Even More on O.S. 20FPs.

While it seems as if a high percentage of you are getting it, or at least there have been no, "Your stoopid," (sic) sorts of remarks made, I did do some testing the other day and thought the results to be of interest.

Baseline: O.S. 20FP, the stronger of two swap meet-sourced engines I am currently fiddling with. It was run for about 20 minutes just to be sure all the parts got to know one another again following cleaning and reassembly.

All runs with APC 9-4 prop. Other variables: SIG 10/25 all-castor fuel, Thunderbolt standard long plug, stock O.S. muffler, ST needle valve assembly. As each variable is changed, it will be noted. The first run is open exhaust.

Open exhaust: 13,170 to 13,260 rpm.
RSM tongue muffler, body modified to fit O.S. port, all outlet holes drilled to .078: 13,140 to 13,230.
Prototype tube muffler from sq8s: 12,360 to 12,480.
Stock muffler with cone-shaped insert for reduced noise: 12,000 to 12,030.
Stock muffler, no insert: 12,360 to 12,390.
J&J Sales NVA filed to .140 cross section: 13,110 to 13,140.
O.S. #8 plug: 13,140 to 13,170.
Bruline "fine" air filter: 13,050 to 13,080.
Bruline "coarse" air filter: 13,080 to 13,110.
K&B green foam air filter: 12,990 to 13,020.
No filter, 10/11/11/68 (castor/Klotz) fuel: 13,020 to 13,050.
Fox Kool-Aid, 20/25/3/52 fuel, the 3% is Klotz: 13,230 to 13,350.

Yeah, yeah, we don't run Stunt motors peaked. Still, there is a pattern here and the differences from one variable to the next will indeed apply at reduced revs. Especially as the revs won't go down that much when dialed in and flying.

Comments: Open exhaust is next to worthless. It's noisy and the revs wander up and down.
Tongue muffler, ditto. Plus the one I was using didn't seem to quiet things enough to bother with.
The tube-type muffler from sq8s, while nicely done, 24 grams light and giving a pleasant exhaust note, did not make for a happy combination. Any time you've got a 20FP and the revs fluctuate over a 120-rpm range, you will be frustrated at the circle.
That insert doo-hickey does quiet things down further.
I was prepared to find more power with .140 cross section spray bar, just not 750 revs worth.
Also surprised to see the old K&B air filters to be so restrictive.
This motor did not take well to half-and-half castor/Klotz fuel, losing about 90 revs. And judging by position of the needle, mileage went down as well.
The 20% fuel sure woke it up, but rare will be the times when this much power is required from a 20FP.

While these and other variables can be used to fiddle with power levels, do please note that it is only when breathing into a stock O.S. muffler that the 20FP straightens up and flies right. I'm not saying one can't run open exhaust or with a tube muffler. You can, but it will take multiple test flights to hit the needle and even at that there likely will be surprises in your near future.

This as opposed to a few flights to determine ideal ground setting with a tach and from then on, whether practicing or contesting, plopping the sucker down, hitting that number and flying with confidence.

Thanks for the meticulous records, Dan. I'll be breaking-in a spankin' new FP.20 this weekend. It's nice to have a few more points of reference.
Derek Moran

Hi Dan,
I too appreciate the information.
With this info from you and Brett Buck's information
on the .20 FP
makes for a killer one, two punch.

Dan,

O.K. Thanks good info. Now for your new assignment, if you choose to accept it? Ha, I know you won't!

Repeat these test on the following engines but with appropirate Bolly and Eather props in all relevant diameters and pitches.

Relevant = useable for Precision Aerobatic purposes.

PA .61 RE, RO Jett .61 RE, BSE and Cast, PA .65 RE and SE, RO Jett .65 RE and SE.

Saito .56 and 72 4 strokers, OS .40, .52 and .70 4 strokers in all relevant configurations.

There that should keep you out of "trouble" (Ha!) for a couple of hours anyway!

OH yes, One more thing! Could you please explain the purple balsa bits to be distributed by a noted Bay Area pilot at the NW Regionals furthur?

