Blenheim operating limits and checklists

[Robo.]

Brilliant thread chaps, especially Blackdog (cheers mate). I've been struggling with the Blenheim and got there eventually by good ol' trial & error method.

And I quite enjoyed it to be honest although there was some cursing involved, obviously...

I came a bit too late as I managed to figure out most of it by reading the pilot notes (and realising they don't really work all that much in CLoD) and simply flying the Blenheim, got her up to 15k feet (colleague Valec got the crate even higher up, apparently to 16.5k!)

I finished reading this thread and I see that everything has been covered in detail, but there might be few bits and bobs missing:

1. Mixture is not working properly (or not at all although you won't be able to start the engine with the lever on Full Lean) - this is an FM issue for all planes at this moment, but it has got rather big impact on the Blenheim if you're silly enough to test the max ceiling.

2. The 2 position de Havilland propeller is not quite working as we tought - and I believe this is actually correctly modelled ingame (I am surprised too!). Apparently, these 2 position propellers were sort of stop gap solution to improve the preformance of the fixed pitch propellers (Watts...) and introduction of the modern CSP props. Oneof the early solutions (considered as temporary even back then) was the 2 position de Havilland propeller acting as a fixed pitch in either full coarse or full fine pitch. BUT: the mechanism was similar to a bicycle pump and the pilots soon dioscovered that it was possible and safe to use the settings inbetween these two preset angles, improving climbing performance and optimising the performance alltogether. The RAF authorities soon encouraged this practice and it was soon included in official manuals. This is obviously a storry of DH 2 pitch prop as used on Merlin III engines, but the props being similar and seing the way the Blenheim propeller is modelled ingame, I can't see why not. Mind you this is completely missing on the DH Hurricane although it certainly should be that way, too!

In fact, apart from the 2 extreme settings intended by designer, there is indeed some limited space for getting a better performance by setting the prop in a sweet spot between coarse and fine (cca between 5 and 35 percent, watch the RPM) just like in real life. It was not as easy as with the Rotol CSP and certain experience was required, but the way it is modelled ingame at the moment (Beta 1.03) is good enough to represent it.

3. carb-heat usage is simplified, engines start to run rough at the alt of cca 5500-6000ft, move the lever slightly to keep them run smoothly and keep doing so in small steps untill cca 7500 feets. Switching it full on straight away robs you of RPM in climb. This is certainly not as per real manual but that's how it seems to work ingame.

My apologies if I happen to repeat some information, I just thought someone might find it helpful.

[41Sqn_Banks]

Originally Posted by Robo.

BUT: the mechanism was similar to a bicycle pump and the pilots soon dioscovered that it was possible and safe to use the settings inbetween these two preset angles, improving climbing performance and optimising the performance alltogether. The RAF authorities soon encouraged this practice and it was soon included in official manuals. This is obviously a storry of DH 2 pitch prop as used on Merlin III engines, but the props being similar and seing the way the Blenheim propeller is modelled ingame, I can't see why not. Mind you this is completely missing on the DH Hurricane although it certainly should be that way, too!

In fact, apart from the 2 extreme settings intended by designer, there is indeed some limited space for getting a better performance by setting the prop in a sweet spot between coarse and fine (cca between 5 and 35 percent, watch the RPM) just like in real life. It was not as easy as with the Rotol CSP and certain experience was required, but the way it is modelled ingame at the moment (Beta 1.03) is good enough to represent it.

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.

[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.

[phoenix1963]

Thanks for a very informative reply Robo
As well as mixture at altitude, the heat flow still seems off. The manuals clearly state that gills should be fully open on the ground, but closed on takeoff and flight.

This is not plausible with the current model, the sensitivity needs some adjustment.

56RAF_phoenix

[IvanK]

The Prop pitch selection in the Blenheim IV was I believe straight out Fine or Coarse without any "wiggle room". In the case of the Spitfire MKI the Wiggle room and technique on how to get benefit of it is clearly stated in the Spit Manual. To all intensive purposes it provided similar pitch control to what we see in the 109E though ergonomically not as well set up. No such mention of this wiggle room is mentioned in the Hurricane I manuals though. Here is an excerpt describing its operation from the Spit I pilots notes:



In the case of the Blenheim MKIV (and MKI) there is no mention at all of any wiggle room in the propeller pitch department in the Pilots notes. Neither is their any mention of it in Graham Warners exquisite book that covers engine handling in great detail. Nor is there any mention of it in the RAE Flight test reports available in the National Archives which cover engine handling in reasonable detail. Two of these reports are in relation to achieve Max possible climb performance and max level speeds.

As to handling with particular reference to Take off as we see it in the Sim the Yaw on take off is grossly over modelled. Again the Pilots notes state ... "Turn into wind, straighten up and opent the throttles together,taking only two or three seconds in doing so. ..... There is a slight tendency to swing to the right which is easily overcome by the rudder." Whats more telling is that the recommended Rudder trim setting for take off is neutral. There is no way we should have an aeroplane that we struggle with maintaining directional control with asymmetric power and stabs of brake.

