OS 20FPS

I have a nice OS 25 FPS in a 22.1 oz. Ringmaster and that motor runs smooth like a well oiled sewing machine. I thought I would try a 20, just to see what all the fuss is about.

Try ebay. If the cost is too high just wait a week and try again. I just paid 28 including shipping for one. I've seen the go much much higher. Try to find a good seller, look at the feedback, most I've dealt with are pretty fair.

I haven't had that much experience with them other than I do have a lot of profile ukies which suit this particular motor very well.

True, the engine of which you speak is no longer in production. I clearly addressed this, and alternative sources of supply, in July/August Stunt News, the column beginning on page 51.

If you have further questions, it has been very strongly suggested that I am no longer allowed to discuss this topic on this forum. Thus you are referred to:

http://hangarflying.proboards18.com/index.cgi?board=wire

See you in Salem, Oregon? I'll have models of interest to you...

And just remember, it works fine, but it is not magic. You will find the 25 to be an even better engine when set up properly (25% more power and less weight.)

Len,

Thank you for the offer. I will politely decline your invitation, however.

If you got the *stock* 25 to work, and not fly at warp 9 (two things that would seem to be incompatible) then you probably already know what it's about. But if it's modified (seemingly must have been if you aren't running slow rat lap speeds), you probably don't. Even a choked-down 25FP managed to pull Pat Johnston's full-sized Green Box Nobler so fast it almost couldn't be flown.

The stock 20FP is moderately hard to handle on the Ringmaster, because it has excess power and wants to fly too fast. I think you would do a lot better with the 15FP, particularly on a 22 oz airplane.

Agreed. But the good doctor is a known user of unmuffled ignition 60s. At contests! When folk are trying to converse in normal tones.

To think he would willingly bolt on a puny 15 and use the muffler is being somewhat optimistic.

No expert here (understatement of the century, in advance), but I can vouch for this. I tried a stock OS 25FP on my Flite Streak... exactly ONCE. Never again...

Tim

A Flight Streak is suppose to be flown fast - - hmmmmm, speed limit combat anyone? 74.99985 MPH Right On!!!

WHY????

And I had an FP 35 on my Flite Streak. It was kinda fun. The one which really did concern me was a Fox Series 5 .36 I put on a slow combat ship then a Combat Flite Streak. Guess I'm not used to that much line tension.

SNIP>>>>.The stock 20FP is moderately hard to handle on the
>Ringmaster, because it has excess power and wants to fly too
>fast. I think you would do a lot better with the 15FP,
>particularly on a 22 oz airplane.

Excess Power? How about not prop-er-ly set up.
Excess Power is an ST -60 on a Rigmaster

Tim

And there is the problem. "Set up the same way." You are either going to have to put a bigger prop (more diameter to take advantage of the 25% greater power) or put a smaller venturi on the engine. Lap speed is a function of pitch and rpm. Sure, drag will also slow you down, but with proper rpm and pitch you can maintain whatever speed you wish, even if you theoretically had no drag.

Just remember, each engine needs to be set up differently. If you have a 35 or 40 you are going to use a bigger prop than you would with the 20. Doesn't it stand to reason that you need to increase the size of the prop going from a 20 to a 25?

Simple solution, more prop or smaller venturi. The stock venturi is too big anyway, but I would opt for more prop anyway just to take advantage of the increased power available.

In this context, power indicates speed. I know no one likes the fact that words have real definitions, but they do. It takes somewhere around .33 HP to fly a Ringmaster at reasonable lap speeds on reasonable lines. Not necessarily shaft HP - HP as measured by the force pulling the airplane forwards (thrust) and the speed. This is a function of the drag, which is a function of the square of the speed, and a number of other things that remain fixed in this situation.

By properly set up, you actually mean one of two things - reduce the shaft power by some modification (i.e. block boost port, choke down venturi, alter timing), or reduce the efficiency of the system to waste excess power, presumably by prop alterations. The first reduces the capability of the entire engine, maybe to good net effect, maybe not. The second is not realistically possible with any commonly-available prop.

