VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics


Go Back   VAF Forums > Model Specific > RV-12
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #11  
Old 07-07-2017, 10:04 AM
rv6ejguy's Avatar
rv6ejguy rv6ejguy is offline
 
Join Date: Mar 2005
Location: Calgary, Canada
Posts: 3,874
Default

Quote:
Originally Posted by rongawer View Post
Sorry to disagree with you, but I'm not talking theory; it's physics. The physical property of friction in play here is simply more wetted area equals more friction, aka "drag". I think you may have missed my point to the concept of matching the engine power to the propeller. You need more wetted, i.e. blade area, to take advantage of more horsepower. Yes, there are some very fast propeller driven aircraft with many blades. I gave the example of a C130, which is the highest horspower propeller driven aircraft in production that I know of - the latest Hercules has nearly 5,000 horsepower per engine with 4 large blades per prop. I could have used any number of WWII aircraft as examples as well. However, that doesn't detract from the fact that more blades have more drag - the engine driving those blades simply has much more power to take advantage of the extra surface area.
I think Tom would still be using a 2 or 3 blade if it was faster since this is Reno and it's all about speed. Every mph counts. Theory is great but it's always trumped by real world experience and results. 284mph average speed around the course with a 360 Lycoming.
__________________

Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, Marcotte M-300, IVO, RV6A C-GVZX flying from CYBW- 415.8 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi.htm


Reply With Quote
  #12  
Old 07-07-2017, 10:06 AM
rongawer's Avatar
rongawer rongawer is online now
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
Default

Quote:
Originally Posted by Canadian_JOY View Post
We can't really compare blade angles because of the difference in the number of blades. It is safe to assume the 3-blade is pitched something like a couple of degrees flatter than the 2-blade. The 3-blade can hit redline RPM in cruise flight quite easily while the 2-blade is pitched such that it can barely make redline in low-level (2500' MSL) cruise flight.

Many of the comments made by previous posters have been borne out in our comparison. The 2-blade runs slightly better cruise numbers - about 3mph. The 3-blade gets off the ground faster and definitely out-climbs the 2-blade by a significant margin (at least 100 fpm better with similar load on a hot day). The 3 blade is quieter and smoother on these direct-drive engines. The 2-blade makes for easier removal of the lower cowl, but in honesty, not by very much.

I opted to go with the 3-blade Warp Drive in hopes it would help in reducing noise and vibration while returning a good compromise on performance. It has delivered on all of these goals with the added bonus of better climb performance. In consideration of these benefits I would opt for a 3-blade again if presented with the same choice to make.
You make a good point, so let's look at this. Without getting into blade shape and a comparison of wetted area, the shorter the blade, the slower the tip speed for the same RPM, therefore the farther from sonic speed (the point at which the blade becomes louder and ultimately start breaking the sound barrier). For fun, listen to an Apache chopper in a high speed descent - at high speeds, the rotor sounds like a high speed cannon going off as each blade comes forward braking the sound barrier in rotation; which is why those pilots aren't supposed to do that....so a three bladed prop with shorter blades than a two bladed prop will be quieter for the same RPM.

Additionally, more blades increases the balance around the crankshaft rotation, due to the distributed weight, making it smoother - in much the same way a heavier flywheel smooths out a car engine. But you don't get balance for free.

Optimizing the propeller for the airframe and engine is why good folks, like Craig Catto, are successful. It's all about modeling for not only what works well, but what you want for your airplane (climb, smoothness, speed, looks, etc...).
__________________
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
Reply With Quote
  #13  
Old 07-07-2017, 10:08 AM
rongawer's Avatar
rongawer rongawer is online now
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
Default

Quote:
Originally Posted by rv6ejguy View Post
I think Tom would still be using a 2 or 3 blade if it was faster since this is Reno and it's all about speed. Every mph counts. Theory is great but it's always trumped by real world experience and results. 284mph average speed around the course with a 360 Lycoming.
You should probably talk to Tom about this then. My guess is a 2 bladed prop sized to take advantage of his power and rpm would either be supersonic at the tips or too long to provide safe ground clearance.

