Blenheim operating limits and checklists

[Blackdog_kt]

After scouring left and right for a Blenheim mk.IV manual or pilot's operating handbook and finding nothing freely available, today i stumbled upon the POH for the Mk.V variant.

I just finished reading through it and while there are certain differences in the placement of controls and gauges, it uses the same engines as the Mk.IV we have in the sim. There are also a lot of stuff that are half-way modeled in the sim (they are modeled but work automatically without input from the player) and it sheds some light on those too.

For example, it says that there is an engine operated hydraulic pump but it can't provide sufficient hydraulic pressure for all systems, so there's a hydraulics selector valve for it. It can be set to provide hydraulic pressure to either gear and flaps or the turret, but not both at the same time. It also has an off switch to prevent it from overheating during cruise.

So, a pilot would take off with hydraulics set to gear/flaps, cruise with them switched off and then switch it to provide power to the turret when nearing dangerous airspace.
This is probably why the turret is inoperable in the sim when the aircraft is on the ground.

There is also mention of a reconnaissance camera and this is also visible in our Mk.IV: go to the gunner's station, press the key that opens the canopy and switch to external view, you will be able to see an opened hatch on the top of the fuselage slightly ahead of the turret and through this opening the camera is visible. I hope it becomes operable at some point.

So, on to the important stuff, engine operating limits. The Mk.V comes equipped with either mercury XV or 25 engines. The manual gives the exact same limits for both, the only thing that's different is the oil temps.

Our in-game MkIV uses mercury XV engines, so the various settings should be the same.

However, the most important difference is that the Mk.V comes equipped with constant speed propellers (20 degrees of motion, plus the ability to lock them in full coarse pitch for cruise by pulling the levers fully back, just like our Rotol Hurricane in the sim).
Our in-game Mk.IV has the simpler two-stage props, so we won't be able to match the limits exactly because we lack precise control over RPM.

The boost cut-out is said to give a maximum of +9 boost when it's on, +5 when it's disabled.

That being said, here's the deal:

Maximum takeoff power to 1000ft, 3 minutes limit:
with 100 octane fuel:
+9 boost
2750 RPM

with 87 octane fuel:
+5 boost
2650 RPM

Max. climb power, 30 minute limit:
regardless of fuel octane rating
+5 boost
2650 RPM
cylinder head temps (CHT) at 210C
oil temps at 80C

Max. continuous power (no time limit) for rich mixture:
regardless of fuel octane rating
+3.5 boost
2400 RPM
CHT at 190C
oil temps at 70C

Max. continuous power (no time limit) for weak (lean) mixture:
regardless of fuel octane rating
+1.5 boost
2400 RPM
CHT at 190C
oil temps at 70C

Max. all out limit (war emergency power):
with 100 octane fuel:
+9 boost
2750 RPM
CHT at 235C
oil temps at 90C
30 minutes time limit

with 87 octane fuel:
+5 boost
2750 RPM
5 minutes time limit

Diving Restrictions:
maximum boost +5
maximum RPM 3120
exceeding 2750 RPM permitted only for 20 seconds with the throttle not less than 1/3rd of the way open

Indicated Airspeed (IAS) restrictions:
Diving 325 mph
Gear down 140 mph
Flaps down 125 mph

Clearance for bomb drop with 500lb bombs (no idea about the 250lb ones):
Dive 55 degrees
Climb 40 degrees
Bank 10 degrees

Exceeding these during a bomb drop probably means that your bombs will bounce around inside your own aircraft.

Carb heat use:
(i) Set to on:
a) For all flying at less than +3.5 boost, unless the ambient air temperature is above +15C. When it is higher than +15C, turn it off regardless of boost settings.
b) For all flying (irrespective of boost and atmospheric temperature) in conditions of high humidity, in or just below clouds, in rain, snow or sleet. It can also be used to help warming up the engines in very cold weather.

