John C
Well Known Member
The low fuel pressure on takeoff appears to be a design issue related to cavitation at the engine driven fuel pump. Controlling the pressure at the engine driven fuel pump appears to be the most direct fix.
The problem is not consistent. At times, the pressure at takeoff does not drop below 4-4.5 psi. At other times, the pressure can drop into the low 2s or even near 1 psi.
Often, I can lower the nose and pull back to 4000 rpm, see a pressure recovery, power up and continue the climb.
The two solutions offered prior were to use a blend of 100LL or just accept the low pressure if there is no engine falter. To me, neither is acceptable. I prefer, and the engine was designed for mo-gas.
Hopefully, the slightly higher rated Facet pump will boost the pressure at the engine driven pump to prevent the pump cavitation, if that is the issue.
The problem may only appear on the RV-12, at least according to two of the Rotax dealers and information from user?s groups or operators of other designs. Thus, it would appear to be a design issue related to the RV-12.
PRESSURE DROP IN SYSTEM:
There are several contributors to the lessening of the pressure at the engine driven pump.
Static pressure with the Facet pump: 3 psi
Pressure with engine and Facet pump running: 5.5 psi
Pull the c/b: 5.2 psi
At higher rpms such as runup: 3.5-4.5 psi, sometimes drops to 3 psi, rarely to 2 psi.
At takeoff, the pressure often drops to 1-2 psi.
Van's, Rotax and others suggested instrumentation or installation issues as the source of the indicated or actual low pressure problem.
1. Bad engine driven fuel pump: changed engine driven fuel pump, same problem. Result, both engine driven fuel pumps are ok.
2. Unusual blockages restricting flow: Using the Facet pump only, the static pressure was 3.0 psi. The pressure transducer is on the right forward side of the firewall, near the top. Flow was 33 gph at the engine driven fuel pump; 20 gph at the fuel pressure transducer; and 1.4 gph at the end of the fuel return line (you should also see 1.5 gph flow on the Skyview.). I did not check the flow to each carburetor as they only need 2.5 gph to each. Result, no unusual blockages.
3. Bad fuel pressure sensor: Static pressure about 3 psi and drops to 1-2 psi on takeoff. A new sensor was added and the results were the same. Dynon pressure was compared to a mechanical gauge on the ground, at takeoff and in flight (see later post for mechanical installation). The mechanical gauge was consistently about 0.1-0.2 psi less than the Dynon sensor. Result: Dynon sensor is accurate to demonstrate that the large pressure drop was real.
The system design also contributes to the pressure loss. It appears that the typical overall system loss is about 1.5 to 2.0 psi attributed to increasing fuel flow when and the rpm is increased from idle to 4000-5000 rpm (see above).
4. Contributing to the overall loss:
Long fuel line runs: 140 inches of 3/8 tube. The Facet pump in the middle confuses the issue for me.
Numerous components/fittings: 4 components and 10 fittings
Six 90-degree bends (plus two 45-degree bends)
5. Column pressure due to column height from tank to engine driven pump: (this is included in the overall system loss)
from full tank: 8 inches or 0.2 psi
from empty tank: 21 inches or 0.5 psi
Takeoff dynamics further contribute to the pressure loss.
6. Increasing column height due to pitching up is added to the overall system loss.
10 degrees pitch at 72 inches: 12.5-inch column increase or 0.3 psi
7. longitudinal acceleration adds to the overall system loss:
0.2 Gs longitudinal acceleration calculated from change in ground speed. With a 90-inch horizontal column, the 0.2 G longitudinal acceleration = 18-inch column increase or 0.45 psi pressure loss during acceleraton and/or initial climb.
The total loss is about 2.5-3.0 psi at takeoff. The portion of pressure loss due to the takeoff appears to be about 0.7-0.8 psi and may be the reason that the problem typically appears during the takeoff and initial climb phase of flight. The pressure drop at takeoff lessens the pressure at the engine driven fuel pump. Perhaps that is the primary trigger for cavitation at the engine driven fuel pump.