My E-Mail to same on the same subject seems to have gotten lost in cyber space somewhere.....

Thanks in advance should you choose to accept such a daunting assignment! (Ha! Fat Chance)

>While it seems as if a high percentage of you are getting
>it, or at least there have been no, "Your stoopid," (sic)
>sorts of remarks made, I did do some testing the other day
>and thought the results to be of interest.
>
>Baseline: O.S. 20FP, (snip)

>All runs with APC 9-4 prop. Other variables: SIG 10/25
>all-castor fuel, Thunderbolt standard long plug, stock O.S.
>muffler, ST needle valve assembly. As each variable is
>changed, it will be noted. The first run is open exhaust.
>
Just making a few comments here. Everything is relative to the prop and the fuel. Other prop and fuel combinations may give decidedly different results, although you are using an optimized prop/fuel combo that you know works in the air and then bench testing with variables. But the venturi on these engines is *huge*. So your choking it down with the ST needle also makes a difference. But if you don't get proper fuel draw in the air, then it is all for nothing. And, as you state later on, it is not peak rpm that we are concerned about, although this does give reference points as to what these various changes do.

>Open exhaust: 13,170 to 13,260 rpm.
>RSM tongue muffler, body modified to fit O.S. port, all
>outlet holes drilled to .078: 13,140 to 13,230.
>Prototype tube muffler from sq8s: 12,360 to 12,480.
>Stock muffler with cone-shaped insert for reduced noise:
>12,000 to 12,030.
>Stock muffler, no insert: 12,360 to 12,390.

The stock muffler is very restrictive. I do not know what the specs on the "prototype" tube muffler are, but if it is less restrictive, this, combined with the too large venturi can add to the greater variations in power. When your muffler is too restrictive, you need to open the venturi to gain power (although you lose suction which affects the in air response). When you go to a properly sized venturi and a less restrictive muffler (and in this case less heavy as well) you will get the same run characteristics on the bench but with a much better set up (better fuel draw) in the air.

>J&J Sales NVA filed to .140 cross section: 13,110 to 13,140.

OK, now you are getting close to the stock set up with the OS needle. Now you have a much larger venturi, but still with the restricted muffler. You would probably notice an even greater gain with a less restrictive muffler to hold it back.

>O.S. #8 plug: 13,140 to 13,170.
>Bruline "fine" air filter: 13,050 to 13,080.
>Bruline "coarse" air filter: 13,080 to 13,110.
>K&B green foam air filter: 12,990 to 13,020.

Adding filters is like closing off the venturi. If you were using a larger prop you would notice an even greater difference in rpm with the added restriction of the filter. And if you were using a smaller venturi to start with, you would notice an even greater difference in using the filter.

>No filter, 10/11/11/68 (castor/Klotz) fuel: 13,020 to
>13,050.
>Fox Kool-Aid, 20/25/3/52 fuel, the 3% is Klotz: 13,230 to
>13,350.
>
>Yeah, yeah, we don't run Stunt motors peaked. Still, there
>is a pattern here and the differences from one variable to
>the next will indeed apply at reduced revs. Especially as
>the revs won't go down that much when dialed in and flying.

The one thing that bench testing cannot tell us is the "friendliness" of the engine. How does it react in maneuvers? Does it pick up rpm when pointed up. Does it "burst" out of the corners? Any of the above changes will change how the engine reacts to increased load and fuel draw requirements. (For instance, the bench test will not tell us at all how the engine reacts when you point the nose up unless you actually point the nose up on the bench. And then you can't peak it out on the bench or it will go overlean.)

>Comments: Open exhaust is next to worthless. It's noisy and
>the revs wander up and down.
>Tongue muffler, ditto. Plus the one I was using didn't seem
>to quiet things enough to bother with.

You used a thin, tinny muffler with big holes. This may be why.

>The tube-type muffler from sq8s, while nicely done, 24 grams
>light and giving a pleasant exhaust note, did not make for a
>happy combination. Any time you've got a 20FP and the revs
>fluctuate over a 120-rpm range, you will be frustrated at
>the circle.

