LA 40 Shenanigans

I've been playing around with LA40's on my Giles(huge flapless stunter) and a couple of other planes. This is a pretty plain vanilla engine. I angled the bypass ports per Tom Hamphshire with a Dremel carbide cutter. The top edges are angled up about 15 deg. Playing around with the head volume made some interesting changes in how it runs. All the fuel has been at least 25% oil, at least half castor.

Box stock, using a .262 LA25 venturi and an OS needle valve it turns about 14,000 rpm on a 9/4 Zinger, 10% fuel, 25% oil. Four cycles very nicely. Screamed in a 2 cycle on a 10/4.

Mod #1- opened up the head using a 3/16 in. Dremel bit to make the squish band about .130 wide(stock is .150 in.) Runs a 10/4 Zinger at about 11,800-12,000 peak, fast four cycles in flight with a 10,000 rpm launch. Nice break between 4cycle and 2 cycle in maneuvers. Does a complete pattern on 3.25 oz. Lap times are 4.7-5.0, depending on exactly how rich it is set.
Mod #2- open up the head to make the squish band about .1 in. wide. .285 venturi. 11/4 Zinger prop 7.5% fuel, 28% oil. Peak rpm is about 11,500. Launches at 9800. Lap times are 5.2-5.4 Noticeably better flying than with the 10 inch prop. Just barely makes the pattern on 3.25 oz. of fuel.
Mod #3- same head, opened up venturi to .292. 12/4 Zinger prop 7.5% fuel, 28% oil. Peak rpm is about 11,500. Launches at 9100. Lap times are 5.8-6.0 Just makes the pattern on 4.25 oz.of fuel. Noticeably better flight characteristics than the 11 in. prop, despite going so slow.

I think a 12/4 is about all the LA can pack. Even starting rich, it threatens to die during the overhead 8 and quit early from fuel starvation. The only thing is to open up the eights to the usual 60 degrees or so. Then most times it will recover and do a nice cloverleaf. On the .285 venturi with a 12/4 the lap times are over 6 seconds and the reverse wingover is just impossible.

We tried an 11/5 Zinger on the .292 venturi with the same NV setting- a nice fat four cycle on the ground. It ran away something terrible turning in 4.5 sec. laps. Good line tension though. I didn't even notice the lines in square corners but getting square bottoms at 5 ft. was a little dicey.

The larger props made very little difference in the control feel. Spillman had a handle with a 3.5 in. spacing, instead of 4 in. That settled the corners down a lot. I moved the pushrod up a notch on the horn with the 12 in. prop because the larger prop would pull better through harder maneuvers and wanted to get a little more up elevator. I haven't played around with APC props because they are a lot heavier than wood and probably would need a Rabe rudder to control gyro precession on this ship.

Ken Armish and I did a lot of experimenting with fuels(~100 flights). The stock 20% oil(50/50) simply did not work. The motors tended to run away and run erratic. Going to 25% oil helped. 28% or so was even better(8 oz. per gallon of added castor oil). Muffler pressure was also a godsend. three engines in 5 different planes, they all ran well with muffler pressure and erratic without using venturis from .262 to .292in.

This all sounds like the experiments are converging on a setup much like the SSW FP 35 and 40 with the ports stuffed.

>This all sounds like the experiments are converging on a
>setup much like the SSW FP 35 and 40 with the ports stuffed.
>
>
>
>Curt

Except I don't think Leonard messes around with the squish band or the head design ....

That's true.

I havent seen what the head looks like on the LA series--I have played with enlarging the "bowl" in FP and Tower heads a bit and thereby narrowing the squish band. Having more head volume resuces the compression ratio but leaving an angled squish are down around 016" provides good event timing and ignition.

I have been pitting a cardinal P with LA46 stock. It really does run fine without mods.

FWIW

Ferousious, I'm a beginnger at working on engines but have been a guided missle tech in the Navy and repair printing press fo a living(33 years) so I concider myself above average in mechanical ability. Could you please explain what angling the ports and opening the head means as I have An OS25 LA, 40LA, 46LA. The 25 is next to be used in a Barnstormer. It broke in fine but I have not run in the air.
Any help or education would be taken seriously from an experience dude such as yorself. Dont knock yourself out if it's too lengthly to explaine, but a brief reply would be helpful.
Thatnks, Jerry
PS: I got plenty big hammers to use!

It's easy to forget that everybody hasn't read every article in the modelling press for the last 20 years.

Angling the ports: the bypass ports in the LA's seem to be cut pretty much square to the cylinder. Using a Dremel carbide straight bit and eyeballing a 15 degree angle, I ground the top, inside edges of the ports a bit to try and put a 15 degree angle on them. This is supposed to reduce the height difference between the exhaust port and the bypasses and make the transition from 4 cycle to 2 cycle less abrupt.

