During the 1996 Galesburg fly-in, the subject of the ground adjustable propellers used on the Stearman came up for discussion. Several owners expressed an interest in seeing an article that covered some of the key issues associated with these propellers. The following article is an updated reprint of that article.
DETERMINING THE PITCH SETTING FOR THE STEARMAN PROPELLER
With the McCauley and Hamilton Standard ground adjustable propellers used on the Stearman, the performance of the plane will, to a great extent, depend on the pitch setting on the propeller blades.
If you don’t know exactly what your engine and desired flying characteristics require in the way of pitch setting, so that you can specify that setting to the Prop Shop, they may set the prop anywhere from 9 to 11.7 degrees (higher for a Hamilton Standard). And that is one huge difference in how the Stearman is going to perform!
The key piece of information that you need in determining what pitch setting you want is the static run-up rpm. This is the rpm that your engine will develop at full throttle tied down, or held with adequate brakes. It should be done with no wind to get the most accurate reading. An often overlooked factor here is the tachometer. A panel tachometer that reads 50 rpm high or low between 1700 and 1900 (and lots of them do) will lead you astray. You need to know the accuracy of your tach!!! If you can borrow an accurate handheld tach, then do so. A Prop Tach is good for this purpose.
A good compromise between climb and cruise is a pitch setting that will result in a static run-up of between 1700 and 1750 rpm. This setting will allow use of full throttle at cruise without going over redline, and still provide adequate climb performance. 1675 rpm or under will be biased for cruise, and anything 1800 rpm or over will be biased toward climb performance.
1700 to 1725 static run-up rpm is a good figure to shoot for. This setting should give close to 95 to 100 miles per hour (statute) at 1850 rpm. Of course there is going to be a fair amount of variation from plane to plane, due to differences in rigging, propeller diameter, drag, etc., but this is a nominal value.
Once you determine the pitch setting for the static run-up that you want, you can specify that setting to whomever does your prop. With a little luck it will come back close to that.
However, just so you’ll know, measure the pitch angle and check the static run-up when you get it back!!! Evaluate the performance of your plane in climb and cruise with the selected setting, and if you are not happy, drop down 25 to 50 rpm for more cruise (higher pitch) and go up 25 to 50 rpm for more climb (lower pitch) the next time (1/4 of a degree would be a good guess for 25 rpm change).
For a McCauley steel prop, either the D-1093 or the 41D5926, the pitch setting at the 42 in. point will probably be somewhere between 11.2 degrees and 11.6 degrees. I use 11.6 degrees. The Aircraft Spec. A743 permits static rpm not over 1975 and not under 1575, with pitch settings from 9 degrees to 11.7 degrees. The diameter must not be over 102 in and not under 100 in.
The Hamilton Standard prop 5404 permits static rpm from 1500 to 1975. The spec does not reference a pitch range. This prop has the same diameter limitations as the McCauley.
The Hamilton Standard propeller requires about one degree more pitch than the McCauley at the 42 in point for the same static run-up. 12.6 degrees works very well on my Ham Std props. There are different blade numbers and shapes that fit the 5404 hub and they will require different pitches for the desired static run up. Remember, use a tachometer that is correct!
run-up rpm that you end up with and from time to time check it to see if it has changed. The rpm can go up with a new engine that is breaking in, or it can go down with an engine that is getting tired, or has a mag or carb problem. The blades can also slip a bit in rotation. When they slip, they will flatten out in pitch and the static rpm will go up. Again make sure that it is not your tach that has changed.
It is a good idea to make an ink, pencil, or white paint mark on both blades just adjacent to and on the hub so that you will know the approximate position of the blade with respect to the hub. Painting a small area on the blade and adjacent hub, and scribing a fine line in the paint works well too. You can then watch for any indication of slip, although a critical amount of shift in the blade position is not easy to see using just adjacent marks on the hub and the blade.
A movement between the blade shank and the hub, at the outside diameter where you make the mark, of just under 1/32 in. (0.026 in.) represents a one degree change in propeller pitch. That means that the total range of pitch from 9 to 11.7 is only .070 in. (a little over 1/16 in) change in distance between the two halves of your mark. And that could vary your static run-up two or three hundred rpm.
MEASURING THE PITCH ON THE PROP
In addition to measuring and knowing your static run-up rpm, you can measure the prop pitch on the plane with a minimum of equipment. Here is one procedure:
1. Par the plane on concrete or asphalt so that the angle of the fuselage to horizontal stays constant. If it is on the grass, let it set for a while so it has stopped settling down.
2. The instruments used to measure the pitch angle on the plane include: a universal propeller protractor, a clinometer, and an electronic level (WEDGE PRO-Smart electronic level). The universal protractor and the clinometer are a bit hard to come by, so the electronic level is a good choice. It is accurate to 1/10 of a degree, and you can get the PRO-Smart or equivalent at any builders supply or good hardware store. Learn the calibration procedure and use it just prior to measuring the prop pitch.
