L2 Prop Surprise

[Ed O'Brien]

I'm taking the Sensenich Wood Prop 72-42 off of my L2 (with a C-85 Don's Dream Machine engine just installed)
I'm working on a new metal prop configuration. For calculating the right pitch I tried a McCauley 71-48 today and generated 2350 static on the run-up. (This is the static upper limit for a C-85 on a BC12D) Mind you, I'm in Denver with PAs at 6500-7200. To get this right the prop shop is recommending a McCauley 71-50... maybe a 52 pitch. My cruise should scream and I'll be passing 140s-170s like a car thief in LA afternoon traffic. Maybe I'll see 105-110, maybe. My question is... Does this sound right to you? Anybody wanna race? Provided I get off the ground. My home field runway is 4700 ft long... I don't want to wait too long to get off the ground... I'd like to use as much of the extra power as I can, without my L2 hugging the ground for a mile or two. What do you think?
With regards; ED OBRIEN

[Ragwing nut]

Re: L2 Prop Surprise

I would not go anymore than the 71-48 and I would double check the actual pitch of the prop, not what is stamped on it. I have run a 71-48 on our stock 85 L-2M and would have preferred it to actually be a 46. Cruise was 105, turned right at redline full throttle level flight.

Mike

[Ed O'Brien]

Re: L2 Prop Surprise

We checked the prop pitch on the old 71-48... it is thinner than a new prop would be and slightly lighter. It's an old one they had around the shop. I'm supposedly slinging 95 HP with this new Dons Dream Machine engine. SO maybe that's the difference. I get one re-pitch for free on the new prop... so once I check it on the plane and with this engine I can repitch it several degrees if need be. By my calculation I should indicate 106-110 with a 50 pitch. Interesting set of decisions and calculations.
With regards;
ED OBRIEN

[VictorBravo]

Re: L2 Prop Surprise

Interesting set of decisions and calculations.
With regards;
ED OBRIEN

Err on the side of climb, Ed. A very wise and learned man once mentioned on this forum the fact that there are flatlander types dotting the local mountains in Colorado

IMHO 97 or 100 mph cruise in an L-2 and still maintaining good climb performance should be a big winner up there.

[Ed O'Brien]

Re: L2 Prop Surprise

VictorBravo;
As is the usual case with you and your advice, I agree. That's why I'm looking for the feedback. Likely, I'll order
the 71-50 and have it pitched back to whatever works. I don't need the screaming speed as much as I need the damn altitude. As I've posted numerous times... bumping into to things is not a good idea, anywhere, particularly here, and I'd be quite shamed by bumping into something myself. You guys would giggle all the way to the prop shop.
With regards;
ED OBRIEN

[M Towsley]

Re: L2 Prop Surprise

Ed,

I will have to do some research to confirm the metal prop pitch but my friends L-2M with an 85 climbed well, but my old BC-12D walked away from it. He had it at redline just to stay up with my cruise, and my BC-12 was not THAT fast, (65 HP). I have his stamped pitch written down somewhere around here.......

[flyguy]

Re: L2 Prop Surprise

I probably missed something, but is there some reason that you can't TEST FLY it with the 48 on it?
Darryl

[Ed O'Brien]

Re: L2 Prop Surprise

Flyguy;
To answer your question -- "can I fly with the 71-48?" No I can't. It's not mine... the logs are missing... and it may not be airworthy... although it specs out OK. The shop won't allow it to be flown. It's too thin for me to buy, overhaul it, and get it recertified... right now it's at minimum width with no extra meat to shave for balancing and the like.

Hey, there always a reason... and there you have that one. Because it's at the thinnest it could be... literally less than 1000th over minimum... they're probably gonna let me have it for mounting at home. Because that thiness makes its minimum for weight too...the shop has calculated a 50 pitch on the new blade to compensate for the lighter blade that we're comparing it to. Ya... it's always something.
Thanks for the try; ED O

[NY86]

Re: L2 Prop Surprise

71-48 is a standard prop for the C85 in a host of 100-110 mph airplanes. 71-50 is a cruise prop in the same planes. It would be standard in a faster plane.

