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Fig. 1
Floatplane Taxi techniques
I have been flying floatplanes for several years, and I hang out a lot in the RCUniverse.com Seaplanes forum. I fly floatplanes whenever I get a chance.
Just about every time the topic of taxiing comes up, the pat answer comes in: “You need water rudders to be able to taxi, especially if it’s windy”.
Well, that IS conventional wisdom, but so was “The Earth is Flat” and “Man will never fly”. The truth is, you don’t have to have a water rudder or
2 water rudders, and bigger is not necessarily better. There IS more to steering a floatplane than leaning on the rudder stick.
Up to a point, water rudders are useful. Sometimes, a water rudder makes things a lot harder. Also, big, non-retractable water rudders add a lot of drag
and can really stretch a takeoff run and make the plane really squirrelly on the water at high speed. Regardless of the size of your water rudder,
it should not stick down behind the tail of the float any farther than the height of the step or at least be spring-loaded to kick up easily.
A seaplane in a crosswind acts more like a canoe than a sailboat. The wind pushes against the sides of the fuselage and tailfin, and the water pushes
back against the float(s). The float hulls do not have a keel like a sailboat to keep them from being blown sideways in the water,
so the wind will make them slide. You can’t just steer a seaplane on the water. You have to sail it. If you learn to sail it, you can taxi.
It is a question of balance. The EAA offers an excellent videotape showing float-flying techniques including taxiing. It is shot with full-scale Cubs,
you will learn a lot from it, and it’s just cooler than heck to watch anyway. The title is “The Wonderful World of Floats”
available online at www.eaa.org. for $24..99. Cheap at twice the price.
What happens when there’s a breeze? Obviously, the airplane part wants to weathervane into the wind. That is why airplanes are pretty successful
at keeping the noisy end in front. Here is where I use Big Words: The center of mass is in front of the Aerodynamic center of lateral area.
Back to small words: The tailfin tries to keep the nose pointed into the wind.
What about the floats? Well, the floats are pretty good at pointing into the wind, too, without a water rudder. Most of the weight is applied right in front of the step,
so the front part of the float shows more side area to the water than the tail of the float. If the wind tries to push the airplane across the surface of the water,
the floats help nose it back around into the wind.
“Well, there it is!”, you say. “That’s why you need a water rudder”. Sure, water rudders can help you overcome the weathervaning tendency in a brute-force sort of way.
Slap a pair of the biggest water rudders you can find on a Cub and watch what happens. On a calm day, it will steer around like a R/C car on a parking lot.
Even in a light breeze, the water rudders provide a lot of authority. This is probably where the rule of thumb came from about adding big honking water rudders.
But when the wind picks up a bit more, the plane will slide sideways faster than the rudders can overcome as it turns crosswind. The big water rudders
will get such a bite in the water, so far back, that the plane will pivot downwind around them. Then the plane will blow off downwind with you trying everything
you can think of to get it to pivot back around into the wind. Full-size seaplanes have retractable water rudders partially because of this.
They retract the water rudder, neutralize all the controls, and the plane snaps around to point into the wind.
Oh, and blipping the throttle to get some rudder authority? Go ahead and try it. Somebody will tell you to. It MAY work. If so: great.
Usually it just makes it noisier for you to taxi into the God-forsaken, swampy, snake-infested, briar patch. After you get the plane fished out of the GFSSIBP try these techniques.
So, let’s break it down into different scenarios:
1. Upwind taxiing.
Taxiing into the wind should be a no-brainer. At idle, neutral elevator, and no rudder deflection, the plane should weathervane into the wind.
In Fig. 1, the plane is idling. The floats are in full displacement mode.
The air forces act on a point (the little black square) aft of where the water’s forces on the floats (the little black ellipse).
Any crosswind hitting the plane will make it weathervane into the wind. . If your plane does not weathervane into the wind,
then your water rudders are probably too big, or you need to buy a retract servo to haul them out of the water.
Fig 2. Water rudders Fig 3: Lifting the water rudders.
With big water rudders as in Fig. 2, the plane will pivot around the water rudders and drift off downwind.