Why did it fluctuate, though? Any engine I run on my test stand will fluctuate greatly unless I just touch the top of the plastic tank I am using. The fuel sloshes so much in that crummy tank that it bubbles through the tube. But if I just touch the top of the tank it will stop. Sometimes we get resonances that at certain frequencies will make a difference. Sometimes our planes are that way on the ground. I cannot see why that muffler would cause a greater fluctuation over some other muffler, especially when your open exhaust test showed only a 90 rpm fluctuation. Still, it showed only an 800 rpm drop while the stock muffler showed over a 1200 rpm drop. I really find this fascinating because when I was doing the tube muffler tests on the Magnum 53 while it was still under development, I could run that engine at launch rpm with the 13 inch 3-blade prop and with open exhaust and then hold the tube muffler to it and it wouldn't drop one bit. Remember, however, it wasn't running wide open. It was running at launch rpm. And I had also done some testing to determine the needed size of chamber and stinger outlet for this engine.

>That insert doo-hickey does quiet things down further.
>I was prepared to find more power with .140 cross section
>spray bar, just not 750 revs worth.
>Also surprised to see the old K&B air filters to be so
>restrictive.

Basically, you are making a carberator out of the venturi. A carberator chokes it off. You were choking it off. I am surprised that you didn't see a much greater difference than what you did, except for the fact that the venturi is too large to start with. There is a point of diminishing returns where when you open the venturi beyond a certain point you will not gain much more rpm but you will lose suction which affects the engine's performance in the air as a stunt engine.

>This motor did not take well to half-and-half castor/Klotz
>fuel, losing about 90 revs. And judging by position of the
>needle, mileage went down as well.

We have also found that SIG half castor, half Klotz, although excellent fuel, does give less fuel mileage than all castor or the castor-ucon blend that we are presently using. Is it the Klotz, or is it something else that is in it? I would think that their all castor and castor - Klotz blends would be the same except for the oil. I wonder if Mikey would have anything to add to this?

>The 20% fuel sure woke it up, but rare will be the times
>when this much power is required from a 20FP.
>
>While these and other variables can be used to fiddle with
>power levels, do please note that it is only when breathing
>into a stock O.S. muffler that the 20FP straightens up and
>flies right. I'm not saying one can't run open exhaust or
>with a tube muffler. You can, but it will take multiple test
>flights to hit the needle and even at that there likely will
>be surprises in your near future.
>
>This as opposed to a few flights to determine ideal ground
>setting with a tach and from then on, whether practicing or
>contesting, plopping the sucker down, hitting that number
>and flying with confidence.
>
>Dan
>

Dan, here is where I have to disagree with you. You tested one tube muffler and a prototype at that. You did not test it in the air and you did not test with even more than one prop. Now, it may be that you have found an ideal prop/fuel combination to be used with the stock engine and muffler, but I certainly wouldn't make a blanket statement that all tube mufflers (other than the stock OS) are no good and that OS, alone, has the wisdom to find a muffler that works with this engine.

There are also good tongue mufflers that work with this as well. Remember, you arbitrarily drilled holes in a muffler intended for a 40, put it on a 20, and then declared it no good after bench testing it only. When I was working on tongue muffler design for our engines (the 35-53) I found that with the same size holes you were using (.078) each additional hole added about 100 rpm to the engine (the 53). But there is a point of diminishing returns once again. I ended up at 15 as a compromise between power and noise, but with a bigger chamber and heavier metal than on the one you were using. The tongue muffler certainly has a different tone over the tube (or stock) muffler, but I found that on the db metter it met the FAI requirements for noise reduction and allowed the engine to develop its power. Some people (especially on the smaller engines) have blocked off a hole or two as an added adjustment (tap for 4-40 screws) and this works somewhat like going to a smaller venturi.

I think you made a lot of valid observations here, Dan, but there are still a lot more tests that need to be made under many and varying circumstances before a final conclusion of any kind can be made. What happens in the air? What happens under different temperature conditions? What about using different props with these various changes? What works the best under these conditions? Will it work at Muncie or in Idaho or Dallas the same as it works there? Stay tuned for the next installment. Y'all out there flying these things, check in.

