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A Wood Propeller For Your Homebuilt?
By Tony Bingelis (originally published in EAA Sport Aviation, January 1991)
A wood propeller for your homebuilt may be safer than a modified metal prop. Certainly one could be more readily available and less expensive, too. Want to hear more?
The Case Against Cut-Down Metal Propellers
Although propeller manufacturers have long produced a variety of metal propellers for light aircraft . . . type certificated light aircraft, that is . . . they are generally ill-suited for many homebuilts.
There are two good reasons for this odd state of affairs.
1. The standard 74 inch and 76 inch diameters of the typical type certificated metal propellers are too long for most homebuilts. The exception might be those shorter 69 inch propellers designed and manufactured for the Cessna 150s equipped with Continental 0-200 engines.
2. In addition to their large diameters, the biggest drawback to using standard certificated metal propellers is that they have insufficient pitch for our smaller, lighter and faster homebuilts.
Consequently, many a homebuilder, determined to install a metal propeller, tries to find a good looking used metal prop that he can have reworked.
By the time he gets his hands on it, and has it cut down (if it hasn't already suffered that treatment) and repitched, it may no longer be a very safe propeller to use.
Furthermore, if that additional twist (pitch) is poorly distributed along the blade length (and it usually is), there goes the touted metal prop efficiency advantage over a wood propeller.
If you are one of the homebuilders who is still unaware of the blade stress and fatigue risks introduced when a propeller is cut down and repitched (especially if the propeller was a damaged one in the first place), take heed!
Propeller manufacturers caution homebuilders not to modify propellers to suit experimental aircraft design limitations . . . at least, not without first having a vibration surrey made. This, for most of us, is not economically practical or feasible.
If you must cut down a stock propeller, be sure it remains in the diameter range that is approved vibration-wise on the engine. However, getting enough twist in the blades to suit your pitch requirement can still get you into trouble.
This metal propeller modification dilemma only makes the wood propellers, in spite of a few shortcomings, more attractive than ever to homebuilders. So, if you have been led to believe your airplane would be faster with a metal prop, don't you believe it . . . there are plenty of 200 mph homebuilts with wood props.
A Bit About Wood Props
First, the disadvantages. To make a fair comparison with metal props, I must point out some disadvantages you have to put up with when using a wood propeller.
A wood propeller is sensitive to humidity changes (wood swells and shrinks) and retorquing its attachment bolts must become a regular practice lest you find you have a loose propeller or failed prop bolts to contend with.
A wood propeller will suffer greater erosion damage when flying through rain. However, newly developed protective tip overlays have done much to minimize this problem. Nevertheless, it would be prudent to slow down if you unexpectedly fly into areas of light rain.
Because a wood propeller is much lighter than a metal prop, it doesn't make a good flywheel - and may, therefore, stop windmilling should you experience an engine failure. You can, however, get it turning again by diving to increase your airspeed to over 120 mph for an engine restart attempt, or whatever. That does take altitude, doesn't it?
A wood prop with a protective metal tip overlay is somewhat better protected against blade damage and erosion but it is less efficient due to the discontinuity or ridge in the airfoil caused by the protruding edge of the metal tipping. Also, another drawback, these metal inserts have been known to fly off if the attaching screws rust out and rot the wood.
With such shortcomings, what good reasons can you possibly have for using a wood propeller? Plenty . . . but decide for yourself. Now, the advantages. A fixed pitch wood propeller is the lightest and least expensive type of propeller you can obtain.
Unlike metal propellers, wood blades do not fail from fatigue due to bending. Wood propeller blades are thicker than metal blades and, therefore, do have sufficient stiffness to resist the development of visible flutter which could be destructive.
You can always get a wood propeller designed and custom built to suit your engine and aircraft, whatever it may be. This could be especially important to you folks who have to find a propeller for a pusher. After all, how many type certificated metal pusher propellers are available?
Even a type certificated wood propeller is far less expensive than a similar metal propeller.
In comparing prices, you will find that a fixed pitch metal propeller costs two to three times more than a custom made wood prop - $1200-$1500 vs. $350-$600, approximately. (My beautiful Warnke prop - 68x69 - cost me $500 plus $30 freight, and a 90 day wait.)