To compensate: try turning as if you had a taildragger with a fixed tailwheel: Kick rudder in the direction you want to go,
then lift the tail by using “down” elevator and a little power, and it should come around (maybe with a spray of water from the propeller).
See Fig 3. Cut some material from the bottom of the water rudder(s) when you get home.
It is really handy for the plane to weathervane into the wind, because it will align itself automatically for the takeoff run.
2. Downwind taxiing
Here’s where the tricks start to work: Watch your floatplane carefully when you start to accelerate. At idle, it just glides along slowly in full-displacement mode.
At high speed, it is up on the step with only a tiny part of the floats touching the water. But watch what happens as it transitions
from displacement to running on the step: As soon as it gets just above idle speed, it starts to push a pile of water, and tries to climb over it.
The rear goes down, the front goes up, and the motor has to provide enough thrust to lift it up over the bow wave. It can reach a surprisingly steep angle.
The actual technical term for this is "humping". Once the plane is over the hump, it's on the step.
Fig. 4, Downwind Taxiing
Here’s the good part: You can hold it like that with the throttle. The tails of the floats are in the water, the fronts of the floats are in the air.
The plane is still moving slowly, and the whole airplane/float assembly weathervanes downwind, or at least is moving fast enough
to neutralize the wind’s forces. The air rudder has ample authority with the air-rudder in the prop blast. So, to taxi downwind,
hold full up elevator, add just enough power, and she’ll answer the rudder and swing around to downwind. You probably will only need the rudder
to select whether the turn starts to the left or right. Neutralize the rudder and the plane will continue the turn until it is taxiing straight downwind.
Note: As it turns, hold the aileron stick to the outside (upwind) wing.
Fig 5: Crosswind taxiing
If you are paying attention this will have already come to you. Crosswind taxiing is halfway between upwind
and downwind.
Set up the plane for a downwind taxi with full up elevator and add a little less power than if you want to taxi straight downwind. Steer it around toward where you want to go using the rudder.
As you get crossways
to the wind, you can steer with the throttle. Add a click or 2 of power to turn
downwind. Back off a little and she’ll turn upwind.
Adding power raises the nose and moves the floats’ contact area back in the water. Reduce power to turn upwind, letting the fronts
of the floats grab a little so the tail-fin makes the nose swing upwind. The whole idea is to balance the point where the water pushes on the rudder
against the point where the wind pushes on the side of the fuselage. Use the rudder to help- it will probably still need to help you push toward the downwind..
If your motor is running, the prop blast over the air rudder gives it a lot of authority. If your motor is not running….it doesn’t matter. Get the retrieval boat.
A caution about taxiing crosswind: You can “cheat” and taxi close to the bank or other wind-blocking structure to minimize the windspeed
and avoid having the plane blown over. The ailerons do not work well when the wind hits them from wingtip-to-wingtip, but you should
always hold the upwind wing down, by moving the aileron stick to the upwind direction. If you are taxiing in a true crosswind path,
the plane will be at a crab angle sliding in the water, and the ailerons will have some bite.
3-channel planes with a lot of dihedral are particularly susceptible to blowing over. The wind will catch and lift the upwind wingtip,
and jam the downwind wingtip into the water. The drag on the submerged wingtip wrenches the airplane around downwind-tail up
and the wind carries the tail over. If it is normally windy where you fly, low wing planes and seaplanes with wingtip floats are more
resistant to blowovers. So on one end of the scale, the most likely to blow over are 3-channel, lightly-loaded planes like the
Craft Air Butterfly or the Clancy Lazy Bee, or a Kadet Senior without ailerons. On the other end of the scale, flying boats such as the ACE Seamaster or Lanier Mariner
are pretty solid in the breeze, with their wings flat and near the water, and the stabilizing floats out at the wingtips.
The BalsaUSA NorthStar is very resistant to blowover since it sits so low in the water and the crosswind almost can’t get under the wingtips
So now that you know all this, go taxi in circles around all your friends, and call ‘em wimps when they complain about the wind.
If they say they need bigger water rudders, give ‘em yours and drive circles around ‘em anyway.