WOOD VS. METAL FIXED PITCH PROPELLERS By Bill Cassidy (EAA 78210) 4652 Montview Blvd. Denver, CO 80207

INf THE

APRIL issue of SPORT AVIATION some guideline data was given for the builder to use when ordering a propeller for his homebuilt. Continuing on this theme of propeller selection, the builder should have some basic knowledge of the pros and cons of wood and metal propellers in order to choose the best type for his aircraft. Although many builders today are going to the constant speed and ground adjustable types of propellers, the emphasis of this article will be placed on fixed pitch propellers which because of their cost, weight and availability still interest most homebuilders today. Propeller Efficiencies According to Fred Weick1 a metal propeller is usually 3 to 4% more efficient than a comparable wood propeller because of the thinner airfoil sections used which result in higher lift/drag ratios for these sections along the entire length of the blade. These thinner airfoil sections also permit higher velocities (tip speeds) before compressibility effects become noticeable and this, of course, allows the designer to use larger diameters than can be used on wood propellers turning the same rpm. Metal propellers are usually limited to a tip speed of 950 fps whereas the wood propeller is limited to about 850 fps. The wood propeller has high strength (for its weight) in compression and tension parallel to the grain, but the resistance of wood to splitting due to torsional loads is very low. The wood propeller must be designed with airfoils approximately 33-1/3% thicker than the metal propeller to provide sufficient stiffness to resist torsional loads as well as flutter and flexing of the blade, and these thicker sections result in the higher drag and lower efficiency of the wood propeller. If a homebuilt aircraft has a maximum speed capability of up to approximately 150 mph and a standard aircraft engine will be used turning from 2400 to 2600 rpm at cruise, and if the ground clearance is such that a standard prop from a certified aircraft can be used, then the builder would probably be ahead to use the standard metal propeller. Slight pitch changes necessary to obtain optimum performance would have no noticeable effect on the efficiency of the propeller. The higher efficiency of the metal propeller assumes, of course, that the airfoil sections are pitched properly at each station along the blade and this is not always possible with the metal propeller. This problem becomes very apparent when the metal propeller has to be pitched up to the very high blade angles that are required to meet the speed vs. rpm requirements of some of the 200 mph homebuilts being turned out today. Advantages of Wood in Coarse Pitch Propellers

Aircraft capable of attaining speed of more than 150 mph will require a propeller with much higher pitch angles and especially so in the center section of the propeller. It is very important to high speed aircraft that every blade station from the spinner to the tip be properly pitched relative to the airflow at that station. Very little help in propulsion can be credited to the center section of a propeller; however, if the center section stations are not pitched properly they can exert tremendous drag at high forward velocities. In order to properly pitch the 38 AUGUST 1979

inboard stations of a propeller capable of attaining 200 mph at 2800 fpm it is necessary to use a hub thickness of from 4" to over 5" thick depending on blade width and airfoil used. The hub thickness of fixed pitch metal propellers is limited to 3'/£". This is not to say that these propellers have not been made thicker (and heavier) but manufacturers of fixed pitch metal propellers have apparently decided that SVz" thick hubs are sufficient to accommodate the center section blade angles for the pitches used on certified aircraft operating up to 150 mph. Many certified aircraft operating in this speed range use constant speed propellers and above this speed range nearly all certified aircraft use a constant speed prop so that hub thickness is no longer a concern. I have checked the blade angles of some pitched up metal propellers and found the angles of the blade from the 75% station out to the tip to be reasonably accurate, however, the stations towards the center section of the propeller start measuring on the negative side at about the 50% station and at the 25% station (this is just about where the blade comes out of the spinner) the blade angle has measured as much as 20° negative. It is impossible to pitch these center section stations near the hub to the high pitch angles required without exceeding the yield in shear. The only way that a thin hub can be used efficiently on a fixed pitch propeller operating at high forward velocities is to raise the rpm to a value where a flatter pitch is required, but then most builders don't want to run their engines at 3500 rpm. A secondary problem surfaces with the higher rpm in that we are now limited to a smaller diameter to keep the tip speed and stresses within limits. Reducing the Diameter of Metal Propellers Most builders seem to be aware of the dangers of operating with a cut down metal propeller and over the years many accounts have been published in SPORT AVIATION about some of these props losing up to two thirds of one blade in flight. The resulting imbalance can tear the engine completely out of its mount and many fatalities have resulted from this type of accident. Every metal propeller has a natural frequency of vibration determined by its length and mass and the manufacturer of this propeller is going to make very sure that the propeller does not resonate at its natural frequency or a harmonic (multiple of the natural frequency) within the normal operating rpms in flight. Reducing the diameter of a propeller changes the natural frequency of vibration and in this instance, the homebuilder can only hope that he is not changing the natural frequency to correspond with any of the rpms that he will be turning in flight. There are ways of checking this, however, the problem is much more complex than described here in that a metal propeller can vibrate in three different modes of vibration and the exciting force causing the vibration can be mechanical and/or aerodynamic. The amplitudes of vibration in each of the modes or combination of modes will vary with rpm and load on the blade; however, the nature of the vibration and its exciting force are really unimportant. The primary concern with a cut down metal propeller is to avoid operating in a harmonic or resonating condition that results in peak stresses which can cause fatigue failure. With some cut down metal propellers there are two or three ranges of rpm that must be avoided to stay away from these resonating frequencies. If a builder decides to change the shape or the length of a propeller he should certainly consult the propeller manufacturer before making a change that could result in a fatal accident. The manufacturer is generally aware of how much change can be made to a blade before a

dangerous condition occurs. One of the biggest plus factors of the wood propeller is the cell structure of wood which gives the material a very high internal friction and makes it self-dampening to vibration. Resonating at a natural frequency or a harmonic is something we don't have to be concerned with in wood propellers. Aerodynamic vibrations are something else, however, and if the blade is too thin and flexes in flight or if the design is such that excessive torsional

loads are imposed on the blade, the resulting stresses can be destructive. Fred Weick states that a slight sweep back will help to present or minimize any flutter at the tip of a wood propeller and provide smoother operation.

