instability in a modern aircraft stability and control standards. That is why the CG are restricted from their wartime limits. The modern definition of sufficient stability has changed as data and the pilot's opinion has been quantified. Mk XVI horizontal stabilizer and elevator area was increased from the wartime 31.46M^2 of the Mk IX to 33.83m^2. This represents a stability margin increase so the Mk XVI is a different animal from a wartime Mk IX.

CG limits have nothing to do with stress or wear on the aircraft.

Good job! Keeping the stick back is key. :thumbup:

If you know this then why do you post the Spitfire Mk XIV data in this thread trying to prove the wartime limits are in effect for Spitfire MkIX's being operated today? As for the Merlin powered Spits, the modification was certainly tested and improved the longitudinal stability. As for what was specifically done to the one you posted...I do not know and neither do you. Something was done however as no post war Spitfire can operate unrestricted with the longitudinal instability of the wartime variants. We have seen enough detailed type certificates to establish that. So far you have been able to produce only ONE aircraft with unknown modifications and only half of the rear fuselage tank installed that comes close its wartime CG limits. http://www.caa.co.uk/AANDocs/17661/17661040100.pdf The operating restriction of "gentle maneuvers only" at weights above 7500lbs and the fact it only mounts 1/2 the rear fuselage tank certainly does not make it comparable to a wartime aircraft. Those restrictions do ensure this aircraft will not have the handling issues of it wartime examples.

57% increase in the horizontal stabilizer and elevator area.

You are selectively reading it and missing important details. The limits on the Spitfire Mk V were the same as the Spitfire Mk IX. 9in reduced to 7.76in is 1.24 in representing a 16% reduction in the CoG aft limit. Once more, the aircraft limits were adjusted to eliminate the onset of longitudinal instability. Yes, there were post war modifications to the tail plane that would have increased the stability margin allowing for an increase in CG limits. Your document does not give any details as to what was done. It tells us nothing. You do not pay attention to the details Bongo, that is what is killing you on this issue. The MkI you linked below has balanced elevators. That means the CG range was 9.0 in for this aircraft during the war and is restricted to 7.9 in post war to eliminate the longitudinal instability. Read the type certificate.... http://www.caa.co.uk/AANDocs/29100/29100000000.pdf Is off topic and irrelevant to the Spitfire Mk IX.

They restricted the CG limits to eliminate the longitudinal instability and the Vne......

The MK XIV is a different animal from the Spitfire Mk IX. Most of them had metal elevators and the tailplane area increased. Of course we know increasing the tail plane area moves the AC and increases the stability margin. That is why wartime Mk XIV were limited to an aft CG of 5.9 inches so our limit is still restricted on this example but not by much. We can't tell why without knowing more details of this particular aircraft.

Yes it has full wartime CG limits but nothing showing it has a wartime elevator and horizontal stabilizer. Where is that information? It will list that on the actual Type Certificate. What you posted is the supplemental type for replacement of the spar booms. So it is not a complete picture or type certificate. It is very far from the "proof" you think. We need more details but since it is a Fighter Collection aircraft, I am sure Yo-Yo can get them. I have no doubt, the post war war elevator modifications have increased the stability margin of the design and would have allowed for safe limits. Do you have more details as the ones I have posted are the wartime elevator and horizontal stabilizer designs. Normal wartime elevator and horizontal stabilizer http://www.caa.co.uk/aandocsindex/29016/29016000000.pdf http://www.caa.co.uk/aandocs/22416/22416000000.pdf http://www.caa.co.uk/AANDocs/29100/29100000000.pdf This one is another Fighter Collection aircraft and does not state the Cg limits but it has some non-standard modifications. http://www.caa.co.uk/aandocs/20279/20279000000.pdf It is just a fact that the Spitfire does not meet modern standards for longitudinal stability and cannot be operated to its wartime CG limits as a result without modifications.

Fantastic! Submit this quantifiable data and lets examine it like adults. I have quantified and measured the argument I have presented. Let's see if we can do the same for your dissension. I have shown the stability margin remains the same as the aircraft's AC and CG limits remain the same. Show me it has changed.

LOL. There is not "two groups" although I do not blame you for thinking that given the ferocity of the personal attacks made against me for raising this subject in the first place. This is a matter of measured data, facts, and science. It is quantifiable and provable. Yes, Yo-Yo knows what he is doing. The misconception from some the Spitfire's flying qualities "destroys" the aircraft is simply without truth. It just means the Spitfire for much of its envelope is sensitive to pitch change and requires double control input same as many modern unlimited aerobatic aircraft. It gives it the unique personality which makes it a Spitfire and helps to complete the dog-fighting picture of the design. It is something that adds realism and depth. The Spitfire was one of iconic designs of World War II and its record speaks for itself. However, the artificially stable computer renditions of the past do not represent what it took to master the aircraft. The longitudinal instability is just part of what makes a Spitfire a Spitfire. Those who mastered it, love it for dog-fighting. It was a rose. All roses do have thorns and being unfamiliar with them leads to being stuck by them. It is one of the most interesting dichotomies of aeronautical engineering. I am sure Yo-Yo will have fun with this one and turn out another great product.

NONE of the Modern Restoration Spitfire Mk IX are allowed to fly at the wartime CG limits. The post war CG limits are restricted for a reason.