Constant-speed props are usually found on high-performance aircraft, but most people don't realize the concept of varying the pitch of a propeller was suggested 30 years before the Wright Brothers flew. John Ruley discusses the history, repair, and alternatives to this staple of aviation.
Editor's Note: This article originally appeared in Cessna Owner and Pipers Magazines.
"It was apparent that a propeller was simply an aeroplane travelling in a spiral course. As we could calculate the effect of an aeroplane travelling in a straight course, why should we not be able to calculate the effect of one travelling in a spiral course? At first glance this does not appear difficult but on further consideration it is hard to find even a point from which to make a start... The thrust depends upon the speed and the angle at which the blade strikes the air; the angle at which the blade strikes the air depends upon the speed at which the propeller is turning, the speed the machine is travelling forward and the speed at which the air is slipping backward... When any one of these changes, it changes all the rest, as they are all interdependent upon one another." -- Orville Wright
Of all the problems the Wright Brothers faced in designing the first successful powered aircraft, they considered the propeller the most difficult. Amazingly, they weren't the first to face up to the complex relationship between blade angle, speed of rotation, speed through the air and thrust -- as early as 1871, a Frenchman named J. Croce-Spinelli suggested that a variable-pitch propeller could improve overall efficiency. He also suggested varying the pitch using hydraulic pressure, and that the ability to change pitch would be most important on takeoff.
Croce-Spinelli was right, as you can see in the graph below, which shows propeller efficiency as a function of speed. Fixed-pitch propellers, which remain common on lower-performance airplanes, can be optimized either for best takeoff and climb performance (referred to as a "climb prop"), or for best speed (a "cruise prop"), but not both. A "constant speed" prop, by contrast, can be adjusted for maximum efficiency regardless of speed or engine power.
Variable Pitch Or Constant Speed?
While early variable-pitch props (the first actually used in flight dates from 1917 at Britain's Royal Aircraft Factory) were mechanically actuated, increasing performance (and resulting wear on complex mechanical linkages) led to the development of electrically and hydraulically adjustable props before World War II. The term "constant speed" refers to a refinement in which the pilot, rather than directly adjusting the blade angle, sets a rotational speed at which he wants the prop to operate; a "governor" operates to maintain this speed.
In modern constant-speed props, this is done hydraulically (kudos to M. Croce-Spinelli!) using engine oil. The diagram below shows a cross-section of a typical constant-speed prop. Engine oil pressure against a piston in the hub causes the blades to move back from flat pitch. The pressure is regulated by a propeller governor (not shown). When the prop reaches the desired speed, the governor opens a valve, reducing oil pressure to the piston. With proper adjustment, this system will automatically adjust blade angle to maintain the speed selected by the pilot.
|Constant-Speed Propeller Mechanism (click for larger view)|
A variation on this design is used in "feathering" props on twins and some aerobatic aircraft: Instead of increasing pitch as oil pressure is increased, feathering props decrease pitch as oil pressure is increased. You can tell which is which when you look at an airplane on the ground with the engine stopped (and therefore, no oil pressure) -- if the blades are flat, like those of a fixed-pitch prop, it's a non-feathering propeller. If the blades are pitched at 90 degrees, it's a feathering prop.
Whether feathering or non-feathering, constant-speed props can have two or three blades in general aviation applications (you'll sometimes see more blades on turboprops, which have to operate in the thinner air at high altitudes). Three-blade props offer some advantages, including increased ground clearance, less vibration, and less objectionable noise (with an extra blade on each rotation, 3-bladed props emit noise at a higher frequency than 2-bladed props). According to the manufacturers, 3-bladed props improve climb performance, but usually make little or no difference in cruise.
Many constant-speed props have "yellow arc" restrictions on continuous operation at certain RPM/manifold pressure combinations. These restrictions are due to resonance effects, which can put excess stress on the prop. Converting to a 3-bladed prop often changes the "yellow arc" limits, in some cases making them less restrictive.
