RE:NACA SPIT TESTS:
see this response from the RAE dated 1943


got this in my files as a pdf somewhere, but the jpeg is legible.
I think I'd go with the RAE numbers.
NOt sure how that would affect your calculations for stall speed...
See below for spit xiv handling data:
..Brief handling trials have been carried out on Spitfire F.Mk. XIV RB.141, one of the first production aircraft. The handling characteristics have been compared with those of the prototype Mk.XIV. JF.319. Special attention was paid to directional behaviour as this aircraft incorporated a fin and rudder different from that fitted to JF.319, but of the type that is proposed for production.

2. Condition of aircraft relevant to tests.

....2.1. General. The main features of the aircraft were:-

A RG5SM engine.
A Rotol 5-blade Jablo propeller, Type No. VP.R19/5F5/1.
Individual ejector exhausts.
A tropical air intake: a Vokes wet air cleaner element was incorporated which could be bypassed for normal use.
Universal wings with small bulge over 20mm. guns.
Armament :- 2 x 20mm. guns, 4 x 0.303" machine guns. Gun ports and muzzles sealed, relevant ejection chutes open.
................"Streamline" blanks over unused 20mm. gun stubs.
Balloon type cockpit hood.
Whip aerial behind cockpit
V.H.F. aerial beneath starboard wing.
V.H.F. blind approach aerial beneath fuselage.
No IFF aerials.
Fin and rudder shown as in Fig. 1. The rudder incorporated a combined balance and trimmer tab.
Retractable tail wheel.
....2.2. Loading. The aircraft was flown at a take-off weight of 8490 lb., with the centre of gravity 4.6" aft of the datum (undercarriage down) which represents service load.

... stall speeds
.4.22 Stalls. The stalling speeds obtained were:-

Flaps and undercarriage up 87 mph ASI
Flaps and undercarriage down 75 mph ASI
The characteristics were similar to those observed on JF.319.
----------------------------------------------------------------------------------------------------------------------------------------------------------
This report does not say whether these numbers are power on or power off.
Since the POH gives 85mph as the power off number, I tend to think the test number above is likewise power off.
Other data reports = again from WWIIaircraftperformance.org state point blank that the xiv turns as well as the ix.
Remember, the 14 only has lbBOUT 3.5 lbs more per square foot wingloading than the spit ix, and it has a LOT more available power.
just more stuff to consider.
finn

Robert your numbers from the poh are a bit misleading still.
you argue that:


You are correct the Spit XIe numbers are for clipped wings, I had to respond in a hurry so forgot to account for this. As you state the stall speed is then around 84-87 mph (3-6 mph reduction) at 7150 lb. I'll go with a 5 mph reduction for non-clipped wings so this gives 85 mph stall speed at 7150 lb. The Spit XIV POH states a stall speed of 85 mph... at 8375 lb. So assuming the POH can never be wrong... a weight difference of 1225 lb makes none or only a single mph difference? It goes against all known physics though. The Spit IX manual also states a stall speed of 95 mph ( 100 mph for clipped wing) at 8700 lb. Calculating by interpolation this results in a stall speed of 93 mph at 8375 mph for the Spitfire IX's POH if using normal full span wings. The Spitfire XIV's POH still states 85 mph at the same weight. So how do we move on with this if we assume the POH can not be incorrect? Also the 85 mph at 7150 lb in the Spitfire IX POH is for an "aircraft without rear-view fuselage". A spit with rear-view fuselage stalls at 5-17 mph higher stalls speeds. Using these numbers a Spitfire IX would then stall at 106 mph at 8375 lb, which is even further from the 85 mph of the Spit XIV at this weight. Since I'm not 100% sure about the "rear-view fuselage" issue, or why it would increase the stall speed by so much, which seems very strange, let's just stick with 93 mph for the Spit XI at 8375 lb vs the 85 mph of the Spit XIV at this weight. We can also look at other trials. This one for example shows a stall speed of 95 mph at 8145 lb, which should result in around 97 mph at 8375 lb.
http://www.spitfireperformance.com/Spit ... ndling.pdf

So at 8375 lb we now have the Spitfire's stall speed stated as 93 mph and 85 mph from two different POH's, and 97 mph from a ww2 trial. Looking at even more trials we'd get even more different numbers for the stall speed for the same airplane. The chaos has just started! I'm willing to commit blashpemy and say that one or both of the POH's might actually be incorrect.

