I got a Chart in my plane that shows fuel burn at different settings for the Avcon Conversion. I flew down to St George last week so I thought I would check it out. At 9,000 ft, 17.4 MP & 2400 RPM it shows 7.4 gal/hr fuel burn. I flew pretty close to those settings for 2.75 hrs (got sidetracked) and then told the kid in St George to fill it all the way to the top when I left. It took 18.1 gallons so it worked out to be 6.58 gal/hr.

I wasn't setting any speed records (100 mph indicated/ 85 kts ground speed). I don't know if that is really the most economical settings or if I would do better going a little faster with more fuel burn. I was wondering what settings you guys use?

Jerry

If you're after economy, then generally speaking you'll want to fly slower and closer to the aircrafts best glide speed (L/D). This holds true in still air or with a tail wind component, but not if there's a significant headwind. Turning lower prop RPM will, depending on the prop, improve economy as well because as the blade tip speed gets closer to supersonic, effeciency drops considerably.

e.g., my Husky, with a 205 mm MT prop, is happiest at around 1900 rpm, 20" manifold and just under 6 gph. That nets 105 knots ground speed. Pushing the knobs forward gains another 15 knots and burns almost twice the fuel. BTW, this is a compromise setting, as I would almost never cruise at best L/D unless I was worried about remaining fuel. I'm willing to trade a little more fuel burn for speed, but this trade is not linear.

Unless trying to break an indurance record, most pilots with a draggy airplane will pick a cruise speed higher than best L/D and lower than max cruise depending on wallet and how time urgent they are.

bumper

Thanks Bumper, I'm usually not in any hurry when I'm flying. I am stuck with using over 2250 RPM because of the Avcon conversion. I don't know why the chart lists fuel burn down to 2100 RPM when it has a placard not to run below 2250?

The chart you are using is for the engine. It is the COMBINATION of the engine and prop that dictates the rpm limitation. And, you CAN run the engine BELOW that rpm range. If you want to give it a try, try a fairly low rpm (2000 or below) and 20 or so MP. If the engine seems happy there (smooth), see what it gives you for fuel flows.

Finally, the MOST unreliable instrument in almost any cockpit is the tachometer. Virtually EVERY mechanical tach I've checked is anywhere from 50 to 200 rpm off. That sucks and is why I switched to an electronic tach. You can probably borrow a digital tach checker from your mechanic, and most likely, you'll find a bit of an error if you have a mechanical tach. Correct for that error when comparing fuel flows to the book figures.

MTV

When you are flying by the hour, I agree with those low settings. My scout when flying with super cubs and everyone is heavy, I burn in the middle of the pack of cubs, high 6's to mid 7's.

But when flying by the mile, I say push every thing forward. I get paid by the mile, not the hour, and it seems that I burn about the same as going slow or fast on the same course, and I end up with an extra hour of free time at the end of the day, and less time on the airplane.

MTV, your right about the tach. When I had the prop balanced we found if the tach shows 2300, the prop was actually running 2440. That's where he set it up.

I haven't run it under 2000 since the prop was balanced but it used to shake pretty bad running it that slow. Wasn't sure how low of RPM's I could run since my Avcon chart only goes down to 2100 RPM. I'll try it out.

patrol guy wrote:When you are flying by the hour, I agree with those low settings. My scout when flying with super cubs and everyone is heavy, I burn in the middle of the pack of cubs, high 6's to mid 7's.

But when flying by the mile, I say push every thing forward. I get paid by the mile, not the hour, and it seems that I burn about the same as going slow or fast on the same course, and I end up with an extra hour of free time at the end of the day, and less time on the airplane.

What Patrol Guy said.

I commute weekly to my client's place of business, and I don't get paid for travel time. Time is money, plus my time is valuable to me on a personal level (there's never enough). I cruise my Cherokee's O-360 at about 70% power all the time, and that yields about 8.3 gpm at about 125 knot TAS. If I cut back to a power level that would burn only 6 gpm, I'd save two gallons an hour but it would cost me about 15-20 knots, adding about half an hour each way on my weekly commute - or an hour r/t a week.

For personal flying, sightseeing, etc., where the flying IS the destination, i.e., where time isn't money, then cutting way back on the power might make sense. Depends on what your flying is for.

Jaerl

"I am stuck with using over 2250 RPM because of the Avcon conversion."

I do not know what prop you are using but most of the O-360 conversions, with a constant speed prop that I have seen, including my original 76" Hartzell, actually had a restricted RANGE of something like 1850 to 2250. Was later told that it is OK in that range for short pattern work just do not cruise in that range. I went with the 80" Hartzell and the DAMPENER. That opened up the RPM range BUT with a lot more weight on the nose. Now have an MT prop. Lost 29 Lbs. off the nose - NO RPM restrictions - alleged "forever" hub etc. Love it so far.
Can actually flare at and below 50 without having to carry the usual 1,500 RPM down to the runway.

Might call Harry Delicker at Delair in Porterville Ca. He has a similar conversion and knows more about these things than anyone I know of. If you do call, be prepared for a rather lengthy education.

Best to ya

Chris C

I have the Hartzel HC-C2YR-1BF prop with the 100 hr AD. The restricted Range for mine is 2000 - 2250. It was the original one from the STC in the 80's. I would love to buy a MT prop but for right now it's hard finding money for Gas.

I did the same flight yesterday at ran 2400 and 20 MP. The chart lists 8.8 GPH and I got 8.42. It is like Patrol Guy said, I went faster and the trip was 35 minutes shorter and only took one tenth of a gallon more. There was a little less head wind but I was really surprised that the fuel was so close.

I did try running at 1900 RPM and 20 MP for a few minutes to see how it felt. It actually felt smooth now. I want to check out a few more settings from the chart and then I will do a run at the lower RPM. I am surprised how close the figures on the chart are to real life. Usually manifacturers seem to be using "Wishful Thinking" to come up with their performance figures.

Jerry

Theoretically, when presented with a choice of combinations of RPM and MP settings to achieve a desired %BHP, choosing one with the lowest RPM will be the most efficient.

IOW once one gets over the whole oversquare myth running high MP and low RPM - within the limitations of the chart - will achieve better fuel efficiency.