Wood propellers are noted for their smoother running due to their excellent internal damping characteristics. They also impose less load on the crankshaft.
Wood propellers are constructed of hardwood laminations, the greater the number, the better, because they ensure a stronger propeller and virtually eliminate any tendency for the wood to warp. Laminations as thin as 1/8" also guarantee uniformity of grain and freedom from hidden defects.
Certificated Wood Propellers?
Are there such things as type certificated wood propellers? Sure, and one would be my first choice . . . assuming I could obtain such a propeller with the diameter and pitch I needed.
The Sensenich Corporation, in addition to making metal propellers, is perhaps best known by EAAers as a source for the fine type certificated wood propellers used on classic and antique aircraft.
Actually, Sensenich seems to me to be the only U.S. propeller manufacturer who is taking an active role and interest in developing certificated propellers suitable for airplanes like the T-18s, RV-4s, RV-6s, Mustangs and fast composite homebuilts.
With a certificated wood propeller and a type certificated aircraft engine installed, the FAA normally imposes a mere 25 hour mandatory flight test period.
NOTE: I believe that many builders who say they prefer a metal prop over a custom built wood prop are influenced by the knowledge that with the installation of a type certificated aircraft engine, and most any metal propeller, the FAA inspector will grant them a shorter 25 hour test period. Well, as I just pointed out that same privilege also holds true when a type certificated wood propeller is installed in a homebuilt.
You have a decided advantage when you can use a stock certificated wood propeller. Unlike a custom built propeller, you can get one almost immediately from the propeller manufacturer or from your favorite homebuilder supply house.
The Custom Built Propellers
The classified section of SPORT AVIATION lists numerous manufacturers of custom built propellers. Many of them have been in business for years and also have the expertise to advise you in the selection of a suitable propeller.
Even so, for many builders the major deterrent to ordering a custom made (homebuilt) propeller is that the FAA will automatically peg their mandatory flight test period at 40 hours minimum. Understandably, that requirement bugs a lot of builders.
This may not be good news to you. Custom made propellers are seldom stocked and must be made to order.
For this reason, timing can become a very important factor, especially if you delay ordering your custom made propeller until you are ready for it.
Take my advice. Try to anticipate your propeller needs well in advance of the completion date for your airplane. Allow yourself a generous several months lead time. One local RV-6A builder has been waiting for his prop for 5 months.
Almost all of these craftsmen (the custom propeller manufacturers) are turning out wood propellers just as fast as they can . . . and the orders continue to pour in. If present conditions prevail, your wait for a propeller will be long.
What To Expect Of Your Propeller
Unfortunately, selecting a suitable fixed pitch propeller for any airplane is complicated by the fact that no single fixed pitch propeller (wood or metal) can be best for all flight conditions.
That is to say that a propeller configured to give you the best takeoff and climb will not give you the highest cruise speed and vice versa.
The ultimate performance attainable for a particular aircraft is, in reality, a compromise between the optimum takeoff and optimum cruise performance. Most builders realize this and reluctantly compromise a little bit of cruise for increased efficiency in their rate of climb.
If you will be operating from an unimproved or short airstrip, you, too, will probably consider your rate of climb to be more important than your top speed.
Years ago a Lycoming installation engineer informed me that if I was using a fixed pitch propeller, the static rpm should be 2300, plus or minus 50 rpm, when the engine is rated at 2700 rpm (most Lycomings are).
This static rpm will vary slightly depending on the type of propeller installed. That is, if it is a climb, cruise or economy propeller.
If the static rpm is too high, there is the likelihood of overspeeding the engine at full throttle in level flight.
It is interesting to note that with a constant speed propeller, static rpm will be the rated rpm of the engine. This is controlled by the low pitch setting of the propeller.
The minimum static rpm is established by the manufacturers for each of their engine installations.
Essentially, reaching this recommended minimum static rpm during your engine run up is a fair indication that sufficient power will be developed for takeoff.