Two things...the numbers include an external drop tank of 45 gallons for the 93mph example, and a 90 gallon drop tank for the 100mph example. The higher number also includes a full rear tank, moving the C of G aft. Seriously so. And ag
This make handling a lot trickier, and can negatively affect stall speed and make spins dangerous.
maybe the problem is less with the POH's than with your reading of them amidst a sea of other data. It can get confusing... but it's important to make sure you account for all the parameters. You make it sound like the poh numbers are representing exactly the same planform and C of G when if fact they are not.

I reiterate that I have never flown an airplane with a known error in it's performance data on the POH. I have seen errors corrected in pen by hand due to typos in the original printing. But those corrections are given to the pilot.
I think there is quite a bit of truth to the need to distinguish between the actual stall moment and the point where a wing drops and the elevator becomes ineffective.
It's a good discussion to have... bearing in mind that the limits of the sim will make perfection unattainable.
finn

The higher numbers for the 'rear' view fuselage spits is likely due to a reduction of yaw stability in comparison to the 'razor back' fuselages.

<S>
I did account for the parameters regarding the weights. The test at 7150 lb was with normal fuel tanks filled, no externals. At 7150 lb the Spit IX POH states 85 mph stall speed for a normal winged Spitfire. The Spit XIV POH states 85 mph at 8375 lb. Please take a minute to really read these weights once again. 8375 lb does not equal 7150 lb. The 8375 lb number is 1225 lb higher than 7150 lb. They are not the same. It is physically impossible that the IX & XIV would both stall at 85 mph, using the exact same wings and airfoil, at such different weights. It makes no sense at all. As such one or both of the POH's is/are incorrect. From all my countless hours of flightmodel programming, I am 100% certain of this.

I also accounted for the Spit IX's at higher weights were using drop tanks etc in the Spit IX POH. I posted a graph in an earlier post where one can see that the stall numbers from the Spit IX POH follows the same line as for my tests where I in game checked the stall speeds at different weights. As such I found no problems making a calculation via interpolation to estimate the Spit IX stall speed at 8375 lb, following the Spit IX POH stall numbers. In the graph below one can see that regardless of having a drop tank or not, the stall speed numbers from the Spit IX POH (blue line) are linear enough to make a rough interpolation for estimating the Spit IX stall speed at 8375 lb.





The 87 mph for the Spitfire XIV must be Power ON. Power OFF would go against the law of physics. I think the best way to prove this is to just say: Ok, let's go with it. Let's assume 87 mph is the power OFF stall speed and run some tests. From the Spit V NACA test we saw that the Power ON stall speed was 13 mph lower than the power OFF stall speed. 77 mph vs 90 mph. Let's use a 12 mph difference to be on the safe side. This gives a power ON stall speed of 75 mph for the Spitfire XIV at 8490 lb (87 mph minus 12 mph).

First off I fully agree with you that the 1.2 CL-max liftco number of the NACA test must indeed be incorrect, as stated in the RAE test due to instrument error. The stall speed of 77 mph at 6184 lb from the NACA report was from flying trials though and not correlated with the CL-max calculated number. I ran a test and a 1.2 CL-max liftco number will give a power ON stall speed of 93 mph at 6184 lb for the Spitfire V, so clearly is incorrect. Now the RAE test states the liftco CL-max as 1.36 at power OFF, and 1.89 at full power. This makes sense. A power ON stall speed is usually for a medium throttle setting. As such our power ON stall speed for the Spitfire series should logically be around 1.65 if using the RAE numbers. The liftco used in game is 1.78 for the Spitfires, so if anything it's at the higher end, and is within the RAE liftco numbers.

