Some Thoughts To Consider On The Lake Amphibian
Written by John Staber – Adverse yaw. The Lake, with its long wing and large aileron demonstrates one of the best examples of adverse yaw. If one tries to make a turn, whether airborne or “on the step”, with just the aileron, the result is a very uncoordinated turn with the nose of the aircraft initially turning the opposite way and then following if the turn is continued.
Consider this effect on the water. Let us say we want to make a right turn while step taxiing. We need to put the right float on the surface of the water with the aileron and when we do that, the nose goes to the left because of the adverse yaw, resulting in a very sloppy turn, if any turn at all.
However, the big rudder at the aft end of the aircraft is there for a reason! Several, in fact. In this case it is to compensate for aileron yaw. Add a little right rudder into the equation and we get a very coordinated turn, both in the air and on the water. In fact, while in the air a turn may be initiated with rudder alone even to a fairly steep bank.
Because of the dihedral built into the wings, and some nose up trim, the Lake will remain in the turn “hands-off” until it asked to stop the turn by using the rudder and a little bit of opposite aileron.
Picture the Lake in a tight step turn and the speed is getting a bit too fast and it feels like it is going to get away from you and you want to stop the turn. Use rudder in the opposite direction, not aileron. If you use aileron, the nose will go tighter into the turn, making it worse.
This same adverse yaw comes into play with a land crosswind landing. The proper procedure is to hold aileron into the wind to stop the drift and use the rudder to keep direction. The Lake is no different, but if one constantly relaxes the aileron, the result is the nose swinging back and forth with each movement of the aileron until the pilot doesn’t know what is happening, and should go around. The aileron pressure must be held constantly into the wind, and the rudder used to change direction and because of the aileron yaw and a strong wind, you might have to use some rudder into the wind also.
Hydraulic landing gear, flaps and trim. The system consists of a hydraulic fluid reservoir, and electric pump, a manual pump, an accumulator, a pressure gauge, a pressure switch, a pressure relief valve. It uses MIL spec 5606 fluid. There is a gear position selector, a flap position selector, and a trim selector. Each landing gear has an actuator at the gear to raise and lower them, as does the flap system and the trim system. The trim is actuated by a lever between front seats sending pressure to the actuator in the tail to raise and lower the trim tabs and is spring-loaded with a center-off position. Hold the lever in either direction until the desired “feel” is attained and let it return to the off position. The flaps are either full up or down and the selector is on the copilot side below the instrument panel with color-coded lights to indicate whether up or down. There are restrictors in the supply lines to the trim and flap actuators to slow the action to a manageable speed.
The landing gear selector is on the pilot’s side below the instrument panel and also has color-coded lights to show the gear position. The 3 landing gear have switches, wired in series, for both up and down. All 3 switches must be in the “closed” mode before the cockpit indicator lights work. There are no restrictors in the landing gear system since the actuators need lots of volume and pressure to operate quickly from one to the other position. Some owners have added restrictors in the nose gear circuit to slow down its operation speed and early Lakes had them from the factory. As a safety feature both the gear and flap selector handles must be pulled out of a detent before raising or lowering. From about 1969 models there are 5 positions to the gear and flap levers: up-bypass-off-bypass-down. Care must be taken that you have selected the proper position.
The pressure is built up by the electric pump which is turned on with the pressure switch when the pressure goes below about 700 pounds and shuts off around 1000-1100 pounds. Should the electric pump fail the manual hand pump is available. The pressure relief valve is set to bypass fluid back to the reservoir after 1500 pounds. The reservoir is located in the nose of most aircraft and the level should be checked before each flight. The level should be between the two little holes in the dipstick and should not be filled to the top as the level rises and falls as the pressure rises and falls, and it is possible to force fluid out of the vent during this process.
The accumulator is usually behind the panel and is the most misunderstood part of the system. In most cases it is a cylinder about 3 inches in diameter and about 8 inches long. One end is connected to the high pressure side of the aircraft hydraulic system. There is a moveable sealed plunger that moves back and forth across the length of the cylinder. At the other end there is an air valve that gets filled with high pressure nitrogen or air to about 350 pounds that pushes the plunger toward the fluid side. As the aircraft system pressure builds it now pushes the plunger toward the air side of the accumulator and compresses the air. With 1000 pounds of hydraulic pressure on the aircraft gauge, when the gear is selected to the opposite position, this compressed air now gives an added boost to the system pressure retracting or extending the gear in a very short time for such heavy gear. With the electric pump shut off and 1000 pounds showing on the gauge, the gear should retract in about 10 seconds or less with some residual pressure left. There are positive mechanical locks at all up and down positions. Conversely, if there is no pre-charge in the accumulator, the system pressure will go to zero when selecting gear, flaps or trim and the electric or hand pump has to do all the work and it is much slower. Having a properly charged accumulator is a huge safety factor as you can see.
It is recommended that Lake pilots extend the flaps when maneuvering at low altitudes since it flies more safely while we are studying the terrain below during pre-landing checks, water or land. It is also recommended that during the initial climb after takeoff, the flaps remain down. It gives us a much better angle of climb and in the event of an engine failure, the flaps are already down making the transition to a steep nose-down attitude before a forced landing much safer as it takes time for the flaps to extend.
With power at idle, gear down and flaps down it is possible in the Lake to point the nose almost to a straight-down attitude and not exceed 100 miles per hour…gear and flap speed is 125 mph. How many aircraft can make 60 degree banked turns at 65 mph?
Text and pictures courtesy and copyright of John Staber, Owner of Colonial Skimmer SN #1; John has flown and worked on Colonial Skimmers since 1964 and has accumulated more than 5000 hours in all different makes and models. In 2011 he published a book on the restoration of N6595K which can be purchased on his website. If you are interested to learn more about Skimmers and Lake Amphibian Aircraft, there will be one new article per week, dealing with a new topic.