Here’s the deal: flying is expensive. Avgas is expensive. Suppose I go flying in my ’74 Sundowner, and want to get the most flying (hours in the air ) for the least bux? What’s the cheapest speed to cruise at? My guess would be somewhat slower than Vy, but I don’t really know.
Of course, there are other costs to flying. Do cheap, slow cruise speeds cause higher engine wear? Do they make sparkplugs glop up? Wear out O-rings? Cause flat spots in the rollerized muffler bearings? Incur added expenses for psychoanalysis of bored pilots?
This is based on an August posting on MML; I thought it might be worth posting on BAC. There have been a number of extensive articles written on this, for both certified planes and homebuilts.
Maximum efficiency will occur very close to your best glide speed, with the engine power quite low to just barely maintain that trimmed speed. Lean as much as you can without excessive roughness. You want maximum “clean burn” conditions, with the highest CHT/EGT for the power setting (which won’t be very high anyway, at the very low power setting you’ll be using). During low power operations, keeping the internal combustion temps up through aggressive leaning helps the lead scavenger additive in the fuel burn all the lead out, despite the low power setting.
Keep the plane as light as you can keep it (no tools, oil, excess fuel, etc. getting hauled around), and loaded as far aft as the CG limit and practicality allow. One ROT is that every 100 pounds of lower takeoff weight will gain one knot in TAS. If you usually fly alone or with one other person, keeping the back seats out will save 28 pounds (seat weight may vary between airframes; check your W&B data). They usually go in and out with thumbscrews in a matter of minutes in our planes.
As long as you keep it very lean while doing this (low-power cruise), and as long as you run it at 70% or above during the climb, this should not cause any problems (aside from the much slower speed, if that’s an issue to you). Ordinarily the significantly lower cylinder pressures and low manifold pressure could reduce ring seating pressure, and leave more of an oil film on the cylinders. This is especially true during a descent, if the prop is driving the engine (rather than vice-versa), when manifold pressure is very low and tends to “suck” oil into the cylinders. If the engine was never operated at full throttle (takeoff), and higher power (70% or more during the climb), over a very long period there might be an increase in varnish in the cylinders. That won’t happen as long as there is some periodic higher-power operation. For the same reason, you don’t want to run low power settings for any extended periods while a fresh engine is being broken-in.
The higher you go to do this, the more efficient you will get during the cruise segment. You’ll get max efficiency, and highest true airspeed for the reduced power and best-glide IAS, if you go high enough for it to require full throttle to maintain an IAS of best glide (my best glide is 105 mph). This may well be up around 12,000 to 14,000 feet for the Sundowner (maybe more, as gear drag declines with altitude). In my Sierra with the constant-speed prop, the level-cruise IAS is still 105 MPH at 15,000 feet (full throttle, full RPM). Your engine power, with full throttle and full RPM, may be 50% or less at these altitudes. Leaning to peak EGT or slightly lean of peak EGT, at these altitudes and power settings, will show very low EGT, CHT, and fuel burn rates. You may even find that your engine can be leaned further than usual, on carbureted engines, without getting too rough. Several reasons for this, that I won’t get into here.
This won’t make sense on a hamburger run, but if you ever need a long extended cruise segment, when you have neutral or tailwinds, this is the way to do it. When you have a headwind, you have to run a higher airspeed and higher power, and it takes more math to figure out the best compromise. This is because if you fly slower, you are in the headwind longer, and burn more total fuel. You can play with this using some of the flight planning software out there. There are additional tricks for use with a Constant Speed prop, when you are high enough to still use full throttle with RPM in the green below the red arc. You can set up for cruise at full throttle, and pull the RPM back to 1,800 or 1,900 RPM, since the manifold pressure will be so low when you are up that high. When you combine full throttle (but very low MP), very low RPM, and lean-of-peak operation, you can get remarkably low fuel burns in a Sierra (and in a Bonanza; my friend saw less than 6 GPH at one time). We have done this during extended cruises out West, when we needed the altitude anyway due to terrain, and we had a tailwind to boot. I’m talking about 5 GPH or so, if I remember right. Haven’t had reason to do this in several years, so I can’t quote the exact lowest number. As a matter of general interest, the stock Bonanza (stock engine baffles) can’t do this for very long. Despite the very low power setting, the associated nose-high attitude causes too much air to bypass the cowl inlet (goes over the top rather than in); the CHT starts to go up. No problem for the Sierra, though.