(ii) Set to off for all other conditions including:
(a) Engine start at all times (seems contrary to what we've been doing in-game so far)
(b) Take off
(c) Landing, except in the high humidity conditions described above in point (i)b)


Lots of other interesting stuff in the manual as well. For example, starting and shutting down the engines is done with the props at coarse pitch instead of fine, it mentions it's longitudinally unstable and that trim should be set slightly nose down for take-off (probably to help raise the tail faster).

I will link the complete manual to be hosted at airwarfare.com as long as their site admin (Gamekeeper?) can assure me there's no copyright troubles involved on their end. Until then, have fun with the abbreviated checklists above, cheers

[bando]

Great stuff thx

[Anvilfolk]

Great info

[JG53Frankyboy]

Thx, VERY interesting summary!!!
I will copy these limits in my personal Pilots Handbook i am writing for every plane

[Blackdog_kt]

A helpful community member sent me a copy of a Blenheim Mk.IV pilot's operating handbook. I'm probably being a bit paranoid, but since it is copyrighted material i'd rather not name him nor link it directly so that we don't draw any flak on the guy and the admins/forums. What i can do is copy the relevant parts here.

Controls and what's different between the real and the in-game Mk.IV:

Propellers:
The real Mk.IV has two-speed propellers. In the sim it has the correct propellers but the slider/control behaviour is wrong. We can move the prop pitch controls through their entire range of travel but it has no effect for the most part: setting it fully aft sets full coarse pitch, setting it to anything higher than that sets full fine pitch.
You can easily see this for yourself when sitting on the ground with the engines off, just pull the controls forward/aft a couple of times and watch how the propeller blades change their pitch.

In other words, there's absolutely no reason to set it about 50% of the way, it has no effect and it's still running full fine unless you pull the controls all the way back.

What the manual recommends:
Fine pitch for takeoff, climbing on one engine and approach for landing. Coarse pitch is to be used for all other phases of flight, unless the engines are throttled way back in which case fine pitch might be needed to prevent rough running/way too low RPM.




Mixture:
Mixture in the real one is semi-automatic. The mixture control positions on the throttle quadrant are labeled normal (for auto rich) and weak (for auto lean). In the sim we can again move the controls through the entire range of motion. I don't know if this means the mixture is semi-auto and it's just the top and bottom positions of the control that matter. It could be that the mixture is fully manually adjustable which would probably be wrong. There were two different types of carburetors fitted, but the manual doesn't state any of them being a manually operated type.

In any case, how mixture works is difficult to test in the sim because we don't have the kind of immediate feedback we can get with the propellers.

There is also a very strong possibility that it is modeled correctly with a strong attention to detail. The manual states "each mixture control lever has two effective positions only", which could very well mean that it could move through the entire range with half of the lever's travel range corresponding to auto lean and the other half to auto rich.

The mixture controls in the sim are reversed as in every other RAF aircraft, not only in regards to animation but keymapping/controller usage as well: you need to press your "decrease mixture" key or pull your HOTAS sliders back to move it towards auto-rich.

What the manual recommends:
Use auto rich (labeled "normal" on the in-game throttle quadrant, levers aft) when running more than +1.5lbs of boost. Use auto lean (labeled "weak", levers forward) when running less than +1.5lbs of boost.
The mixture levers snap back to the normal/auto-rich position when closing the throttles to prevent rough running, just like in the Hurricane, but i don't remember if this is reflected in the sim.



Flaps, gear and hydraulics:
The hydraulics (flaps, gear and turret) are driven by a pump which is in turn driven by the port engine. If you lose or have to switch off the port engine you must set the flaps/gear controls to the desired position and then use (as in ,repeatedly click on) the emergency hand pump in the cockpit, situated to the right of the pilot's seat.

The engine driven hydraulic pump is not powerful enough to drive everything at once, it's either the turret or the flaps and gear. This explains why the turret refuses to move when the aircraft is on the ground, the sim models the correct procedure without input from the player. In reality they could even switch off the pump completely to prevent it from overheating.

