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  #1  
Old 07-15-2022, 02:18 PM
Mike S's Avatar
Mike S Mike S is online now
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Location: Dayton Airpark, NV A34
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Default Carson number and contiounous RPM

The new issue of AOPA has an good article by Mike Bush about how to maximize speed, range, and fuel use.

With the current cost of avgas, I am looking to try this out.

It all revolves around flying at the Carson number and leaning. For those not familiar with the Carson number, here is an older article explaining it.

https://www.aopa.org/news-and-media/...ue-cheap-speed

One of the ideas brought out is for those with a C/S prop, you pull the prob knob back at WOT until you get to the Carson speed.

This will most likely end up having the engine running at a RPM lower than most of us usually fly at, and I suspect potentially getting into some restricted RPM zones.

Anybody know if an IO320 with a Hartzell C/S prop has any specific RPM to avoid continuous running?
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Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."

Last edited by Mike S : 07-15-2022 at 06:55 PM.
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  #2  
Old 07-15-2022, 02:24 PM
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mike newall mike newall is offline
 
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23/23, lean and crack on.

We are paying $12 a US gallon at the moment in the UK.

I just go flying and enjoy it
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  #3  
Old 07-15-2022, 02:56 PM
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rv6ejguy rv6ejguy is offline
 
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Dave Anders flies WOT and as low as 2100 rpm. He knows more about best MPG than just about anyone through extensive flight testing and documentation. 158 knots TAS on 4.8 GPH at 17.5. Of course his RV is pretty slick.
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Last edited by rv6ejguy : 07-15-2022 at 03:01 PM.
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  #4  
Old 07-15-2022, 03:31 PM
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Untainted123 Untainted123 is offline
 
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Quote:
Originally Posted by Mike S View Post
Anybody know if an IO320 with a Hartzell C/S prop has any specific RPM to avoid continuous running?
This was discussed a long time ago on VAF, but I don't think there is any limitation with an O-320, only O-360's, and it's limit continuous use between 2000-2250 RPM.

I found this document, while not having anything official in it, seems to be from Hartzell, basically saying that with EI, they found the stresses increased in that same RPM range, but this was already documented, so they didn't recommend changing the existing limitation.

Overall, in another article or book from Mike Busch, he recommends running at the lowest possible RPM anyway (ie "over square") due to other efficiencies gained from the slower rotation of parts in the engine. We had a discussion about it here.
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  #5  
Old 07-15-2022, 03:40 PM
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Carl Froehlich Carl Froehlich is offline
 
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One thing that I found on my data runs is for a constant fuel flow, lower RPM is not necessarily better efficiency. I can only assume the ability of a prop to transform engine power to thrust is not linear and dependent on the prop design.

This leads me to operate high at RPM between 2450 and 2500, WOT, 20-30 degree LOP. I’m using a 74” Hartzell BA CS prop on a IO-360-M1B engine. I suspect I can squeeze out 1 or 2 more MPGs at a lower RPM but for me the curve is fairly flat, and for that small gain I’d take the trade off for higher cruise speed.

I’d be interested in other builder’s data.

Carl
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  #6  
Old 07-15-2022, 06:03 PM
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Quote:
flying at the Carson number and leaning
Well, this is what I’m doing back home where fuel is kinda double price of here in the US.
Carson speed for my bird is around 116kts IAS, still gives me like 130kts TAS at 4Kft on 5.5USG. Running well lean of peak on my carbed dual electronic ignition. Many do believe injection is the key, well, ignition is the real key to LOP.
My typical settings being 19/2000, flying in a world of silence

Right now touring the US, I’m using higher MP settings like 23 or 24. But of course FF goes to around 7.4, and the speed is way more than Carson‘s ideal…
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  #7  
Old 07-15-2022, 06:22 PM
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Quote:
Originally Posted by rv6ejguy View Post
Dave Anders flies WOT and as low as 2100 rpm. He knows more about best MPG than just about anyone through extensive flight testing and documentation. 158 knots TAS on 4.8 GPH at 17.5. Of course his RV is pretty slick.
Isnt Dave running an angle valve 360?? And touched by the magic of LyCon? Different animal from mine.

As I have yet to try getting into the Carson number flight regime, I can only speculate what the engine RPM and MP will be. I expect to be flying at 8,500 or higher at WOT, and would not be surprised to see RPM at 2000 or maybe even lower. AFR leaned down to 16:1 or more.

Just thinking ahead for a cross country trip in a few months.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #8  
Old 07-15-2022, 07:40 PM
Kyle Boatright Kyle Boatright is offline
 
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Carson's number, IMO, is just a weird number. It isn't the fastest, or most efficient, it is just a specific point on the efficiency curve. Why choose that point on the curve, as opposed to any of the others? Is it a "thing" just because there's a calculation for it?
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  #9  
Old 07-15-2022, 07:53 PM
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Mike S Mike S is online now
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Quote:
Originally Posted by Kyle Boatright View Post
Carson's number, IMO, is just a weird number. It isn't the fastest, or most efficient, it is just a specific point on the efficiency curve. Why choose that point on the curve, as opposed to any of the others? Is it a "thing" just because there's a calculation for it?
“Accepting that excess fuel is to be traded off for airspeed during normal operations, there is a method of operation which represents the ‘least wasteful way of wasting fuel,’” Carson wrote in a pioneering 1980 study.

Well, the way I take it is anything faster than best economy speed is using more fuel. There comes a place where your use of fuel increases faster than the speed increase. That is what I am shooting for.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #10  
Old 07-15-2022, 08:18 PM
Richard Connell Richard Connell is offline
 
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Ive just finished my new RV-10 testing - with about 35h or so on it now.
Ive definitely noticed that lower RPM is more efficient, but Im yet to collect extensive data on this. Currently I cruise at at 8500' running 2200/24" which is about 70% power. LOP 11 GPH gives 168 TAS, ROP 14 GPH gets 174 TAS. I need to do some more nozzle tuning as i still have one cylinder peaking 0.75 GPH earlier than the rest. I think ill get it down to 10GPH and perhaps a few kts slower.
Ive GPS box tested 100 through 150 IAS and the PEC is <1 KCAS (actually under-reads at 150 by 0.6) so I'm confident in the numbers.

It has 9:1, CAI, showplanes cowl and a Hartzell 3 blade.
The 3 blade was a conscious choice as I intend to use some short strips as I did with my 7. Intuitively I thought that this combination should see the extra HP hopefully offset the extra blade to some degree. To my pleasant surprise it comfortably meets Vans published numbers or exceeds them. FWIW I made my own plenum as well which when combined with slightly smaller intake rings than the stock SP cowl possibly helps.

I notice almost no degradation in performance from 2400 down to 2200 RPM at constant MP. above 2600 it gets slower.

Anyway, all of this is to preface that the 3 blades are obviously much bigger than a 2 bladed aluminium (lightweight climb performance is quite something).
Ive come to operate it quite a bit differently than my IO360/Hartzell BA equipped RV7 so I think that any RPM discussion needs to consider the propeller choice. I never ran my RV7 slower than 2400. But fuel was cheaper then!
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