What is the relationship between speed and prop pitch? How much pitch do I need?
Search strings: re-pitch my prop, de-pitch my prop, change prop pitch, prop overhaul,
propeller modification, improve climb rate, rate of climb, rate-of-climb
Technical Editor:
The following is a valuable extract from a posting on MML by Bob Steward, A&P-IA:
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I wanted to ask what people have done to increase their climb rate on the 150HP O-320.
The two proven choices:
* De-pitch the prop (cheapest)
* Increase compression ratio (not STC’d for the E2C, but it has been done with a field approval many times) which is most conveniently done at engine OH.
An additional choice MAY be available:
The rumored PF exhaust may also be an option, but there are so many exhaust systems on the Musketeer series, I don’t know if they have or will get around to approving the exhaust system for all models. This is probably the most expensive mod, some $4000 plus installation.
TECHNICAL EDITOR’S NOTE: The PFS exhaust system is now available for all 1962-1963 Musketeers, all O-320 Sports, and all O-360 Sundowners. Several BAC members have posted enthusiastic praise for the results on their planes.
De-pitching the prop allows the engine to spin up faster on T/O and climb, but in theory decreases the cruise performance.
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I don’t know what Beech engineers were smoking when they pitched the prop on the A23-19. The plane can not ever take advantage of the stock prop pitch in cruise. At 7500 feet Density Altitude, where wide open throttle and 2700 RPM equal 75% power, the plane would have to fly 145 mph INDICATED to be using the 58″ prop pitch. Each inch of pitch is worth ~2.5 mph indicated airspeed at 2700 RPM. You could easily de-pitch the prop to 55 or 54 and gain much T/O and climb, and actually lose NO cruise speed. (54″ x 2.5 mph =135 mph). Who has an A23-19 that will do 135 Indicated?
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Reducing the pitch is what will give you the climb you are seeking. At a cost of ~$150-$200. Another mod you may want to consider is the “Art Matson” tip mod. Your Sensenich 74DM6-58 prop can have 2.5″ cut off the tips (1.25″ off each tip) and the ends beveled to give a Hoerner wing tip profile. This allows the prop to spin faster with the same base pitch, and gives a nice climb improvement, without killing the cruise performance. Pictures are available for your prop shop to work with. No special approval is needed, as this is within the repair limits in the Sensenich prop data.
The basic O-320 power change is to replace the pistons with Lycoming #75089, which is the piston used in the 160 HP version of the O-320. The only STC for this that I have experience with limits the max RPM to 2650 and still rates the engine as 150 HP. However, the T/O and Climb are dramatically improved.
Bob Steward, A&P IA
Birmingham, AL
TECHNICAL EDITOR’S ADDED NOTES:
1. Before making prop pitch changes or tip modifications, please check with your prop shop and A&P-IA to make sure all the legalities are complied with for your plane. The A1CE TCDS spells out the approved props. The prop shop can verify what specs they can meet while still complying with the TCDS for those props.
2. Keep in mind that drag reduction efforts via airframe ‘clean-up’, and horsepower improvements via new pistons and/or the PFS exhaust, may in fact turn your existing prop into a perfect match for the new operating parameters. A higher IAS via reduced drag, and more power available from the engine, will combine to make good use of more prop pitch. If you plan to ever make these improvements, you should make them prior to making prop changes.
3. Keep in mind that each prop has a cumulative ‘lifetime pitch allowance’. There should be prop log entries for any previous changes made to the prop. Once the pitch has been altered by the cumulative max allowable inches, it can no longer be changed. The limit is quite low, so don’t take these changes lightly.
4. Check your current static run-up RPM. Make sure that you can reach at least the 2,300 minimum RPM shown in the POH and TCDS. If you cannot, something else may be wrong that is causing reduced power output.
The A1CE TCDS prop segment follows:
Propeller-
Sensenich M74DM-0-58
Propeller limits Static r.p.m. at maximum permissible throttle setting:
Not over 2400, not under 2300
or
Sensenich 74DC-0-58
Static rpm at maximum throttle setting:
Not over 2400, not under 2300