Engine Experts... C-85 Question

[VictorBravo]

Re: Engine Experts... C-85 Question

The tach that was in the airplane originally when I got it is a "normal" 3 1/8 size unit, with an integral hour meter / Hobbs. I don't know if this is affecting anything as was suggested.

I have a brand new NOS tach in a box that I can try, but if this "new unit" is 40 years old then could the magnet be weak already?

To the chagrin and annoyance of everyone in the FAA, this is a job for a good $20.00 model airplane tachometer!

[Vasey fan]

Re: Engine Experts... C-85 Question

Coming in on this one a little late, but I'll throw in my 2 cents anyhow.

I'm with Eric as far as the too much prop opinion, 50 inches of pitch is a pretty big bite. I'm running a 71-48, and most days I barely make static. (Yeah, I check it every time - I fly from a 5k field elevation).

As far as all the theories about the rpm/map relationship, I'm just gonna sit on the sidelines & watch..maybe get some popcorn...

[astjp2]

Re: Engine Experts... C-85 Question

The A65 tach and the C85 tachs are calibrated different. I would guess your tach is wrong.

The tachs are calibrated for the HOBBS if it has one, if not they rotate at a 1-1 ratio for both, The only thing that can really cause a Tach to go out is the flying magnet may lose some of its ability to pull the needle through the swing. Some Tachs do have an adjustment for the spring tension on the needle, if in doubt send in your tach for test. Most speedo shops also have that ability to check it. If you are lush for funds, you can buy a photo tach and compare the acft tach at several different rpms for comparison. Tim

[astjp2]

Re: Engine Experts... C-85 Question

The C-90 and O-200 have EXACTLY the same crank by part number...The same rods and even pistons.....per the manual that I currently have. The C-150 had a 74+ inch seaplane prop approved on the O-200. Its a 1A90CF (seaplane only) not over 75 and not under 73.5, on page 4 of the TCDS

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/a01fa66c9a3bc9498625745100674f44/$FILE/3A19.pdf

[NY86]

Re: Engine Experts... C-85 Question

You can request a field approval to install the Cessna Sea Plane prop, its 76-74"

Where do you find that? The only one I'm aware of is on the C90 where up to 73" is permitted. The C90 has a different crank and RPM limit, so vibration characteristics are different. AFAIK, there's no approved 1A90/1B90 installation on a C85 longer than 71".

The markings on the A65 tach will be wrong, but if you get it to turn the right way it should read right. Both are "1:1". Problem is most old tachs are really inaccurate.

[crispy critter]

Re: Engine Experts... C-85 Question

The A65 tach and the C85 tachs are calibrated different. I would guess your tach is wrong.

[VictorBravo]

Re: Engine Experts... C-85 Question

OK, ignore me. I don't care.

Wouldn't DREAM of it ! I appreciate your input.

My tachometer is somewhat suspect because there is a reversing adaptor on it. Meaning that I am using the same tach that the A-65 used, which I should not be able to do with an 85. I suspect that this was a common thing where people wanted to use the same tach as the previous 65HP engine used.

I will check the RPM's soon with some sort of an optical tack checker.

[Ragwing nut]

Re: Engine Experts... C-85 Question

There is a hundred issues that can cause low rpm, not as simple as just the wrong pitch. 71-50 is too much, I don't like anything more than a 71-48 for cruise prop.

Flat engines not like to be ran over square, unless you have a constant speed. Without the CS, you are burning more gas without increase in prop efficiency.

Performance will be about the same if the stampings are true. The increase in length with offset the decrease in pitch.

Mike

[astjp2]

Re: Engine Experts... C-85 Question

You can request a field approval to install the Cessna Sea Plane prop, its 76-74" (I am running a 74-43) depending on which model you want and they do CLIMB:yahoo:

Your BMEP dictates what prop rpm and loading you want. Temporarily install a manifold gauge and see what you are running and use those numbers as a comparison....

[NY86]

Re: Engine Experts... C-85 Question

.....or your prop might have more pitch then it says on the hub.

Good point. Trust no one!

