The A2A Simulations Community

When taxiing -- specifically in the Mk.Ia with DeHavilland two-speed prop -- is it better to use the coarse or fine pitch setting?

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120th Fighter Wing, Montana Air National Guard

G'day Mate,

The DeH needs to be shut down in coarse pitch (low rpm, just to make it clear to everyone). This has to do with the actual mechanism and stressing. Then at start up you will be in coarse still (required) as you can not move TO coarse without significant rpm due to the counterweight arrangement.

So before shutdown it needs to be moved to coarse...exactly WHEN I can't recall just now however - I - always taxi in fine picth (high rpm) as that is where you will taxi to takeoff after you move the blades to fine during warm up and test.

Coarse moving around is hard on the engine and naturally, the lower rpm for same power will lead to plug fouling problems etc.

Hope that helps

Darryl

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

Always fine pitch position when engine is running (post takeoff checks prior takeoff, temeratures has to be in "green" arc). Due to low idle RPM it is not possible to get propeller to coarse position.

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FSX SP2 DX9.Addons:FSG2010,REX OD, AS2012, PMDG, DA Do27, Dodosim, A2A accusim all, Katana4X,Coolsky DC9 Classic, Swiss Pro,ORBX all, Fly Tampa, DRZEWIECKI DESIGN, FS Dreamteam, FranceVFR Haute Normandie 3D...

Nic081,

Just remember NOT to bring the pitch up BEFORE min oil temps and radiator temps (15C and 70C respectively) as to do so you must increase rpm above the low idle required until fter those temps are reached!

Darryl


EDIT..you can bring pitch up immediately after the engine "settles" (my Altzheimers cutting in again), you MUST though make sure that you reduce throttle as you increase rpm so it doesn't go above low idle. This can be tricky, particularly in teh cold..so SAFEST just to wait until min temps....which is what I do.

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

This all makes sense now, especially since I discovered that I've been erroneosly taking off with coarse pitch.

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I like Thunderbolts! (Link to MySpace tribute page)


120th Fighter Wing, Montana Air National Guard

Hi Darryl,

sure..

Edited prior post little bit just to clarify.

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FSX SP2 DX9.Addons:FSG2010,REX OD, AS2012, PMDG, DA Do27, Dodosim, A2A accusim all, Katana4X,Coolsky DC9 Classic, Swiss Pro,ORBX all, Fly Tampa, DRZEWIECKI DESIGN, FS Dreamteam, FranceVFR Haute Normandie 3D...

Nic081,

N.P.


Skycat,

A not uncommon mistake and one that nearly cost Douglas Bader his life (again!).

Grumpy Unwin said that the trees bordering the base were missing several top branches from pilots doing the same.

We tend to forget that both variable pitch AND retractable undercarriage were very new to single seat pilots in 1938/40

Darryl

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

Good thread guys.... I didn't realize about the temps after start-up, so I'd always get her started- then turn off starter mag, close primer, close ignition button cap, and put prop to fine- 1, 2, 3, 4 done as soon as the motor was on. And I always forget to switch back to coarse before shutting down, without fail. So far 40 hours on her and still running fantastic.

And I still can't remember to NOT tap the brakes prior to retracting the landing gear... old habits die hard!

Joe

Joe,

The most critical temp is Oil temp and in most of our flying (minus the guys in cold areas) the oil will already be at min before you get the engine rpm to stabilise so you can increase it.

Start up in -15C rpm up and take straight off and see how you go!!

The main thing about radiator temp is that it indicates how the engine is warming up much more quickly than oil will, which can take quite a while to reflect the actual temp of the enigine. So OK oil but low Radiator wil mean that the Merlin may not run as smoothly as it should but at least you are getting oil circulation so damage is not so much of a worry.


For the same reason, high OIL temp is much more serious than high radiator as the radiator can be cooled much more quickly than the oil.....and it (high oil) probably indicates that the Merlin has been running hot for a while already!!

Note Spit I is 70C, Spit II is 60C due to different coolant makeup.

In real life, you would NOT run a Merlin at anything less than minima simply because any problem will usually show up when the engine is stressed..and you are about to put one of the greatest stresses it will ever have onto it...takeoff. Failure then leaves you liable to be out of speed, out of height and out of ideas...

Hope you had a good Christmas mate

Darryl

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

Thanks Darryl... I like to keep it as close to real life as I can, within reason, so I'll definitely let her warm up more before I switch to fine.

I guess I'm gonna have to not taxi until it is warm enough to switch over (I usually taxi out slowly under absolute minimal RPM to save time and to avoid having to shut down the engine halfway through taxiing to cool off). I may want to have the ground crew walk my Spitfire down the taxiway closer to the runway... the other traffic will just have to deal with the wait and appreciate watching a classic Spitfire in action!

Christmas was great, I hope yours was too! Tomorrow I'm headed to Boston for our annual New Years debauchery! Have a Happy New Year, I'll be back on the forum next week (hopefully with a shiny new P-40 to dive into!)

