I'm contemplating trying to build a schroud around my prop. Are there any hard rules that will work for a 13" prop? (shape, inlet length etc)
Thanks again fpr all ther past hints.


Peter D

To do anything more than just produce turbulence the shroud needs to have a venturi like shaped lip that extends quite a ways in front of the prop blade. It's been a long time since I saw the article on inlet venturis for ducted fans bu I seem to recall that it needs to be at least 1.5 to 2 blade chords wide before it sees the prop disc. And have a good flare. Just adding a simple cylindrical ring won't do diddly and will actually hurt more than it helps. Another big issue was that the fan or prop needs to fit closely to the shroud. Gaps of even 1/16 inch caused so much drag adding turbulence that it took away a lot of the advantages. So you'd need to clip the blades to produce a flat end and then profile them on a lathe or some jig to shape the tips to match the shroud curve and leave only about a .020 to .030 gap. As well the shroud should extend back for a couple or more blade chord lengths to again ensure that the airflow doesn't suddenly turbulate due to the blades passing by.

So obviously to maintain such tight tolerances your shroud design needs to be structurally stiff and strong. And there's no guarantee that the gains will be worth the time and added weight. Not to mention that such a shroud has quite a bit of side and top view area. The tail surfaces would need to be enlarged to take this into account.

A shrouded propeller may nearly double static thrust per horsepower, which is great for VTOL airplanes, but the thrust advantage rapidly deteriorates to nearly zero as speed increases. In general, at normal cruise speed, the shroud's additional weight and drag negate any advantage.

And there is a major detraction. The suckers weigh a lot and structures to support them do as well. And none of that structure usually can be utilized to do something else.

One of the few applications I have seen them used on is pusher aircraft to protect the blade during rotation (prop strikes) or as has been mentioned when used for VTOL or STOL type of aircraft.