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09-18-2009, 01:49 PM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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Quote:
Originally Posted by dedgemon
If all I wanted was 1000'/minute I could fly a spam can. 
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Good luck with that one on RV fuel burn rates! 
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09-18-2009, 03:32 PM
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Join Date: Dec 2008
Location: na
Posts: 1,457
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[quote=L.Adamson;359755]Oh really......
Personally, I'd been flying C/S props before I had the notion to build an RV. I knew the difference then, and the difference now. So being polite, I'll put it is way, ...around here................it is NOT well proven that there just isn't much of a performace gain between F/P & C/S.
One of my subtle points in the discussion of Ps was that you CANNOT use two different airplanes to make the analysis..this is a fundamental Flight Test process foul. If all -8's were built to the same standard and configuration then yes it would be ok.. but this is not the case. One of the benefits of the FP is in most cases is it will be over 30lbs lighter, maybe not true for some of the lighter composite c/s props tho?. Does anyone have Climb Rate vs. Gross Weight and CG data?
The other thing I hear a lot of is the efficiency gained with a C/S prop..yet I can never get anyone to agree what efficiency they are talking about. I had the Hartzel software while I was doin a lot of test work that predicted the prop efficiency of the 76666 blade (I think thats the number??)..and depending on the DA, RPM, and A/S and prop diameter the efficiency (in this case the ratio of THP/SHP) varied from as low as 78% to as high as 86%. Comparing that with some of the more common FP props that often have efficiencies in the mid/low 80's you can see that depending on the flight regime the FP can and will have a higher efficiency.
Efficiency can be measured however you choose to measure it..
How about MPH/Fuel Rate? Or MPH/Total Acquisition cost?
Oooh I like that one... So if my max speed is say 200MPH with a FP and it cost me $50,000 total to "build" my plane I have an efficiency of .4% (200/50,000) *100)
If I put a C/S on and I now get 210MPH at $56000 my efficiency is .375% !
Its not hard to devise definitions to PROVE your point
Also, if I match my my airplane, engine, FP climb prop correctly it will do as well as an C/S in climb...but will certainly suffer in cruise.
But you might say yeah but I get to my destination faster..ok so maybe there is a benefit over the long haul for fuel savings. But with a 30lbs lighter plane I might be able to bring ALL of my wife's shoes, purses and hair products with me saving me that money at the destination end of the trip!!
BTW I had a guy with a 200hp RV-8 challenge me to a race in my 180hp BD-4. I told him no way he could possibly win and I was right...I kicked his butt!!! Actually he backed out...which is an even better win.
I told him the race was from St. Mary's, MD (2W6) to Ocean City straight line of about 70 miles...his plane cruised at 220 Mph mine was 180 Mph. He backed out cause the race was to see who could get their family of 4 from St. Marys to Ocean city with all their beach gear in the quickest time..HA!
Even SPEED needs to be defined to show one is "faster" than the other...
Honestly...this is not an effort to prove one is better than another...because I KNOW it cannot be done..because the criteria (as I've just shown above) can be tailored to fit your personal objectives... This is just for fun
But to make it really fun why don't those of you with flying RV-8 create a CAFE style fly-off
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09-18-2009, 03:35 PM
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Join Date: Dec 2008
Location: na
Posts: 1,457
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the first couple lines in my previous post are the quotes from the other guy..I don't know how to do that quote thing I guess
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09-18-2009, 06:36 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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[quote=RV8R999;359918]
Quote:
Originally Posted by L.Adamson
Oh really......
Personally, I'd been flying C/S props before I had the notion to build an RV. I knew the difference then, and the difference now. So being polite, I'll put it is way, ...around here................it is NOT well proven that there just isn't much of a performace gain between F/P & C/S.
One of my subtle points in the discussion of Ps was that you CANNOT use two different airplanes to make the analysis..this is a fundamental Flight Test process foul. If all -8's were built to the same standard and configuration then yes it would be ok.. but this is not the case. One of the benefits of the FP is in most cases is it will be over 30lbs lighter, maybe not true for some of the lighter composite c/s props tho?. Does anyone have Climb Rate vs. Gross Weight and CG data?