This really isn't theoretical. Physics doesn't change. What does change is the implementation of those physics. Note Tom's blade design.
__________________
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

Last edited by rongawer : 07-07-2017 at 10:12 AM.
Reply With Quote
  #14  
Old 07-07-2017, 10:48 AM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,051
Default

Ron,

In Subs we used to get cavitation at high screw RPM running shallow, but it was due to the water flashing to vapor in the low pressure regions around the blade and then the bubbles collapsed as they were shed from the blades. It was a phase change phenomenon. It made a lot of distinctive noise, so we always tried to avoid it. How do you get cavitation in air if there is no phase change?

Rich
Reply With Quote
  #15  
Old 07-07-2017, 12:31 PM
rongawer's Avatar
rongawer rongawer is online now
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
Default Cavitation

Quote:
Originally Posted by RFSchaller View Post
Ron,

In Subs we used to get cavitation at high screw RPM running shallow, but it was due to the water flashing to vapor in the low pressure regions around the blade and then the bubbles collapsed as they were shed from the blades. It was a phase change phenomenon. It made a lot of distinctive noise, so we always tried to avoid it. How do you get cavitation in air if there is no phase change?

Rich
Rich, it's all fluid dynamics.

I'm quite familiar with submarine propellers, which are just fancy pumps. Whether a propeller on an airplane, a propeller on a submarine or a house fan, the mechanics are all the same. Air and water are just fluids. Whenever pressure on the outlet of the pump is less than the pressure on the inlet, Net Positive Suction Head (NPSH), goes to 0 pounds, and cavitation starts to occur. The goal is to create a higher pressure on the outlet than the inlet. You can do this several ways, but in the case of a submarine, a smaller sub-set (pun intended) of blades is inserted between the larger blades to increase the overall surface area, hence wetted area, of the propeller and therefore the force exerted to the water is greater, keeping the pressure up and reducing cavitation. The fancy schmancy submarine propellers are also why a cover is always kept over the sub's screw when brought into dry dock to keep prying eyes from the classified design.

When you say phase change, you imply going from a physical state of liquid to a gas, solid or plasma state, however in cavitation, the water isn't going through a phase change, but rather the gas in solution is expanding rapidly due to the sudden pressure reduction behind the eye of the impeller. Interesting fact: this is also why ice forms at the center of the propeller first - it's the lowest pressure, hence coldest, region of the propeller.

The noise you hear in cavitation of a water driving screw (propeller) is a result of the gas bubble suddenly collapsing as it passes into the slip stream behind the screw. It's all a function of pressure transients. And power, and the force applied as a result of that power is what causes that transient.

To your question, you get cavitation in air the same as you do in water. As an experiment, take a home fan, turn it on high speed with your ear next to the center of it. It will be loud immediately, but quiet slightly as the fan motor settles at speed and the air flow stabilizes. This phenomena is cavitation. The difference is that air is less dense than water, so less of a pressure transient occurs.
__________________
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

Last edited by rongawer : 07-07-2017 at 12:46 PM.
Reply With Quote
  #16  
Old 07-07-2017, 12:48 PM
RFSchaller RFSchaller is offline
 
Join Date: Oct 2010
Location: Phoenix, AZ
Posts: 2,051
Default

Ron, got this from Engineering Toolbox. It describes the phenomenon I'm familiar with in water systems, but I wonder if in aerodynamics it has a different meaning since there is no phase change. I'd like to understand what is physically going on when the aircraft prop blade number is increased. I have heard it described as the blades "getting in each other's way" with regard to air motion, but that doesn't really explain it.

"What is Cavitation?
Cavitation may occur when local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature. According the Bernoulli Equation this may happen when a fluid accelerates in a control valve or around a pump impeller.

