Fan

[Breeang]

Is there really that much difference, in the amount of air handled, between a 12 blade fan and a 5 blade ?.Doesn't air cavitation factor in there somewhere as in a boat prop ?.
I have no idea why this question suddenly popped into my head.

[NoLift911]

I am not clear on the direct volumetric efficiency of the 12 over the 5 but...

What moves more air, a jet engine or a prop?

-Jeff

Originally posted by Breeang
Is there really that much difference, in the amount of air handled, between a 12 blade fan and a 5 blade ?.Doesn't air cavitation factor in there somewhere as in a boat prop ?.
I have no idea why this question suddenly popped into my head.

[DW SD]

I don't remember my thermodynamics that well, but I don't think gasses cavitate. Cavitation occurs in liquids when the pressure drops enough for them to change state (to gaseous).
It is my impression that the 12 blade fan moves FAR more air and does make a big difference. I don't have research or first hand knowledge to confirm the difference, though.

Even four blade propellers are more efficient than three (marine applications) and smoother in their delivery of power.

Bill Verburg (or Wil Ferch) will probably provide a dissertation on the subject that will boggle the mind.

Doug

[Bill Verburg]

Previous fan threads

[silverc4s]

Why do all of the top secret submarine props have so many blades???

Bet there is a reason that has to do with both efficiency & performance.

[Bill Verburg]

Originally posted by silverc4s
Why do all of the top secret submarine props have so many blades???

Bet there is a reason that has to do with both efficiency & performance.

Quiteness is priority 1

[sammyg2]

There is a thread going on about this on the rennlist 911 mailing list right now. I don't know what the final conclusion will be.

[silverc4s]

Originally posted by Bill Verburg
Quiteness is priority 1

Bill,
MOST quiet would be no propellor at all, right? There have been rumors of that type of drive system, do not actually know of any in existence.. (ion drive?)

[Early_S_Man]

12-blade fan???

Typo, or halucination???

[dorschman]

come on, didn't anyone see The Hunt For Red Oktober?

[silverc4s]

Originally posted by dorschman
come on, didn't anyone see The Hunt For Red Oktober?

We won right??

NOTE to Breang:
This thread has spun out of control. I gues none of us know the answer to your question. Sorry for the hijack..

[AtomicDog]

silverc4s,

There are any number of options, but magnetohydrodynamic drive is not viable due to its low efficiency and a field that can be seen from space (without GREAT pains in shielding...).

Seriously, the Aero Engineers will have a better line on the fan question, but the critical factor is sonic velocity. As soon as a portion of the blade tip reaches the speed of sound, the induced shock wave will either disrupt the flow drastically, or the blade will shatter.

Adding SIX more blades to the fan allows for a higher developed pressure than the five-blader. The flow is more like that produced by an impeller vs. that from a propeller...

Chris

[ianc]

I've wondered this myself, but in relation to WWII fighter planes. Some, like the Mustang have 4 blades, and some, like the Spitfire have only 3. It seems to the layman that a 3 bladed prop would be inferior, but there must be a reason...

ianc

[HawgRyder]

More blades = more HP to drive.
The reason for more blades on the Mustang was the HP available needed to be harnessed of the prop would have cavitated.
Bigger blades (more cross-section) or more blades need more power to spin them as fast as a smaller/thinner/or less blade prop (or fan).
You do get more air, it will take more power, it will cool better, you choice.
Bob

[1972_911T]

As far as planes go its all to do with the intended RPM of the engine. In essence the prop size and pitch is the gearing ratio, When I used to fly RC aicraft if you was building a plane with a 2 stroke engine the engine rpm would be higher than the torquier 4 strokes thus you used a smaller prop at higher rpm where the 4 stroke put the power out at lower rpms so you used a bigger blade a less rpm. On some planes the size of the prop needed with the 4 strokes meant that with a 2 blade to get the right surface area the prop would hit the ground in this case you can use a smaller diameter 3 blade and get higher surface area.

[Wil Ferch]

I posted on all this ( mostly)..

BTW...it's an ELEVEN blade fan....

I also comment on how far away we are on sonic conditions....tip speed for the fans at max engine RPM is remarkably similar in all configurations...something like 350 mph, from memory.

Do a search...or I'll edit this post with a link when I find something...

Wil

EDIT- here's one...notice the fan tip speed calcs and the hp /flow calc relationships...

Feeler: Electric fan

[AtomicDog]

All,

Has anyone given serious thought to improving the airflow out of the bottom of the engine? Lower shrouding with an incorporated venturi or electric fans (or both)...

Chris