Friedrich-4B

Just clearing up a myth that Merlin 66 pistons regularly failed after 7 1/2 to 50 hours of running at high boost. It demonstrates that the Merlin wasn't made of glass and stops some folks from believing that Merlins should automatically self-destruct after 5 minutes and 5 seconds running at +18 lbs boost during combat.

Not that the Spitfire IX used a bobweight for the elevator; that was a feature of some Spitfires with the "small" elevator...

If Kurfurst is referring to Fig. 28 of Lovesey's article, nowhere does the chart or Lovesey's text say that the pistons "kept failing" after 10 hours of stress testing (ie: continuous running at +18 pounds and 3000 rpm). On reading available material (see below), Rolls-Royce did not clear "+18 lbs while the pistons kept failing under 10 hours of stress at the said load" because they tested and altered the piston's design (and the crankcase and other vital components) until it wasn't failing at the low limits claimed. What the article does point out is that "some ring gumming" occurred after 7 1/2 hours during one test (plus four other tests ranging from 9 1/2 hrs to 50), without stipulating whether this was enough to cause a serious loss of compression, or how many piston rings were affected and to what extent. As it is, the tests for the Merlin 66 referred to occurred between November 1942 and April 1943; as noted by Alec Harvey-Bailey in The Merlin in Perspective - The Combat Years (Rolls-Royce Heritage Trust, 1995) pages 67 & 68, piston rings gumming up and other piston-related problems were solved in production Merlin 66s: Thus, the DCS Merlin 66 of 1944 shouldn't have pistons gumming up, or have other piston related problems after a mere 7 1/2 to 50 hours of running at +18 lbs @ 3,000 rpm.

No such comment was made or implied - it was merely an observation that the early Merlins relied on 100% glycol cf. the 30/70 glycol & water of later series Merlins.

What sorts of conditions and what sort of engine handling in DCS could promote over-cooling? Dead right; as noted here the Pilot's Notes General make it clear that the pilot had some discretion during combat or emergencies. NB: A.P. 2095 "Pilot's Notes General" were issued to RAF pilots along with the Pilot's Notes on specific aircraft types - attached are some pages from Part II: Engines and Propellers

:thumbup: From Pilot's Notes General (2nd Edn, 1943)

This highlights how much more developed and tougher the Merlin 66 of 1943-45 was than the Merlin II/III of 1940 (noting that the 66 ran on a 30/70% mix of glycol and water rather than the 100% glycol of the II/III): it also effectively illustrates how the pressure of wartime experience and development can lead to a much better aero engine.

Attached is the entire Lovesey article on the development of the Rolls-Royce Merlin. In addition, Crumpp didn't provide "illustration 27" (Fig. 27) which shows that by September 1943, 395 hours was used on developing the Merlin 66 itself into a more durable engine than the one that first appeared in mid-1942. Hopefully, Crumpp doesn't expect ED to model their engines to automatically fail at 5 minutes & 10 seconds! It is highly unlikely that a properly maintained Merlin would choose to die if run at +18 or +25 pounds for a few seconds longer than 5 minutes; as all the testing highlighted by Crumpp shows, by November 1943 Merlins were being run in overboost for 100 hours with no signs of component failure. The fact is that most combat situations lasted for seconds, and few pilots would have been forced to operate their engines at such high boost pressures for even one minute, unless in a dire emergency. (iii) is an extract from A.P 2095 Pilot's Notes General: Part II Engines and Propellers; Note A. Engine Limitations (see below) Paragraphs i(b) and ii of 3. Duration and Flight Condition Limits of Use state thus, in para ii, pilots were given a degree of flexibility during combat or in emergency situations. ED should model their Merlin 66 accordingly.

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

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. 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 (7.2+19.5)/84 = .3178 * 100 = 31.8% MAC Besides, undercarriage down isn't a normal flight condition... Does any of this prove the Spitfire IX was longitudinally unstable?

