It’s interesting to take a look at driving for economy from a distinctly different angle, and to do this we’ll take a look at the characteristics of the Merlin engine used in the Supermarine Spitfire ñ one of the heroes (along with the pilots and the Hurricane) of the Battle of Britain. Spitfires used in the Battle of Britain were early versions, lacking many of the refinements, improved engines and interlocked controls brought in on later variants. Many of the pilots were also very green, as we know, with very few flying hours in their logs and, sadly, their gung-ho enthusiasm often outweighed their knowledge and flying experience. So many pilots lost their lives, or crashed their aircraft, when their 90 gallons of fuel, good for maybe 90 minutes of flying, ran out before they got home. It was established that this was often due to ignorance of the Merlin engine’s characteristics, and not adopting recommended engine settings. Precious fuel was often wasted when cruising home from action zones, rather than when involved in dog-fights with Messerschmitt ME109’s, and the errors were in fact pretty basic, and avoided in later Spitfire variants by building in automatic systems that ensured maximum fuel economy, and increasing fuel capacity.
We need to know a little about the Merlin engine though, and how it operated in the rarefied atmospheres at high altitude. Fundamentally, it relied on a supercharger to boost performance at altitude, to draw enough air into the V12 engine to develop adequate power for combat. Of the many engine controls, there was a control for weak or rich mixture, one for propeller pitch, and the boost control for the supercharger, effectively the throttle, which would need to be increased with altitude, and for maximum performance at any altitude. There was no direct control of engine speed, which was dependent on these control settings, much as we don’t directly control engine speed in our cars, other than by changing gear, the Spitfire equivalent of which was propeller pitch, whilst the accelerator equivalent was the charge boost. The sharper the angle of the propeller blade, the stronger it bites into the air, and in effect the lower the effective gear. If you were cruising at low altitude you could feather the propeller and in effect drop into a high gear, but need to increase propeller angle and supercharger boost as you climbed into less dense air. Notes from RAF Fighter Command memos explain the story in their terms:
ìPilots have been told to fly at low rpm and high boost to economise on petrol. All pilots must know (and adopt) the correct rpm and boost at which to obtain the longest duration of flight or range.
Wings must still fly at the most economical rpm when flying under enemy RDF screens (it’s often not recognised that Germany had effective radar at the outbreak of WW2), but it is essential that, as soon as they are liable to be detected, they open up to maximum power for formation flying.
The acceleration of the Spitfire is relatively poor. It is therefore dangerous to cruise at low boost, say, +2psi boost, and at 1,900rpm, when the Hun is about (love that 1940’s language!), because the time taken to accelerate up to maximum speed will allow him to quickly close into firing range.î
Later writers added ìA Mk V Spitfire could fly at 225 knots at 10,000ft and burn only 29 gallons an hour of fuel, but that was a suicidal speed and altitude over enemy territory, even if a reasonable strategy to adopt over the last part of the Channel on a homeward flight, or while looking for the home airfield.î
So, there we have it. The critical settings were best adapted according to prevailing conditions, and the economy settings were really only advisable when safe. The Merlin engine could, of course, switch from low engine speed and high boost to prepare for combat, or to escape, but the few seconds delay involved in building power and speed were critical, so pilots needed to be ready for the ìHun coming out of the sun,î but often failed to use more economical settings when the threat was low. It reminds me of times past, when I drove quite aggressively, and learnt from my father to sit in a low gear behind any car you were waiting to overtake, ready to put your foot down and get instant acceleration. Effective, but not good for economy, which rarely topped 30 miles per gallon in my VW Beetle.