Handling of the engine
To convey the 274 bhp to the propeller, the Lycoming GO 480B1A6 has a reduction gearbox with a ratio of 120/77. This means that the propeller RPM is 64% of the engine RPM. At a take-off power setting of 3400 RPM the propeller turns at 2176 RPM .The disadvantage is that this engine with its system of gears, bearings and counterweights is highly vulnerable to power fluctuations.
For the pilot, this means that he should try to maintain a constant speed as much as possible and not apply too large power swings. If he does so, then the gears will wear out more quickly, which in turn may induce a resonance in various parts of the engine. This problem may occur when the manifold pressure gets below 0.5 atm. This happens especially in older engines that are near or over their maximum tolerances.
If the manifold pressure gets below 0.5 atm at an airspeed over 80 kts, then the propeller may drive the engine.
In the worst case, you may experience a strong resonance, counterweights can then break out of their bearings on the crankshaft, which will result in an acute failure of the engine, with all its consequences.
Many aircraft, equipped with these engines (Piaggios, DO27 's Republic Seabees, Helio Courier's) have encountered this problem, so this engine has gotten a bad reputation.
Lycoming has recognized this problem and issued special AD´s on this subject.
Seagull over the past 25 years has learned to deal with this problem and now flies without any resonance problems all the military manoevres from the years 1940 to 1960.
The pilot therefore never can throttle back below 0.5 atm. Only during the touchdown and below 80kts he is allowed to do this.
For formation flying, which Seagull practices intensively, this is a big problem, but also a huge challenge to do it right.
When a formation of aircraft equipped with "normal" engines (ie without the resonance problem) flies past, it is striking how often the engine sound varies from full to no load at speed and position changes.
When Seagull practises formation flying the pilots cannot throttle back below 0.5 atm. inlet pressure and also not easily give full throttle to stay in position. This results in the fact that once you´re too far out of position you cannot not quickly correct.
The only solution is to stay "close" in position, leaving only small power changes possible. One will not hear large power swings when we are flying!
Once you´ve learned to fly formation with Seagull then you´ve learned to do it very well!