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Prop question for C90 J3 w/w/o floats

K

KevinSinny

Registered User
Hello. I’m converting my J3C to a C90-8F w the ultimate intention of putting it on straight floats. The prop that’s available w the engine is a McCauley 1B90-CM7148. Would this be a good compromise for wheels/floats (if there is such a thing) or should I go straight for a 74 inch and use it on wheels until I get the floats?
Thank you for any help/time/information.
 
Check the type certificate. With the C-90 engine you are limited to a 71 inch diameter metal prop. The only longer prop approved is a 76 inch wood prop.

see the section of the TCDS which references “Applicable to all”.

SO, unless there’s an STC out there somewhere which allows a longer prop, 71 inches is as long as you can go with that engine.

MTV
 
alaskadrifter said:
I believe the TCDS allows for a 74” Sensenich.
Click to expand...

Here's the pertinent section of the TCDS which provides approval of the "alternate engine" Continental C-90-8:

(10) C-90-8F (landplane, skiplane and seaplane) Use actual weight change
Engine Limits: Takeoff, 2475 r.p.m. (90 hp)
All other operations, 2375 r.p.m. (80 hp)
Fuel: 80 minimum octane aviation gasoline
Oil Capacity 5 qts.
Propeller Limits:
Static r.p.m. (with Item 101, 5 or 6 propellers) at maximum permissible throttle setting:
Not over 2300; not under 1950 for land and skiplane, and and not under 2100 for seaplane.
No additional tolerance permitted.
Diameter:
(a) With Item 5 or 6 propellers:
Not over 71 inches, not under 69.5 inches.
(b) With Item 101 propeller:
Not over 76 inches, not under 70 inches.
Carburetor heat system must be modified to provide required heat rise.
Approved modification made by M. Mullis, York, South Carolina.

Here are the "Item 5 and 6 propellers":


5. Propeller - McCauley 1B90 or 1A90. +21 lb. (-53)
Eligible with Item 311C(10) and Item 311D(5) engines.

6. Propeller - McCauley 1A90 to 1B90. +21 lb. (-53)
The Item 101 Propeller is the 76 inch wood prop.

Trust me, I went round and round with this on my PA-11, which is on the same TC.

So, again, unless there is an STC or other approval out there which approves a longer propeller, if your prop is metal, its limited to no more than 71 inches.

MTV


 
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The TCDS refers to items 311(c)9 for the C85 only which is the M76AK-2 prop which is the most highly regarded prop but for the C90-8 you're limited to items 5 and 6 and 101 props which are Mccauley 1B90 and 1A90 props (what you already have) and wood props not exceeding 76".

Your Maccauley 71/48 is on the more cruise oriented side of things but is otherwise fairly well suited for the C90. If you're not making on the high side of static RPM you might consider having it re-twisted down a couple inches so it pulls harder on floats but otherwise that's a fine prop for general use. You could go with a 76" Sensenich wood prop but I think any gains from going to a longer prop would be lost on the less efficient blade profile of the wood prop. Some folks will try and get a Catto 76" on there under the item 101 wood props but it does say specifically it needs to be a certified prop which the Catto is not.
 
If you have not purchased the replacement engine yet, I would recommend the C85 over the C90. In addition to the prop limitations being less desirable for climb performance, you are also static rpm limited on the C90 to 85 HP. So you can't even use those extra 5 ponies (legally). The C90 has no benefit over the C85 in the J3. In fact it's less desirable for STOL and float operations because of the prop. Plus if you ever want to go to a stroker STC (O-200 crank with longer stroke gets you 100HP for real), you can only do that with the C85. Things to consider.
 
I would disagree with that fairly strongly. The C90 makes MUCH more torque than the C85 in addition to more horspeower. JimC has the torque and HP graphs and the C85 doesn't look great on those in comparison. The C90-8F is generally regarded as one of the best if not THE BEST of the small continentals due to it's making nearly as much horsepower as an O-200 but at 2450rpm instead of 2750. The valve lift of a C90 is much higher as is the cam profile much more aggressive as well as having more displacement. You have to go fully to a "stroker" C85 to come close to a C90 in performance.
 
