two-speed props on the Empire Boats

[attaway]

Hello all:

For years I've been frustrated at having to use the Dh-88 Comet config entry to model a two-speed prop - since the Comet prop cannot be shifted back into fine pitch after take off, but a real two speed prop such as the DeHavillands on the Empire boats could.

I have found a way to implement a true two-speed prop using fs9 config entries. The first key is to set max_rated_rpm in the engine section so high the engine will never reach it:

max_rated_rpm=4000 //to simulate two spd prop was 2600.0

The second key is to keep your prop lever fully advanced (fine pitch) for takeoff and then slam it right back to the minimum (course pitch) after takeoff or climbout. The real two speed props had a conventional prop lever with only two settings, full fine pitch and full course pitch. With the

The third key is to change the beta_max and beta_min settings to give you the rpms the actual two speed prop gave. For one thing, the DeHavillands could not shift more than 9 degrees. But I've found 9 degrees is more than enough.

For instance, after changing Jen's S.23
propeller_diameter=12.75 //value from Flight article on the Empire boats, was 11.53

I first set the take off rpm by trial and error. I knew max to rpm was 2475 rpm at 3 3/4 lb boost, and that the first power reduction was to 2 1/2 lb boost which gave 2250 rpm. I tried a number of settings between beta_min=15 and 19 and settled on beta_min=17, which gave me about 100 rpm under max at the end of the take off run and a nice 2250 rpm on reducing power.

I could go to beta_max=26 with my DeHavillands, but that gave me an unrealistically low cruise rpm. I have one cruise rpm data point in 'Flying Empires', the pdf ebook: 2200rpm with -1 1/8 lb boost at 10,000 ft. Settling in after climbout at 10,000 ft with altitude hold st (ctrl-z), I experimented and found beta_max=22.5 gave me 2200 rpm with -1.1 boost.

There is my methodology. Not too complicated to follow, I hope. Here is the complete set of my config file edits to Jen's S23 in one place:

[piston_engine]
compression_ratio=6.0 //from the Pegasus iii - was 5.5, a bit low for 1937
max_rated_rpm=4000 //to simulate two spd prop 2600.0

[propeller]
propeller_diameter=12.75 //value from Flight article on the Empire boats, was 11.53
beta_max=22.5 //gives 2200rpm w -1.1 boost at 10000
beta_min=17 //keeps to rpm under max and hits 2250 rpm at 1st power reduction to +2 1/2

That it.

[attaway]

one more thing:

critical_altitude=1300 //was 7000

Its hard to get the critical altitude right for prewar British engines because they gave altitude at rated boost, which is always less than maximum boost ... and then there are issues with the fs9 supercharger model.

The Pegasus XCs on the empire boat gave 2 1/2 lb boost at 2250 rpm and 3500 ft (rated boost)

And we have a single datum point from flying empires of -1 1/8 lb boost at 10000 ft.

1300 ft critical altitude for 3 3/4 lb take off boost hits both those points and is in agreement with the curves published in 'Flight' for other Bristol engines with 'medium supercharging'.

Also 'Flying Empires' tells us that the take offs from Lake Victoria at 3,720' took twice as long as at sea level. So we know the 'critical altitude' for max boost is well below this.

[attaway]

yet one more thing:

[propeller]
min_gov_rpm = 0 // no governor on a two-speed prop

[railrunner130]

Sorry, this is the first time I've noticed this. I really enjoy the Empire.

How do these changes affect the performance?