Propeller Selection for A75 on BC12D

[barnstmr]

I am looking for comments from others who have experience with the A75 on a BC series. I have a BC12D currently with an A65, preparing to change to A75. I would like to keep my metal prop. So I am trying to determine what I need to do about the propeller pitch. The A65 and A75 power curves are the same up to 2300 RPM. The extra 10 hp comes at the higher RPMs, so you need a prop that will allow the additional RPM. I want to ensure optimal climb without too much sacrifice in speed.

I currently have the 1A90 CM7044 propeller. (I just determined there is no log entry for this prop) With the A65, I currently barely manage to get 2050 RPM at full-throttle static on the ground. With the A75 I don't expect this to change, so I am considering flattening the pitch. The Piper J5A is certified with a CM7038 prop with the A75. Yet the airplane is does not need to be as fast as the Tcraft. The Luscombe 8C is certified with a CM7144 prop for climb and CM7146 for cruise with the A75.

I have flown a BC65 with an A75 and CM7443 prop. It climbs great... much better than my airplane. Granted the BC65 is a bit lighter than my BC12D. But this prop does not let the engine turn up fast enough to take advantage of the extra 10 hp. On the ground, it will turn up to 2100 rpm. and in flight will not go over 2250. I think if that prop was re-pitched to 7441 it would actually turn up even better.

Taking a SWAG at these considerations without an actual test, I am thinking I will have my CM7044 re-pitched to CM7041 and see how it will climb. Anybody have any comments?

Laz,
How's your A75 project coming along?

Sabrina,
1. Which propeller do you have with your A75.
2. Whats your full-throttle max RPM on the ground?
3. Whats your L and R magneto timing degrees setting?

Any other A75 BC fliers out there?

[jgerard]

Re: Propeller Selection for A75 on BC12D

The A65/75 engine I built for my BC12D turned a 1B90 74-42 at 2300 climb out and 2600 in level flight.

[Guest]

Re: Propeller Selection for A75 on BC12D

Hi Terry,

Appears that you have the 85 hp prop on there now.

The a65 is approved for min. pitch of 72.5".

The C85 is appoved for min. pitch on 69.5"

See TCDS for a-696 103 c,e and g.

I have a BC12D w/a65 but have 1B90-CM7049 (with a field approval, it's legal) and it turns static 2100 rpm measured with optical tach using 100 LL. I think turns 50 rpm slower on auto gas.

Mechanical tachs vary a lot from one to another (by 50-100 rpm in my experience) and differ from the optical by 50 rpm easily (also my experience). Temperature also effects the mechanical tach readings.

In flight it will run up to nearly 22xx (nearly 2300) full open throttle.

Terry, hate to be the bearer of bad news but it sounds like the prop is not legal for your a/c. A field approval could remedy that.

Dave.

[barnstmr]

Re: Propeller Selection for A75 on BC12D

Jason,
Whats your mag timing? My father-in-law has a BC65 with an A75, but he sets his timing L & R both to 30 deg BTC... like an A65. I wonder how much more RPMs he may be able to turn if we change his timing to 29 (R) and 32 (L) deg. BTC like the A75 book says. I plan to use the staggered timing on my A75 by the book.

Dave,
Yes you are correct... the 70 inch prop has never been approved. I am planning to do some testing and may approve it on the airplane, but need to find the correct pitch. If money were no issue, I would buy a longer metal prop. I also have a Sensenich wood prop 72-42 that I may try as well. I was saving it for my Pietenpol project.

[jgerard]

Re: Propeller Selection for A75 on BC12D

+.015 cylinders, New cam, I did all the top end valve work, I match ported the cylinders/intake system, and set the timing to 29/32. It was an extreemly strong A65.

[Guest]

Re: Propeller Selection for A75 on BC12D

Jason,
Whats your mag timing? My father-in-law has a BC65 with an A75, but he sets his timing L & R both to 30 deg BTC... like an A65. I wonder how much more RPMs he may be able to turn if we change his timing to 29 (R) and 32 (L) deg. BTC like the A75 book says. I plan to use the staggered timing on my A75 by the book.

