BC12-D prop question

[Craig]

Re: BC12-D prop question

Good evening

I am in the final stages of purchasing a '46 Taylorcraft BC 12D with a C85-8 engine installed that has had the 0 200 stc completed.

The last question to be answered deals with the prop that is currently installed.

It is Sensinich 74CK-0-42. The problem is the prop only measures 72"

I am having the records researched for any paperwork showing that this mod was documented and approved.

My questions are
Is this prop legal
What would be the expected performance with this prop
If this prop is legal can it be re-pitched for a better cruise
If not What prop would be better suited to provide the best tradeoff for
climb and result in the best cruise

Thanks for your responses

Craig Bixby

[astjp2]

Re: BC12-D prop question

The M74CK-0-46 is not certified at 74", I tried to STC this prop for the O-200 and I dont have the funds to get the data needed. So with it cut shorter, check the type certificated it may be approved but I doubt it. Tim

[CJ Ross]

Re: BC12-D prop question

Ok, here's a little summary. I have owned 3 BC12's. Two 65's and my present 85.

1. Forget the 100 mph cruise unless you run 100% power. Tcraft airspeed indicators are notorious for being all over the board. Likely due to the stock location of the "static" port in the wing, where it really isn't very static. Cut the one you have now some slack and take anything over 90 as good.

2. Ok, enough of that. Props: Since you can run a 74, and since they are LONGER, they will give you more thrust and are the best legal/safe choice.

3. On a land BC12-65 the 46 pitch is a good all around choice. I have run one on 2 different -65's and both worked well for cruise and climb.

4. Maybe some airplane guys can loan you a 7443 to try, which, according to a response above, works well on a 65. I also suspect that is so.

That is my best shot on land 65 props.

Good Luck,
Darryl

I agree with the accuracy of the airspeed indicators, but a T-craft should not have to work too hard to outrun a champ. My old 65 would get out better and easily go faster than my nieghbors champ so I'm hoping this one will perform atleast as well with the 75. I'm going to try a 0200 spider on it sometime soon and see if that will wake it up a bit. Has anyone tried the bigger spider? any noticable change?

Chuck

[flyguy]

Re: BC12-D prop question

Take this for what it is worth. I had a very light -65 that had the 7446 prop previously mentioned on it. I once took it to 12,500 and it was still going up at 300 ft/min. stabilized. It would go right past 65 redline in level flight and just keep winding, and the 46 prop is not that flat.

Had to rebuild the carb and when we took it apart, surprise, surprise, it had an -85 jet and venturi. I didn't ever pay much attention to the spider, but I suspect that it was for a 65.

What does it take to make a 75?

One of the manufacturers list a 74 inch for C85, but it has a dash number that means it is cut down to (72", 71"??)

Being my goofy self I have always wondered if I guy couldn't get away with running a flat 74" on an 85, but only use it to 85 RPM's on take off and back to 65 limits for climb and cruise. Makes me wonder if the problem is the RPM's or simply the engine/prop combination.

DC

[CJ Ross]

Re: BC12-D prop question

I have been told an A75 is a 65 with a slightly different cam grind and it just turns a bit harder. I haven't done the research to findout what the difference is first hand, but there are people on here that have that knowledge from what I have read in other posts....

[astjp2]

Re: BC12-D prop question

The cam is the same. You need to read the bowers flybaby site about the small cont. The pistons are the same according to some parts manuals, its mainly an RPM thing. Most of the 65's had the rods modified with the spurt hole, I used to have all of the difference memorized but the info in on the web. Tim

[NY86]

Re: BC12-D prop question

Makes me wonder if the problem is the RPM's or simply the engine/prop combination.

The issues are pretty complex. In a nut shell, the prop and crank constitute a resonate system. If you hung it from a string and hit it with a hammer, it would ring like a bell. You have 4 hammers (pistons) attached to the system and they are hitting it at different speeds (frequencies) depending on RPM and different force depending on throttle, cam grind, etc.

When McCauley says a prop is limited to a certain length for a certain engine, it's because more (or less) length can change the resonance of the system into a range that coincides with the hammers attached. This is why you'll see some prop-engine combinations placarded against operating in a particular RPM range.

