Mosquito 100 Oct Supercharger shift height

[Bert_Foster]

in Game with 100 Octane Fuel and Supercharger set to AUTO the Shift altitude is 8000'

This is incorrect for Operational WWII Mosquitos with Merlin 25 installed.

 

Wartime Pilots notes quote 12,500' for the SC Shift change. Some confusion may exist as a post war set of pilots notes lists the Shift change altitude as 7000'. These post war notes do not make any mention of Octane rating.

 

War time Test reports also list 12,500' both in text and in the graphs:

http://www.wwiiaircraftperformance.org/mosquito/hj679.pdf

 

The In game Mosquito represents a WWII operational aeroplane whose Auto 100 Oct Supercharger Shift height should be 12,500'

 

 

[Fizzix]

7000' is correct for +18 boost and 3000 RPM, according to pilot notes I have ( amendment list 1 ).  12500' is probably for "climbing" power of +9 and 2850.  The shift alt varies with boost, rpm and airspeed ( ram effect ).  It is not a constant.

[Bert_Foster]

The Supercharger Auto shift Altitude is driven by a simple Aneroid device (interestingly located in the rear fuselage) it looks at Altitude only and current Boost/RPM setting doesn't have anything to do with the Shift change. Below is an excerpt from the Maintenance manual ... though given is dealing with the Merlin 21/23 hence 15,000' being referenced here. In the Sim it also only looks at Altitude (8000') and doesnt care about Boost/Rpm

 

As to the various pilots notes. I have 3 different sets all with slightly different dates. the only one that talks about 7000' is the post war version dated 1950. I have posted images from two above. It would be interesting to know the publication date on the set you are referring to Fizixx. Publication date is in the top left corner of Page 3 opposite the aircraft profile image.

 

 

 

 

 

Edited July 12, 2022 by Bert_Foster

[Fizzix]

The date is Jan , 1950.  AL.1 says,

 

These heights are the correct supercharger gear change heights only when using maximum power ( operational necessity ).  When using low power settings, the selection of high gear should be carried out by switching to AUTO as recommended in para. 41...

 

So, while the boost and rpm may not affect the altitude, the altitude is optimized for a specific power setting.  The 'optimum' shift height does depend on boost, rpm and airspeed, which is why the manual says to leave in MOD, until boost falls to a certain level.  It would seem that the AUTO function is more to make sure that it shifts to low on the descent or in a dive, rather than switching to high.  If the system were optimized for climb power ( +9, 2850 ), then the altitude would be higher.  Perhaps that's how it was originally set up earlier in the war?

[Bert_Foster]

The 1950 post war manual is not relevant. Para 41 that you refer to in the post war manual has no equivalent in the the WWII version of the manual. In the WII manual the closest is para 63 and 64 shown in the graphic below. .... The intent to use AUTO in both Max performance climb and normal climb. The AUTO function is actually optimized for the climb, not the descent shift down.

 

The optimum Shift height is chosen by the performance engineers to optimize performance. This value is then set in the aneriod unit. All the aneriod unit then looks at (In the Mosquito installation) is static pressure. When it drops to the chosen and set value the Shift occurs.

 

Auto 100 Oct Supercharger Shift height should be 12,500'

 

 

[Fizzix]

Except there's a problem with that, probably why it was changed at some point.  When was it changed?

 

Here's the problem.  Let's say supercharger is set to AUTO and you want to perform an emergency climb at +18/3000.  You CAN'T, because you can't shift at the optimum 7000' and boost is falling as you climb.  It won't shift to high gear until boost has fallen to less than +9 at 12500'.  You have significantly less than full power between 7000 and 12000' in that case.  That's 5000' where you can't use emergency/max power settings....

[JtD]

But that would be nothing special, a lot of WW2 engines let potential performance go to waste for simplicities sake and conservative concepts.

It may also be that the high gear change at high boost was only possible after a clutch or some other component was changed in a post war engine modernisation. We don't know that.

It's probably smart to go with documented performance, even if it goes against common sense.

[the_emperor]

23 hours ago, Fizzix said:

You have significantly less than full power between 7000 and 12000' in that case.  That's 5000' where you can't use emergency/max power settings....

Not necessarily, it is a bit more complex than just looking at the boost gauge, as the second stage of the supercharger itself uses up power of the engine and increases air/fuel mixture temperature, so you have to find the spot, where the second charger stage actually increases performance over the drop in boost and not exceed the allowed boost pressure. as in the Merlin engines overboost is just dumped to keep the engine in its boost limits, in other words power from the drive shaft directed to the charger goes to waste (a disadvantage of the fixed charger coupling of the merlin engines opposed to hydraulic coupling of the DB engines).

and also keep in mind that you also want to have the plane be able to reach a certain FTH.

And then there is of course reducing the workload of the pilot by automation (same for mixture, radiators, rpm etc). We maybe dont see the benefit in our quest by trying to squeeze out every extra mph for max performance out of our virtual planes but in real life its benefits probably outweighed that

 

Edited July 14, 2022 by the_emperor