Opening up the head: The LA engines all seem to have heads with a fairly wide, nearly flat squish band and a circular-shaped combustion chamber about 1/3 the area of the head. I used the 3/16 Dremel to make a circular cut right on the lip of the combustion chamer, all the way around. This cuts away the corner and reduces the compression ratio. Lower compression retards the ignition timing so the motor can make more power on larger props at lower rpm(theoretically). Overall, the motor probably makes less peak power, but it can run decently(or even really good) on alarger prop.

Going from a 10/4 to 11/4, to 12/4 the plane went from 4.9 sec/lap to 5.4sec/lap to 6 sec/lap. At a 5.5 sec lap on the 10/4 it was unflyable, no pull through maneuvers. With the 12/4 at 6 sec/lap it flys well. It needs to go about 5.8 though.

>It's easy to forget that everybody hasn't read every article
>in the modelling press for the last 20 years.
>
>Angling the ports: the bypass ports in the LA's seem to be
>cut pretty much square to the cylinder. Using a Dremel
>carbide straight bit and eyeballing a 15 degree angle, I
>ground the top, inside edges of the ports a bit to try and
>put a 15 degree angle on them. This is supposed to reduce
>the height difference between the exhaust port and the
>bypasses and make the transition from 4 cycle to 2 cycle
>less abrupt.

"Playing" with the timing will make a difference of how the engine runs, no doubt. Although (to a point) it can increase 4-cycle power, it will reduce 2-cycle power so you get less increase at the break of a 4-2 run. I am hoping that this is what you wanted. Othewise angling the ports doesn't seem to make any difference over simply cutting them square.

>Opening up the head: The LA engines all seem to have heads
>with a fairly wide, nearly flat squish band and a
>circular-shaped combustion chamber about 1/3 the area of the
>head. I used the 3/16 Dremel to make a circular cut right
>on the lip of the combustion chamer, all the way around.
>This cuts away the corner and reduces the compression ratio.
> Lower compression retards the ignition timing so the motor
>can make more power on larger props at lower
>rpm(theoretically). Overall, the motor probably makes less
>peak power, but it can run decently(or even really good) on
>alarger prop.

Have you really tested this? Do you know how much you are changing things and what these changes are? I have tested a number of heads on OS and Fox engines, and have never come up with a chamber shape that I felt was better than what comes on the stock OS engine. Even on the Fox engines which have a head shape that is totally different, we tested a number of heads that Lew Woolard made on his lathe, and there was a tremendous difference between one and another. Some worked better. Some worked worse. And a lot of times you don't know until you actually test.

I would hesitate to grind with a Dremel tool, and most especially to grind without knowing what change this was making. Even when setting the head height with a stock head, three or four thousandths can make a lot of difference.

One thing you will find out, however, is that this, too, lessens the difference between the 4-cycle and 2-cycle power. Sometimes you want a little more "kick". Sometimes you want a little less "kick". Unless you know that the squish band is too wide or that the shape is way wrong, it is much easier to accomplish your goal with a simple adjustment of (thin) head shims which you can take out again when you go too far. If you grind too much out of the head, the only recourse is to get a new one.

>Going from a 10/4 to 11/4, to 12/4 the plane went from 4.9
>sec/lap to 5.4sec/lap to 6 sec/lap. At a 5.5 sec lap on the
>10/4 it was unflyable, no pull through maneuvers. With the
>12/4 at 6 sec/lap it flys well. It needs to go about 5.8
>though.

The larger diameter will give you more thrust. However, it seems to me that you are way too slow. What are your line lengths?

I have to do something to keep my interest up. Just flying patterns is way too boring. I opened up the head leaving just a 1/16 in. squish band and went back to a .282 venturi(OS NVA).

I'm currently using 60ft. lines. The sweet spot as far as line tension vs. airspeed seems to be around 5.4-5.8 sec/lap. 5.8-6 is what the 12/4 props seem to give. I was pretty amazed that if flew quite well at that speed, with just a little coaxing to get through the wingover. the slow speed certainly makes it a lot easier to fly the plane through maneuvers.

I did try an APC 12.25/3.75 meat slicer. The motor could turn that faster, giving a 5.5 sec lap which feels pretty good. Didn't seem to give any excessive gyro effects. It launches at about 8200 and peaks at 9200 with the low compression head, so it can't really runaway even if it wants to.

I may have to go back to a bladder though. At these speeds the plane outflies the tank in the overhead eights. Either open them up to 60 degrees, or the motor loses fuel head and quits. The plane could do 15 ft. overhead eights if the motor would keep running.

Len, you gotta remember that us Combat fliers like to grind on engines! Phil's done a lot in this line if you read his earlier columns in Flying Models.

Dremel tools aren't bad, there's worse - you should've seen Doc Passen's head modifications for the Mark IV Combat Special - he used an X-acto knife to convert the heads into trumpet heads.

