Spit 1a

[furbs]

Just used the FMB to start with a Spit 1a at 18,000 ft @ 300MPH

The spit then started at 200 MPH and wouldn't go any quicker.

Can someone else please test the same?

[Welshman]

spit IIa has the rotol prop and i just held 310 , without full trimming etc at sea level

so i guess we just need the rotol spit 1 not the 1938/9 version

[Plt Off JRB Meaker]

furbs,will check my settings properly tonight and get back to you,I'm sure I've had my Mk1a over 250mph.

Will confirm this later if I get the chance.

[furbs]

Welshman, well im sure the spit 2 should be quicker than that.

[ATAG_Dutch]

Originally Posted by furbs

Can someone else please test the same?

Just done it.

Entered 500kph @ 6000m in FMB, and once the 'plane was trimmed out for straight and level, with boost through the gate and coarse pitch, after a bit of a wait and some re-trimming the airspeed indicator read 240mph, which allowing for IAS/TAS calculations, put the speed at about 350mph.

This looks about right to me. Unless my maths is wrong, I had to convert to metric and back also.

Which begs the question, when you enter speed in FMB, are you entering IAS or TAS? Does FMB calculate the difference?

Interestingly, she really wanted to climb so lots of down trim was needed. Might test the altitude again to see if anything has changed.

[Welshman]

starts loosing boost above 18000 too

[Welshman]

yes i know but no good for top speed

[Plt Off JRB Meaker]

Originally Posted by Dutch_851

Just done it.

Entered 500kph @ 6000m in FMB, and once the 'plane was trimmed out for straight and level, with boost through the gate and coarse pitch, after a bit of a wait and some re-trimming the airspeed indicator read 240mph, which allowing for IAS/TAS calculations, put the speed at about 350mph.

This looks about right to me. Unless my maths is wrong, I had to convert to metric and back also.

Which begs the question, when you enter speed in FMB, are you entering IAS or TAS? Does FMB calculate the difference?

Interestingly, she really wanted to climb so lots of down trim was needed. Might test the altitude again to see if anything has changed.

Well done Dutch,got there before me,top job

[ATAG_Dutch]

Originally Posted by Plt Off JRB Meaker

Well done Dutch,got there before me,top job

Thanks Meaks!

It doesn't explain the performance at sea level though.

If the 2-bladed fixed pitch prop achieved 295 @ 2000ft, 250 for the 2-pitch prop at sea level seems a bit dismal.

Can anyone find any figs/graphs? Most google references point to either 1C or Ultrapack!!

[Blackdog_kt]

Almost everything except the Spit Mk.II is slower than it should and that's what Luthier probably referred to when talking about FM fixes (i think he was replying to a thread about this very issue).

With that out of the way and without going into the octane/boost levels debate at all, the root cause of the problem is the fact that most people are not really familiar with how prop pitch works and are getting confused with the two-speed prop.

The trick is simple but a little time consuming, however it should be no big deal for any dedicated Spit drivers until we get a Rotol (constant speed) variant:
start a free flight mission and just fly for half an hour with the sole aim of seeing how much you can get away with without frying the engine.

I'm almost not flying the Spit at all but i have no problem getting 250mph IAS at medium altitudes (which would translate to about 300 mph) with very conservative cruise power settings: no over-boost, just maximum continuous power (the white triangle marker in the instruments red zone), coarse pitch and being careful to trim well and minimize radiator use.

The problem many people have is that they don't know much about how the propellers really work and what that means for ease of maintaining performance. It's no biggie, i didn't know either, there are a load of articles on the web and after some reading my handling with all aircraft improved considerably.

I've seen three excellent articles posted here some time ago, some of it might be too technical (actually the mixture article is mostly about modern civilian prop-aircraft from a point onwards) but it's a worthy read. Just take your time when reading it and don't rush through it, you'll see a lot of mysteries getting explained

Here are the links
manifold pressure: http://www.avweb.com/news/pelican/182081-1.html
propellers: http://www.avweb.com/news/pelican/182082-1.html
mixture: http://www.avweb.com/news/pelican/182084-1.html