Terminology



Terminology

Weight and Balance

Center of gravity (CG): is the point at which a hovercraft would balance if suspended.

Center of lift (CL): is the center of area.

Reference datum: is an imaginary vertical plane from which all distances are measured for balance purposes.

Station: is a location along the hovercraft hull given in terms of the distance from the reference datum.

Empty weight: is standard weight plus weight of optional equipment

Maximum weight: is the maximum weight approved for general operation.

Maximum flight weight: is the maximum weight the craft will fly in ground effect.

Operational

Belt Drive: Engine is connected to the propeller via a belt through a pulley system. Bet drives are typically used instead of gear boxes to reduce weight, noise and to lower hovercrafts center of gravity.

Cushion delay time (cdt):
is the time it takes for the hovercraft to go from full hover (or 10 inches of hover height) to the landing skids touching the ground. This action is done by rapidly reducing the engine rpm with the throttle or by turning the key to the off position.

Direct drive:
Refers to a propeller or fan being directly mounted to an engines drive shaft.  The largest recommended direct drive propeller is 42 inches and the largest recommended lift fan diameter is 36 inches. Using larger diameter propeller or fan will increase tip speeds and make the drive system very loud and susceptible to blade tip erosion.

Hover height: distance between the lower hull and the surface when the craft is on cushion.

Landing skid: surface on the bottom of the craft in which the craft rests when not on cushion. Landing skids are also used to reduce wear on the bottom of the craft.

Plow in: is when the hard structure in the bow of the craft comes into contact with the water. The contact has the ability to generate a high level of drag causing a controlled deceleration of the craft.

Plow plane: Angled bottom surface of the craft that first comes into contact with the water. This surface helps to break water from bottom helping to reduce drag.

Reduction ratio: Belt or gear drive system that reduces the rpm of an output shaft.

RPM:
Revolutions Per Minute. The amount of revolutions an engine, propeller or fan turns every minute.

Scooper hole: Hole or damage to the skirt resulting in water continuously being scooped into the skirt. These holes are typically found in the skirt contact area.

Side plow: is when the hard structure on the side of the craft comes into contact with the water. The contact has the ability to generate a high level of drag causing a controlled deceleration of the craft.

Skirt drain hole:
Hole located in the rear center section of the skirt. This hole remains open allowing water to continuously drain from the skirt.

Skirt contact area: Portion of the skirt that comes into contact with the terrain. This portion of the skirt incurs the most wear.

Tail heavy: When the rear of the craft is excessively loaded. The rear of the craft squats in the water causing excess drag and hindered maneuverability.

Thrust duct: is the tapered ring around the diameter of the propeller. Thrust ducts add protection; increase overall efficiency and stability of the craft.

Tip Speed: This term refers to the speed at which the tip of the propeller is traveling. High tip speeds (400+mph) will result in excess wear to the blades. Keeping tip speeds low through a reduction drive system will decrease erosion and noise. Most UH propeller drive systems are designed to run a tip speed of 180 to 250 mph.

Trim wing / elevator: Horizontal wing located behind the thrust duct. This wing helps to maintain proper trim of the craft while operating. When the craft is in Hoverwing mode the horizontal wing them becomes an elevator controlling the altitude and trim of the craft.




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