Len,

While I appreciate you expanding on some of the variables, most of the things mentioned I found relatively obvious, although they may not be obvious to all. Which is why your efforts are appreciated.

You do, however, seem to have missed a fairly salient point and that is that I have in fact done a fair amount of in-air testing *and* have proven to my satisfaction that, of the mufflers I have tested, they are significantly inferior to the stock O.S. muffler. I don't care if they are lighter, cheaper or plated with gold. They simply do not work.

So part of the reason for this testing was to see if bench-running would uncover undesirable characteristics with mufflers I had on hand. And so it was that I was pleased to see the revs bouncing all over the place--not just at peak rpm, although those are the only numbers published here--with open exhaust and the tongue muffler. It only got worse with the prototype 24-gram tube-type muffler, leading to the fitting conclusion that it wasn't going to work in the air. Have I tested it on a model in flight? Nope. And see no need to do so.

Not when the revs just lock in with the stock muffler. There is in fact something going on here. It may be unique to the 20FP, but I very much doubt that.

Agreed, there are a great many other tests which could be run, the first on my dance card being the MACS muffler, another 1/4-wave muffler design. However, my scope is much more limited than you seem to think might be the case. I know an unmolested 20FP works. I know the APC 9-4 prop works. I know 10/25 fuel works. I know the stock muffler works. I have shown the revs to be expected from various combinations, this leading to a more accurate tune-up for each individual. I have given a big, big hint as to what one should see from any muffler tested, both in raw output and in steady-state running.

From there it is on to bolting a 20FP to a Skyray or Flite Streak, enjoying a superb combination of model and engine, leaving the extensive test programs for PA models and engines.

Finally, while I did not mention it, I depend upon an exceedingly solid test stand. Remember that I was involved in some pretty serious test programs during development of the first VA .049, prototypes of what became the AME .049 and .061, several iterations of an RC Pattern-specific two-stroke .60 and several other engines which never saw the light of day. Touching any part of this test stand, even the fuel tank, will not change indicated results; not even drop-kicking the sucker will change things, although I do admit to not having tested this theory.

One test that I would suggest that you didn't try would be a smaller venturi. The stock venturi is huge on those things, and even though you have choked it off to a great deal, it is still overly large for the engine in stunt. You were surprise by the amount of increase you gained by narrowing up the needle valve. The only reason you didn't get a larger gain is because of that overly large venturi. At some point you get diminishing returns when you go larger.

However, if you went to a .228 venturi with an OS needle or a .248 venturi with the ST needle and then tried one of those less restrictive mufflers you might find that they work. The stock muffler is so restrictive that it is holding the engine back. Granted, some engines need a certain amount of back pressure. But then the answer is not to open up the holes on the tongue muffler or to maybe add a couple of screws to plug a couple of existing holes.

These aftermarket mufflers can be made to work, and once you find the set up, they are plug and play just like the set up that you like so well.

>One test that I would suggest that you didn't try would be a
>smaller venturi. The stock venturi is huge on those things,
>and even though you have choked it off to a great deal, it
>is still overly large for the engine in stunt.

Too big, eh? Based on what?

>>One test that I would suggest that you didn't try would be a
>>smaller venturi. The stock venturi is huge on those things,
>>and even though you have choked it off to a great deal, it
>>is still overly large for the engine in stunt.
>
>Too big, eh? Based on what?
>
> Brett
>

Based on the size of the engine. If it is too big for the 25, it is certainly too big for the 20. The reason it is working for you, as I see it, is because you are using a restrictive muffler. I am not knocking your set up when I am saying this, just stating an opinion based on observable fact.

There is a certain range of choke area that works for stunt motors, and it works whether it is the 20 FP, the 40 FP, or your PA or Ro/Jett 61. Matt's venturi on his PA 61 for instance, with the big prop and 5% nitro and all, is still at the bottom of that range. Your spigot venturi, with your smaller prop and higher nitro will still fall within that range. There is really not much difference between them.

