Crumpp

RAE aerodynamic data on the Spitfire says the aft CG limit is 2.638 feet from the LE of the wing root. 2.638ft *12in = 31.656in - 9in = 22.656 difference in the Leading edge distance to Aft limit in the NACA and RAE 22.65in - 21.89in = .766in -.766 = 8.2 (-8.2-21.89)/85 = .354 x 100 = 35.4% MAC is the rearward limit of the Spitfire at 9 inches aft of the datum. Let's check it against the RAE MAC... -9.9-.766 = 9.1 (-9.1-21.89)/85 = .364 * 100 = 36.4% Hopefully this will reduce the questions as to where the .766in correction comes from... That is what is needed to align the RAE moment/arm datum point with the NACA LEMAC.

That is actually measured without much to argue. I am sure Yo-Yo will get things right. Why do you keep supposing? Just work the math yourself. .314*85 = 26.69 (-4.8-21.89)/85 = 31.4% MAC .766 inch difference between the Moment and Arm Datum on the RAE load sheet and the LEMAC of the wing root used by the NACA. 9-.766 = 8.2 (-8.2-21.89)/85 = .354 x 100 = 35.4% MAC is the rearward limit of the Spitfire at 9 inches aft of the datum. Let's check it against the RAE MAC... -9.9-.766 = 9.1 (-9.1-21.89)/85 = .364 * 100 = 36.4% Well, that is excellent agreement with RAE MAC at 9.9 inches aft of 36.3%. Less than 3% error which is probably due to rounding.... :music_whistling: Modern Spitfire Mk IX are only approved with a restricted CG limits. http://www.caa.co.uk/aandocsindex/29016/29016000000.pdf http://www.caa.co.uk/aandocs/22416/22416000000.pdf

What are you talking about and why are allowed a pass making such innuendo? :music_whistling: I did that math. Why don't you do the same?

Yep... You know that was pages of conversation too, LOL. :megalol:

BTW, It is not hard to convert the NACA MAC to RAE arm and moment.

Not compared to any modern example of the Spitfire. None of the modern flying examples of the Spitfire are allowed to have the same CG limits as an RAF service Spitfire. Modern flying examples are restricted to a much narrower CG range due to the stability of the design. Your bicycle comparison is actually spot on. Taking pilot anecdotes from pilots of the early 1940's in context, we find they had very different experience on what constitutes pleasant to fly compared with a modern day Cessna driver. An experienced pilot in the early 1940's had little experience in stable aircraft. Most of the designs up until the late 1930's did not have the speed or the weight required to make stability and control engineering very critical at all. Many airplanes were unstable and some had some pathological handling qualities. The need to quantify pilot opinion was realized rather late. The engineers simply found themselves at the mercy of the test pilot's ego. That is why the NACA made deliberate effort to professionalize the test pilot. Part of the process was to expose them to flying a variety of aircraft. They would do so with close communication with the engineers to improve their understanding of stability and control. Test pilots were being transformed from the "most macho guy at the aerodrome" to a precision instrument and valuable member of the design team. https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/HCooper.pdf Gates and I agree the RAE would have greatly benefited from a similar set of standards and approach during the war. As it was, the RAE's "flying qualities" program was based on the opinion of a few ego's and at the mercy of that experience.

A more stable aircraft would have improved their survival chances while they gained experience.

Which is exactly there was a need for stability and control standards. Most nations adopted them post war but only a few had some sort of standard during the war. Opinion is not quantified without a standard. Even to the point of differing conclusions: The history of the development of the Cooper-Harper handling qualities rating scale: https://engineering.purdue.edu/~andrisan/Courses/AAE490A_S2010/Buffer/HCooper.pdf

40.4cm to 62.9cm = 11.3 inches That is a good CG range.

Increasing the area of the elevator/horizontal stabilizer is one of the design changes that allows for movement of the AC and subsequent increase in the stability margin. There is no details as to the design changes of the new elevator. It is one of those design changes that allows for the CG limits to be moved. Unfortunately, it just did not happen in the Spitfire. The rearward CG limit remained 9.0 inches aft of datum.

No response JtD?

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

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.

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

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? 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!

MMMM......:smilewink: Looks like I am not the only one who understands the issue. Dynamic stability is not even the subject, JtD.

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?

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. 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. Neither have you, what is your point? No, you can run the math and understand the science. Plus you can look through textbooks on stability and control! :) 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 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:

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.

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?

Same area so no movement of the AC, just aerodynamically balancing the elevator in place of the bobweight. 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. http://www.charleslindbergh.com/plane/spirit.asp

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.

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

740kph EAS with 2 SC500 bombs!! :music_whistling:

It is and also one of the most misunderstood controls on the airplane. In fact, I would even say more pilots DO NOT know how to properly use a rudder today compared with those who do. Modern tricycle gear trainers do not teach you to properly use a rudder nor work to develop the muscle memory. http://flighttraining.aopa.org/magazine/2005/December/200512_Features_Choose_to_fly_right.html http://www.jetairgroup.com/2013/03/19/how-to-make-coordinated-turns/ http://www.aopa.org/aopa-live?watch=hpa202YTpqZ-bn2yNikbfXO2iUdAZLbD#ooid=hpa202YTpqZ-bn2yNikbfXO2iUdAZLbD&ootime=08m30s