The angling of the ports is something I've heard about for Combat engines - supposedly helps smooth out airflow. Remember we're going for maximum power, not Stunt runs, most of the time. Sounds like Phil's going the high RPM/low pitch route.

Ferocious Phil - Your post on cutting the ports very closely mirrors the results I typically find. One suggestion though, after cutting the ports, and deburring with great care, you'll still miss a burr now and then. (At least I do.) So when you fit the piston into a newly cut liner, put it in upside down, skirt first, for the first try. That way, if you hit a burr, you've put a scratch on the bottom of the skirt, instead of through the compression band at the top of the piston. Won't make a bit of difference as to how it runs, and you can now find and remove the burr. Has saved my neck a time or two. Hope this helps, Tom H.

If you put the piston in upside down, what should I connect the con-rod to?

Good idea Tom. These mods are basically what you and others have been doing for years. My biggest "contribution" has been matching the rpm and venturi size to the prop. Just realizing that the engine only has so much to give its important to back off on the prop size when the engine gives indications that it is overpropped- running away after launch, not smoothly following the needlevalve adjustment, surging too much when it goes from 4 to 2 cycle, and just sounding like it is running hard.

I think the idea of lowering the compression to run bigger props came from Dave Gierke's book. I'm sure other folks have tried it. Just saw it mentioned in the English model mag. It sure works well on the LA engines. I prefer grinding on the head to shims cause I find it easier than cutting gaskets and cheaper than buying them. It doesn't seem to be a critical change. The LA's seem to be pretty darn uniform and the same changes seem to work the same way on different engines. Not like the old stunt 35, where one wrong tweak on a bolt could bollix up the whole engine setup.

In my limited experiencee, I have found that lowering the compression by way of added head volume is better than doing it with shims. Shimming alters the squish clearance and seems to affect the run in a generally reduced power way.

keeping the squish where it should be and adding volume seems to add power at the lower RPM ranges that we like.

FWIW

I don't think it is just your limited experience. Part of this experiment was to see if there was any difference between shimming the head and hogging it out. At least on the LA 40, can't say that I see any notable difference. Of course, I'm not looking for 100 rpm differences, just the quickest, easiest way to get the motor running satisfactorily- and some good ideas on what the limits are regarding prop size,venturi size, fuel, etc.

did some more experimenting on another couple of engines.
with the ~.130 wide squish band on an LA 40, .282 venturi, Cardinal profile, muffler pressure, and high oil fuel- ran very well on an 11/4 Zinger, about a 10,800 peak rpm, 9700 launch, steady four cycle, 5.4 lap time.

Switched to a 12/4 Zinger. peak rmp dropped to 10,200, launch rpm 9200. Ran a steady four cycle, 5.8 lap time, but the plane held out on the lines and flew just about the same, although it is easier to aim at the slower speed. The motor does sound like it is running "hard" though. The exhaust note is sharper and it seems to be laboring a bit. We'll try opening up the head some more and see if that helps.

The distance from the glow plug to the top of the piston can be a fairly critical factor that we often don't keep track of. Simply shimming the head changes three things at once, the compression ratio, the squish band height, and the plug height. Carving out the combustion chamber only changes one and is easier for my feeble mind to keep track of. Must plan a good experiment on this one of these days.

>did some more experimenting on another couple of engines.
>with the ~.130 wide squish band on an LA 40, .282 venturi,
>Cardinal profile, muffler pressure, and high oil fuel- ran
>very well on an 11/4 Zinger, about a 10,800 peak rpm, 9700
>launch, steady four cycle, 5.4 lap time.
>
>Switched to a 12/4 Zinger. peak rmp dropped to 10,200,
>launch rpm 9200. Ran a steady four cycle, 5.8 lap time, but
>the plane held out on the lines and flew just about the
>same, although it is easier to aim at the slower speed. The
>motor does sound like it is running "hard" though. The
>exhaust note is sharper and it seems to be laboring a bit.
>We'll try opening up the head some more and see if that
>helps.
>
>The distance from the glow plug to the top of the piston can
>be a fairly critical factor that we often don't keep track
>of. Simply shimming the head changes three things at once,
>the compression ratio, the squish band height, and the plug
>height. Carving out the combustion chamber only changes one
>and is easier for my feeble mind to keep track of. Must
>plan a good experiment on this one of these days.

If you feel that the distance from the glow plug to the top of the piston is a "fairly critical" factor, trying adding an extra glow plug gasket to your plug and see what happens. Also make not of whether nor not the plug is now being recessed into the head. Maybe try this with a 4-cycle plug that normally extends down into the chamber a bit.

Just remember, you are changing the compression a bit, and you are changing the angle at which the fuel hits the plug. You idea of planning a good experiment on head shape or squish band size or height are valid ideas, but you need to have a series of heads in hand that you can switch and run one after the other to really tell what is happening.

