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Slow Cruise

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sonicapollo

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If you just want to loiter, how low is your fuel burn on the O-360 in the rv-8?

I keep looking at these LSAs with their 4-5 gph and 90kt cruises and wonder what the RV-8 will do when you simply pull the power back. The new 912is costs what a 360 does. Where are the savings?
 
I usually keep my comments to myself when someone comments how efficient their light sport is sipping 5-6 gph at 100ish knts. My carbureted fixed pitch 7 with electronic ignition isn't as efficient as an 8 or constant speed prop but Cruising LOP at 155ish knts yields an avg of 8 gph or less for an avg 3 hour leg. I see low 7 gph in cruise, And I burn lots of 91 ethanol free car gas.
I haven't spent enough time that slow for an accurate answer but I would assume 90 knts is to slow for an 8 at "best range" or lowest fuel burn, I'm guessing it would be 125-140knts at 5ish gph.
 
Depends on your definition of loiter.

For me, today's definition included two ship formation practice. Me solo in my RV-8 (IO-360), started out tanks full.

At about 3500 ASL, 60 degrees F, CS prop at 2550 RPM and MP at a range of around 18 - 20, we were about 125 KIAS. Sometimes higher sometimes lower depending on the maneuvers. Fuel was about 7.8 GPH during ops check. My plane uses 100LL.

I have no experience with LSA.

Are you thinking of buying or building an RV? Maybe 12 vs 8?
 
Here's the deal. I can poke along at 110 knots burning 5 gallons of premium from the corner gas station, with or without ethanol, per hour. That's some pretty cheap flying. But I'm not loitering, I'm cruising. I can push the throttle in but I'll only get another 10 knots. Not 70. A 7, 8 or 9 can slow down, but I can't speed up by much.

Just my humble opinion, but there's not really a tough choice to be made. Regardless of WHY you're an 8 guy or a 12 guy, you're either an 8 guy or a 12 guy. There's not a lot of commonality there other than the family connection. Again, just my opinion.
 
I can cruise at 45% power with around 126 KIAS, that makes almost 150 KTAS at 10000 ft and 6 GPH. I found 2000 RPM more efficient as like 2400 RPM, makes around 4-5 KIAS difference at the mentioned 45% power.
These are no final numbers, however very similar to my RV-7.
 
close, so very close

crabandy said:
I usually keep my comments to myself when someone comments how efficient their light sport is sipping 5-6 gph at 100ish knts. My carbureted fixed pitch 7 with electronic ignition isn't as efficient as an 8 or constant speed prop but Cruising LOP at 155ish knts yields an avg of 8 gph or less for an avg 3 hour leg. I see low 7 gph in cruise, And I burn lots of 91 ethanol free car gas.
I haven't spent enough time that slow for an accurate answer but I would assume 90 knts is to slow for an 8 at "best range" or lowest fuel burn, I'm guessing it would be 125-140knts at 5ish gph.
Click to expand...

Yep, there is a break over point at which you start slow flight and start burning more fuel to stay below the step, just like with a boat in the water. And I agree, this is one of those that you don't need to brag about to a 12 driver, but yes 5-5.5 is something that can be done with the idea of just seeing the sights as you fly along. For us it is about 42% on the power meter and somewhere between 120-125 Kts. on the GPS ground speed as I don't always trust the "Q" at that set point. I try to stay at 50-55% on CX trips to save fuel if I don't want to hurry up and get there. The IO-360 with a fixed pitch light prop is a very good selection to go slow, go fast and carry a good load with what ever FF you want to spend that day. Hope this helps, Yours R.E.A. III #80888
 
Fuel Efficiency

Apples to Oranges but Klaus Savier did Santa Paula to Lakeland nonstop at 252 statute ground speed using 6.6 gallons hour at 17'500. Parallel valve modified 360. From Kitplanes.
Max Conrad did 4.5 gallons hr. per engine in the IO 320 powered Twin Commanche during his 60's record flight. The IO 320 and the IO 360 will burn almost exactly the same amount of fuel at a given horsepower.
 
SuperCubDriver said:
I can cruise at 45% power with around 126 KIAS, that makes almost 150 KTAS at 10000 ft and 6 GPH. I found 2000 RPM more efficient as like 2400 RPM, makes around 4-5 KIAS difference at the mentioned 45% power.
These are no final numbers, however very similar to my RV-7.
Click to expand...

