Noblarf/OS 25fp progress

Well, all it took was changing to a new engine, opening up the cowl cooling holes double the size, opening up the tongue muffler holes to 4 times the size, and going to a smaller prop. I finally got a really good engine run!

That's the good news. The bad news is that the plane feels like the available power is marginal. The weather was perfect, cool, dry, very light breeze. I don't think I could do a pattern at higher altitude, warm day, heavy breeze, etc.

I will pull out the venturi blockers (currently .218 dia), and try even higher nitro (currently 15%), but I am concerned that I will have to switch to my Double Star 40 to get the performance I want. That will probably require yet another tank, as the one I made has only 3-oz capacity (way more than enough with the .25, btw).

Larry,
Isn't that venturi size small? I can't recall offhand what the canonical size should be.

Larry,

I'm a little surprised by this. To the point that unless your DS40 is known to be a good 'un your time will be better spent coming to grips with the 25FP.

I remember Pat Johnston flying his scratch-built Nobler with a 25FP and it hardly lacked for power. On the other hand, Pat builds quite light models, as confirmed by his leaving the outboard panel in close proximity to his tool box, thus giving us Lookie Louies a peek inside.

Pat was using a B-Y&O 10-5, albeit one which Clarence Bull had a hand in producing. Personally, I am still trying to come up with a prop for the 25FP that works better than the APC 9-4, although the application is an ARF Flite Streak for sport flying.

Larry,
Try replacing the tongue muffler with a Mac's muffler.
Derek Moran

>I will pull out the venturi blockers (currently .218 dia),
>and try even higher nitro (currently 15%), but I am
>concerned that I will have to switch to my Double Star 40 to
>get the performance I want. That will probably require yet
>another tank, as the one I made has only 3-oz capacity (way
>more than enough with the .25, btw).

Uh, .218, even with an OS spraybar, is like trying to suck a watermelon through a soda straw! The stocker is marginally too large at .257(w/135 spraybar) but a filter will cure that. Maybe try something like a .250 without a filter. Besides, I thought you were running pressure so you could run any venturi you wanted?

Yes, yes, the venturi is too small. As Brett points out the stock venturi (really too large) is .257 which gives a choke area of .01717916 inch. I usually recommend a .238 for this, or .242 if you want to push it. The .238 gives a choke area of .012358. Now, you are using a .218? A .219 (standard bit) bore gives a choke area of only .0081 in comparison. so:

.257 venturi gives .0172 choke area
.238 venturi gives .0123 choke area
.219 venturi gives .0081 choke area.

This is less than half of the stock size and only 2/3 of what I would recommend. And this is where your power has gone. It is like running the engine at half throttle. You probably didn't need to open the holes in the muffler. You needed to open up the venturi.

I agree that the venturi is too small, and will open it up to the next size. When I got a full-bore run, the plane was running 4.8 second laps with the modified 10x4 Zinger. But, it boggs down in the turns, and the run wasn't reliable. I think I will go to a stock venturi but with an ST needle so that I have a bit more positive control.

Currently it has a MA Scimitar 9x5 on it, and it ran great. Even worse bogging down in maneuvers, as you would expect.

I have a MA 10x3 that looks interesting, and should unload the engine, while giving lower top speed and more pull.

However, even as is, the plane flies better than most others I have ever had <G>

Larry,

I missed the girlie venturi size, inexcusable as I am currently spiffing up/converting a couple Swap Meet-sourced 20FPs for use on a (scratch) Super Combat Streak and an ARF Flite Streak.

However, it has been made very clear numerous times that the APC 9-4 is the prop of choice for the 20/25FP. Until you both free up the intake and use the suggested prop--as a benchmark if nothing else--in your entry into the world of "big" engines, you will continue to cause me to pull my hair out. Oh, wait! Too late already...

I didn't get the performance I was looking for on my Gieske Nobler (45 oz) until I went to an 11-5 and .280 venturi. I have a stock McCoy NVA and a hybrid McCoy 40. I run about 8400 rpm.

Larry Renger wrote:
>
>I have a MA 10x3 that looks interesting, and should unload
>the engine, while giving lower top speed and more pull.
>

APC and Graupner both make 10-3's. Either will most likely work better than a MA 10-3. Easiest sources are the APC factory and Hobby Lobby, respectively. Steve

>I agree that the venturi is too small, and will open it up
>to the next size. When I got a full-bore run, the plane was
>running 4.8 second laps with the modified 10x4 Zinger. But,
>it boggs down in the turns, and the run wasn't reliable. I
>think I will go to a stock venturi but with an ST needle so
>that I have a bit more positive control.
>
>Currently it has a MA Scimitar 9x5 on it, and it ran great.
>Even worse bogging down in maneuvers, as you would expect.
>
>I have a MA 10x3 that looks interesting, and should unload
>the engine, while giving lower top speed and more pull.
>
>However, even as is, the plane flies better than most others
>I have ever had

Might instead try an APC 9-4. The key to making these guys run is to unload them to the maximum extent possible. A 10-3, particularly an Master Airscrew, is both draggy and has to be spun fast - a deadly combination.

Brett