Speedracer2112

Shes great. More flights than I can count now. Well over 400. Shes a for stroke tho.

Have fun and build her right and you will have just as many if not more.

SR

Fidelity101

My Feedback: (13)

Did you build with those 95s? How was balancing, AUW and wing loading?

Mustang Fever

My Feedback: (2)

I relocated both throttle servos when I put a pair of 55AX's in mine. Was really simple and resulted in short, direct pushrods. Synch is perfect at wide open, and within 200rpm at idle, so I'm well pleased.

Fidelity101

My Feedback: (13)

I just bought a set of retrace only to find out I bought 85 degree mains and the 105 degree nose setup. Apparently I need 90 degree mains and 100 degree nose. Does anyone know if I can change the throw amount myself...or if Robart would be able to adjust them? I bought the retract set for the top flite mentor t-34, thinking they would be the same thing.

RCGuy41

Fidelity101,

You might be able to exchange the T-34 gear for B-25 retracts; I would talk to your local hobby shop first then to Robarts. As for the gear, all throws are determined by the amount of metal removed from each side rail that the actuating camb travels. Talk to Robarts they might be able refurbish your T-34 retracts into the Top Flight B-25 retracts you need, at a cost to you of course.

Fidelity101

My Feedback: (13)

Thanks RCGuy!

My last step before building starts is picking the right engines.

http://www.osengines.com/engines/osmg0900.html
Powerful, cheaper than 81s

http://www.osengines.com/engines/osmg0981.html
Good performance, good fit, expensive!

I know the 81s pull the b-25 around well on 12x6x3 props. Can anyone comment on the 95v installs and performance? I'm guessing the opening in the cowl will have to be wider and the engine will stick out more. Are there any other issues?

Mustang Fever

My Feedback: (2)

The OS 55AX's on mine turn a pair of 12x6x3 MAS props at close to 11,000 RPM on 15% Cool Power. That's good for over 60mph and a total of almost 20 pounds static thrust. Two of them go for $340. (List)

The four strokes you referenced total between 600 and 740 bucks. (List) Am I missing something here? [:-]

Fidelity101

My Feedback: (13)

My Cessna 310 has twin 55s. It's a great performer and fun to fly. But I'm really going for the four stroke sound with twins. Better sound, better fuel efficiency.

I am curious about the use of the 95v in the b-25. I'm thinking the prop would have to be bigger, but that probably affects ground clearance. I would get the os 81a, which are 600 for two through tower...but that's a lot of cash! The 95vs are 50/motor cheaper

Mustang Fever

My Feedback: (2)

I guess I don't understand why modelers think four strokes sound more realistic.

The Wright R-2600 Twin Cyclone used in the B-25 turned at 2600 RPM, which works out to 9100 exhaust pulses per minute per engine half, a half being one row of 7.

A two stroke turning 10,000 rpm or so comes a lot closer to that than any four stroke.

Weeso

As you can see it is an ASP 91 engine which fits easily with no modification

Weeso

ASP 91 4stroke

Weeso

Tuning the engins
http://youtu.be/rz3hpzCp2pk
http://youtu.be/QLTjeRm8AvY

jdhughen

My Feedback: (1)

"I guess I don't understand why modelers think four strokes sound more realistic."

Unfortunately your math doesn't add up to actual overall "sound" properties. In sound properties the overall engine "Sound" is the result of many things besides "exhaust pulses", a big one being RPM. another is the fact that with a 4 stroke there are 2 prop revolutions per "exhaust pulse". Added to that is the difference in prop noise. Then there is torque differences that allow the 4 stroke to produce the same thrust at a much lower RPM. This is why most observers describe a 4 stroke producing the same amount of thrust as a "purr" while a 2 stroke as a high pitched "whine". Now weather or not one appeals to you more than the other is a different question. But IMO the purr of an 7 cyclnder 4 stroke turning 2800 sounds nothing like a 2 stroke rev up to 10-12k

A ground based analogy would be the difference in sound properties of a 2 stroke Yamaha Ninja bike and a Harley. While we would all love to have mini Cyclone radials turning 2800 in our "model", that's not realistic either. so it's just a preference of which you feel sounds more realistic or pleasing in your WWII bomber, a ninja or a Harley

Just my $.02

gregory.aldrich1

My Feedback: (9)

I have flown my B25 ten times. Unfortunately, because I was so absolutely busy this summer, I only had the chance to fly her last season. Anyway, a friend of mine has the same bird and we have met a few times to fly them together. He uses 2 stroke power and i use 4 stroke. So I have heard both in person. Besides that, I have obviously heard both in birds of other kind. I personally prefer the sound of the two Saito 82b's in mine, but his bird sounds cool too. I think I have always likes the four strokes and generally, that's what I put in most of my birds. I have some two stroke planes and I have had a gasser as well. They all sound cool complared to electrics, but then I have several of them as well. It's all about preference, but I have to say, the twin four strokes sound pretty cool popping off on the taxi ride out and when you run em up on the roll out, they really hum. Just sayin.