A few recommended minimum static rpm for several popular aircraft engines follow:
Continental A-65 - Not under 1960 rpm
Continental C-85 - 2200 rpm
Continental C-90 - 2125 rpm
Continental 0-200 - 2320 rpm
Lycoming 0-235 - 2200 rpm
Lycoming 0-290 - 2200 rpm
Lycoming 0-320 - 2300 rpm
In general, your propeller should be one that gives you the best cruise (full throttle) at approximately 7,500 feet altitude without unduly reducing the rate of climb.
This means a Lycoming with a design 2700 rpm red line should reach its red line rpm with full throttle at that altitude.
If the engine overspeeds, your prop could absorb more pitch. If you cannot obtain the 2700 rpm in level flight, the engine is being overloaded by a propeller with too much pitch.
In bragging about their top speed, some builders neglect to mention that they are operating their engines over the red line rpm to get more power. Although this practice may not be regarded as being overly stressful on the engine, I believe engines can endure just so many power strokes between major overhauls.
It's up to you. You can use them up quickly or at a more leisurely rate.
Tips For Selecting The Right Prop
A propeller represents a considerable outlay of cast . . . even for an uncertificated wood propeller that can only be used on a homebuilt.
That being the case, you had better order the right one the first time. Consider the following:
Your aircraft's weight and balance can influence your propeller selection.
For example, if you find your airplane has a tail heavy condition, a metal propeller would be most beneficial in getting more weight up front. That is because a typical 74 inch metal propeller for a four cylinder Lycoming engine weighs approximately 30 pounds. Compare this to the feather weight 10-12 pounds for an equivalent wood propeller.
Aircraft balance just about mandates the use of a wood prop for most of the popular composite pushers. They cannot tolerate the excessive weight of a metal prop in back.
But that works both ways, doesn't it? If you have a nose heavy condition, substituting a lighter wood propeller in place of a heavier metal prop could help alleviate that problem.
To help you in selecting the right propeller, you'll need to have some basic information about your airplane.
You should, for example, know the maximum diameter propeller you could install and still have a minimum ground clearance (in level attitude) of 9 inches.
You should also know the maximum rpm you could turn a maximum diameter propeller without exceeding its critical tip speed.
The critical tip speed is generally accepted to be 75% of the speed of sound which, at sea level, is 1100 feet per second (at standard temperature and pressure). Above that speed a prop loses efficiency and noise increases considerably.
For this reason, a wood propeller's tip speed should be kept below 850 fps.
With that basic information in hand you are ready to look for a propeller:
1. You could try to find a type certificated propeller designed for an aircraft that has a design top speed, rated rpm and horsepower similar to what you expect to have.
2. You could rely on the aircraft designer's recommendation for a particular propeller diameter and pitch for each of the engines he has approved. He may even suggest several sources for propellers. You can rest assured the designer wants his airplane to perform well for obvious reasons. His propeller recommendations should, therefore, be seriously considered.
3. Find out from other builders what props they are using for your type of aircraft. Compare notes with as many of them as you can before ordering. Be prepared to hear conflicting information. The performance figures quoted may not be the same from each of the builders using like propellers. However, you can safely assume that if they are satisfied with the propellers they are flying, a like propeller should be fairly close to what you need.
It's O.K. to be a bit skeptical about performance figures because many builders do not have calibrated instruments and the performance numbers they quote may not be as accurate as they choose to believe.
Tachometers, too, are notoriously inaccurate. Quoted airspeeds especially may be suspect due to poorly located pitot and static ports. Of course, your own instruments should also be suspect unless you have them calibrated and checked for accuracy.
4. Select a propeller maker with a good reputation and rely on his recommendations for a propeller. He will ask you to provide him with some essential information, namely:
What airplane design is the propeller for?
Which engine and horsepower do you have installed?
What's your preference? Best cruise/top speed. Best take-off/climb/high altitude or a standard/compromise prop.
What kind of airspeed are you hoping or?
It's comforting to know that most prop makers are very cooperative, and often mill repitch your wood prop slightly to fine tune its performance without charge.
And, finally, if your propeller ever develops a serious defect, it should be returned to the manufacture for repair.