BC12-D prop question

[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

[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

[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....

[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

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

[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

[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

[flyguy]

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

BIG OOPS here. Sorry, C.J., I missed the 75 HP part. Thought you were talking about a -65.

I will leave this here, but I am talking about a -65 only for you guys who might profit from this info.

[CJ Ross]

Re: BC12-D prop question

I'm flying off of a 1700ft grass strip so I need it to climb pretty well but without sacrificing cruise too much. My first T would take 2 200+ people out on a warm day and still criuse just above 100 on an A65. This one has the A75 so I would like to get at least close to what I had before. Still trying to locate 43781 to find out what prop it had.

[akndrifter]

Re: BC12-D prop question

I run a 74x38 on a 65hp, I think it's a sensi. Works awesome for t.o, I cruise at about 85mph. My t.o.s are usually under 250' when light.

Re: BC12-D prop question

Recall the debate a few months ago about whether or not to use the wood prop's front plate with a metal prop?

See note 1b in P842, the answer is in there;

NOTE 1. Installation. The model 1A90/CH propeller is to be assembled only with the Continental Motors tapered
crankshaft hub detail No. 3745. The propeller nut and snap ring, Continental Part Numbers 21202 and
21203 are also used. The model 1A90/CF and 1B90/CM propellers are equally adaptable to the Continental
tapered crankshaft (CMC 3745 hub) or the flanged crankshaft (SAE No. 1) using bushings A-1170 as
required. The model 1C90/LF and 1C90/LM propellers are eligible only with McCauley adapter C-1210
which permits installation on an SAE No. 2 flange.
Bolts, nuts and front plates furnished by the engine manufacturer are not to be used. The propeller
manufacturer will furnish the necessary bolts, bushings and adapters or spacers as needed for the desired
installation. See NOTE 2 for installation drawing requirements.

It doesn't make any sense to me but it needs to be complied with.

My field approval falls within their rules in NOTE 9; on a
Continental A-65 diameter between 76" to 69 ½" for an 1B90/CM prop

[NY86]

Re: BC12-D prop question

Diameter: not over 75 in., not under 73.5 in.

We're not talking about an O200.

see the type cert. for the prop, you can run from 78" to 70" @2600 on a 85 hp engine

but not any 85 hp engine. and specifically, not a C85.

Read note 9 of the prop TCDS.

I'm all for field approvals, but there are times when they should not be granted. When McCauley refers to vibration they aren't talking about a minor annoyance, they're talking about something that can cause the crank or prop to fail.

[astjp2]

Re: BC12-D prop question

so....on a BC12D...65 horsepower what would be the ideal prop for decent climb out and cruise? JC

Sen. M74ck-0-43, the 46 is standard but the reduction in pitch will increase climb without sacraficing much in cruise. Tim

[astjp2]

Re: BC12-D prop question

Do you mean Sensenich? The longest 1A90 allowed by Cessna on the C85 or McCauley on the A75 or C85 is 71"...

III - Model 150G, Model 150H, Model 150J, Model 150K (cont’d)
2. McCauley 1A100/MCM 21 lb. (-32)
Diameter: not over 69 in., not under 67.5 in.
Static r.p.m. at maximum permissible throttle setting:
not over 2475, not under 2375
No additional tolerance permitted
3. McCauley 1A90/CF (seaplane only) 24 lb. (-32)
Diameter: not over 75 in., not under 73.5 in.
Static r.p.m. at maximum permissible throttle setting:
not over 2600, not under 2500

see the type cert. for the prop, you can run from 78" to 70" @2600 on a 85 hp engine.

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/B551B448ACFF577B8625726D0061EBDF?OpenDocument

Or if you look at item 5 of the Taylorcraft F-19 allows for a 74-43 with the O-200 and this would be justification for a FIELD APPROVAL to use the 1A90CF on the 85 hp engine. Tim

[jim cooper]

Re: BC12-D prop question

so....on a BC12D...65 horsepower what would be the ideal prop for decent climb out and cruise? JC