Take the 95 hp claims with a grain of salt, unless you're flying a dynometer or a controllable pitch prop.

Given your altitude, I'd stay with the 7148. This will give you "standard" cruise performance and slightly improved climb due to the increased displacement and compression (assuming you have the O200 crank STC installed.)

[LLBOYD]

Re: L2 Prop Surprise

Ed, I have a DC-65 with a C85-12 and a SEN MK74AK-4-50. This gives me a static of 2150 and redlined at 2375. This was the limitation placed on the one time STC when we installed the 85 in 1965. This is at sea level. Even here on the coast the climb is somewhat subdued but at 5k and above the cruse is good. I have consistantly crused at 107 to 113 MPH with the modified enclosed cowl, fairing on the struts, landing gear and as much wash removed to allow hands off. Yes it will get on the step. The only issue we have had is when solo you are bumping the forward CG as it has the standard mount.

Larry

[flyguy]

Re: L2 Prop Surprise

Larry, What is the real diameter of your 74? I understand that some of the props with 74 part numbers are shortened by the manufacturers.
How is a 74 signed off on an 85? I Don't mean that critically, I'm just wondering how it is done.
Darryl

[Forrest Barber]

Re: L2 Prop Surprise

Always check real dia. then remember that McCauley & Sensenich measure pitch at different stations, then remember that the BC12D is fatter and faster and likes the 74 in prop.
The L-2 ( Model D) is narrower and slower just like C.G. said it would be , the 71-48 was a popular and approved prop for the 85 HP. Try different props and lets see what happens....... I use the 74-45 for 65 hp & the 71-48 for 85 Hp and the 70-38 for the A-75 & A-80 . All this has been posted before some where in cyber space .

[Ed O'Brien]

Re: L2 Prop Surprise

Thanks Forrest;
The prop shop has measured everything and said it to be just as I explained. I'm still alittle dubious but have ordered the McCauley 71-50. That said, I get one repitch for free... so we'll have to wait for the next static test with the new prop. If there's any room for pitching it to 48... I'm for that. My question to you would be... does the 93-95HP Dons Dream Machine make the difference and therefore while the 71-50 would accelerate slower the HP would make up the difference and therefore the take would be about the same? Or is there something in this calculation that I've missed? A non-technical answer for a simpleton
such as myself would do fine.
Thanks for your help with this -- I appreciate all the feedback on this issue!
With regards;
ED OBRIEN

[Ragwing nut]

Re: L2 Prop Surprise

Larry, What is the real diameter of your 74? I understand that some of the props with 74 part numbers are shortened by the manufacturers.
How is a 74 signed off on an 85? I Don't mean that critically, I'm just wondering how it is done.
Darryl

He does not have a 74 on there, he has a 70" prop. Sensenich stamps the full length of 74 which is part of the model number then it lists a -4, which is 4" shorter making it 70" long. If it was a 74" prop, it would read 74AK-0-50. I have done several stroker installs and I don't repitch them anymore. Most are happier to have the better climb performance for the 100*+ days. I think the 71-48 Macauley to be a little much and would probably work great with you stroker motor. I would order the 71-48 then pitch up if you decide later. The L-2 is heavier (by a bunch) over a BC-12D on the same HP and can benefit more from the flatter prop.

Mike

[LLBOYD]

Re: L2 Prop Surprise

ED, Mike is correct. The prop is 70 inches by 50 pitch. When I get the restoration finished I plan to repitch to 48. My installation is a one time STC which has the RPM limited to 2350/65 HP. I am interested in other STC's that will allow for the full 85 HP. Are you using a shortened mount or the original. By the way N70B has a empty weight of 834 lbs. with a +16.1 EWCG.

Larry