Merlin negative G cutout too quick?

[Osprey]

I'm not sure of what was fitted to the aeroplane in the video but what I immediately noticed is that on an inverted flypast the engine spluttered, but not terribly, and maintained power. In game, even with minor moments of low/negative G the prop will grind to halt. So the question is really - what is fitted to this aeroplane?

I also notice the how easy it is to drive the Spit on the ground, never had that in game from any aeroplane. At Duxford the P51's waltz about at taxi but in 1946 they were far harder to move.

[Crumpp]

an inverted flypast

It was a positive G maneuver most likely and certainly did not have a float carboretor. AFAIK, Spitfires were not outfitted for inverted flight.

Float type carburetor's are not capable of inverted flight as fitted to the Merlin.

When a carburetor is inverted, it can no longer meter fuel, and the float rises and cuts off the incoming supply.

http://www.airspacemag.com/how-things-work/upside.html

[Crumpp]

In game, even with minor moments of low/negative G the prop will grind to halt. So the question is really - what is fitted to this aeroplane?

The relative wind will drive the propeller and it will continue to spin. A CSP will act as huge airbrake immediately slowing the airplane down at a rapid pace until there is not enough wind to drive the propeller.

A fixed pitch propeller will slow the plane down at a much slower pace.

[fruitbat]

Something not completely related, but interesting non the less. This is a quote from sir Stanley Hooker, no doubt someone from here will be along to say he's wrong, and doesn't know what he's talking about, but hey

Before the war, when the Merlin was designed, I don't think anyone even considered the possibility that fighter pilots would ever want to bunt their aircraft across the sky. The first thing we heard about the problem was from a whole lot of complaining fighter pilots at the time of the Battle of Britain.

Now the fact is that if we had fitted fuel injection to the early Merlins we should of lost power. The evaporation of the fuel squirted into the supercharger reduced the temperature of the fuel-air mixture by about 25 degrees C, giving an increase in the order of 60 HP - which meant 6-7 mph more on the top speed of a spitfire. It may not sound much, but for many a pilot it made the difference between life and death.

Quite apart from that, the fitting of fuel injection to the Merlin simply was not on as a short term measure, it would of taken a year or so to get such a redesigned engine into service because the balance of the engine, carburettor and supercharger was critical and one could not alter one part alone without reducing the efficiency of the whole

Quoted from 'The Spitfire Story', by Alfred Price.

[drewpee]

I think that in many of the maneuvers the throttle would have been cut back to reduce prop torque and aid control.

[TomcatViP]

Yes. We shld not forget here that float carb was the norm at the time and pilots were used to fly that way.

[Crumpp]

Interesting, cold air induction does have its merits, but never would have thought it mattered what point you introduced the cold. If it made a 25C difference at the supercharger intake, why couldn't it be the same 25C at the cylinder head?

Sir Stanley Hooker's quote in context is referring to a single point injection. It has nothing to do with direct injection.

[Viper2000]

Originally Posted by cheesehawk

Interesting, cold air induction does have its merits, but never would have thought it mattered what point you introduced the cold. If it made a 25C difference at the supercharger intake, why couldn't it be the same 25C at the cylinder head?

The supercharger compresses the charge adiabatically.

In the isentropic case, (T2/T1) = (P2/P1)^((gamma-1)/gamma)

Isentropic Supercharger work = W*Cp*(T2-T1)

Actual Supercharger work = (Isentropic work)/(Isentropic Efficiency)

Reducing the temperature upstream of the supercharger therefore reduces the supercharger work at fixed supercharger efficiency, and therefore increases the overall efficiency of the machine.

Furthermore, because the supercharger temperature ratio is > 1, it follows that the the temperature reduction in the induction manifold will be correspondingly greater than that due to the evaporation of the fuel alone (though in this case the 25 K figure was measured in the induction manifold, the point is that you wouldn't actually get a 25 K temperature drop from direct fuel injection at TDC).

[TomcatViP]

Well... they call it cold air intake with most of them being not so cold

It works also with a turbocharger

[Osprey]

Originally Posted by Crumpp

It was a positive G maneuver most likely and certainly did not have a float carboretor. AFAIK, Spitfires were not outfitted for inverted flight.

Float type carburetor's are not capable of inverted flight as fitted to the Merlin.



http://www.airspacemag.com/how-things-work/upside.html

Did you watch the video? I was referring to the video.