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PA-18-150 Max RPM question (again)

jwflys

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Hi,

I'm new to Super Cubs. Love it.


I hope you'll forgive me if this question was asked before, but I can't seem to find an answer buried in the forums.


I have a PA-18-150. (does not have the 160 HP mod) Has the Sensenich 74DM6-0-56 prop with fewer than 50 hours and in good condition. Compression is within mid 60's to mid 70's.


Before and after the recent annual, I am unable to produce RPM over 2500 in level cruise (2475 ish). Static RPM is about 2250.


We checked the RPM at a few settings with a hand held device. The tach is very close to indicated, although the needle does swing occasionally at high RPM settings. It always settles.


After flying a square on cardinal headings, I'm getting about 100 mph according to calculations. Airspeed indicator seems to be off as well. Pitot has a very slow leak.


My mechanic says he doesn't think I'm developing full power. Another mechanic says its about right for the age of the engine (3000 + TT, 275 SMOH) and standard prop. They will be taking a closer look soon. Yet another mechanic thinks I should get more RPMs.


While I sort it out, I got recommendations to use Marvel Mystery Oil in fuel, check valve clearance, check for intake manifold leaks, check carb butterfly for full opening. The throttle and mixture cables have stop-to-stop travel.


I understand all planes are different but I wonder what max RPM are other PA-18-150 with standard props are getting (Are there any PA-18-150's with standard props anymore? I plan to order a Borer 8242)?


I know this topic has been discussed here before. I appreciate helping a newbee.


Thanks!
 
I believe a PA-18-150 TCDS requires static RPMs of 2350 minimum and 2450 max. That's for land planes. I recall seaplanes are +100 RPM for the min and max limits. Redline is 2700 and that's where it makes rated power. Your prop and power mods may alter the static speeds a little but they should be higher. Rated power is defined by RPM. RPM is determined by prop pitch. That's a pretty good topic to discuss with your mechanic. Mine wants the static RPM on a 160 to be 2400+.
 
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TC 1A2 says:

Note 5.a. Not over 2450, not under 2350 (landplane and seaplane)
Not over 2550, not under 2450 glider or banner tow
No additional tolerance permitted

Note 5.b. Not over 2550, not under 2450 (seaplane)
No additional tolerance permitted

Compression in the mid 60's sounds a bit low. A -56 prop sounds a bit course for a tired engine. I get about 2500 with a 72.5" dia. -52.
 
What concerns me is getting "data plate" RPM's at takeoff (the little tag that engine builder is required attach to the engine tells you everything).

Gasoline engines are rated by RPM. The engine tag is what you look at. If it is not getting rated RPM you are not getting the full HP. Ground run-ups were mentioned to help A&P's determine engine health in the TC. But, flying and reaching rated HP on takeoff can only be meassured by doing just that.

(recip, non-turbo, carbureted engines, ambient fuel)
 
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Change to a flatter pitch prop.

Has the engine been sitting? 275 is very low time for an overhaul. If it has been sitting, it sounds like it needs to have about 300 gal. of avgas run through it.

a 56 is way more than any cub I know of. We discuss 41 and 42 size. 46 would be a fast prop for me.

Sounds like a big bite, reduce that and fly the plane. Engines like to be run, not sit.
 
Same engine/prop combo on mine ( 150hp A2B with 74 DM6-56).
Static 2450 rpm, cruise 2800rpm.
Did lots of bannertows with that prop.
Thore
 
jwflys, Your 74-56 prop is the standard prop which was installed when the PA-18 came from Piper. One thing which has not been mentioned is magneto timing. Are your mags properly internally timed? And properly timed to the engine? If you have a sharp A&P he should be able to tweak the mags for optimum performance.

This engine propeller combination will not achieve 2700 rpm at take off and climb as 8GCBC seems to imply. You should be able to make close to 2700 rpm at full throttle in level flight.
 
confirm the prop pitch
look up into the muffler tail pipe and check for blockage
electrnic tach tool to check mechanical tach
 
electrnic tach tool to check mechanical tach

If your airport has a mercury vapor light you can use it to check your tach. Point the airplane so that you are looking through the running prop towards the light. The prop will appear to stop turning when it is turning at any rpm number which is divisible by 60.
 
With 275 SMOH I'd think the compressions would be higher than the mid-60's on all of the cyls. How recent (calendar-wise) was the overhaul?

Good suggestion to check the prop is truly pitched at 56, I'd definitely do that if other causes arent evident. It should make a little closer to redline RPMs in level flight, full throttle and low altitude.

Let us know what you find out, and good luck.
 
My first statement was very oversimplified and was not up to speed (post #4). Let me add...

...By design a fixed pitch propeller rarely will let an engine produce rated HP (RPM). It is a compromise. The pitch will determine the airspeed the prop "may" get to 2700 RPMs, but that envelope is really "really" small, certainly not on takeoff (from what I have flown).

TCDS static run-up(s) (airspeed ZERO) are for 0' density altitude, 0 humidity, perfect tracking propeller, no alternator load, no wind,, or accessories. These condition are not always available.

