Thinking about converting your gravel-dinged and eroded two-blade prop to a sexy looking three-blade model? Theres probably a three-blader out there that will fit your airplane and, surprise, it may even be cheaper than a two-blade prop.
Fair warning, though. Under the harsh light of subjective analysis, the decision to swap a two-blade prop for one with three is probably a guy thing.
For all the crude remarks about women airplane-and car-owners making purchase decisions based on paint scheme, the three-blade upgrade seems to occur sans anything approaching an engineers cold, calculated analysis.
A Freudian would probably have a field day with the underlying psychology but the undeniable truth is that three-blade propellers simply look jazzier than two-bladed models but may offer little else to the prospective buyer.
In this article, well try to nail down the technical and economic issues that bear on switching to a three-blade prop.
At the outset, we have to agree with one retired engineer we spoke to who observed that there are three kinds of pilots: those obsessed with speed, come hell or high water; gadget freaks who arent happy until they own every accessory found in Trade-A-Plane; and last, the down-to-earth, bottom-line types who are willing to exchange performance in one area to gain it in another if they can save a few bucks. By and large, owners who have converted to three-blade props seem to be in the third group.
Nonetheless, in asking questions about performance, vibration, noise, blade erosion and such, we pried open the back doors of the aviators psyche and discovered something weve always suspected: Many pilots buy things because they look cool and then work like hell to rationalize the decision.
The Push For Three
While there have been a few exceptions, most GA airplanes had two-bladed propellers until the late 1970s when Beechcraft and Cessna developed new versions of the Bonanza and 210. When three-bladed propellers were offered as options, buyers snapped them up despite the extra weight of the third blade and marginal, if any, performance gains.
Through the 1980s, a modest aftermarket for both three- and four-blade prop conversions developed through supplemental type certificates pursued by Hartzell Propeller, Inc. of Piqua, Ohio and McCauley Propeller Systems of Vandalia, Ohio. A handful of prop STCs are owned by other companies but Hartzell and McCauley are the major players.
McCauley developed a potent marketing strategy when it developed four-blade props for the King Air line, painted them jet black and called them the BlackMac. The idea was an immediate hit. The aura of rugged, macho props had arrived, giving King Air pilots (and the execs who paid them) certain swaggering rights.
McCauley followed up by certifying a three-blade BlackMac line for piston singles and twins. The list of STC conversions is considerable and still evolving. Check with McCauley or your prop shop to see if your airplane is listed. In addition, the company already has four-blade conversions for some turboprops and is developing a five-blade system for the King Air B100.
Hartzell has been aggressively and successfully marketing its Top Prop conversions. It has an even longer list of STCs than McCauley as of this writing. Hartzell also makes a four-blade conversion for the Cessna 441 with -10 engines and at the request of Alaskan operators seeking greater prop clearance, a four-blade conversion for the Cessna 208, reducing diameter from 106 inches to 96 inches.
These lists are changing constantly, as both companies tussle for market penetration. Weve tried to make them as accurate as possible at the time of publication, but check directly with Hartzell and McCauley as well as your prop shop to confirm your airplane is on the list. New additions occur almost monthly.
Given the complex forces involved, developing and certifying a propeller is frighteningly expensive. The engineers first come up with a design based on what has worked in the past, they refine the idea and then make a prototype. The new propeller undergoes various ground tests to determine its resonate frequencies, resistance to stress and overall strength.
For the vibration stress survey-a critical part of prop safety-strain gauges are placed on virtually every square inch of the propeller and many flights are made to obtain data from each of the strain gauges.
Tests are repeated with the propeller blades progressively shortened to determine the minimum length, or how much the prop can be safely reduced in size during maintenance. (Blades that are too short may have catastrophic vibration characteristics.)
Often, the test airplane is run on the ground with another airplane running nearby, using its prop blast to load the test propeller from different angles to check propeller response in ground wind conditions.
