Props 101
By: Ralph & Jeff Shultz


How did all this prop stuff get started anyway?
Ever since the Wright brothers realized that a propeller could be designed having the same shape and properties of a wing, a rotary wing if you will, other designers have used the principles they pioneered. (See Appendix A for more information on the Wrights design insight.)

Judging by the questions asked concerning props, there is quite a bit of misunderstanding in the flying community about these devices. Simply put, a prop converts engine torque into forward thrust for propulsion. Some pilots wonder what all the numbers mean and how they tie in or relate to each other and performance. Others ask what prop is best in this or that situation. How does one decide the right prop for their plane?

This article will attempt to make things a little clearer. No high tech concepts or complex math will be presented, just plain basic prop relations and information. (See Appendix B for some basic prop design information.)


Lets start with some nomenclature and terms defining a prop and its performance.

Hub - Center of the blades where the prop attaching bolts are located.

Hub Thickness - Axial dimension measured at the prop centerline. This dimension can be a factor in the props designed pitch. In some combinations it may be difficult to fabricate a wooden prop having a thin hub with a high pitch value.

Platform Width - Width of the blade as seen from the front or rear of the prop.

Blade Section - Cross sections located at various distances along the blade length.

Section Airfoil - A streamline shape designed to produce lift or thrust for the prop.

Blade Cord - Length of that blade section's airfoil, leading edge to trailing edge.

Tip - That portion of the blade at the very outermost part from the hub.

Tip Speed - The speed at which the prop tip moves in its circular path expressed in ft/sec.

Blade Thickness - Thickness of that blade section when measured perpendicular to that section's cord.

Geometric Pitch - The distance the prop would move forward on each revolution, much like a screw, if it operated in a solid substance.

Effective Pitch - The forward distance the prop actually moves on each revolution when operated in air.

Slip - The distance difference between geometric and effective pitch. Usually stated as a percent of the geometric pitch.

Blade Section Angle Of Attack - The blade angle above that required in the geometric pitch helix, it's this angle that develops the prop thrust.

Blade Angle - Prop geometric angle plus the angle of attack.

Prop efficiency - A measure of the ability of the prop to convert engine torque into thrust to propel the aircraft forward.

¾ R Station - Located at 75% of the length of the blade, hub to tip. It's at this station that most prop properties are referenced to.

Prop designation convention -The first number is the props diameter (in inches) and the second is the geometric pitch (also in inches). These numbers are usually marked on or near the props hub. Most will look something like these: 60 x 44 or 54 x 37.


Through the years many different materials and combinations of materials have been tried and successfully used. Today, in light aircraft, the most common materials used are metal, composite, wood and wood laminates. Usually the more flexible the blade is, the quieter and smoother the prop will be. Lets look at each of these materials.

Metal is really only used in “Spam Cans” and higher performance aircraft. Metal props are heavy, subject to fatigue, are expensive and nosier, but usually have longer useful lives than most other materials.

Composites are synthetic man made materials. Their use typically takes two forms. First is the solid composite, and the second is a foam core incased in a composite sheath. Solid composite props like the Warp Drive are the more flexible of the two, but also the heavier. Foam cores like the Powerfin are lighter, but can be difficult to repair. Composites usually hold up well and have a long service life. Most ground adjustable props available on the market are composites.

Wood is arguably the most used material for light airplane props. Wood props such as Tennessee Props' are light and relatively easy to fashion into props. It is probably the easiest material to damage and the easiest to repair. It too is flexible, but not as much as the solid composite. Wood does not fail from fatigue. It is, however, prone to splitting along the grain, which is always oriented radially hub to tip. When splits do occur they are usually in the tip area. The hub bore and mounting bolt holes should also be inspected for cracks from time to time. All wood can and does fail in bending and will simply break in two if over stressed.

Wood laminates are made from several thin sheets of wood that are glued together under high pressure. Some are made from several different types of wood and some have different sheet grain orientations all in an attempt to customize the flex properties of the finished prop.

