RAF airfoils?

[tomo pauk]

...where RAF does not mean Royal Air Force, but it is a name for the airfoils originated from National Physical Laboratories in the UK. For example, the De Havilland company used the RAF 34 airfoil for the wings of many of their aircraft, eg. the DH 88 Comet, or the Mosquito.
I'd like to find out more about those airfoils, and how they stack vs. other oftenly used airfoils, so please contribute

 

[pbehn]

I found this


For such a famous airplane, details of the wing section geometry are very hard to come by! Without any manufacturing drawings or access to an existing airframe, all I can do is try to reverse engineer the profile. From " The Mosquito Manual ", I know there were two sections involved. The first was a Piercy section ( Norman Augustus Victor Piercy ) and the second was the RAF 34.

It is important to understand how the RAF 34 came about and so we refer to the Aeronautical Research Committee Reports and Memoranda No. 946, by H. Glauert, November, 1924. A description of the airfoils is as follows,

The basic symmetrical section ( R.A.F. 30 ) was calculated by the method described in R. & M. 911, using the constants k = 1.08, n = 1.95, B = 0. The aerofoil shape so obtained ends in a sharp angle, and so the last 1 percent of the chord was cut off in order to avoid a thin trailing edge. The form of the aerofoil was also adjusted slightly towards the trailing edge in order to remove a slight reflex curvature. The aerofoil has a maximum thickness of 0.13 of the chord at a distance of one third of the chord from the leading edge, and its shape approximates closely to the symmetrical Gottingen section 459 which was known to possess good aerodynamic characteristics. The aerofoils R.A.F. 31 and R.A.F. 32 were obtained by curving the centerline of the symmetrical section R.A.F. 30 into circular arcs of camber 0.02 and 0.05 respectively... Finally, as R.A.F. 32 has too large a value of km0, a fourth aerofoil R.A.F. 33 was designed, using the centerline 19.36y = x(1-x)(7-8x) and the same symmetrical fairing. This aerofoil has the same center line camber 0.05 as R.A.F. 32, but should have constant center of pressure ( zero km0 ).
So, where's the RAF 34? Apparently, Glauert didn't think that the center of pressure movement would become excessive until the camber exceeded 2 percent. Well, someone did and so in R&M 1071, we find the RAF 34, with a reflexed camber line of 2 percent.

48.4y = x(1-x)(7-8x)

This is significant as we look at the Piercy section. The aerodynamics department ( Richard Clarkson ) at de Havilland obviously thought that having a constant center of pressure was of great importance, since several aircraft preceding the Mosquito also used the RAF 34. Bristol chose the RAF 28 airfoil for several aircraft, including the Blenheim and Beaufighter. That section was considered superior to the RAF 31, with 2 percent camber in which the amount of curvature went to zero at the trailing edge.

In 1937, Piercy came up with a new type of wing section, inverted from a hyperbola. Cambered sections of the first family had circular arc mean lines ( and thus undesirable center of pressure movement ). The second family had the ability to "twist" the camber line to provide reflex, but looking at a Mosquito shows no signs of reflex. That leaves only a symmetrical section to be used in the interests of maintaining a constant center of pressure. Piercy had written,
The 1937 profiles provided the first laminar flow wings, having according to tests by the National Physical Laboratory, some 35% less drag than normal wings. They were adopted for certain war-time aeroplanes and promoted research into the subject at home and abroad.When the position of maximum thickness is moved back along the chord, the leading edge radius becomes smaller and so it is necessary to increase the thickness of the profile to maintain good stalling characteristics. A symmetrical Piercy section with thickness of 0.15 at 0.4c will have a leading edge radius similar to one of 0.13 thickness at 1/3 chord ( RAF 34 ). What happens when the two are merged together? Hopefully, it looks like a Mosquito wing section!

 

[pbehn]

An interesting list of aerofoils used here

The Incomplete Guide to Airfoil Usage

 

[pbehn]

This is a good read. RAF 34 aerofoils were used on the Supermarine 224
https://www.aerosociety.com/media/4953/the-aerodynamics-of-the-spitfire.pdf

(P 61)
The Type 224's aerofoil section for the inboard anhedral portion was a symmetric section of 18% thickness/chord (t/c) ratio (NACA 0018) (10). For the dihedral outboard portion an RAF 34 section (12.5% t/c ratio) was modified to a t/c ratio of 18% to match the anhedral NACA 0018 inboard section and tapering linearly to 9.6% at the tip (Pegram (8)). Thus the Type 224's t/c distribution was similar to that of the Hurricane (see Table 1 below).

 

[tomo pauk]

Hope you like bacon

 

[pbehn]

Hope you like bacon

num num, Considering I hadn't heard of RAF 34 aerofoils until yesterday I have read some interesting stuff. good topic.

My take on it is that the later aerofoils were better but de Havilland, building a plane from wood decided to go with the "devil they knew" with RAF 34 modified in line with best known practice at the time.