Constant speed propellers can achieve 2700 RPM during most takeoffs.
 
Hi,

I'm new to Super Cubs. Love it.


I hope you'll forgive me if this question was asked before, but I can't seem to find an answer buried in the forums.


I have a PA-18-150. (does not have the 160 HP mod) Has the Sensenich 74DM6-0-56 prop with fewer than 50 hours and in good condition. Compression is within mid 60's to mid 70's.


Before and after the recent annual, I am unable to produce RPM over 2500 in level cruise (2475 ish). Static RPM is about 2250.


We checked the RPM at a few settings with a hand held device. The tach is very close to indicated, although the needle does swing occasionally at high RPM settings. It always settles.


After flying a square on cardinal headings, I'm getting about 100 mph according to calculations. Airspeed indicator seems to be off as well. Pitot has a very slow leak.


My mechanic says he doesn't think I'm developing full power. Another mechanic says its about right for the age of the engine (3000 + TT, 275 SMOH) and standard prop. They will be taking a closer look soon. Yet another mechanic thinks I should get more RPMs.


While I sort it out, I got recommendations to use Marvel Mystery Oil in fuel, check valve clearance, check for intake manifold leaks, check carb butterfly for full opening. The throttle and mixture cables have stop-to-stop travel.


I understand all planes are different but I wonder what max RPM are other PA-18-150 with standard props are getting (Are there any PA-18-150's with standard props anymore? I plan to order a Borer 8242)?


I know this topic has been discussed here before. I appreciate helping a newbee.


Thanks!

if your prop is a true 74/56 You have a weak 150HP it should run at cruise flight around 2500 t0 2600 is what i have seen inn 150 HP there ar alot of things that could be wrong haven't read thru the thread yet but check exhaust may be starting to warp a bit causing loss of power
Steve C
 
Some good suggestions already posted. I would check mag timing ,spark plug condition, muffler for loose baffle and confirem prop pitch before i decided I have a weak engine. Also with compressions in 60's more importantly where is the compression being lost? Valves or rings. Always check the easy(cheap) stuff first, don't let your mechanic talk you into spending unnecessary $$$.
 
Lots of good suggestions, one more very simple one-----check your throttle linkage and make sure it is going all the way to the stop.
 
My friend in Honolulu had an 0-320 that had not been overhauled for 35 years. It even was painted the old Avco Blue (before there was "Lycoming Gray"). It had about 4000 hours and no oil filter (screen only). It rarely got an oil change on time. But, it had compression in the 70's. The Cessna 172 it was on had a "climb" prop and it did 850'/min at gross. Always made TCDS RPM numbers no problem (PHNL is 13' MSL, Density Altitude is usually about 2500').
 
Although that engine must be pretty tired, check for weak valve springs and then the dry tappet clearance.
 
Thanks to everyone for suggestions.

Sorry for taking so long to respond.

Over the course of troubleshooting my mechanic and I tried most suggestions but did not find a solution.

I decided to mount an 8242 Borer prop anyway and go on a trip. I got marginal improvement in RPM and performance with the new prop. It still wasn't what I expected compared to other PA-18-150's I've flown.

Cub pilots I met along the way thought my Cub sounded like it wasn't developing full power.

As the trip progressed, a small but messy oil leak developed on the right side. Oil burn/loss was about a quart in 9 hours.

The engine ran as usual at the beginning of a flight but got a little rough after an hour or so.

Mechanics I met during the trip diagnosed that I had a leak in the right mag seal (This mag had been overhauled just before the trip ). Oil accumulated in the mag until it covered the points.

Long story short - I replaced the old mags with new mags and replaced wire harness. We were able to pour oil out of the old mag.

I now have expected RPM. Can hit redline with no trouble. Power is much better and fuel burn is down as well (2 gph at 2600 RPM - never able to hit 2600 before). Mag drop at runup is now 50-75 RPM on each side (100+ per side with old Mags). I know that some performance difference is due to the prop, but it seems that the new spark has a significant benefit. Perhaps the timing is set properly this time as well?

Now that the problem appears to be resolved, I'm curious. Have others seen immediate and significant improvement after rebuilding or replacing Mags? Is it really difficult to get timing set properly? What about spark plug harnesses? Do they really degrade significantly and if so, over how many hours?

I have some better performance numbers as soon as time allows.

Thanks again for your help.
 
Mags act up if the cooling tubes become blocked and they get too hot (Slicks are the worst). Checking the timing is a no-brainer: you need a $2 6-12V door buzzer, a 9V battery and some wire. An extra pair of hands to tap the prop fwd is useful. MAKE SURE THE LEADS ARE OFF THE PLUGS AND ONE SET OF PLUGS IS OUT because you'll be working with HOT MAGS. To set it, start by setting your 25 degs and then rotate the mags one at a time as needed to find the spot where the points open. Don't try to adjust the points gap. Harnesses usually have cigarette tip problems if they get dirty, mishandled or damaged. Other than that, they should last to TBO (but in real life seldom do). Glad to hear you found and resolved your problem. Who o'hauled your mag?
 
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