Monday, August 31, 2009

Density Altitude and Seaplanes

Summer is really here in Alabama. With humidity like warm butter and temperatures heading into the 90s, the AWOSs are now reporting the density altitude is in the remarks section.

Any airplane will have to contend with loss of cool dense air to burn in the engine and the wings and prop will have to work harder for lift and thrust. Planes will continue to fly off the runway with about the same indicated airspeed as a 10 degree winter day, but the ground speed and true airspeed will be considerably higher as the density altitude increases since the air is thinner. This is just like what happens when the airplane climbs. Our airspeed drops and our true airspeed clims. The engine will produce less horsepower, the prop will be less efficient, and the wings will have to work harder to fly in the less dense air.

Seaplanes have one extra piece of drag as the ground speed, or water speed increases, water drag. Water drag is the seaplane's worst enemy as far as accellerating enough to take off anyway. When you take off from glassy water you can feel the suction of the water's grasp as it finally gives up and lets the airplane fly. Water drag increases at the square of speed, so as we accellerate, the water's effect on the plane increases dramatically. Land planes do not have this issue. This is another reason that seaplanes need to have extra horsepower, flatter pitched props, and STOL kits. It is not just that their owners have extra money to throw at their planes. The planes really need this stuff to get airborne. There is a weight and density altitude that will just not be able to gather enough ground speed to take off.

What can we do to mitigate density altitude. We can fly earlier in the morning or later in the afternoon. We can reduce weight as much as possible, by pumping the floats thoroughly before flight, leaving gear or fuel behind. We can fly directly into the wind to increase airspeed as much as possible. We can fly down river which will increase airspeed as well but it will also reduce water speed. Using good glassy water techniques can help, including raising one float on take off and getting the pitch just right to minimize drag. One final thing we can do if the water is calm, we can make some waves to reduce the drag on the floats. Glassy water is the worst. So leave yourself more room on these high and hot density altitude days and keep your cool.

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