Spitfire IIa prop pitch

[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