3. Turn the prop to level by measuring across the top of the hub with the level if it is flat enough. Another method is to use “v” blocks on the shanks of the blades and measure across the top of the “v” blocks.
4. Measure the full width of the hub accurately with a scale (about 10 in. for the McCauley and about 14 in. for the Ham. Standard). Take half of the hub width and subtract from 42 in. Measure out that distance from the outside edge of the hub and make a mark in the center of the back of both blades (about 37 in. for the McCauley and about 35 in. for the Ham. Standard). This gives you the 42 in. point from the center of the hub.
5. Using a machinist’s square with a built-in level, line up the long blade of the square with the mark you made on the blade, and with the bubble showing level on the right angle portion of the square, draw a 1 to 2 in. line on the back of both blades near both the leading edge and the trailing edge (in line with the center mark and vertical as shown by the square level).
A white fine line paint marker that you can get at an artist supply shop works fine. It is fairly permanent and easy to see. A pencil line on a black blade will drive you crazy.
When you order the white font letters for the information on the front of your prop, ask for several straight lines of the white, 1/16 inch wide and 7 inches long. Applying one of those to the 42 inch point lasts a long time and is very visible.
6. Keep checking to see that the prop is Jack Davis Tribute 30 Jack Davis Tribute | STEARMAN FLYING WIRE level. Hold the ProSmart Level against the front surface of the prop nut with the level as near vertical as you can get it (line it up with the #1 cylinder), and take a reading. Turn the level around 180 degrees and take another reading. If it is not the same, you need to recalibrate the level. Record the vertical prop hub nut reading on the chart. Later you will take another hub nut reading with the prop turned 180 degrees. You will average these two to cancel out any alignment error in the hub nut surface and get the angle of the propeller plane with respect to horizontal.
7. Figure # l is a chart which can be used for recording the data when measuring the prop pitch.
8. Now on the back of the right hand blade, align the left edge of the level along the vertical white line that you made, with the level body outboard of the white line. Rock the inboard edge of the level toward the blade until it first just touches. This gives a three point support for the level, with the left hand edge of the level right on the white line. Take an angle reading and record under Blade #1. (Somewhere around 67 degrees.)
9. Rotate the prop 180 degrees and relevel.
10. Take another vertical hub nut reading and record. Take an average of the two hub nut readings (halfway between the two) and record.
11. Now measure the angle of blade #2 which is now on the right and record.
12. The average value of the hub nut readings minus the angle for blade #1, is the pitch of that blade in degrees at the 42 in point.
13. The hub nut reading minus the measured angle for blade #2 in the pitch of that blade.
14. They of course need to be the same. It is worthwhile to go through this measurement procedure two to three times to be sure the readings are consistent.
The reason for using the right hand blade is that the angle of the left hand blade with respect to horizontal when the fuselage is on level ground, is very close to 90. About 89.5. You have to be very careful in knowing what side of vertical you are on and there is the danger of a brain cramp trying to visualize it. This problem goes away using the right hand blade.
RESETTING THE PROPELLER PITCH Setting
the pitch on a ground adjustable propeller requires that an A&P perform the work, but here is one way that it can be done.
1. Again, be sure to make a mark on both blades just adjacent to, and on the hub so that you will know the position of the blade with respect to the hub before you start.
2. Make a similar mark on each clamp and hub so that the clamp can be put in the same rotational position when it is retightened. This is important to retain the vertical balance.
3. Remove the cotter pin on the clamp bolt and loosen the nut a couple of turns. Always keep the blade with the loose nut level or pointed down. If you point the blade upwards, it can move in away from the hub shoulder.
4. With a rubber hammer strike the leading edge or trailing edge of the blade to give a very slight rotation (depending on whether you want more or less pitch). Watch the mark that you made on the blade and hub for sign of movement (remember-1/32 in. is a lot!!). If it doesn’t move, loosen the nut another turn. If it moves too easily, tighten the clamp nut a bit. Level the prop and take another pitch measurement to see where you are. Repeat this process until you get the pitch that you want.
Make sure that the clamp is in its original position and retighten the nut to specified torque. (1100 in-lb. +/- 100 in-lb. or 83 ft.-lb. To 100 ft-lb.) I somewhere have a technical notice that says to use the 100 ft lb level of torque, and that is what I use.
Insert cotter pin and measure the blade pitch to see if it is the same as prior to tightening. They will sometimes move on tightening, and not always in the same direction. Record this angle. You will match the other blade to this value.
