John Simic:
Standard prop on a 63 160 Lyc is M74DM-0-60 (climb prop option is 58). Another member has a Sport (pumped up) 150 Lyc with a standard 56 pitch prop & has been mulling increasing pitch as motor is making way more power than standard, or stock configuration.
Understand this, but then, have been paying attention to the parting out of an A23 with a 165 hp engine & prop is M74DC-0-56. Confused here. 160 hp prop comes with a 60 pitch. Understand why a 150 hp would come with a 56 pitch,don’t understand why a 165 hp would have a 56 pitch?
What about re-pitching a prop to a different value?
Phil:
What about putting a constant speed/variable pitch prop on an O-360, or any of them for that fact? Sounds like a good idea to me but I’m not all that knowledgeable about it.
Bob Steward, A&P-IA:
Its just the cost.complexity issue. A fixed pitch prop requires no special pilot training, and is “cheap” at about $2500 today. A C/S prop and governor requires additional pilot training to make sure that the ability to control the prop does not end up damaging the engine from overspeed or lugging. There is the cost of the prop ($6500-$7500), plus a governor ($1500-$2000), plus a MP gauge so the pilot can set the proper % power. Considering that this ($8500-$9500) is a significant percent of the purchase price of the plane, most light aircraft do not have or need a C/S prop.
The advantage would be improved T/O and climb performance, and more flexibility at altitude in cruise. In actual practice it’s cheaper for the manufacturer to put the next larger engine in the plane to get the T/O and climb performance.
Its not until you break the 150 mph mark that you really start to see the need for the range of pitch that a C/S prop gives. Interestingly enough, that is right at about 60″ of pitch on a clean airframe. Most of the Musketeer series are “over propped” in my opinion. Take the pitch in inches, multiply by 2700 rpm, and divide by 12 (to give feet per minute instead of inches), multiply by 60 to give feet per hour, and then divide by 5280 to give MPH. This gives the theoretical top speed of a given prop. For example, 60″ = 150 mph.
(You can also convert feet per minute into mph by dividing by 1.46)
Anyone got a 150 mph Musketeer or Sundowner? Then you are trading off T/O and climb performance for no gain in top speed. In fact, over propped planes lose speed, since they can’t pull the prop to engine redline in level flight; and they can’t spin that high pitch prop as fast at altitude, so they lose some service ceiling, too.
To see how well matched your prop is to the airframe and the engine horsepower, fly at full throttle at 8,000′ density altitude. You’ll have to break out the E6B to do the conversion, but its for a good cause. Note the max RPM you can get after leaning to MAX POWER by peaking the RPM. If your tach is calibrated, then you are ready to see if you are in fact getting all the power that the engine has, by turning 2700 RPM redline. At this altitude that is about 75% power because of the reduced air density.
Getting LESS than 2700? This means your prop isn’t optimized for either climb OR cruise; it is less than ideal for both. You could actually use a flatter pitch prop, and get better climb AND faster full-throttle cruise. Figure on 1″ of pitch for each 50 RPM, to see how much change you need. (Caution, 20 year old mechanical tachs often run 100+ RPM slow!) Consult the Type Certificate Data Sheet to find your allowable range of pitch, and then talk with your prop shop. Each prop manufacturer allows some total number of inches of pitch to be bent into the prop over its lifetime. The blank is forged and machined at some neutral pitch, and then the initial bend is made at the factory to set the pitch as ordered. This is part of the total bending that can be done. Usually its 8-10″ of pitch, and it is not uncommon for 4-6″ of that to be used up in the manufacturing.
If your 74DC-60 was forged and machined a 56″, and then bent to 60″ before shipping out, then that 4″ is subtracted from the lifetime bending allowed. The prop shop can tell you the exact limits on each prop model. Or call Sensenich with your model and serial number and they can tell you how many inches they bent it new, and how many are left to be bent in the field. Just because the prop has some bending left, doesn’t mean it is legal to tweak it to just any old setting. You must consult the AIRFRAME Type Certificate Data Sheet to know what the legal pitches are for that particular model. The TCDS can be easily viewed on BAC.
The O-360-A4J/K/M series used on the Sundowner have a solid shaft, and would not be suitable for a C/S prop. However, even if they didn’t, a teardown and change of front bearing, and in many cases the addition of the fitting for prop oil pressure, and the drive gear for the rear mounted governor, would all have to be installed. Its not like
it would be a bolt on conversion, even if it were approved.
The relative cost of swapping a good A4K solid crank for the correct P/N hollow crank with counterweights would be minimal, when compared to the $5000 in labor to remove, teardown, modify, assemble and hang the engine,
plus the $8-10K for the prop, governor, etc. If one wanted to do it, the hold up would not be a $1000 price to swap cranks with an engine shop.
Willis Cooke:
The 56 pitch is an optional climb prop for the Continental. My Continental has a M74DC-0-60 and will turn about 2600 in level flight with it. I cruise at 2500 and about 105 knots. My worn out M74DC-0-60 would turn about 2750 in level flight and cruise was about 100-102 knots. Climb was better. The key arrangement is different for the Continental and the Lycoming, so you can’t use the M74DC on your 160 Lycoming.