RV-6: Performance figures at 8000 feet

Leonard_Smith_nz · #1 12-05-2006, 01:04 AM

All,

Went up over the weekend to do some high speed runs at 8000 feet with aircraft at MAUW. Ground temperatures 15 degrees C (62F) and QNH 1008. Did 4 runs on N, E, S, W with full throttle and mixture full rich.

For all runs noted figures in the order of: MP 22.2, engine RPM 2630, fuel flow 57 liters/hour and IAS 144 knots. Once back on terra firma I entered the GPS ground speed data into Doug Gray's spreadsheet to calculate TAS and came up with 164 knots (188 mph).

My aircraft is fitted with all fairings and spats. It has an empty weight of 1081 lbs and has flown pretty straight from day one. While these figures are a little below Vans numbers I'm happy with my aircraft's performance.

A few points that I wondering about:
1. If this test scenario sees my TMX O-360 producing 75% power how agressively can I lean the motor (if at all). I recall a Lycoming directive to not lean at 75% or above.

2. As various experts on this list have noted, the fuel flow is indicative of the power produced. 57 liters/hours is EXTREMELY high for 75% power. I see these sort of numbers when I run her flat out at 1500ft...and there I get nearly 170 knots TAS

Just wondering out loud...and learning as I go. Any observations appreciated.

wv4i · #2 12-05-2006, 04:39 AM

At 8k', I see 160-165ktas, 7.5 gph fuel flow, and about 23-24" WOT, with best RPM about 2350. These are avg numbers at various weights, winter/summer, seen during 3 trips to OSH fm South FL, local flying, etc.. My airplane is 1014lbs EOW, and has a more fwd empty CG than most. More aft CG and colder air make airplane go a little faster of course.

Link McGarity
RV6/N42GF/flying, for sale actually
RV10/N41GF rsvd/tailcone

ChiefPilot · #3 12-05-2006, 06:23 AM

Quote:

Originally Posted by Leonard_Smith_nz

All,
For all runs noted figures in the order of: MP 22.2, engine RPM 2630, fuel flow 57 liters/hour and IAS 144 knots. Once back on terra firma I entered the GPS ground speed data into Doug Gray's spreadsheet to calculate TAS and came up with 164 knots (188 mph).

Doesn't 57 liters/hour work out to something like 15 gallons per hour? That sounds awfully high for fuel burn, especially cruising at 75% @ 8000.

yakdriver · #4 12-05-2006, 06:54 AM

Running it full rich at 8000' is probably taking away around 15% of your power. Lean it out and you will see a marked improvement in performance. Don

db1yg · #5 12-05-2006, 07:27 AM

Leonard,

Lycoming says not to lean if above 75% power. At 8K' density alt your engine is probably putting out around 75% unless you have a significant pressure recovery from a ram air system.

Lean the engine to around 125 degrees F rich of peak--this should give you best power (vs best economy) and you will see a significant improvement in performance. Some fp prop pilots have offered me another approach, which I have not tried, and that is to lean to peak rpm on aircraft with fp props (only in smooth air) which should also come out to best power. Anyone out there who have used this approach please chim in.

Either way you are going to see more speed.

Cheers,

db

ps. Don't forget to close your cabin vents--it does make a small difference.

Leonard_Smith_nz · #6 03-19-2007, 03:17 AM

All,

Took the 'girl' out for a run last week to put into practice some of the advice given after my last 8000 ft test run. Figures are shown in the graph below....FF in liters/hour, EGT and CHT in degrees C and speeds are in knots

db1yg · #7 03-19-2007, 03:46 AM

Leonard,

Congratulations--nice performance improvement!!

BTW, which method of leaning did you use--by the egt or by rpm/airspeed?

Cheers,

db

wv4i · #8 03-19-2007, 05:19 AM

FF's, EGTs look about right. With a CS prop, you'd probably see 1 to 1.5 gph less fuel flow at cruise, roughly same speed. And, the takeoff and climb performance would be significantly improved as well. I went from a FP Sensenich to Hartzell 2 blade CS prop and so noted on my fmr O-320 equipped RV6.

Ron Lee · #9 03-19-2007, 08:34 AM

Also ensure that you are flying at 8000' DENSITY ALTITUDE. Definitely lean. One way is to lean until the engines runs a little rough then enrichen until smooth. Obviously monitor EGT if you have it.

az_gila · #10 03-19-2007, 10:58 AM

If I'm reading the small detailed Lycoming Chart for the O-360-A engines in the Operator's Manual, it appears that the figures you are giving above are operating a little above 75% .. perhaps at 80% of rated power - I wish Lycoming would publish their graphs on larger paper...

If you want to regularly operate in this area, perhaps you should either climb a little higher, or use a few less RPMs, and get below the 75% power number?

Nice figures and performance.... similar set-up to my RV-6A...

gil in Tucson