 

[pbehn]

Just thinking, the aerofoils on the mosquito really only applied to the wing outboard of the engine, whatever they were, it had the radiator inlet on the wing leading edge and the outlet on the underside.

 

[tomo pauk]

That is probably true, the Mosquito's wing root was (IMO including the radiator housing) noted as with 13% thickness-to-chord ratio. Link to the excellent thread, root TtC are noted in the photo #2 there. Note the hight TtC of 19% for the Whirlwind and Beaufighter, as well as apalling 21% for the Welkin. Both Westlands with Cd0 greater than 0.03.

NACA proposed the same radiator layout for the YP-38 as it was on the Mossie, part of the increase in chord (= lowers TtC) aimed to cure it's problem with sharp rise of drag above mach .6.

 

[pbehn]

That is probably true, the Mosquito's wing root was (IMO including the radiator housing) noted as with 13% thickness-to-chord ratio. Link to the excellent thread, root TtC are noted in the photo #2 there. Note the hight TtC of 19% for the Whirlwind and Beaufighter, as well as apalling 21% for the Welkin. Both Westlands with Cd0 greater than 0.03.

NACA proposed the same radiator layout for the YP-38 as it was on the Mossie, part of the increase in chord (= lowers TtC) aimed to cure it's problem with sharp rise of drag above mach .6.

My post was purely intuitive, I am in no way an expert. It is based on this.

1. The research into aerofoils is on an aerofoil with the flow going either above or below the wing. Putting a radiator in the wing leading edge changes all of this airflow, that may or may not be a disadvantage but since I know jack all really I am confident that overall it was a compromise worth making on overall cooling drag etc.

2 The RAF 34 profiles used were also modified by de Havilland. If the P51s success was due to its wing aerofoils, its low cooling drag and its refined overall aerodynamic finish then the major difference between the two amounts to the difference between the modified RAF 34 profiles and the P51s NACA profiles, which is very little.

 

[tomo pauk]

My post was purely intuitive, I am in no way an expert. It is based on this.

1. The research into aerofoils is on an aerofoil with the flow going either above or below the wing. Putting a radiator in the wing leading edge changes all of this airflow, that may or may not be a disadvantage but since I know jack all really I am confident that overall it was a compromise worth making on overall cooling drag etc.

I'm no expert either
FWIW, here is the link about NACA recommendatios for the YP-38. One of suggestions is, with chord increased by 20%, installation of radiators on the new portion of the wing. Reduction of overall drag vs. 'classic' radiators was noted.

2 The RAF 34 profiles used were also modified by de Havilland. If the P51s success was due to its wing aerofoils, its low cooling drag and its refined overall aerodynamic finish then the major difference between the two amounts to the difference between the modified RAF 34 profiles and the P51s NACA profiles, which is very little.

Agreed.

 

[J.A.W.]

Later NPL wing profiles were of improved, 'laminar flow' type,
( albeit not 'trumpeted' as such - until post P-51 fanfare, by Supermarine, with the Spiteful).

Hawker utilized one, for the Tempest ( & carried it over to the Fury) but called it a 'Hawker high-speed wing'.

With both the Hornet, & Vampire - de Havilland had jumped on board, but the Meteor, oddly - dipped out..

 

[Simon Thomas]

I am not sure I subscribe to the idea that the Mosquito aerofoil is a blend of the RAF 34 and a Piercy aerofoil.
There were many Piercy aerofoils around at that time. Piercy developed his transformation based on the hyperbola, using two factors to define the shape.
Looking at a nose section from the Mosquito handbook, to me it is very unlikely that the proposed blended aerofoil matches the Mosquito aerofoil.

My WAG is that Piercy used his two factors to produce an aerofoil that matched the RAF 30 aerofoil as closely as possible, and then DH tweaked the camber to suit the Mosquito wing. The upper and lower surfaces of the flaps and ailerons are flat, so the camber in that area has to be a straight line. Possibly DH kept the Glauert's camber formula until near the start of the flap and then transitioned to a straight line.

All conjecture on my part, but I remain unconvinced that the Mosquito aerofoil is just a simple blend of the RAF34 and a Piercy section.

 

[nuuumannn]

Mosquito's wing profile, the best I could do since I'd have to be standing on a Harvard in order to get this square on.

TV959 24

Mustang and Spitfire wing profiles for comparison.

Wing profile

Wing profile

The Mosquito's wing was built as one continuous piece, on which the fuselage sat. In this image you can make out subtle changes in its surface profile across its span.

TV959 02

More detailed Mosquito images here, including control surfaces and wing sections:

http://warbirdswalkaround.wixsite.com/warbirds/tv959

 

[Simon Thomas]

Finally found the photos I was looking for earlier.
The first photo is the wingtip of KB161 at Windsor, the toonie is for scale. The second is the outboard leading edge shot from about where the motor sits.

 

[Wurger]