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  #21  
Old 09-07-2016, 04:50 PM
judoka5051 judoka5051 is offline
 
Join Date: Aug 2010
Location: McAlpin, FL
Posts: 253
Default Got me thinking, and now my head hurts!

I've been reading both sides of the debate, and agree with both sides for the most part. But,,, I'm curious about the HP vs constant speed performance.

Lets say that I have two identical RV's, I put a standard O360 (180hp) CS on one, and a 200hp FP on the other, will the extra 20hp overcome the CS advantage? That's assuming a FP cruise prop, and hoping to regain the climb through extra HP.


That being said, I'm going with a FP just because I like to fly cheap.

As a side note, the RV8A (160hp) I built was super light with a whirlwind ground adjustable. It climbed 1700 with 2 up and half fuel on a hot day and cruised 205mph at 10,500 @ 2700. The only gripe I had was on initial roll, it took about 300-400 ft to spool up and start pulling hard.
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McAlpin, FL
Plane at FL10

Scratchbuilt Biplane
Preceptor N3 - Sold
Zenith Zodiac 601HD - Sold (good riddance)
Kitfox IV - Sold my share, but loved that plane
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Cherokee 180 pickup truck
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  #22  
Old 09-07-2016, 06:20 PM
FasGlas's Avatar
FasGlas FasGlas is offline
 
Join Date: Mar 2014
Location: Prescott, AZ
Posts: 741
Default

Quote:
Originally Posted by RVbySDI View Post
Actually, I would suggest you do a little more analysis of current FP prop performance. You have twice now mentioned performance characteristics. Yet you still miss the point I am making. I am simply trying to provide realistic analysis of information that a builder/pilot may wish to examine in determining which prop would work best for a particular airplane's configuration. You are intent on arguing that in all aspects a CS is better than a FP. I am saying that you should step back from the emotional "macho" aspect of faster faster faster (faster climb; faster cruise; faster emptying of the wallet) as the ONLY determination of "better".

However, I will nibble on your bait a bit and say that in not ALL situations will a CS outperform a FP. There are always variables to consider. And I am not just talking about going fast at a top speed, or climbing fast. There are FP designs that do indeed perform in multiple aspects of the performance range and not just remain "stuck" in a particular performance regime due to the restricted design configuration.

I would contend your concept of FP is indeed "fixed" on an adage that: "What has been, will always be", when it comes to performance. I am not arguing at all that CS props are a "wrong" choice to make. What I am saying is that there are multiple concepts of "best" that just talking about faster this or faster that do not address. There is very much a bigger picture analysis that should be evaluated when making the determination. If you are convinced there is nothing but a CS prop that will work on an airplane then you have pretty much made it clear your views. I am writing to simply open the eyes of someone, clearly you being the exception, that might begin to understand that there are indeed very good reasons someone might choose a FP prop over a CS prop.

Oh yes, I almost forgot: Live Long and Prosper!
Ok, I get it.. If you would have just said you were too cheap to go with a CS setup I would have understood. Look, I don't build planes to go slow, climb or cruise. I have a car for that. I've said over and over (since you can't find it in my posts) that each type is mission specific. Since the OP appears to be building a plane he plans to fly and not race, a CS is more prefered for ALL the right reasons. If he stated that he wanted to roll slow, climb slow and go fast at the top end, spinning 3000 RPM, I would have suggested a FP. Or climb like a bat out of he11, and cruise slowly, again I would have suggested a FP... But he asked about a good combination for his plane, an RV-6 to go flying. This is a prefered setup. Go try one and learn something. Some builders actually don't mind spending some money on the airplane they really want in the first place.
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  #23  
Old 09-07-2016, 06:58 PM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
Posts: 200
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I seem to recall reading somewhere in my plans where Van's highly recommends a CS prop on the RV. So, there's that...
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  #24  
Old 09-07-2016, 07:29 PM
David Z David Z is offline
 
Join Date: Aug 2010
Location: Thunder Bay Ontario
Posts: 335
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RVs stalls at 50 kts and cruise at 150, that's quite the speed range. A 172 stalls at 50 and cruises at 100, much narrower speed range. FP props are optimized for one speed, CS props are much more flexible. Is that flexibility worth the purchase and operating costs? That can only be answered by you.
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  #25  
Old 09-07-2016, 09:09 PM
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RVbySDI RVbySDI is offline
 
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Location: Tuttle, Oklahoma
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Quote:
Originally Posted by FasGlas View Post
Ok, I get it.. If you would have just said you were too cheap to go with a CS setup I would have understood. Look, I don't build planes to go slow, climb or cruise. I have a car for that. I've said over and over (since you can't find it in my posts) that each type is mission specific. Since the OP appears to be building a plane he plans to fly and not race, a CS is more prefered for ALL the right reasons. If he stated that he wanted to roll slow, climb slow and go fast at the top end, spinning 3000 RPM, I would have suggested a FP. Or climb like a bat out of he11, and cruise slowly, again I would have suggested a FP... But he asked about a good combination for his plane, an RV-6 to go flying. This is a prefered setup. Go try one and learn something. Some builders actually don't mind spending some money on the airplane they really want in the first place.
Ok it is clear that you are passionate about your constant speed prop. I will refrain from posting further on the topic after this post. I have refrained from commenting on specific performance data but since you are adamant that a fixed pitch prop is slow in climb, slow in cruise and burns more fuel in doing so, I will end my comments with data about my slow to climb and slow to cruise fuel guzzling RV9A. I routinely climb at 1400 FPM on 95 deg Oklahoma summer days to cruising altitude where I routinely cruise around 170-175 MPH and burn around 6.5-7.0 GPH all while doing this with a fixed pitched prop. Here is a thread with some numbers I posted a few weeks ago (post #9) if you are interested in seeing numbers:
http://www.vansairforce.com/communit...light=gph+club

I am sure a constant speed will do somewhat better in one, or maybe even all of those paremeters (although not too sure in the fuel burn paremeter), but the point I have been trying to get across is the concept of whether the extra "everything" a CS prop could provide is worth all the extra one has to pay, dollar wise and otherwise, for the small gain in performance seen with a CS prop over what the FP will deliver on the RV.

