Cyclic:
The guy converting the Seneca is going for an
STC.
I would think diesels engine be heavier than the gas (Petrol for the Brits) engines and consequently one would run into center of gravity problems.
Last time I flew a Seneca II the owner had a
50 lbs piece of metal for ballast in the aft baggage compartment. He said the airplane was to nose-heavy with only pilot and co-pilot onboard. Perhaps even more so with heavy engines bolted on?
Good luck to the project anyway.
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Men, this is no drill...