Yes, a Bolly 9.5-4.5, depitched to maybe 2.75 or 3", will have such low efficiency at the desired speed that it will be flying at a reasonable speed, and of course the maneuver performance will be sparkling (since the efficient climbs rapidly as the speed drops, providing excess power into the airframe that will result in a very tight limit on how far the speed can drop). But that's a $27 prop that has to be mail-ordered.

The closest reasonable choice is a 4" pitch prop of around 9 or 10 inches diameter. This will have to turn *too slowly* to get a reasonable speed - a speed the engine will not reliably turn without modifications. In fact, it want to an usually does spn faster, leading to excessive airspeed. Bigger prop? It will have to turn *still slower*. As some point, you bog it down so that it will spin slower. But then it will stay bogged down in the maneuvers, in fact it will get even worse, and have terrible maneuver performance.

That's why a 15FP ( that will spin a 8-9"-4) is a better choice. Smaller diameter will have to spin faster to get the required .33 HP. Faster enough that no you can let the engine run where it wants to, and still have some reserve in the maneuvers.

If you think this is all just theoretical BS, you might be illuminated to find that I figured out by experiment. The data came first, and I worked out the theory later. And it works *the same* on 40VFs, PA61's, and RO-Jett 61s.

Y'all want a 25 fp, fine. Just order the stinkin piston and sleeve from tower and stick it in your LA 25.

Geeze, facts. What are they going to think of next?

>From somewhere near Parkville, Mo.
>William Crane
>AMA 6733

That will only work if OS put the boost port in the Case of the LA 25 and I do not think they did.

Actually, Ted doesn't have a Medic. The one we experimented with was built by Bob Hazle, and was later sold/given to Paulette Ericson (Mrs. Uncle Jimby). We ended up with a 25 and a 9-4 APC, and a "fine" aif filter element. The difference, of course, is that it's a significantly larger airplane that requires more power in level flight at a given speed, or alternately, the same power at a lower speed. Hence the balance between power available, prop efficiency at a given forwards speed and rpm, were completely different. And, this *particular* engine was one of the ones that actually liked running at the tuned speed. The ABC 20's most all like to run that way with a 9-4. The ABC 25's typically want to run faster than the tuned speed, and that results in unacceptable performance (too fast and greatly reduced regulation). If you put on a bigger prop, it helps it run in regulation, but the efficiency is higher enough that the airplane *still* flies faster.

Note that this entire discussion violates the common conception that "more efficient" = "better". Maximizing efficiency is only necessary when power is limited. When you have an airplane that requires 1/3 horse to fly at a reasonable speed, and an engine that could realistically put out close to 3 times that, power is not a limiting factor, therefore the problem becomes one of how do you waste or limit the available power in the most effective way possible. It's no different (in fact it's even more important) when you are running piped 40's in airplanes we used to think too small for an ST46. And also why most airplanes don't fly significantly better when you go from a piped 40 to a piped 51, to a piped 61, to a piped 77. The *requirement* for power is the same in all cases. You only need so much to fly at a given speed, and there's only so much power boost you can stand in the maneuvers. It then becomes a matter of how easy or effectively you can get that amount of power, and other, non-power-related run characteristics.

This is the same airplane (on the same flights) where we got rid of a huge amount of inboard tip weight and massive hinging by adding a micro-teeny rudder tab (half a glow plug cardboard thingie bent at about 15 degrees). That was actually more of a revelation than the engine.

As you yourself have said (previously), it takes a certain amount of thrust to haul an airplane at a certain speed. This is the theoretical top speed of the system. Drag goes up when speed increases. Thrust goes down when speed increases. When thrust equals drag, that is it. But when we match everything properly with rpm and pitch, we are also nearing a very real line where if there were no drag the airplane would go no faster. RPM times pitch is the maximum speed that we are going to get out of the system, drag or no drag.

What I am proposing is the RPM x pitch factor in limiting speed. There is no reason we have to turn 14,000 rpm so that we can get the maximum "power" out of the engine. The engine will develop more torque at slower speeds which is what turns the prop anyway.