Quest for Efficiency
Like everything else in aircraft design, building the perfect prop is a series of compromises. And heres where things get sticky. In gener-al, two-blade props are considered to be the most efficient for the power ranges used on GA airplanes, although some argue that single-blade props, however impractical, are more efficient.
A number of counter-weighted, single-blade props were tried in the 1920s, but these simply didnt work well enough to hang on production airplanes. Besides, they looked awful.
Two-blade props have been the traditional starting point for most general aviation designs simply because they represent the best compromise between looks, cost and aerodynamic efficiency.
Efficiency is, however, relative. The big variable is horsepower; when theres a lot of it available, an extra blade (or two) definitely improves climb performance without hurting cruise speed much, if at all. This is consistent with reports from owners who have converted to three-blade props. Those with engines of less than 300 HP often report cruise speed losses of two or three knots.
Prop data we obtained from Alfred Scott, president of Sequoia Aircraft, which makes the popular Falco kitplane, confirms this real world observation. Scott has developed a computer program called Benchmark, which projects performance with constant-speed propellers.
In a nutshell, Scotts research shows that the crossover point for performance between two- and three-blade propellers occurs at about 300 HP. Below that, the two-blade propeller is better, delivering faster cruise if a little less climb performance. Thats largely the reason why Falcos have two-blade props, with the exception of a single three-blade model flown in Germany to meet noise abatement rules.
If youre interested in doing computer analysis of aircraft performance for yourself, Scotts program is commercially available from Sequoia Aircraft, 2000 Tomlynn Street, Richmond, Virginia, 23230, 804-353-1713. (www.seqair.com, Macs only.)
Recent test work by Mooney validates the theoretical underpinnings Scotts program uses. Mooneys chief engineer, Tom Bowen, told us that in general, cruise speed losses on three-blade propellers were unacceptable for use on anything other than the Ovation anf TLS.
He noted that all of the 252s operating in Europe have been converted to three-blade props for noise abatement and all have suffered from 4 to 6 knots loss of cruise speed. Quite a few 201 owners have tried three-blade props, only to switch back to two-blade props to regain lost cruise speed.
Why the lower cruise speed? Its due to a combination of induced drag from the additional blade and the higher weight of the three-blade prop, which requires greater tail-down force and thus produces additional airframe drag. In climb, at lower speeds, this is less of an issue, thus a three-blade prop tends to deliver the same or slightly better climb performance.
How They Fly
Blade shape also affects performance. A few three-blade propellers have squared-off tips. On aircraft with less than 300 HP, these seem to have the most adverse effect on cruise speed. Better are the propellers with elliptical tips and best yet are the new scimitar propellers which show the most promise for not taking a bite out of cruise speed.
McCauley uses a swept leading edge called the QZP for Quiet Zone Prop. It was designed as a noise-reducing propeller and was developed to equalize the loading along the blade to minimize noise. In testing, says McCauley, it tends to perform as well as the two-bladed propeller in cruise. Hartzells scimitar is more banana shaped and is the result of work to optimize cruise speed through evaluating the aerodynamics of the propeller blade.
The good news is that prop shops dont seem to be overpromising on three-blade cruise performance, although a report published in a recent issue of the Cessna Pilots Association magazine told the tale of a Cardinal owner who had been promised higher cruise speed but, in fact, experienced the opposite when he converted to three blades.
Interestingly, the only pilot we interviewed who reported higher cruise speeds was Cardinal owner Stephen Conners, of Minot, North Dakota. He told us an early three-blade STC for the Cardinal was tested on his airplane. However, he also noted that the speed increases were slight and not consistent across all power settings and ambient temperatures.
Conclusion: Be skeptical of any claims of faster cruise speeds due to a three-blade prop, especially for engines of less than 300 HP. Well be watching to see if the new scimitar props actually improve the cruise numbers.