McCauley doesn't approve the 1A90/1B90 props on the C85 in lengths over 71 inches... 'Course, if it won't turn over 2000 RPM you can call it a C75 and lengths up to 74" are approved there...

[crispy critter]

Re: Engine Experts... C-85 Question

OK, ignore me. I don't care.

A Cessna 140 will turn a 7150 prop around 2150 static. Static RPM is airframe independent, so you should get the same numbers with a Taylorcraft (or a Luscombe, or an Ercoupe,..., etc.) RPM is always higher than static if there is any airspeed, so climb in the 140 is going to be higher than 2150. The same numbers apply to the Taylorcraft; you should see static higher than 2100 and climb higher than static. If you don't, (and you don't since you're climbing at 2000), something is wrong. Check your tach. If it's not reading low then your C85 is not making rated horse power.

.....or your prop might have more pitch then it says on the hub. I think you m ight like the 74/47 better anyway.Let us know how it turns out.

[NY86]

Re: Engine Experts... C-85 Question

OK, ignore me. I don't care.

A Cessna 140 will turn a 7150 prop around 2150 static. Static RPM is airframe independent, so you should get the same numbers with a Taylorcraft (or a Luscombe, or an Ercoupe,..., etc.) RPM is always higher than static if there is any airspeed, so climb in the 140 is going to be higher than 2150. The same numbers apply to the Taylorcraft; you should see static higher than 2100 and climb higher than static. If you don't, (and you don't since you're climbing at 2000), something is wrong. Check your tach. If it's not reading low then your C85 is not making rated horse power.

[flyguy]

Re: Engine Experts... C-85 Question

Good to see information on such a wide variety of props used on 85's. Pretty much tracks with what I get with my 7146. Have to copy some of this.
Darryl

[VictorBravo]

Re: Engine Experts... C-85 Question

My experience with C85 powered Taylorcrafts is that you are using way too much pitch. I had a 71x43 and it did really good.

Thank you, Eric. My gut feel is that there was too much pitch also. But since I just paid $5.35 a gallon for avgas, the thought of a 100 mph cruise at 1950 or 2000 RPM was very very tempting.

Someone yesterday told me that the 71-50 is the standard cruise prop on a Cessna 140. Since a T-craft has roughly the same amount of drag as a 140, and is lighter, wouldn't a 71-50 be way too much prop on a 140 too?

Right now the cowls are off for some long overdue paint and body work, but the next time I fly I'll try the 74-47 and see what that does.

I have not done full power static runs, so I don't know what that number is. On takeoff it comes up to 2000 and maintains 2000 at 58-62 mph IAS. I had no problem getting it to 1700 and 1800 on a runup for mag check. So I can only guess that the static was about 1900?

[Larry Lyons]

Re: Engine Experts... C-85 Question

I read a story once about a civilian pilot who went around flying along on missions during WWII in Corsairs as tech advisor. After each mission they wondered where he had been and why he didn't fly the entire mission with them because his tanks were still half full while everyone else was coming back in on fumes. The entier squadron confirmed he was with them the entire trip. When asked how he did it he said it was simple....he ran a very high manifold pressure(full throttle)and pull the rpms waaaay down with the prop. The maintance crew had a fit and told him he couldn't do that because he would burn the engine up in no time. After several more missions his engine was completely problem free,plugs were cleaner,cold and hot starts were easier,and he hadn't changed his flying habbits at all. He also told them that it got him across the atlantic ocean non-stop....yep,it was Charles Leinburg. He explained that while the manifold pressure was running very high,the engine is turning very slow all while running the same speed or faster then everyone else. With the throttle open wide it would take big gulps of fuel with each rotation but it turn hundreds less rpms then everyone else,the extra fuel at each rotation helped cool the cylinders and the lower rpms at the same speeds run the oil temps cooler and making the engine more efficent.

I also believe this story includes his running the engine way lean. So lean the power, ergo the heat, was not there. Larry
PS: I also believe it was a P-51! But whats the difference; a sweet airplane is a sweet airplane, is a sweet airplane. :dance: