N

N79PT

Active Member
I have started testing for a possible max-range flight circumnavigating the state (the 7 major islands, about 738 NM). The target performance I need to for this flight, the FAA, momma and the kids is at least 135 knots TAS and 6gph.

My RV-6A has an O-360 with fixed pitch prop, one Lightspeed ignition, and the Ellison Throttle Body Injected (TBI) carburetor. My usual cruising profile is 155-160 KTAS (~67%) at 7gph (30 deg LOP) at 10,000 ft. I have no engine monitor to calculated real-time FF or % power.

I ran a test yesterday at 9500MSL and 2300RPM on the full tank for 30 minutes trying to save some gas. Upon refueling and calculating TAS using Kevin Horton's GPS spreadsheet the results surprised me, 6.1 gph at 143 KTAS.

Strangely, during the test the engine wouldn't run smooth at 2300 RPM without the boost pump on. (I usually cruise at 2450 to 2600 RPM at this altitude with no problems.) At first I'm thinking that the engine driven pump may be having a bad day. But turning the pump off and adding power at altitude and the engine is smooth. The same smooth operation prevails at lower altitudes and lower power settings.

Engine temps were all standard for cruising at 9500MSL. OAT was a little warmer than usual at ISA +18 (usually +12 year-round). The DA on the Dynon was 11,730 ft. The engine was running at Peak EGT to 10 deg LOP.

Notes:
1. The Ellison TBI is susceptible to fuel vaporization as it has no carburetor bowl. Occasionally the engine will hesitate for a split second in cruise flight. Also with a hot engine running too long on the ground and the oil temp above 200 deg I have to keep RPM at 1000 to keep it running smoothly.
2. The TBI is very efficient and can run about 30 deg LOP, though it does require a little inflight adjustment to keep it lean but not rough.
3. Per Ellison the TBI requires 2-6 psi fuel pressure to operate.
4. Don't get me wrong, I think the Ellison is a fine unit. I wouldn't trade it for anything with rapid throttle response and good power with economy.

My next test is to try a higher altitude and higher RPM to try to find the sweet spot with a smooth engine. I understand the FP prop is most efficient for one RPM and altitude. My RV6 runs max speed about the same (180KTAS) at 1500MSL or 7500MSL, both at a max RPM of 2700RPM. The percent power is obviously different.

Any ideas out there on what's going on here?
 
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When you are at partial throttle the pressure in the cylinder after the exhaust stroke is near ambient. Then when the intake valve opens, there is a short burst of air from the cylinder into the intake duct which is at reduced pressure. If there is a 2:1 pressure difference across the intake valve when it is first opening, the gas velocity will be at M1 for a brief interval. That causes a pressure wave out toward the throttle valve. Since the throttle valve is somewhat shut, some of that wave will reflect from the TV and some will go on out to the inlet, where it reflects back. That is even more likely with the Ellison since it uses a slide plate perpendicular to the flow as opposed to an angled disc. It could be that the combo of inlet temperature which establishes the speed of sound, along with the rpm which establishes the periodicity of these waves, is having some kind of destructive interference at the Ellison's mixture tube. Just conjecture, but I have seen pictures of a standing wave of fuel-air mixture bulging out of carbs at certain MAP-RPM combos.
 
Interesting points. I don't quite understand how at this power setting there isn't enough fuel pressure from the engine driven pump, assuming the two conditions I mentioned earlier are related.
Greg
 
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Grigson said:
Interesting points. I don't quite understand how at this power setting there isn't enough fuel pressure from the engine driven pump, assuming the two conditions I mentioned earlier are related.
Greg
Click to expand...

Maybe a little vapor lock due to reduced fuel flow & high density altitude?
 
Yes to the 100LL. Also as a further note, the boost pump eliminates vapor lock hiccupping after a long taxi back to the hangar or after hot soaking from a post-flight refueling.

I never had much luck with insulating the gascolator bowl or the short metal fuel line on the hot side of the engine compartment. I just modified my ground procedures. It might be beneficial to look into these again for low FF at high altitudes.

Thanks for the inputs so far. Any ideas to quickly zero in on max range for a FP prop? I'm just sort of iteratively searching at altitudes and airspeeds to find the ideal mix.
 
Don't know about coming up with max range settings, but for your engine cough, do you have Carb heat? If you do, try cracking a little carb heat on, it can help vaporize the fuel once it's in the intake. If it's not vapor-lock, this could help. If it is vapor-lock, this probably won't. Going higher would make vapor lock worse.
 
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