Engine Experts... C-85 Question

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

Hi Randy,

I have one you can use, its a "One Touch Tach' same as you get from Aircraft Tool Suppy

Dave

[barnstmr]

Re: Engine Experts... C-85 Question

Ken,
To address your question about an engine with non standard parts, like a C85 with O200 crank, rods, pistons, etc.... First of all the factory isn't likely to have tested this configuration, so theres no data on it... this certainly isn't in any TCDS that I know of.
Now lets talk about what might be different than configurations that have been certified. John is on the money about "power pulses". Even though the rotating mass is like the O-200, there could be other configuration differences from the O-200. For example.. the cam was not mentioned. Cylinders were not mentioned... nor spark plugs, timing, valves.... These are all items that can cause changes to the power pulses. If you change one or more of these things, you will have a whole different characteristic harmonic for the engine. Perhaps better or perhaps worse. Again you cant be sure without testing.

Now lets look at the practicality of having this configuration. Engineers design engine parts using the "safe-life" design method. (see previous post). We know the engine parts are over built so much that they have an unlimited life (except in some cases for life-limited parts). So these STCs that get issued for O-200 crank in C85, and others are OK for that reason. Everyone knows that the parts are already approved and were shown to have no life limit by the factory. And engineers know that the difference in operating loads between a C85 and O200 are small. So from the standpoint of the engine STC... it is safe to operate and approved per STC.

But the engine STC does not consider which prop you plan to use. Thats why there needs to be a separate approval in consideration of installing that engine on an airplane. If you were coming to me with this configuration and suggesting a metal prop, I would be cautious about which one and about its limits. I would probably contact McCauley and see if they might help. But the most practical and safest bet for these non-standard engine configurations is to use a wood propeller.

Practically, the wood propeller makes sense for two reasons. One.... wood is strong and can flex a long way within its elastic region. It is therefore not subject to fatigue. Two...wood tends to absorb vibration and so therefore the engine and airframe are not subject to as many harmful stresses.

[barnstmr]

Re: Engine Experts... C-85 Question

Bill,
To answer your question on safety. It is relative. You could theoretically operate the longer prop just fine within its fatigue life and declare it life-limited. In other words, if you knew when the material would begin to crack as a result of fatigue, you could actually set a service life with a comfortable margin below that limit and certify it that way. It should operate just fine but should declare it unsafe after so many hours. But to do this, would require you to have the test data like the prop manufactures have. In this scenario, your prop would actually be a safer due to the added performance.
To answer your question on service life, the TCDS limits are set with the intent to have unlimited life. How is this possible? They have defined RPM, horsepower, diameter limits so that as long as the operator does not exceed them, the prop will never be flexed beyond its material's elastic limit. This design technique is called "safe life" design (as opposed to "life limited"). Wing designers have used the "Safe Life" method on many airplanes...they really over design the spar so much that it will never flex that much. Some of the old birds like DC-3 etc were designed this way.
On the other hand, nowadays, wing designers can use an approach to predict the fatigue life and design wings with a lighter spar. Knowing that the lighter materials will someday reach their fatigue limit, they'll set a "life limit".
One last thing that can throw a wrench into all of this is a thing called "damage tolerance". If, for example, your prop gets a pebble nick in the blade, it changes the strength of the material, and can actually concentrate the stresses into that local area. This can significantly shorten the fatigue life. The same thing can happen on a wing or other structure if a mechanic lets his drill slip and cause a nick in the material. They call this a "stress riser". All sharp gouges are apt to cause this. Its always best to file them smooth or burnish them so that the strength compromise is minimized and to provide a smooth path for stress to flow in the material.

Bottom line is - if you operate outside the factory limits, you'll probably get away with it for a while... maybe for a long time. But how long is a guess unless you know the material, its geometric characteristics, and stress levels.

[birdlegs]

Re: Engine Experts... C-85 Question

Larry, Kevins memory serves him well, his quotes are from Wolfgang Langschwasa's book "Stick and rudder" Wolfy was a test pilot for Chance Vought during corsair development during the navy's pre-purchase evaluations. Navy test crews were unable to get the specified range from F4U1's at cruise which was I belive 4.0 hs. Navy pilots could at best get 2.5 hs.
Charles Lindburg showed up as a tech rep for Pratt & Whitney and increased the cruise altitude to angel's 20,kicked the second stage blower in and lugged it down till you see the prop blade flicking by, adjust mixture to keep temps in the green arc and thus doubled the birds range.
My late father was in the navy's first Corsair units and flew every variant until retirement in 1959. He recounted this bit of legend often.
Brad

[jgerard]

Re: Engine Experts... C-85 Question

After reading about your High temp issues and now this thread about your low rpm at full throttle I am putting my self on the "you're running too much pitch" side of the fence. Fixed pitch props gain pitch from the aerodynamic loads imposed during use so over time they "fatten up" Never trust an old prop to be what is stamped on the hub. Also lots of props get run through a prop shop with out beeing re-stamped and there is room for human error while re-pitching and measuring a prop. There is even a variance on new props from the factory. Check your tach first, find static rpm with the 71" prop, compare static to max RPM in level flight, tell the prop shop how many more RPM's you would like to get (must be able to reach red line in level flight) and have them adjust the pitch accordingly. Roughly it's 1"=50rpm

[NY86]

Re: Engine Experts... C-85 Question

Stick with the long Prop, most IA's wont pull a spinner and measure a propeller anyways. Tim

Your IA puts his neck on the line every time he signs off your plane as airworthy. The least you can do is not try to trick him. If I were your IA and I saw this statement I'd ground your plane till you got another IA to sign it off.

[NY86]

Re: Engine Experts... C-85 Question

Thanks, Terry, for taking the time to flesh out what I've been trying to say. I don't have the patience or typing skills to do as well...

...I have weighed the rods and pistons for both engines and they have some that were EXACTLY the same weight. So the Harmonics would be the same for both, and the props then would interchange. O-200 is the same as the C-90 and you can get a 74 for the 90. Stick with the long Prop, most IA's wont pull a spinner and measure a propeller anyways. Tim

On the surface you may think this is the case, but, let's take the O200/C90 as an example. The O200 produces 90 HP at about 2675 RPM. The C90 produces the same HP at 2475 RPM. Doesn't this tell you that the C90 has stronger but less frequent power pulses? One might suspect this could affect the vibration characteristics.

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.

I think a better choice of words would be "could be approved". "Would" implies it's automatically covered. Not the case. The STC holder for the engine modification might be able to include additional props as part of the STC, or someone else could get an additional STC to cover different props on modified engines. This, however, is a bit of a can of worms, though, as props are also approved by the airframe manufacturer, so an STC for each airframe would be required.

[astjp2]

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.

Yep, maybe, depending on who you talk to on here. People seem to think that unless it was a factory certified propeller combination the propeller will fail. There have been LOTS of FIELD approvals for different combinations that the factory never tested for. The key to the factory approved combinations is the factory did perform testing for thier combinations and did not approve similar combinations because they did not demonstrate any testing data. The C-85 crank is interchangeable with the A-65, I have weighed the rods and pistons for both engines and they have some that were EXACTLY the same weight. So the Harmonics would be the same for both, and the props then would interchange. O-200 is the same as the C-90 and you can get a 74 for the 90. Stick with the long Prop, most IA's wont pull a spinner and measure a propeller anyways. Tim

[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?

[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.

[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?

[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.

[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.

[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

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.