The effect of backpressure

What is the effect of backpressure on a 4-2 break? Are there any reasonably predictable options that create the same effect without robbing power? For instance, an engine has a reasonable 4-2 break with a muffler that creates a specific backpressure. If the muffler is opened up to lower the back pressure and we see a power gain but can no longer get a 4-2 break are there any options that get us back to the 4-2 that do not rob the new found power? What is it about the backpressure that allows, or creates, the ability to 4-2? The reason I ask is that in playing with a K&B/Brodak .40 that has a Holcroft muffler that was made for these engines I am able to get a nice 4-2 break when the muffler has one of the Du-Bro silicone exhaust extensions attached. When the extension is removed I can no longer adjust the engine for a 4-2 run (although it runs very nicely in the 2 cycle mode). Other specifics are: K&B plug, Sig 10% nitro 20% lubricant fuel (50-50 castor-synthetic), Uniflow plastic clunk tank w/muffler pressure (although I've run it without muffler pressure - and without uniflow for that matter with the same result), 11-5 APC prop, .295 dia venturi, stock K&B needle valve. --- One other characteristic thats worth noting: when it's set up for the 4-2,it has to needled on the ground for a deep 4 cycle (to the point where it has very little power) -- once in the air, after a few laps (3-6) the 4 cycle picks up to a point just short of a 2 with the expected power increase, and from there on you can go through the pattern with a nice 4-2. -- any ideas?

Dad,
Very interesting question.... I wonder why there are no replys? Let me know if you get any good answers.

>Dad,
>Very interesting question.... I wonder why there are no
>replys? Let me know if you get any good answers.

Yeah, I hope some of the engine experts will chime in. I'd like to learn more about how and why the 4-2 break works... and how to make it work consistantly... Also, it seems that the change in power with the 4-2 break is very different with different engines. On my old Fox .35 it just seems like the noise changes but not much change in power. They may be getting worn out or something... other engines just seem to have a large jump in power when it goes from 4 to 2 cycle... Do the older "baffel" engines have less of a change in power? Or is it due to the more modern engin just moving into a much more powerful part of it rpm curve? I think the old Foxes had a pretty broad, flat power curve...

>...I'd like to learn more about how and why the 4-2 break works...
>and how to make it work consistantly... Also, it seems that
>the change in power with the 4-2 break is very different
>with different engines. On my old Fox .35 it just seems like
>the noise changes but not much change in power. They may be
>getting worn out or something... other engines just seem to
>have a large jump in power when it goes from 4 to 2 cycle...
>Do the older "baffel" engines have less of a change in
>power? Or is it due to the more modern engin just moving
>into a much more powerful part of it rpm curve? I think the
>old Foxes had a pretty broad, flat power curve...

A lot of this depends on engine design and how it is set up and how you are running it (doesn't everything?) First off, on the original question, the muffler, itself, will affect the power output of the engine. The engine should be capable of the 4-2 break with or without the muffler, and the only effect the muffler would have on it might be to make it break at a lower rpm merely to a lack of power (if the muffler becomes too restrictive) or to help with combustion (see the note on the Fox muffler following).

But the softness or harshness of the break (when it breaks, how it breaks) will be dependent on timing, compression, venturi size, nitro content, density altitude, prop you use, and, of course, where and how you set the needle. If you set it too lean or too rich, no break. The Fox 35 is mildly timed and low in compression which causes a quick break, but hardly any increase in rpm (there should be an increase in power, however).

If an engine is jumping too much in rpm between the 4-cycle and 2-cycle mode, you can detune it (lower the blow down period, which should increase the 4-cycle power but decrease the 2-cycle power), decompress it (which decreases the "jump" and maximum power), go down in nitro or venturi size (same effect), and, I suppose, increase back pressure. You can also go to a larger prop (uh-hu, but only to a point). Smaller props will give a larger jump in power, larger props will give a lesser jump in power (when going from 4-cycle to 2-cycle) on the same set-up.

Everything works together, and I like bigger props which will make a different set-up than someone running smaller props (even when running a pipe set-up with the same engine on the same pipe). With the pipe set up you adjust pipe length (and pipe restriction. some pipes are a lot more restrictive than others. And that "rubber duckie" at the end does NOT help), venturi, head shims (which definitely affect the amount of break), and even nitro content according to the heat of the day.

If you really want to see the effect mufflers and back pressure have on an engine, get a tongue muffler with about 15 holes drilled and tapped for 4-40 bolts. Start with the engine in a "normal" set up (for that engine) and all holes open. Then put a series of short 4-40 bolts into the inner holes one at a time and see what affect it has on the engine run. The early ones will have a minor difference (maybe 100 rpm) on the run. But the more you add, the bigger the difference becomes. And you can't plug them all (which should be obvious)

I might add that certain engines, such as the Fox 35, might run better with a little (that is little, not a lot) back pressure simply because the back pressure builds up the heat and helps to keep the plug lit. All back pressure, however, does lose power.

