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How does a pilot flying the scooping maneuver in a CL415 firefighting plane deal with the changing forces from taking on such a large mass of water (nearly 50% of the plane's own mass) as quickly as these planes do?

Some basic math indicates that the scooping maneuver would generate $F=\dot{m}v = 18.4\ \mathrm{kN}$ of additional backwards force just from accelerating the $\dot{m} = \frac{\Delta m}{\Delta t} = \frac{6140\ \mathrm{kg}}{12\ \mathrm{s}} = 512\ \mathrm{kg/s}$ of scooped water up to the plane's $v = 36\ \mathrm{m/s}$ scooping speed.

Or using a different calculation, with no additional thrust added to compensate, the plane would slow from 70 to about 50 knots from taking on the water. Plus, there's other forces generated here too — a downward force from accelerating the water vertically to lift it into the plane's tank, probably a pitch-down torque from the scoop placement, the additional weight from the changing mass, and also just the hydrodynamic drag forces that seaplanes also deal with landing or taking off.

Some of the control inputs seem clear, like needing to throttle up and probably pitch up slightly, but how does the pilot determine the adjustments to make here, and how do they coordinate the timing for making these adjustments?

Also, how does adjusting trim for the empty vs full states work? I'm imagining that there'd be some way to mark down separate trim settings for empty vs full and then use that to quickly switch to the appropriate trim after scooping or dumping the load, but is this done manually or is this automated in some way?

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  • $\begingroup$ I don't know how to fly a water bomber, but I can put your numbers in perspective: 18.4kN at 36m/s is 'only' about 900hp, which is less than 20% of the total installed power of the CL415. $\endgroup$
    – Sanchises
    Nov 8, 2021 at 15:31
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    $\begingroup$ The 13,500 lbs of water that is scooped is very near the center of gravity, and is is only about 1/3 of the 47,000 lbs total weight of the aircraft. The trim changes are handled by “seat of the pants” manual adjustments. $\endgroup$ Nov 8, 2021 at 15:55
  • $\begingroup$ @Sanchises Sure, and I have no doubt that the plane has enough power to handle it. This is more about how the pilot coordinates applying that power while transitioning from the scoop approach to making contact with the water. $\endgroup$ Nov 8, 2021 at 17:02

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I have experience on seaplanes, although not flying boats, and used to teach float ratings. The techniques for flying boats and floatplanes are more or less the same except for the differences due to the tip floats.

When you fly floats, you learn to apply up elevator to compensate for a pitch down from the drag of the floats at water contact (not too aggressively, or you pop back into the air; just enough). In fact, when landing on glassy water, which is normally done with a special procedure where you fly on very gently because you don't know your precise height, the pitch over is the signal that you have touched down.

Same with a flying boat. When you have scoops sticking down, it just makes the pitch over and deceleration stronger, and you have to apply more up elevator to compensate, but the timing and application is exactly the same as a normal landing, just more. On some waterbombers the intake probes are extended after stabilizing on the step at touchdown, so there two separate pitch reactions, but again, you are just responding to an action you are expecting.

You normally land seaplanes with power on until you've settled on the step after water contact, so the procedure if you're scooping water is to check the pitch over and add more power, enough to keep the plane from decelerating too much, and just hold that until you retract the scoops. Whatever the pitching tendency is from the scoops going down and up, you just learn to counteract it with control inputs intuitively, and similarly with power.

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    $\begingroup$ I used to fly PBY water bombers in the 1980s. After touchdown on the water, we would add a little to power to stabilize the aircraft on the step, and then lower the single probe, while adding full power. When the tanks were full we would raise the probe and let the aircraft accelerate and takeoff. $\endgroup$ Nov 8, 2021 at 21:00
  • $\begingroup$ My boss at the bush air service I flew at in the early 90s was an ex Canso (commonwealth name for the PBY) waterbomber pilot and he said it was like "waltzing with a gorilla" with the heavy controls, and his copilot did the job of providing aileron boost during heavy maneuvering. Were they that bad? I was pretty sure the 415s touch down with the probes out, but now I have to go check. $\endgroup$
    – John K
    Nov 8, 2021 at 22:10

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