Noblarf vs OS 25. No winners!

I took the OS 25 fp out of my Noblarf, and am replacing it with a Double Starr 40. My suspicion is that the previous combo just isn't compatible.

Sure enough, I put the OS back on the Flite Streak from whence it came, and it is completely controllable and docile, gobs of power again. (too much, actually with a 10 x 4, even on low nitro, high oil, and restricted venturi).

I suspect that the OS runs in a resonance region of the Noblarf that causes uncontrollable fuel foaming. Even with Armor-All in the fuel.

It will be interesting to see how the DS performs.

I would go with an OS 40-46 or a Brodak 40. Never liked the DS 40. Mine was a dog.....just my miniscule $.02 worth.....but let us know how it works....

I was talking to a gent at the Texarkana meet three weeks ago and he said by opening up the venturi about .010" he improved the performance of his DS .40. Worth a try. It does have a rather small venturi.

I'm thinking about going back to the guitar. Resonance is a lot more pleasant and easier to control!!

The DoubleStar, running slower, on a bigger prop, should do better.

My Double Star 40 suffered a lot of heat problems inside the Noblarf's cowling and wouldn't run worth a lick. I then went the Atlas V route and dropped a RO-Jett 40 in. Much better performance; happier pilot.

My Double Stars seem to run much better when they have a lot of moving air around them - such as in the Oriental, or on my Old Timers.

Two more cents.

Yesterday my DS 40 suffered heat issues in my 52 Nobler. But I suspect in my case it was an issue of high outside temps (93 degrees) and not running enough nitro (I was using sig champion 5%) I forgot to bring my 10% with me. In high temps, 15% nitro is not out of order for a DS. When in doubt, throw nitro at it.

I was running 10 percent, with 23.5 percent lube. It was also very hot here in Virginia, but I would be reluctant to try more nitro. Since it's an oxidizer, it can really cook the inside of an engine quickly.

You must be in Northern Virginia.....maybe all the hot air from Washington reduces power still further.

I know the DS 40 will work on the Nobler, cause I've seen too many good setups here on the SE contest circut. I just have to find what is best on mine. I have a 42 ounce plane, so overeight is not an issue. The engine may just need more time on it.

I agree with you on both counts

Also, I screwed up. "On further review..." I was only running 7.5 percent nitro. I whipped up some 10 percent the next day. I'll try it and see what happens.

Larry,
We have always been of the opinion that the 25FP (or LA) is outmatched on "normal" weight (~> 40oz) planes of the Nobler Arf size.

>Larry,
>We have always been of the opinion that the 25FP (or LA) is
>outmatched on "normal" weight (~> 40oz) planes of the
>Nobler Arf size.

I don't share that opinion, although I am more than willing to at least once acknowledge that Larry pretty much knows what he is doing.

And it is heartwarming to see Larry move into the world of really big Stunt motors.

My Skyray, fully trimmed, came in at 34.5 ounces. Powered with a box-stock 20FP. Plenty of power, I even bulked it up to 36 ounces for a couple flights. No problem.

Well, who's flying a "normal" weight Nobler (Arf or not) with the 25FP? Like I said, it's my opinion.

Pat Johnston. It was light, got a lot lighter as outboard panel whacked his tool box on the way by.

You are welcome to your opinion, even though you initially said "we," not "I." And I simply do not agree.

If I could get my 25FP to run *exactly* as do my 20FPs, just with the bump in power one would expect from a 25% displacement advantage, it would quickly be bolted into my Zilch X. And all the rest of the NW OTS fliers would be dead meat.

Verrrrry interrrrresting. Correct me if I am wrong, but don't the 20 and 25 FP share the same case and crankshaft? Only the cylinder wall thickness is changed?

Hmmm, relatively longer stroke compared to the bore, more metal around the combustion chamber to absorb and distribute heat. Could that be the difference?

When it (briefly) ran right, the fp 25 had plenty of power for the Noblarf. Keith Trostle did consecutive, square, 4-leaf clovers with it. It was power controlability and consistency that I could not achieve.

If I had more time, I would have tried a tetra type tank to see if the problem really was fuel foaming. But I have fiddled around more than I can afford to. So the DS 40 is going in.

