Plain Bearings Motors and Heavy Props?

Does anyone have any experience concerning bearing longevity with the use of heavier props (like the APC) on plain bearing motors (like the OS LA40/46)?

I am now running my LA40 with an APC 11x4 which is a pretty heavy piece of composite spinning up front! Yes, the prop is balanced. And, yes I understand that once running, the oil film in the bearing provides quite a bit of strength to prevent metal-to-metal contact. But, it seems that every time the airplane corners, gyroscopic forces from such a weighty prop are going to really force the shaft into the bearing (at 90 degrees to the direction of movement, of course). And, we do a frequent bit of cornering in stunt! I suspect this would accelerate bearing wear in comparison to running the motor with a lighter prop such as one made of wood.

Can you provide your experiences or thoughts in this regard?

Thanks!

The load differences between a light and heavy prop are likely insignificant given the large bearing surface in a plain bearing motor. Bearing unit loading is much greater in ballbearing motor applications.

On an empirical note I have an LA 46 with more than 3000 flights on an APC 11.5 X 4 prop and the shaft and bearing are like new. Always run on 10-10-10 fuel.

The APC props are much more effecient, in general, than any available wooden props, and providing the extra weight doesn't create a CG problem will almost always produce better flights with the proper selection, as compared to "old technology" wood props.

Just an opinion based on facts!

The gyro moment itself is not that strong, but problem is that the moment is not homogenous and two blade prop in corner vibrates. That dynamic effect eats the bearing very quickly to some extent and then it stops. Surprisingly it does not have any ill effects to run of the engine. You can check it on tips of prop on hot engine after flight.

It is much better to use 3 blade prop, the gyro moment is homogenous, prop does not vibrate and it gives better chance to bearing.

Igor,
I believe this to be more of an issue with front ball bearings than with plain (bushing) bearings engines. It is also frequency dependent, and as such varies with RPM, prop size and engine dynamic balance.

If you think on this a bit, you will realize that the time spent doing hard corners is insignificant compared to the time just going 'round (round?).

GMA used to complain that too many good engines were ruined by not using enough castor oil. If you use R/C fuel, then you get the wearing out before its time.

I prefer wood props, only because it is better to break a prop than bend a crankcase-- if it comes to that.

>The load differences between a light and heavy prop are
>likely insignificant given the large bearing surface in a
>plain bearing motor. Bearing unit loading is much greater
>in ballbearing motor applications. Randy C

I take it this would also apply to the use of heavy hubs?, which can be especially useful on snub-nosed Old Timers.

Well...
Heavy hubs are a lot more weight further out on the moment from the bearing...Hmmmm
I've never used these things personally, but have seen others use them with no ill effects.
Without running any numbers my guess would be that for a plain bearing engine they wouldn't have much ill effect.
But...that's only an educated guess...
They certainly do increase precession, etc...
I would reccommend avoiding them for that reason.

A heavy hub is a good way to blow the shaft on a Fox 35, esp. if the prop is unbalanced, or you run the motor fast. We've had a couple break in our club just running at stunt speeds.

I haven't heard of or seen any serious main bearing problems in the LA series. That includes several 25's and a couple 40's with hundreds of runs at 15k+ on combat planes, which is much tougher on the bearing than at stunt rpms.

>I haven't heard of or seen any serious main bearing problems
>in the LA series. That includes several 25's and a couple
>40's with hundreds of runs at 15k+ on combat planes, which
>is much tougher on the bearing than at stunt rpms.

Phil,

I think I know why you are saying this, as intuitively your comment would seem to be correct: High revs, high loads.

I'm not so sure about this. On the other hand, I have no meaningful data.

Only the anecdotal evidence of running into rear bearing failures with some prototype RC Pattern 60s when they were loaded with APC 12-12 props, fed 45% nitro fuel. Peak was 10,200 on the ground.

This sort of failure never happened when the engines were propped to run at high revs.

Incidentally, the balls themselves would break; we are not talking about a mere failure of the retainer here.

Same sort of experiences in RC cars. The over-run revs at the end of the straights are horrendous, and we would break rods--even the steel rods, although not as frequently--but crank and bearing problems were nearly unheard of. My theory is that while the engines were revving hard, they really weren't doing much work, thus not near the load one would think was being generated in the lower end.

Anybody have real-world data on this subject? I tend to believe our CL Stunt motors carry a lot more load than we think...

LAST EDITED ON May-09-06 AT 07:46 PM (CST)
 
Shaft runs really don't put much load on the main bearings, mostly the conrod.

45% nitro is a bunch and puts a lot more torque(bending load) on the crank. The most likely thing to break would be the rear bearing or the crankpin. Cracking the balls is also a sign that the bearings were likely fitted too tight.