Dave,
Yes you are correct... the 70 inch prop has never been approved. I am planning to do some testing and may approve it on the airplane, but need to find the correct pitch. If money were no issue, I would buy a longer metal prop. I also have a Sensenich wood prop 72-42 that I may try as well. I was saving it for my Pietenpol project.

Hi Terry,

FYI-

I have two props, each with a hub installed on it so I can change easily.

The other is a wood ck72-42. It cruises about 5-8 mph less.

But also weighs 10 lbs less.

Dave

[Forrest Barber]

Re: Propeller Selection for A75 on BC12D

A-75 uses a 70-38 , for you, try whatever will meet the specs for the A-75 in your neck of the woods.

[flyguy]

Re: Propeller Selection for A75 on BC12D

Forrest,
Do you know the reason for not being legal to use a 74 inch prop on an 85HP engine? I had one on a 65 that had an 85 carb on it and it worked fine. You get an extra 2 square feet of area out at the tip and that can't be bad for performance. I figured it out and the tip doesn't go supersonic and if you use the correct pitch it should turn up ok. I just have never understood the problem.
Darryl

[Guest]

Re: Propeller Selection for A75 on BC12D

Hi Darryl,

Pehaps its a vibration issue due to RPM & aerodyamics or due to engine power pulse & RPM or both?

Dave

[flyguy]

Re: Propeller Selection for A75 on BC12D

May be, I would like to get my hands on a few 74 inch props with different pitches (is that a word lol?) to see what would happen. Bet you could get climb or cruise up quite a bit, but probably not both.
DC

[Guest]

Re: Propeller Selection for A75 on BC12D

My conjecture is that many props have not been approved merely because no one made the attempt because it was not needed or not profitable at the time.

When getting feild approval (FA) for the 70" on an a65 that appeared to be the case. I started out going for an STC but gave up on that path. But found no technical reason to prohibit it. Was less expensive to do FA and the FAA engineer I dealt with was retiring.

Dave

[Acroeric]

Re: Propeller Selection for A75 on BC12D

The C85 and props as long as 76 inches are approved on some other aircraft- I have field approved a few 74" props on C90's for seaplane operations. For the A65 with the short prop- find another "Approved engine and prop" combination on another make of aircraft. It should be no big deal getting a field approval if the approved data exist.

For some interesting reading look at the prop limitations for the really early Tcrafts. .. . very liberal- the 1939 BC was limited to a tiny 83" diameter!

[Len Petterson]

Re: Propeller Selection for A75 on BC12D

A 65 with 1750 hr prop is Mc Cauly B90 74*45 turn 1980 Static. Cruise is 88 Mph IAS at 2150, wide open 2300 = 98Mph IAS.
I can still get in and out of 1500Ft strip alone.
Len

[Forrest Barber]

Re: Propeller Selection for A75 on BC12D

Part of the answer to the above conjectures have been printed many times...
The 69-50 on the F-19 was BECAUSE the factory ( Dorothy, Charlie, Kenny, John & Chris) never thought to order a prop for the prototype. I was out of country playing Law Enforcement. I came back in time for them to ask to borrow the 69-50 prop off my Cessna 150 and use it to see if the prototype 3541T would fly , it did, it used that prop for all tests, it passed, there you go another fine screw up.
Later on we needed to run the tests again for a "seaplane" prop. I insisted on the 74 in prop and we ended up with the 74-43 for the F-19 on floats at 1500# gross. Remember any prop change listed on a Type Certificate needs to do most all tests over again, cooling, fuel comsumption, ROC, take-off distance, and many others. THIS takes time & money. I set up the program For doing a lot of tests over again at Lock Haven to improve the prop ( offer choices) fuel tanks, redesign and re test., and other things too! not done , no money! ps.... please take no offense to my snide remarks to new folks who only want some knowledge, KEEP ASKING THE QUESTIONS WHEN I HAVE TIME I WILL ANSWER TO THE BEST OF MY KNOWLEDGE I have doing this all my life and do wish I could download my brain ( & Chet's & Dick & others) into a searchable data base for these answers. Got to run; snow, cold, winter is here!