Author Topic: OS 40 LA run quality question, venturi size? (Read 1323 times)

Dane Martin · « **on:** July 03, 2020, 07:45:59 PM »

So on my TEOSAWKI, I've always had an OS 40 LA. It's been a bit of a test bed, so it's had a couple. It is stock internals, I know because I've actually checked. I just installed a new backplate as well (plastic OS, actually new). The venturi is a Delrin 0.281 (I actually checked that as well) and a 4mm ST NVA. That's how I got it. I don't even know where it came from.

I ran my normal 11x4.5 prop. I ran 3.25 oz of 10%, and after the pattern, I had over 30 laps before it died. When richened up to keep that down, it had no power.

I then switched to an APC 11x5 prop. 3.25 oz, same fuel. I richened it about 1/4 turn. It consistently ran 10-12 laps after the pattern before landing.

The engine is a little warm, more so than the other OS 40 LA"s we ran today. Also, mine uses much less fuel.

Question is, should I enlarge the venturi some, to get the rpm up and fuel usage up enough to run the lower pitch prop?

Steve Fitton · « **Reply #1 on:** July 03, 2020, 07:56:20 PM »

do you have a tach to see how many RPM you are turning?

Dane Martin · « **Reply #2 on:** July 03, 2020, 07:57:59 PM »

Quote from: Steve Fitton on July 03, 2020, 07:56:20 PM

do you have a tach to see how many RPM you are turning?

Somewhere. I can dig one out

Tim Wescott · « **Reply #3 on:** July 04, 2020, 09:32:19 AM »

I simply would not try to adjust an engine for fuel usage. If it flies correctly and uses 2 oz of fuel to finish the pattern then fill it up with 2 oz of fuel and go fly.

Now, if there is some other problem that you are seeing with the big venturi that you are not telling us, then tell us and we will help you correct that.

Having said all of that, it seems like a really big venturi for that airplane and motor. I would at least try the venturi that Motorman is suggesting.

Dane Martin · « **Reply #4 on:** July 04, 2020, 10:01:55 AM »

Quote from: Tim Wescott on July 04, 2020, 09:32:19 AM

I simply would not try to adjust an engine for fuel usage. If it flies correctly and uses 2 oz of fuel to finish the pattern then fill it up with 2 oz of fuel and go fly.

Now, if there is some other problem that you are seeing with the big venturi that you are not telling us, then tell us and we will help you correct that.

Having said all of that, it seems like a really big venturi for that airplane and motor. I would at least try the venturi that Motorman is suggesting.

It's running too lean to fly this plane, is the problem. It runs hotter than the LA 40 that's pulling a larger heavier plane. The smaller venturi would give me overall less power, causing me to run it harder to get the same power, correct?

Dane Martin · « **Reply #5 on:** July 04, 2020, 10:51:31 AM »

Quote from: Motorman on July 03, 2020, 10:00:42 PM

I had the same problem. You need a smaller venturi so you can lean it down. It will use less fuel and have more power with the 11x4.5. I use that prop and a .265" - .272" on my LA46.

Motorman

What size is your spray bar

Dane Martin · « **Reply #6 on:** July 04, 2020, 12:26:30 PM »

Let me see if I can explain this better

With the 11x4.5, and this engine running "properly" it will not generate enough RPM to suit this plane flying the pattern. Nothing has changed but the engine. Same plane, same lines, same tank, even the same actual prop I've used. I've flown this plane with an OS 40 LA in every contest it has been to. This engine is slower.
If I lean it out to get the RPM up to meet what this prop needs, it's too lean and gets boiling hot.

I set the engine to run properly, and switched to the 11x5. It was a lot better. More pitch with the lessened RPM helped get the plane speed back up. Patterns looked much gooder.

The question is, to get the RPM up to run the 11x4.5, wouldn't that mean larger venturi?

Dane Martin · « **Reply #7 on:** July 04, 2020, 12:33:27 PM »

Also, the results I found were very consistent. We tried it all day. Well until the heat got to us... the engine will run consistently to its setting all the way to a slight lean on the last lap and a clean shut off. When I checked the tank after every flight, there was no fuel left. So I have a low RPM issue, not a fuel draw issue.

Tim Wescott · « **Reply #8 on:** July 04, 2020, 01:24:08 PM »

I'd have to go measure my venturis to compare with mine, but the symptoms are of a too-small venturi.

This number is close (but doesn't work if the needle is really big compared to the venturi): Venturi area = diameter * diameter * 0.785 - diameter * needle_diameter

I'd either do the math on your problem engine and one of you happy 40LA engines, or I'd swap the venturi from a happy 40LA onto your problem child and go fly.

