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What It Looks Like

When An Airplane Has A Q-Tip Propeller

Did you hear the one about the FAA inspector who grounded an airplane when he saw it on the ramp and noticed that the tips of the propeller blades were bent over? The inspector assumed the airplane had made a gear-up landing, which typically results in the prop blades being bent back as they strike the runway surface.

Unfortunately for the inspector and his embarrassed employer, the FAA, he apparently had never heard of Q-tip propellers. A Q-tip propeller is distinguished by a 90-degree backward bend in the last inch or so of the blade tip. The airplane that the overeager inspector grounded had not been in a gear-up landing accident; it had merely been outfitted with Q-tip propellers.

The bent tips on a Q-tip propeller accomplish two things. First, the 90-degree bend acts like a wall to block air from flowing spanwise along the face of the blade (the side you see when sitting in the cockpit). On a normal straight blade this air mixes with air spilling off the camber side of the blade (the side you see when standing in front of the prop) to create tornado-like tip vortices. Aircraft wings also produce tip vortices when high-pressure air flowing spanwise across the cambered top of the wing mixes with lower-pressure air flowing spanwise across the flatter bottom of the wing. These wingtip vortices can produce potentially hazardous wake turbulence for fol- lowing aircraft. Just as a winglet blocks tip vortices from forming off the end of an aircraft wing, a bent propeller tip blocks the formation of prop-tip vortices.

The other main function of the Q-tip propeller is to reduce the diameter of the blade, typically by about two inches.

Shrinking the blade's diameter and blocking the formation of tip vortices leads to several benefits. For one, the prop produces less noise. That's one reason Q-tip propellers are popular on aircraft in Europe, where noise restrictions are more stringent. Also, there's less chance the prop will stir up dust, dirt, and other foreign objects that can be sucked into the air cleaner or sent flying to strike another aircraft.

Hartzell, the leading manufacturer of Q-tip propellers, makes them for aircraft ranging from 160-hp IO-320-powered Glasairs and Piper Twin Comanches to the 850-hp Pratt & Whitney turboprop-powered Piper Cheyenne III.

By Mark Twombly

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