The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately after evaporation collapses when velocity decreases and pressure increases."
Reply With Quote
  #17  
Old 07-07-2017, 01:09 PM
artrose artrose is offline
 
Join Date: Jan 2011
Location: San Antonio area
Posts: 68
Default

"Whenever pressure on the outlet of the pump is less than the pressure on the inlet, Net Positive Suction Head (NPSH), goes to 0 pounds, and cavitation starts to occur. The goal is to create a higher pressure on the outlet than the inlet."

You might have meant to say ? ..........

NPSH Net Positive Suction Head is the minimum pressure required at the suction port of the pump to keep the pump from cavitating.
Reply With Quote
  #18  
Old 07-07-2017, 02:23 PM
rongawer's Avatar
rongawer rongawer is online now
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
Smile

Quote:
Originally Posted by RFSchaller View Post
Ron, got this from Engineering Toolbox. It describes the phenomenon I'm familiar with in water systems, but I wonder if in aerodynamics it has a different meaning since there is no phase change. I'd like to understand what is physically going on when the aircraft prop blade number is increased. I have heard it described as the blades "getting in each other's way" with regard to air motion, but that doesn't really explain it.
Rich, it's still just a fluid process. The only difference is the density of the fluid, which is why a water propeller will erode, or "pit", as a result of continuous cavitation and aircraft propellers don't (if you find pitting on your blades, that's from solid objects, such as dirt or other debris that went airborne). The air is already in a gaseous state, so it doesn't collapse to the extent that a air bubble in water will, but it does undergo a pressure transient just the same.

Blades "getting in each other's way" is one way of saying that turbulent air exists between the blades, causing a low pressure area that the next blade cavitates through. Imagine a blade turning in a vacuum; there is no friction of the blade on air, therefore no force applied, therefore no thrust occurs. To a lesser extent, this is the condition that occurs when a blade is cavitating - it's not making thrust. More correctly, it's not making full contact with dense air, but rather moving through "bubbles" or pockets of high and low pressure air. At the risk of way over simplifying, it's much like a tire spinning on mud - it's not getting traction. When cavitating, the blade isn't getting traction on solid air.

This will happen all the time if the blade is too small for the application. Friction, i.e. drag, is an essential part of propeller function. Without it, the prop would just spin and do nothing. This is the main reason I have to take pause when someone says a propeller doesn't have drag. The key is having the right amount. Just as having too large a blade exerts excessive drag and reduces efficiency, too small a blade creates excessive cavitation - insufficient friction, and also obviously inefficient. All of these phenomena can be solved with correct blade size, count and shape for the power, rpm and airframe.

So, back to the OP's question: it depends on your application and desires.
__________________
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

Last edited by rongawer : 07-07-2017 at 02:47 PM. Reason: clarification
Reply With Quote
  #19  
Old 07-07-2017, 02:24 PM
rongawer's Avatar
rongawer rongawer is online now
 
Join Date: Dec 2007
Location: Brentwood, CA
Posts: 185
Default

Quote:
Originally Posted by artrose View Post
"Whenever pressure on the outlet of the pump is less than the pressure on the inlet, Net Positive Suction Head (NPSH), goes to 0 pounds, and cavitation starts to occur. The goal is to create a higher pressure on the outlet than the inlet."

You might have meant to say ? ..........

NPSH Net Positive Suction Head is the minimum pressure required at the suction port of the pump to keep the pump from cavitating.
Read what you said, then reread my statement and see if it makes sense. I simply stated it in basic terms. NPSH is the net pressure difference between suction and discharge. When NPSH is less than or equal to 0 - you no longer have positive suction head, cavitation occurs, and ultimately the pump doesn't discharge; you might say it needs priming...
__________________
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

Last edited by rongawer : 07-07-2017 at 02:49 PM.
Reply With Quote
  #20  
Old 07-07-2017, 02:40 PM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Los Angeles
Posts: 301
Default Back to reality

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.
__________________
Thank you
John S

WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for thier use.

Extra dues paid 2017, worth every penny

RV9A- Status: tail feathers done less tips
Wings 50%
www.pilotjohnsrv9.blogspot.com
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:32 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.