Not quite; Crumpp's calculation doesn't show that the A&AEE's (NOT the RAE as stated by Crumpp) MAC was 84"... (-8.2-21.89)/84 = .3582 * 100 = 35.82% rounded down = 35.8% MAC @ 9" aft of datum (-9.1-21.89)/84 = .3689 * 100 = 36.89% rounded up = 36.9%

Actuallly, NACA calculated the MAC at 85"; see sentence stating MAC may be in error: while the A&AEE drawing shows the MAC was 84" This is the drawing referred to by Crumpp as "RAE (sic! should be A&AEE) aerodynamic data"; it has been adapted for a flight sim (among other things, the data is for the Mk. VB Trop while the title says Spitfire Ia & Ib); nor does it show a MAC of 85".

As well as peddling the same theme, in one form or another, on this forum. Actually, the OP would help himself were he to stop implying that other forum members, who have considerable real-world aviation experience (engineering and/or flying), are idiots who don't know what they are talking about. Unfortunately that is the OP's most prevalent MO.

Crumpps "information" in #203 (either on a Mk IX, G-ASVJ or Mk. VC G-BUWA? Crumpp seems to be mixed up on this) actually comes from a restored Mk. VC, G-MKVC, which has been rebuilt to completely different specifications to G-BUWA, and has been given completely different conditions for flying. In fact, the following applies to G-BUWA (ex AR614) (From the Airworthiness Approval Note 23951 issued in July 1999): So much for all post-war/restored Spitfires not being allowed to fly with the original CG limits. :music_whistling: And what Crumpp's throwaway comment means is anybody's guess. :smilewink:

Yeah, right :smilewink: ; the only "reports" on the IX posted by Crumpp are a few paragraphs from Morgan & Shacklady, most of them dealing with a Spitfire IX tested with full rear fuselage fuel tanks and hand-made elevators, plus the report, courtesy of Mike Williams, that deals with the latter - in other words, no typical wartime Spitfire IXs are dealt with throughout Crumpp's entire "presentation". And nothing measured and quantifiable, except some stuff about Mk Is and VAs that relate to the Spitfire Mk IX only insomuch as they belong to the same family. (Closest Spitfire relatives to the Spitfire IX are the VC, from which the IX was derived, and the XVI) :lol: How about making it "10 million" or even "100 million bobweights" just to emphasize that some experiments were conducted and the bobweights rejected. The only bobweights adopted were on some Spitfire Vs to help overcome poor loading practices on some squadrons. The bobweights were removed when the new elevators (mod. 789) were introduced on the Mk IX. As if the Spitfire was the only WW2 fighter to experiment with or alter elevator design and balances; nor is there anything unreasonable about being concerned about manufacturing tolerances on such vital components. To claim it was an obsession, based on one report, is as much of an overstatement as Crumpp's "A million different bobweights". The only evidence posted by Crumpp is the certificate for one restored Spitfire V - otherwise this continual assertion, implying that Supermarine were too stupid to fix the Spitfire IX's purported longitudinal instability until post-war, is rubbish. There was no problem to fix. Except for redesigned wings and elevators, plus the longer fuselage and heavier powerplant, including the propeller, engine and cooling system - most of which Crumpp mentioned in his OP, waaay back then: Funny, too how the Mk IX needed to be ballasted to restore the original handling characteristics...plus there were those pesky modified elevators that helped shift the forward CG limit, as well as enabling the aft limit to be moved back to 9". So, no cigar for Crumpp.:smoke:

How about Crumpp provides that report on the Spitfire IX's stability that he claims to have, so that 18 pages of debate can be brought to an end? No doubt, it will satisfactorily vindicate Crumpp's POV, thus ensuring that the said POV will be respected and not open to further debate, should the subject come up again. At this point, still waiting. :music_whistling:

All good things take time; for now relax and enjoy some of what's coming: https://www.youtube.com/watch?v=0erLVkqvPb0

Nor is longitudinal instability mentioned as a reason why restored Spitfire IXs and XVIs are not flown to their wartime limits. Now as I recall, right at the beginning of this year, Crumpp assured us that he had at least one definitive report stating that the Mk IX suffered from longitudinal instability http://forums.eagle.ru/showpost.php?p=2279493&postcount=71 So here we have a problem; surely it would have been far easier on everyone had at least some of this information been posted right at the start of this thread, instead of subjecting everyone to endless debate and speculation based on partial documents of earlier, single-stage Merlin Spitfires, and what has happened since to restored examples?