Look, for example, at the C75 and C85 combined with the 1A90. The engine is exactly the same except for the carb. Why the different prop lengths? Well, the C75 redline is 300 RPM less, which apparently keeps the longer prop out of the danger zone. Now, look at the 85 vs the 90. Here you're allowed a slightly longer prop on the 90. Why? Well, different crank, cam and RPM range. Now look at the C90 vs the O200. Here we have a different RPM range and different cam and a much shorter prop length.

HTH

[T-Rex]

Re: BC12-D prop question

This does not answer the original question but may help? I went through the whole prop search for T-Rex c85-12 just over a year ago. I wanted climb for floats but did not want to sacrifice to much in cruise speed. I tested a few different props. I had a Sen M74CK40 field approved and it worked great. Cruise just above 90 and great takeoff. This prop has no legal business being on this engine because of the harmonics it produces but the FISDO approved it anyways. I don't like or want to run it because of the fear of doing unseen harm to the internals of the engine. There is no installation where this prop is approved at this length on this engine on any TCDS that I have been able to find. IF you want to run a 74" Sen on a C85 Use the M76AK-2-38 or 40. It has a much larger/ beefier hub (center) section and a completely different blade profile. The chord is longer and is also thicker. This prop is approved on the C85 TCDS and is also approved on Cubs, Cessna 120 and 140's with the C85. In my opinion the Sen AK series do not spin up as good as I would prefer for float opps. where RPM on takeoff is king. The 90hp champs generally use a M76AK-2-42 for floats and they work great, keep in mind 95 takeoff horsepower. What I decided on was a Mac 1B90 CM71-44. Not a climb prop but not a cruise prop When light takeoff performance is in the 200 to 300 foot range with a 100mph cruise at 2400 RPM. At 2300 I get about 95mph, GPS confirmed. I looked into running the Mac 1B90 CM 74-38 or 40 but once again I came into the problem of harmonics and no instances of the being used on the C85 anywhere. It does not make sense to me that prop can turn 2700 RPM on an O200 but cant turn 2575 safely on the c85. I talked to Butch at Maxwell propeller repair at length about using one of the longer unapproved props and even in good old boy conversations he did not recommend it because of harmonics and vibration. IT is just the differences in the engine that make it work on one and not another.

[cvavon]

Re: BC12-D prop question

I wonder if any one has looked in to harmonic balancer's like automotive engines Cummins diesel will brake crank shafts if the balancer goes bad also would act like a fly wheel and i think that would make more tork

[NY86]

Re: BC12-D prop question

...The 90hp champs generally use a M76AK-2-42 for floats and they work great, keep in mind 95 takeoff horsepower...

This is another "urban myth". The only way you can get 95 HP is if you can turn the engine up to 2625 RPM. The only setup that is certified to do that that I am aware of is the Beech-Roby controllable prop. All fixed pitch installations are limited to 2475 RPM and won't go higher in climb anyhow.

It does not make sense to me that prop can turn 2700 RPM on an O200 but cant turn 2575 safely on the c85.

Many significant differences between the two. The rods and crank throws are longer so the resonant frequency is different and the two engines produce like power at different RPM's, so the force of the hammer blows is different.

[Dano"T"]

Re: BC12-D prop question

I have 2 BC12D's....one with C90-12F and one with C85-8. The C90 has a Sensenich 76AK-2-40...works great....I tried it on the C85 though and barley got off the water....would only make 2150 or 2200 rpm...It's a great running C85 with only 200 hrs SMOH...I use a McCauley 71-42 on the c85 and it is a real performer....105 cruise and awesome takeoff. I'm surprised anyone could use a Sensenich 76AK-2-40 on a C85 unless it's a stroker.

[jim cooper]

Re: BC12-D prop question

Quick answer? Those who can tell you quick are either North East propeller in Lancaster Pa. Sensich dealer or Leavens prop in Toronto. Or look up closest Sensich dealer on internet. JC

[Forrest Barber]

Re: BC12-D prop question

THE prop for the A-75 is the 70-38 according to the 337's we ahve here on file. the A-75 has to turn faster than the A-65 check the TC.

[cropduster]

Re: BC12-D prop question

Dano, You are right. The 76AK-2-40 prop I am running on the C85, is a stroker!

[Len Petterson]

Re: BC12-D prop question

In this years annual ( 2010)I would like to remove and reinstall the propp hub on my A 65 just to make shure it dues have any corosion on the taper.
I dont see a requirment for it but as my plane is stored out side I think it would be good to check it.
Any suggestions as to what rust inhibitor to use. When I installed this engne 3 years ago we used CRS or similar.
Len