The C90 is the bee’s knees for a J3 or PA-11, full stop. Every 90 I’ve flown burns less gas than every 85 I’ve ever flown, and outperforms the 85s. Anecdotally, I feel like they were smoother in general too. I wish I had one but my Cub came with an 85 so we make do. I would not mind if the universe brought me a C90 though. (Hey, universe? It’s me, Amy…)

—Amy
 
I hate to argue - actually I love it so I will anyway. ;-)

The C90 may makes great torque - IFF it's on a different airplane that allows full RPM. Redline on the C90 is 2475 in the J3, compared to 2575 in the C85. And the 3" extra prop of the Sensenich matters in converting the hp/torque to usable thrust.

I think JimC has argued that you should get the O-200 - but his was done with a field approval a long time ago along with a bigger Mac prop that is not legal any other way - something many of us feel isn't so easy to do any more. The C85 stroker STC is the path to O-200 level of performance in the J3. It is for these 3 reasons that I argue the 85 is the one I wanted on mine.

But I'll concede to splitting hairs. I've flown the C90 - it's pretty dang awesome - and wouldn't kick one out of my hangar if she was otherwise good to me.
 
I have a C85 stroker in an Aeronca Chief. It is just a hoot to fly, it virtually levitates off the ground when you throw the throttle in.
It makes it into an impressive airplane (as far as Chiefs go). Crazy what an airplane can do when you come close to doubling the horsepower from the original pre war airframe.
 
Can't argue with you on the prop front. Short of field approving one, the C85 is the easiest way to get the 76AK-2 on there and yes, the prop makes up a large amount of the difference. My brother has a 65 horse Tcrate with the 76AK on it and it'll give my J3 with C85 a run for its money when empty.

The C85 stroker is definitely a great choice on a J3 since it's easy to make legal and with good props but most people don't immediately run out and rebuild a running C85. For engine choices right out of the box the C90 makes good sense on a J3. I'm sure you could field approve a better prop on there using prop manufacturer data or the engine/prop combo from a similar aircraft as data.
 
I've had 2 C90 airplanes, 1 C85, 1 Stroker and an 0200 Cub. Flown another 5 or 6 C90 Cubs/Champs. C90 properly propped makes the best usable power. Just flew a C90 Pa11 76-2- 38 on floats last Monday and even 2 up we were off the water very quick. A very prized engine if you ever get lucky to find one for sale. The right prop on any little C engines is the key to finding the power.

Glenn
 
The Guru says with the C-90 the 71-42 is the perfect seaplane prop and not a bad prop all around.

I tend to agree with him.
 
Crash said:
Can't argue with you on the prop front. Short of field approving one, the C85 is the easiest way to get the 76AK-2 on there and yes, the prop makes up a large amount of the difference. My brother has a 65 horse Tcrate with the 76AK on it and it'll give my J3 with C85 a run for its money when empty.

The C85 stroker is definitely a great choice on a J3 since it's easy to make legal and with good props but most people don't immediately run out and rebuild a running C85. For engine choices right out of the box the C90 makes good sense on a J3. I'm sure you could field approve a better prop on there using prop manufacturer data or the engine/prop combo from a similar aircraft as data.
Click to expand...
My first Tcart had a Mac 74-38 with an A65 on the front. It was very light and I was always impressed by the performance. Not much behind a C90 PA-11 on take off that I used to fly with (faster in cruise too). When I upgraded to an F19 it didn’t perform much better when flying lite, but the starter and real baggage space sure was nice. Those were great planes.
 
I owned 2 Taylorcraft at the same time…one had a C-85-8 with McCauley 71-42…the second nearly same empty weight had a C90-12F (I still own this one) with a Sensenich 76AK-2-40…..the 90 was a far better performer. I’m my case I did the 85hp STC….did a 337 for that, but listed the C90 Substitute under the 10% hp allowance rule and added the 76 AK to The 337. It’s a really great combo. Piper Type certificate is great with tons of approved stuff….just that one little oversight. Re-pitch that 48 prop or trade it for a 42 if you’re going on fliats. My 90hp T is on Baumann 1500s
 
The 76AK-2-XX prop really is the ticket though. It can turn an okay performing plane into a great one. I'm saving pennies and searching for one currently. It was a night and day difference on that A65 Taylorcraft. Reduced takeoff from 800ft to less than 500.

