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I'm in the Mark IIb, and the "Start Up" instructions say to set the prop to "Fine". Is "fine" low or high RPM?

So essentially, what RPM setting should I be using for startup?

Full Fine is fully forward on the Mk II control (fully back on the Mk I "Bike pump" )

It means HIGH RPM, low blade angle.

There are reasons that the Rotol starts in Fine and the DeH 20 in coarse but they are beyond your question and Cody won't let me give longwinded answers any more !!


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

For the Rotol CSU;

For start you want to select fine pitch/high RPM. Coarse pitch/low RPM are at the other extreme of the prop range available. This is the British terminology for propeller settings.

In the United States, we call the prop settings Low pitch/High RPM and High Pitch/Low RPM. Of course with us you have to consider we call where the car is sitting the "driveway" and where the car is moving the "parkway"

It's always been a bit confusing but once you get used to it it gets even more confusing ))))))))))

Dudley Henriques

Well, Darryl, I am all ears - I know the Rotol starts in Fine because w/o oil pressure, the prop hub has no pressure to move the blades, but why does the DH prop start in coarse?

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Cheers,
Craig Tatley

Craig,

You just want to get me in trouble with Cody

Ok, the Rotol you are correct on and for the same reason a govenor failure will generally lead to the prop moving to full fine, high RPM. (Dudley, jump in if an old bookworm gets confused ) This can be a very dangerous situation. Starting in Full Fine has the advantage that the blades are moving less air, so the engine is not "loaded" on start...not that the throttle settings should be high enough to load the airscrew much anyhow.

Now the DeH 20 is a different animal. It is a "bracket" type prop. A counter weight moves along a track to move the blades. This system uses centrifugal force to throw those weights forward to rotate the blades to coarse pitch. The "Bike Pump" has a Bowden cable that connects to the system and when it is "in" the cable allows the weights to move forward. when it is pulled "out/back" it drags the weights back, under tension and revolves the blades to fine pitch.

So you can see that if it is in Fine Pitch/ low blade angle there is actually no way to move the blades to coarse / high blade angle, unless the engine is running at a reasonable RPM. (on landing, this is the reason that a "burst of power may be required to change to coarse pitch"). The cable can not "push" weights forward.

I suspect that the reason it was shut down / parked, in "Coarse", was that in Fine the cable was under tension, so it was better to leave the system with a bit of slack. Also if in fine when you start and you happen to have moved the Bike Pump control "IN", the moment the engine "takes" I would imagine you would get the most horrible bang as the centrifugal force and the torsion on the blades due to application of air pressure to the blade by the rotation, throws the blade weights from the back of the "track" straight forward full travel to the front. Nasty!

A failure on the part of the DeH Govenor (ie, the pilot) which breaks the cable will result in a move to coarse pitch...IF you are lucky. If not it will result in terrible vibration, stress fracture of the rods (not necessarily all at once!!) and general mayhem if weights etc come loose.

As I've said before...vital parts of the aircraft may adopt independant descent profiles.


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

I will never understand British engineering Dudley....

Darryl, I'm keeping an eye on you.

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Thanks for that very informative description Darryl

Cody, why should Darryl give short explainations? You learn something everyday, so you may as well learn it well.

BTW What Altitude/RPM/Boost combination is required to get maximum speed out of the DH Prop? Currently, at 18000 feet, +4 PSI, 2800-3000 RPM I get roughly 240-250 MPH. In the Weybrige 2 bladed prop at the same Altitude/RPM/Boost combination, I get 260-270 MPH, obviously depending on the condition of my engine . Did the DH prop actually give any performance increase, or did it only reduce take-off distance?

Sorry about all the questions, but seeing as you are a Spitfire expert, you would be the one to ask these things.

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Cheers,
Craig Tatley

Craig,

Just to clarify....Cody is REALLY ANNOYING ...but this is an "in joke" between us, not a real disagreement or anything close.......


The proper cruise for the DeH is to pull her all the way back to coarse, as it was designed as a "two pitch" airscrew. In reality it was used as a variable. You DO need to watch the "minimum RPM at maximum boost" figure if you are going flat out and pull into a climb, for instance.

Somewhere between 2250 and 2600 is the combat cruise range at 4 1/2 boost. With a little lower boost (say max weak mix 2 1/4lb) you can dial her all the way back and may get a lower RPM count...BUT don't go below about 1,800 for any length of time.

1500 is "slow flying" but it is difficult to achieve.

Climb setting...best at +6 1/4 boost and 2600. You need to adjust the control every once in a while as boost falls to maintain the 2600 though. Remember this is not a CSU but a variable...more of a "flexible fixed pitch" . (and all these are ex Boost override cutout as that is emergency and not "useful" in normal flight or for sustained periods.)

Also remember that your ground speed in still air is around your height, divided by 1000 and times 5 plus your calibrated airspeed. Calibrated airspeed is adjusted for position error...where on the aircraft the Pressure Head is (Pitot) and airflow factors etc that interfere with a real reading. A list of these adjustments is in the manual. So at 250mph indicated at 18k you are actually getting pretty close to top speed. This is actually about where the DeH shines!! Don't run at 2800 or 3000 for too long...it is outside allowable limits! I can't recall the boost achievable but full normal throttle should buy a couple of extra miles per hour. I'll have to test fly it.

Climb performance, performance at height and takeoff run were the major beneficiaries of CSU VP prop. Have another look at Scott's propellor video now you have some flying time.

The weight of the new props plus the other gear added at that time meant that top speed actually dropped....this is modelled by Accu-sim.

AND remember this is not "modelled" by tables, lookups or anything..this is modelled by Scott building the engine then making it perform to spec. It is a measure of his skills that when you take this to a given altitude and test it...it calculates and performs to the actual engine WITHOUT tampering!!


Darryl


Oh, Watts / Weybridge airscrew, want to check that figure for me again (260-270mph at 18000ft) off the top of my head sounds a touch high.

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Hi Darryl,

Yea I sort of realized that it was a joke

I will also check that figure again, and report back tomorrow, as I have to log of now. So the DeH prop is slightly slower flat out. Hmmmmm. Very interesting. So 250-260 is what I should see? With the DeH prop?

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Cheers,
Craig Tatley

Round about...exact figures are hard to give because Accu-sim will give you a Wednesday morning plane and me a Friday afternoon one...and THEN they will run differently on different days too.

Let's say within 5 mph or so...

Darryl


PS..note I fixed the speed calc above ....( 20,000 / 20 ) x 5 plus airspeed !!!! Not likely. Height divided by 1,000!!!!

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

OK Thanks mate, i'll get those figures back to you asap tomorrow.

Have a good night,

Craig Tatley

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Cheers,
Craig Tatley

O.K Darryl, here are the figures for the Fixed pitch prop on the Mark 1a:

Spitfire Mk.1a,
Weybridge 20 degree prop,
+4 PSI Boost,
RPM Fluctuated between 2700-2800 RPM,
Floating between 17900 - 18000 feet,

I got 267 MPH, held consistently for 5 minutes. Radiator was full open, temp was about 90. If I closed i may have got 270 MPH. I gave my Spitfire a new engine for this record, so this probably is not representative of a combat-wary Spitfire. I'll try and get you the figures on the DeH Variable.

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Cheers,
Craig Tatley

Thanks mate,

Tomorrow night I will have some time to drag out some figures.


Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Mostly for Darryl,

My wife is from Japan, but learned English from a British professor in Japanese college. I had a heck of a time figuring out what a "bonnet" and a "lift" was, "petrol" I understood, and getting that out of her vocabulary was even rougher.

Glenn

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