12x4 prop on OS Max 40 LA

Has anyone tried a 12x4 prop on an OS Max 40LA engine?

www.osengines.com says an OS 40LA will turn a 11x5 or 11x6. If it will turn an 11x6 it should turn a 12x4 maybe even a 13x4.

I am running an 11x5 prop slow RPM turning a 5.5 second lap. I think I should able to run a 12x4 prop, speed the engine up, and still get a 5.5 second lap.

What I am trying to do is go as slow as I can and still have power to fly through the manovers. Hobby shop is open today I need some props to experement with.

Currently I'm running a rev-up 12x5W on an LA-46, and very happy with it. It wouldn't hurt to try a 12x4 on the 40. Maybe try the easily obtainable APC 12.25x3.75???? Then you could trim the diameter down to something its happy with if 12.25 is too much.

The 11-4 or even 11-3 APC would be a better match for a .40LA, in most cases. Sometimes an 11-4 APC will be too fast, but a stock, crumby, cheezy, inefficient 11-4 Zinger will work just right. Several guys in our area have reported this. If you put on too much prop, it will sag in the overheads. Don't! You need to launch about 1-1.5k rpm below peak to avoid overhead sagging and loose lines. Steve

I talked to the guys at Zinger and they told me they deliberately send the cheap, crummy, inefficient props to the Pacific Northwest.

Personally, I've had good success with the smaller Zingers and would second your recommendation for the LA .40. The 11-4 APC, Zinger, and RevUps all run very well.

>I talked to the guys at Zinger and they told me they
>deliberately send the cheap, crummy, inefficient props to
>the Pacific Northwest.

Ron, You're probably right. I think we get the cheezy ones only by special order. Steve

Hello Gary, and Happy New Year,
I have run 12x6 B-Y&O props on my O S LA 40 at Dee Rice's coachin'.. He showed me how efficent these engines are and that they will work without over heatin'.. I added an extra head shim (Gasket) to mine and was runnin' Powermaster Fuel (5% 22% oil 11&11) and it ran fine.. Try it and see how you loke it..Try on ground first and don't set the needle too lean..

> I have run 12x6 B-Y&O props on my O S LA 40 at Dee Rice's
>coachin'.. He showed me how efficent these engines are and
>that they will work without over heatin'.. I added an extra
>head shim (Gasket) to mine and was runnin' Powermaster Fuel
>(5% 22% oil 11&11) and it ran fine.. Try it and see how you
>loke it..Try on ground first and don't set the needle too

Gil;

With those numbers, you must be running a good sized venturi. Can you tell me what the diameter of the opening is?

>Has anyone tried a 12x4 prop on an OS Max 40LA engine?
>
>www.osengines.com says an OS 40LA will turn a 11x5 or 11x6.
>If it will turn an 11x6 it should turn a 12x4 maybe even a
>13x4.
>
>I am running an 11x5 prop slow RPM turning a 5.5 second lap.
> I think I should able to run a 12x4 prop, speed the engine
>up, and still get a 5.5 second lap.
>

That's likely to be true, but you never know. I would recommend the 11.5-4 APC. Very light load, so it should be easy to turn fast enough.

Bob,
If memory serves the venturi is a .282 and the n v a is Super Tigre
Works well.....Gil;)

i use a 11x4 prop on mine it works just great running the 4-2-4 break

steve

the superglue fool

I have recently tried an APC 10.5 X 4.5 on my LA .40 with excellent results on a Brodak FW 190. The release rpms are aprox. 10,400 and once up and unloaded the engine just purrs along quite nicely. Fuel is 7% nitro, 22% castor oil and 3% Klotz.

I second Brett's reccomendation for the APC 11.5-4. I run one on my unmodified FP40 with excellent results.

I've been running the following setup on a couple LA 40's and it has proved to be reliable and repeatable.
.282 venturi with OS needle valve.
Brodak chip muffler for LA 46 etc.
the head is modified by carving out the squish band nearly to the edge using a 3/16 in. round Dremel bit.
at least 25% oil, half and half synthetic and castor. We add castor to Sig 10% fuel.
Muffler pressure. the venturi is too big to run steadily on straight suction.
12/4 Zinger carved up. I used the 11 in. blade as a template and narrowed the 12 in. prop. Thinned the outer third a lot. Stock blade is 1.12 wide .223 at the widest part and .105 at the tip. Modified is .95 wide, .175 at mid point and .075 at the tip.

This is on a BIG, DRAGGY plane.

A similar setup with a .13 squish band works well on 11 in. props on something smaller like a Cardinal.

The stock muffler seems to want to run around 11,000 rpm in the air. It can be erratic when running larger props. The motor wants to run at 9800 and the muffler at 10,800

>I've been running the following setup on a couple LA 40's
>and it has proved to be reliable and repeatable.
>.282 venturi with OS needle valve.
>Brodak chip muffler for LA 46 etc.
>the head is modified by carving out the squish band nearly
>to the edge using a 3/16 in. round Dremel bit.

