Spitfire IIa prop pitch

[nakedsquirrel]

The Spitfire IIa prop pitch seems to behave differently than any of the other aircraft in game.

Most aircraft will gain some speed with a slightly coarse pitch, but the Spitfire IIa is fastest with the prop pitch set to fully fine.

Is this unintended, or is there something about the Spitfire IIa prop pitch that makes it different from the other aircraft? (IE is it supposed to be a version of auto-pitch?)

[41Sqn_Banks]

This is because the Spitfire II has a CSP (constant-speed propeller). The lever in the cockpit doesn't set the propeller pitch, it sets the desired engine RPM. The constant-speed unit/governor tries to keep the desired RPM by adjusting the propeller pitch. Of course the constant-speed unit/governor only allows to set RPM values that don't overstress the engine.

So if you set the RPM lever to the forward position the highest possible RPM is maintained. In combination with full throttle this will give you the highest possible engine output power and therefore the highest possible speed*.

*I assume the at highest possible engine RPM the propeller is spinning with an RPM where the propeller has the best efficiency. This is achieved by choosing a suitable propeller reduction gear.

Note that there are other aircraft that have a CSP, e.g. the Hurricane I Rotol and most (if not all) german bombers have a CSP.

[JG53Frankyboy]

flyable palnes with CSP units in the moment are:
-Spitfire II
-Hurricane I_Rotol
-G.50
-Br.20
-Ju87
-He111P&H
-Ju88

Manual pitch, in what you control the propeller anlge directly:
-Bf109
-Bf110

Two speed propeller, actually fixed propellers with two settings (fine and coarse):
-Hurricane I_DeHaviland
-Spitfire I and Ia (weird , but it still is this way ) , most propably the heartbreaker too, never flew it
-Blenheim

fixed propeller:
-Tiger Moth



lets hope in one of the future patches at least the Spitfire Ia gets a CSP and the Ju88 a manual one.

[Crumpp]

Spitfire IIa is fastest with the prop pitch set to fully fine.

Vmax occurs at maximum rpm and manifold pressure in any aircraft. Although it is called "course" and "fine" pitch, as the other posters have explained it is really maximum rpm.

The blade pitch will open as velocity increases and maintain rpm. In a selectable pitch propeller, the pilot must open the blade pitch to maintain maximum rpm as the aircraft speed increases.

This the reason for the VDM clock. It gave the pilot a measure of his blade pitch to set to achieve desired performance.

[Crumpp]

Manual pitch, in what you control the propeller anlge directly:
-Bf109
-Bf110

The VDM propellers are constant speed propellers with an electrical manual pitch that is used for emergencies.

I am not the Bf-109 expert but I believe only a small number of purely manual pitch propellers came into service early on in the 109 series development. IIRC, it was during the Battle of France that a handful were delivered "ohne automatik". It was then replaced with the "mit automatik" which is the VDM hydraulic-electric series used throughout the remainder of the war.

[JG53Frankyboy]

During BoB there were A LOT of Bf109s in active service still with manual pitch.

I cant count anymore how often we had this in the forums
here is just one example
http://forum.1cpublishing.eu/showthread.php?t=22204

So , its all do the decission of 1C what propeller system they will give the BoB time 109s - they cant decide wrong !

[Crumpp]

Ok, from December of 1939 it was introduced.

I read that thread.

You have a copy of the Bf-109E Flugzeug handbuch where the automatik propeller is covered and included from December 1939. That means it was in use on the aircraft from December 1939 forward. Probably even sooner as the normal procedure is a Technical bulletin is issued until the POH is republished and updated. It also does not tell us the extent of operational use.

IIRC, VDM did not produce both types concurrently. The automatic propeller superseded the variable pitch only propeller in VDM production and as the propellers were overhauled, they were upgraded.

It also seems the claim is being made the propeller was not introduced until September 1940. That claim introduces a translated OKL document that raises the rpm limit for the engine and instructs that only Mtt and VDM can modify the governor for the new rpm limits. Both companies are working as fast as possible to make the necessary modifications to comply with the new rpm limits.

That document does not say a thing about the introduction timeline of the VDM propeller "mit automatik" and does not shed any light on the extent of propellers use operationally.

[nakedsquirrel]

I'm still learning about prop pitch, so bear with me, but I'm still a bit confused because the Hurricane Rotol behaves much differently than the Spit IIa.

While flying the Rotol, I find I get a better cruise / dive speed when I pull the prop pitch lever back to 40-50%.

When I do this, the Boost needle goes up while the RPM gauge drops down. (so, high manifold pressure, low rpms)

This is similar with the Hurri DH, Spit I and Ia except that those Spits have the 2 settings, full fine/ full coarse

In the Spit II, however, when I pull back on the pitch, my RPM drops and my speed drops. Even in a shallow dive, pulling back on the pitch lever still reduced my speed. That's the opposite effect that the control has on the Hurri Rotol.