Flaps in the real Mk.IV were adjustable. In the sim we only have fully retracted or fully extended flaps (much like the Spitfire), but it should probably work more like the Hurricane, with up, neutral and down positions so that you can set it to neutral mid-way through the extension sequence and get partial flap extension.

What the manual recommends:
Use 15 degrees of flaps when the outboard tanks are full for long range missions (14500lbs total weight), they are not essential for lighter load-outs (up to 12600lbs total weight).

This is why when turning on the autopilot/handing control to the AI you see them flipping like mad between flaps up and down, they are trying to get partial flaps. Until we get a revised flap control logic for the Blenheim, we can't use partial flaps in the sim.

Maximum speed with the gear down is 130mph IAS.


Cowl flaps:
During normal flight they allow enough air to pass through to cool the engines even when they are set to the fully closed position. They must be open for ground running to prevent overheating when there's no airflow to cool the engines. For climb, high speed (probably meaning high power settings) and flying in warm weather they should be partially open.
Keep closed as much as possible to minimize drag, unless the cylinder temperatures (the two instruments on the right cockpit wall above the fuel selector switches/wheels) are climbing above specified limits (to be given further on down, keep reading )




Boost over-ride:
Similar to the boost cut-out in Spits and Hurris, the lever on the instrument panel to the right of the RPM gauges.
Used for takeoff when the aircraft is configured for long range flight and thus heavier (outboard tanks filled, 14500lbs total weight). When activated, it raises maximum available boost from +5 to +9lbs.
As you may have guessed this needs 100 octane fuel.

Important: Interestingly enough, since +9lbs is only to be used for takeoff when the outboard tanks are full, only these outboard tanks carry 100 octane fuel.

This could even explain why some people have more trouble taking off than others. The default loadout has all tanks filled up. If the difference in octane ratings is modeled in the sim it's very important to execute the takeoff with the outboard tanks selected as this enables us to use the boost over-ride for an extra 4lbs of boost, then switch to the inboard tanks once the boost over-ride has been disabled and we are running normal boost values again.


Various controls:
There's a fuel dump switch for the outboard wing tanks to the left and behind the pilot, right there with the carb heat and pitch controls that are obscured by the pilot's seat. You can only jettison fuel from the outer tanks (which are only filled up for long range missions), so the suggested procedure is to use up the inboard tanks first.
I don't know if this is functional in the sim.

The third fuel selector is used for cross-feeding. If you need to shut down an engine you turn on this fuel selector and the remaining engine is fed from the tanks on both wings, so that you don't get an imbalanced airframe with more weight on the "dead engine" wing. In general, it should be left to the off position in most other circumstances.

Trim tabs should not be used to assist in maneuvering, especially dive recovery, as they can cause overstress of the airframe.

The brake lever could be locked in the "down" position, effectively functioning like a parking brake, pressing the lever again unlocked it. I don't know if this is reflected in the sim, it would be very useful if it was because we wouldn't have to keep the brakes pressed while warming up our engines.



I'll cut off here and supply the engine operating limits in the next post for ease of reading (this one's a wall of text as it is).

[Blackdog_kt]

I always end up postponing this, but i see a lot of people have many questions.
I finally decided to bite the bullet and do it, these are the most important bits as a quick and dirty reference

So, without further ado, here are the Blenheim MK.IV operating limits and abbreviated checklists, according to the proper Mk.IV pilot's operating handbook:

Take-off:

• Mixture auto-rich (levers back)
• Prop Pitch fine
• Cowl flaps 1/3 open when on long range load (long range load = all fuel tanks loaded), closed when on normal load (normal load = inboard tanks only)
• Flaps at 15 degrees when on long range load (can't be done in the sim yet, it has two-position flaps but they should be more like the Hurricane's with up/down/neutral settings to enable the pilot to set them partially open)
• Boost cut out on for long range load, off for normal load. With the cut-out enabled you can get +9 psi boost BUT it needs 100 octane fuel. In the real one, the outboard tanks were 100 octane and the inboard ones 87 octane. I don't know if this is reflected in the sim, so i don't use the boost cut-out just in case it's modeled with 87 octane across the board for all fuel tanks. Normal max boost with the cut-out disabled is +5 psi.
• Let the tail rise and the aircraft to lift off. Then, gear up and keep it mostly level to accelerate. Once you hit 120mph switch to coarse pitch, then climb only mildly to let it accelerate to 150 mph which is the best climb speed.
• Disengage boost cut-out before throttling back if you had it engaged.

