Engine Experts... C-85 Question

[Ragwing nut]

Re: Engine Experts... C-85 Question

Now I guess I'm confused. I was talking about the C85 vs. C90. Different cranks. Different cams, too, but I doubt if the cam has much bearing on vibration characteristics.

I stand by my original statement (for now). I know of no certified C85 installation with a McCauley 1A/B90 over 71 inches.

You quoted a reference to a Cessna so I thought you were referring to the differences between 90 and O-200. Nothing is the case is the same between 85 and 90 except gears and rear case.

[VictorBravo]

Re: Engine Experts... C-85 Question

I did my first test flight with the C-85 engine and the McCauley 74-47 prop yesterday instead of the 71-50 I had flown before.

Climb at 60-65 MPH was indicating 2150 RPM. Climb rate was quite good, no measurements taken.

Cruise at 2200 - 2250 RPM was 105 MPH IAS at 3000 feet.

No static full throttle RPM was observed because the brakes will not hold against full throttle.

Tachometer has not yet been checked for accuracy. That is the next little project.

Assuming that my tach checks out accurate and my ASI is not off, then I'm cruising at 22.8 miles per gallon at 4.6 GPH. Reducing RPM to 2000 gave me an IAS of 88-89, which the book says is 3.7 GPH, which gives me 24 MPG.

Right now I'm happier than a pig in poop

[Forrest Barber]

Re: Engine Experts... C-85 Question

Pig in poop, now that is Happy! tie the tail to something or chock her.
Use a light , hanger or street or?? behind the prop late evening or night time, the blades will apprear to stop at multiples of 60 cycle/ sec. Like 1800 RPM & 2100 RPM. Quick easy check.
I now have an electronic tach from Harbor Freight that is very accurate.

[rcefird]

Re: Engine Experts... C-85 Question

Forrest
Which model electronic tach from HF? Some of their stuff works good, others don't and I could use an electronic tach for several things.


Randy

[M Jones]

Re: Engine Experts... C-85 Question

Someone may be interested in trying out some free-ware that is suppose to measure engine RPM from the sound. It can be downloaded at the link below and installed on a laptop or pocket PC. A member/friend on this group sent it to me. I still haven't tried it out...

RPM From Sound

[Larry Lyons]

Re: Engine Experts... C-85 Question

Forrest
Which model electronic tach from HF? Some of their stuff works good, others don't and I could use an electronic tach for several things.


Randy

I am interested too as I have several other uses for a decent tach that did not cost me a first born! Larry

[T-Rex]

Re: Engine Experts... C-85 Question

Does anybody know of somone who has goten approval to use a longer Mac1B90 on their C85 powerd plane. I would like to run a 74" 1B90, but I cant make the payperwork work out.

[astjp2]

Re: Engine Experts... C-85 Question

look at the seaplane prop approvals, they have a 75-35 1B90 that I know has been used on Tcrafts....Tim

[NY86]

Re: Engine Experts... C-85 Question

Does anybody know of somone who has goten approval to use a longer Mac1B90 on their C85 powerd plane. I would like to run a 74" 1B90, but I cant make the payperwork work out.

If you get it approved, you're pulling a fast one on someone. McCauley lists the acceptable lengths based on engine/prop vibration characteristics and RPM ranges. And they aren't talking about some small, annoying vibration that makes your compass hard to read. They're talking about destructive harmonics that make the crankshaft fracture.

For the C85 the limit is 71 inches. Caveat Emptor.

[astjp2]

Re: Engine Experts... C-85 Question

If you get it approved, you're pulling a fast one on someone. McCauley lists the acceptable lengths based on engine/prop vibration characteristics and RPM ranges. And they aren't talking about some small, annoying vibration that makes your compass hard to read. They're talking about destructive harmonics that make the crankshaft fracture.

For the C85 the limit is 71 inches. Caveat Emptor.

