tom3097

My Feedback: (2)

I have an OS 40 VR-P that I plan on putting in a Screamin Demon.Unfortunately I can't find much info on the engine.I ran it yesterday with 15%,APC 7.4x7.6 carbon prop and a Macs fuse top header and 7.5cc unmuffled pipe.I started with the header full length and cut off 1/2 inch at a time until I ran into a snag and ripped the silicone coupler.I did get 20,400 out of it but I think the pipe needs to be shortened more.I did find a little info on the engine from a response to a post my friend put up about the engine before I bought it.The post mentioned that it was considered an F1 engine and they were designed to run on high nitro and open exhaust.Any info on this engine and also suggestions on servos for the Screamin Demon would be greatly appreciated. http://www.rcuniverse.com/forum/m_73...tm.htm#7363959

combatpigg

My Feedback: (3)

1/2" cuts are pretty aggressive unless you are pretty experienced.......[:-]!!
If the rpm is steadily rising, carry on....but 1/4" chunks are a little bit safer to try.
Just about any full sized servo works fine. I've put close to 100 flights on a pair of HS 225s which are considered "minis".......they are also beginning to get a little sloppy, the output shafts have a little wobble. My usual flight pattern is pretty gentle, bad launches and less than perfect arrivals at my rough field can't help.
Vibration takes it's toll, too.
So, any reputable servo works for me.....not sure about that Hobby King stuff.......

Be real careful with ground running and looking for extreme rpm.....monitor the exhaust spray and be brief as possible with the runs. Let the tuned length do the work, once you hit resonance the fuel delivery system should kick into high gear to keep up with the extra fuel demand...always be prepared and mentally rehearse the quickest and safest shutdown by pulling or pinching the line from behind the engine. Never do any knee jerk movements that will get you sliced up.

I'm not sure what your own experience level is, but I wanted to throw in the safety talk to any new comers looking in.

tom3097

My Feedback: (2)

I actually didn't cut a 1/2" off the whole way.I did make smaller cuts towards the end.The HS 225s are what I use in my clipped wing Predator2 which has a Nelson Q500 LS.It's fast and the servos have held up with that but I wasn't sure how they would hold up in the SD.

combatpigg

My Feedback: (3)

Figure that 3/16" to 1/4" is enough flap movement to fly pretty aggressively. So having the push rod in the inner hole at the servo arm and about 1 1/8" or so up on the control horn gives the servo good leverage and holding power.
The only servos I would recommend not to use are the JR Sports...I've never seen 2 servos wear out so fast.

Sessholvlaru

Use Hitec HS-5245MG's. They're high speed digitals. Can't go wrong. I am also curious as to prop recommendations on the VR-P. We tried an APC CF Q40 prop that did not have the pitch marked. Are these good for this motor? This SD will compete against my Magnum w/ Q40 power.

tom3097

My Feedback: (2)

I did verify what prop it was that I ordered which was the 7.4x7.6. Oh and compete might not be the correct word

Sessholvlaru

My mistake. The only info I can find is this: http://www.mh-aerotools.de/airfoils/...F3DEngineOSMAX

If we go by what the author says, our readings of just over 20k on the ground might be reasonable.

tom3097

My Feedback: (2)

Yeah but if you look at that post he was talking open exhaust and 60% nitro

Sessholvlaru

Here are some shots of the motor and pipe. Mind you the coupler blew out on the last run. Any suggestions on pipe//header length?

combatpigg

My Feedback: (3)

To the naked eye that exhaust system is looking pretty short.....[:-]
I would eliminate some meat from those prop blades...I'll bet I remove 30% of the original material overall from hub to tip.
The engine might thankyou with better unloading.
I've found that a 7 inch diameter prop will launch a Demon OK at about 22-23,000....once you have the plane dialed in to practically fly on it's own.

MJD

My Feedback: (1)

The center ribs of the SD are laser cut to fit HS225's. They work. There are mightier and fancier servos for sure, but they work alright esp. on 6V. The torque requirements are less than you imagine.

Also, many fast fliers recommend against using MG servos on high speed/high rpm stuff - the metal gears self-erode under conditions of buzz faster than plastic. You can use MG's but monitor the geartrain for slop. Some may not have any issues. Others definitely have.

MJD