Everyone we spoke to reported perceived climb rate increases after the three-blade conversion, ranging from 100 to 300 FPM, which we have to believe was with an airplane loaded below gross weight. Again, were somewhat skeptical of these claims, since owners admit they dont have reliable before and after data.
Theres little question that the additional drag of the extra blade is an effective air brake at idle power but most owners dont seem to report much operational difference, once theyre used to the new prop.
Mooney owner Robert Mann told us the three-blades additional drag tames the airplanes tendency to float on landing. And Sequoias Scott notes that many aerobatic pilots prefer three-blade props simply because they need the drag to stay in the speed envelope when flying descending maneuvers.
We were told by seaplane operators that low-end acceleration and performance coming off the water with three-blade propellers on the Cessna 180 and 185 is not as good as with a two-blade prop. This is consistent with the reduced diameter of that propeller. One engineer commented that for takeoff performance on seaplanes, diameter is everything.
Speaking of which, some owners-especially of Mooneys-convert to three-blade props to provide more ground clearance. While a three-blade prop is of a smaller diameter on some airplanes, thats not necessarily true on all applications. Check the specific STC for details before making the decision.
Vibration and noise are two more key points. As discussed in the sidebar, this is a mixed bag. Three-blade props may have a different vibe and noise signature due to frequency shift, but the actual levels may be only perceptibly lower.
What It Costs
With no great gains in performance claimed, cynics might argue that three-blade conversions are simply a way for the manufacturers to sell more blades. That may be so, but Hartzell and McCauley are pricing the three-blade conversions at about the same price as two-blade models, for equivalent props. Sometimes the three-blade is cheaper.
We heard from more than one owner who said that for his airplane, it was cheaper to buy a new three-blade prop than a two blade. Steve Alquire of Suttons Bay, Michigan has a 180 HP M20C and needed a new prop following a prop strike. He was quoted $6900 for a three-blade model and $7600 for a two-blade.
The decision, to him, was obvious. While his conversion is fairly recent, he has noticed the airplane seems quieter and climbs better but he admits hes skeptical enough to withhold judgment on an apparent speed gain. Theres no reason to believe youll get lucky and beat the theoretical limitations of three-blade props.
We did some checking on representative three-blade propellers through some prop shops. For a Cessna 177B, the list price for the McCauley scimitar propeller is $6475, for the Hartzell its $7195. For a Mooney 201, the list prices are exactly the same.
Prices have tended to be the same throughout a prop manufacturers piston line, regardless of the size of the propeller. The shops we spoke with said that they discount list prices so that the actual prices paid by an owner for the Hartzell or McCauley propeller, out the door, are closer than the list price disparity suggests. At least one shop includes a new spinner in the price of the propeller, but still discounts the entire assembly below list.
However, before signing the check for the STCd three-blade, ask some questions. Most conversions are bolts out, bolts in replacements. The prop price covers everything except your A&Ps installation charge.
But on some airplanes-the Cessna 185, for instance-additional mods may be required. According to 185 owners we contacted, Cessna used one engine mount for its two-blade landplane, a different mount for the two-blade seaplane and yet another mount for the three-blade. Despite that additional cost, the 185 owners we spoke with were happy with the conversions.
The above mentioned mods notwithstanding, prop conversion downtime is minimal. Pulling one prop and hanging another requires only a few hours of shop time. A number of owners and shops commented that the prop flange dowel pin location means that the propeller can be installed in only two ways. It cant be set at just any position on the shaft.
Although owners generally report improved vibration signatures, we were also told that if the prop turns out to be a problem to balance, reversing the mount 180 degrees usually fixes the problem. But we did hear from prop shops that report occasional unresolvable vibration problems with three-blade props.
Its generally true that any prop can be balanced, its also true that some engine-prop combinations run smoother than others, even after a competent A&P has emptied his bag of balancing tricks. In such a case, we think its reasonable for the shop to assure satisfaction or offer an exchange for a two-blade replacement.