Well if you look at a C-150 which has the O-200, its rated for 75" on floats, and the McCauley type certificate on page2 has a 1b90ECM listed for 85 HP@2600 rpm limited to 76-70" diameter and 68-38" of pitch the 1B90CF is 78-70" diameter on page 1...Tim

[NY86]

Re: Engine Experts... C-85 Question

The O200 is a completely different rotating mechanism, with different vibration characteristics.

The HP rating is only a small part of the issue. Look farther down, in Note 9, "Special Limits", where they specifically call out the Continental C85. (The word "limit" as a very specific meaning when used in type certificate data sheets).


It seems some of you guys don't want to hear this. It reminds me of Reggie Lewis, the Captain of the Boston Celtics Basketball team. He had a heart condition. He shopped around for a doctor who would tell him it was OK to play basketball. Dropped dead during a practice session at age 27.

[barnstmr]

Re: Engine Experts... C-85 Question

John is correct about this being a fatigue life issue. It is the same characteristic you see when you bend bailing wire back and forth until it breaks. What happens is... the vibration harmonics (i.e. frequent pulses) cause the blade to flex on every peak of the vibration "pulse". So each time this happens, forces (stress) are imposed that can breech the material's elastic limit. Repeat this enough times and you get a crack..and soon a failure. Usually it is the aluminum propeller blade that is the limiting factor, since aluminim is not as robust against fatigue in comparison to the steel crankshaft.

Think about the RPM of a prop and imagine how many of these pulses might happen every second - minute - hour. Multiply this out to see how many times the prop sees a harmful force in an hour. It adds up really fast. Consider also the difference between a C85 with a rated RPM of 2575 vs. a C75 rated at 2275. Because of this RPM difference, a prop used on a C85 cannot tolerate as much vibratory stress (force) as the same prop can tolerate on a C75 because it will reach its life limit much quicker.

Every engine model has different harmonic characteristics. And every prop model has its own harmonics. The engine and propeller play together and "talk" to each other. The aircraft and prop manufacturers have to run vibration tests with strain gauges mounted on the prop blades. This is repeated for each engine model that they certify as eligible for a certain prop model. The diameter limits and the RPM range restrictions you sometimes see are a result of the tests where it was shown marginal or unsafe stresses were measured.

Many folks get suckered into the belief that a certain prop or engine combination should be certified because they tried it for quite a while and nothing went wrong. What they are missing is that this is a fatigue life issue. Everything might work just fine until that fatigue life limit is spent. It might ruin your day if you were cruising along just fine, half way to TBO thinking all is well when this happens out of nowhere.

-------------------
So after all that, to look at whether a given prop is safe with an engine or not, look at the propeller TCDS, (typically under note 9). Here is whats given in the McCauley TCDS for 1A90 and 1B90 props and C85 Continentals.

1A90/CF
1A90/CH
1B90/CM
1B90/ECM
Continental C-85 series,
85 hp and 2575 r.p.m. or less
Max dia - 71
Min dia - 68 ½
Placards - none

This means that McCauley did testing to substantiate the prop within these diameter limits and the max horsepower/RPM shown with no unsafe RPM ranges.

If you'll notice, the same props can be used up to 74 inches max dia on C75 engine. Even though the C75 and C85 are basically the same engine, the difference is that the C75 max RPM is considerably lower than the C85, so fewer cycles occur over a period of time. This permits the prop to tolerate higher stresses and still have a safe lifetime.

[VictorBravo]

Re: Engine Experts... C-85 Question

1A90/CF
1A90/CH
1B90/CM
1B90/ECM
Continental C-85 series,
85 hp and 2575 r.p.m. or less
Max dia - 71

So Terry does this mean I can just cut my 74 inch prop down to 71 and achieve the same level of safety/service life?

[flykenken]

Re: Engine Experts... C-85 Question

If you have an 0200 crank, rods and pistons in the C85 then would not the harmonics be based on an 0200 instead of a C85? If so the longer prop would be approved.

[flykenken]

Re: Engine Experts... C-85 Question

I should add the sensenich 76AK metal props are good to 74 inches on C85 but again does the 0200 crank/rod/piston change that?