Quote:
Originally Posted by PE_Tigar
I agree with Klem. I guess the effect starts now with anything less than 1G. In my around 300 hours of flying, most of which I've spent flying atmospheric piston engine airplanes with carburetors pretty much similar to WWII design (no neg G capability), I haven't seen this happening, not in heavy turbulence, nor in powered stalls (which is a zero G maneuver on full RPM).
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I can show you plenty of YouTube videos of people doing silly things in 172s etc at zero g where no engine misbehaviour is evident, and I can tell you that back in my younger days I played map catching games* in a Grumman AA5 at zero g on a couple of occasions without the engine missing a beat.
What I haven't done is taken the carburettor apart to investigate its design, so I can't tell you whether the lack of a zero g cut was by accident or design. I don't recall reading anything about carburettor performance in the POH for the non-aerobatic types I've flown; I suppose that it's not considered to be relevant information.
However, I have managed to find a video of some rather foolish people (no HASSLE checks**) obviously pushing into negative g in a 172, which does produce what sounds like a lean cut:
There are actually quite a few YouTube videos out there of people confusing zero g with negative g and getting these sort of engine cuts, which perhaps explains why most rental aeroplanes are so clapped out. Of course, this would also imply that airframe and engine safety factors are being rapidly consumed by people incapable of judging what they're doing to the aeroplane, which isn't a happy thought...
OTOH, genuine zero doesn't produce misbehaviour in this video:
Of course, this sort of comparison is hardly scientific because not all 172s were created equal, and even if they were, they certainly aren't Merlin powered! But perhaps it can inform the discussion by analogy if drawings of the carburettors concerned can be produced for comparison with the
particular carburettors the simulator intends to model.
That, of course, is the other important detail; there were several different carburettors which might be fitted to the Merlin, so it's important that we are specific as to which one we're trying to match, because clearly an early SU carburettor will behave differently from for example an RAE anti-g carburettor.
My understanding is that all of the Merlins which saw service during the height of the Battle were made at the Nightingale road factory, and so they're probably more likely to have a consistent set of ancillary components than later engines which were built at a dazzling array of Rolls-Royce and shadow factories in both the UK and USA. So arguably our task is easier than would be the case for later engines, provided that we can find the required source material.
*Place map on instrument panel glare shield. Pitch up 20-30ยบ, then push to zero g and catch the floating map between your teeth before getting uncomfortably close to VNE.
Obviously,
very slight negative g is required to get the map off the dashboard (say -0.01 or something), but once it has floated up a couple of inches you obviously have to stay almost exactly at zero if you're going to catch it. So I never pushed deep into negative whilst playing the game; doing so would probably be unwise in a non-aerobatic aeroplane, though I'd be more worried about the lubrication system than the carburettor TBH. Of course, I've pushed into negative g in aerobatic aeroplanes, but I can't say I've ever been a fan of negative g; it always used to give me a headache...
**Yes, I know most people would say HASELL/HELL, but the alternative is another one of the "interesting" habits I've picked up over the years; I find it easier to remember HASSLE because the checks are a pain. So, before starting aerobatics:
Height - sufficient for recovery/legality/insurance, whichever is greater
Airframe - capable of safely executing the manoeuvre intended in its current condition (snags, weight & CoG etc).
Straps/Security - straps tight, no loose objects in the cockpit, especially near control runs. Positively identify strap quick release box and parachute quick release box, since mixing them up in case of emergency would be
terminally embarrassing.
Situation - not over built up areas, close to danger areas, restricted airspace etc.
Lookout - clearing turns and all that jazz, making sure to check both above and below. Set lights & transponder as required.
Engine - set power required, ensure throttle friction nut tight, check instruments for abnormalities (Temperatures, Pressures, Manifold Pressure, rpm)
Then for subsequent manoeuvres:
Height
Engine
Location
Lookout