Pander S4 Postjager colors and markings

Around 2010 Execuform issued a 1/72 scale vacform model of the Postjager. A 1/72 resin model is expected before the end of 2012. I initially planned to build the Execuform vacform, and started researching the subject. For some time, the black and white photos confused me thoroughly. But patterns emerged slowly, and it is now possible to analyze the colors with just five photos. The result was slightly surprising.

Black and white film types

Color analysis of the Postjager markings

The red-white-blue fin tip shows clearly that this is an ortho photo: red is black, blue is rather light. (From: Hazewinkel: Pander - een Haagse vliegtuigbouwer, page 117)

This photo is clearly made with pan film: the red portion of the fin tip red-white-blue has a normal grey value, the blue part is slightly darker. Now look at the PH-OST registrations on the rear fuselage and the wings: they have a grey value, and are clearly not black. Since red and orange are the only colors that show this difference in ortho and pan films, they must be in that color range. Since they are lighter than the red part of the fin tip, they must be orange or orange-red. The Pander factory logo on the tail appears to be the same color. (From: Hazewinkel: Pander - een Haagse vliegtuigbouwer, page 113)

This photo does not offer a direct clue whether it was made with ortho or pan film. (From: Hooftman: Nederlandse vliegtuigencyclopedie - Pander S4 Postjager, page 89)

However, combined with this photo we can now be sure that the previous photo was made with ortho, and this one with pan. The different representation again shows us that the lower side was painted orange or red. Considering the grey value, orange is most likely. The ST lettering has the same grey value as the rest of the aircraft, and thus appears to be silver dope. Note that 'POSTJAGER' in large capitals on the nose is painted in the same color as the lower side of the wings - orange. (From: Hooftman: Nederlandse vliegtuigencyclopedie - Pander S4 Postjager, page 75)

This photo shows the demarcations of the orange areas on the lower side. Note that the tailplanes are also orange on their lower sides. (From: Hooftman: Nederlandse vliegtuigencyclopedie - Pander S4 Postjager, page 41)

Other bits of evidence

Circumstantial evidence that the lower side was (going to be) painted orange. (From: Hooftman: Nederlandse vliegtuigencyclopedie - Pander S4 Postjager, page 45)

More details on the painting: the plywood was first painted with 'red paint' (maybe red oxide dope?) followed by English nitro-cellulose paint with aluminium powder. (From: De Ingenieur 8 december 1933 - Constructieve bijzonderheden van de Pander-Postjager PH-OST, page 227).

C. ter Horst, in 'De Modelbouwer' (June 1987) reports that the complete aircraft was covered with Madapolam, a bleached cotton fabric. This avoided the use of serated ('pinked') linen rib tape to cover joints in the wood structure, and ensured a very smooth surface. The beech plywood was Bakelite (phenol formaldehyde) bonded, a fairly recent innovation that made it waterproof and improved the durability greatly.

Wikipedia reports that Madapolam is a soft cotton fabric laid out in linen weave. It was famously used as the covering for the de Havilland Mosquito, tautened and stiffened with aircraft dope.

The finishing techniques of the de Havilland Mosquito make an interesting comparison: "The tailplane, and fin, as well as being wooden, were covered in wood, then Madapolam fabric. The fabric was treated to Scheme "Z," which consisted of two coats, of half-strength, followed by three coats of full-strength, red dope, followed by, at least one, but, often, two coats of light-reflecting aluminium (not silver) paint." A great photo of the application of the Madapolam fabric can be found on the webpage dedicated to William Herbert 'Bill' Grace (4/5 down the page).

Later I remembered that de Havilland also used fabric over the wooden fuselages of the Vampire and Venom. It is described in The History of the De Havilland Vampire. Photos of the wooden structure can be seen at Uschi van der Rosten's De Havilland Vampire References.

I tried to find more information about the practice of covering the plywood skin with fabric. I consulted the following references, but found almost nothing, suggesting it was not common.

General markings

Additional markings during the Indonesia flight

Additional markings during the London-Melbourne race

Markings designed so far

I'm still working on them!