If not badly damaged, all props are repairable regardless of the material. Small nicks and shallow dents in a wood prop can be filled and sanded, others materials can be filed smooth without materially weakening them. By following the instructions in the article titled “Wood Prop Repair & Balancing” authored by Jim Hayward, you have a method for minor damage repair on wood props. The article can be found on the Kit Building page, under the menu heading of "Engine Related Articles". In most other cases they should be returned to the manufacturer for assessment and repair.

Special Considerations

In selecting a prop, its intended use must be considered. Such things as the complexity factor, the type of plane it will be used on, whether a floatplane or a land based aircraft. Will it be used primarily for low speed, climb, cruse, or maximum level speed or for all around compromised performance? Things like fixed pitch, ground adjusted, in-flight adjustable, prop material, pusher, tractor, and whether a 2 or 4-cycle engine will be used to drive it, all will play a part in the decision process.

Let's take a look at how some of these might affect the prop requirements.

The complexity factor is probably the first thing to consider. In-flight pitch adjustment is a complication that a basic light airplane does not need. Potential overall performance gains, when used with a 2-cycle engine, are difficult to achieve. The engine power band, tuned exhaust effects versus rpm and the varying load the prop puts on the engine all make it difficult to set these props in flight. 4-cycle engines seem to handle the in-flight adjustable prop much better.

Ground adjustable props are favored by some mostly because they can be adjusted for climb, cruise or max speed. . Many who use them like them. The ease with which the various makes can be adjusted varies widely. Some take much fiddling to get the blades set exactly alike.

Fixed pitch props are designed for maximum performance (efficiency) at one specific rpm and airspeed. If operated at other rpms and aircraft speeds it will not deliver optimum performance. It is impossible for a fixed pitch prop to be equally efficient at a different rpm or aircraft speed than it was designed for.

Consider whether it will be used in a tractor or pusher configuration. It is important that a determination be made for the maximum possible prop diameter that will work, as well as it's normal operating rpm. For any specific aircraft speed, a small diameter prop must be turned at a higher rpm and have less pitch. As the prop diameter increases its design rpm can be reduced and its pitch can be increased. A general rule of thumb is that the largest, slowest turning prop for the desired airspeed will result in the most efficient propulsion.

All prop tip speeds should be kept below the speed of sound, which is approximately 1,100 ft/sec. The maximum tip speed for a metal prop should be kept below 950 ft/sec, composites below 900 and wood below 850. At these tip speeds the stresses in the prop hub can be dangerously high. For safeties sake some prop designers' design to only about 80 % of these limits. Be aware that the higher the tip speed and the more rigid the blade material the nosier and less efficient the prop tends to be.

What other Challenger owners are using

What props are others using and which is best? Now that's a loaded question. Ask ten pilots and you will get at least five different answers. Discussions on the merits of different props usually take on the sound of talk about “girlfriends”. Everyone has his own idea of what makes one “better” than the rest. The following information amounts to a summary of many such discussions.

Seriously though, there are two primary groups or categories of aircraft use or operations in our light aircraft world. They are: floatplanes and land based planes. Each of these categories requires unique propulsion solutions.

Floatplanes typically are relatively heavy and draggy. They require a prop design optimized more toward a climb prop that delivers high thrust at high rpm and at relatively low aircraft speeds. This is necessary first to get off the water and then to overcome the high air drag of the floats and their support structures. This suggests a three-bladed prop with low pitch. Protection from water spray should also be provided.

Land based planes are usually lighter and not as draggy as their floatplane brothers. Except for rate of climb, they generally don't require as much thrust at take-off and climb. Higher cruise speeds can be designed for as a benefit of the lower drag. This suggests a two-bladed prop with a higher pitch.

Operating off water, snow, grass, gravel or a hard surfaced runway presents its own problems in so far as the prop is concerned. Problems arise from objects being kicked up and struck by the prop. If water or gravel is in your planes diet, consider a composite or laminate prop for their bruise resistance or a wood one with leading edge protection. It might be wise to select the prop with the lower inertia when deciding between two equally performing props.