Phil;

Do you have a print on this head mod?

you gotta be kidding!! a drawing for a hand-carved head? Dremel makes two different ball bits, one is about 1/8 in. the larger one is about 3/16. Use that. I just hold the head in one hand and work the grinder around the lip of the squish band until it is almost all gone. The plug depth is only slightly deeper, so you really can't go too deep in the head. This seems to be a very forgiving modification. As I posted before, the stock squish band is about .15 wide and works well with the stock muffler and a 9 in. prop. Grinding out the head to a .13 squish band runs better on 10-11 in. props. I had to take all the squish out and go to a chip muffler to get consistent runs with a 12 in prop. As Brett pointed out, the LA mufflers seem to have some tuning effect. On a 12 in. prop the motor sometimes trys to run away with the stock muffler, but it can't due to the size of the prop, so it just runs erratically.

The LA 40 makes a lot more power on a 10 in. prop, but the plane flys a lot better on the 12 inch at a similar airspeed.

>you gotta be kidding!!

No Phil, I was quite serious.

When making mods, I've found that things that sound simple can be misinterpreted. As an example, when you say "until the squish band is almost gone" - almost can mean different things to different people - as I have found out the hard way. Because of that, when I worked as an engineer, I never sent anything to the machine shop for modification without a drawing.

I suppose I am just funny that way....

My thanks for the description.

you're right Bob, a picture is worth a thousand words. The grinding is on the inner lip of the squish band.

I wonder what would happen if you ground off the "outer" edge of the squish band. That would make a weird head configuration. Maybe I'll waste a $10 head and find out!

Sorry Bob, didn't mean to be real flip. A picture is definitely worth a thousand words. Been there, done that trying to describe something. But, the picture wouldn't upload.

Maybe I can do it in text:

____________________________plug________________________________
| / / \ \ |
| / / \ \ |
| / / \ \ |
| | | | | |
| | | | | |
----- ----

The inner "curve" is the stock head. The outer curve is the mod. It can be taken out until the squish band is nearly non existent.

>Sorry Bob, didn't mean to be real flip. A picture is
>definitely worth a thousand words. Been there, done that

Phil;

You may be "Ferocious", but never a "flip". I was just trying to get a little more precise detail. I actually have a couple of these LA 40s that I got from RC flyers for almost nothing that would be worth experimenting with

I must be "visually challenged" today, but could not make out what you were trying to do in text. If you want, file attach the picture to me in an email, and I will see if I can get it to upload.

The problem with drawing pictures in text is that the software takes all the spaces out. Ask me how I know.

I'm "spaced out". Nothing works right today. I guess a bad picture is worth 2000 words maybe? Anyway, open up the ##### bowl of the combustion chamber- you can take it all the way to the cylinder if you want. A couple of used RC engines are perfect for experimenting. At most it might cost you another $10 for a new head. When you are done I expect you will have an engine that will run in a fast 4 cycle and have a soft break into a 2 cycle and won't have the schneurle "runaway" syndrome. Then put it in a plane the correct size and weight for its horsepower- something in the range of 45-50 oz. 550-600 squares. Or build a Giles- 60 in. 750 squares, and 45 oz. I know that flys well with the modified 40.

If I understand what is going on right, lowering the compression ratio lowers the rpm for peak torque. Since an LA 40 has a hard time turning a 12/4 prop much over 10K and the stock torque peak is around 11K, a lower compression lets the motor run stably on a big prop. It can't jump from a rich 4 cycle below the peak torque to a lean 2 cycle above the peak torque rpm.

"If I understand what is going on right, lowering the compression ratio lowers the rpm for peak torque. Since an LA 40 has a hard time turning a 12/4 prop much over 10K and the stock torque peak is around 11K, a lower compression lets the motor run stably on a big prop. It can't jump from a rich 4 cycle below the peak torque to a lean 2 cycle above the peak torque rpm."

Phil C

I'm going to keep on doing this until you guys get it right:

Peak torque *always* hits at the point in the rev range where the motor breathes the best. If you are not altering the flow of air through what must be viewed as--and in fact is--an air pump, you have not changed where peak torque hits.

Lowering compression lowers the amount of torque, even though a great many of you--and this certainly includes Phil--have known this for many years.

I was mostly going by several of Peter Chinn's famous engine tests. Several times he tested engines with different heads from the manufacturer and it appeared that the higher compression head moved the torque peak up. Similarly opening the venturi moved the torque and hp curves up. Changing the compression would tend to reduce torque, but it also retards the ignition timing so the motor can run better on a large prop at lower rpm.

Where is Mr. Burn when we need him. He did(does) this stuff for a living.