And I don't see how, if that engine is laboring on that prop, that opening up the head some more is going to help you. You either need to add nitro (or compression) or cut down the prop.

The experiments you are doing will really change the characteristics of the engine. And grinding a little here and there with a Dremel tool does not give repeatability.

"laboring" is what I call the sound the engine makes when it is trying to turn too big a prop for how it is set up. The ignition timing is too advanced. On higher revving engines it goes from just sounding like it is running hard to that frying egg sound. At lower rpms it never seems to get that far. From what I've seen on the LA40 engines increasing the combustion chamber size by narrowing the squish band lets the engine run more smoothly and sound much more relaxed on the larger props. Raising the head is all that is needed on the 46. Opening up the head works on the 40, haven't tried shimming 'cause the grinding worked pretty well.

Adding a gasket to the plug exposes the threads, which greatly interferes with combustion, so that wouldn't be really a valid experiment. Ideally I'd want to experiment using Nelson plugs to avoid the threads altogether, but that is getting away from the idea of seeing what simple things I could do to get a more tractable run. Anyone want to make me a one piece Nelson plug head for the LA 40?

A basic experiment with head volume, squish band width, and plug height would need something like 15 different heads and at least 5 well-controlled runs on each head. That is more work than I'm looking for too.

More nitro is something I can try. I'd like the LA 40 to run a 5.6-5.7sec. 60 ft. lap using the 12/4. That appears to need 96-9700 rpm at launch, which is 5-600 more than I can get. It will do that on an 11/4. Maybe nitro can get the rpm on a 12/4 without overloading the motor and wearing it out prematurely. If not, then I've got to scale the plane down a bit.

Hi Guy,
I have only a few questions concerning your application.
(1) What is the weight of your airframe?
(2) What length lines are you using?
(3) Weather conditions?
(4) Did you fly the engine on the airframe in the stock configuration?

From my experiences with this engine, I have no complaints. I have flown a box stock OS40LA on two different airframes.
(1) Tom Morris Cavalier at 39 ounces total weight, Powermaster 5/29
fuel, 12/5 prop, 60 foot lines, 4.5 ounce uniflow tank,
tounge muffler on pressure.
Performance: Full pattern + ten laps good line tension all around the pattern. Excellent 4-2-4 break. After the flight the engine in warm to the touch

(2) A scratch built profile Zero with a lincoln log wing total weight
43.2 ounces, Power master 5/29 fuel, 11/6 wide prop, 58 foot
lines, 4.5 ounce uniflow tank, tounge muffler on pressure. This
airframe is slightly larger than the cavalier.
Performance: Full pattern + extra laps, take off at 9000 rpm, 4-2-4
break is less pronounced due to faster 4 stroke speed. After the flight the engine is very warm.

This engine has been tested with Powermaster 5/29 and Sig 5/25 fuels. The only differences in the needle valve setting.

My OS40LA has performed straight out of the box.

OH YEA; I did omit one detail, I did change back to the front needle valve position. I did have trouble with the rear needle valve position.

I hope this information helps you out of your problem.

Most of this testing has been done on the my Giles- 45 oz. 750 squares. Also some on a 46 ounce Cardinal, both on 60 ft. lines.

We just couldn't get a reliable run on the stock motor with bugger size props. Dead stock an LA40 wants to run 13-14000 rpm on a 9 or 10 inch prop. We just couldn't get consistent results on 11 and 12 inch props running rich. Sometimes fine, other flights it would run away lean.

I'm glad you are happy with your motor. Those are nice, light planes and should work well with theLA40. Sounds like the 11/6 is running hard though. It shouldn't get noticeably hot.

I milled all the excess metal from the outside of the case. It was 1/4 thick where they left out the boost port. 240g

Giffy

I've thought about doing that, but haven't taken the time. It looks like there is a lot of meat under the exhaust too. Maybe we could talk RO or Jett into programming the milling. I'll bet they could take the whole engine down to 210 grams or so.

Taking metal off might be a reason that it seems to be running hotter than expected. With less metal mass the motor can heat up quicker and the cylinder may be more flexible resulting in more heat. Running 12 inch props I've been thinking of trying to either fit a heli head for better cooling or increasing the gap between the fins on the cyl. and head for better cooling. I'd like to see .1 or so between the fins so the air gets in there better.

Mike, your engine should be warm, or even "very warm" immediately after the flight. There is a lot happening up in that cylinder, which is why it needs cooling fins. Your car engine gets hot, too, even though it is water cooled.

You should also be able to run on longer lines (65 or 66 feet) with that engine with lap speeds of around 5.3 seconds. Give it a try. It is a cheap experiment do do. Just choose a prop that gives you the power and lap speed that you need. You can always go back if it doesn't work for you.