prop on ducted fan?
 

Hi all,

I was woundering if it is possible to put a prop on ducted fan engine ( BVM 91) and use it on a prop plane. Has any one had any experince with that? . wouldn't be fast?

RE: prop on ducted fan?
 

I have seen props on OS 46VX and they work very well. I turn a prop on my OS .25VF-DF and its a awsome combo. The BVM 91 is a powerfull engine! You might run into the problem of finding a fast 90 size airframe. What airframe were you thinking? I would look for a pattern plane. Not sure where to start with a prop. Sell the BVM to me and buy a Nelson

RE: prop on ducted fan?
 

That's pretty standard practice for go-fast folks. The very real problem you have is finding a suitable propellor. As examples, the OPS and Picco 10cc engines used by many Euro speed guys were actually set up originally for ducted fan use, so we've been told. The requirements are much the same, except for pure competitive FAI speed flying the engines may get tweaked even more. Stock is "quite adequate" however.

Ducted fan engines are high-timed engines, meaning ported/timed for peak horsepower at high rpm relative to typical propellor sizes - usually in the low to early 20's versus mid-teens. Therefore, although they are solid engines down in the mid-high teens regime, that is not where they run most happily, and you must be careful not to overload them. To run in the rpm range you want for all-out speed AND to keep prop tip speeds within reason, you need to run composite props in the 8" - 9" - 9.5" diameter range such as 8-11, 8.5-10, 9-10 or thereabouts, these are just ballpark examples. Propped appropriately that 91 is capable of pulling a clean & decently designed prop job aircraft 200+.

If you install a stock APC/MAS/whoever's-molded-nylon prop and run it at those rpm, stand way behind the engine and wear eye protection in case the blade fragments ricochet back at you. Seriously - don't do it. Fan engines are good at shredding regular props. You can cut down larger props to take advantage of the hub size, but a the probable expense of efficiency. An example is the use of a Zinger 11-10 trimmed to 8.5" or so to present the proper load for run-in of high performance 10cc+ engines.

There is a bunch of info along these lines stashed around this forum, since it's what most of the meatheads here are all about (unless it's pokey ARF prop jets [sm=tongue_smile.gif]). Search VRDF, fan engine, ohh look what papa got (that thread morphed into much general speed information), OPS 65, what else.. those should get you going.

The advice re older pattern designs is good - they're solid, clean, fly great, and they'll do 110-120 on a pattern .61. Put one of these on one with the right prop you could probably fly aerobatics at 165 all day.

MJD