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#1638931
Ive been reading alot about ww2 aircraft recently and i find it interesting that the spitfire couldnt keep up with its german counterparts in a dive ie me109 and fw190. The german aircraft as far as i know didnt have a typical constant speed unit more of a lever to set the pitch to where it stayed unlike a CSU where an increase or decrease in prop speed would automatically be counteracted by increasing or decreasing propeller pitch. My understanding of the constant speed propeller is that the pilot sets the prop speed by adjusting the spring pressure on the fly weights inside the CSU which control a valve either allowing oil to flow into the propeller hub increasing pitch or return from the propeller hub decreasing pitch, i understand how this works so well in normal flight but in a dive when gravity overtakes the work of the propeller in accelerating the aircraft in my mind this then makes the propeller sort of like a turbine turning the engine, so this then would reverse the effect of the CSU as when the aircraft enters a dive as it begins to accelerate under gravity the back of the propeller will become somewhat unloaded allowing a slight increase in rpm which would then cause the flyweights to slightly open the valve allowing oil into the propeller increasing the blade pitch, under these conditions increasing the blade pitch will just exaggerate this turbine effect so where the CSU is applying a coarse pitch in order to slow the propeller its actually speeding it up. Ive also read that pilots were advised to select coarse pitch before entering a dive to prevent the CSU jamming into fine pitch, going back to what i mentioned before with the propeller speeding up in a dive and with the CSU applying maximum blade pitch when the pilot pulls out of the dive this will induce a sudden perhaps harsh operation of the CSU if left in fine pitch as suddenly the engine will be dealing with high rpm with max pitch, a combination unachievable in normal flight, this to which the jamming of the mechanism could be associated with. So all this would happen at a cost in the name of drag and this would explain why the germans would get away in a dive.
Any thoughts on this?
Last edited by User22169 on Wed Sep 19, 2018 4:13 pm, edited 1 time in total.
#1638949
A sentence 16 lines long and very little punctuation/paras makes this really difficult to read and comprehend.

Just my thoughts .


But.......
The spitfire had carburettors which became starved of fuel during a bunt/dive. It was partly mitigated by 'Mrs Shilling's orifice', but not completely. Aircraft could also be turned on it's back to chase them down. German aircraft fuel injected.

This may be of no relevance at all. :wink:
#1639058
The constant speed unit holds the prop rpm constant by coarsening the blades slightly in a dive. The engine power output thus stays constant, while gravity provides a slight increase in speed.

Conversely a fixed pitch prop in a dive unloads and allows the engine to turn faster, creating more power, so both gravity and the engine provide an increase in speed - which in turns unloads the prop more, causing the engine to turn even faster, increasing speed, and so on until a new equilibrium is (eventually) reached, the engine overspeeds, or the pilot pulls the throttle back to compensate.
#1639117
T67M wrote:Conversely a fixed pitch prop in a dive unloads and allows the engine to turn faster, creating more power,

Is this actually true?

Consider pulling the mixture to ICO at best glide. Presumably we agree that the engine is now developing zero power.

Now dive. The increased airspeed will for sure turn the prop faster. Presumably we still agree that at ICO, the engine is still developing zero power.

So the fact that the engine is turning faster does not, on its own, mean that the engine is developing more power.

So what happens when the engine is running, and the increased airspeed turns the prop faster? Is there more fuel being burned per second? I don't know enough about carburetors to answer this, but the engine can only be developing more power if there is.
#1639987
TopCat wrote:
T67M wrote:Conversely a fixed pitch prop in a dive unloads and allows the engine to turn faster, creating more power,

Is this actually true?

Consider pulling the mixture to ICO at best glide. Presumably we agree that the engine is now developing zero power.

Now dive. The increased airspeed will for sure turn the prop faster. Presumably we still agree that at ICO, the engine is still developing zero power.

So the fact that the engine is turning faster does not, on its own, mean that the engine is developing more power.

So what happens when the engine is running, and the increased airspeed turns the prop faster? Is there more fuel being burned per second? I don't know enough about carburetors to answer this, but the engine can only be developing more power if there is.


I think the first thing to realise is that even in a dive, the engine is still producing all of the power which is turning the prop - the prop isn't "driving" the engine, it's merely that the increase airspeed is "unloading" the prop slightly, allowing the engine to turn faster. With the mixture at ICO and cruise speed, the airflow is probably just enough to overcome the compression, but only at a few tens of RPM, not the 2000+ RPM that the prop normally turns at. At glide speed, the propeller will probably stop turning completely, and an "air-start" of the engine generally requires a very steep dive and an airspeed above the normal cruise speed.

Research shows that the power produced by an internal combustion engine is a direct proportional function of manifold pressure and engine RPM according to the formula HP = k x MP x RPM where "k" is a constant based upon of the engine design (cylinder size, number of pistons etc).

In a dive with a fixed-pitch prop, the manifold pressure increases by a tiny amount due to increased ram effect and increased air density, and the RPM will increase significantly due to the reduced drag (load) from the prop. The increased RPM will also reduce the manifold pressure slightly because the cylinders will be "sucking" in the fuel/air mixture more frequently, and thus there will be an increase in the amount of fuel consumed per second.

It is an interesting aside to realise that in an aircraft with a fixed pitch prop, the throttle control actually changes the manifold pressure (through the effect of the butterfly valve in the carburettor/inlet) but the effect is usually measured by the pilot using the tachometer which measures RPM. In an aircraft with a constant speed propeller, there are separate gauges for MP and RPM, and separate controls for each.
#1639993
"it's merely that the increase airspeed is "unloading" the prop slightly, allowing the engine to turn faster"

Yes and what happens inside the constant speed unit when the engine turns faster, it allows oil to flow into the propeller hub increasing blade pitch. Also note the merlin was known to hit 3600rpm in a dive, this seems abnormally high considering there was barely an increase in rpm on the 109's db600 engine.
#1640010
It is worth being very specific when talking about the Spitfire as the different marks had hugely different characteristics. The earliest Spitfires had fixed pitch propellors, replaced by coarse/fine two-position variable pitch (not constant speed) propellors, then fully variable pitch units until finally they were equipped with constant speed propellors, although I'm not sure if they were conventional or aerobatic CSUs, and both types will eventually hit their coarse pitch stops anyway, effectively becoming a fixed-pitch prop at that point.

I do know someone who has flown most marks of Spitfire (lucky bugger!) so I may be able to find out more...
#1640023
"and both types will eventually hit their coarse pitch stops anyway, effectively becoming a fixed-pitch prop at that point."

Thats what im getting at though if it stayed at the pitch setting it was in on entry to the dive rather than apply max pitch perhaps it would of kept up. Also im aware of the prop dilemma but im sure throughout all the marks it was just taken that the spitfire couldnt perform as well in a dive, im fairly sure by the mkII all the props were CSU equipped.