Difference between Max-S and Max-H

Helicopters

OK...any idea what is different about the engine itself? Other then one has S embossed on the side, and the other has H? You can run an S engine is any pretty much any position. I wonder if the H has better cooling? I would think a Helicopter engine would not have as much air blast on the engine. Or maybe it has some internal design difference to run at lower speeds...I'm guessing...I have no clue LOL

Yes, typical "H" variant engines have much larger head assemblies for cooling. There can be other heli-centric features as well, but the big head is key, the others don't mean squat if the engine is being fried. It wouldn't take but a couple of minutes on any of the major model engine manufacturers to educate yourself a bit.

It ain't rocket science - it's airplane science ;-)

"AFAIK" they tend to be set up for a different midrange mixture via needle geometry, as well as the head issue. I think the midrange tends to be richer than a sport engine carb setup.

"daytripper" wrote

Along with the better cooling head, it is made to be shrouded, with a fan blowing air onto it. I think (I'm not sure on this one) that the threaded crank is made longer, to mount the fan and the gear or cogged pulley to drive the gearbox or main shaft. The thrust washer is most likely a different type, too.

The Heli engine is likely made to run at higher RPM's, to get a maximum weight to power ratio, which is so very important for a heli, and run those higher speeds for longer periods of time than an airplane engine. There are likely to be a larger bore carb, with the crank having a different timing, optimized for high RPM, and peak power, meaning sooner opening, and more overlap, which could mean subtle differences in the porting, too. It is just not as important for a Heli engine to idle well. They usually run the same exact RPM, or close to it, for their whole flight.

Some guys do run heli engines, but put a regular head on them, to take advantage of the higher power output. They are usually the kind that race, or run around wide open, like they are pretending to race.

The most visible, and perhaps the most important, difference is the bigger, usually square, cylinder head. The square shape is much better suited to fit inside a shroud, so that the air flows between the cooling ribs, and not around the engine.

Another important difference is in the carburetor. The load/throttle relationship is substantially different in a helicopter, compared to a propeller driven plane. Very simply speaking, the heli engine has a richer midrange, but it is much more complicated than that.

I have OS Max-S 35 which has a sleeve bearing and OS Max-H 40 wiht ball bearing. So I gues it is the kind of bering making the difference.

How about an OS-Max 35 FP? I have two of those, and don't really know what the differences are.

The FP series are sleeve bearing, instead of ball main bearings.

Ahh...and therefore more susceptible to damage from lean burns, more susceptible to wear, etc. ?

To the extent that such engines are mistreated, yes. But you should know the venerable FP series probably powers more models than any other line (as opposed to, say, the dreaded FX series...)

/daytripper

On Tue, 27 Feb 2007 18:55:33 -0800, "Zootal" wrote in :

The plain bearing engines in my experience are more rugged.

It's pretty easy to ruin ball bearings. Please don't ask me how I know. :o(

The FPs tend to rattle at low throttle.

They are more susceptible to breaking things if you flood them and get hydrolock. I don't know why that should be, but it seems to be the case.

The FPs with which I'm familiar also use an air bleed carb. To lean the low end, you want to close the air hole (tighten the adjustment screw). To righen the low end, open the air hole (loosen the adjustment screw). This is the opposite of a dual-needle valve system.

For starters on the air bleed carb, put a pin or the tip of a fine blade into the air bleed hole and adjust the screw so that the hole is half-open. Just a few turns will close the hole completely or open it completely, so make many small adjustments instead of large adjustments. If in doubt, get out the pin or the knife and find out where the adjustment screw is.

If I ran a lot of FPs, I would probably use castor blend. For some reason, I think they would like the extra protection castor gives. I've run them successfully on all-synthetic

As a general rule, FPs are lighter than the same displacement ball-bearing engines. They also tend to be less powerful. I had fun with a TT GP .25 (an FP clone) in a Sig Wonder. It worked a whole lot better than the Magnum .15 ball bearing engine that I started with.

I took out a .40 ball bearing and put in a .40 FP in a GP Fun One. The change saved six ounces overall, but I had more fun with the more powerful engine and switched back to it.

Marty

As far as I can tell, ball bearings are just another way to squeeze $20 out of the customer's pocket. The FP series was just about the perfect model airplane engine. I still have quite an extensive collection of them.

On Tue, 27 Feb 2007 22:14:54 -0500, daytripper wrote in :

I don't own any of the new OS plain bearing engines, but I don't like the looks of them. I've seen a lot of them at the field, both .40 and .60 size.

This is sheer prejudice. I've stripped too many Philips head screws in my day, I guess. I also don't like remote needle valves very much.

Marty

"Zootal" wrote

Not necessarily.

I think most here would say that the plain bearing is more durable, and tolerant of less than ideal conditions. The piston and cylinder is what suffers from lean burns, mostly. Ball bearings are susceptible to damage from moisture and corrosion if not stored well, and protected by after run oils, or castor oil fuels. (watch this bring out the discussion)

The penalty for the plain bearing is mainly less HP for the weight and size than the ball bearing twin.

What you say is true, but it is not the plain bearing that causes lower power output in such engines. It is the overly mild timing/porting that is the culprit. How would it look for OS' plain bearing engines to be keeping up with their high dollar ball bearing engines? It would not help sales of the expensive engines at all.

There have been some very powerful plain bearing engines made by Fox and several other companies. These were usually special event engines, of which classes totally escape my mind at the moment. I'm sure that others will remember.

I just obtained two NIB OS LA engines. One is a .40 and the other a .46. Much to my surprise, both engines have a pronounced pinch near TDC. I truly did not expect to experience this, as the FP series that I'm familiar with lacked such a pinch. At least they did by the time they got to me (used/beginner engines).

Ed Cregger

Don't worry Ed, as soon as you get them good and broken in that will disappear! ;-)) Seriously that is entirely normal for FPs and is actually a good sign. I wouldn't reccommend rotating them much by hand however to feel that pinch until you get some lubrication into the motor. You might damage the liner and/or piston cranking it dry like that. Proper break-in is key for an FP if you really want it to run well for a long time. Had a couple and did (like most newbies I suppose) everything bad I could do to them... crank them with an electric starter while hydro locked, ran them over lean etc... The lasted about 3 years before the bearing up front started to leak and made them hard to tune. Considering the abuse I gave them they did great. They are lighter but less power than the ball bearing models of course but they work well within their designed use. I'd take a ball bearing over a sleeve every time for the average sport flyer who wants a good reliable long lived motor but for the dollar an FP can be a great buy. Similar to an LA in my book, just lacking the remote needle valve and plastic backplate... a plus in some folks book!

Jack

You got that one backwards. To richen the idle on an air bleed carb, you close the air hole. To lean the mixture, you open the air hole.