Unless you are conceding that the 20FP is truly limited in power and that the airframe is holding it back which is why it works so well, then there is no difference in a 20 FP turning a certain pitch at a certain rpm and a 25 FP turning the same pitch at the same rpm. And if we do not decrease the size of the venturi (which is one very real answer to the situation) we can increase the diameter of the prop (maintaining the same load as if it were a 20 FP) and the maximum speed will thus remain the same. Instead of using the drag of the airplane to hold us back and maintain speed, we will be using rpm times pitch. The efficiency of our airplanes is going to be much better if the rpm times pitch closely matches the desired maximum speed of the plane anyway.

You have run as small (as I recall) as an 11.3 inch diameter 2 blade prop on a PA 61. No argument, as the system worked for you. Matt has run a 13.5 inch diameter 3-blade prop on the same engine. Same (approsimate) airplane size. Same (approximate) line length. Same (approximate) lap times.

Why can't the same argument hold true for the 25 FP or LA? A 35 FP and a 46 LA should use different props. Shouldn't the same hold true for a 20 FP and a 25 FP or 25 LA? Use a larger diameter prop to load the engine properly and harness the extra (not excess) power available, and lap speeds will remain the same. But, as you yourself pointed out, you now have an added advantage in that the airplane will not slow as much in maneuvers or in the climb. This has always been my argument for using the 25.

I applaud you for finding a system that works for you on the 20FP to your (and others) satisfaction. But there is a system that can work on nearly every engine. Yes, we need to make changes. But once we make those changes and set it up properly it will work. It is possible that the only change we need to make between one system and the other is prop.

And as a follow-up to the above, Matt had a 35 FP in his 570 square inch Stuka driving a 12.5 x 5.5 Bolly prop. Lap speeds were around 5.3 seconds. Line lengths were about 66 feet. We used this same plane as a test bed for the development of the Magnum 53. This engine is a full 50% bigger, so we put on a prop and a half. We used a 13 inch 3-blade Bolly, pitched to the exact same 5.5 pitch. It worked beautifully (VERY nice!) And lap speeds remained 5.3 seconds. Launch engine speeds were nearly equal. Prop pitch was nearly equal (as equal as we could get it). We used the same plane, the same lines, so the drag was the same. And lap speeds remained equal. But the ability to maintain maneuver speeds was impressive. So impressive that we never looked back. His next plane was powered by the .53 and the 13 inch 3-blade combination.

Steve IT works for you. Same here, But I've found the the 9.5x5 Clubman works so good on the FP-25. If you ever find one pick it up and try it. ANYWAY your post sounds just like what I found out. It's not a world beater but it fly's ..and your having FUN!

Scott(It's about FUN) Riese

From what I understand, Brett has empirically determined the peak torque RPM of the OS-20FPS, the exact prop (brake/load) that puts the engine at or near that peak RPM, other factors (stock venturi, stock muffler with cone, stock timing, etc) that produce the desired shape of the torque curve, and finally, the optimal drag/load (airplane size, mainly wing) that matches the horsepower produced by that engine at the peak-torque RPM (important: note that peak-torque RPM is way below the peak-HP RPM). All these "numbers" comprise the now-famous "20FP setup". Any deviation of any of these factors will predictably result in the need to re-tune the rest of the system.

Leonard's suggestion of using the FP25 instead, with smaller venturi and possibly a bigger prop, is one way to re-tune the above setup. Venturi is one of the many factors that affect an engine's torque curve, and prop is what loads the engine and puts it at a certain operating point on that curve. It is certainly possible to "choke down" the FP25 to make it produce the same peak-torque HP as the FP20 - what we don't know (OK, what I don't know) at this point is how this modification affects the rest of the torque curve. If it suppresses the peak torque and widens the curve, there is a danger of a runaway / windup. However, from my <limited> past experience and observations, just the opposite may happen - the "choked-down" 25FP may acually have a steeper torque curve drop-off at above-peak RPM and a flatter curve at below-peak RPM, which would make it a better choice for the stunt application.
I will have the data once my new torque engine stand gets assembled and operational.
One final note. Some of us like more line tension than others. One possible way to increase line tension (besides flying heavier planes at faster speeds) is to use part of the engine thrust (perhaps with a little offset), which would call for a larger diameter prop and more power to match it. So, if Brett is OK with a 11.3" prop on a full-size plane and Matt wants 13.5" on the same size plane, with everything else being equal (?), this could be just a matter of personal preference and flying style.
Hence, no ground for a heated argument.

