Blenheim operating limits and checklists

[Robo.]

Originally Posted by 41Sqn_Banks

Can you give a source for this (official manual???) and a better explanation on how the angles between fine and course could be set? It sound like you do not mean the official modification that converted the DH 2pitch props to CSP.

No, this is definitely the DH VP props prior to famous and swift conversion to CSP by DH chaps. The actual mechanism was almost identical on both DH VP and later DH CSP.

...instead of pushing the control in to fully coarse pitch as the RPM rise after takeoff, the control should be moved slowly forward until the RPM drop to the maximum permissable and then pulled slightly back; this will leave the airscrew pitch at the position which gives these rpm until the power begins to drop off with altitude. As power drops off the rpm can be maintained by again fining the airscrew pitch as required

This was still not proper CSP but some sources say that well flown Merlin III with 2-pitch DH prop was not that far from the CSP performance (the whole apparatus being considerably lighter than CSP, too). The early DH was adopted as stop gap solution, improving the fixed pitch prop performance a lot (Watts / Waybridge on Merlin II & III), but still acting as fixed prop pitch effectively unless the pilot exploits the sweet spots between the 2 preset angles.

Still:
DH VPP - to 20.000ft 9.4min
Rotol or DH CSP - to 20.000ft 7.7min
(Mk.I Spit on 100 octanes)

As for the manuals - the original manual presumed that Fine pitch will only be used on take-off run and will be switched to coarse straight away for climb. In reality, pilots apparently used the sweet spot settings in order to get up faster (climbing on more appropriate RPM on the same boost). Newer version of the manual encouraged such a practice (the quote above) before the full conversion to the CSP happened.

This starts to get a bit confusing if you get into details and some points are quite impossible to prove. I am not stating that this was the same like later CSP (Rotol RMS-7 or DH Hydromatic, but with some awkward effort, experienced pilot was able to squeeze more RPM. Just like Blenheim with, perhaps, identical VP mechanism and very similar bracket - give it a go and just before you coarsen the full fine pitch, there is some small space to fiddle with, around 10-25pct if you care to switch engine controls in your info window.

Valec got the Mk.IV up to 17.000ft today. Service ceiling should be 22k, top level speed of 266MPH achieved in 11.8k. All in all, if they fix the mixture issues, the Blenheim could be pretty well modelled as far I can tell.