>Verrrrry interrrrresting. Correct me if I am wrong, but
>don't the 20 and 25 FP share the same case and crankshaft?
>Only the cylinder wall thickness is changed?
>
>Hmmm, relatively longer stroke compared to the bore, more
>metal around the combustion chamber to absorb and distribute
>heat. Could that be the difference? (snip)

Larry, You are right on your first analysis, but Dan (and Brett and others) are running the 20FP awfully fast, not at all like a long stroke taking advantage of the available torque which you might think would be there. You don't hear a lot of complaints about the 35 and 40 FP, but interestingly the 25 FP has a bore/stroke ratio that is about halfway between that of the 35 and 40. The LA 40 has the same bore/stroke of the 40FP and the 46 is the same engine bored out further. Guess which one is the darling of the lot? Yup, the 46. So I wouldn't make too much of the bore/stroke situation.

I keep pointing to two things, the restrictive muffler and the large venturi.

The stock venturi on a 35FP produces a .055 choke/displacement ratio.
On the 40FP the stock venturi produces a more reasonable .048.
On the 25FP it is .068
and on the 20 FP it is .085
The "normal" ratios for most stunt engines are .045 to .052

What we have there is a too large venturi being held back by a too restrictive muffler. On the 20 FP they have apparently found a combination that works. On the 25FP, trying to run it the same way with the same venturi and muffler, the engine produces more power.

If you take the simple expedient of going to a smaller venturi, sized correctly for the displacement, you will find the engine much more friendly, and the power available much more useable. Or put on a tongue muffler and tap the holes and add screws in the holes one at a time until you get it tamed down. I would much rather see it done with the venturi, however, as this will improve fuel draw and the friendliness of the engine.

I am running a restricted venturi (.250 dia) and a tongue muffler drilled out like Swiss Cheese. Seems to work just fine on the FS, but not on the Noblarf.

>I am running a restricted venturi (.250 dia) and a tongue
>muffler drilled out like Swiss Cheese. Seems to work just
>fine on the FS, but not on the Noblarf.

Is it because one is sideways and one is inverted? I am wondering what the differences are in the run. What is it that you don't like about the run on the Nobler?

The engine would run away and get so hot that the exhaust was black (like an overcompressed diesel running cheap fuel). You could not get it to run rich enough not to do this (it would suddenly quit rich in the air as an alternative). Total lack of needle control on a practical basis.

>The engine would run away and get so hot that the exhaust
>was black (like an overcompressed diesel running cheap
>fuel). You could not get it to run rich enough not to do
>this (it would suddenly quit rich in the air as an
>alternative). Total lack of needle control on a practical
>basis.

Maybe it is overcompressed. The deck height on these engines will vary from engine to engine, even of the same model.

But back to the venturi, go to a smaller venturi to increase suction and needle control. But it still seems a bit wierd that you would get run away on teh 25 using the same set-up as the 20. Have you checked for fuel foaming and also to make sure the needle is clean?

I did try reducing compression, I used Armor-All in the fuel. I switched to a 2nd OS 25 fp with identical results. I tried all sorts of stuff if you look at my earlier thread of a month or so ago.

I believe that killer fuel foaming IS the problem, and switching the engine back to another model where it now works again is another clue.

If I have problems with the new engine in the Noblarf, I will tear the nose open and do Mike Pratt's suggested 1/2" cross-grain balsa thing.

>I did try reducing compression, I used Armor-All in the
>fuel. I switched to a 2nd OS 25 fp with identical results.
>I tried all sorts of stuff if you look at my earlier thread
>of a month or so ago.
>
>I believe that killer fuel foaming IS the problem, and
>switching the engine back to another model where it now
>works again is another clue.
>
>If I have problems with the new engine in the Noblarf, I
>will tear the nose open and do Mike Pratt's suggested 1/2"
>cross-grain balsa thing.

Are you running a fuel filter between the engine and tank? Try one of the small sullivan crap trap types. It will stop the bubbles if your fuel is foaming. Works for us.....

Dan,
The "we" includes another flyer in my area, so not the "royal" we which would include (gasp!) you!

And as I recall, Pat Johnstons Nobler was in the low 30's oz (as my poor memory recalls), which is why I said "normal" weight Nobler.

>If I could get my 25FP to run *exactly* as do my 20FPs, just
>with the bump in power one would expect from a 25%
>displacement advantage, it would quickly be bolted into my
>Zilch X. And all the rest of the NW OTS fliers would be dead
>meat.
>
>Dan

DON'T THINK SO!!!!!!!!

Scott(my McCoy 29 will kill)Riese

snip...

>When it (briefly) ran right, the fp 25 had plenty of power for the Noblarf. Keith Trostle did consecutive, square, 4-leaf clovers with it. It was power controlability and consistency that I could not achieve.