Agreed, my point exactly. You may not like what I like and I may not like what you like, but that's why God gave us color instead of just black and white. Choices, many many choices!

BRETT

Theoretical BS? There are no Theoretical BS here. It's all trial and era.
Do what comes naturally. We all have some kind knowledge on running an engine; It’s not that hard.
As a person that work hard at TRIAL AND ERA new engine set UP’s it just comes down to HOW much time do you want to take in making the engine work right. I have logs of INFO on most of the engines out there. OH there is a lot of in-fo that comes for others such as you and kiwi. OOHHH I used the word YOU Please be kind I’m disabled…. lol

Scott (a Brett fan) Riese

I'm not against trial and error fiddling, do it all the time. I do rail against "Wing of bat, eye of newt"* approaches toward "improving" a setup known to be very nearly perfectly optimized. And I think that's a part of what Brett is saying, although I say it a lot funnier.

Dan

*Palgiarism, pure and simple. Reference: Kevin Cameron's TDC column in current Cycle World magazine. Worth everyone's while to check it out.

Actually, that's not it at all. I merely meant to distinguish on some level that what stunt fliers mean when they wsay power, and the accepted defintion of power, are two different things. My comment about "therotical BS was to address, premetively, the oft-stated position, that this stuff it made up in some ivory tower and doesn;t actually relate to anything in the real world. Trial and error is an excellent way to determine whether something works, or doesn't work, particularly when it's cheap and easy to do, like it is in this case. Understanding the results from one experiment (as opposed to simply finding one thing that works and just repeating the same idea over and over)helps to determine the next experiement.

In the specific case of the 20FP, I think I have pretty well exhausted the possibilities of things that work, and come to the conclusion that the stock stuff is pretty much as good as you can do. Stating the optimum setup allows people who neither know, nor care, how it works or what underlies it to get very good performance without a lot of fiddling or expense.

For those who care to delve into the details, it's an excellent case study that really taught me a lot, and might presumably be helpful and illustrative to others as well. The physics are pretty simple, and can easily be extrapolated to other, more complex, situations.

There's nothing magic about the 20FP, and I don't care one way or another whether anyone uses it. But it makes a good case study, and the fact that the 15, 20, and 25 are all of more-or-less same thing in different sizes makes it easy and valid to explore the differences.

I think (for me at least) it would be extremely educational for Brett to publish a set of Infinity plans with not only building instruction, material selection, power requirement(s), etc. but explanations as to What,Why,How,When,Because,Where and the Reason(s) behind the design. A Case Study so to speak.

It's obvious to me, (I don't know about anyone else) but Brett has an ability to explain the "Wing of bat, eye of newt"*. "Doing the Math" is tough for allot of us and I have to say I learn allot of "Stuff" from this guy. My old man used to tell me that education was the only thing nobody could take away from you. So publish the drawings allready...Better yet, get Brodak to Kit the Infinity. A Brodak kit with "reason". I'm buyin $$$.

Brett, you should really consider it.

I've never met Brett Buck (or John Brodak for that matter) face to face but Brett has a knack for explaining things (in writing) that I understand. John Brodak makes some pretty good kits and "sells" them to every "Sport" flyer on the planet. PAMPA membership forms in Brodak kits, what about "Real Instruction" not just glue this to that. A part of the proceeds could even go to "The USA Team", PAMPA, or The Giant Scale R/C Hospital fund. ("Braveheart quote" Join the Clans")

I have never been to or entered a model airplane competition of any kind and have no agenda. In fact, I rather "enjoy" being on the other side of the continent away from Dirty Dan and the "West Cost" Chapter of "The Blood Brothers of Stunt".

However, if I really "knew" what I was doing and "why" I might consider entering a competition and utilize PAMPA in the ways for which it was created.

Hmmm, educating "Sport" flyer's to become "Competition" flyers, why appearance points are "Earned" and not "Given" why the rules and the charter that were set-up by our forefathers "are" the way they are, That's what you want to put in a Brodak Kit.

