David Hill

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Hi Guys,
I am working on a little project here and I would very much like your thoughts on it. I thook a Sig Kobra kit and I am fitting an OS 46VRF in it. The plane comes in just at 3lbs 3 oz ready to cover. The engine is turning an APC 7.8 x 7 carbon fiber prop at 21,300 rpm. Any idea what kind of speed I can expect from this combination?? The plane is set up as a tail dragger.

Thanks,
David

combatpigg

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Hi DAVID, If the assumed inflight plusses and minuses cancel out, the calculator says 141 mph. It's going to be impressive no matter what!

David Hill

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Hello,
Thank you very much. I was really hoping for a little more than that. Any suggestions?


David

Rudeboy

Never seen the Kobra from up close, but it seems like a fairly fat plane... so I don't know if the APC 7.4x8.25 prop will work, but it's worth a try.

In your original post you mentioned a .46 VRF engine... did you mean VRDF perhaps? If so it should turn up the 7.4x8 or 7.4x8.25 pretty high with a properly set pipe.

combatpigg

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Hi DAVID, I suggest that you enjoy the plane for what it is! Should be AWESOME ! If you want to take the same power to a higher level, you need to get into a purpose built speed plane, like the ones that are in the GERMAN SPEED CHALLENGE [thread]! Yea, a 7 pitch prop is easy on the engine but if you want speed you have to move up in pitch. I'm going to take some lumber this weekend and start carving the mold for a .40 powered speed plane. Once you get set up and familiar with the methods, making a fiberglass plane from a raw idea is pretty easy and fun.

David Hill

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Hello,
The engine is an OS Max 46 VRF , frontintake, rear exhaust engine had in the late 1980's. They are very strong engines, but is not the ducted fan engine. I also have a prop I picked up 8 props in Europe a couple of years ago that is a 7.2 x 9.4 would that be a better choice? I really want to break 150mph.


David

combatpigg

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Only you can answer those questions by bench running it to see if it will jump on the pipe, and if the needle is too critical or not, with what ever prop you choose. Carbon props can have the blade area reduced and thinned for fine tuning. Don't expect a raw off the shelf prop to be the right one. You have to be willing to experiment. A good strong level throw at launch can make up for a lack of prop diameter [thrust], but if your airframe is not sytreamlined enough, you are beating a dead horse with a stick.

BruceDana

Reduce drag wherever you can.

bob27s

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Thats a good engine dave.......

that 7.2x9 prop may be key to getting up to 150. You may also want to try one of the APC 8.8x9.25 props, clipped down to 8" diameter. You are going to have to have the engine turn up over 17,500 on the ground to get that sort of speed.

set it up on the bench......start with the pipe at about 10" (plug to fat part). Slowly work your way in .25" at a time. My guess is you will end up somewhere close to 9".

Although that engine was intended for somewhere around 15K rpm, its sleeve is pretty well pipe timed...... timing should let you turn it up well above that. That little 7.2 prop is going to easily turn over 18K rpm I'd imagine. The clipped APC will probably get there too, and may pull harder. Only way to know for sure is on the bench, then in the air.

As for the airframe......the little Kobra is fairly clean... smaller airplane helps.....although the wing is a little bit thick. But with that sort of power, perhaps retracts....... you might get it upwards of 130-150.