>Larry

Aha! We now have the answer to a question we were too polite to ask: It's owner/operator error from beginning to end.

more snipping...

>And as I recall, Pat Johnstons Nobler was in the low 30's oz (as my poor memory recalls), which is why I said "normal" weight Nobler.

>Alan

Alan,

I believe my statement included the fact that Pat's Nobler was light. Certainly not low-30s light, but quite nice. In any case, Larry has confirmed that when the sun, the moon and most stars were aligned just so **and Keith Trostle was involved** the 25FP had plenty of power for Larry's ARF Nobler. Which was my point.

As for Scott's McCoy 29 vs. my 25FP, sounds like a wager is in our future. Will be interesting.

>>>Larry,

>>My Skyray, fully trimmed, came in at 34.5 ounces. Powered
>>with a box-stock 20FP. Plenty of power, I even bulked it up
>>to 36 ounces for a couple flights. No problem.
>>
>>Dan
>
>Dirt,
>How in the %$ did you get it up to 34.5 ounces????? It
>must have been the lead bellcrank LOL. Seriously, I have
>built around 10 Skyray .35's and they all were between 25
>and 31 ounces. The 31 ounce version was a carrier model
>with a three line bellcrank, tailhook, and two main wheels.
>
>My SuperStunt Skyray has a Fox .25 and tongue muffler and
>two main wheels and weighs in a 27 ounces. I even have a
>version with tricycle gear and flaps that is not finished
>but I would bet it is under 34 ounces LOL.
>
>Later Dude
>
>Mikey

Mikey,

Effort, lots of effort, and wood that will never get used otherwise.

1) Cut lots of holes in stock fuselage, skinned it with 1/64" ply from nose to tail.

2) Sheeted LE over kit-dimension outer panel ribs, these from 1/16" balsa. Again, heavy wood. And thicker airfoil for more drag.

3) Capstrips.

4) Thicker tail surfaces, horizontals increased in area to match numbers from your Primary Force.

5) Big ol' block tips.

6) Muffler alone weighs 2 1/3 ounces.

7) Real tail wheel and stooge attach.

8) Longer motor mounts.

9) Stupid-strong spars.

10) Lead bellcrank.

No complaints from me, the Skyray 20 flies great! Remember, with fatter foil and other stuff, area went up as well, to about 450.

Nice design well executed, incidentally.

Dirt,
>How in the %$ did you get it up to 34.5 ounces????? It
>must have been the lead bellcrank LOL. Seriously, I have
>built around 10 Skyray .35's and they all were between 25
>and 31 ounces.

Wow! My straight-from-kit version was 36 oz. And that was with a Fox, monokote, and very minimal finishing on the fuse. The rebuilt version with balsa wing parts is about 29. It's amazing, also, what little difference 7 oz (20%) made in the performance.

>Dirt,
>>How in the %$ did you get it up to 34.5 ounces????? It
>>must have been the lead bellcrank LOL. Seriously, I have
>>built around 10 Skyray .35's and they all were between 25
>>and 31 ounces.
>
> Wow! My straight-from-kit version was 36 oz. And that was
>with a Fox, monokote, and very minimal finishing on the
>fuse. The rebuilt version with balsa wing parts is about 29.
>It's amazing, also, what little difference 7 oz (20%) made
>in the performance.
>
> Brett

I think we need to refresh Mikey's memory as to how those of us in the real world have our choice of, at best, two or three kits from the shelves of Local Hobby.

See, Mikey thinks that what a discerning person should really do is first have The New Guy weigh 100 kits from inventory, selecting the lightest five or six.

These "average" kits just like we can all buy in any hobby shop are then opened and each component of significance is weighed on a gram scale.

Just like that and one has quick, ready access to a couple pretty nice kits. And they are stock, fresh-from-inventory pieces, nothing special at all.

Well, that might be what I would do. Mikey, however, is known to have much higher standards.

That's very funny.

I built a light Magician at 31oz. around my OS 25FP and it is not a great combo. I'm digging out my OS35S soon. I know the 25FP has done well stock elsewhere...but not here!

Here I am posting again at the end, like I should.

I have found the OS 25 FP to be possibly the smoothest-running engine I have ever used. If yours vibrates, it just has to be a bad prop, or wimpy motor mounts.

I don't have a RTF Nobler, but I would suspect the 25 is a bit under power for it.

The props were perfectly balanced, and no visible vibration. That engine (2 of them, actually) just would not run on that airplane. The engines run fine before and since on a Super Clown and Flite Streak.

