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  #21  
Old 07-07-2017, 03:07 PM
artrose artrose is offline
 
Join Date: Jan 2011
Location: San Antonio area
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In basic terms it might be less confusing and more accurate if we leave out the discharge pressure, and say.... NPSH is the difference between inlet pressure and the lowest pressure level inside the pump. More simply put, if there's not enough positive pressure on an operating pump inlet, the pump will cavitate. Doesn't matter the fluid, or the pump design.

I'm trying to understand how 3 blade airfoil performance can be explained with 3 phase current? I'm confused.
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  #22  
Old 07-07-2017, 03:32 PM
RFSchaller RFSchaller is offline
 
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Ron,

OK, I see what you are saying. If I paraphrase: each blade disrupts the air as it passes with a low pressure in front of it and high pressure behind it. The next blade passes through the disrupted air hitting the low pressure area and decreases it's efficiency. The fewer the blades or lower the RPM the longer the disrupted air has to return to a uniform pressure. Has that got it in Sesame Street fashion?😁

Rich
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  #23  
Old 07-07-2017, 04:14 PM
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rongawer rongawer is offline
 
Join Date: Dec 2007
Location: Brentwood, CA
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Quote:
Originally Posted by artrose View Post
In basic terms it might be less confusing and more accurate if we leave out the discharge pressure, and say.... NPSH is the difference between inlet pressure and the lowest pressure level inside the pump. More simply put, if there's not enough positive pressure on an operating pump inlet, the pump will cavitate. Doesn't matter the fluid, or the pump design.

I'm trying to understand how 3 blade airfoil performance can be explained with 3 phase current? I'm confused.
Hmm...I'm not sure how best to respond to you, but essentially it depends on the type of pump - and there are many. I'm thinking this discussion is outside the scope of this topic and should likely be in a different forum, but briefly - if you're talking about radial flow pumps, what you're saying is correct. However, an aircraft propeller for an RV-12 is a single stage, axial flow pump - which doesn't have an internal pressure; only inlet and outlet. You could talk in terms of the pressure just prior to the prop compared to pressure at the blade edge, but that would just be more complicated.

What's in front of the prop is the suction (inlet) and the backside of the prop is the discharge (outlet), and if the prop has turbulence on the outlet, it's cavitating...and that's simple.

As for your 3 phase AC versus 3 bladed prop. A serious response is that you can't; the potential between all three phases is explained in terms of the square root of 3 versus line voltage where all three phases will have a different potential. The pressure at all three blades on a spinning propeller in a normal atmosphere will be the same.

The non-serious response is more like "it don't rust, don't bust, don't collect no dust and runs cheaper, lighter, faster and more efficiently".
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Ron Gawer

- RV-12 N975G, SN 120840, Build in progress...finally on the finishing kit, so, something like 90% done and 90% left to go.
- 1975 B58 Baron, N1975G (a bottomless money pit that makes building an RV look like lunch money, but it's a great airplane, hauls the family and my wife likes two engines...go figure)
- 1961 A33 Debonair, N433JC (R.I.P.)
- RV7A; didn't finish it and donated to kid's club
- Zenith CH601XL; flying somewhere in Louisiana https://youtu.be/wa_Y_A_rP_8
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  #24  
Old 07-07-2017, 04:19 PM
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rongawer rongawer is offline
 
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Location: Brentwood, CA
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Quote:
Originally Posted by RFSchaller View Post
Ron,

OK, I see what you are saying. If I paraphrase: each blade disrupts the air as it passes with a low pressure in front of it and high pressure behind it. The next blade passes through the disrupted air hitting the low pressure area and decreases it's efficiency. The fewer the blades or lower the RPM the longer the disrupted air has to return to a uniform pressure. Has that got it in Sesame Street fashion?😁

Rich
Close enough. Just know that with more power and bigger blades, you move more air undisturbed, because you're moving more of it in a single blade pass.
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Ron Gawer

- RV-12 N975G, SN 120840, Build in progress...finally on the finishing kit, so, something like 90% done and 90% left to go.
- 1975 B58 Baron, N1975G (a bottomless money pit that makes building an RV look like lunch money, but it's a great airplane, hauls the family and my wife likes two engines...go figure)
- 1961 A33 Debonair, N433JC (R.I.P.)
- RV7A; didn't finish it and donated to kid's club
- Zenith CH601XL; flying somewhere in Louisiana https://youtu.be/wa_Y_A_rP_8
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  #25  
Old 07-07-2017, 04:59 PM
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rongawer rongawer is offline
 
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Originally Posted by PilotjohnS View Post
I hate to stop this submariner party but lets talk about propellers.
Since we are talking about subs and I know nothing about them, except they are cool, the idea that three blades are smoother than two I see differently. With thre blades or two, the moment of inertia determides the flywheel effect, not the number of blades; one may argue three blades have one more blades worth of inertia thou. I think three blades will tend to be smoother because of another reason too. With a propeller, each blade nevers sees the same angle of attach as it rotates around, mainly due to pitch angle, flow field differences, etc. with two blades each blade will go in and out of this area on each cycle and might be synced to a 2x rpm harmonic. With three blades, there is a higher probability that at least one blade would be in each different flow field at a given time, making for smoother operation. Similar to how 3 phase AC works. Similar to why a Hughes 300 is smoother than the old Bell 206 Jet rangers. JMHO. Now let me get my beer and enjoy the rest of this blog.
John, John, John...

...for the record, I'm not a submariner - although I did a lot of engineering work on them many years ago (think Rickover era). But you're missing the point; a propeller on a submarine works just like a propeller on an airplane - it screws it's way through the fluid, whether water or air.