Climb:

• Keep within the engine limits by adjusting cowl flaps position. Oil temps up to 80 degrees, cylinder head temperatures (CHT) up to 200 degrees.
  In the sim you can push the CHT up to 250 and the engines feel mushy below 180, so i keep it between 180 and 230.
• Best climb speed at full throttle is 150 mph up to 10000ft. For every 5000ft extra, reduce climb speed by 10 mph:
  • 150mph IAS up to 10000ft
  • 140mph IAS up to 15000ft
  • 130mph IAS up to 20000ft

Cruise:

• It's possible to fly at very low throttle settings, resulting in less than 1600 RPM.
• Absolute 5-minute limit for level flight is 2750 RPM at +5 psi boost. It shouldn't be exceeded unless in an emergency.
• Engine power limits for cruise
  • for auto-rich/normal mixture (mixture levers fully back): 2400RPM at +3.5 psi boost
  • for auto-lean/weak mixture (mixture lever fully forward): 2400RPM at +1.5 psi boost
• Lowest fuel consumption cruise: mixture auto-lean/weak and throttles back as far as it's possible to maintain level flight.
• Maximum range (most miles per gallon) cruise: Slightly higher speed than the one for lowest fuel consumption.
• Maximum range at 15000ft is obtained at 110mph IAS. In lower altitudes it's impractical to fly that slow (due to disturbed air), so 130 mph is used for extended range when flying low.
• Cowl flaps should be closed for economy cruise. I don't know if this is reflected in the sim, but running on rich and between +0 to +1 psi boost i can certainly close them more than halfway (i keep them about 1/3rd of the way open).

Diving:

• Maximum permitted speed: 285 mph IAS
• Engine RPM: up to 2750 if throttle is less than a third of the way open, at more than 1/3rd throttle up to 3100 RPM is permitted for a very short time (manual says momentarily).
• Prop pitch should be coarse (prop pitch controls fully back, everything above that in the sim corresponds to the fine pitch position).

Landing:

• Throttle back to bleed off speed, close cowl flaps if necessary to maintain CHT.
  
  This is very important, because if you just idle it and keep it there on a steep, long descent for an immediate flare and touchdown afterwords, your engines will cool way down and not develop any power when you need it for the flare and touchdown and you'll pancake onto the runway.
  
  It's also very important to remember to open the cowl flaps again once you apply throttle, in order not to damage the engines. Try to make it a habbit of adjusting them together: Throttle up- open cowl flaps some, throttle back- close cowl flaps a bit
• Once below 150 mph IAS, lower gear and set fine prop pitch.
• Enter the airfield circuit at 120 mph IAS.
• Drop full flaps when nearing the end of the circuit. The manual doesn't state more but full flaps sure is draggy in the sim, so i would say fly a normal rectangular pattern with gear down and drop flaps when turning from base leg to finals.
  
  Turning with both gear and flaps extended presents a ton of drag and shaves off quite a bit of altitude, leaving you to execute a fairly flat approach instead of a normal glide. And to keep it flat (ie, level) at such high drag you need a lot of power while you're at a low airspeed-->overheat danger.
  
  So, it's better to wait until you are aligned with the runway before lowering flaps.

Missed Approach and Go-around:

• Raise the gear immediately after leaving the ground to minimize drag.
• Don't raise the flaps until you reach a safe altitude of 500 feet. First, nose down a bit to accelerate to 100mph IAS, then raise the flaps.
• If one engine fails during the go-around, there is not sufficient power to complete the take-off at this high drag configuration. In such a case, closing throttles and landing at whatever cost is the only option.