> Wow! My straight-from-kit version was 36 oz. And that was
>with a Fox, monokote, and very minimal finishing on the
>fuse. The rebuilt version with balsa wing parts is about 29.
>It's amazing, also, what little difference 7 oz (20%) made
>in the performance.
>
> Brett

That seems heavy for the Skyray, I have built two and both of them were 29oz with 25LA's and using all of the kit wood. I guess the kit would varies by a ton or at least 7oz's.

>Bret,
>I think the difference in weight is FP-20 w/ muffler,
>Monocoate, two wheels, tail wheel, and maybe a few other
>things I don't know about.

It weighed 36 with a Fox/RSM tongue muffler, single-wheel gear, etc - basically stock. I built it for my work buddies to fly at a picnic, and I figured spending a lot of time on modifying it was going to turn out to be a waste of time. The single wheel was one of those giant Williams Bros. "vintage" wheels, but those don't weigh very much. There was *a bunch* of nose weight. I am pretty sure I built it cleanly, and there was not (and still isn't) much finish on it.

Once I went to the 20FP, I took out most, but not all, of the nose weight. I added the two-wheel gear but that was close to a push. I would guess it was only +1/2 oz net change. But of course the performance went through the roof with the superior engine/prop.

Then it got wrecked at a stunt clinic, and I rebuilt it with balsa parts. Lost about 7 oz, and now it balances correctly with only a plastic spinner as additional nose weight. The only other change I have made is to make the elevator wider - but it didn' help that much. The turn is rate-limited, not lift-limited, so shortening the tail moment, or making the elevator larger and travel more, would help the turn.

>Dumb question, Brett, but you mention "only a plastic
>spinner" in your latest rendition. I assume that means you
>had a heavier spinner (and possibly weight) on the engine
>earlier.

Actually, I had a bunch of nose weight in various places. I would guess it was about an ounce on the shaft and about the same on the body. Bear in mind that the Fox had a lot of shaft weights, and the FP did not, so I switched engines at the same time as spinning mass.

I only reason I run the spinner now is to aid in crash damage. The spinner keeps the needle valve from getting bent, at least where it matters. You have to remember that I let literally *anyone* fly the airplane, including rank beginners and it gets crashed a lot. Like at least 40 times so far. It also has an air filter to keep the crah crud out of it. I have run it both with and without and it makes no detectable difference in the run (of course, we would both agree that the muffler is the limiting/regulating factor in performance even if we don't agree with the mechanism).

The combination of the balsa wing, Monokote, regular landing gear, spinner, and filter seem to make it close to crashproof over grass. It can hit nose in and take a divot, the wings flex forward but don't break or split the covering, and then it just bounces out of the hole. It's also a good thing that APC props only cost $2.00 and the factory is a mile and a half from the flying side.

With the plywood ribs and spruce spars, everything just blew into a million pieces on the first nose-in crash.

> Did you notice any difference in the run between
>the lighter spinner and the heavier weight?

I've run it at various times with nothing except the prop, the plastic 1 3/4" spinner and as much as a 2 oz "flywheel" weight. Can't tell the difference in the run (but of course can tell the CG moving around). Of course the "flywheel" effect is pretty negligible from a moment of inertia standpoint since it only has a radius of gyration of about 5/8".

I'm also unconvinced that the shaft weights cause wear, but I don't know that I have run plain bearing engines enough with large weights. to encounter the problem.

Even though it may be a minor point now, it is worth noting that I spoke with Pat Johnston concerning his 25FP-powered Nobler during the NW Regionals. Pat says it came in at 37 ounces, so it was light, but not ridiculous.

I think he had other things to say about the model, but by then I was totally engrossed by a beautiful pistol Pat had pulled out of his pickup.

Considering it is you, Sir Dirty, I presume you were engrossed because said, aforementioned pistol was pointed at you?

Really got to watch that tongue of yours, me-lad!

Anyway, I think that the FP could pull the Noblarf had I made the recommended mods to eliminate some of the vibration, and controlled the fuel foaming.

But I was sick of messing with it and the DS 40 is working a treat.

>Considering it is you, Sir Dirty, I presume you were
>engrossed because said, aforementioned pistol was pointed at
>you?
>

Larry,

My man! Of course the pistol wasn't pointed at me. After all, the sucker was loaded.

Wait until pictures show up of my special NW Regionals handle. Do not misconstrue said handle to infer that I might be so stupid as to take a knife to a gun fight.