I'm interested to see Leonard's comment about too much 4/2 break. I have heard this mentioned before, but I need a little convincing. Ideally we consider the pattern should be flown at constant speed. This means in horizontal, climbing, and diving flight, speed should not vary. We use the break from 4 to 2 to give increased power in the climb to compensate for having to overcome gravity in addition to aircraft drag at flight speed. I have noticed quite expert fliers stating we need the smallest jump in rpm possible, which does not make sense to me. For a start, the smallest jump possible is no jump, so why bother with it at all? The answer to the question "Can you have too big a jump in rpm when switching from 4 to 2?" is "Yes". The jump is too great when you have a model which accelerates in the climb and slows down in the dive. I have yet to see such a model. Anyone have any other thoughts? Regards to all, Ian Russell.

Matt has a term in which he describes it "zippy". That means there is too much jump. With his pipe ship he can really tell it. Now, that can mean too large of a venturi (go down a couple of thousandths of an inch) too much nitro (he usually uses 5%, but will go to 7 1/2% or even 10% in hot weather. Too much, however, and it gets "zippy") or not enough head clearance. (again, it may mean merely raising 2 to 3 thousandths.)

Conversely, there are times when it does not have enough "drive". (That's the opposite of being "zippy". Gotta get those terms down, you know.) When there is not enough drive, it is slow in coming out of the corners or doesn't have enough authority in the climb. The solution, when we know we have the correct venturi, is to either go up slightly in nitro or remove a 2 or 3 thousandths head shim. When all is working right, you should be able to adjust with nitro between a cool morning and a hot afternoon. (Usually nitro doesn't go up until it gets over 85 degrees or so, or the wind comes up.)

I might add that the pipe ship is set up for just a slight break in the top of the loops, above 45 degrees, and in the corners. Some engines, like the Fox 35, will break when you point the nose up at 45 degrees. You get a sooner, quicker break, but not nearly as noticeable powerwise. These are not "zippy" and would actually benefit from a little less blow down, more compression (hemi-head) or moving the fence over slightly on the piston. (Fitting the engine with a .36 piston helps immensely because of the fence location.)

Paul,

You may want to reconsider the prop choice. I used 11x4 APC and it was too much for the motor. 11x5 APC is probably going to be worse. You may want to consider going down to 10 inch prop. To be honest, I am the only one trying to run an 11 size prop on B40 around here. Other pilots use APC 10x5 on their ships with very good results.

I did find the answer to B40-APC matchup though. Take 12x4 APC prop and cut it down to 11x4. The reason this prop works better than a stock 11x4 APC is because of its shape. 12x4 APCs have the 'new' APC shape that's different from older generation of APC's. It has a narrower blade tip and doesn't have that scimitar look to it.

The performance gotten so much better after switching to 12x4cut, I switched to 7 1/2 nitro content and still maintain a very good 4-2-4. I used to fly on 11x4 TF wooden prop as it gave me good results. After trying the new APC, I have a new favorite!

Steve,

I realized after reading your message that I had not been clear enough describing the engine. The one that I'm talking about is the controline version of the K&B .40 (model 4055 - I think) that Brodak was selling a few years back not the newer B40. Sorry about the confusion.

Paul,

For what its worth, and before the flames start, let me suggest two things to try on the K&B 40. First, try a fuel with 25% or so lube. The original Brodak instruction sheet that came with the K&B 40 ABC stunt actually recommended 29% oil (ie Fox Superfuel). More lube should help the 4-2-4 run. Then try a very hot plug (thundervolt, etc) to keep the motor lit with all that oil.
The late Mike Garmon worked with K&B and Brodak to get a stunt version of the K&B 40 ABC and 61 ABC marketed by Brodak. He wrote the supplemental instruction sheet Brodaks included with each engine and ran all his prototypes exactly as he said to do in the instructions. He used the factory muffler on all of the test motors. Every one I saw him fly purred like a kitten with a perfect 4-2-4 break, all the while leaving that Superfuel smoke trail. His K&B 61 powered job had an awesome sound and was great to watch in action. Its a shame K&B folded after only a few hundred of each were produced.
I'm sure there are other ways to get the ABC 40 to 4-2-4 on 20% oil, but if you really need that run, the superfuel route can get you a baseline 4-2-4 run without too much fuss. I had a K&B 40 ABC myself, which always ran a 4-2-4 on Superfuel, and would run 2-2-2 with just a needle tweak, with no sign of sagging hot or any fuss. My only beef has been with the considerable weight of the motor. I've got another new K&B 40 installed on Mike's Brodak profile warbird P-51 prototype I restored, but have not had a chance to fly it yet. I hope I can fly it as well as he could.

Steve,

Thanks for the input. Am I correct in assuming that the extra oil helps the cooling and therefore the changes the way it breaks?