The composite Warp Drive two and three-bladed props are reported to be very tough and arguably the best performers on the market. They also may be the most costly. The three-bladed props are said to be particularly smooth running. For use on floats and loose gravel many prefer the standard tips with the nickel leading edges. Taper tips seem to get the nod for higher cruise on land based planes operating from other types of fields.

There are many other brands of props being used, the Power Fin, IVO, GSC and the TPI to name a few. The Power Fin and IVO are foam cored and are not highly thought of in some quarters. Both are difficult to repair. The GSC gets good marks from many users.

The two-bladed Tennessee (TPI) wood prop that the Quad City factory supplies with the Challenger kit is very hard to beat for all-round use. It probably is the best performer for the money. The most popular two-bladed all-round fixed pitch prop for use with the low redrive (2.2:1) is the 54 x 37. If more climb performance is wanted, usually at the expense of some cruise, try the 54 x 35. For the high redrive (2.6:1) the two-bladed 60 x 44 fixed pitch gets lots of praise for all-round flying. Again for more climb try the 60 x 42.

Well there you have it, everything physical you wanted to know about props. Not quite everything you say? Well you're right, there are things about the engine/prop marriage and use that can greatly effect their combined performance, but those are great subjects for another article.

             Prop Manufacturer Web Links

Appendix A.

The Wrights had reasoned that by applying the principals of wing lift generation; the prop would provide the propulsive thrust they needed. Using their prop design principles they just about doubled the thrust efficiency over that of any other prop design of the day.

From their earlier glider flights, they had determined how much hp was needed to propel their flying machine in level flight at a speed roughly equivalent to its gliding speed. They had reasoned this out by observing the height the flying machine descended at its gliding speed and its weight. That rate of descent, (ROD), they thought was due to the earths' “gravity motor” and all they had to do to fly level was provide an equivalent thrust force. They also knew that 1 HP was equal to 550 ft/lbs/sec.

They figured the required HP would be = (Gw x ROD) / (550 x 60) at a prop efficiency (Pn) and a chain drive system efficiency (Dn) of 100% each.

The numbers below are guesstimates on this author's part but does show the process used.
These calculations seem overly simple today but they do testify to the ingenuity of the Wrights.

Gw = the flyer gross weight = 750 lbs
ROD = Rate of Decent - 400 feet per minute.
Pn = 70 % approximately
Dn = chain Drive efficiency = 85 % approximately
Engine RPM = 5,000
THP = Thrust Horse power = engine power delivered into the air by the prop for propulsion.
BHP = Brake Horse Power = engine horse power after subtracting power lost to heat, friction and other internal losses. This is the actual power delivered by the crankshaft to the prop.

HP = (750 x 400) / (550 x 60)
      = 9.09 at 100% Efficiency

THP req'd = hp/(Pn x Dn)

THP = 9.09 / (.70 x .85)
        = 15.28 (at the crankshaft)

It is generally known that the BHP developed by an engine is not what propels the plane but rather the torque it delivers. The engine must deliver a torque equal to or greater than the drag on the prop at an RPM the engine is to run at. This drag is affected by the rpm and is the result of air drag on the rotating prop, it's lift (thrust) generation, gyroscopic and inertia forces on the prop.

The torque BHP relationship is:

T = (BHP x 5252) / RPM

In the Wrights case:

T = (15.28 x 5252) / 5,000 = 16 ft / lbs produced at the crankshaft

Appendix "B", A Basic Method for Prop Design and Preformance Calculations

When deciding on what fixed pitch prop you want for your bird you must answer the following questions:

  1. How much engine HP and torque is available and at what RPM.
  2. What airspeed do I need the prop to normally operate at?
  3. What rpm will the prop normally operate at considering tip speed?
  4. What is the max diameter prop that will work in my configuration?

