Converting OS 46VR Marine engine to Air - RC Groups
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Jan 23, 2013, 07:10 PM
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Converting OS 46VR Marine engine to Air

Is it possible to convert an OS 46VR-M (marine) to aircraft use? The goal is to make a rear intake/rear exhaust airplane engine -- to run open-faced.
I have an OS 40VF aircraft engine I can use for parts. I have not purchased the marine engine yet.
I would really appreciate advice from anyone who has done this.

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Jan 23, 2013, 07:59 PM
Two left thumbs
Since the first 7.5cc ducted fan engines were converted marine engines, going the other way should pose few problems. I don't own a 46VRM, but do have a couple of D/F versions. I think the port timing is identical (check the OS parts list to be sure). You'll need an air cooled head and probably a couple of additional head gaskets. If the crank is too short you can use a prop adapter such as was used for D/F engines. Buy some good single strand carbon fiber props - on a pipe it'll turn over twenty grand in the air.

Jan 23, 2013, 08:48 PM
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Thanks Geoff.
I see the 46VF and VRM take the same crankcase. Does that allow me to rotate the case 180deg to get the exhaust to the rear?
Then will the prop drive washer, prop, and nut off the VF fit on the VR crank shaft?
I will be running with no pipe and no muffler.

Last edited by rbush; Jan 23, 2013 at 08:49 PM. Reason: spling
Jan 23, 2013, 08:57 PM
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downunder's Avatar
You'll need a head from a VF46, or possibly another 46 from the same era, because the 40VF has a smaller bore. Apart from swapping front and rear ends around you'll need to swap around the conrod so the chamfer on the rod big end faces the crankweb. But that also means you have to swap around the piston because the wristpin has one end blanked off and the blank faces the boost port.
Jan 23, 2013, 09:46 PM
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downunder, thanks for the details.

Can I remove the water jacket part of the head to allow air cooling? Maybe do some machining?

I'm thinking the project is a non-starter if I can't put a prop on the crankshaft. Any ideas?

Jan 24, 2013, 12:51 PM
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Hemikiller's Avatar
Crankshaft is shared by the VR-M and VR-DF versions, so it might be long enough for a standard prop driver and washer/nut. You'll need to remove the flywheel to see what'll be needed. If it's too short, a "reach through" type prop nut is available from Tru-turn for around $10. I use them for my YS heli to air engine conversions.
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Jan 24, 2013, 01:36 PM
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Thanks for the good info.

Jan 25, 2013, 08:03 AM
Two left thumbs
You'll need to ream the prop to 3/8" when using the Tru-Turn adapter. (I'm glad Hemikiller remembered who still makes them!) I think Fox still makes the reamer to do the job fairly precisely. It used to be pretty common. Dooling and McCoy racing engines all used this setup.

Why no tuned pipe? That's where the power is! Annnnnd, it sounds soooo cool when they come on the pipe!

I may have a scrap .46 VRDF if you need parts.
Last edited by GeoffinIN; Jan 25, 2013 at 09:37 AM. Reason: Add photo
Jan 25, 2013, 03:03 PM
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Good photo. I may have one of those adapter nuts. I definitely have the reamer.
Originally Posted by GeoffinIN
Why no tuned pipe? That's where the power is!
Good question ---
I need an engine for Control Line Carrier competition, which has two main tasks:
1. Standing-start half mile. I'm looking for 110 to 120mph average speed.
2. A half mile of the slowest flight possible. I'm aiming for 5 to 15 mph. This is done at 60 degrees angle of attack and requires dependable, precise throttle control.

The rules allow a pipe with 10% max nitro, OR unlimited nitro with no pipe -- my choice.

The planes weigh 40 to 60 ounces and are scale models of navy carrier aircraft. Wing spans about 28 to 36 inches. The models and control lines are very draggy. In the flight score, low speed is weighted heavier than high speed. Here's a link with pix:

So I'm looking for the best compromise. Dependable throttle is necessary when one is flying below stall speed at 2 feet AGL in a varying crosswind (hanging on the prop.)
So I guess a question is how does the 46VRM idle and transition?
Jan 25, 2013, 07:30 PM
Two left thumbs
Ahhhh.... Well, that's a problem! This engine is timed for very high rpm, but you need a prop much larger than it will handle. Unless they'r now flying ducted fans in Navy Carrier, and you fly something like a Grumman Cougar (Gobs of wing area for slow flight) I'm afraid this engine's impractical.
Jan 26, 2013, 02:41 AM
Registered User
I was afraid of that. My experience is that anything smaller than a 9" dia. prop hurts your acceleration in this event, unless the plane is very light. As a rough guess, a 9X6 turning 20k would give a good average speed.

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