Engine Experts... C-85 Question

[VictorBravo]

ENGINE EXPERTS... Questions I'd like an answer to:

With a McCauley 71-50 and a stock C-85, when I take off and climb at 60 MPH I am only getting 2000 RPM. That's 575 RPM short of it's rated RPM and power.

A fairly engine-educated person has said to me that lugging the engine down that much on a full power climb is absolutely not good for the engine. I told him that if I put a lower pitch climb prop on the engine, the fuel consumption would go up at cruise due to higher RPM, and he said "probably not".

His feeling was climbing at an RPM closer to rated power would be much better for the engine, less stress on the engine, etc.... and that cruising at a LITTLE higher RPM with less load on the engine would allow the engine to use less fuel because the throttle would not be as far open.

The Continental engine book implies that fuel burn is solely dependent on RPM and not throttle position. (The Continental book's fuel graph apparently tells me that at 2000 RPM in a full throttle hard climb I will burn the same 3.7 gallons an hour as if I was cruising at 100 MPH with the throttle halfway closed.) That seems suspicious.

This friend of mine (off-road race car builder) said HOGWASH, the different loads on the engine would give different fuel flows at the same RPM. He figured the Continental book fuel graph was based on level cruise flight only.

SOOOO....My question for you engine experts is twofold:

1. If I am only getting 2000 RPM in a hard climb after takeoff with a 71-50 prop, is that harming my engine or is it not... do I need to go to a lower pitch prop for engine health as well as climb rate?

2. (I still have the old 74-47 that came off my 65 HP engine) If I put the 3" lower pitch prop on, I know I will see a better climb, but would I also see a lower fuel burn at an equivalent cruise speed?

[3Dreaming]

Re: Engine Experts... C-85 Question

I'm no expert, but an engine is just an air pump. The increased load will change cylinder pressures, but the carburetor meters fuel by air through the venturi. There fore the engine RPM effects the air moving through the carburetor controlling the fuel flow.
Now for the prop these are just my thoughts. I like to see red line or above for full throttle level flight. I like to cruise at 10% below maximum level flight RPM for 75% power. If your engine will turn red line in level flight that would give you a cruise RPM of just over 2300. You also need to make sure that you have a static RPM within limits. If all that is OK I would not worry about the lower RPM's on a slow climb. My guess is if you lower the nose some the RPM, speed , and ROC will all increase. Tom

[NY86]

Re: Engine Experts... C-85 Question

Bill:

A C85 with a 71-50 should turn a static RPM greater than 2000 RPM. If static is greater than 2000 then propellor load at 60 mph is lower and RPM should be higher. Ergo, check your tach. My guess is you're turning faster than you think.

BTW, what is your static RPM?

[flyguy]

Re: Engine Experts... C-85 Question

Just a few general comments: For any given HP engine or any engine developing a given HP the engine turning the lower RPM's will burn less fuel. Read, more displacement, slower RPM's is more efficient. Reason; ring friction that is dependent on piston velocity. That is why large slow turning stationary engines are so effiecient. So low RPM for given power is good , EXCEPT; is stressful, mainly on the bearings. Your friend is right on that.

You are correct about the second part though. Letting the prop wind up will reduce the efficiency of cruise. The reason is a cruise prop (or overdrive transmission) allows you to pull a given amount of power at a lower RPM which in turn reduces your pumping losses because the throttle is open wider and the carb. velocity is slower. That is why 75% power works best at 8,000 ft or so; that is where minimum pumping loss occurs because 75% power is wide open throttle at that altitude.

The 3.7 gal per hour at 2000 RPM cruise sounds correct. I believe I got about that on a quick and dirty check one time. Didn't believe it then. Have it written down somewhere.

Tom may be right about lowering the nose some.
Darryl
Edit: I have a 71-46 and get 850 ft per minute on std day at about 70 MPH. What are you getting at 2000 rpm? Also if it helps I get about 500 ft/min. when I come back to 2100 or so. I think my normal climb is about 2300 RPM full throttle, and the plane is on the heavy side.

[Karl Rigdon]

Re: Engine Experts... C-85 Question

Bill,
A long time ago I gave up on a set climb speed on a normal depature. I now lower the nose untill I get 2300 RPM (A65 mcalley 74/45 prop) and climb at what ever speed happenes that day. I like the higher FWD speed for better cooling.

Karl

[crispy critter]

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.
Bill,I know they had a constant speed prop so they could adjust it for climb and cruise but I still think the same principal applies. If you want to take a little strain off the engine while climbing then all you have to do is lower the nose and gain a little speed. Once you are above obsructions and out of ground effect you will find that your rate of climb will increase with more airspeed. Drop the nose and climb her at 70-80 and see what happens,I think it will surprise you. Also,since you already have it, try your 74/47 prop and see what that does....I think that will really surprise you too. I also agree with whoever above said it...running your engine on a hard load all the time is not healthy for it,but you don't have to run it under a load all then time. Climb a little faster and find that happy cruise setting that the airplane likes with as little throttle as possible. Don't be affraid to run a little harder and burn a little more fuel.Remember,with your old engine you were barely getting 90mph at 3.5-4gph,now you are getting 110 plus at 4-5gph.If I've done my math correctly you are getting as much or better milage even thou your burning more fuel per hour. Try the other prop,then try the current one again.....or just enjoy the hell out of it and don't worry so much about what everyone else says.

[jCandlish]

Re: Engine Experts... C-85 Question

I too would try the 74/47 prop.

Lugging the engine is bad for any number of reasons. It means the air/fuel mixture has become post-critical, so you are either washing the oil from the cylinder linings with too much fuel (forget about any notion of extra cooling), or your tending toward detonation and burning the snot out of your exhaust valves and seats.

The type certificate says with regard to the c-85:
7. McCauley 1A90 with the following limits: Static r.p.m. at maximum
permissible throttle setting: not over 2350, not under 2170.

What are you getting static?

Cheers
jCandlish

p.s. Anyone got infos on a short mount for the o-200?

[Acroeric]

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. I could get 200 rpm over redline in calm full power runs and it cruised about 105 mph. Climb was very impressive, static was 2350. I kept it here on my 800' strip with no problems- even with 2 on board and lots of gas.

Again- I think you are just using way too much prop.

[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.

[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?

[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

[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.

[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

.....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...

[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

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....