Now, your 3-phase AC comparison...that's engineering humor for me I'll have to use on Monday.
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Ron Gawer

- RV-12 N975G, SN 120840, Build in progress...finally on the finishing kit, so, something like 90% done and 90% left to go.
- 1975 B58 Baron, N1975G (a bottomless money pit that makes building an RV look like lunch money, but it's a great airplane, hauls the family and my wife likes two engines...go figure)
- 1961 A33 Debonair, N433JC (R.I.P.)
- RV7A; didn't finish it and donated to kid's club
- Zenith CH601XL; flying somewhere in Louisiana https://youtu.be/wa_Y_A_rP_8
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  #26  
Old 07-07-2017, 05:24 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 488
Default Back to Lycomings and propellers !

I personally experienced the conflict of theory and reality with my two blade prop. The dynamic balance guy in Indianapolis counceled that the prop and crank should be aligned for best balance and smoothness. For my 0320 with 3/8" bolts, this was not possible with the Saber extension. Because, the crank bolts and the prop bolts are offset 30 degree to provide metal for the propbolt inserts . The goal is described to be concurrence of combustion moments with propeller centrifugal forces.

Enter the three blade theory. It is my understandinding, that in addition to the pressure pulse syncronzation presented above, the power pulse moments can never coincide or harmonically differ from the 3 blade inertial forces ???
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RV-3B N243RV flying
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  #27  
Old 07-07-2017, 05:41 PM
RFSchaller RFSchaller is offline
 
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When I was in Navy nuclear power school one guy was having a tough time understanding the physics of a chain reaction. The instructor dumped 100 paper clips on the desk and started throwing some away, sticking some in his pocket and then adding some to illustrate each physical process on the subatomic scale. We used to call that reactor physics for Sesame Street!😜
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  #28  
Old 07-07-2017, 05:44 PM
rongawer's Avatar
rongawer rongawer is offline
 
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Location: Brentwood, CA
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Quote:
Originally Posted by RFSchaller View Post
When I was in Navy nuclear power school one guy was having a tough time understanding the physics of a chain reaction. The instructor dumped 100 paper clips on the desk and started throwing some away, sticking some in his pocket and then adding some to illustrate each physical process on the subatomic scale. We used to call that reactor physics for Sesame Street!😜
Having taught at NNPS, I completely agree. Thanks for the memory.
__________________
Ron Gawer

- RV-12 N975G, SN 120840, Build in progress...finally on the finishing kit, so, something like 90% done and 90% left to go.
- 1975 B58 Baron, N1975G (a bottomless money pit that makes building an RV look like lunch money, but it's a great airplane, hauls the family and my wife likes two engines...go figure)
- 1961 A33 Debonair, N433JC (R.I.P.)
- RV7A; didn't finish it and donated to kid's club
- Zenith CH601XL; flying somewhere in Louisiana https://youtu.be/wa_Y_A_rP_8
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  #29  
Old 07-07-2017, 06:00 PM
rongawer's Avatar
rongawer rongawer is offline
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
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Quote:
Originally Posted by Larry DeCamp View Post
I personally experienced the conflict of theory and reality with my two blade prop. The dynamic balance guy in Indianapolis counceled that the prop and crank should be aligned for best balance and smoothness. For my 0320 with 3/8" bolts, this was not possible with the Saber extension. Because, the crank bolts and the prop bolts are offset 30 degree to provide metal for the propbolt inserts . The goal is described to be concurrence of combustion moments with propeller centrifugal forces.

Enter the three blade theory. It is my understandinding, that in addition to the pressure pulse syncronzation presented above, the power pulse moments can never coincide or harmonically differ from the 3 blade inertial forces ???
Ok, you've got a lot going on there... first off, if we took a three bladed prop and a two bladed prop and spun them on a shaft with some constant force, such as an electric motor, if either prop were out of balance, there would still be harmonic imbalance applied to the shaft. Once both props are properly balanced, and assuming both props weighed the same, then the issue of "smoothness" of a 3-bladed prop driven by piston engine "power pulses" comes down to the mass of the prop being evenly distributed and the force of the crank shaft being applied in three locations around the circumference of the shaft instead of just two.
__________________
Ron Gawer

- RV-12 N975G, SN 120840, Build in progress...finally on the finishing kit, so, something like 90% done and 90% left to go.
- 1975 B58 Baron, N1975G (a bottomless money pit that makes building an RV look like lunch money, but it's a great airplane, hauls the family and my wife likes two engines...go figure)
- 1961 A33 Debonair, N433JC (R.I.P.)
- RV7A; didn't finish it and donated to kid's club
- Zenith CH601XL; flying somewhere in Louisiana https://youtu.be/wa_Y_A_rP_8
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  #30  
Old 07-07-2017, 07:42 PM
N49ex N49ex is offline
 
Join Date: Apr 2015
Location: Wheaton, Illinois
Posts: 23
Default How a 3 blade can be just as fast as a 2 blade

While in general the physics that says a three blade will climb better but be slightly slower in cruise is true in a general sense, all else being equal, there are instances where you can beat this.

I had the classic Hartzell 7666-2 two blade prop on my 200 HP. IO-360, and when it came time for overhaul, the shop told me it was too close to spec to be be overhauled again. I was faced with the need for a new prop. So, I bought one of the new Hartzell scimitar top-props, and guess what - it climbs noticeably better AND achieves the same cruise speed as the old two blade. And the reason is technology - an improved newer design that simply is more efficient.

Ironically though, while in cruise the three blade is perfectly smooth (and it is dynamically balanced to < 0.03"/sec), when dropping down to low power in approach to landing, the 3 blade is not as smooth as the two blade was, until below about <14" and 2000 rpm. Some kind of harmonic interaction with the counterbalanced engine....

Reinhard Metz
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