With respect to Engine RPM in SIM versus Real world there is a large mismatch. Again the RAE reports give good details here. I "think" the issue in the Sim is how propeller pitch has been set this results in out of wack RPM values for a Given Boost and TAS. There is a fair bit of discussion in the RAE reports on setting up the Prop pitch stops for coarse with the intent that Full Coarse at Full throttle height should provide max RPM i.e +5Lbs Boost Prop Full Coarse around 2700RPM:



Here is the test data on RPM Versus Altitude V TAS again Coarse pitch



The RAE reports also provide copious data on Engine cooling. Clearly the Sim is way to critical in this area, I think Phoenix is spot on. Cylinder head temperature and Gill position is way to sensitive/critical.

A great insight to the Blenheim IV and its handling and engine operation etc is the Flying Machines "Flight of the Blenheim" DVD.

http://www.flyingmachinestv.co.uk/page2/page2.html

Warning slight but related thread drift follows.
With respect to Propeller pitch issues this imo is a common oddity in the Sim that may go some way to explaining the shortfall in performance in the 2 pitch Spits even at only 6Lbs boost (avoiding the 100 Octane debate . Looking at multiple RAE reports that provide RPM versus IAS/TAS V Altitude for the early Merlins then repeating the tests in the SIM will see the SIM RPM consistently 300-400RPM less than real world.

[Robo.]

Originally Posted by IvanK

The Prop pitch selection in the Blenheim IV was I believe straight out Fine or Coarse without any "wiggle room". In the case of the Spitfire MKI the Wiggle room and technique on how to get benefit of it is clearly stated in the Spit Manual. To all intensive purposes it provided similar pitch control to what we see in the 109E though ergonomically not as well set up. No such mention of this wiggle room is mentioned in the Hurricane I manuals though. Here is an excerpt describing its operation from the Spit I pilots notes:

Brilliant info, IvanK, thank you very much for that! The quote I provided above origins from this very same document. Older version of Spitfire Mk.I Pilot's notes didn't contain such info because it was assumed that pilots would only use fine pitch for take-off and switch to coarse pitch thereafter. This was apparently discovered later on. In the very first phase, when these fighters were new with their VP, some pilots used to fixed props kept forgetting about it:

''It was easy to forget the propeller adjustments that had to be made to the Spitfire, the same as they did to the Hurricane. Brian Considine (...) had only flown fixed-pitch propeller biplanes when he was sent to join 238 Squadron at Tangmere. (...) He 'took off in fine pitch and promptly forgot to put it back into coarse pitch, and did a few circles round the field thinking how marvellous it was... I made a nice landing and as I taxied in I could see the CO jumping up and down like a monkey in a rage. When I got out he told me U had wrecked the thing. I hadn't but it was all covered in oil.''

Then they found out what was later incorporated into official pilot's notes. Now the question is if the airscrew had the same bicycle pump mechanism (and bracket), can it be assumed that a bit of wiggle space has been available on Mk.I Hurricane and Mk.IV Blenheim, too? I agree that if there is no mention at all anywhere, there is not much space for speculations. It was perhaps the particular control lever allowing such a practice in Spitfire only.

Originally Posted by IvanK

In the case of the Blenheim MKIV (and MKI) there is no mention at all of any wiggle room in the propeller pitch department in the Pilots notes. Neither is their any mention of it in Graham Warners exquisite book that covers engine handling in great detail. Nor is there any mention of it in the RAE Flight test reports available in the National Archives which cover engine handling in reasonable detail. Two of these reports are in relation to achieve Max possible climb performance and max level speeds.

I am reading through the Mk.IV pilot's notes - it says ''the aircrew pitch controls should be pulled out to put the airscrew in coarse pitch'' (obviously) and it mentions later that the RPM indicators are not very precise. I wonder how much resistance these airscrew controls had when pulled and if it was at all possible to set them inbetween. Even if it was, it is clear that it has not been used then and it is indeed wrong in the Sim.

Originally Posted by IvanK

As to handling with particular reference to Take off as we see it in the Sim the Yaw on take off is grossly over modelled. Again the Pilots notes state ... "Turn into wind, straighten up and opent the throttles together,taking only two or three seconds in doing so. ..... There is a slight tendency to swing to the right which is easily overcome by the rudder." Whats more telling is that the recommended Rudder trim setting for take off is neutral. There is no way we should have an aeroplane that we struggle with maintaining directional control with asymmetric power and stabs of brake.

Absolutely! You need full left rudder trim for take-off and almost full left for climbing at the moment. It also says it can be airborne in about 400yards (14.5000lb. long range full load in still air) which is also quite impossible even on +9PSI.

