Zwilling

Hi there,

I have read all info on the RCV SP series, and I am about to buy a 60 size.
Otto, I am very intrigued on your comment you could improve the sound into "merlin"...
Could you give me comment? I suppose they don't do this at RCV company themselves?
My main question; the engines give lots of torque, so you have to be careful on take off. Does this stay the same in a push-pull configuration? (Dornier do 335, probably a .90 front, and .60 rear)
Ever heared of prop shaft extension? I don't know if this is possible and safe (vibrations), and how long this shaft may be.

Otto Kudrna

My Feedback: (1)

Hello
I do all the modifications here in the US. I do not suggest using a 60 and a 90, use two of the same engines. If you use an extension longer than 2cm it will have to use its own ball bearings and have to be balanced for the RPM vibration. You can install a harmonic balancer on the shaft to make this automatic. If you fail to do this you will knock out the front bearing on the engine. The only concern you have to take into consideration is that you get enough air to the rear engine. This must be positive air flow meaning forced air not just the air flowing into the air scoop. The engine would overheat on the ground in about 60 seconds. You will have to make a internal fan to accomplish this. this fan can be electric and thermostat operated.

Thank You
Otto Kudrna

Zwilling

Hi Otto,
your first point: could we discuss this in private email?
Different engines: I saw the model fly with two different saito's, with good effect. I thought about ball bearings, but this has to be really balanced, I've never heard of "harmonic balancing", sounds interesting, but heavy... Do you know an URL where I can inform?
Electric fan: Otto, if you read this whole thread through, the fan is becoming very famous... I would like to see a pic of it; though it all sounds very heavy, and I can't use this at the tailside...
If someone has an option, I would be very happy to hear about it.
Overheating: I thought I had a solution during flight with an air channel through the airscoop, via engine to outlet holes, but didn't think about the so fast overheating on the ground; doesn't this apply for the normal front engines too?
thank you very much, good research is half the work...

Otto Kudrna

My Feedback: (1)

If you use different size props on the front and back props, I guess you could use a smaller engine in the back. I know that the 234 used the same size props, so I presumed that you wanted to do the same on your model. The problem will arise though, because you will be using such large props ,much larger than on a Saito, that the one engine will cause undue drag for the other engine and will result in reduced performance for both. A Saito turns a much smaller prop with a much lighter pitch and this fact is nowhere as evident and critical.
When I spoke of a fan for this application, I was speaking of using a small electric motor like a Graupner Speed 280 turning a small fan blade to move air threw the upper cooling scoop. The rear engine will have to be baffled very carefully to provide the right cooling air flow. I would study the air flow baffles on a full size radial air cooled engine for this. To see how the air flow from one cylinder is prevented from flowing over the cylinder behind it. The front engine will be exposed to airflow at all times due to the radial opening in the front of the aircraft, and the prop directly blowing air threw it. You will have to create a rear exit hole for this air, due to the fact that the cowl flaps on the aircraft are much too far forward. The cowl flaps were there to accommodate a radiator in front of the engine, not to accommodate an air cooled engine, but the radiator opening lends itself nicely to the installation of the RCV engine. You will also have to create a baffle to flow the air fro top to bottom. If this is not done than the fire wall will prevent the engine from cooling correctly also.

Thank You
Otto Kudrna

Otto Kudrna

My Feedback: (1)

I would like to discuss this subject in more detail on the new web sight www.rcscalebuilder.com in the engine section. I think that the guys on that web sight would be very interested in this entire subject. This is a very good subject and should be shared with the scale crowd who can use your questions to their benefit. I will see you over there.

Otto

Zwilling

Hey Otto,
you have touched some very good suggestions here. For the fan I see an opening that can be discussed wiht my friends flying indoor. They are using very light, self winded brushless motors (taken from an old cd-rom).
The front engine is causing no troubles I think, that couldn't be overcome. It is the rear part that needs more thought. I need to loose some wheigt over there. Anyway, I will start this the soonest next year, so I hope for more feedback from everybody; as I said before... good research is half the work!
thank you and see you at scalebuilder.

rcgreg

I am also a fan of the DO 335 and bought two of the 60 size RCV for this application. I researched the rear engine installation and came up with the following - but as yet untried - method of mounting the rear engine with a shaft extension. I went to my local Hobby store and chatted with the high power boat chappies. They all use a shaft extension in a high stress, high RPM and very cowled in application. What they suggested was to use a power boat cable (steel cable with a teflon shell), attach one end to the crank of the engine with the same brass piece (The name escapes me at the moment but it looks like a drill chuck). the cable doesn't require perfect alignment - in fact in power boats it tends to not be. On the prop side I plan on using the front crankcase from a forty size engine, removing the flywheel from the crank, removing the carb, and JB welding the end of the cable into the hollow crank. This will provide two bearings at that end, along with a method to attach the prop and the front crankcase even has mounting holes !. Using a small quantity of lube oil in the crank to lube the bearings then seal the carb opening with a plastic cap. To provide cooling - simply use a helicopter fan on the engine crank ahead of the ferrule (Hey I remembered the name !). Baffle to ensure a good flow in from the rad duct opening and continue the baffle with a duct to exhaust out the rear cowl flaps. I have all the parts - just need to find the time. The model is the 80 inch version of Art Johnson's plans.

