NEED INPUT -- A75 Prop Specs -- Final Answer?

[PhilP]

SO after searching the forum threads high and low again and seeing the various selections, I wondered if we could get the final answer(or at least opinions) on what the BEST A75 prop to swing on the Taylorcraft BC-12 would be...need to make a purchase soon.

A last point to consider...the J3 cub TCDS seems to make it clear that a J3 with an A75 shouldn't have a metal or wood prop longer than 70 in...though the former guru over at the sensenich metal forum suggested 74 is just fine...very confusing.

(from J3 TCDS)
Engines and Engine Accessories - Fuel and Oil System (cont.)
311. C. (8) A-75-8-9 (land and seaplane) Use actual weight change
(See NOTE above regarding installation changes.)
Engine Limits: For all operations, 2600 r.p.m. (75 hp)
Propeller Limits:
Static r.p.m. at maximum permissible throttle setting:
Not over 2425, not under 1950 (with Items 101 and 6).
No additional tolerance permitted.
Diameter: Not over 70 inches, not under 68.5 inches (Items 101 and 6).

Items 101 and 6 are both metal and wood propellers.

Please reply with your prop diameter/pitch if you would! Does anyone here swing a 70/38?? What is your cruise and climb performance like?

Thanks All....

[Guest]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

I have cm70-49 on an a65 landplane that climbs and 2100 rpm and criuses at 2150 to 2250rpm and high 90's mph. I like it a lot. It will run up to 2350 if you fly level and full throttle.

In case you noticed it is not in the tcds I have a field approval for it, I forgot the static rpm,could look it up in the logs, I recall it is within metal prop limits for a cm but would have to look up to be sure.

Dave

[Ragwing nut]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

The reduced diameter is required to develop 75hp at 2600 rpm. 70-38 is WAY to flat. That would be a lycoming 65 prop dimensions. Wrapping an "A" series continenal that far is not pretty as the weak link is the vavles and spings. A-80's were notorious for popping valve stems and valve float. We run a 71-48 metal on our last 65 and run perfect, so clipping it to 70 should yield a little more rpm for takeoff and good cruise numbers.

[PhilP]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

The question remains....what prop are folks currently turning in A75 powered Taylorcraft that allows the engine to achieve the max 2600rpm in level flight and full power?? (I'd rather not guess here based on theories of what 'should' work before buying the prop...)

Just to compare to other applications:

For the A75 Mccauley lists the 1A90/CF7038 & /7040 for the piper cub, aeronca champ, chief etc.

Sensenich lists the 74CK-2-36 for the J3 with an A75(i.e. a 72/36).
(even though the J3 TCDS says max 70in Diameter )

[Ragwing nut]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

You can't compare J3 prop specs to Taylorcraft as they cruise 20 mph faster. As I said before, I run a 71-48 on our 65 and turned around 2400 straight and level, a 70-48 should be perfect

[LostnSpace]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Hi Phil, when I got the approval to put the C-90 in the Tcraft, the guy at the FSDO wanted to know what prop or props I was going to be using, have you discussed this with anyone in ATL? O.T.

[PhilP]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Ragwing,

I'm just using J3 and Aeronca specs to get a ballpark idea of what is correct. The Taylorcraft and J3 are usually about 2 degrees apart. A climb prop on a J3 is an economy prop on a Tcraft. IE climb props 74/44 and 74/46 on the A-65 models of both.

You might very well be right but a 70/48 sounds a lot more like a C85 prop pitch to me...I'm just trying to get some specifics from folks currently running them...

[waltermrich]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

I have used a Mc Caully 1B90 70/48 on an A65 on my BC12D and it will climb great and cruise at about 95-98mph and will rev up to about 2400rpm in level flight but takes a minute, not sure about Type certificate info but it does say not less than 70 inches in the TC.So it might be OK

[Guest]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Hi Walter,
that prop is not legal, it does note meet the TCDS requirements;

McCauley 1A90 with the following limits:
Static r.p.m. at maximum permissible throttle setting:
Landplane: not over 2210, not under 1960
Seaplane: not over 2210, not under 2070
No additional tolerance permitted.
Diameter: not over 74 in., not under 72.5 inc

1A90 and 1B90 are equivalent.

As I said I have a feild approval for a 1A90-7048 so I know that it can be had (btw-my performance is similar to your's). So that prop can be made legal pretty easily.

Seems like your IA isn't looking very closely, that prop/aircraft combo should have been identified as not meeting the aircraft type design.

Dave

[barnstmr]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Walter,
What is the static run-up max RPM on the ground with your 1B90/7048 prop on the ground with A65? I have tried the same prop and only see about 2050 RPM. I suspect either my tachometer is out of calibration or I have something else wrong. I checked mag timing and it is OK. I think I should be seeing close to 2200 rpm. Thats why I am curious about yours.

[Edwin Otha]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Phil,
I use a 70/38 with a C85 but not on a Taylorcraft. It's as flat as you need to run. Any less pitch and you will over speed the engine. It provides great climb performance but needs about 2450 for a cruse setting.
The A75 is just not a good selection. To get rated power you need a little Sensenich GK blade which doesn't seem to work well with these draggy little planes. The high RPM's needed put your fuel burn very close to the C series engines. Generally people will modify the A65 to A75 specs to take advantage of the reliability mods made to the engine. The 75 horsepower rating is basically for marketing. I feel the A65 will out perform a properly set up A75 on a Taylorcraft, with a smaller fuel burn.
EO

[waltermrich]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

Terry , As I remember The prop I have does run up a bit more than that on static run up but not a hole lot more, maybe 2100 or 2150 my plane is down for an annual right now so I cant check and its off the engine, Also it is a 70/47 not a 70/48 I was inncorrect when I first posted

[AndyDuke]

Re: NEED INPUT -- A75 Prop Specs -- Final Answer?

IH had an A75 (now replaced with a C85) and used a McCauley IB90/CM 7143 prop. As I remember I only got around 2300 rpm out of it and I don't have details of the static rpms, although I believe they were in limits.

Hope that helps. Andy.