Fidelity101

My Feedback: (13)

I've decided to go with OS 95v engines and 13x6x3 master airscrew props. Break in props will be 14x6, assuming I can use that big of a prop on the b-25.

Mustang Fever

My Feedback: (2)

The 14's will leave you without about 1.5" ground clearance, using the fixed gear and factory tire sizes.

jdhughen

My Feedback: (1)

If your using robart retracts with robostruts and land on grass you might have 14" prop tips dig in on anything but soft touch downs Depending.

My B-25 is about 3 years old now and I'm thinking of a partial rebuild getting new nacels and converting to 4 stroke gas. I currently have satio 72s and while they are sufficient, if the grass gets a little tall it take every inch of the runway to get her up. I'd like to go with the satio FG 14B gas which will give me a little more power. Is any one using the new electric robarts ? Is robart making a elec replacement for the B-25 ? I've never been a fan of air retracts as they are not very realistic motion and are pone to problems. I've tried electric retracts on some of my smaller planes and love them. What I want is a converion kit, not a total replacement. anyone seen this for the B-25 retracts?

Joel

Fidelity101

My Feedback: (13)

The electric conversion set should be available on Robarts (and Towerhobbies) website shortly. It's a pretty simple do it yourself conversion and it should be under $300 for all three drives and the control board.

I'm trying to convert my robart t-34 mentor gear to b-25 electric gear. If anyone needs some robart gear for their t-34 mentor, it would probably be easier to sell it and buy new gear. I'm thinking I will need about $170 for the conversion, which is half of the price for a new set of b-25 gear.

Also, has anyone flown with the 13x8x3 props? I generally fly in grass and sometimes it's tall grass. If there is any concern, I need to cancel my th order soon and just wait for a deal on the os 81a engines.

ronbell

Joel - check with Wingspanretracts.com I bet they have the conversion kit you need, or they can convert them for you.

Fidelity101

My Feedback: (13)

Robart will be selling, if they don't already, the do it yourself conversion kit for 270. Includes the three drives and the control board. That's what I'm going with.

Fidelity101

My Feedback: (13)

I'm trying to buy an onboard glow driver setup from sonic tonics and I need to know the length of wire required from the glow plug to the center of the fuselage or any logical place where the main box will be installed. Has anyone done this already? I haven't started the build process yet, so a rough estimate would be greatly appreciated.

gregory.aldrich1

My Feedback: (9)

I used two sonic tronics glow drivers in my B25. I mounted them on the hard framing of the nacelle underneath the fiberglass structures. I didn't need any extra length of wire other than what was included in the box. The only problem I had was adjusting them after install because of them being mounted under the fiberglass, so I drilled holes to access the adjustments and put american flag stickers over the holes to hide them.

Fidelity101

My Feedback: (13)

Thanks Greg. I read through the thread and noticed one person commented about using 40" wires for their twin engine setup. I have two Sullivan on board glows for my Cessna and I did what you did, which is to put one in each nacelle. It works, but I'd rather have one switch to deal with. I also hope to do that mod I read about early on in which people made a hatch using the front cockpit windows and installed all the switches in there.

gregory.aldrich1

My Feedback: (9)

Fidelity, you can get a McDaniels glow driver that will run both motors out of one box and have SonicTronics create longer leads for the actual connrection to the ground and glow plug. I think anyway. Having double boxes is no problem though because once you set your come on setting and switch the reverser, your done. I wanted the ability to adjust even after placing the fiberglass covers, so I drilled the holes. Like I wrote though, a couple well placed american flags and shazaam! the holes are gone. lol

Fidelity101

My Feedback: (13)

Thanks Greg. I'm working on ordering a mcdaniels with 48" leads right now. I'm also thinking about the twin sync...anyone else use it? I was going to go with two optical rpm sensors via my hitec telemetry unit, but I'm not sure if I can activate a warning system using that. I wonder if the voice system would send a warning if one rpm went very low?