Hello,
I have a home-built UFO that uses 4 propellers for vertical lift. the props are 10x4.5 gws style props and are spinning around 4500 rpm wiht 5.33:1 gearboxes. they sit in a short venturi pipe with about 1.75" from the bottom of the pipe. there is about 1/4 inch between the prop tips and the sides of the venturi. (i can not bring it any closer than that because it is made of foam and is very flexible) would i increase or decrease the STATIC thrust by constricting the pipe after the propellers a bit? i know that it could move faster if pointed horizontally, however it is a vertical ship with very little forward motion, so static thrust is what is important. Most people have questions about the UFO that do not regard the question. If you have one, PM it rather than ask it here, i dont want this thread to get off subject.
thanks for any and all help,
-spaceclam

Clam,

Since ducted thrust relies heavily on efficency, your prop tip clearances are the problem. Try reinforcing the ducts with .58 oz glass cloth and finishing epoxy resin, Pacer Z-poxy works great. Line the ducts with glass and resin. Squeegee out as near to 50% of the resin as you can get. The weight of the glass/resin will be nothing compared to the thrust you will gain after re-fitting props for a tighter fit. Also, since you are forced to use a smaller diameter prop right now, you are losing thrust there as well. Use the largest diameter prop you can without them hitting the duct walls. Try changing the pitch as well. Hope this helps ya.[8D]

Dan

Detail Man is right! You could gain 3-4% by using a 10.5" prop. In theory, the slipstream contracts after passing through the prop, so your idea might work. If the vehicle is close to the ground, the increase in slipstream velocity caused by the contriction may help the ground effect, but that's just a guess. However, I suspect the real problem is the propeller itself. VERY few propellers are optimized for static conditions. I don't imagine that the GWS props are particularly efficient anyway. The best propeller will NOT have a constant pitch distribution along the blade. It will have an increasing pitch towards the tip, making for a flatter twist. You may be better off to try and make your own, like a helicopter blade, maybe tapered so the tip is 1/2 the root or something.

I don't suppose you know the torque or power of the motor? Current draw?