Prop design forward speed, rpm, blade angle, pitch and aircraft speed are all related. A prop's efficiency is not measured by the slip but rather the ratio of the air speed entering the prop vs air speed leaving the prop. This ratio (efficiency) is very difficult to calculate. Slip and prop efficiency are however closely related. Some designers find the use of a slip factor in preliminary calculations to be advantageous.

By using the props tip speed limit and the engines best power/torque rpm, the prop tip speed (Ts) and the best prop speed reduction or redrive ratio (Dr) can be calculated by the following formulas:

N = prop rpm
RPM = best engine speed for power/torque
Dp = prop diameter (inches)


Ts = (N x Dp x 3.14159) / (12 x 60) = ft/sec

Dr = (RPM x Dp x 3.14159) / (60 x 12 x Ts)

Figure 1 below is a composite drawing illustrating the relationships of prop design characteristics. Some artistic license has been taken in order to make these relationships more visible and thereby understandable. Keep in mind that all stations of the prop need to advance at the same rate (Pitch). Props are usually measured and rated at their ¾ R blade station.

The concept described below is a very basic and simple method of prop design and performance. It will return fairly good representive data and is very helpful in seeing how a prop works. Although this method is correct as far as it goes, make no mistake about its BASIC NATURE. There is much, much more to prop design and performance than indicated by the method. Such things as the prop profile thickness and platform width distributions, airfoil shapes, thicknesses, cords, angle of attacks and the distribution of loads just to mention a few. OK, with that out of the way let's get into the concept.

Figure 1

It can be shown that prop thrust increases by the square of its rpm; but goes up by the 4th power of its diameter. Therefore it is easy to see why the diameter is so important. Reducing the props diameter by 1", when all other things are equal, will increase the engine speed by approximately 75 rpm. On a fixed pitch prop, it may be possible to increase the effective pitch by decreasing its diameter.

Referring to figure 1, the rotational distance the blade section travels per revolution is denoted as CR.
CR = Dp x .75 x PI
Geometrical pitch, denoted as GP, is the forward distance in inches a blade section advances for each complete revolution.
GP = (Aircraft design speed in MPH x 1.467 x 60 x 12) / (Engine RPM / Redrive Ratio)
EP = Effective pitch (actual forward prop advance in inches) = GP x (1.000 - % of Slip)

Let's see how these formulas work.
Let's assume:

    A/C design speed = 102.5 MPH
    Dp = Prop Dia = 60"
    RPM = 6,400 (Max RPM)
    Dr = Redrive Ratio = 2.6
    Sp = Slip Factor (at props highest efficiency) = 16 % = .16
    PI = constant = 3.14159

From above:

CR = 60 x 0.75 x 3.14159
      = 141.4"

GP = (102.5 x1.467 x 60 x 12) / (6,400 / 2.6)
      = 108,264.6 / 2,461.5
      = 43.9825 " Geometric Pitch (This would represent a 100% efficient prop)

So, if you were ordering this prop, it would be designated 60 x 44.

The effective pitch however would be:

EP = GP x (1.000 - .16)
      = 43.9825 x .84
      = 36.945 " Effective Pitch (This represents the actual advance at the prop design speed)

Then the actual aircraft speed should be:

Airspeed = (EP x Engine RPM x 60) / (Dr x 12 x 5280)
                 = (36.945 x 6400 x 60) / (2.6 x 12 x 5280)
                 = 86 mph
Note: Off-design operations such as changes in RPM and/or airspeed will change the prop slip factor and will affect the calculation of off design airspeed.

If you want to check the pitch you have set into your ground adjustable prop, be sure to use the angle at the .75R station, you can use the following formula:

Pitch = (tangent of the blade angle) x (Prop .75R section diameter) x PI

After following all of the above, it should be clear that everything involved with the production of an airplane's airspeed is related to and dependent on everything else. That may be why simple performance calculations are often found to be misleading.

For those who would like to play with this method of prop design and performance check out the included spreadsheet.