For those of you who have switched to a climb prop or are running a ground adjustable prop. How do you compute fuel burn?

Finally ordered an 8042 for my 170. While I wait the several months for McCaulay to build it... I figured I would ask how to figure out the fuel burn with a climb prop. Is there some rule of thumb I can use? (ie follow the burn for RPM/Airspeed)

Orrr... I'm overthinking this.

Several factors involved. Fuel burn charts should be available for your motor (you did not say what it was). With a fixed pitch prop you are basically looking at RPM. HOWEVER! Proper leaning and altitude make a huge difference. We are only talking fuel burn per hour. Once you figure out fuel burn per hour then add in the mph/RPM factor to figure how far you can go at a given altitude/RPM/airspeed. Airspeed will change between aircraft due to several factors Prop/Tires/ect. So you can go off stock airframe and adjust as needed. Do you have a fuel flow and all cylinder EGT/CHT for proper leaning?
DENNY

SmokeyTheBear wrote:Orrr... I'm overthinking this.

Haha...the first thing I thought when i saw this was that I would wait for the closest day to standard conditions and dip the tanks (if not fuel flow instrument) then just go cruise the area at your favorite leaned cruise power/RPM for at least an hour.

I appreciate any techniques for calculating, especially modifier conditions. But there's a lot to be said for the empirical.

Denny: I've got a C-145. My understanding is that fuel burn is a function of load. At 2450 where I'm normally cruising I'm going to be going slower and while the engine is turning the same RPM it should in theory be under less load and thus burning less fuel. How much less is probably more a function of where I put the red knob than anything... But I figured there might be a rule of thumb for such things.

ZZZ: Topping the tanks, cruising for a couple hours, and refilling is certainly the plan.

One of the fun things about puttering around in a 70 year old airplane is that while everything is new to ME it is certainly well traveled ground. While its fun to figure things out the hard way its cool that so many folks on here have already done it. I'm sure there are a bunch of folks who have run an 8042 on a 170 on here.

I have a climb prop on my 172, O-300 with 7650 prop, it basically works out to 5 mph less than the numbers in the book for cruise speed. All attitudes, all throttle settings, 5 mph slow. Fuel burn per hour seems to be very close to what the owner's handbook says it should be.

Best mod to the O-300 I did. I really didnt notice any change in fuel burn, but I am very conservative with fuel planning. Cruise dropped about 7 knots to 92 with 26" Goodyears. Light, on a cold day with half tanks at sea level you will get a feel for why you really want an O-360.

O-300, 8042 Climb Prop. Engine Montitor and Fuel Flow Gauges.

I typicically cruise at 2550RPM. That is where my engine/prop is smoothest. I still plan for 7.5 gals per hour and regularly see that. RPM's may higher, but load on the engine is less. I'm not an engineer or engine expert, so I won't guess the exact science involved. The simple single-cell fighter pilot brain I am equipped with flight plans the same and see's the same basic fuel flow with both my 7555 Cruise Prop and 8042 Climb Prop. 7.5 GPH/100 MPH. PS. I utilized a Flight Engineer for much of my engine management. The Flight Engineer resembles a 69LB Black Lab Hunting Machine, but he knows when the RPM hits 1500RPM, cuz he wakes up and starts barking.

I have been flying a 1955 C170 with my friend Sonia for the last year. She had a 75 inch prop on previously that was a total dog in the climb. She switched to the 8042 and it is a real game changer for a C170 with a stock O300. She doesn't have an engine monitor with FF so hard to know the difference in FF but I think it is negligible.

I flew it in Idaho with her last summer and although we didn't have camping gear or heavy loads in it - we were flying full fuel at times and it performed really well. I like to geek out on this stuff too but I believe she is still seeing about 7.5 gph. Not sure how much she uses to get to altitude. It did result in a 3-4 knot cruise speed loss. Rate of climb at sea level was improved significantly 200-250 fpm.


Josh

With a fixed pic prop at a given RPM leaning is what is going to have the most effect on the fuel burn. Easy + or - 2 gal per hr. But leaning is dependent on altitude. Without proper instrumentation I would do a ZZZ says, but don't screw with a dip stick. Fill tanks to top Go fly at favorite altitude and RPM, with your preferred leaning procedure. When you land refill to top of tank. Now you know exactly what the fuel burn per hour is at that mixture setting/RPM/altitude. Log it. Any time you are going to change from those settings do the same procedure and after a while you will have a good ideal what the fuel burn is in different operational settings. You could also look in the engine operations manual most have the charts all ready done.
DENNY

You guys rock! Thanks for the info!

This is a good thread for me to share a somewhat recent and relevant tangent re: moving to a finer pitch "climb" prop. It's been a few years so my memory is a little spotty for details.

When I got my 1956 PA-22 taildragger it had the Lyc O-320 150hp with Sensenich 7456 prop. I was impressed with the performance. Local Tri-Pacer guys commented that this was the sweet spot for props on the 150hp bird, but my personality defects soon saw me trying to improve the shortfield performance by going to a finer pitch.

I took it to NW Propeller, where they ask you not "what do you want this prop to do?" or "how quickly do you want to takeoff and cruise?" but rather "how much RPM do you want to add or subtract?"

Quick goat thinking... I asked for just +100 RPM because I knew that would get me to 2700 at Vy, which I assumed would be great for short takeoffs and climbouts. When it was all done (I got to watch the process and even shot a whole video on it) the prop was stamped a 52" pitch. By adding 100 RPM I had gone from 56" to 52", which was a scale I understood better. I was a little nervous.

The first test seemed to work great, but it was only a few laps around the pattern. The next week my buddy (and partner in the plane at the time) and I flew it 2 hours to have some maintenance done.

Cruising at 6000 MSL I was struggling to find a good cruise RPM. With the 56" I had been getting 125 mph at about 2450 with 8.50 tires on. Now, I was at 2600 RPM struggling to maintain altitude and fighting the trim. The prop just needed to spin faster to "keep it on step" and make the tail effective. IAS for more like 95ish now. This is on 31" ABW.

Some other thoughts: Many people I know just firewall the throttle in cruise and go high enough to lean to an efficient fuel burn. I'm not sure that's the best idea for a draggy airplane with big bushwheels. Maybe it doesn't matter. Looking back, the 56" pitch did fine on the ground and it cruised great. Should have stuck with it.

My new prop, which is a wood composite Sensenich fixed pitch 8038 will be an interesting comparison. 4" longer, different chord, but even finer pitch. Haven't run it yet. Fingers crossed.

TL;DR I added [much] larger tires and repitched the prop too fine for any kind of efficient cruise.

SmokeyTheBear wrote:.......... I'm overthinking this.

IMHO yes you are.
Why not just fly the airplane and keep track of your fuel burn & tach time over the course of a month or two.
Then do the math.
For example, in my C180 I burn about 11 gph on short local flights, running at about 65% power (22 squared),
and closer to 12 gph on longer flights (when I might run it a bit harder).

I stick my tanks after every fueling,
and write the tach time & fuel-on-board on a piece of tape stuck to the panel.
I am generally within a gallon of guessing how much fuel I burned on any given flight--
in other words, how much fuel I need to put in to get the fuel level up to where it was.