The outlet of the stock muffler is .375 diameter compared to the Holcroft at .312, however the stock muffler has considerably more internal volume. I'm not sure what all of this means since it appeared that increased backpressure helped the break, but I'm going to give it a try and see what I can learn.

By the way is there a prop that you found that the K&B was particularly happy with?

Paul,

Its always been explained to me that extra oil helps a motor drop back into a four stroke from the break. I suspect that timing , load,construction (steel vs ABC, AAC, etc) etc plays a large role in this, too, but if a motor that is timed to run 4-2-4 and is not, then more oil should bring the break back. (Obviously, if somebody was running 29% already and had no break, there is another problem.) The general rule of thumb is that ABC engines don't need 29% oil, being perfectly happy on 22-24% in most cases. But in the case of the K&B, you have an engine that really was not designed for stunt-a happy (for C/L) accident very much like the LA-46. As a result I think the timing is a bit more on the R/C side than a Fox, etc, and the 29% oil helps keep the motor tamed down. You have to remember that at the time, Brodak's were kicking around for a good sport motor to sell, and Mike Garmon, who was friendly with K&B's Bill Wisnieski (sic) had come to the realization that the K&B 40 and 61, with only the substitution of a venturi for the R/C carb, would make perfectly adequate, albeit heavy, stunt motors. Since Mike was helping Brodak with his product line at that time, the K&Bs became the logical choice for a Brodak stunt motor. Mike never saw the need to run them on anything other than Fox Superfuel, so they were never really tested on anything but that,(JT, I know you probably tried something else, but thats another story!) and any issues regarding tendency to run away, etc, were never explored. Mike was a practical guy, and since they ran well on 29% oil he saw no need to suggest anything but that.
I don't think the holcroft muffler should be too restrictive. My flying partner has run them on FPs without any sign of trouble. I used a tongue muffler for a few flights on my K&B, and it would still 4-2-4 ok, although to me it felt a bit down on power. A tongue has got to be more restrictive than a tube muffler. I actually tried to obtain a holcroft muffler at that time, but heard they were no longer available. Unfortunately I wrecked the airplane and engine shortly thereafter. I have not yet run the new K&B I got, although I still have the tongue and factory mufflers. I plan to try the tongue first. For props, I was using BY&O 11x6s (these were old Clarence Bull units, lighter and with thinner blades than the Brodak offering) Also, the Top Flite 11x6 power prop. This ran a stout 4-2-4 run with no trouble. I tried a zinger 11x5 but this seemed a terrible setup, mostly due I think to the quality of the prop than anything else. I was flying a severly overweight Brodak Cardinal with this setup. I have a picture of Mike's P-51 when he was flying it, the prop appears to be a rev-up, maybe a 10x6, I'd have to look again to be sure. His airplane currently weighs 41 ounces, it would have been about 43 as he flew it-my Cardinal was probably 48 ounces, so I tried more prop than he carried. An APC 10x6 might be a place to start, although that would add to any nose heavy condition brought on by the motor. Mostly I'd just avoid zingers unless you are skilled at thinning the blades and removing the square cross section of the leading and trailing edges of the prop. The bottom line on the prop selection is going to mostly depend on what plane you are trying to move with that engine.....For serious stunt, any time I looked at the new K&B sitting on my shelf the past year, I immediately replaced the thought with "LA-46" since for low cost you got lighter weight and more power. But acquiring Garmon's old prototype Brodak warbird was a perfect excuse for the K&B to see the light of day once more.

Steve,

Thanks for all the information. Last night (a blackout is the perfect excuse to go flying) I got a chance to try the engine with some 10% nitro 29% castor Powermaster. Could not believe how well it worked - your advice was spot on. The 4-2-4 was about as good as one could wish for and the needle adjustments were positive without being sensitive. And, this was without the exhaust extension, which, up to now, was necessary to get a decent 4-2-4. I'm going to do a bit more playing with props to evaluate the differences. I was using an APC 11-5 which worked very well and also tried a Robbe 11-5. This was the first time that I've tried a Robbe prop and was surprised, since the blades are more flexible than would seem ideal, at how well it worked. Interesting stuff.

Thanks again,
Paul

Steven just for grins bolt an APC 10.5 x 4.5 on that B-40, set the needle so it just breaks when you point the nose up and see what you think.

Will do! I just called LHS and asked to put one aside. ARF Nobler has very short legs and can't take off grass with 11 prop. Perhaps 10.5 diameter will work. Should I expect a surprise?

>Will do! I just called LHS and asked to put one aside. ARF
>Nobler has very short legs and can't take off grass with 11
>prop. Perhaps 10.5 diameter will work. Should I expect a
>surprise?
>
>Steven Yampolsky

Remove the wheels and put on some 3 inch Lite Flite wheels. They are light weight and will give you more ground clearance. (Better roll in the grass, too.)

Thank you for the idea Leonard! I can't beleive I haven't thought of that!