In service, a three-blade prop shouldnt require significantly more maintenance than a two-blade model. Some owners report reduced rain erosion, but this is anecdotal evidence.
Overhaul times on three-blade props are comparable to two-blade units but the overhaul cost will be marginally higher, perhaps between $300 and $500 on a $1200 to $1500 no-surprises overhaul. Remember, youve got an extra blade, seals and other related parts, plus three blades require more dressing and finishing time than do two.
As the manufacturers see it, three-blade conversions have a safety and liability benefit not apparent to owners. The three-blade trend and sweetheart prices have convinced many owners to replace older two-blade designs that havent been overhauled in 20 years with new, state-of-the-art equipment. The idea of getting those old props out of circulation (pun intended) has undeniable appeal to Hartzell and McCauley and both are pricing their products accordingly.
For the short term, the manufacturers have shot their parts sales in the foot with the conversions, however, since rising three-blade sales have noticeably reduced the volume of parts sales for two-blade props. Presumably, the three-blades may also require fewer replacement parts because they benefit from improved metallurgy and machining.
One last thing to consider is weight. That extra blade will add an additional 10 pounds at the extreme forward end of the CG envelope. In an airplane that already has a forward CG-such as a Cherokee 6-this wont help much, although the installation of a lightweight starter could offset the weight gain.
A Cardinal owner reported a significant forward CG shift but said he had learned to live with it. On the other hand, a Bonanza with a notorious aft tending CG, might benefit from the additional weight up front.
As we said, converting to a three-blade prop may be as much an emotional decision as a practical one. If youre due for a prop overhaul on your old two-blade, replacing it with a three-blade simply isnt cost effective, unless the prop is completely shot and in need of replacement.
Following a gear-up or other major damage, switching to a three-blade makes more sense but may be purely a matter of price. The increase in weight and potential loss of cruise speed are negatives, albeit minor ones on most airplanes and perhaps something you could live with.
Theres simply not enough reliable data to predict performance. (If the invoice is $700 less for the three-blade, the trivial negatives may fade to the background.) We think its reasonable to expect an equivalent if not better climb rate with a three-blade prop, plus substantially reduced vibration or at least a more tolerable vibe frequency. This, in our view, may be the single best reason to convert from two to three blades.
Increased ground clearance is nice, but none of the owners we interviewed reported any difference in rates of blade erosion for landplanes, although the reduction was significant for seaplanes. What may tip the scale in favor of the three-blade is the increased level of safety afforded by a new prop over an overhaul.
And dont forget the look. Three-blade propellers just plain look sharper than two-blade models, just as four blades look cooler than three. Just watch the male pilots at any airshow where a Hawker Sea Fury shows up with its five-blade prop; eyes glaze and they make men-in-a-hardware-store inarticulate grunting sounds.
Having decided to convert, should it be McCauley or Hartzell? Frankly, although were not shy about making choices, from the user feedback standpoint, its near a dead heat in price, although McCauley enjoys a slight margin.
In our conversations with prop shops, theres a clear preference for McCauley based on service policies. Shops told us McCauley is simply more responsive. Further, the McCauley one-piece hub is seen as a better design than Hartzells two-piece hub.
The decision may be made for you by who owns the STC for the airplane youre flying and, of course, by who has the best price. Also, if your prop is subject to recurring ADs, a three-blade replacement may solve that nuisance for good.
At last falls National Business Aviation Association convention in Dallas, McCauley showed a scale model of the five-blade prop it builds for the Jetstream 41.
The booth was packed with piston pilots asking if they could get one for their 172s, Bonanzas and Arrows. After researching this article, we dont doubt that if someone builds a five-blade prop for a piston single, owners will buy them, even if they weigh more and reduce cruise speed.
Like we said, its a guy thing.
Also With This Article
Click here to view the Three-Blade Prop Checklist.
Click here to view “Vibes and Noise: Generally, Less of Each.”
Click here to view Propeller Addresses & Contacts.