Modeling links

Wing profiles

There is not much data available on the technical design of the Postjager. One of the questions is the wing design, more specifically the wing profiles. The NVM drawings state that the center section was 'M12 17%' and the tip 'M12 11%', and it contains a table of profile ordinates (coordinates) for both. The use of the M12 profile is confirmed by one of the old newspaper articles listed below. But I found the use of M12 in two different thicknesses strange and confusing. To answer that question, I looked into the design of the M12 profile. Normally, the ordinates of a profile are all you need. But in this case, to understand the 'thickened' version', the camber line and thickness distribution must be known, and preferably their mathematical equations known. A long search followed..

The 'M12' profile is the 12th Munk-designed profile, in a series of 27, for NACA. Their profile data and performance are all listed in NACA Report 221: Model tests with a systematic series of 27 wing sections at full Reynolds number. Table XXVIII of that report lists which combinations of camber lines and thickness distributions were used. Table XXIX list the ordinates of all 27 profiles. It includes an obvious typing error for the 80% chord lower side ordinate. Furthermore, I found that M12 has a Gôttingen equivalent, the Go 676 profile, but I could not find out which came first!

Unfortunately, the paper does not contain the mathematical models for the three camber lines and the three thickness distributions, it only identifies them as "straight, a and b" and "I, II and III" respectively. Therefore I entered the data of six profiles in Excel, and starting doing various analyses to try to understand the way these profiles were constructed.

The good news is that I was able to establish the camber line equation of M12. Note that M10 and M11, with the same 'b' camber line, don't have their start and end point on the chord line - welcome to the world of old profiles. Seeing the slight reflex at the rear, and knowing the German background of Munk, I remembered the equations that were used to design the Me-163B wing profile, using a P-curve and an S-curve combination for the camber line (Skeletlinie), see Göttingen 765: attempt 6: camber line plot.

And indeed, the combination of n = 1 and h = 1.75 for the P-curve and m = 1 and lambda = 1.5 gave a fairly close approximation of the camber line calculated from the profile data.

I think that the thickness distributions I, II and III are identical except for their thickness, but I have not yet identified a model that describes them. It's not the NACA 4 distribution, I checked that. Considering Munk's German background, it's most likely one developed in Göttingen. Also, there information on the leading edge radius is also missing.

Luckily I got expert help from Keith Pickering, who provided the excellent report 'Recovered equations and ordinates of the Göttingen 765 airfoil' for my Me 163B Komet website. He used the same method here, optimised the results with a least-squares method, and his analysis resulted in the following model for the thickness distribtion.

The thickness distribution ahead of the maximum thickness point x = m is described by:

The thickness distribution aft of of the maximum thickness point x = m is described by:

I tried to find more information on the Gottingen 676, which is the M12 equivalent, although I still don't know which came first. I consulted 'Aerodynamische Profile - Windkanal-Messergebnisse, theoretische Unterlagen' by Friedrich Wilhelm Riegels from 1958. On page 150, the ordinates are listed, and they do not agree with the numbers in NACA Report 221. On page 128 it lists performance data of 676, and quotes 'E IV' as the source. Unfortunately no list of references is provided, so the trace ended there.

Lastly, worthy of mention is Parametric airfoil catalog Part II Goettingen 673 to YS930 by Thomas Melin. Melin presents a method to represent airfoils by four Bezier curves, including the Go 676 (equivalent to M12) profile. It might be useful for a 3D model.

Now back to the Postjager profiles. From NACA Report 221 I conclude that Munk himself combined various thickness distributions with a camber line. Since thickness distributions I, II and III appear identical in shape, you could also say that he scaled the thickness as desired. In that sense, a '17% M12' profile would fit in Munk's approach. But the drawback is that no aerodynamic characteristics are know for a self-defined thickness. It would seem much more logical to me to use profile M15 then, that was an 18% profile using the same camber line, and for which all aerodynamic characteristics were measured and report in NACA Report 221. Therefore I still don't think I understand the reasoning for the 'M12 17%' and 'M12 11%'. Possibly some old reports saved at NLR could tell us more.

Research links

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