Regards,
John Jolley

I think I'd paint "Buck Off" on the wing of my model, Just Kidding (smiles)

In the late fifties, early 60's a certain saying was popular especially when
one would be in crisis, a person might ask themselves "What would Brando do?"
As in cool, tough guy actor Marlon Brando.

For at least one day of the year one could if he or she wanted, subsitute Buck
instead of Brando as in,

"what would Buck do?"

I appreciate what Brett and many others give and that's their time.

Thank, this is all very flattering! To make a kit, you first have to have plans, and I have that but haven't checked then fully yet. I am making an airplane from the plans now, which is a good way to check them If there is an article (and I have been approached on the topic by one of the magazines), it will be kind of like you describe.

Failing that, there are plenty of good other articles to read, from more accomplished fliers. The Nobler article, "Stunting can be Smooth" by Aldrich is absolutely excellent and of course, seminal. The Olympic article by Gialdini is also quite illustrative, in a similar vein. The Imitiation article by Fancher would be like a prototype for an Infinity article - it has what amounts to a "core dump" of the state of the art when written. And the Impact article by Walker represents what amount to current information - seeing as how there have been no significant breakthroughs since it was published. It also shows the level of attention to detail required to be competitive these days.

And heck, I'll tell you exactly why I did anything. It make not make sense, and it may not be right, but there are hardly any secrets being held here. Between Ted and I we have pretty much told you what and why we did anything.

>However, if I really "knew" what I was doing and "why" I
>might consider entering a competition and utilize PAMPA in
>the ways for which it was created.

If you look at SN these days, there's an awful lot of that in there already. I just don't know what more we could do as an organization to support sport fliers and casual competitors. The notion that these members have somehow been short-changed or ignored in a way that needs a "champion" to "fix it" is, AT BEST, a political theme and/or diversion from real issues.

>Hmmm, educating "Sport" flyer's to become "Competition"
>flyers, why appearance points are "Earned" and not "Given"
>why the rules and the charter that were set-up by our
>forefathers "are" the way they are, That's what you want to
>put in a Brodak Kit.

I think that's a bit off the basic theme, there...

Brtt

Dan

*Palgiarism, pure and simple. Reference: Kevin Cameron's TDC column in current Cycle World magazine. Worth everyone's while to check it out.

I hope Wing of de BAT and eye of newt was not pointed at
ME young MAN.
What I find about running engines is I work at it. I might Fly 10 flights in one day with an engine I'm trying out. 10 Flights sounds low .However, with so many poeple I'm teaching (beginers)I HAVE to most of my testing in the back yard. I rambling.....

WHY WOULD AN OLD AND SLOW GUY LIKE YOU READ CYCLE WOURLD MAGAZINE? LIKE THE PHOTO'S.....lolllllooollll

This thread rapidly degenerated into pros and cons of the OS 20 vs the OS25, and suitable planes. That was not my concern. But at least I now know how to get a hot discussion going, with a lot of personal animosity among those who tradionally aspire to those things.

Floyd

EDIT: I tried to print a reply from Dan R. My printer coughed up 15 pages and I couldn't stop it. I finally had to pull the plug on the computer. My reply to DAN:

I like to fly large spark ignition motors without mufflers because that is what we did back in the '40s. My OTS models are exactly like the ones I used to fly, including simple paint schemes and original engines. That is what OTS is all about! I am pained to see something like a S.D. Zilch with a honking glo motor in it! Jim Saftig didn't use glo motors, and neither do I.

If you don't want to mess with the bay they are listed for $61.50 here with Muffler.

http://www.udisco.com/hobbies/index.htm

I'm not sure this is the muffler with the cone though.

There was an interesting thread on some other board (Phil Browns? sorry, I don't have the url) where someone who had a muffler without the baffle cone put in a big flat fender washer instead. And liked the results. I might try that at some point.

It makes some difference, but it works pretty well without the baffle cone, too. Definitely sounds different! The tuning effect that it uses seems to be the one associated with the tail cone, not the baffle.

UDISCO is a pretty good place to get stuff, and they actually seem to know what they have, and what they don't, and will tell you!