I had a setup where my todds models 10x4.5 electric propeller was turning about 4000 rpm and was pulling about 10.75 oz of static thrust. static thrust is the most important element in my application. i now have the propellers inside a ducting tube. the tube is 11 wide (1/2 inch clearance on each side) and is two inches long. the propeller tips are 1/2 inch behind the front of the tube, giving me 1.75 inches behind the propellers. how much more thrust will i gain by this? any good estimates? this is critical for my application

Not a clue about what you'll see but I can say that the clearance to the duct is probably way too much. Ducted props work best if the tips are shaped to use the duct as a fence so the tip effects are minimized. So the tips should be cut off square (slightly arced to match the duct diameter actually) and the clearance to the duct should be no more than about 1/16 inch. Also you did not mention what the duct shape is. For best performance the duct should have an exagerated venturi shape at the intake side. 1/2 inch of duct before the prop sounds like it may work but then again it may not be enough at that point. There was some info in an old glow ducted fan article I read one time that had the ducted fan unit up above the wing and the guy found that with the sharp edged and short entry much noise was present and the thrust was poor. Adding a venturi intake lip ring added about 1/2 of an inch to the mouth to bring the lip to fan distance up to what appeared to be about an inch. This venturi and extended intake mouth length smoothed the airflow entering the fan and kept the turbulent blowback within the duct mouth where it could be re-swallowed by the fan rather than forming a tubulent area that pushed the incoming air away. I also suspect that you'll need a longer exit duct.

If this is important to you I would start by clipping the prop tips and sizing the duct to shroud the prop much more tightly. Play with some cheap GWS props if you do not want to cut up the Todd's prop. Also play with the duct length and prop placement within the duct. This is not cut and dry stuff where you can predict numbers. You'll get the maximum from playing with the parameters.

If the purpose of the duct is to reduce losses between vortex sheets (to approximate an infinite number of blades), that 1/2 inch clearance at the tips needs to be reduced to absolute minimums (1/8" maybe?). And even then, under static conditions, the benefits are almost non-existent. The difference shows up at high advance ratios according to Prandtl's effective diameter theory.

The full-size Miller Special used a prop ring, but when the Pushy Cat race plane Pushy Galore was built it was found to give too much of a rudder effect and removed.

Yes, too much clearance at the tips. THe way to make the prop fit is to trim it to the same size as the duct then sand the end down until they no longer rub under power. Depending on RPM, the blades can stretch while turning, so a small clearance while static will disappear.

Also, in the static condition, any gains made with the small tip clearance will be lost if the intake end has a sharp or square lip. The intake lip needs to be rounded or bellmouthed.

hmm. i do have the guy cutting me some shims to help with lining it up. he is cutting them to 3mm so i can reduce it by a little bit. i could probably get him to cut them to 1/4 as well, but in a static thrust situation, will it benifit enough to bother with it? i can easily round off the intake lip. do i just need to take off the sharp edge or do i really need to round it off a lot for it to work? the duct is round.

A fan duct really doesn't give you much benefit at high speeds, and only improves static thrust if properly designed. You need a nice smooth bellmouth to properly guide the flow to the blades. A simple tube, even with rounded edges, will hurt your performance.

static thrust is what i need, i could care less abotu high speeds. could any of you give me a cross section drawing or something so i know how to shape it? i can't really bring my clearances down below 1/4 inch.

Then you're wasting your time with that duct. The tip losses will still be there and in all liklyhood the turbulent interaction may even slow the prop down further and ruin the airflow more than no duct at all.

For static thrust the front end needs to be a very prominent bell mouth as stated and as I tried to hint at. And by bell mouthed we are talking seriously wider at the intake than the main throat. This pic below is a sort of example of what we mean but for static conditions the venturi flare should be even more pronounced than these. Oddly enough it's not easy to find a picture on the web of what I'm looking for. But look at these prop shrouds and you'll get a much better sense for what we are trying to describe. Note that the length of the total shround is about 1/3 to 1/2 of the diameter. I would suggest that this is also what you should be using and with a similar shape.

No short cuts allowed. Do it right or there's no point in doing it at all with something like this. You'll have to re do your duct and get those tip gaps down to as little as possible.

Also note the fan placement of the foreground hovercraft. The fan is well back in the duct to ensure the air is a column shape as it enters the fan disc.

well, the shims i have coming will be about 8" long. i can bellmouth it but the clearances i cant do much about becuase all this is hand cut. then again, 8" is way too long for this so i may be able to put a second shim inside and bring the clearances down even further. the problem lies in the fact that this is styrofoam and i need room to wiggle. for efficiency, is ir really critical that the propellers are situated a lot farther back under the lip?

maybe it will help a bit if i explain my situation.
I am in the middle of constructing a UFO body for my 4 rotored vtol that i made myself. it has flown in the past, but the UFO body will be replacing all the structure i had earlier. here's a picture of what the ducts look like at the moment. i can easily bellmouth it outward at a very shallow angle so that the tip of the prop is right as close as possible but it would be bellmouthed so much that there would essentially be no duct above the prop tips. that can be done. i can also extend my exit ducts about 2" before i begin to experiance ground clearance issues. Would a setup like that be at least as efficient as something with no duct at all or would it hurt it? like i said, i have many shims at my disposal so i can bring down the clearance. i am just concerned about flex becasue if for some odd reason one of the propeller tips hits the side, it will slow it down dramatically and cause the model to flip over and crash. being that the body is styrofoam that may be a concern. it is perfectly okay if the ducts arent doing anything, as long as they are not hurting anything i am okay. it would be prefferable to increase thrust but in any case a decrease is not acceptable as my thrust margins are very close.

What about increasing the prop diameter and decreasing the pitch? Trim the prop blade tips to suit the duct dia.

Well, you should figure out a couple of braces so the prop won't hit the sides.

Now, to make the best of this setup, install your shims, which I assume are foam strips to fill the space around the hole. It may be easier to sand the round corner into the shim before putting it in. Just knocking the corner off won't do, but making a big giant radius isn't necessary either. maybe .25" radius.

that's it? all i need is a bellmouth that opens up 1/4 inch on either side? i can probably bring the shims down pretty close come to think of it. unfortunatly i can not increase my prop diameter because this is one of the precious few propeller blades that come in matching counter rotating sets or are available. the draganflyer blades are the only ones that are 11" (1" larger than mine) but they are made for spinning at 800 rpm not 4500 rpm and even at 3600 rpm they would go into negative pitch, even though i stiffened them and such. i dont really have any propeller options.[] also, about how long do you think the exit duct should be? and about how much static thrust do you guys think i should gain permotor?

Let me know how your UFO turns out Spaceclam. Can I ask how your doing the electronics? I'm working on a Ufo project myself. My design has an inner ring and an outer ring. The inner ring is for the propulsion and the outer for navigation. Since I work on gyros for an avionics facility it gave me the idea. I will probably use the new Draganflyer board that is due out at Spectrolution and attach
it to the non spinning portion on top of my saucer. Then use 4 small micro motors in a duct config.

come to this thread
http://www.rcuniverse.com/forum/Buil...1487065/tm.htm
That forum is more or less about how we have all build ufos or are helping others do it. i already have several control schematics posted there for diferent configurations. If you can give me a drawing of what you want accomplished, propeller positions and such and post it there with a more detailed question, i think i can help you out a bit. BTW, i probably wont have my UFO flying for a while. I ended up needing to go brushless so at my current income rate it will take me about 17 weeks to come up with the money i need unless i get some cash for helping out my parents with the move or my can collection etc. come and discuss it with us!
Regards,
Clam