The stock venturi on the 20 FP is about 70% larger than optimum. Now, you can put a highly restrictive muffler on an engine and open up the venturi and get away with it, which is in effect what you are doing. And that is also why a less restrictive muffler seems to work less efficiently for you--the venturi then is too big. But you can use a lighter, less restrictive muffler with a smaller venturi and accomplish the same thing. And you will get better fuel draw as a bonus. George Aldrich used to do timing changes and hog out the head to lower the compression and then open up the venturi to bring back the power. And with the too large venturi and lower fuel draw, you get a quicker switch between 2 cycle and 4 cycle that he so loved. (Does Fox 35 come to mind?) And I know that you aren't running yours anywhere near like a Fox 35--nor would I recommend it--but the principle of a restrictive muffler and a too large venturi still holds.

By the way, Matt is building a Rabe Mustang that he is planning on trying a rather restrictive header muffler on (because of size and weight restrictions.) You know Randy's comment in using this muffler? Go to a larger venturi. Same thing I am saying here.

This is a great discussion.

Is there really anyway to bench test an engine to find out how it might compaire to some known engine or how well it might preform as a stunt engine?

I would love to play with venturies and head shims on a couple of engines but have no idea how to tell if one set up might be more desiarable, have more torque, will have a better run than another. Heck even flight tests are confusing, I have a hard time telling one setup from another unless it really falls apart.

So OK it will fly the pattern and we have good tension overhead but what will it do in a 20 MPH wind? Can it be better? How the heck can we measure it?

Bob,

It is my opinion that the best guys in our event have developed "The Touch" to the point where they have almost got the equivalent to a dyno right in their flying hand. I know there are quite a lot of these folk, but in my personal experience the list begins with Walker, Don McClave, Brett, Ted, and ends with David. Not necessarily in that order.

For you and I, the great unwashed, bench-running tests can certainly nicely and effectively augment what we see at the field. Further positive reinforcement, if you will.

A series of tests such as that which began this post can help determine the most promising of various combinations to try when out flying. For example, it was not until I made this series of runs that I had any idea at all how many revs were killed off when using an air filter; now I know the effect of the three different filters being carried around in my SSF tool box. If nothing else it's a lot easier to fiddle with filters than to turn a whole bunch of venturis in steps of .002.

You can certainly determine if the engine is fully run-in. You can see how much load it will take before going all wonky (technical term) on you. If buying a small dyno you can probably learn a lot more.

Or you can sorta reverse engineer things, which is part of what I did, testing in a more controlled environment those variables noticed at the field. In this case, I *knew* open exhaust and tongue mufflers were inferior to stock mufflers on 20FPs, based upon flying experiences. But how to demonstrate this, how does this manifest itself on the test stand? Wandering revs, from what I can see.

First time I got a grip on the increased power and mileage from all-castor fuels was at a test stand. At the time there was no way I could have felt this while flying, although I could of course meaure run time quite easily. Even a couple years ago I would have just automatically fed 20FP and 25FP motors SIG 10% "Champion" fuel and thought I was some kind of hero as I wasn't using Kool-Aid fuels like the RC guys do.

And there is a developed feel for what an engine is doing on a test stand, even though it takes a lot of back-and-forth, flying field to test stand, in order to see lights going on over your head.

A number of years ago GMA had one of my motors, mainly as I was not only having some trouble with it, but had blurted out the name of the fellow who had originally set it up. Details not now, but the latter piece of information did not sit well with George, he *insisted* I sent him the offending piece. I did, and when it came back it was really nice.

But during test runs he was impressed enough with the reliable break and very reliable four-stroke running that he called just to find out what sort of weird glow plug I had in it. Must have misplaced his glasses that day, as it was merely an O.S. #8 as could be clearly seen by the stamping on hex portion of plug body.

The point is that simply by running the thing, listening to it, maybe holding up a tach to the prop, George knew it was going to work well for me. And he was right. No need at all for flight testing.