The other thing I hear a lot of is the efficiency gained with a C/S prop..yet I can never get anyone to agree what efficiency they are talking about. I had the Hartzel software while I was doin a lot of test work that predicted the prop efficiency of the 76666 blade (I think thats the number??)..and depending on the DA, RPM, and A/S and prop diameter the efficiency (in this case the ratio of THP/SHP) varied from as low as 78% to as high as 86%. Comparing that with some of the more common FP props that often have efficiencies in the mid/low 80's you can see that depending on the flight regime the FP can and will have a higher efficiency.
Efficiency can be measured however you choose to measure it..
How about MPH/Fuel Rate? Or MPH/Total Acquisition cost?
Oooh I like that one... So if my max speed is say 200MPH with a FP and it cost me $50,000 total to "build" my plane I have an efficiency of .4% (200/50,000) *100)
If I put a C/S on and I now get 210MPH at $56000 my efficiency is .375% !
Its not hard to devise definitions to PROVE your point
Also, if I match my my airplane, engine, FP climb prop correctly it will do as well as an C/S in climb...but will certainly suffer in cruise.
But you might say yeah but I get to my destination faster..ok so maybe there is a benefit over the long haul for fuel savings. But with a 30lbs lighter plane I might be able to bring ALL of my wife's shoes, purses and hair products with me saving me that money at the destination end of the trip!!
BTW I had a guy with a 200hp RV-8 challenge me to a race in my 180hp BD-4. I told him no way he could possibly win and I was right...I kicked his butt!!! Actually he backed out...which is an even better win.
I told him the race was from St. Mary's, MD (2W6) to Ocean City straight line of about 70 miles...his plane cruised at 220 Mph mine was 180 Mph. He backed out cause the race was to see who could get their family of 4 from St. Marys to Ocean city with all their beach gear in the quickest time..HA!
Even SPEED needs to be defined to show one is "faster" than the other...
Honestly...this is not an effort to prove one is better than another...because I KNOW it cannot be done..because the criteria (as I've just shown above) can be tailored to fit your personal objectives... This is just for fun
But to make it really fun why don't those of you with flying RV-8 create a CAFE style fly-off
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I've read everything you've wrote.....................and just have to say "so what..." But I'll even place a smiley here ....  , as I don't actually get upset, as it might appear. I'm smiling as I write this...
As Rick has mentioned; yes I do fly out of a 4200' msl airport, and most flights are in the 8500 - 10500' range. No matter how you look at it, from dollars to weight, the C/S will outshine the F/P everytime in performance from takeoffs to landing. Pilots from around here will simply look at all your figures and say "bah". We are going from RV experience; not what we think might happen.
As you've said, the only way a fixed pitch is going to better perform on an RV over a C/S is to have one that's either pitched for climb or cruise. With a lighter plane, you might win in either climb or cruise, but never both....
And of course, the reality is, that most F/P's are compromised between climb and cruise. That is fact, as the owner is never quite content with just one or the other.
So fine, fly at sea-level with your wooden prop RV8, and I'll happily continue with my C/S. You said that you wouldn't have a top line C/S if it was given to you. Most F/P pilots around here could only wish for a top line C/S. They'd take it in a second if their engine allowed for it. And I'm not making this up. I've flown in many F/P equipped RV's in the last 15 years. I know exactly what these pilot/owners have said. But then............yes, we live in the mountains...
L.Adamson --- RV6A
Last edited by L.Adamson : 09-18-2009 at 06:43 PM.
Reason: improve grammer...
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09-18-2009, 06:41 PM
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Join Date: Apr 2005
Location: KSLC
Posts: 4,021
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Quote:
Originally Posted by Brantel
Good luck with that one on RV fuel burn rates! 
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Of course the fuel rate will be up, because we can develop more horse power.
But it all averages out...
L.Adamson --- RV6A
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09-18-2009, 09:05 PM
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VAF moderator
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Join Date: Dec 2005
Location: In Walter Mitty's dreams
Posts: 947
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My eyes hurt....