On the other hand, 3-blade props are usually heavier than 2-bladed props, which can not only reduce your useful load, but also create center of gravity problems (remember that the weight of the prop is at the extreme end of the nose). Hartzell's recent 3-bladed STC prop for Piper's Malibu uses blades made of Kevlar, in order to keep the weight down. And 3-bladed props are usually more expensive than 2-bladed props. For these reasons, until quite recently 2-bladed props were provided as standard equipment on virtually all single-engine airplanes.
All propellers -- whether fixed-pitch or constant-speed -- have a manufacturer-recommended time between overhauls (TBO). As a practical matter, fixed-pitch props are generally inspected during the engine overhaul, and usually require no more than minor balancing and dressing (unless the prop has been subjected to excessive wear). Constant-speed props are another matter -- they're mechanically complex and require a complete teardown, inspection, and replacement of worn parts in much the same way as an engine. That can be expensive -- according to Merle Furry, an A&P at the Modesto Flight Center (my local flight school), prop overhauls generally run "about $4,000 or so," but he's seen them run over $7,000 if a governor has to be replaced. That approaches the price of a brand-new prop, ($6,000-$8,000 for most single-engine constant-speed props; feathering props for twins are more expensive).
Joe Nelson, OEM Account Manager for Hartzell, says that's a significant selling point for Hartzell's "Top Prop" series of STC'd props: "A lot of times, you can buy a new 3-blade prop for not much more money than you'd spend to overhaul an old 2-blade prop; and you get a better warranty and a longer TBO for the money."
His point about TBO is well-taken -- and points up a significant issue about constant-speed props: The TBO is listed in terms of both hours of operation, and calendar time. Typical new props today have TBO limits of 2400 hours and six years, respectively, while many aircraft engines today are rated for 2000 hours and 10 years. Nelson notes that "Props are generally the highest stress piece of a typical airplane, and if a failure occurs it may give no advance warning. You really have to do preventative maintenance. Part 91 owners aren't required to follow the manufacturer's recommended TBO -- but to be safe, you should. Our new production props are good for six years or 2400 hours. The time limit is based on corrosion. Old props were good for five years or 2000 hours -- we've improved corrosion protection, and recently overhauled hubs get the longer time limit."
Aside from the Kevlar prop for the Piper Malibu -- which Nelson says "adds only seven pounds to the weight, and cuts takeoff distance by 25%," recent Hartzell STCs include an improved prop for the Cessna Caravan. "It has a long TBO (five years or 3000 hours) and unusually wide blade tolerance. You can file a lot off one of our new blades before it has to be scrapped! If you can go through two or three overhauls before you have to replace the blades, there's a big cost savings!"
Most Cessnas, on the other hand, use McCauley propellers, because Cessna has owned McCauley since 1960. McCauley's factory is in the process of moving from Ohio to Georgia. Sondra Taylor, a McCauley sales representative, told me that because of the move, "We're getting quite a backlog. Our standard lead-time is now 120 days" for props and parts that aren't available in stock. McCauley currently provides original-equipment props on all Cessna singles, and has a line of "Black Mac" STC'd props available for many different makes and models.
While researching this article, I discovered an alternative to the constant-speed prop that may be of interest to pilots of airplanes with fixed-pitch props. Tarver Propellers LLC holds the type certificates for the Aeromatic prop, descendent of a prop design from the 1930s. It's an "air-controlled" variable-pitch prop, which requires no governor, and does not depend on engine oil pressure to vary the pitch. In the Aeromatic prop, blade pitch is controlled by a combination of air pressure and centrifugal force. At full throttle, the blades automatically go to flat pitch. At reduced throttle in cruise, the blade angle increases -- all automatically, without any need for a prop control.
The downside of the Aeromatic prop is that it uses wood blades. Kent Tarver told me, "My present supplier is using maple. This kind of blade was used on early Spitfires and Hurricanes. It was certified on most Piper, Cessna, Fairchild, Swifts, Stinsons, Bellancas, Aeroncas, Ercoupes and others, including the Cessna 170. I plan to get them certified on the 172 both with the Continental and the Lycoming engines."
That's probably a year away at this point -- Kent's had quite a time trying to get the FAA to recertify the design. He describes the process as "agonizing and expensive," but feels confident that he will be able to offer STC'd props sometime next year.