a big NO to all of that, really.
The worst thing you can do to these engines is to shock cool them. Keep the CHT between 150 and 210 degrees at all times. If you approach in cold and moist weather they will cool off rapidly if you open the cowls. At low power settings they’ll need to be closed up to full. If your engine temp drops and you need to go around you may easily lose them completely. Beside all that, the cowl flaps aren’t effective anyway.
Never turn off an engine for the sake of slowing down!! It has four for the reason that they are unreliable, not to fix your mismanagement. It won’t help much anyway, you’ll feather a shut down engine, otherwise you break it.
If you ned to descend steeply there are way better and much more effective options:
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Push the RPM up. A faster spinning propeller acts as a very effective wall of drag. These are constant speed props, use them. Set the propellers to 2600 RPM and set the manifold pressure as low as possible according to engine temperature and BMEP. For the latter don’t go below 70 lbs/in² or you may hurt your bearings. Yes, all that is simulated.
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Use your undercarriage. Fly level, reduce to 174 knots or less and drop the gear and set flaps to 20. With good engine management this will give you a descent angle of up to 5-6 degrees which is steep! If you really have to you can even slow down into the white speed range on the ASI and drop the flaps to landing position. That’s more than sufficient drag.
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Last but not least: Be smart, use the time. Increase your flight time and fly a 360 or simply do left/right turns to increase the way for the given distance and all you need to do is wait until it comes down. If you planned right you’ll have plenty of fuel. There is no rush flying a DC6 anyway. =)
BTW… All that can be used for any aircraft, not just the DC-6. The propeller part won’t work with a 737 obviously but beside that all you have to deal with is energy and it can be decreased with many methods.