Combat of any type puts much more offcenter load on the prop. The stunt pattern has has 12 loops worth of hard corners in it. Any combat match probably has that much in a minute or less, and at much higher rpm. Combat will wear out the main bearing in a Fox 35. Also much more likely to break a shaft in the Fox, or almost any engine not designed to take it(G21-35 or any 12 mm shaft engine at 15K+ rpm). Henry has the right idea, 17 mm shaft in a 36 engine is good, since it is putting out about the more horsepower as a 61-75 pattern engine.

Real world data is that shafts in stunt motors are extremely unlikely to break. The only ones that do are known marginal design(Fox, maybe a few far east 28's). High rpms on a combat plane almost routinely break shafts unless they are very large like the Nelson.

Cars have none of the off-center loads you get with two-bladed props, so the shafts rarely break. But you can see what damage over-revving can do. something else breaks before the shaft.

>Shaft runs really don't put much load on the main bearings,
>mostly the conrod.
>
>45% nitro is a bunch and puts a lot more torque(bending
>load) on the crank. The most likely thing to break would be
>the rear bearing or the crankpin. Cracking the balls is
>also a sign that the bearings were likely fitted too tight.

Yep. But in this case they were not. When I was doing this testing the O.S. Hanno Not-So-Special .61 RC Pattern motor was also known to break balls in the bearings.

>
>Combat of any type puts much more offcenter load on the
>prop. The stunt pattern has has 12 loops worth of hard
>corners in it. Any combat match probably has that much in a
>minute or less, and at much higher rpm. Combat will wear
>out the main bearing in a Fox 35. Also much more likely to
>break a shaft in the Fox, or almost any engine not designed
>to take it(G21-35 or any 12 mm shaft engine at 15K+ rpm).
>Henry has the right idea, 17 mm shaft in a 36 engine is
>good, since it is putting out about the more horsepower as a
>61-75 pattern engine.
>
>Real world data is that shafts in stunt motors are extremely
>unlikely to break. The only ones that do are known marginal
>design(Fox, maybe a few far east 28's). High rpms on a
>combat plane almost routinely break shafts unless they are
>very large like the Nelson.
>
>Cars have none of the off-center loads you get with
>two-bladed props, so the shafts rarely break. But you can
>see what damage over-revving can do. something else breaks
>before the shaft.

You might be right about this, I am simply going by other examples and a gut feeling.

It would still be nice to have real data.

Ron B.
F4Fguy

Phil:

I agree,mostly.

A little nitpicky, maybe, but, while there's certainly some increased bending load, the main loading increase (from higher nitro) is torsional.

In answer to the original question,I think Igor has it right. The basic problem is the non-uniform nature of the rotation. In (simplified) form, the torque is peak at the upper end of the stroke and essentially 0 at the bottom. This translates to an angular velocity difference which is in turn dependent on the rotating mass. This, in turn, produces an assymmetrical thrust pattern causing a periodic side force on the shaft. The resulting load is always in the same direction regardless of maneuvering and is rpm and load dependent. The size and weight of the prop are, relatively, not a factor. The direction of the force can be altered by changing the angular position of the prop on the shaft.
That's not to say that prop weight isn't a factor in shaft life. With a relatively long shaft such as is normal in a front intake two stroke, a heavy prop can make for some very high torsional loadings due to the inertia of the shaft vs that of the prop. The "heavy hub" can only add to these differences. Look at the failure point of most cranks. They tend to fail at the rotary valve port opening. If the problem was bending moment one would expect failure at the crank web or the crankpin, as the loads at these points are far higher than the prop can generate, either from gyro, or P-factor.

Dan may well be right in that stunt engines, operating much nearer the torque peak than combat or other high rev events, may well have higher torsionals.

All the above relates only to bearing and shaft life, not to aircraft performance. There, prop weight is a major factor. Not only do heavy props generate higher precession, they also inhibit accelleration, a major asset not to be thrown away lightly. Heavy hubs and or shaft weights only add to the problem.

LAST EDITED ON May-10-06 AT 08:25 PM (CST)
 
When I put a heavy hub/heavy prop on a Fox 35 the shaft broke right at the front of the case, under the prop driver.

When I broke one from too much nitro, it broke across the port.

I'd bet you can break almost any engine with a 12mm ball bearing shaft simply by letting it rev. Most of them, from 25-36 will happily turn an 8/5 or 8/6 prop upwards of 18-20,000 before breaking the shaft across the port. I've proved to myself more times than I should have that a 12mm shaft is not strong enough to pull a combat plane over 90 mph. I have never heard of anyone breaking this size shaft in a stunt application.

A front intake shaft port is a horrible design solution- huge stress risers, twisting loads, terrible fatigue life, bad resonance problems at common useful rpms(typical FI engine will vibrate badly at ~7500 rpm, 15,000 rpm, 22500 rpm, and something over 30,000). The G-21.35 would often break the shaft just when the motor started running well, around 15,000 or so. The AKM folks specifically tell you not to run the their motors over 30,000 because the shaft will break.