If we meet at OSH or somewhere lets drink a Spotted Cow together and tell stories about how fun it is to fly our RVs!

And so I shall finalize my comments on this thread by again signing off with:
Live Long and Prosper!
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  #26  
Old 09-08-2016, 07:13 AM
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Toobuilder Toobuilder is offline
 
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Location: Mojave
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A properly set up EI can provide anywhere from "improved" to "dramatic" performance gains over magnetos and are a no brainer as far as I'm concerned.

Fuel injection offers performance and operational benefits at a higher initial cost than a carb. The choice is not as clear on that one - it is mission specific and a trade study should be conducted.

The very wide speed range of the RV line suggests a CS propeller, but as others have pointed out, RV's are generally overpowered enough that a fixed prop capable of supporting a fast cruise will also drag the airplane off the ground acceptably. CS propeller systems are complex, heavy and expensive - yes, but they offer a great deal of versatility to the airplane. This is an area that does not lend itself to internet opinion - we are not picking out a color scheme here. You need to know what you need and equip the airplane appropriately for your mission. If you have not flown a CS prop enough to understand what they do for you, then get that experience, make your decision, and move on. Everyone here is going to defend their own decision as the "right" one, so we are the wrong people to ask.
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  #27  
Old 09-08-2016, 07:55 AM
brad walton brad walton is offline
 
Join Date: Jul 2007
Location: Cypress, TX
Posts: 526
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What I recall Van saying is build light and keep it simple.

I did. RV8 at 1067 pounds with a Superior IO360 and a Catto 3 blade. I have a plane that is a joy to fly and well balanced for fun flying. It won't accelerate from 0 to 125 knots as quickly as a constant speed prop or climb as well in that speed range, but that is about it as far as drawbacks and I only fly in that speed range at the beginning of each flight for maybe 60 seconds. After 125 knots, it will climb with a constant speed.

If my primary mission was long cross country, a constant speed would allow same cruise speeds at lower rpm's. But mine is definitely not slower or less efficient.

As the previous poster noted, everyone defends their own decision as being the right choice. I made the right choice for me.
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  #28  
Old 09-08-2016, 08:54 AM
Bighorn Bighorn is offline
 
Join Date: Dec 2013
Location: Mn/Tx
Posts: 25
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Brad what are you getting for ave numbers ie take off roll at gross, cruise speed,
fuel burn

Thanks
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  #29  
Old 09-08-2016, 09:29 AM
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ssmdive ssmdive is offline
 
Join Date: Jul 2012
Location: FLL
Posts: 120
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From the FAA: "a fixed-pitch propeller achieves the best efficiency only at a given combination of airspeed and rpm, the pitch setting is ideal for neither cruise nor climb. Thus, the aircraft suffers a bit in each performance category. The fixed-pitch propeller is used when low weight, simplicity, and low cost are needed"

When I was looking for an RV, I wanted CS. I had friends that argued with me that a FP prop could outperform a CS prop in all areas of flight. They could not explain why, they could not give details, they could not provide data to support the claim. I could buy that a good FP could outperform a CS in ONE area of flight, but not all.

FP props are set for either cruise, climb, or a hybrid. But its performance range is limited in some area. Let's say the plane was like my old Challenger LSA. It took off at about 40 and topped out at about 95. A CS prop would have been a waste there since it would have added cost, weight, complexity, and there were not really good options.

Now look at my S1S Pitts. It has a climb prop for acro. Because of this, T models with the same engine as mine TO in less distance, cruise about 24 MPH faster, have longer vertical uplines, and burn less GPH in cruise than me.

A FP prop is a compromise somewhere, and the higher performance and larger speed range, the higher the compromise somewhere.

I would only go FP if cost was a major factor. I am quite happy with he RV6 180HP/CS that I am flying.

As for FI or Carb. Love the FI in the Pitts. But didn't really dislike the carb on the Citabria or in the RV. Inverted fuel carbs are picky and if you are going to spend time inverted at 0G or -G, I'd get FI.
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  #30  
Old 09-08-2016, 09:48 AM
brad walton brad walton is offline
 
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Location: Cypress, TX
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Hello Bighorn
I am not sure what takeoff roll I get at gross. I would guess 600 to 800 feet off my grass home field. This time of year with temps in the mid 90's and about 1500 pounds gross I can be at 1400 AGL in 60 seconds from a dead stop without zoom climb.
I have a screen shot of my EFIS at 16000 DA at 178 KTAS and 2700 rpm's burning 7.7 GPH. And another screen shot at 12000 DA at 158 KTAS burning 5.85 GPH. Both are ROP.
At low altitudes I burn 10.5 to 11 GPH at 178 KTAS at 2700 rpm's. If I turn 5% more rpm's, I get a corresponding increase in speed. My usual cross country cruise is 162 KTAS at 2450 rpm's and at 6000 DA I burn about 8.5 GPH. I've never had the prop balanced, but it is turbine smooth at all but idle speeds where I do get vibration

Last edited by brad walton : 09-08-2016 at 09:55 AM.
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