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.
orthochromatic film ('ortho') is the oldest black and white film. It represents red and orange as black, and makes blue appears lighter. Blue-eyed people have nearly white eyes in ortho photographs. Anecdote: since this film is not sensitive to red light, it can be processed in a red-lighted 'dark room', as you will still see in movies!
panchromatic film ('pan') was introduced in 1906 and slowly replaced ortho film. It represents all colors with a realistic gray value
complicating matters is the use of the color filters
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)
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.
ANC-18 Design of wood aircraft structures, 1951. Does not mention it
ANC-19 Wood Aircraft Inspection and Fabrication, 1951. Only the sections 5.767 - 5.769 and figure 5.104 discuss fabric covering of plywood
Werkstattpraxis für den Bau von Gleit- und Segelflugzeugen, Jacobs, 1942. Does not mention it, possibly because the practice is too heavy for gliders
Flugzeugbau: Handbuch für die Werkstattpraxis, Schneider, 1962. Does not mention it
aluminium dope overall (including the many Elektron sheet metal parts) with orange lower wings and lower tailplanes
PH-OST in orange on the side of the fuselage
PH-OST in orange on the upper wings
PH-OST in aluminium dope on lower wings
Pander logo on tail in orange
presumably Fay-Egan logos on props (other sources report Hamilton Standard)
rear side of the props (outside cowling contour) are painted black
'POSTJAGER' in large capitals on the nose, most likely orange
Dutch flag on fintip, black '6' on white area on rudder
'PANDERJAGER' inside a red/white/blue stripe on the nose, both sides, most likely black, possibly orange
Fay-Egan logo absent on center prop
I'm still working on them!
Rob Hamann's model on the Modelbrouwers forum
Gabriel Stern's Execuform Postjager. Gabriel also concluded that the fuselage registrations and the wing lower sides were red or orange-red
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.
yp = h * ( 1 - (1 - 2x) 2n ) called P-curve
ys = lambda * ( (1 - 2x) - (1 - 2x) 2m + 1 ) called S-curve
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:
± yt = T /.2 * (a0 * x0.5 + a1 * x + a22 * x2 + a3 * x3)
m = 0.323075441, so the maximum thickness is a bit aft of the reported 30% chord
T = 0.119434915
a0 = 0.296308751
a1 = -0.185708798
a2 = -0.01012475
a3 = -0.218443619
The thickness distribution aft of of the maximum thickness point x = m is described by:
± yt = d0 + d1 * (1 - x) + d2 * (1 - x)2 + d3 * (1 - x)3
d0 = 0.002
d1 = 0.147610807
d2 = -0.058247106
d3 = -0.050013811
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.
YouTube: 'Luchtvaart en Shell' with Postjager footage. Pushed out of LVA Hangar I, Mr De Kok (Shell director) arriving, passenger on test flight on 19 October 1933.
YouTube: 'The Great Air Race - England to Melbourne 1934 ' with some Postjager footage.
Historische Kranten - Nederlandse dagbladen uit de 17e, 18e, 19e en 20e eeuw by the 'Koninklijke Bibliotheek' allows you to search 400 years of Dutch newspapers online. If you go to 'Geavanceerd zoeken' and check 'Losse illustraties' you will search photos. Some random Postjager results:
Telegraaf 8 August 1933: 'Panders snelle Postjager in aanbouw' "Heeft thans nog een rood jasje aan" and "Een [motor] is er reeds gemonteerd, de beide anderen dobberen op het oogenblik op den oceaan".
Haagsche courant 8 januari 1934: Briefwisseling "De onderkant van den vleugel van den Postjager is Oranje-kleurig"
Haagsche courant 8 augustus 1933: Briefwisseling "Vleugel en romp worden heelemaal met aluminiumverf bespoten, zodat de machine zilverkleurig wordt. Onder deze laag bevindt zich nog een roode laag, welke de ultraviolette stralen uit het zonlicht absorbeert."
Bataviaasch nieuwsblad 4 October 1933: 'Panders Postjager op Schiphol' with unique photo
De Tijd, 18 August 1933: 'Pander's Postjager' "Het profiel (N.A.C.A. M12) is bikonvex"
Bataviaasch nieuwsblad, 22 August 1933: 'De Postjager van Pander' with unique engine installation photo in Rijswijk.
Soerabaijasch handelsblad, 16 October 1933: geen titel "Bovenop heeft het vliegtuig de kleur van aluminium. De onderkant is oranje gespoten"
De Tijd 9 december 1933 "De zilver glanzende vogel, die in grootte roode letter den naam "Postjager" op den romp draagt, ...."
Het Vaderland 25 September 1934: 'Incident met den ex-Postjager' right landing gear retracted on second flight on 24 September
Algemeen Handelsblad, 16 October 1934: "Uiver en Postjager naar Mildenhall", new photo, no prop logos??
Algemeen Handelsblad 9 December 1933: 'Postjager om 4.15 vertrokken' with description of the starting preparations