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View Full Version : Spitfire with Merlin engine - Zero g



Abbuzze
01-20-2005, 04:37 AM
I know that the later versions of Merlin equipt Spits had no problem with neg. g anymore due modifications of the carburator.

But what would happend if such a spit would fly a zero g arc?? I can´t imagine that in such circumstances a carburator would still work...
Is here anyone with more/better informations?
Thanks!

Wiking1
01-20-2005, 04:59 AM
I think that the fuel pump delivering a header to the injection carburator canceled that problem. See the text copied below.

BSS_Tintin

Found at http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20Merlin%20V-1650%20Engine.htm#Allison%20V-1710%20Description

Induction
A Bendix Stromberg PD18 injection carburetor is used on the V-1650-9 engine. This double-barrel twin-boost venturi carburetor has an air metering unit consisting of the two throat throttle body with boosted venturies, a fuel metering unit consisting of a regulator which provides a fuel head proportional to air flow and metering jets which provide fuel flow proportional to fuel head (and thus airflow), and an injection nozzle which directs the discharge of fuel into the eye of the supercharger and provides a reference metering pressure for the fuel metering unit. The system includes and accelerator pump to provide fuel during momentary interruptions as power settings are changed, an automatic mixture control which adjusts the airflow metering signal to compensate for temperature and altitude, a manual mixture control which changes jet selections and bypasses the automatic mixture control, an automatic fuel enrichment valve to increase fuel flow under high demand, and an ADI derichment valve which leans the mixture when a pressure signal is applied to the valve.

Fuel pressure is from an engine driven eccentric-vane type pump with an integral pressure regulator. An electric boost pump of the same type provides initial priming and for racing applications runs while the engine is on.

Engines using ADI (anti-detonation injection, usually 50:50 mixture of methanol and water) use some form of an ADI regulator to generate an appropriate flow of ADI for the engines operating condition. While various ways of doing this exist, most boat racers use a system developed by Dixon Smith Systems, Inc which provides an ADI regulator which is controlled by boost pressure. A needle valve inside the ADI regulator opens as pressure increases, the needle valve is shaped to provide the required flow for a given boost setting. Eccentric-vane type pumps provide relatively constant ADI pressure to the regulator regardless of demand, and the derich valve of the carburetor is activated when the ADI system activates. Each type of engine requires an ADI regulator calibrated for it, since the specific ADI consumption at a given level of boost is a function of engine parameters. Generally, ADI systems are calibrated to provide about 1/2 pound of ADI per pound of fuel consumed. ADI is used because it provides very good charge cooling, dramatically reducing detonation at high boost levels. ADI is usually injected into the eye of the supercharger (often its just a pipe extending into the intake elbow). ADI is very corrosive and is very hard on pumps, hose ends, sensors, etc ... frequent flushing and pickling with water soluble oil is required.

Air-fuel mixture passes through the two stages of the supercharger and through the ADI tube (or aftercooler) to the intake plenum which sits deep in the "vee" of the engine between the two cylinder banks. From here, the mixture is distributed to each bank by an intake plenum which bolt directly to the intake ports. Flame traps are contained within the intake manifold immediately adjacent to the intake ports which contain backfires and flames which could devastate the plenum if its high pressure mixture were to ignite. The intake manifolds also provide atomizing priming jets which are connected to a solenoid on the PD18 metering unit to facilitate starting by direct injection of fuel into the intakes.

VW-IceFire
01-20-2005, 06:57 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Abbuzze:
I know that the later versions of Merlin equipt Spits had no problem with neg. g anymore due modifications of the carburator.

But what would happend if such a spit would fly a zero g arc?? I can´t imagine that in such circumstances a carburator would still work...
Is here anyone with more/better informations?
Thanks! <HR></BLOCKQUOTE>
No plane is going to be happy with flying neg-G for a long time. Even the Allisons on a P-38 were not recommended for negative-G or inverted flying because the oil would stop going to the engine properly.

Having a fuel injector just means you can do sharp dives and negative-G manuvers without cutting the engine out. But it doesn't mean you should do it for more than several seconds.

Arm_slinger
01-20-2005, 08:36 AM
What Ice said about the oil. I think the max time a Spit should be subjected to negative G's is 7 seconds

VW-IceFire
01-20-2005, 12:41 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Arm_slinger:
What Ice said about the oil. I think the max time a Spit should be subjected to negative G's is 7 seconds <HR></BLOCKQUOTE>
I suspect thats true for any WWII era prop aircraft. May not be the same time all around but probably even the German engines, which were so good in the neg-g regard...have the same oil issues.

Abbuzze
01-20-2005, 12:58 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by VW-IceFire:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Arm_slinger:
What Ice said about the oil. I think the max time a Spit should be subjected to negative G's is 7 seconds <HR></BLOCKQUOTE>
I suspect thats true for any WWII era prop aircraft. May not be the same time all around but probably even the German engines, which were so good in the neg-g regard...have the same oil issues. <HR></BLOCKQUOTE>

I´m not talking about any negativ g-forces!!!

0-Zero-Nada-g is what I mean! http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

VW-IceFire
01-20-2005, 04:32 PM
You started by talking about negative-G forces.

To achieve Zero G? Why even bother asking? Instances where zero g occured would be for such a short period of time that its not even worth thinking about. Nothing I can think of anyways. Your inevitably going to dip into a negative g area or a positive g area.

Arm_slinger
01-20-2005, 07:36 PM
Zero G is negative G..... At this present moment we are all experiencing 1 G, there is no between, its either positive or negative.

Is that right?

Cragger
01-20-2005, 09:08 PM
No.

The measurement of 1 G is the force the earth's mass exerts on your causing you to accelerate towards its center at 9.18 m/s^2 if I remember correctly. This is neutralized by a normal force (the ground under you which you cannot go thru) accelerating you at 9.18 m/s^2 away from the earth's center.

Now in an aircraft this would relative to the body of the aircraft that would maintain 1G thru resisting the acceleration of the Earth's Gravity by an equivalent lift acceleration vector perpendicular to the earth's surface. If this lift vector was reduced to 0 the aircraft would be falling at a rate of acceleration of 9.18 m/s^2. BUT everything in the aircraft would be falling at the same rate and would in effect be under 0 G because there would not be a normal force resisting their decent (in relation of the aircraft's enviroment).

This is not the perfect example but it does illustrate that science as it was put by a great mind is very relative to what your applying it to.

And to the topic, yes WW2 engines could not be induced to negative G force for long periods because they wouldn't oil properly and would friction lock.