LOP Fuel Flow vs. TAS/ BSFC

[rv6ejguy]

The DH diesel discussion has me intrigued with what more people are seeing in the real world with their Lycomings running LOP. Perhaps we can work out approximate BSFCs from the drag polars and Van's basic speed data.

Feel free to post your numbers here, fuel flow, TAS, type of RV.

[petehowell]

Hi Ross,

RV-9A dual EI/Carb/Hartzell

LOP at 2 settings:

6.0 GPH 148-150KTAS 6000ft and above

5.0 GPH 128-130KTAS down low

I run at one of these 2 settings 90% of the time.

[ao.frog]

RV-7, Extreem IO-360 from Eagle Engines.
Silverhawk FI, dual P-mags, 2.blade M/T prop.

22 litres/hr (5,8 US GLS), 130 KTAS, 50* LOP at 5000' and below.

24 litres/hr (6,3 US GLS) 150 KTAS, 50* LOP at 10.000.

[JonJay]

Temperature and/or density.

[hevansrv7a]

1. My real SFC when LOP is about .40 but the GRT tends to exaggerate because it is using RPM and MP for its calculation along with FF and RPM & MP don't produce the same actual power when LOP vs. ROP. The GRT will compute it down to .38 or even .37. Lycomings tend to do a little worse - maybe .41 or .42. Higher compression or electronic ignition tend to improve it. See GAMI articles on this Forum for extensive information on that.

2. Deducing SFC from the drag curves is only possible if you have a correct value for propulsive efficiency, most of which is the efficiency of the prop itself and some of which is losses due to interference between the prop wash and the airplane. You have a better chance of deducing your propulsive efficiency using a fixed value for SFC. For much more on this see my article, link on my website or signature below. Also, in order to deduce anything from the drag curve you have to know what it really is. For a 6-A you could refer to the CAFE article. Ditto for the 9A. All others must punt. There is an 8 or 8A write-up but the result on drag do not IMHO compare correctly to the others. For most of us, propulsive efficiency will be in the range of 82 to 87 %. Feel free to debate this.

[AlexPeterson]

6A, 180 FI/EI/CS, 158 to 162 ktas, 7.2 to 7.5 gph, 22"/2300, 7500 to 9500'. Same settings lower, probably lose around 3 to 5 knots. Been cruising with those settings for 1100 hours. It is probably around peak to 50F LOP, depending on conditions. Sometimes I run WOT at 7500', but not often. I set the throttle and rpm to those settings, then adjust for fuel flow. Rarely look at egt's any more (I do the fuel flow vs egt series every year or so to keep an eye on mixture distribution). CHT's are powerful indicator of mixture settings, adjusted for OAT. If I see cht's in the above 350 range with OAT of 70F or so, I dial out the mixture. Lose a few knots speed, but cools things down in to the 330 range right away.

[rv6ejguy]

If we work back to 135 knots, we're seeing averages of around 4.4gph so far on various RV airframes up in the 6000-10000 ft cruise altitudes. Not bad! My Subaru setup is pretty bad then at about 6.6 leaned to 1400F EGT.

[rv6ejguy]

Originally Posted by hevansrv7a

1. My real SFC when LOP is about .40 but the GRT tends to exaggerate because it is using RPM and MP for its calculation along with FF and RPM & MP don't produce the same actual power when LOP vs. ROP. The GRT will compute it down to .38 or even .37. Lycomings tend to do a little worse - maybe .41 or .42. Higher compression or electronic ignition tend to improve it. See GAMI articles on this Forum for extensive information on that.

2. Deducing SFC from the drag curves is only possible if you have a correct value for propulsive efficiency, most of which is the efficiency of the prop itself and some of which is losses due to interference between the prop wash and the airplane. You have a better chance of deducing your propulsive efficiency using a fixed value for SFC. For much more on this see my article, link on my website or signature below. Also, in order to deduce anything from the drag curve you have to know what it really is. For a 6-A you could refer to the CAFE article. Ditto for the 9A. All others must punt. There is an 8 or 8A write-up but the result on drag do not IMHO compare correctly to the others. For most of us, propulsive efficiency will be in the range of 82 to 87 %. Feel free to debate this.

This would be approximate of course due to variations in prop efficiency vs. speed and rpm plus airframe drag differences. I was more interested in ballparks using Van's or CAFE published data.

[SteinAir]

Originally Posted by AlexPeterson

6A, 180 FI/EI/CS, 158 to 162 ktas, 7.2 to 7.5 gph, 22"/2300, 7500 to 9500'. Same settings lower, probably lose around 3 to 5 knots. Been cruising with those settings for 1100 hours. It is probably around peak to 50F LOP, depending on conditions. Sometimes I run WOT at 7500', but not often. I set the throttle and rpm to those settings, then adjust for fuel flow. Rarely look at egt's any more (I do the fuel flow vs egt series every year or so to keep an eye on mixture distribution). CHT's are powerful indicator of mixture settings, adjusted for OAT. If I see cht's in the above 350 range with OAT of 70F or so, I dial out the mixture. Lose a few knots speed, but cools things down in to the 330 range right away.

Ha! That's funny....because when people ask me what my RV6 cruises at, I often say: "about 7.5 gph". Sometimes that's 160 kts, sometimes less, sometimes more, but in general my numbers are almost spot on with Alex. I too end up cruising around based on fuel flow settings and not so much speed. I usually shoot for 7.5 or 8 gph and whatever that gives me for speed is what it is!

I rarely spend time putting down at the 135 range, but I'll have to do that and see what it ends up like. I know Pete has some pretty good data too with his 9, I'll have to ask a few other 9 guys where they are at while going slow.

My 2 cents as usual,
Stein.

[hevansrv7a]

Originally Posted by rv6ejguy

This would be approximate of course due to variations in prop efficiency vs. speed and rpm plus airframe drag differences. I was more interested in ballparks using Van's or CAFE published data.

My 7A has its speed for best L/D at 96 kts=110.5 mph whereas the CAFE 6A had its best L/D at 106 mph. The 6A had drag at that speed of approx. 134 pounds. My drag at 110.5 is approx. 133 pounds as close as I can estimate it. That is using .40 for SFC and .85 for propulsive efficiency. These are at comparable weights or with mine a little lighter. CAFE's 6A was at 1650#.

I have not completed proving this yet, but I believe that my speed for best L/D is directly comparable to the 6A's, method to method. CAFE used the Zero Thrust technique. I used a number of alternative techniques designed to emulate the result. If you don't get the L/D right then the curve won't be right.

I won't take up the space, here, to show why .40 is the correct result for my airplane, but I believe it is. Kevin Horton thinks it is not that good - we have discussed it - so his data and calculations would say it is .4125 or thereabouts but that would mean my propulsive efficiency would be even higher since my flight data is solid or even conservative. Superior's data says I can do it. GAMI says I can do it. Lycoming not so much. Mine is a Superior XP IO-360+ 180 HP. No matter how you slice it, the individual components (drag curve, SFC, propulsive %) must fit together such that the combination produces the correct flight data (TAS, fuel flow).

My 7A goes 160 kts tas on 8 gph at 8000' with IFR calibrated instruments and the TAS verified with GPS.

I hope this is what you were looking for. If not, come back for more.