Originally Posted by IvanK

With respect to Engine RPM in SIM versus Real world there is a large mismatch. Again the RAE reports give good details here. I "think" the issue in the Sim is how propeller pitch has been set this results in out of wack RPM values for a Given Boost and TAS. There is a fair bit of discussion in the RAE reports on setting up the Prop pitch stops for coarse with the intent that Full Coarse at Full throttle height should provide max RPM i.e. 7200ft +5Lbs Boost Prop Full Coarse around 2700RPM:

That explains a lot then - at the moment you're pretty much climbing with engines idling at odd 1800RPM even at full +5lbs., coarse pitch. Unfortunately the manual only provides very vague figures. On the ground (few seconds engine start-up).

+5lbs - 2300-2400RPM (static)
+9lbs - 2500-2600RPM (static)

And only refers to Boost from then on, saying that pilot should switch to coarse pitch at the speed of 120MPH and then climb at 150MPH at coarse pitch. Coarse pitch + full boost were not recommended unless necessary. None of these is possible in the Sim at the moment. But I am only repeating what you have said already anyway...

Originally Posted by IvanK

With respect to Propeller pitch issues this imo is a common oddity in the Sim that may go some way to explaining the shortfall in performance in the 2 pitch Spits even at only 6Lbs boost (avoiding the 100 Octane debate . Looking at multiple RAE reports that provide RPM versus IAS/TAS V Altitude for the early Merlins then repeating the tests in the SIM will see the SIM RPM consistently 300-400RPM less than real world.

It seems some of these are global FM issues concerning more planes in the Sim.

At the moment (beta 1.03) - the temperatures and failures are far too aggressive, mixture is not working at all (except when starting the engine and is still animated other way around on Hurricane) and the RPMs are, just as you say, far off at given boost / TAS / alt. 300-400RPM is a lot and in case of Mk.IV Blenheim we speak about 500-600RPM difference to the real life data!

Regarding the grills, it says that even with these fully closed, there was enough air streaming to keep the engines cool. CHT limit was 210C.

Thanks for your post, IvanK, that was excellent.

[phoenix1963]

The pitch does now seem to be variable in the one third portion closest to coarse setting.

Right or wrong, it proves quite useful, I can now climb at about 2150 rpm, +2lbs boost, +1000 ft/min. But I still have to have the gills open ~70%.

As Robo says, shaking sets in at almost 5000 ft, indicating the need for carb (intake) heat.

Best news (unless I missed it before the Beta) is that the sight now has sideslip adjustment!

56RAF_phoenix

[Blackdog_kt]

I think sideslip correction was only mentioned for the German Lofte sights? I could be wrong, but i didn't see it with the mechanical sight in the Blenheim.

In any case, the correction that does get applied automatically in the Blenheim is an angle of attack correction. In reality i guess it was manual, what happens in the sim is that it needs some time of straight and level flying while the bombardier automatically adjusts this. In short, it's just a matter of correcting for how up/down your nose is pointing when on level flight due to weight differences between sorties (a heavier aircraft will need more nose up trim than a lighter one to maintain level flight, assuming they are flying at the same speeds).


As for the propellers, i was under the assumption that the method of obtaining semi-adjustable RPM didn't involve setting the prop controls mid-way, but manually and constantly alternating between the two positions, ie a case of increased pilot workload and as such, useful only for specific phases of flight.

There's a video on youtube by A2A simulations (the guys who made the Spitfire add-on for FSX) aptly titled "propellers" which demonstrates the method. I tried it in CoD with a Spit Mk.I and it works, but you have to be wary of your boost: coarsening the pitch as much as the two-stage props do results in a substantial rise of manifold pressure, so set your throttle for an in-between boost value that won't exceed maximums when at coarse pitch.

[IvanK]

"As for the propellers, i was under the assumption that the method of obtaining semi-adjustable RPM didn't involve setting the prop controls mid-way, but manually and constantly alternating between the two positions, ie a case of increased pilot workload and as such, useful only for specific phases of flight."

I think that assumption is incorrect Blackdog Kt The Spit manual implies that its just a matter of using the Prop pitch control as a Vernier device that enables you some additional control of Prop pitch other than just the Coarse and Fine stops. The location of the Prop pitch control in the Spit I makes it a little easier to manage.

As to the Blenheim with the controls mounted behind and under your left elbow and 2 to fiddle with that would be imo virtually impossible. As indicated in my earlier post there are no references at all to the Blenheim IV (and I) props being capable of intermediate settings other than Fine and Coarse.

[phoenix1963]

Originally Posted by Blackdog_kt

I think sideslip correction was only mentioned for the German Lofte sights? I could be wrong, but i didn't see it with the mechanical sight in the Blenheim..

Blackdog - There's an adjustment knob/nut on the right hand side of the Blenheim sight that rotates it left or right to allow for wind/sideslip. I don't know much about sights, so maybe I simply missed it before!

56RAF_phoenix