After landing:

• Open cowl flaps while taxiing if the temperatures are high.
• If the aircraft is going to storage, change to coarse pitch before shutting down. If the props don't change over to coarse while taxiing, step on the brakes and throttle up a bit until the pitch changes (throttling up builds the necessary oil pressure for the prop governors).
• Close throttles for about a minute to allow the oil to settle in the sump.
• Stop engines by pulling the carburetor cut-outs. In the sim they are called slow running cut-outs and they are obscured, behind and to the left of the pilot's seat.
  Just map a key to it, select engine one and hold, not press, hold it down the slow running cut-out key until it stops. Select engine two and repeat to turn it off as well.
• Turn off ignition by switching off the row of magneto switches on the lower part of the instrument panel.
• Switch off the fuel supply by setting the wheel-shaped fuel selectors on your right-hand side cockpit wall to off.

Engine Limits:

• Power settings:
  
  • Take-off on 100 octane fuel (long range load - up to 14500lbs - select outer fuel tanks), up to 800-1000ft or for two minutes using max RPM at max boost (boost cut-out on): +9 psi boost, 2750RPM
  • Take-off on 87 octane fuel (normal load - up to 12500lbs - select inboard fuel tanks, outboard tanks empty), up to 1000ft or for three minutes: +5 psi boost, 2050-2650 RPM
  • Climb: +5 psi boost, 2650 RPM
  • Maximum cruise on rich mixture: +3.5 psi boost, 2400 RPM
  • Economy cruise on lean mixture: +1.5 psi boost, 2400 RPM
  • Maximum level flight power (5 minute limit): +5 psi boost, 2750 RPM
  • Maximum Dive (20 seconds limit): +5psi boost, 3100 RPM

• Oil Pressure:
  • Normal: 80 psi
  • Emergency minimum (5 minute limit): 65 psi
• Temperatures:
  
  • Oil Inlet Temperature:
    • Minimum for opening up throttle: 5 degrees
    • Maximum for continuous cruising: 70 degrees
    • Maximum for climbing: 80 degrees
    • Emergency maximum (5 minute limit): 85 degrees
  • Cylinder Head Temperature:
    • Maximum climb: 200 degrees
    • Maximum cruise: 180 degrees
    • Maximum level (5 minute limit): 240 degrees

[JG53Frankyboy]

the coarse pitch setting i dont use, at least not at height till ~6000ft.

the rpm i can achieve with coarse pitch are so low , even with 100% throttle, the temperatures become too low.
I only use fine pitch in CoD's Blenheim

[bando]

Originally Posted by Blackdog_kt

.
The brake lever could be locked in the "down" position, effectively functioning like a parking brake, pressing the lever again unlocked it. I don't know if this is reflected in the sim, it would be very useful if it was because we wouldn't have to keep the brakes pressed while warming up our engines.

It is modelled in the sim and indeed very usefull.

Thank you very much for another great piece of writing about the planes in this sim and the Blennheim in particular. Looking forward to the engine limitations. I got a rough idea as to what they must be, be nice to see it confirmed.

Thanks again Blackdog.



Bando

[JG53Frankyboy]

Originally Posted by bando

It is modelled in the sim and indeed very usefull.

.................

how do you lock the brake ?

[ProHib]

The only way I've found to keep the engines running while airborne is fly as follows:

-Full Boost +5 (full throttle) to heep Oil Temp at least 40 C. Anything less than full boost and oil temps fall to below 40 and things die.

-Full coarse pitch: To keep rad temp below 230 C I need to keep RPM's low to 1600 etc, otherwise rad overheats.

-Cowl flaps full open: to keep rad temp below 230 C.

Mixture full lean seems to have no effect, mind you I haven't been higher than 6000 ft.

I've tried running with 60-70% throttle but then boost is too low and oil temp dies off.

What am I doing wrong here? Nothing I've tried other than the above can keep the big bird in the sky. Is it somewhat broken? I don't want to turn off CEM or disable engine temp management.