No comment on the German aircraft because they all seem to make sense to me.

[Blackdog_kt]

First of all, pitch is not the slider's position but the angle of the blades in relation to the oncoming airflow.


In keeping with the analogy of gears in a car, a fixed pitch prop is like having a car with a single gear, the early Hurri and Spit dual-position props are like having two separate fixed pitch props that you can change in flight or like having a car with two gears (one for low speed and initial acceleration and one for high speed and fuel economy).

The manually adjustable props found in 109s and 110s are like a fully manual car transmission, with the added caveat that it's continuous and not in discrete steps: you don't only have 1st, 2nd, 3rd, etc gears, you also have all the in-between settings and this gives slower but finer control.

The car analogy is preferred simply because it exhibits similar use, even though the actual workings of it all are fundamentally different:

If you want to drive uphill or limit your speed when driving downhill you use a lower gear that gives higher RPM for the same position of the gas pedal, you do the same in the aircraft by selecting a finer pitch with higher RPM.

If you want to pick up speed when driving downhill or attain your top speed you switch to a higher gear, RPM drops initially but as you keep accelerating it builds back up, it's the same with selecting a coarser pitch.

And if you try driving uphill on a high gear/low RPM without having enough speed your car can't do it, similar when trying to climb in an aircraft with coarse pitch it only works if you have enough starting airspeed.


A CSP prop is one where the pilot affects the governor state (either through oil pressure/quantity in the governor or a mechanism of flywheels and inertial balancing), each governor configuration corresponds to a certain RPM value and then it's a case of balancing forces:
if the airflow is acting against the prop, pitch changes one way, if the airflow effect is lessened pitch moves the other way.

In this way, depending on forward speed the pitch (the angle of the blades) is constantly changed to maintain steady RPM without intervention from the pilot. The lever might not move, but the blade angle is changing whenever your airspeed or throttle does.

In other words, it's like having all the in-between gears again, but this time it's shifting automatically whenever you exceed or drop below the RPM specified by the governor. If you set a CSP for 2800 RPM, whenever you hit that RPM the blade pitch will coarsen a bit more and this ensures you keep accelerating optimally at all times. If on the the other hand you are climbing and your airspeed drops and RPM drops too, the blade angle will automatically move to a finer pitch to bring the RPM back up.

Imagine driving a car where you can select at exactly which RPM it will shift gears.
If you wanted a responsive drive with lots of speed changes and "nervous", immediate response to your gas pedal (whether accelerating or decelerating), you'd tell it to shift at an RPM range that you know your car behaves with more "nerve". This is like being in a dogfight (sudden speed changes and need for responsiveness) and that's why we select a higher RPM for the prop governor to maintain.
If you just wanted to go on an easy ride without burning a lot of fuel, you'd tell it to shift gears at a lower value. This is similar to cruising in the aircraft and that's why we pull back the lever a bit and have the governor settle the RPMs a bit lower.

Then on top of that, imagine that gears are not discrete steps but a continuous adjustment in the transmission, so that you only have to worry about pressing or releasing the gas pedal to maintain the speed you want and you are automatically shifting gears to ensure optimum usage of your fuel and motor depending on what you want to do. Pretty neat, huh? It's no wonder pilots considered it a big deal



The auto prop on the 109s is not like that however. The final effects are similar to a CSP but the operating principle is not. Instead of using a governor mechanism, they actually "tied" each throttle position to a certain pitch (or was it RPM?) value and further fine-tuned it by adding instrumentation that measured the amount of load on the engine.

In other words, the 109's auto-prop is much like having an early analog computer operating the prop in the same manner that the pilot manually would: by directly influencing the blade angles through motors and not by indirectly commanding a certain RPM via a governor mechanism.

Here's a pretty good link explaining how it works (i got it from the other thread linked a few posts above): http://marseillegruppe.com/foro/view...f328cb9d3b84f4

[Crumpp]

The auto prop on the 109s is not like that however.

NO...You have confused automatic devices.

The VDM hydraulic electric propellers are CSP in hydraulic mode and variable pitch in electric mode.

Schwarz-man, whom I know and he is correct in his explanation of how the SYSTEM works.

He is describing the interaction of the CSP PROPELLER with MANIFOLD PRESSURE of the ENGINE!!

This eliminated the pilot from having to adjust PROPELLER RPM as with ANY CSP and then have to adjust ENGINE MANIFOLD PRESSURE. It reduced his workload when compared to a using a CSP on regular aircraft engine with manual controls.