As great and fun to fly as the Tcrate is my J3 will still spank it any day in measured to/landing distances and going in/out of short and rough strips. My brother claims it's just my extra 20 horsepower but we both know the Piper badge on the side just makes it fly better.;-)
 
Crash said:
The 76AK-2-XX prop really is the ticket though. It can turn an okay performing plane into a great one. I'm saving pennies and searching for one currently. It was a night and day difference on that A65 Taylorcraft. Reduced takeoff from 800ft to less than 500.

As great and fun to fly as the Tcrate is my J3 will still spank it any day in measured to/landing distances and going in/out of short and rough strips. My brother claims it's just my extra 20 horsepower but we both know the Piper badge on the side just makes it fly better.;-)
Click to expand...
If I remember correctly back in the ‘08 Valdez stol competition I had one take off that was about 250’ and the other was maybe 212’, I was still on the shopping cart tires (6.00x6s), but the 26” Airstreaks didn’t add much if anything to my take off roll. Maybe that 65hp was pumping out a little bit extra, or the mac is that much better than the sensenich.
 
My numbers could very well be off if you're comparing to a competition type situation. I was judging the take-off while circling above the strip and the Tcrate was heavy with gas. The sensenich 76AK-2-38 certainly woke the plane up in comparison to the Sensenich wood 72" prop that was on before.
 
To add confusion to this thread, a friend pointed out to me that the PA-11 is approved, with the C-90 engine, with a Sensenich M76AK-2 propeller, at a length of 74 inches. Here's the pertinent sections, the first being from the engine listings for the PA-11:

Engines and Engine Accessories - Fuel and Oil System (cont.)
311. D. (5) C-90-8F (with propeller Item 5, 6, 7, 9, or 101) Use

And, this is the Item 9 propeller:

9. Propeller - Sensenich fixed pitch metal model M76AK-2. +24 lb. (-53)
Eligible with Items 311D(5) and 311C(9) engines only.
Static r.p.m. at maximum permissible throttle setting:
No additional tolerance permitted.
Item 311D(5): Not over 2425, not under 1950.
Item 311C(9): Not over 2550, not under 1950.
Diameter: Not over 74 inches, not under 72.5 inches.

So, we're trying to figure out why a Sensenich prop would be approved in the -11 with a C-90, but not on the J-3 with same engine. As a side note, the PA-18-95 type certificate, also approves the Sensenich prop at 74 inches.

Structurally, the primary difference between the J-3 and PA-11 is the position of the main fuel tank.

So, would it be possible to get a field approval for the Sensenich prop at 74 inches on a J-3, based on the approval of that prop on the -11 and -18-95?

MTV
 
I unbolted a 71/42 off my PA11 90 and tryed a 75/38 and it was same difference as unbolting a 74/56 off a 150/Cub and bolting on a 82/41 Borer. I had a friend that ran the 75/35 on his "95" PA-18. It was like nite and day .
Those props are a must for big loads on hot days or outta
High lakes.
E
 
The J-3 was pretty much a pre-war airplane (excluding the wartime Grasshoppers) while the PA-11 began later in 1947. When the two 74" diameter Sensenich props 76AK-2-** or 74CK-** were made available to airframe manufacturers I don't know. Might have been late pre or post war?

Edit: Here's a timetable for Sen metal prop production. Looks like post war. So maybe Piper didn't bother to re-certify the J-3 with the long 74" prop?

Have a look at TCDS 1P2 Rev. 15 which applies to those propellers. In Note 9. it mentions which propeller-engine combination has been approved but from only a vibration standpoint. That might aid in obtaining a Field Approval w/or w/o further engineering assistance.

TCDS P-842 Rev. 16 describes the popular 1A90CF 75** discussed above. Primarily a C-150 seaplane prop, in Note 9. it shows an approval for only the O-200 from a vibration standpoint at 75". For the C-90 it lists the max diameter as 73". However at the top it shows approval for 90hp at 2475 rpm.....the C-90's rated specs. Not sure what that means but there might be some options.

I've run a Sen M76AK-2-40 on a PA-11-90 and now a Taylorcraft C-85 (Field Approved from a vibration standpoint) now a 201ci 7:1 comp Stroker (engine Data Plate still says C-85-12). On my first Taylorcraft I had the 1A90CF 75/38 on a C-85. Both good props on floats.

Edit: Yet another reason to stroke a C-85 (C-90 equivalent) to allow the approved use of a 76AK-2-** prop with Item 311. c. (9) C-85 engine's on a J-3.

Gary
 
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