The Aviation Historian — January 2018

56 THE AVIATION HISTORIAN Issue No 22

was just the rate of change of the velocity (and the

pressure gradient) that was reduced.

There is much to be gleaned from this direct

comparison (the pre-Mach drag of the Lightning

against the Westland Welkin speaks volumes, for

example). The North American P-51 Mustang had

a wing designed to be as compressibility-friendly

as that of the Spitfire; it was thicker, but with a

lower design lift coefficient, the maths conspiring

to give them very similar critical Mach. Some

have claimed the elliptical planform of the

Spitfire’s wing allowed it to dive so much faster

than the Mustang; but, on the basis of the above,

it is much more likely to have been the Spitfire’s

“optimum” fairing from Caltech via Langley that

made the crucial difference.

A mention in the report of a Mach-improving

“spoiler” for the Mustang I is of great interest.

The text mentions this was just in front of the

radiator intake, and that the reason it did so

much for the aircraft’s dive limits was unknown.

Reference is made in one edition of the Pilot’s

Notes to an extendable deflector on “AG664 and

earlier aircraft” (this being the last of 320 Mustang

Is delivered during 1941–42). It also appears on

a J.H. Clark cutaway published in The Aeroplane

in November 1943, but seems to have been

largely forgotten about in Mustang lore since.

The Mach-improving effect (possibly a result of

the pre-separation of a turbulent boundary layer,

much like a vortex generator would provide) was

almost certainly accidental, the flap probably

being fitted to prevent over-cooling in the North

European theatre. That the type might be made

to go even faster with the right concessions to

high-speed flows was not lost on everyone;

racing Mustang conversions at Reno have

and by Mach 0·6 the differential has increased by

58 per cent — and this is for a standard mid-wing

design with no “pocket”.

Practical examples

In 1946 the British Aeronautical Research Council

publication Research on High Speed Aerodynamics

at the Royal Aircraft Establishment from 1942 to 1945

detailed the research and testing of numerous

aircraft types.^17 It also provided a handy summary

of the Mach-induced rise in drag of various con-

temporary fighter types, clearly showing the

speeds at which some kind of compressibility

drag effect kicked in above normal drag (although

this is not the same as critical Mach).

The report made it abundantly clear that real

aircraft start to experience compressibility drag

— caused by increased pressure gradients — well

before critical Mach. These compressibility effects

do not just suddenly kick in at critical Mach, they

come on progressively, as Delano had discovered

in a NACA windtunnel in 1939.

The most telling parts of the report are the two

Gloster Meteor curves. While the fuselage-join

was fairly low-drag and compressibility-friendly

(a simple expanding fillet), it seems the Gloster

team overlooked the two other bodies attached to

the wing. The first engine nacelle design was a

very draggy “egg”, following Muttray to the

letter and making a similar mistake to the Lock-

heed P-38 Lightning team [to be covered by the

author in a forthcoming article in TAH — Ed]. When

the nacelles were lengthened and the join thus

straightened, the speed at which compressibility

affected drag on the aircraft was raised dra-

matically, although critical Mach was not. This is

because the air was not slowed by the change; it

LEFT & BELOW The Gloster

Meteor initially sported two

teardrop-shaped engine

nacelles (as seen left), creating

separation and drag, as

discovered in wool-tufting

trials in 1944. Altering the

shape so that longer nacelles

(as below) gave straighter

junctions and reduced

pressure gradients delayed

separation and added up to

50 m.p.h. (80km/h) to the

aircraft’s maximum speed.

PHILIP JARRETT COLLECTION x 2