RESEARCH FOR AERONAUTICS—FARREN 259 
Comparing the two fighters in more detail, we find first that the 
wing surface is the same for both. Disregarding induced drag (or 
assuming it to be the same fraction of the whole in each), the top 
speed at the same height will be proportional to the cube root of 
the thrust power divided by the drag coefficient. Since the pro- 
peller efficiency is near enough the same for both, we may use brake 
power. Taking ground-level powers in both cases—180 hp. for the 
Hispano and 1,250 for the Merlin—the ratio is about 7. Thus the 
contributions to increase of speed are: 
g 0.039\14 
by reduction of drag — Suleal 
by increase of power (7)? =1,92 
The product of these figures is 2.33. 
If we assume that by supercharging it is possible to keep the Merlin 
power constant up to say 25,000 feet, where the density is approxi- 
mately halved, we shall get a further rise: 
by supercharging™ (2)! = 1.26 
The total ratio of increase is therefore nearly 38. 
At this point I feel that the engine people are feeling very pleased— 
and we have good reason to acknowledge the success of their effort. 
But these figures as they stand do less than justice to the aerodynamic 
contribution. All the cooling required by the seven-times increased 
power has been provided and yet the aircraft has no more than half 
the drag per square foot of wetted surface. 
How have these improvements been made? Let us look first at 
the drag account (table 1). To the saving of 47 pounds, the most 
obvious contributions are from the elimination of wing bracing and 
undercarriage—31 pounds in all. But the body and cooling drag is 
actually reduced by over 10 percent in spite of the sevenfold increase 
of power. 
TaBLe 1 
8. E. 5 drag | Spitfire drag 
at 100 ft. at 100 ft. 
per sec. per sec. 
Pounds Pounds 
‘ey sel en ora aes Aa Su Neca ES SR ae Re il aR a 28 20 
Mynoaraninie seo oe Fe oe eae a ots os MS A UNG ea Sue RS SEAR 
iBomysand cooling *#. 2 1 2Sc uk Tote ae Se aE ieee ee ee a ae 44 38. 6 
PE RIISIITIACES Masses Ute Le ts FA eee nea ebay res a ae! 4.4 
Unt erearniagve soe eS ee a 6 LN eet Ne ae he Nn ee BP ae ee Gh) ioe Cee 