All the best,

Joe

Taxiing with coarse is bad to plug fouling but also straining as it means high torque on low rpm. But to get from coarse to fine pitch you HAVE to apply high rpm with coarse WHILE the airplace is on the ground, stand-still. Sound like a bad solution for 2-pitch prop that you can't operate it at low rpm. (Obviously you can in the sim - it's not properly modeled.)

I always wondered why MkI had to be revved up prior to shutdown but MkII it wasn't necessary. Now it's pretty obvious. (Radials should be revved higher to improve oil scavenging from the lower cylinder walls but an upright V12 doesn't have issues with oil leaking into combustion chambers after shutdown. B-109 with DB-601/605 sound outright horrible on that aspect as it's an upside-down V12 and ALL cylinders point downward. I wonder what a job it must be to free the cylinders of leaked oil & clean plugs prior to each start-up. Or dunno, maybe they've come up with some improved scavenging.)

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So to verify I got the facts straight:
A) moving pitch from coarse to fine requires high rpm
B) moving pitch from fine to coarse requires high rpm
C) start-up must be done with coarse setting for some reason related to stressing of the prop pitch control mechanism
D) shut-down must be done with coarse setting because of reason B)
E) warm-up must be done with coarse setting because of reason A) to prevent over-stressing a cold engine

And when it comes to A2A Spitfire none of the above is modeled, AFAIK. Some things would be impossible to model properly without a force feedback prop pitch lever on the controller. And I don't think such a thing exists. Or does the "plunger" resist movement with engine still or at too low rpm? Or does it move freely, just delay effect until rpm are raise adequately?

I must be too much a nerd that my brain keeps asking "why" C)...

That was what I was wondering, both in the sim and real life. If the mechanism can't move until there's sufficient rpm then clearly it's important to make sure the prop is in coarse on shut down but does that also mean that upon start up you can immediately kick it into fine pitch and leave it or is the lever physically locked until sufficient rpm is reached? If not, it seems like an easier way to make sure you're in fine pitch for take off rather than finding out halfway down the field during a scramble.

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From what I've read and seen, the A2A Spitfire has *ALL of the above modeled through Accusim, including the damage done if you abuse the engine due to the prop. That was one of the things that Scott highlighted in his video on the propeller system in the Spitfire.

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Correct Mate, subject to a couple of small FSX limitations (you should in RL be able to drag teh prop to fine pitch even with teh Merlin shut down..but can't to coarse.)

Some people need to RTFM, use a "search" or give up Trolling. I've answered all these questions before (I've also always found it useful to not automatically assume something is wrong..but what would I know..I just get paid a lot by the hour to advise people and don't have the luxury of spouting off things "AFAIK".)

r4y30n...correct the trick is "sufficient" RPM, not "high" RPM.... BUT there is no control lock..it is simply a matter of not being able to "push" a bowden cable against the weight of the prop and swivel gear. That is what teh counter weights are for and they need centrifugal force to work to coarse. Getting to coarse is important before shut down and this may require a "burst" of power (ie above taxi power). In teh sim I recall that this had to be compromised to "some revs" vs "no revs". I recall that the guys could also not get FSX to accept that teh prop would move one way but not the otehr in case of certain failures.

Darryl

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

I was not talking about "damage to engine due to prop" but "damage to prop due to engine". There's a heck of a lot of difference between the two.

If we were actually talking about prop pitch damaging the engine (and not the other way around) then OBVIOSLY we'd start the engine with "fine" instead of "coarse"... as we do with every other plane except Spitfire MkI. Wasn't the reason why you had to put it into coarse due to the counterweight mechanism in the prop pitch control? Then obviously it's not about damaging the engine if you shut down and start the engine with fine prop - it's about damaging the prop control mechanism instead.

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Regarding my list of assumptions (based on Killratio's response):
A) was in incorrect. You can move from coarse to fine at low rpm. Even stand-still... but you should do it because it's impossible to return it to coarse before starting the engine.
B) was correct: you might need to slightly rev up to be able to push the plunger in. In simulator, having any revs is enough (no need to raise the idle) but with engine shut down, it doesn't work.
C) was is... correct? Even though CapFlyer says it damages the engine instead of the prop pitch mechanism. (I just fail to see how it could possibly damage the engine to start in "fine".)
D) was correct
E) is a contradicting now with A). See contraction analysis below.

Regarding E), I'm going to quote a few lines, both from Killratio:



So, you CAN drag the prop from coarse to fine (but not the other way around). Therefore I assume you should be also able to drag the prop from coarse to fine with low rpm idle, before your engine has warmed up.

Therefore: why do you recommend that after you've started it with "coarse" it should NOT be set to "fine"? If you can switch from coarse to fine with ZERO rpm, you should be able to switch from coarse to fine with low rpm as well. Therefore, I don't see why you claim:


Does the last quote not contradict the fact that you said you can move the pitch from coarse to fine even with engine shut down? Because I see an elephant sized contradiction right there.

EDIT: seem like you also noticed the same contradiction as you later added


It's things like these that make my mind go WTF. Because I have that nature that I seek possible contractions in everything I read. It makes me a dickhead, perhaps (and I admit being one). But am I a troll for mentioning it when I see it?