5. Rotate the prop 180 degrees so that the second blade is on the right and repeat the procedure for that blade. It is important here that the pitch angle on the second blade match the pitch of the first, after both are tightened.
Balancing a ground adjustable propeller again requires the services of an A&P or certified prop shop. This is the general procedure used.
These propellers are balanced by adding weight (clay) on the outside of the light blade when in a horizontal position on a balancing stand. The amount of clay is adjusted until the propeller stands in a horizontal position without moving. Put a piece of tape with a mark on the blade at the position of the center of the clay. The distance from this mark to the hub end of the blade will be measured later. The propeller is then disassembled. Measure the distance to the mark on the tape from the hub end of the blade (D1). Then measure the distance from the same hub end down the inside of the blade to the center hole where the lead is peened in (about 11 inches) (D2).
Carefully weigh the clay ball that provided the correct balance, to 1/10 of an ounce.
Determine the weight of the lead to be peened down into the center hole of the blade as follows:
Clay Wt X D1 = Lead Wt.D2
Weigh out the correct amount of lead and peen it down into the center drilled hole on the interior of the light blade shank.
Heavy lead wool and a 1/2 inch diameter steel dowel works well. If there is a lot of lead, a length of 1.2 inch diameter lead dowel can be used.
The propeller is then reassembled and put on the stand to recheck the balance. If it is not in balance, then the procedure is repeated until horizontal balance is achieved.
The McCauley D-1093 has a ring on each end of the hub that looks like a bicycle sprocket. Fine balancing is accomplished by turning the appropriate ring in or out and then moving the clamp up against the ring. This greatly simplifies the final balancing procedure.
Vertical balance is achieved by turning the propeller to a vertical position on the stand. If it moves off of vertical, then the vertical balance needs adjusting. This is done with the hub clamp and bolt combination. (See diagram.) To start with, both bolts should be positioned forward symmetrically about the center line of the hub. The bolt head is supposed to be pointed toward the leading edge of the blade that it holds. The clamp has a flat that captures the bolt head so that the nut can be turned for tightening. If the flat on the clamp is pointed toward the trailing edge of the blade, then the clamp is on backwards.
Now to adjust the prop for vertical balance, rotate both clamp/bolts in the same direction toward the light side, until the prop stands in the vertical position without moving.
Rotating the clamps in opposite directions, may look more pleasing, but does nothing to achieve balance. When you see a propeller with both bolts directly on the front of the hub, or symmetrically positioned in opposite directions, you can bet that vertical balancing has not been done. The technical order that covers propeller balancing, states that the clamps should never be positioned so that the bolt is across the parting line of the hub (which would be 90 degrees from the forward position). It also states that if the bolts have to be moved more that 45 degrees from the front position, the blades should be balanced internally. On either side of the center hole in the blade shank that was used for inserting lead for horizontal balance, there are two holes for inserting lead to achieve vertical balance. This is a separate balancing procedure, and shouldn’t be required. If lead for horizontal balance is inadvertently put in one of the two outside holes, it will throw the vertical balance out, and make it difficult to adjust with the clamp positions.
One more tip. When the propeller is installed and the hub nut is tightened, there is supposed to be a clevis pin in the matching holes in the end of the crankshaft and the hub nut. The pin is installed with the head on the inside, and a washer and cotter pin on the outside.
A preflight inspection includes wiggling the clevis pin to see that it is still loose. That tells you that the hub nut has not moved. A bolt and nut that is not tightened down, but has a cotter pin, will serve the same purpose, but will add more off-center weight. A bolt and nut that is tightened defeats the purpose.
Having been through the ordeal of having prop blades slip when the shanks are painted with the same primer and black paint as the rest of the blade, the problem went away (with a lot of testing verification) with the use of the following paint on the shanks. This paint was specified by a prop shop that we were using at the time.
The blade shank is painted about 1/8 inch beyond where the hub ends. Completely stripping the shanks to bare metal also prevented slipping, but going without any surface protection is asking for corrosion trouble. These are the coatings that have given the non-slipping result. They are Sherwin Williams Industrial coatings that are not available in the normal retail store. There are color choices, but we used gray.
One thin coat of Industrial Wash Primer, P60G2 (1 part) and R7K44 catalyst/ reducer, (1.5 parts) Top coat, (keep it thin), Polane S+ urethane enamel, (6 parts), V66V29 Catalyst (1 part), and R7K84 Reducer (1 part)
These paints require professional skills in mixing and application. Also, again, the clamp bolts should be torqued to just under the 100 ft-# maximum. The 83 ft-#, which is the minimum called out by the spec, is just not enough.
After torquing, you should be able to slip a thin feeler gauge or piece of paper between the two halves of the hub on the front, back or both. This insures that the blade shanks have not been reduced in diameter, and the hub halves are not bottoming out.