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!

[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...

[ATAG_Dutch]

Originally Posted by DK-nme

Hmmm they are good though...

First saw them in 1979, about 25 times in total, and will be going to the Bloodstock festival this year. Played pool with Lem, Philthy and Brian Rob once in Sheffield.

As much an English institution as Fish n Chips.

Lemmy for PM!

So anyway, manifold pressure and prop pitch......

[MadBlaster]

I read somewere a while back that the constant speed prop is based on oil hydraulics and aerodynamic forces on the prop from airspeed. Basically, the faster you go, the prop wants to run fine pitch from the aero forces, so to counteract that, there's a governor mechanism that runs off oil pressure to make it run course pitch. So you have the two forces constantly balancing to keep the rpms in range so the engine doesn't overspeed. So whether your running fine or course gears, its the same principle. It's a passive control more or less. Not like a variable pitch prop in manual mode ala 109.

Manifold pressure, I'll take a guess as I'm not a pilot either. I think it's just like a car. Natural aspirated, the engine creates vacume on the piston downstroke and sucks in the fuel/air mix. The higher the rmps, the more sucking power and the higher the manifold pressure. In supercharger setup, pump is belted somehow to the drive shaft and forces air/fuel into the manifold via a pump. I guess what the guys are saying up above to explain your observation about manifold pressure going up when prop pitch is changed to 'course' is that at high rpms, output of the supercharger pump somehow hinders the engine vacume (ala "law of diminishing returns"), causing it to be lower than it would be if the engine was naturally aspirated. So, it's a trade off when you use a supercharger. What you lose in power at the high end of the rpm scale you gain on the low end of the scale (i.e., higher manifold pressure at course pitch setting where the load on the prop in static state (no acceleration or deceleration) is highest).

[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!

[Blackdog_kt]

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

As another interesting bit of information, the less amount of stress on the manifold is not with the throttle closed. At low throttle settings the intake "tube" is trying to implode, because the inside pressure is less than the outside pressure. In fact, the lower amount of stress on the intakes occurs when running throttle that gives a manifold pressure equal to the outside (ambient) air pressure: at sea level this would mean running full throttle on a non-supercharged engine.

Just goes to show how things are not that much set in stone but there's a lot of inter-dependency between different conditions.

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


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