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Just using internet sources Boulton and Paul was estimating the P.94 speed as 360mph with the Merlin XII (single stage single speed) and 364 with the Merlin XX (two speed engine)
I would suggest 10mph be knocked of for the armoured wind screen, IFF aerial and mirror that cost Spitfire Mk.II over 5mph compared to the Mk.I
So we have a 347-350 mph P.94 with a Merlin XII versus a 357mph Spitfire II with the same engine. That 7 mph represents 2.0% speed difference equal to the P.94 having 6% more drag.
Assuming the fuselages are equal it means the P.94 wing has 18% more drag. Spitfires more raked windscreen and better filleting has less drag but fully covered wheels make a big difference in favour of P.94 There is less wetted area in the P.94 fuselage as the thicker wing accommodates more equipment. The rear variable area ejector flap on the belly radiator can be seen. It seems to have improved and changed on Defiant II.
Note:
the dorsal turret of the Lancaster cost it 12mph even when spread across 4 merlins, while a Lancaster is not Defiant it does give an indication of the serious impact of the big dorsal turret. That's a 4% cut in speed equal to about 12% cut in power ie worth 440hp across the 4 Merlins.
Below I will calculate an estimate of the power the Bouton and Paul Turret would absorb if its drag were added to a 360mph Spitifre II
The equations I will use are F=1/2.Cd.A.p.v^2 where F = drag force, p = air density, A = frontal area.
and Power = Force x Velocity. P=F.v where drag F = 0.5 x Cd.A.p.v^2 which gives:
P = 0.5 x Cd.A.p.v^3 (P = Power in kw, p = air density in kg/cubic m, which is 1.2kg/cubic meter at sea level or 0.6 at 20,000ft)
Lets calculate the Cd.A of the P.94 cockpit and
its razorback. I estimate an windscreen area of 0.5m x 0.5m = 0.25sqm. I estimate an Cd of 0.15. (A perfect circular streamline is 0.05, a thin symmetrical wing 0.04, Fatman Nagasaki bomb teardrop about 0.15).
So Cd.A = 0.0375 (from 0.25 x 0.15)
Lets calculate the Cd.A of the Defiant Cockpit/Turret combo. I estimate an area of 0.75 x 0.6m = 0.40m. I estimate a Cd of 0.5. (A sphere is between 0.48 and 0.6, a half sphere flat face forward is about 0.5, a cylinder flat face forward is nearly 1) Its hard to get a Cd under 0.5 for a circular/spherical object because of suction on the back and turbulence. A sharp cut off is better. Note 0.6m width barely allows a human to fit between gun breeches. (assuming 75cm/28 inch turret ring)
So Cd.A = 0.20 (from 0.40 x 0.5)
The difference is about Cd.A of
0.1625.
If the Spitfire had the extra drag of the Defiant's turret at 360mph/160ms @ 20,000ft it would need 200kW = or about 250kW assuming 80% prop efficiency. That's 340hp, about 37% more than the 900 hp Merlin XII could provide at that altitude. Using a cube root law would produce a 13% speed reduction of the Spitfire i.e. 314mph.
Likewise the Defiant II at 315mph/140m/s @ 20,000ft the removal of the turret reduces power demand by 134kW which is 167kW assuming 80% prop efficiency or 227hp so the aircraft can operate at 670hp instead of 900hp and that 227hp 33% increase in horsepower over 670mph would raise speed by 10% i.e. 346mph.
So the Boulton and Paul Estimates are Plausible.
My educated guess is that the P.94 with Merlin XII with 12 guns, IFF aerial, windscreen armour, mirror would be about 10-12 mph slower than a Spitfire III so 345-347 mph instead of 357.
It certainly wont be below 340mph, which is a lot fast than a Hurricane with the Merlin XX.
The p.94 aircraft would gracefully handle a Merlin 60, Griffon, Sabre or Vulture engine and would handle carrier operations (tough undercarriage, thick wing, good visibility) and could handle longer ranges if gun space were converted to fuel space.
Note If I use a Cd.A difference of 0.14 instead of 0.1625 the Spitfire drops to 325mph and the defiant increases to 343mph.
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