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A seaplane cannot use brakes to slow down, like a land-based aircraft. So how does one slow it down?

I am asking this in the context of landing, taxiing, aborted takeoff etc., as long as the plane is on the surface of water.

I imagine a few possibilities:

  • Land on a really long "strip" to ensure that water friction slows the aircraft down enough, then remember to not taxi too fast because you cannot slow down
  • If the plane has a variable pitch propeller, probably you can go to beta range to produce reverse thrust (what to do if the plane has a fixed pitch propeller?)
  • Steer left and right repeatedly (but wouldn't this cause stress on the airframe / chance to flip the plane?)
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    $\begingroup$ The very fact that the seaplane is on water does an astoundingly good job of slowing it down. Sometimes too good. (yes, okay, that one was pilot error) $\endgroup$ – Greg Hewgill Nov 1 '17 at 0:16
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    $\begingroup$ If slowing down quickly is a necessity, wouldn't it be a simple matter to design in "water brakes", along the same lines as air brakes? $\endgroup$ – jamesqf Nov 1 '17 at 3:01
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    $\begingroup$ Related, if not a duplicate: How do seaplanes do run-up checks without brakes? $\endgroup$ – Ron Beyer Nov 1 '17 at 3:39
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    $\begingroup$ Water is about 8 times more viscous than air. Granted, there is less aircraft in contact with the water on landing than there is with air when flying, but it does a great job of acting as a brake. To see this in action, head out to a popular water skiing lake during the summer and watch how quickly ski boats stop when the skier falls - boats don't have brakes. Boats can reverse direction on the prop, but there's no need to except when backing away from the dock. $\endgroup$ – FreeMan Nov 1 '17 at 12:04
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    $\begingroup$ @FreeMan viscosity is neglectable at the relevant Reynolds numbers; it's the higher density that makes water a so much better brake. And the density is not merely 8 times higher... $\endgroup$ – leftaroundabout Nov 1 '17 at 20:37
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As already mentioned, decelerating from flying speed to a slow taxi speed occurs naturally at a much higher rate than when touching wheels down on a runway. But there are tricks to minimize forward motion with the engine running when that matters (e.g. approaching a dock). One method is to switch off one magneto, which decreases RPM and thus thrust (if you use this make it a habit to verify both mags are on right before EVERY takeoff). Another is to head into the wind as much as possible and for maximum effect open one or both doors fully.

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Unless you're planning on trucking a seaplane out of where you have landed it, you usually don't need to worry about slowing down on landing since you can land in a far shorter distance than you can take off. In other words, the landing water run is far shorter than the takeoff water run.

If you do want to shorten the water run or reduce your taxi speed, holding up elevator will do that. A few seconds after touching the water and at idle power, you can use full up elevator without fear of the aircraft lifting off again, and it will add the drag of the elevator plus change the attitude of the floats or hull to a high-drag, plowing attitude. I used to do that if where I wanted to dock was aft of the touchdown point and I wanted to quickly slow to a safe turning speed.

Most constant speed props (variable pitch) on reciprocating engines don't have a reversible pitch or beta, but that can be purchased as a modification on some. The major use of such is as an aid in docking, and for that it's really useful.

Steering left and right repeatedly doesn't impress me as a very good idea, but I suppose it would help a little, but very little since it wouldn't be advisable to do it until you're well off the step.

Deploying the water rudder adds a little additional drag. Depending on the wind conditions, putting in a little rudder to match cross-controlled ailerons also adds drag, but that's not generally used to slow down as I remember. It is used, though, when sailing the aircraft, for which it's standard procedure.

See also Chapter 4 of the FAA Seaplane Handbook.

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Your first flight in a seaplane will impress you as to how much drag is offered by the floats / hull during water operations.

In general, seaplane pilots have three modalities of water operations.

  • Idling position. In this position, the floats / hull have a similar displacement as when the plane is at rest in the water. Low power is used, as on many planes it is possible kick up water which substantially erodes propellers.
  • Ploughing position. The aircraft is nose high, and the floats have less displacement than at rest, causing less drag. Generally ploughing is utilized when on rough water, as it helps keep the propeller out of water spray, reducing erosion and probability of damage.
  • Planing or on the step. The aircraft is nose high, and the speed is greater, permitting take off. Water rudders are retracted, and as float/hull displacement becomes less, the drag from water displacement and surface tension becomes less.

When landing, water displacement causes substantial drag and there is no need for brakes as might be needed on a land plane with a hard surface runway.

Beta is available on some seaplanes, but they are normally larger planes, which sit higher, and most frequently have turbine engines. Even in a Cessna Caravan with floats, use of beta on the water is discouraged as it can increase prop erosion. Again, it is not necessary for slowing the plane in normal operations.

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A floatplane landing on water will experience much more drag than a normal plane on a runway as the water wraps around the underside of the floats, due to its surface-tension. This, combined with the vastly higher area of contact between the float and the water (which is much more than the area of contact between a tyre and a solid runway), means that the drag experienced by floatplanes skimming around on water is several times higher than the drag experienced by planes that land on a solid strip of asphalt.

In order to slow down the airplane even faster, you can perform aerodynamic braking, by using your elevators to disrupt the flow of air and hence increase drag in a controlled manner to slow the airplane down. Deploying water rudders also increases the drag to a noticeable effect.

Please do comment if you find any discrepancies in my answer.

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