After reading all those responses my eyes hurt.... I've got quite a lot of time behind both and enjoy them both. Any day I get to go fly is absolutely fantastic. I like being able to get the most horsepower out of my engine on takeoff with a CS because it feels good to me and I like it. If I can only get out of a field with a CS then I probably should not be making that flight that day. Most of the "Ladies" and "Guys" that one takes flying to impress them don't know what the blue knob does..(UNLESS you are Mrs Dye  or my GF).. they are simply impressed that you can fly an airplane. The fuel burn difference is not enough to cause anybody to bounce a check to the FBO. The maintenance cost as well as the initial cost will be more than a FP. Usually there is a weight penalty of some sort with the CS... but if you did some yardwork every night then you could lose 30 lbs couldn't you? I was 250... now I'm 230... from doing yard work. 195 here I come. So if I may quote some non-descript redneck, "RUN WHACHA BRUNG" and be happy. And if it counts for anything... my helicopter has a CS
Best
Brian Wallis
__________________
Brian Wallis
(Exempt AND VAF dues paid 02 FEB 16)
Callsign: VOODOO sold RV3 to pay for ratings  !!!
AP/IA COM/Multi/IFR/350 type
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09-19-2009, 05:24 AM
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Join Date: Oct 2008
Location: Schaumburg, IL
Posts: 2,053
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There's no question to me about the performance advantage of the CS. I am on the fence though if it is worth the extra $$. Of course convincing my wife that we need to climb 200 fpm more and cruise 4 kts faster all for the bargain price of $10,000 is the real challenge. 
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Tony Phillips
N524AP, RV 9 (tail wheel)
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09-21-2009, 11:49 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by tinman
I put a fixed pitch on my -8 and am very pleased. I think that the "one-gear transmission" analogy is not quite on target...it is more like a speed boat prop since it "swims" in air rather than stuck in positive traction mode on pavement. I give up a bit of climb rate since I have biased towards cruise. This isn't a problem for me because I don't fly in the mountains. If I had that mission, I would have chosen the CS prop.
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The "single speed transmission" analogy is used because a fixed pitch is only "optimal" at a single speed - period. Assuming we're talking about a prop pitched for typical RV cruise speeds, can this same propeller allow the engine to turn to full rated HP at brake release? No. Can it do it from stall speed? No. Can it do it at Vy? No. The FP certainly has a few discrete advantages and the right prop will even outperform a CS at a single speed, but it also makes a less capable RV on the whole. For some, the added capability is not worth the added cost - I'm just not one of those people...
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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09-21-2009, 01:02 PM
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Join Date: Feb 2006
Location: Grand Prairie, Texas
Posts: 232
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Not both
Quote:
Originally Posted by apkp777
There's no question to me about the performance advantage of the CS. I am on the fence though if it is worth the extra $$. Of course convincing my wife that we need to climb 200 fpm more and cruise 4 kts faster all for the bargain price of $10,000 is the real challenge. 
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You aren't likely to get both improved climb and cruise. Most fixed pitch propellers are optimized for one or the other. My order from Catto is for a cruise pitch three-bladed prop, and Craig Catto assures me that it will equal or exceed the cruise speed of a constant-speed prop.
Climb is another story, however. I'm accustomed to climb of about 600 fpm in my Bonanza. I anticipate that the Catto will exceed that by a big margin, but I know it will not equal the performance of a constant-speed prop in climb pitch.
I'd love to have the braking power of a constant-speed prop. Money's tight right now, though, and I think the performance of the RV will vastly exceed that of my Bonanza, so I'll be happy.
__________________
Mike Gray, A&P IA, AET, KGPM, Grand Prairie, Texas
1956 Bonanza for sale.
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09-21-2009, 01:25 PM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,768
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Mike,
My RV-6 with an O-320 with a 3-blade Catto is giving me about 3 times the climb of your Bonanza. I think you will be very happy with it. With the proper technique you will be able to slow down in the pattern with no problem. Since the diameter of the 3-blade is less, drag is less. It took me about 5 landings to get used to the new "drag pattern". I fly into my 1500' strip with no problems.
__________________
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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