Please show, with documentation, that post-war Spitfire IXs and XVIs had their horizontal control surface areas increased by 57%. From http://www.caa.co.uk/aandocsindex/29016/29016000000.pdf It is just a fact that restored Spitfires are not operated to wartime standards because, like any other restored WW2 aircraft, they are rare and extremely costly to repair or replace. It has nothing to do with purported "longitudinal stability" standards. (From Spitfire: Return to Flight )

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]): and what was written in the report that he cited, http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf : Supermarine's Datum, as set by the company: So, we have now established that the Datum set by Supermarine wasn't at the tip of the spinner; But, Hey, :pilotfly:?! :music_whistling: 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:

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. 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 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.

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. 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. :smilewink: :thumbup: 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. 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.

Crumpp has already claimed that he has a definitive report on the Spitfire IX - not the Mk I or VA - proving that it was longitudinally unstable. Surely Crumpp would make things easier on himself and everyone else by posting this purported information on the Mk IX, instead of regurgitating the same old material on a very early Spitfire I, and a VA tested by NACA, thus forcing himself and others to "do the math" using incomplete information? As it is, it is far more likely that DCS has ready access to far better, more accurate information on the Spitfire IXs flight qualities, making this thread nothing more than an exercise in futile speculation, based on a random sampling of material gathered from other websites and Morgan & Shacklady.

Exactly what happened with the Mk IX cf the Mk I or VA - the wing was redesigned (including introducing extended or clipped wingtips, changing the internal layout and weight distributions, etc), the fuselage was lengthened and the design of the stabilizer was changed. In other words there was nothing abnormal about the Spitfire's MAC cf other "conventional aircraft" (or is Crumpp saying that the Spitfire wasn't a conventional aircraft design for its time?) Crumpp has never flown a Spitfire IX in real life, nor do we have accounts from IX pilots stating that the IX "oscillated and required double controlling" under normal loading conditions. Had the Spitfire IX been such a handful at normal, or at it's rearmost CG limits, it would never have been used for low-medium altitude ground attack missions while loaded up with bombs and/or drop tanks We only have Crumpp's word on this, with no evidence to prove that the Spitfire IX was/is a "hot topic". (The only one who has so far made this a "hot topic" is Crumpp, who has pushed this subject many times, both here and in other forums.) No mention of a 7 1/4 inch bob weight - the CG limit was 12.2" aft, cf Crumpp's 6.8" (see below) so it shouldn't be surprising that the stability margin was dicey; Quill did not specify that the IX in this condition was acceptable for formation flying - his comment was "just about acceptable" - hardly a ringing endorsement, but not surprising for an aircraft being tested with its CG 3.2" aft of its normal rearmost limit. Note that Crumpp maintains that: So why should it be such a big surprise that an aircraft loaded to 5.4" inches aft of its normal take off datum proves to be more unstable than normal? None of this "proves" anything, other than the Spitfire IX was surprisingly forgiving to fly, even under extreme loading conditions.

Bottom line Crumpp has never provided documented evidence of the IX's so-called instability; last time he was asked point blank whether he had documents stating that the IX was unstable, he said Yes - then got stuck in South America... The purported document on the Mk IX's instability was never produced. :smilewink: Anyway, I'm not interested in feeding Crumpp's lone obsession with this non-issue, nor should this thread be diverted into further "discussion". :yawn: