It depends
Hi all...
If I put an O-320 FP in my RV-7A(to-be), what kind of fuel consumption / performance should I expect? Van's page only lists for a CS and suggests 6.7 gallons per hour at 150 knots 8000... Thanks, JCB
I think you meant 150 mph not knots.
For the Lyc 320 (160hp) generic max FF: 75% (120hp) = 10 gph, 65% (104hp) = 8.8 gph. To get down to 6.7 gph, about 70-75hp or about 45% power is simple, just reduce power and lean the heck out of it. Do that by flying higher (which is best if winds are favorable) or do it with the black knob. Yes you can fly at 45% power very nicely but probably not 150kts. Van actually list 171 mph, just shy of 150kts (172.5 mph) at 65% power, which is more the 6.7 gph, but not too much much.
From Vans RV-7A, w/ 160hp numbers:
Cruise [75% @ 8000 ft] 190 mph (solo); 189 mph (gross)
Cruise [65% @ 8000 ft] 171 mph (solo); 170 mph (gross)
150kts (172.5 mph) at 6.7 gph? From the above 65% is 8.8 gph! Well if you lean, use a lower RPM and are flying at 8,000 ft. FF can get down into the low 7's gph. So I am thinking 6.7 gph at 150 mph is very doable.
The bottom line keeps coming back to the black knob and red knob. Most pilots fly their RV at/above 65% power most of the time unless flying above about 13,000'. Also pilots don't lean early, often and enough sometimes.
The KEY to better speed for less fuel is lower drag as much as possible. Build it straight, light and clean, it will give you better MPG gas milage than most cars, easily in the 26 mpg or better range. Lower drag, prop efficiency, engine efficiency (electronic ignition, tuning, fuel injection). All these add a few percent better fuel econ here and there, LOP ops or not. Every few percent counts.
Constant speed props have a wide sweet spot in cruise, an ability to achive higher efficiency & thrust than a fixed prop, which has one sweet spot. Still many fixed props are optimized very nicely for a good efficient cruise, Sensenich being one of them. You lose control of RPM and take what you can get for that throttle setting. The ability to lower RPM in cruise is where the constant speed prop shines. For best engine/prop efficiency 2400-2500 rpm is about right. Most fixed props cruise at 2600-2700 rpm.
A compromise is called for with fixed pitch props. If you increase fixed pitch prop too much for a lower cruise RPM, you lose takeoff / climb performance. Sensenich has done a great job making that compromise work well. Their RV prop is customized and designed specifically for the RV. Fixed prop top speed has more to do with over revving the engine slightly than total efficiency. Go fast or burn less fuel, its up to you.
Keep in mind a O-320 at full throttle, sea level on take off is burning 16 gph! So for trip fuel, all that cruise fuel efficiency has to be averaged with your takeoff and climb. Good flight planing, climb, descent, arrival, approach and landing will save lots of fuel as well.
To brings it back to the pilot, most don't want to fly at 55% power much less 45% power. Most pilots don't want to fly at 150 mph much less 120 mph which is even better. You get use to doing 180 kts.
Some claim modest to significantly better SFC by running LOP (lean of peak). LOP usually almost means a well balanced FI system verses a Carb, which usually does not have sufficient fuel distribution precision to allow for smooth LOP operations. Some Carb guys have been able to run LOP. That is a whole other topic, but it can mean a few percent lower fuel burn. Even if you use "normal" lean technique and conservative power settings, you can achive greater fuel savings. Fly at say 55% power and lean to rough, enrichen enough to get smooth ops.
If you really want to save fly around at 110-120 mph! That is cessna speed but the fuel savings will be excellent. The most fuel efficient RV is the RV-9 with either the 235 or 320. If flown real slow you can get about 30 mpg (zero wind).
