Question for the Motor Heads

[ARM505]

Funny that A2A is mentioned, because I seem to remember that LOWERING RPM in the A2A Spit LOWERS boost (ie, lowering prop RPM's via the prop control lever at a constant throttle setting results in less boost indicated). This is explained in the A2A docs - the supercharger, being directly linked to the engine, is now being turned at a lower RPM, hence a lower manifold pressure - the opposite to unforced induction, where lowering RPM at a constant throttle setting will result in higher manifold pressure, the 'car going up a hill in a high gear' analogy.

Yup, just checked the A2A Spit, and that's what happens. COD seems to behave in the opposite way - lowering RPM's lowers manifold pressure. One is right, the other wrong I suppose. Funnily enough, once again we seem to have a multitude of inputs explaining why COD is correct. Oh well. I'll just go and fly my Spit IIa IRL to check it out quick

To be honest, my vote goes to the A2A Spit, which is frankly awesome, even though the engines tend to wear too fast!

[Seeker]

Originally Posted by ARM505

Funny that A2A is mentioned, because I seem to remember that LOWERING RPM in the A2A Spit LOWERS boost (ie, lowering prop RPM's via the prop control lever at a constant throttle setting results in less boost indicated). This is explained in the A2A docs - the supercharger, being directly linked to the engine, is now being turned at a lower RPM, hence a lower manifold pressure - the opposite to unforced induction, where lowering RPM at a constant throttle setting will result in higher manifold pressure, the 'car going up a hill in a high gear' analogy.

Yup, just checked the A2A Spit, and that's what happens. COD seems to behave in the opposite way - lowering RPM's lowers manifold pressure. One is right, the other wrong I suppose. Funnily enough, once again we seem to have a multitude of inputs explaining why COD is correct. Oh well. I'll just go and fly my Spit IIa IRL to check it out quick

To be honest, my vote goes to the A2A Spit, which is frankly awesome, even though the engines tend to wear too fast!

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

[ATAG_Dutch]

Originally Posted by Seeker

There's a thread about the Spít I and II in the FM forum where people are beating each other over the head with docs and charts as usual.

In the thread there's a comparative test between 109 and Spit, and the RAF docs clearly state that the Spit pilot reduced revs to 2600 which raised the boost...

If you watch the A2A vid referenced by louisV, the A2A spit is provided with three different props, so it would depend on which one was being used. From what I can make out, the spit in the comparison would have the constant speed in order to set revs at 2600. I think!

[ATAG_Dutch]

Thanks for the replies chaps, all good information!

So going back to fixed pitch props for a sec, presumably the throttle then controlled an air/fuel valve in the carb, so boost/manifold pressure and rpm would both go up or down according to throttle setting?

The constant speed prop set up is a) set required engine revs and b) set throttle to achieve faster or slower airspeed at those revs, making the throttle effectively the pitch control, and there's some automation in making sure enough fuel/air is fed to the engine for those settings?

The two speed prop's throttle then controls what?

Sorry, I always was a bit slow with this stuff!

[DK-nme]

Originally Posted by Dutch_851

No, not Lemmy's gang!
Thanks in advance!

Hmmm they are good though...

Originally Posted by Dutch_851

Also is the RPM gauge telling you prop RPM or engine RPM?
Thanks in advance!

RPM is for the engine, I think. And this is why. A quick calculation:

If prop lenth is 1.5 meters (too long, too short?)
And max RPM is 2400-2500 r/min (too much?), then, the speed of the propeller tip will be:

2*radius*Pi (m/r) * 2400 (r/min) * 60 (min/h) *1/1000 (km/m) = 1357 (km/h), which is a bit above speed of sound.

Edit: Oooops, a small bug in the calc. Now it has been fixed

The propeller should not exceed the speed of sound, cause the compresability will eliminate the lift (of the propeller, that is) and thus, the RPM must logically represents the revolving of the engine...

[kimosabi]

Propeller tips are known to reach the speed of sound but not the whole blade. Ever wondered what the "flapping" sounds from helicopters are?

[DK-nme]

Originally Posted by kimosabi

Propeller tips are known to reach the speed of sound but not the whole blade. Ever wondered what the "flapping" sounds from helicopters are?

Yup, I know, but the idéal speed is just slightly beneath the speed of sound - I think, that A2A actually stated the same, in the video about prop pitch and constant speed propeller...

[DK-nme]

Originally Posted by kimosabi

Propeller tips are known to reach the speed of sound but not the whole blade. Ever wondered what the "flapping" sounds from helicopters are?

Ahem, this is a quote from the following link, just to confirm my earlier statement about prop tip speed, not exceeding the speed of sound:

"You will not see prop tips moving much faster than 600 knots, as this is getting too close to the speed of sound, and all sorts of nasty aerodynamic things start to happen in the vicinity of the prop tips. The noise alone is bad enough!"

http://www.avweb.com/news/pelican/182082-1.html

[Blakduk]

So CoD is correct and A2A is wrong????

Syn_Bliss makes a very convincing argument for the accuracy of CoD- but all good theories must be proved by experiment. Does anyone have a Spitfire they can take for a spin to check this out?
I could ask my brother's mate who occassionally flies a MkV at Temora, but i feel really stupid asking him questions like this.

[ATAG_Dutch]

Originally Posted by Blakduk

but i feel really stupid asking him questions like this.

Why? I'm sure he'd be only too happy to give you the benefit of his experience.