My RV-6 is similar. 122 KIAS @ 6 GPH regardless of altitude. (2,100 RPM, 22-Inches)
 
Most efficient slow cruise will be at best L/D (lift over drag) speed. In my RV8, that is around 100 to 110 knots. Minimum sink speed will yield most time aloft. In my RV8, about 85 knots.
I don't know my fuel flow at best L/D or minimum sink speed, but I have a screen shot from early this summer at 143 KTAS at 6600 feet DA burning 5.53 gph rich of peak. This is a Superior IO360 with a Catto three blade.
Do a Google search also for Carson speed. It is about 140 ktas for my RV8
 
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Bottom line is that if you have the discipline to pull the knob back, you can fly in a 4/6/8/9 at roughly the same speed as a 12, at fuel burn so close that the cost difference is lost in the 'financial noise'. But you can't land quite as slow.

If you need to buy & fly now, & can't get a new medical, you still need the -12. Otherwise....

Charlie
 
5gph

When I have no destination other than simply going flying, I set the throttle @2000 rpm, lean the mixture back a bit,which gives 120k IAS at the usual 3000'. This consistently shows 5 gph. As previously stated, about the same fuel burn as the 12 at the same airspeed, but with the option to push the throttle open and leave it in the rear view mirror.
Lycoming o360-fix pitch 7a.

Jim Diehl 7A
 
Here is my data for for my 6A with a 180HP IO-360-B2B (O-360-A2A with Bendix RSA-5AD1 fuel injector) Lots of great flying at 120-130 kts and 5-6 gph but look at all that great flying on the right side of the plot. It is not likely in an RV to fly at 40% power (although it flys great there) to realize the cost savings of lower fuel flows.
RV-6A%252520performance%252520charts.jpg

TAS is in Knots
Decision on buying an LSA probably should not be made based on the cost of buying fuel but everyone should choose based on their own situation (and ability to rationalize).
 
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Wonder if

Philip,
This is good data. I wonder what it would look like if you plotted the data going thru zero on both axis and using a second order polynomial? This should better show the in increase in fuel flow at higher air speeds. Also didn't se any LOP data.

Also the scatter suggests either a data error or a aerodynamic thing a my jig at 160 knots max power.

Thanks for posting, during my flight test I will need to remember to plot this data.

John s
 
My Data

I took a short flight this evening and collected some slow speed data flying around 100 kts and 110 kts:

IMG_2249small.jpg



IMG_2252small.jpg


I was heavy at about 1830 lbs flying weight (full fuel, two large guys aboard). 3500' MSL.

5.1-5.3 gal/hr for approx 100kts true and 5.3-5.4 gal/hr at 110 kts. The air was smooth for these tests, but the data is still ballpark.

Interestingly enough, my actual fuel mileage was about the same or slightly worse than it is for 170kts economy cruise at 10,500'.

Skylor
 
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Skylor's data is consistent with the O-470 on my C180, adjusted for the difference in displacement. My typical loiter point is 15", 2,100 rpm, 6,000 MSL and about 6.1 gph. This works out to about 41% power.

Different engine series by a different manufacturer on a different airframe, but roughly the same fuel burn adjusted for engine size and power setting.

Why do it? Sometimes I just want to fly along some particularly interesting place and have a look. No point hurrying on by at cruise then.

Dave
RV-3B, now on the fuselage
 
49clipper

I almost hate to say it after seeing the numbers shown. If you want to slow down to 90mph, my fuel flow shows 3.2gph on my O-320_d1A. no flaps needed, and it flys very nicely. I had a sport cub pass me once on a short flight. that was LOP also, before installing the Surefly Electronic Ignition. I duplicated that once, but later only got it down to 4.0 gph. Have not done that lately.
My EIS and transducer are very accurate on my normal flying.
 
49clipper said:
I almost hate to say it after seeing the numbers shown. If you want to slow down to 90mph, my fuel flow shows 3.2gph on my O-320_d1A. no flaps needed, and it flys very nicely. I had a sport cub pass me once on a short flight. that was LOP also, before installing the Surefly Electronic Ignition. I duplicated that once, but later only got it down to 4.0 gph. Have not done that lately.
My EIS and transducer are very accurate on my normal flying.
Click to expand...

At what weight did you see those numbers? At these speeds and power settings, differences in flying weight makes a pretty significant difference required power. I will try again the next time I have partial fuel and an empty back seat.

Skylor
 
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