The higher rated Facet pump may fully offset the takeoff effects.
The problem is not consistent. At times, the pressure at takeoff does not drop below 4-4.5 psi. At other times, the pressure can drop into the low 2s or even near 1 psi.
Often, I can lower the nose and pull back to 4000 rpm, see a pressure recovery, power up and continue the climb.
The two solutions offered prior were to use a blend of 100LL or just accept the low pressure if there is no engine falter. To me, neither is acceptable. I prefer, and the engine was designed for mo-gas.
Hopefully, the slightly higher rated Facet pump will boost the pressure at the engine driven pump to prevent the pump cavitation, if that is the issue.
The problem may only appear on the RV-12, at least according to two of the Rotax dealers and information from user?s groups or operators of other designs. Thus, it would appear to be a design issue related to the RV-12.
PRESSURE DROP IN SYSTEM:
There are several contributors to the lessening of the pressure at the engine driven pump.
Static pressure with the Facet pump: 3 psi
Pressure with engine and Facet pump running: 5.5 psi
Pull the c/b: 5.2 psi
At higher rpms such as runup: 3.5-4.5 psi, sometimes drops to 3 psi, rarely to 2 psi.
At takeoff, the pressure often drops to 1-2 psi.
Van's, Rotax and others suggested instrumentation or installation issues as the source of the indicated or actual low pressure problem.
1. Bad engine driven fuel pump: changed engine driven fuel pump, same problem. Result, both engine driven fuel pumps are ok.
2. Unusual blockages restricting flow: Using the Facet pump only, the static pressure was 3.0 psi. The pressure transducer is on the right forward side of the firewall, near the top. Flow was 33 gph at the engine driven fuel pump; 20 gph at the fuel pressure transducer; and 1.4 gph at the end of the fuel return line (you should also see 1.5 gph flow on the Skyview.). I did not check the flow to each carburetor as they only need 2.5 gph to each. Result, no unusual blockages.
3. Bad fuel pressure sensor: Static pressure about 3 psi and drops to 1-2 psi on takeoff. A new sensor was added and the results were the same. Dynon pressure was compared to a mechanical gauge on the ground, at takeoff and in flight (see later post for mechanical installation). The mechanical gauge was consistently about 0.1-0.2 psi less than the Dynon sensor. Result: Dynon sensor is accurate to demonstrate that the large pressure drop was real.
The system design also contributes to the pressure loss. It appears that the typical overall system loss is about 1.5 to 2.0 psi attributed to increasing fuel flow when and the rpm is increased from idle to 4000-5000 rpm (see above).
4. Contributing to the overall loss:
Long fuel line runs: 140 inches of 3/8 tube. The Facet pump in the middle confuses the issue for me.
Numerous components/fittings: 4 components and 10 fittings
Six 90-degree bends (plus two 45-degree bends)
5. Column pressure due to column height from tank to engine driven pump: (this is included in the overall system loss)
from full tank: 8 inches or 0.2 psi
from empty tank: 21 inches or 0.5 psi
Takeoff dynamics further contribute to the pressure loss.
6. Increasing column height due to pitching up is added to the overall system loss.
10 degrees pitch at 72 inches: 12.5-inch column increase or 0.3 psi
7. longitudinal acceleration adds to the overall system loss:
0.2 Gs longitudinal acceleration calculated from change in ground speed. With a 90-inch horizontal column, the 0.2 G longitudinal acceleration = 18-inch column increase or 0.45 psi pressure loss during acceleraton and/or initial climb.
The total loss is about 2.5-3.0 psi at takeoff. The portion of pressure loss due to the takeoff appears to be about 0.7-0.8 psi and may be the reason that the problem typically appears during the takeoff and initial climb phase of flight. The pressure drop at takeoff lessens the pressure at the engine driven fuel pump. Perhaps that is the primary trigger for cavitation at the engine driven fuel pump.
The higher rated Facet pump may fully offset the takeoff effects.