C-90-12 rpm

[Jon B.]

Greetings.

My Champion 7EC - '57B - is powered by a C-90-12, which has about 70 SMOH. My take-off rpm is low; in climb-out, it runs only up to 2200 rpm at 60 mph indicated. The only way to get the thing to redline is to leave the throttle pegged and enter a dive.

The prop is also new; a Sensenich 76AK-2-44. I've called Maxwell, in Minneapolis, to have them check it. If it is, indeed, at 44 inches of pitch, I'll have them take 2 inches off. A local guy, with an 85 hp Chief, gets 2700 on climb-out! He has a wood prop but I don't know what pitch. It's likely to be 72 inches long, as that's pretty much what's spec'd. That Chief really jumps off the ground, too.

A friend with an 85 hp 7DC can easily redline his engine in level flight. He cruises at 2350 with the throttle just over halfway open. Mine's shoved to the stop to get close to 2400. The 7DC has a McCauley 71 x 43, I believe.

I think that by taking out some pitch, I can also reduce my fuel burn. Running wide open takes about 6 gph. Brad, in the DC can't even burn 4 gph! I doubt my speed will suffer a lot, as I can barely get 90 out of it anyway. My interest is in short-field stuff, so if I cruise at 85-86, so what? On skis, the engine *really* doesn't wind up on takeoff, especially with a passenger. I had Brad fly her the other day to compare; he noticed immediately the low rpm.

Jon B.

[MikeB]

Jon,
Before I'd start messing with the prop (and you have maybe already done this) check a couple of things. Be sure the tach is reading accurately. It's not unknown to be off 100-200 Rpm. Most AP/AI's have a electronic "tach checker" to check your's against. If nothing else, Wag Aero sells one for less than $50. Check your carb heat box to be sure the flapper isn't in a partial heat position. Another spot to check is the exhaust for a partial blockage. And, believe it or not, I've found at least a couple of engines that had been rebuilt by AP's where the timing was off as much as 20 degrees. One A65 would only rev up to 1800 RPM.

Point is: don't panic and check all the obvious first before condeming the propeller pitch. It might be the prop, but I'd check a few things first.

Hope this gives you some ideas, anyway.

Mike

[Don Eide]

Jon,

I think you have the wrong prop.

The Type Certificate calls for a Sensenich 72GK-48 or 49 or 50.

The Sensenich brochure I have calls for the same props.

I looked under STC's and can't find your prop.

You might want to check.

Hope this helps,

[joea]

My C85 has no problem meeting redline in level flight. No idea what prop its got on it but I as well burn 4.5 gal/hr on average.

Would check what Mike noted above first. As well I would try to borrow another prop first if all the other things check out.

JoeA

[MikeB]

Oddly enough, the Univar book lists the 76 AK 2-44 as climb prop for the 7EC w/C90 . One column down they also list a 76 AK 2 42 as a standard prop for a 7EC (depending which model, I guess). I have a 7EC manual at the airport but not sure it lists the correct prop.

Interesting regarding the TC.......who is correct here?? 48-50 seems a big bite even if it is shorter.

I have to drive bus in a few minutes. If I have time I'll stop at the airport and see if I can find the manual.........for whatever it's worth.

You'd think Maxwell would know what the correct prop is... .

[nowlen]

this is info is out of american champion type certificate data sheet.
max rpm for a 7ec with a c-90 is 2475. static rpm limits are: not under 2125 or over 2350. that should be easy to check. max prop diameter is 72" and min diameter is 70". the props listed for the 7ec are the sensenich 72GK-48 or 49 or 50 and the mc cauley 1B90CM.
these are according to the t/c the only"legal" props and none have a diameter of 76". univair is wrong. what else is new ? put a 70-72" prop of appropriate pitch(lowest if you want climb performance) and your problem may be solved. good luck bernie

[Nathan K. Hammond]

Here's what I'm seeing on a C-85 with the O-200 Crank/Rods/Pistons (considered a bastardized C-90 ???)