Dane Martin · « **Reply #9 on:** July 04, 2020, 05:11:40 PM »

On this engine I get 0.01788
0.281 venturi, 0.157 nva

On another engine that I enjoy the run quality, I believe I get 0.0214
0.256 venturi, 0.118 nva

Tim Wescott · « **Reply #10 on:** July 05, 2020, 11:47:17 AM »

That's a significant difference.

If you knew someone with the machining chops to make their own engines, you could have them open that venturi up, or make another one.

diameter = needle_diameter / 1.571 + sqrt((needle_diameter / 1.571)^2 + venturi_area / 0.785)

(Note that 0.785 is pi/4, and 1.57 is pi/2 -- so if you have a calculator you can stick in the "correct" numbers)

((And I really ought to post the version that involves trig, but it's out in the garage, and I'm sitting here keeping company with a cat)).

To match your 0.0214 square inch venturi, I get a diameter of 0.293 inches. Probably 0.290 or above will get you to your happy zone, or if you go over by a bit you can pile layers of nylon mesh over the thing to stop it down a bit.

Dane Martin · « **Reply #11 on:** July 05, 2020, 12:04:08 PM »

As it turns out, just messing around last night, I made another venturi. I ended up about 0.296, because of the bit, not the math. I brought both with me, just in case the 0.296 was too large. I started with the large one. As soon as I took off, I knew it was at least better. With 4oz of fuel, I got about 7:30. I wanna say I finished the pattern in 4:30 but I was using my phone, and it was a little difficult to pay attention to. Lap times were 5.00, maybe less? Like I said the phone was hard to hold and check.

So with the matching choke area, a 0.293 might be actually the correct way to go. But this is close enough to leave and try for a few days.

Tim Wescott · « **Reply #12 on:** July 05, 2020, 01:03:24 PM »

I actually prefer to use at least one layer of nylon net, to screen out large debris (this is the fine-screen net, with a pitch of around 1/32 or 3/64 -- not the hexagonal nylon net with the 1/8" holes). If I've got more than about five layers I figure it's time to make a smaller venturi.

Usually the difference between "right" and "almost right" is one layer, so it gives a nice way to tune the venturi "size" to the plane.

Tim Wescott · « **Reply #13 on:** July 05, 2020, 03:50:14 PM »

The formula that I gave Dane is off by a tiny amount -- it's really insignificant, unless the spraybar is really big in the venturi. But, it's not exact (horrors!).

So here's the math, in all it's accurate glory.

I'm going to define d_v as the venturi diameter, and d_s as the spraybar diameter, because otherwise things get too wordy. I'm using the "Fortran power" notation, x squared = x ** 2, sqrt is square root, and 'asin' is arc sine.

area = d_v ** 2 * (pi / 4 - asin(d_s / d_v) / 2) - d_s / 2 * sqrt(d_v ** 2 - d_s ** 2)

If you now ask for the reverse calculation -- i.e., getting the venturi diameter given a desired area and spraybar diameter, the answer is that it can't be solved algebraically. You'd need to get that number by plugging and chugging guesses into the above formula (or letting a computer do it for you). If you do it by hand, start with the approximate one I gave Dane, and plug and chug from there.

Dane Martin · « **Reply #14 on:** July 05, 2020, 04:19:00 PM »

Quote from: Tim Wescott on July 05, 2020, 01:03:24 PM

I actually prefer to use at least one layer of nylon net, to screen out large debris (this is the fine-screen net, with a pitch of around 1/32 or 3/64 -- not the hexagonal nylon net with the 1/8" holes). If I've got more than about five layers I figure it's time to make a smaller venturi.

Usually the difference between "right" and "almost right" is one layer, so it gives a nice way to tune the venturi "size" to the plane.

I have some pantyhose.... I mean I specifically bought some for this, and no other purpose, is what I meant to say....

I'll experiment a little. I was pretty excited to feel such a difference though.

Tim Wescott · « **Reply #15 on:** July 05, 2020, 04:43:05 PM »

If it seems too eager to "run away" then you want to run leaner.

If it "thins out" at the tops of the loops, in the overheads, and at 45 degrees, then you want to run it richer.

If it's too powerful when it's in the sweet spot for rich vs. lean, make the venturi smaller (or pile on nylon).

If it's not powerful enough when it's in the sweet spot for rich vs. lean, make the venturi larger (or remove nylon).

If you just can't get it to be powerful enough and rich enough, then you need a bigger engine (or you need to crock-pot the thing -- I did have a Tower 40 that came to me all varnished up).

Author Topic: OS 40 LA run quality question, venturi size? (Read 1323 times)

Dane Martin

OS 40 LA run quality question, venturi size?

Steve Fitton

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?

Dane Martin

Re: OS 40 LA run quality question, venturi size?

Tim Wescott

Re: OS 40 LA run quality question, venturi size?