Now... since we pretend that 87 mph at 8490 lb is the power OFF stall speed... I have made tests and in order to get the Spitfire XIV to power ON stall at 75 mph at 8490 lb we must give it a liftco of 2.60. This is quite a freak liftco for a wing area of 242.1 sqft and quite outside the RAE numbers, especially as the Spitfire series is using the thinnest airfoil of all fighters of ww2. The Spitfire is using the NACA 2213 (root) and NACA 2206 (tip) airfoil. Some sources claim NACA 2209 at the tip, but even with this tip thickness it's still the thinnest wing of all fighters in ww2. With a liftco of 2.60, it's a superfreak airfoil, having a better liftco/sqft than any other airfoil of ww2. If setting all fighters to a weight that gives a wingloading of 30.0, the Spitfire will now have a 9-15 mph lower stall speed per wingload than any other fighter. At 30.0 wingloading (7263 lb) the Spitfire would now have a stall speed of only 69 mph. The Hawker Typhoon that has the thickest wings of all ww2 fighters, stalls at 75-76 mph at a 30.0 wingloading. The Spitfire wing (thinnest of all) now stalls 6-7 mph below the thickest wing of all ww2 fighters. It's completely illogical.

This makes the Spitfire a UFO. The Spitfire I, that fought in the Battle of Britain, would then have a stall speed of 62 mph at full weight with this 2.60 liftco setting. The Spitfire I would now turn inside a Ki-27. This isn't logical. A stall speed of 62 mph is like 8 mph lower than the Hawker Hurricane, which has a thickness/chord ratio of 18% (root) and 12% (tip). Compare this to the 13% (root) and 6% (tip) numbers for the Spitfire. Anyone who knows aerodynamics understands that the Hurricane should have a lower stall speed than the Spitfire at the same wingloading. I ran some tests on the Hurricane. I believe the Hurricane should stall at 5-7 mph below the Spitfire if both are flown at the same wingloading, owing to it's way thicker wings. From my tests, in order for the Hurricane to be correct in stall speed relative to the Spitfire, now that we assumed that the RAE numbers for the Spit XIV are for power OFF, the Hurricane, which has a wing area of 257.6 sqft, would pretty much have to be modeled with a wing area of 378.5 sqft. Otherwise the Spitfire I will easily turn inside the Hurricane I, which goes against common knowledge.

I hope that by going with your statement that the 87 mph would be for power OFF, and showing with actual tests how this completely makes the Spitfire wing a total freak wing compared to all other fighters of ww2, shows you that it is highly illogical and goes against the laws of physics. I wish I could hire an aerodynamical engineer's teacher to better explain it. What is worth checking though is if the Spitfire XIV stalls at 87 mph power ON at 8490 mph. This would result in a more generous liftco number than the 77 mph at 6184 lb from the NACA test, which requires a liftco of 1.78. From my tests, for an 87 mph stall speed at 8490 lb the Spitfire series would need a power ON liftco number of 1.95, which is above the RAE CL-max numbers of 1.36-1.89. I believe the issue is a matter of their stall test being done by a real life pilot trying to feel the stall, which gives a lower stall speed number than the clinically perfect, and mathematically correct auto-pilot stall tests I can make in the game. If anything the Spitfire series should currently be right in line, or better, than the real life Spitfires regarding their current stall speed. As they are now (CL of 1.78), they make perfect sense when compared to all the other fighters in the game. By giving them more liftco (CL of 1.95) they somewhat stop making sense, and would cause them to produce to much lift relative to how other airfoils have been modeled. Assuming that 87 mph at 8490 lb is for power OFF (CL of 2.60), the Spitfire wings would have to be made of fairy dust and enchanted by Professor Dumbledore and Harry Potter.


<S>
/Robert

Rephrased my above post a bit, should hopefully be easier to follow my thinking.