Question for the Motor Heads

[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.

[Blakduk]

Dutch- He makes me feel like i'm a 12yr old playing games while he's a grown-up doing things for real (which is all very accurate, except i'm well past 12).
I'll humiliate myself and ask him, i can handle his disdain.

Back a few years ago i met a guy called Alec Henshaw. He was a test pilot for Spitfires during WW2 and flew hundreds of them- now he would have been the one to ask!

[winny]

Originally Posted by Blakduk

Dutch- He makes me feel like i'm a 12yr old playing games while he's a grown-up doing things for real (which is all very accurate, except i'm well past 12).
I'll humiliate myself and ask him, i can handle his disdain.

Back a few years ago i met a guy called Alec Henshaw. He was a test pilot for Spitfires during WW2 and flew hundreds of them- now he would have been the one to ask!

Just for you.. Alex Henshaw in action over Castle Bromwich 1941
The 1st Min of the video is him.

http://www.youtube.com/watch?v=nCmzYccyBYM

[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).