I am very much preparing to plunge into the world of DF Jets, hopefully as a stepping stone to turbines. I am considering power plants and had a thought concerning fans and impellers.

Bare with me here:
As a long time prop flyer I would never use the same prop on a 61 as I would on a 91 or 120. Each engine delivers its optimum horsepower at a different RPM range, and we prop them to take advantage of that torque curve. That is magnified by the use of a tuned pipe that works best at a particular RPM. All that being said, why don't fan units come in different pitches. I know that there is a restriction on diameter, but couldn't we take advantage of the added torque of a bigger engine by adding a higher pitch fan? I know that much smarter people than I have probably mulled over this issue many times, but I was not privy to what they came up with and remain curious. I am also aware that the fan works more on a speed principal than on torque, but it seems like an awful lot of stress on equipment to run at such a high RPM all the time. If I am not mistaken isn't the speed of a plane a function of the pitch anyway? Would an OS 61 DF turning a 5x6 fan at 25,000 RPMs make a given plane fly faster than my YS 120 turning a 5x14 fan at 12,000 RPMs?

I am sure that there are pitch limitations, and probably some kind of mechanical engineering issues involving a lot of numbers and formulas, but it would seem like a good idea for noise reduction, longevity of equipment, and efficiency that we would want to slow the fans down and increase the pitch of the blades, or at least have a choice of pitch combinations. And what about the huge available torque of 4 strokes?

Thank you for reading this long post, and forgive me if I soud like an idiot. I am one of those poeple that think the only stupid question is the one that doesn't get asked.

Anthony

Anthony, while you post good questions, and I don't have the answer to it, what I can tell you is that no matter what you want in the DF world, it's been done on the equipment that is available to you from a Harrier, to an F-15, F-18 and other scale models to other models that are fully capable of 200 MPH and way beyond.

So even if you could repitch anything, which I don't think you can, all I can tell you is that it's not necessary. Reinventing anything in the DF world (unless that's how you get your willies) is not necessary. Especially if all you intend to do in DF is use it as a stepping stone to turbines.

Regardless, welcome to the Jet Forum and good luck with any project you take on. Take pics, share them, and ask lots of questions. Myself and others here have a world of experiance in DF and many who frequent here fly DF routinely.

Good Luck,

Sean

Thank you very much Sean.

I am sure that if it were possible to get more power and less wear out of a DF setup using huge pitch and four strokes, counter rotating feeder fans (I made that up) or some other combination, people like Bob Violet, Philip Avons and the many other brilliant minds in RC would've figured it out already! My background is pattern, and I can certainly tell you that form follows function. Good ideas slowly drive evolution, and bad ones quickly become extinct. My plan is to stay mainstream where all the support and knowledge is, and build a light straight plane that I can get my DF wings with.

More than anything it was a matter of curiosity. I am sure that someone has done the testing, I would just like to know how it came out.

fans were designed around available powerplants. there is a bit of range within each. for 40-60 sized engines you have the turbax units. for 77 to 91 you have the dynamax. for 91 and larger inlets use the ramtec. for BV stuff, use the viofan (BVM fan units are a smaller diameter) these are loose rules of thumb. there were just not that many different size fan engines manufactured

Pat, can you email me your address, a couple of nights ago, I realized I have your fancy DF tuning tool and mirror, and I'd like to send it to you.

Sean

Some interesting info:

On 7/19/04, Gary Koester wrote:

>Can you tell me about a Balsa USA enforcer with the Byron unit which is
>just sitting on top with no ducting.

Well, Byron is a bit of a special case.. the fan has to eat the garbage air off the back of the engine, and that hurts.

The key thing in that is to get some kind of a good inlet lip radius.
The byron fan has a bit of one, but bigger is better.

>Does an OS 77 and the Byron unit get close to 13 Lbs static thrust?

I have no idea. The 77 is less power, but the larger diameter of the Byron fan can increase thrust compared to the smaller Ramtec. (note, bigger diameter and more static thrust does NOT mean a better performing airplane.. the Viojett is the smallest of the .91 class fans, and gives the best performing airplanes)

>The duct has a slight taper toward the exhaust. Does this help thrust?

For a given engine and fan, set the exit diameter to properly load the engine with whatever pipe you have. It should be on the low end of the pipe range at static so it doesnt overspeed in flight.

On a .91, the Byron normally overloads the engine a bit, so I would not taper the pipe very much. On the 77, I am not sure where it puts out best power.

For a nacelle like that, one option is to make a tapered pipe, and then trim it back to larger diameters and see what happens.

bob

I heard of some hobby shops repitching dynamax/ramtec fans.. but its not nessacery.. just get a jet or kit.. if it requires a .46+ put a os.91 in it if it requires .77-.99 put a os.91 in it always go with the most power you can fit into the jet of your choose.. Ive found its not fun to fly a DF jet that needs all its possible power just to takeoff... underpowered jets are not fun to fly, and your chance of messing up&stalling are greater.. thanks my .50 cents for ya...

Im running a dynamax/os.91 combo, with a trust tube with 4inch exit diameter.. when i power up, u cant hold this thing down!! Ive have read that the thrust diameter, if smaller exit size gives you more thrust / less speed.. or the other way arround.. u could make two different sized thrust tubes and test them both out to see the difference..i could be wrong..[:-]