Weight

The fixed pitch metal propeller is quite heavy for the small wing area of the average homebuilt. A metal propeller will weigh as much as 37 Ibs. and when "cut down" will still weigh about 30 Ibs. as compared to the 10 to 20 Ibs. that can be expected from wood propellers. The weight of a wood propeller cannot be estimated by simply comparing the density of wood against the density of aluminum. Wood propellers can have a wide variation in weight because of the many different ways in which they are manufactured. The number and thickness of laminations, metal or plastic leading edges, fiber glass or plastic sheathing, type of wood and even the finish can effect the final weight of a wood propeller. It has already been pointed out that the airfoil sections of a wood propeller are usually 33-1/3% thicker than the sections of a comparable metal propeller and much thicker hubs are used on coarse pitch wood propellers. Most of the homebuilt wood props are constructed of 3/4" thick laminations and have very little tip and leading edge protection. These propellers are very light, weighing from 4 to 12 Ibs. depending on diameter, blade width and hub thickness. Impact Resistance Many wood propellers have some form of leading edge protection which tends to make the propeller more resistant but certainly not impervious to impact damage. A metal leading edge on a wood propeller will give much better impact resistance than either plastic or fiber glass, however, it is only a matter of time until the right sized rock gets picked up during a static run up and punctures the metal leading edge. About the only repair that can be made to dents and punctures in a metal elading edge

is to use an epoxy paste and then sand or file the leading edge to contour and this is the same technique that is used to repair plastic or fiber glass leading edges. The metal propeller is just as susceptible to impact damage by rocks and foreign objects as is the wood propeller; in fact, the metal propeller is probably more susceptible when the possibility of fatigue failure is considered as a result of nicks or cracks in the blade caused by rock damage. Resistance to Rain Damage

intensity, the size of the rain drops and the time duration of exposure will all have an effect on the amount of damage incurred. Many types of leading edge protection have been tried with wood propellers. A metal leading edge gives good protection but will result in a noticeable loss of performance unless the metal is recessed flush with the airfoil surface of the blade. Hoffman uses this type of metal leading edge on their propeller. The metal leading edge is epoxied into position and lies flush with the airfoil contour so that there is no disturbance to the airflow over the blade. Burt Rutan has been experimenting with Kevlar fabric on his wood propellers. This material has a much higher resistance to impacting than fiber glass so it might be a good material to resist rain damage. Sensenich uses a plastic leading edge on some of their wood propellers which is also recessed to lie flush with the airfoil contour and I understand this has very good resistance to rain damage. Regardless of the protection applied to wood propellers, it is not recommended here that any wood propeller be flown through rain — stay out of it if at all possible! Resistance to rain damage is where the metal propeller really stands supreme, however, many pilots are not aware of the fact that some rain erosion also occurs to metal propellers — ask any pilot who flies in heavy rainfall areas or operates an aircraft on floats. Rain will get to a metal prop, too, but it takes a lot longer; so much longer, in fact, that it is of no real concern to the pilot. Leading edge erosion on a metal prop can usually be attributed to rain but several hundred hours of flight through varying weather conditions are required before leading edge erosion becomes noticeable.

In summing up, we can say that no propeller — whether metal or wood — is resistant to rock damage or damage sustained by foreign objects passing through the blades. The metal propeller will show less damage from sand and small particles than wood propellers, however, the metal propeller is subject to possible fatigue failure from nicks or cracks caused by impacting. Where rain damage is concerned, the metal propeller is practically impervious and can be operated in this environment with complete confidence. The wood propeller can

get you through if it is protected with some kind of rain

resistant leading edge and the pilot uses judgment by throttling back to minimize the erosion effects of the rain or mist. Weathering

This is another area where the metal prop is superior. The wood propeller is subject to the normal deterioration that takes place on wood when exposed to the weather. Ultraviolet from the sun will usually begin to roughen

the finish within a year if the propeller is exposed for

long periods. Wood propellers should be protected from the weather if the aircraft is to be tied down outdoors. A canvas sleeve, sewn so that it can be slipped over the blade and tied to the spinner will give the necessary

Flying through rain with an unprotected wood propeller could be a fatal experience as wood has almost

protection against normal variations of weather. Frequent cleaning and waxing of the finish will also serve to lengthen the life of the propeller. In very hot and humid

the blades several times but no damage has occurred

protected and dry. There is always the possibility of a wood propeller warping and especially so if it is manufactured in one climate and then shipped to a buyer and operated in a completely different climate. In spite of these apparent weaknesses, wood propellers have an excellent history of giving reliable service and will give many years of this service if the pilot will give some consideration to the effects of weathering and provide some protection and care to minimize these effects. 'Aircraft Propeller Design by Fred E. Weick, B. S.

no resistance to erosion by rain or mist. I have accumulated approximately 10 hours of flying time through rains of varying intensity in my Midget Mustang using a 60" diameter wood propeller with the outer 14" of each blade sheathed with fiber glass and Whitewater resin (impact resin). I have always throttled back to 2350 to 2400 rpm in rain and I have lost the paint off the tips of to the fiber glass sheathing as yet. This does not mean that fiber glass will protect a wood propeller from rain damage because it will not. Fiber glass will retard the erosion effects of rain if the pilot exercises judgement and throttles back while going through rain. The rain

climates a wood propeller can warp if it is not kept well

SPORT AVIATION 39