PROPELLORS (Propellers)

[dacker]

My propellor is an MDM 7653. Is that the same as the 1A170DM76XX that the TCDS lists? I am not sure what the serial numbers are, but I can check the next time I go to my hangar if that has any bearing.
Also, since I am rebuilding my engine, what are thoughts on sending it in to a propellor shop for testing and repitching?
David

[dacker]

Oops! I goofed, it is a DM7653. Is that the same and could I get it repitched to a 51, and what kind of airspeed can I expect?
David

[Bruce Fenstermacher]

This is what I've learned from this forum. The DM and the MDM are different props in that the airfoil is different but they are both approved for the 170

The following figures I borrowed from Eric's post in the carb thread and are his numbers for his plane. He claims the constant 1056 works for his plane and if yours where flying (I seem to remember it's not) you could adjust the 1056 so the formula gives you the mph you see for a given rpm. Once the variable is established you can change the prop pitch to see how it wold effect MPH

Pitch X RPM / Variable = MPH
51 X 2400 / 1056 = 115.9 mph
51 X 2550 / 1056 = 123.1 mph
51 X 2700 / 1056 = 130.4

Following the above examples but substituting a 53 pitch prop would give the following:

53 x 2550 / 1056 = 127.98 mph

About a 5 mph difference from the 51 pitch for any given rpm.

I must have a very SLOW airplane or Eric's is fast because mine only does about 118mph at 2600 rpm with a 54 pitch DM prop. I have no idea how mine compares to either George's or Joe's plane.

This all has been discussed before so do a search and I'm sure you find other examples of airspeeds you can use.

[zero.one.victor]

From what I've heard, when you take your prop into the prop shop for repitch they ask you how much more static rpm you want. Seems like they must have a method of measuring the pitch, so you couild tell the "51" or whatever. FWIW, I have a DM7651 on mine, and I wouldn't want any more pitch than that if I was gonna operate off short strips or in the hills. A flatter pitch doesn't necesarily mean you'll go slower, you just have to turn it up a little more-- which it will do easier, and with less manifold pressure. So you don't sacrifice speed or fuel economy either. IMHO.

Eric

[FredM]

Here is the break down for the complete model number. 1A170 is the basic design number. DM means it is drilled for installation on a number 3 flange (8 bolt) and has square tips. DF means it is for installation on a number 3 flange and has elliptical blade tips. GM means it is for installation on a number 2 flange (six holes) and has square tips. The next two numbers (76) are the diameter of your prop. The last two numbers (53) in your case, indicates the inches of pitch measured at .75 of your props radius. There is not a MDM designator for the 1A170 prop. A 1C172 prop does use MDM to indicate use on a number 3 flange. In other words if you are comparing a 1C172 MDM to a 1A170 DM the difference is the basic design of each prop 1C172 vs 1A170. the MDM means the same to a 1C172 as the DM does to a 1A170. Sorry if I have confused anyone even more.

[N1277D]

The 1056 number provides the True Airspeed at standard temperature and pressure; for most of us the indicated airspeed is usually lower. The trusty E6B or another calculater is needed to make a one for one comparison; if you plug in the temp, pressure and altidue and do the conversion you should find closer agreement to the 1056 conversion from pitch/rpm to mph.

[dacker]

Fred, I think you are hitting on what I was asking. I can't get to my prop for a few days to double check the markings that are on it, but if my memory serves correct, it just has DM7653 stamped on the front of the hub, and serial numbers on the side. I don't recall seeing the 1C170 part stamped anywhere. I am trying to establish that I have the right prop before I send it in for inspection and possible rework. My logs show the engine coming from a 56 C172 way back in its history, it is possible that the prop came with it.
I am curious about the MDM props. Dave, you say that the MDM props are approved as well, and it seems to me that they should be, but the TCDS only shows the 1A170DMxx props. Am I missing something on the designation, how are the MDM props certified for the 170?
Thanks for the prompt replies, part of the fun of ownership is the learning experience.
David

[zero.one.victor]

" Aircraft Specification No. A-799
...............................................
item 1. Propeller
(a) McCauley 1A170........
.............................................
item 7. Propeller- McCauley 1C172/MDM "


Eric

[dacker]

Duh... I R Goin tu larn tu reed wun of thez daz!
Thanks, Eric. I looked at the TCDS several times and missed it each time!
Mystery solved!
David

[dacker]

I was able to lightly buff away some of the paint and read the designation on my prop better. What does Klip tip mean?

[Bruce Fenstermacher]

It stands for a prop design where the prop tip is cut straight at the tip. When compared to a standard style prop that has a rounded tip the Klip Tip looks like the tip is "clipped" off.

[GAHorn]

Just to add a bit more to this discussion, ... the original DM series had a slightly thicker blade than the MDM series. (MDM series blades, being thinner, are more efficient and lighter.) The EM series is essentially the same blade as the MDM, but being a 6-bolt hub, it's smaller/lighter still, which is why Continental went to the 6-bolt design. So in other words, if you have the 6-bolt O-300 C/D engine, then your EM series prop is the same blade as the old 8-bolt MDM series prop, but with a slightly more efficient hub-to-blade profile, therefore the pitch is a bit more for the same performance/hp output (rpm). An EM7655 is about the same performance as the MDM7653, therefore the 6-bolt design is slightly more fuel efficient, all other things being equal.