McCauley 1B90 CM71-46 (very much a cruise prop)
2250 Full Power Static
2300 once rolling and climb-out at 60mph
2500 Full Power in Level flight

Speeds in Cruise: RPM - MPH
2200 - 85/90
2300 - 95/100
2400 - 100/105
2500 - 110/115

When I'm light, it get's up and goes like a monkey up a tree. When fully loaded (13 gal, 250, 250, 10 in baggage) it breaks ground and climbs like a strong 65 horse thats lightly loaded.
Clear as Mud???

nkh

[Nathan K. Hammond]

Jon, thats a 74 inch prop correct? That might be your problem, too much length and not too much pitch. Might see if the prop shop or a buddy has a different screw you can put on. just a thought.

nkh

[Jon B.]

Let's see if I can answer all of you in one message... I've made these same suggestions to others, so I hear what you're saying.

Bernie & Don: The numbers you're seeing are for the *wood* Sensenich prop. Mine is metal, and is the correct model and pitch for a 7FC/EC with the C-90. Wood: W72GK-48/49/50. Metal: 76AK-2-44/46. The numbers for wood and metal can *not* be compared directly. The -2- means that 2 inches is removed from a 76-inch model. The overall length is 74; the max for a metal prop on this combination.

Nathan: This is the correct prop for the C-90. I'll take a long prop over a short one. Long and flat is better than short and steep. The Super Cub guys use 82 x 40 or 42 inch Borer props for short-field stuff. (Note to self - a Champ is not a Super Cub!)

Mike (and others): The tach may be off; I'll borrow an electronic one to check it. However, when I left Brad's place on Saturday, I noticed that the engine just doesn't wind up at full throttle. This was the first time I've had a passenger while on skis. Brad commented on it, too. The 2200 on climb-out is solo; half fuel, so I'm well under gross. I'm fairly confident that the airbox and flapper are OK but will check that, too. I haven't a clue as to the mag timing. Nothing weird happens when switching between the two, though, at run-up or in flight. I can't easily check my static rpm, as I'm on skis. I can tie the tail to some immovable object, I guess...

I've heard from several people that new propellers sometimes come from the manufacturer with the actual pitch way off from what's on the tag and stamped at the hub. In some cases, they found the blades two or three inches different from one another! Or worse. On guy claimed a new brand 'M' had one blade at 46 and one at 41 inches. Some send their new props to the prop shop before they install them. Jim Fix, in Nebraska, checks each one he receives from the maker. If nothing else, I can have mine checked for pitch and balance.

Jon B. - who'd like to have a 76 x 40 wood prop...

[Paul Agaliotis]

I hope you get this one figured out. Nobody really knows how a prop works so good luck. For my two cents worth, it looks like you need a shorter or flatter prop.
Brian, my prop guy has a great quote on the wood props. He says, The wood props HAVE to be good, look how many are sitting around on the hangar shelves.
Paul
.

[Jon B.]

I bolted the re-pitched prop on last evening. This afternoon, an A&P/IA will check the torque, safety-wire the bolts, and complete the paperwork. I hope to get in the air tomorrow for a test of the reduced pitch. I'll let y'all know next week how things turned out.

We're quickly running out of snow, and '57B is sitting on straight skis. North of here, in the far north of Minnesota, they've got piles and bunches of snow. I hope we can keep enough for me to continue operating for a while.

Jon B.

[Jon B.]

Well, I was able to fly the Champ on Saturday. The conditions were less than ideal for a test flight though. The wind, although only 10 or so degrees off the runway, was at 17, gusting to 28! I hadn't been up for two weeks and needed a 'fix'. I had also just made the switch from skis to wheels. (Naturally, it started snowing Sunday night...)

Anyway, having repitched from a 44 to a 42 prop. here are the results of my uncontrolled test (not an uncontrolled flight, though):

My full-throttle static rpm went from 2150 to 2250 (engine not yet at full operating temp).

At 60 mph climbout, the rpm went from 2200 to 2300. It was hard to hold a steady speed & pitch in Saturday's wild wind, though. A calm day will return better information, I think.

When opening the throttle, the tail was flying immediately. Liftoff into the wind was quick! I doubt that any takeoff exceeded 100 feet. The deck angle at 60 was ridiculous; no forward vis at all.

As I mentioned, a calm day will make the observations a bit more meaningful. I shall keep y'all posted of my findings.

Jon B.