(1)* >When I put a heavy hub/heavy prop on a Fox 35 the shaft
>broke right at the front of the case, under the prop driver.
>
>When I broke one from too much nitro, it broke across the
>port.
>
(2)* >I'd bet you can break almost any engine with a 12mm ball
>bearing shaft simply by letting it rev. Most of them, from
>25-36 will happily turn an 8/5 or 8/6 prop upwards of
>18-20,000 before breaking the shaft across the port. I've
>proved to myself more times than I should have that a 12mm
>shaft is not strong enough to pull a combat plane over 90
>mph. I have never heard of anyone breaking this size shaft
>in a stunt application.
>
(3)* >A front intake shaft port is a horrible design solution-
>huge stress risers, twisting loads, terrible fatigue life,
>bad resonance problems at common useful rpms(typical FI
>engine will vibrate badly at ~7500 rpm, 15,000 rpm, 22500
>rpm, and something over 30,000). The G-21.35 would often
>break the shaft just when the motor started running well,
>around 15,000 or so. The AKM folks specifically tell you
>not to run the their motors over 30,000 because the shaft
>will break.

Ron B.
F4Fguy

Phil:

My criticism is not with the idea that the crank will fail, or even, where. It was that the mode of failure was torsional in nature. Certainly operating at a nodal frequency will bring failure earlier, that doesn't alter the mode of failure. Whether coincident or not, the part will fail when the fatigue limit is reached at the point of highest stress.

*(1) Adding rotational mass forward on the shaft will move the point of max stress forward. As an example: Consider what would happen if you moved the flywheel on a car to the final drive (assuming front engine, rear drive). The power delivery to the wheels would not be significantly changed but, the reversal loads on the transmission would quickly destroy it or the drive line in between. If you doubt this, talk to an ALFA owner with an Alfetta, which has the clutch in the rear at the final drive. They eat drive lines. The high nitro example is actually an example of what I'm trying to convey; the resistance is primarily from the prop, there's no great inertial mass, the force at the crankpin is dramatically increased, so the failure point is as predicted, at the port.

*(2) Only true if you are still running a prop for load, in which case the failure will still be torsional fatigue, and most likely at the port. It will be induced by the cyclic load imposed by the rotating prop and the nature of the pressure curve. If you shaft run with no load the failure will probably be at the crankpin (or, more likely, the rod). That is assuming the bearings will allow continued operation at this speed. Once a bearing failure occurs, all bets are off, as the crank is now subject to eccentricity and friction which changes everything.

*(3) No argument! On the other hand, it's the lightest, simplest, least complicated, lowest cost solution to the two stroke conundrum. I don't know about you but, I don't want to return to side ports.

Floyd,
Don't plan to crash...

Thanks to everyone for the responses. Good information

I've run heavy hubs a thousand or so flights each on various Fox 35's with no problem. I went through 4 crankshafts running a heavy hub and APC 12 x 6 on a dieselized Tower 40. Same engine has had several hundred glow flights on it with heavy hub and Supercool carbon 11 x 5. No problem. It is a little wet nose, which I attribute to honing the main bearing after the broken crank incidents.

I have actually seen two Fox 35's break, one a rod, the other a crank, neither with a heavy hub, both flying stunt.

The fact of the matter is that some stuff works for one person and not for another.

>I went through 4 crankshafts
>running a heavy hub and APC 12 x 6 on a dieselized Tower 40.
> Same engine has had several hundred glow flights on it with
>heavy hub and Supercool carbon 11 x 5. No problem.

Jim,
What part(s) of the shafts broke on your dieselized Tower 40? Because glow conversions weren't designed for diesel, do you think they are more likely to break than a purpose-designed diesel.

Looking at the tight OT scores at last weekend's contest in Woodland, CA, I'm about ready to take the diesel plunge.

The Tower shafts have a grinding relief groove around the shaft at the base of the crank wheel. All four shafts broke fairly cleanly in that groove. I think the heavy hub was a real contributor. I ran the engine diesel on the bench, (without heavy hub) a couple of hours without incident and put maybe 50 flights on it before the first break. The time between breaks decreased after each break. I don't remember, but maybe 5 or 6 flights between the last two breaks. Others running conversions report no problems.

Randy. Nobody ever "plans" to crash, but it happens. Windy seems to crash regularly, and I suspect he does it on purpose so that he can write another repair article.

Just my opinion: I think many of us are happy to get 1000 flights over the course of a long time. For those of us in that category, any breakage might just possibly be due to a faulty part. I have never had the problem of a broken crankshaft on a Fox 35 or other engine. I did break a rod in a well used Veco 19BB while flying a R/C plane once, years ago. I have "worn out" very few engines over the course of 40 plus years, and most of those were from not taking proper precautions around dirt fields, not throughly cleaning engines after a crash, etc. many years ago, so maybe it isn't a problem for many. And using improper fuels played a large part back then, too, I'm sure.