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  Last Update: 12 Mar 2010

- Additional Conversion Considerations -

More Critical Issues to be Handled

In addition to the installation weight (covered HERE), there are several other important criteria which should be considered during the design phase of an engine conversion. Although many of these considerations may seem to apply only to certified installations, it is prudent to keep in mind that they apply equally well to experimental installations.

In the experimental world, the FAA isn't looking over your shoulder to assure compliance, but keep in mind that the laws of physics tend to be quite unforgiving, and are not the least bit dependent on whether or not a project is certified by the Government.

  1. When designing an engine retrofit, it should be a high priority target that the empty CG of the aircraft with the new powerplant is very close to that of the original aircraft. That simplifies the certification structural issues, and helps to preserve the flying qualities with which the airplane was certified.
  2. The engine mount structure should be designed and tested to meet FAR-Part 23 specified criteria for all loading conditions. In addition, it is prudent to consider whirl mode effects, even though that analysis is not required for piston engine installations.
  3. Even if the weight of the retrofit installation is similar to that of the original, there is still the potential issue of greater overhung moment due to the typically-greater length of the liquid-cooled installation. If there is a substantial increase in the overhung moment of the installation, it will be necessary to analyze (and perhaps strengthen) the aircraft structures to which the engine mount is attached.
  4. One of the most challenging aspects of the retrofit of a liquid-cooled powerplant is to achieve suitable cooling capacity to meet the FAR hot day climb performance requirements, as well as the necessary margins to assure cooling at high altitudes and high power settings.
          There are typically two heat exchangers to be considered: engine coolant and oil. If the engine is supercharged, a third heat exchanger ( the aftercooler, aka "intercooler") will probably be necessary.
          The inlet and exhaust airflow ductwork for these heat exchangers will require skilled design and implementation work, especially if good pressure recovery and low cooling drag are important. Sometimes the cooling drag can be reduced by the use of an oil-to-coolant heat exchanger instead of another air-to-liquid heat exchanger.
  5. If the conversion includes a turbocharger, there will be significant challenges with the location and insulation of the turbocharger, wastegate, controller, and exhaust plumbing. There must be well-designed accommodations for exhaust plumbing flexibility to allow for the differential expansion of components in direct contact with the exhaust gas. If the turbocharger is not mounted directly on the engine, a separate mount attachment and suitable support for the g-loads and gyroscopic loads must be incorporated into the mount structure.
  6. If the location of the turbocharger does not provide sufficient vertical height above the oil sump, then a scavenge pump will be needed. Special provisions are necessary to prevent the scavenge pump from overpowering the turbocharger internal seals and sucking large amounts of oil into the compressor, and of course, then onward to the engine.
  7. Many fuel, oil and coolant plumbing components such as lines and hoses (fireproof for oil and fuel), fillers, expansion tanks, filters, air-oil separators, tanks, etc will need to be designed, drawn, fabricated and tested.
  8. There will be a design and fabrication effort required to implement a suitable alternate air system for the converted aircraft.
  9. New cowlings must be designed, fabricated and tested. Mathematical Lofting and Computational Fluid Dynamics (CFD) capabilities will aid significantly in designing low-drag cowlings with efficient diffuser and nozzle ducts for the ingress and egress of cooling airflow. Cooling air must also be provided under-cowl for accessories such as dry vacuum pumps and alternators.

BOTTOM LINE: An engine swap on an aircraft is a complex effort, and necessarily involves alterations to several major systems, including the structural characteristics and various resonant frequencies on the aircraft.

If it is intended to certify (STC) the retrofit, especially if the retrofit is on a multi-engine aircraft, be prepared (both technically and financially) for the FAA to require, as part of the STC program, a ground vibration test (GVT) on the conformed prototype.

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