glowplugboy

My Feedback: (17)

You also might want to go down to a 9 X 5 prop. In Open B Combat, I run a 9 X 4 on my Magnum .28's with tuned mufflers. With this set up, they unload to around 18K. The Magnum .28 is one of the toughest little mid size displacement engines I have ever run. They get hot. REAL hot. Just make sure yours is not too lean.

jib

GPB,

Thank for the response. It's nice to hear they are tough, but do run hot. I run it on 18% oil and 1/3 of that is castor.

I hear you on the props. A pair or 9x5's (one APC and one MA) are already in my flight box, balanced, tips painted white and ready to go.

This engine is going to get some fuel pumped through it this weekend to insure it's fully broken in and get a prop selected, as it will be powering a coro-stang in Open B next weekend, Sunday the 23rd.

jack

Sport_Pilot

When I think about it a zero pitch prop will work like a centrifugal fan. That is air moves in near the center of the prop from both sides and is flung out. So that is moving mass and is work. But strictly speaking from physics definition friction is a loss not work, but it really doesn't matter, both cause the prop to slow down on the ground.

jib

tomlee,

I agree 100%, but the problem was that I bought the engine used and did not realize that it had not yet been fully broken in, so it was running hot. I treat any used engine as if it's new, but I was surprised at how hot the little guy ran, and was concerned.

Jack

Sport_Pilot

Your instincts and experience is not really incorrect. We have been arguing a very minor point about what consists of work, which really doesn't make a difference how the engine acts. For example you can load an engine up on a dyno with a disk brake or you can put a pump, or electric motor on it. Ether way you can measure power. But with the pump and motor the work can be measured to get a more exact reading.

glowplugboy

My Feedback: (17)

Ah, ha! Yep, if you are using this engine with the stock muffler in Open B, you will want to come down on the pitch. A 9 X 5 will help you turn faster with reduced straight line speed, whereas a 9 X 4 will give you more straight line speed. I prefer the 9 X 4 and the accleration it provides. I would also suggest opening up the muffler exhaust stack to reduce back pressure. After all, you are interested in high end and idle is not really an issue in a combat engine. 15% nitro with 18% oil is good for this engine. You don't really need any more nitro than 15%.

jib

I've thought about opening up the exhaust a bit, but that can wait. I'd like to play with radiusing things and improving the flow inside the muffler first. My son and I are just dipping our toes into combat. I have time to nail every last 1% of HP. Right now it's about keeping the planes easy to start and dependable flyers.

I'm confused by your 9x5 vs. 9x4 discussion. When changing props 10x6 to 11x5, I understand the relationship, but I'm not sure I follow why a 9x5 will be slower than a 9x4, unless the .28 can't get the 9x5 up to full RPM.

Jack

NM2K

My Feedback: (14)

We assumed he did that before he bothered writing RCU about it. But you are right, sometimes folks don't RTFM.


Ed Cregger

jeffie8696

Are you allowed to port and polish ?

glowplugboy

My Feedback: (17)

The 9 X 4 allows the .28 to fully unload and has less torque whereas the 9 X 5 has more torque which will allow faster turning. In Open B, speed is your friend, and any speed advantage you can have is good. In serious Open B competition like the Texas Nationals, some Open B models will be flying near 100 mph. Engine mods are allowed, but reliability is more important than redline performance.

jib

In Open B, speaking in very GENERAL terms, I don't believe there is much of a restriction beyond the overall weight of the plane, wing area and engine displacement.

Jack

glowplugboy

My Feedback: (17)

The restrictions are as follows: 3 1/4 lbs. weight limit, .30 maximum displacement. After that, kick the tires and light the fires....

draftman1

3.1 Combat will be broken down into these classes:
Class Maximum
Weight
Electric Maximum
Wt
A = up to .15 cu in 2.5 lbs 3.0 lbs
B = up to .30 cu in 3.5 lbs 4.0 lbs


any engine mods are ok as long as it isnt over .30 the open really relates to the airframe style

PacificNWSkyPilot

My Feedback: (19)

Jack,

This is not illegal. It works great. Find the largest drill that will fit into your muffler's pressure tap, and then take the next size up and drill the tap out to that.

See if it doesn't give your motor that little something extra.

Oh, and WORK? That's what it amounts to reading all these nutso posts about what work is and isn't!

AAAGH!

LOL

Jim

fizzwater2

My Feedback: (61)

I like this one....

jeffie8696

Port and polish it, it will sound totally cool when you do. My GP42 sounds totally badass now.

jib

Yikes! I have to much to do already, but I'll add these to the list.

Thanks everyone!!!!!!

Jack

PacificNWSkyPilot

My Feedback: (19)

Port and polish it? That's cheating. Also a LOT of work for a small cheapie motor!

Jim

jeffie8696

It is fun and it turns a cheapie engine into a screamer.