Basic info on props

[joea]

Paul,

Question for you or anyone knowing about props. This is really about all props but asking about the Sedan prop located in the parts for sale section here: http://www.joea.com/phpBB2/viewtopic.php?f=10&t=4044

74CD50 Prop

The large number (in this case 74) pertains to the diameter of the prop.

The smaller number (in this case 50) pertains to the pitch or bite of the prop.

Is there any chart or info on which type of prop this one is? Or for that matter, any other prop?

The prop on the Sedan right now is a "DM76-49" McCauley Met-L-Prop version. I also have the old Sensenich wooden prop that you turned me onto on Ebay a couple of years ago that needs overhauling. Its a 73BR44. The Flottorp on the "for sale" thread is a 74CD50. Am guessing that the prop on the plane now has less pitch than the Sensenich making it a climb prop, but that the Flottorp is more of a cruise prop.

What I am getting at is, is a 50 pitch prop a cruise, climb or standard prop? I know it changes from airplane/engine for each application but hopefully the basics are the same.

As well while on the subject, are the numbers on a wood prop the same with a metal prop?

I see on the chart on ACS's website that an early Cessna 172 that uses a Continental C-145 like the Sedan recommends:

DM 76-55 Standard
DM 76-56 Cruise
DM 76-53 Climb

In other words the larger the number, the less "bite" the prop has on the air, and thus the higher the RPM's and hopefully the horsepower. The "56" version above would have a deeper bite on the air and faster cruise, but lower RPM.

This sound correct?

Thx,

Joe A

[seaheli]

Joe,

Hopefully I can explain this without confusion. (fixed pitched propellers "not" constant speed)

Climb Prop: larger diameter-low pitch = lower rpm = torque. (large dia. = increased area = lower rpm)

Cruise Prop: shorter diameter-high pitch = higher rpm = speed. (increasing rpm increase horsepower)

There can be a fine line in the equation (rpm) with to much or to little pitch. Also the angle of attack (climb & cruise attitude) affects the function of performance.

Charles

[joea]

Charles,

Thanks but all of these props used on the Sedan are the same diameter from the way I understand it. Was under the impression that the props were the same diameter and the pitch was different.

The prop on our Sedan has been re-pitched one time, from a -47 to a -49. Am assuming that this made it a cruise model?

Joe

[seaheli]

Joe,

That is the practice, to increase cruise speed you increase pitch.

Hopefully Paul will have a bit more wisdom on this!

Charles

[joea]

Thanks and next question.

How do pitch rates compare on a wooden prop versus a metal prop?

Would a metal prop with a 50 pitch be the same as a wooden version with a 50 pitch?

[Paul Agaliotis]

Joe,
First, you can only compare the length/pitch information to the same manufactures. A McCauley 76/55 will not perform the same as a Sensenich 76/55. The reasons are endless but to boil it down, blade profiles and airfoil shape play into the differences. Now you can only compare wood/wood and metal/metal.
In a perfect world the prop above will move forward 55" per revolution, but it's never acheived. At this point torque and drag show up. As drag increases the prop starts to slip in the air and only moves forward 50" per revolution and the airplane slows down. Increase the torque of the engine and you will gain back the drag loss up to a point. Decrease the drag and you will need to add prop pitch to keep everything constant. It's kind of a cube root problem. Boy this is getting confusing. In a nut shell your 145HP Continental uses a 6 1/2 ft prop but my 145HP Warner uses a 8ft prop. You can't spin my prop and I over torque yours.
To find the correct prop you need to have the maximum manifold presure at rated RPM. This is for fixed pitch and non-boosted applications.
Props are a confusing subject due to the different forces acting on the planes. I hope this helps to understand the problems of selecting the perfect prop. What works good on a Taylorcraft might not work on an Aeronca, even though they have the same engines.
Now my head hurts.
Paul

[joea]

Paul,

Thanks and I stopped thinking as it gave me a headache!

Talked with the guys at the museum prop shop today. Very nice to chat with and hope to be doing some business with them.

Joe

[Paul Agaliotis]

Joe,
That was either Herb or Guy, both great guys. I look over their shoulders on the aircraft being rebuilt at the museum. Herb is just about done with a prop for my Jones replica Chief.
Paul