FrankC29

I have a Tower Hobbies .75 ABC, have run about 1/2 gallon through it, and it's turning 12,700 RPM with a 12X6 APC prop. This is the recommended prop for break in and I haven't tried any other props yet. I'm wondering if this number is average, above average, or poor for an ABC engine of this size. Can anyone give me your RPM figures for this, or any .75 2-stroke, ABC or not, like the ST75 or the GMS76, etc. Thanks.

CMFLY40

Frank,

I have the Tower 75, and have tried the following props.

APC 12 X 6 = 11,900 RPM
APC 13 X 6 = 10,320 RPM
Master Air Screw 12 X 6 = 12,150 RPM
Master Air Screw 13 X 6 = 10,320 RPM

These numbers are not really the maximum RPM, since my
engine was still fairly new when I tried the different props and
I didn't want to lean the engine out too much.

I still use the APC 12 X 6 prop as I like it best, and it will now
turn well over 12,000 RPM, but I prefer to fly it at 11,900 RPM
so it will be a little rich. Your engine's 12,700 RPM's sounds
like it is performing great. This is a good engine. Enjoy it.

Charlie

majortom-RCU

My Feedback: (40)

I am just breaking in a Tower .75. Had fuel draw problems with my fancy dual tank system for the first 12 oz or so of fuel, couldn't get it above 8,000 rpm. Switched to a single tank with simpler plumbing, got it over 12000 on a stock (not balanced) APC 12x7 with K&B L-1 plug and 15% nitro/20% lube, with plenty of exhaust trail visible. After about a minute at that rpm the muffler blew off its end cap and for a few seconds I had maybe 18000 rpm and a mighty roar. I found the end cap about 20 yards away, put it back on with a nylon lock nut, started up and again went to max throttle. Within another minute the muffler blew again, this time the center rod stripped the threads out of the muffler's aluminum front bell. Same big noise, didn't scare me so much this time.

I have run a #6 threaded rod through the muffler, put lock washers, jam nuts and thread lock on both ends, and will take it out today with a balanced 13x6 Zinger and tune the low end.

No question this is a very strong engine, ample power for my .60 Stinger, so I will not be pushing the limit on rpm at all. The plane is under 8 lbs dry, just over 8 lbs with 12 oz of fuel, and the Zinger will give me close to 10 lbs of vertical at even 11000 rpm, which I expect will be no problem.

As to comparisons with other .75 size two-strokes, I have a GMS .76 which is a comparably strong performer although I haven't tached it lately, and an MVVS .77 which turns in the 12's on a Zinger 12x8 with 10% nitro.

CMFLY40

I had a problem with the muffler end cap working loose, but it
never came off. I ended up buying a couple of extra thru rods
and used a new one with plenty of lock-tite and that has held
ok so far.

Good luck with your flight today. Please post some RPM readings
with the new prop if you don't mind. Thanks.

majortom-RCU

My Feedback: (40)

So I tuned the low end, tried the ground handling, decided to give it a shot, took off, made about a dozen circuits and brought it in. Aileron flutter, servos seemed a little twitchy.

Anyway, before and after the flight I tached the Zinger 13x6 at 10,800, smooth & steady idle at 2500, still on 15% nitro, 20% blended lube. The midrange is not as solid as it should be, so I will see if I can tune it up a bit finer--after I install separate aileron servos and re-route my antenna. These readings were at rich setting on the high needle, plenty of exhaust visible, no sagging at all with the nose straight up, which was mostly what I was interested in at this stage. At this point the engine has almost half a gallon through it.

CMFLY40

Thanks for posting the RPM figures for the Zinger prop.

My Tower 75 wants to lose rpm when I am in a turn, unless
I run it rich enough to leave a visible smoke trail.

Would this indicate that the low end needs a little adjusting?

Thanks.

majortom-RCU

My Feedback: (40)

I consider a visible exhaust trail to be the normal condition, absence of which makes me suspect the engine is too lean. Most of my crashes have been a result of engine failure, and most of those have been a result of running too lean. I am happy to compromise a few hundred rpm at the top for no flame-outs. In spite of my vigilance in this regard, my first attempted flight of this season ended in flaming out on take-off at about thirty feet of altitude on what I thought was a trusty old HB .61 hauling a vintage Lanier Dart. As soon as it lost power, it tip-stalled and came straight down. I didn't like that plane anyway.

Optimum tuning of the low end needle to me is a.) via pinch test, in which you pinch the fuel tube closed and hold it there until the engine does something different--revs up a little (or a lot), which means you can lean it out some more, or sags and dies, which means you can richen it just a bit, or just keeps running in spite of the pinch, in which case I leave it where it is OR b.) a smooth, non-sputtering run-up from stable idle below 3000 rpm to full bore when you punch the throttle. I like to see an idle down around 2500 rpm, but I usually settle for whatever revs do not quit and do not pull the plane forward on our grass (and dirt) field.

Optimum setting on the high-end needle is a.) I can see the exhaust stream when the plane is flying toward or away from me (but not always when it is flying with its wingtip pointing at me); AND b.) I can hold the plane with nose straight up and less than half a tank of fuel, at full throttle for 30 seconds, and there is no sign of sagging. If I can't pass the nose-up test, I put it on the ground and go through my pinch test for the high end. This is pretty much the same as for low end, which is pinch the fuel line firmly (usually only a brief pinch of one second or so) and listen for an increase, decrease or engine stops. If rpm increases markedly, I lean the n.v. a click or two; if rpm increases only slightly, I leave it where it is; if rpm sags or engine dies, I richen a few clicks.

Anytime anything changes in the combustion scenario (fuel mix, glow plug, prop size, change in temperature or humidity, etc.) it's worth going through at least the high-end pinch test. But if I am in a hurry and want to fly real bad, I rely on the nose-up test. When I don't (like with my Dart the other day), I often regret it.

CMFLY40

Majortom, thanks for the helpful hints on adjusting the mixture
on high end and low end.

This Hobby is relatively new for me since I didn't get involved until
two years ago, and old enough to qualify as a senior citizen, so
I am still in the learning process. This is a fun hobby, and this
forum is very helpful. Thanks again.

Charlie

FrankC29

After running almost a gallon through this engine, I am now getting 13000 rpm with a 12X6APC prop, about 4 clicks off peak, and still getting a nice smoke trail. Powermaster 15% fuel, and 2 shims in the head. I haven't tached the 13X6 lately, but honestly I like the performance of the 12X6 over the 13X6, though I suppose the extra 2000rpm means the engine won't last as long?? Very powerful engine, excellent transistion, rock solid idle at 2500rpm, and my shuts off, at least for now. Deffinately not bad for $100. I have it in a 7 lb. plane, and unlimited vertical is a forgone conclusion!