Tango Juliet

I'm trying to get more speed out of my O.S. 25 FX mounted on my Combat Gremlin. I currently have a 9x6 prop installed. A local club member suggested removing the baffle from the muffler, but that only gave me mixture issues. I could run about two ounces through it before it would die, and it was very sluggish. I'm not sure which way to go with prop sizes. Suggestions?

stg61abc

Hi TJ, the 25 FX is a nice sport engine.
Taking the baffle out from the muffler will help, but it also may change the carburetor setting, you may need to re-adjust it.
Try a smaller prop and let the engine rev more. You could experiment a 8X6, 8X7 as a start point. But to determine which is the best prop the best path is experiment various props to find the best to your use and needs. There are various other pylon props that may help you in the speed range, mainly some APC racing....
You may also cut your present 9X6 to 8.5 or even 8 diameter in order to gain some rpm, dont forget to balance it!
nice flyings

Tango Juliet

stg61abc - Thanks for the reply. I've never strayed from the Mfg's suggested prop ranges for a given engine, so I didn't know which way to go without damageing the engine. I'll have to buy a few props and see what happens.

flygilmore

build yourself a mousse-can pipe with Macs header and see a tremendous improvement!!!!

Tango Juliet

Yes, that would work, but this is on a Combat Gremlin and that isn't practical. I'm not flying a racer, I just need more speed and power in manuevers so I can fight in the horizontal and not always in the vertical.

badger5964

My Feedback: (6)

We fly combat nearly every weekend with phencepost airplanes. The OS 25FX is the most popular engine. No one runs a 9x6 on this engine. I'd guess 65% of fliers run a 9x4 Master Airscrew. The rest spin 9x5 Master Airscrews. Don't be afraid of it. All the dirt combat engines digest will ruin it long before the extra rpm. You said yourself that you need more speed and power in manuevers. Letting this engine rev up gives you that. It is the same old trade off: the 9x4 pulls harder, keeps the speed up in tight extended turns but is slower on a straight run, the 9x5 will make you a little faster in a straight run but the plane slows more in manuevers. I experimented with a 10x4 and found it to pull the hardest, nearly vertical performance with a light airframe. However it is a very slow airplane. When I broke the last one I never bought anymore.

Mouse can mufflers can be purchased from a company called air scharnell at [link=http://www.airscharnell.com]www.airscharnell.com[/link] $35 and shipping. Will add a thousand rpm. I removed the baffle in the OS muffler, but put it back. I could not get consistent runs. A GMS 25 muffler fits too and adds a couple hundred rpm and is only $8. I never put it on a tach but it sounds like it. The mac one peice muffler adds rpm similar to the GMS but at $35 your better off with the mousse can as it adds much more rpm.

Scott
[link=http://www.fayettevillercclub.com]www.fayettevillercclub.com[/link]

scott17

Very good info right there!!![8D]

stg61abc

Hi TJ, dont be afraid to rev your engine a little more.
Is important that your props are all very well balanced, and you must/may use a fuel with a "decent" oil content, at 20, 22% by volume. If you use a fuel with a lower oil percentage, add castor oil accordingly.
The 25 FX develops its maximum power of 0.84 HP at 18.000 rpm, as per OS Max website. So, consider this as a maximum limit in flight. I guess that if you launch your engine at 15.000/16.000 revs in the ground, it must give you around 18 K in the air giving you some very nice speed.
about props, i strongly suggest you to use ones that have strong hubs, because the high rpm use, or even racing props that are more reinforced than the common ones.
Great success pal..

jaka

Hi!
More nitro (up to 15%) is an easy way of getting more power. More nitro than that and you have to add a head shim (0.1mm) or two.
Second always use APC props! Those are most of the times best for most sport flying purposes. I would try a 9x5 or 9x6. 8x6 -8x7 if in a smaller model (1m in span).

speedster 1919

My Feedback: (3)

The MAS GF/2 made a 8x5 combat prop for .25 . These props are tuff as nails. You probably have to go to GF/3 8x5..........

Ed_Moorman

My Feedback: (1)

If you want to spend some money, get a Jett tuned muffler for the OS. I have 2 on a twin. Goes pretty good.

vicman

My Feedback: (10)

We use the .25FX in quickie racing. The spec prop is an APC 8X6.
Before doing the Q25 thing I experimented extensively with everything from 6X8 to 10x4, wooden Zingers, MAS, and Scimtar tips. The APC 8X6 gave the best all around performance in my tests. I get a nice fat running 16K if I lean a little more I can see 16.5. Baffle in. Obviously a Jett pipe or similar will help things out a bit.

Go with the 8X6 APC you will get plenty of speed in the straight line and enough grunt for a sharp climbing turn. I run into more fuel deliver problems than power problems when turning hard around the pylons.

[8D]