Otto Kudrna

My Feedback: (1)

Hi guys
About the flex shaft idea: I used to run boats also and am quite familiar with them. I have seen one of my boats catch a piece of string in the prop and load up just a little. The affect was the same as a rubber band snapping in a stick and tissue model, literally explosive. The cable would have such unbelievable back lash the first time you would advance the throttle on that big prop that it would collapse the back of the airplane or brake and cut it in half. You must remember that you will be turning a 16" prop which weighs about a half pound or more not a little 2"-3" prop which only weighs an ounce or two. You would have to use a cable of a MINIMUM of 1/4" diameter. You are better off using a large diameter chrome molly tube in its own bearings to eliminate any potential vibration. you want to make sure that you do not tear out the front bearing in the engine or worse yet, split the front housing in half.

Otto Kudrna RCV Tech Support.

rcgreg

Hi Otto, Yep thats what the boat guys told me too, which is why I have the 1/4 cable, I was thinking about placing the cable assemble inside a brass or alum tube to prevent just such an occurrance. Of course a steel drive shaft with sealed universals would be just the ticket - but where could I find such a thing ?

Incidentaly the full size A/C used a drive shaft within a shield arrangement too!

Thanks for the advice - I just gotta find time to build it !

rcgreg

The Don Smith Plans I have do show the engine installation, a couple of 3W 24's I believe, placement of servos are your choice, He did answer my emails, I got the canopy and both cowls from him and Century Jet makes a custome set of retracts - last quote from them was $419.00 US a year or so ago - maybe more now - they never seem to go down !
The Al Masters plans are more detailed and are answer more questions, but if you have ever scratch built before the Don SMith plans should not be a problem.

Wing loading on the Al masters was not a problem, this A/C has a huge wing - more area than most twins of the same WS, the flaps work beautifully - no pitching - it just slows down as you deploy them. I built and flew the smaller Al MAsters DO335 with a ys 45 in the front and a .25 in the rear and that was obscenely over powered - can't image it with a couple of Saito 4S - YIKES !
Its only rated for two .25's and the YS could fly it just fine by itself.
The 80" one flies well enough on the YS 120.

Damnathius

No one has mentioned that the RCV SP design might not be suitable for a pusher configuration. Consider what exactly will happen with thrust pushing back on the shaft instead of pulling out on it.

Dave

Zwilling

Greg,
I replied to you about how much postage was for Belgium, europe?
Hans

rcgreg

I talked to the factory when I ordered the engines, outlining the use of one as a pusher and they didn't see a problem with that at all, in fact they even put me onto a chap who custom made me a 14 X 14 pusher prop !

Damnathius

Interesting... Another fellow here at RCU ruined his gears by running it as a pusher. I'm sure the factory knows best. I just have to wonder if they've ever tried it.

Good luck.

Otto Kudrna

My Feedback: (1)

Hi guys
The push configuration will not have any effect on the engine mechanically. The prop shaft / cylinder, is locked in the front housing, and cannot move. In fact the front housing expands more than the cylinder and moves the gears farther apart if anything. This is with the exception of the older 60SP engines which were locked against the front bearing and could move some, but not enough to ruin gears. The most pressure you could put against the gears in this situation would be the weight of the aircraft. This is nothing compared to the pressure placed against the gears in a tight engine. The only thing that might be damaged in a tight situation would be the two small crank shaft bearings. You would cause more damage if you ran the engine in very cold weather, which could cause insufficient thermal expansion. The only concern I would have in a push situation would be the flow of direct cooling air, which may be lacking in this situation, nothing els.

Otto Kudrna

Square Nozzle

My Feedback: (1)

Hi All,
I am reading this commentary with great interest. My installation of a 90SP in a P6E is minor compared to what you are attempting. I too am in model boat racing and am very familiar with flex shafts. Otto is correct in suggesting the 1/4" flex shaft. Also, I suggest you strongly consider housing the shaft. Another option is the style of drive shaft that we boaters used before flex shafts. It's a solid steel shaft with universal joints at each end. I believe Dumas Boats still sells them. Look them up on the web and see what they offer. Good Luck with your endeavor.

rcgreg

John, how did you find the wear factor on the universal joints with boats - are they lubricated in any way ?