Assuming that the power and diameter are the same, how much extra thrust can be gained by placing a propeller in a duct? How does it influence the RPM?

To clarify: take any prop, place it in a well-designed duct, what can we expect? I expect higher RPM for same power, but also duct enhanced thrust. An approximation based on sources would be great.

  • 2
    $\begingroup$ That "duplicate" question basically says, "maybe even twice" the thrust, which is more an intro to, rather than an answer to, this question. $\endgroup$ – Ralph J May 13 '17 at 14:36
  • 1
    $\begingroup$ @mins I don't think he does mean "wing lift." How would a ducted prop produce any major effect on the wings. When he says "lift" I believe he is referring to thrust. Which makes it a dupe of the other question except that it's asking for some more specific quantities. $\endgroup$ – TomMcW May 13 '17 at 20:29
  • 1
    $\begingroup$ @whiteknight can you confirm if you are referring to thrust, not lift? (well technically the thrust is the "lift" of the prop blades, but thrust is less confusing) $\endgroup$ – TomMcW May 13 '17 at 20:32
  • $\begingroup$ @TomMcW: I've reverted to the OP's version. BTW I haven't seen the tag 'helicopter'. $\endgroup$ – mins May 14 '17 at 20:36
  • $\begingroup$ I do not understand your question. You want to know which is the power of a propeller in a wind tunnel that completely encloses it or just of a propeller mounted on a plane with a structure that encloses the prop? $\endgroup$ – Gianni Alessandro May 19 '17 at 7:46

Relative to MAV, as opposed to large scale rotors:

Compared to the baseline open rotor, the shrouded rotors showed increases in thrust by up to 94%, at the same power consumption, or reductions in power by up to 62% at the same thrust.

At rotor collectives above 10°, all shrouded-rotor models showed improvements in performance over the isolated (‘open’) rotor — that is, higher thrusts and lower power requirements. At the same power consumption, increases in thrust over the open rotor by up to 94% were observed, or, conversely, up to 49% reductions in power at the same thrust, for the condition where the open and shrouded rotors are required to have the same tip speed. If, instead, the rotor disk areas are required to be the same, then up to 90% increases in thrust at the same power.

The effects of changing any one of the four parameters (tip, rlip, d, Ld) on the shrouded-rotors’ performance became more pronounced as the other three parameters were changed to degrade the performance. It is therefore not possible to categorically state that any one particular parameter has a greater effect on performance than any other, since the amount of influence of any parameter depends on the values of the other parameters. The exception to this seems to be the lip radius, changes in which had almost same effect no 258 matter what the values of the other parameters are. Thus, in general, it would seem that changing the value of any shroud parameter so as to improve performance results in less sensitivity of the performance to changes in any of the other parameters; similarly, improving the efficiency of the rotor itself would seem to result in less of a performance benefit from the presence of a shroud.

Extracted from: HOVER AND WIND-TUNNEL TESTING OF SHROUDED ROTORS FOR IMPROVED MICRO AIR VEHICLE DESIGN Jason L. Pereira Doctor of Philosophy, 2008 Dissertation directed by: Professor Inderjit Chopra Department of Aerospace Engineering University of Maryland College Park

Rotor parameters including tip gaps, tilt and relative wind, collective, and motor characteristics will influence rotor RPM, and are indirectly addressed in Pereira's dissertation. Again, there are very broad assumptions in the OP question, but Pereira substantiates generalized efficiency improvements for specific configurations.

  • $\begingroup$ Could you please elaborate on the test setup? It would be of worth to the question, I think, to describe the orientation of the shrouded rotor in the wind tunnel. Also, nice pick...I should've thought to check this. I walk past the results of this guy's thesis work every day... $\endgroup$ – Marius May 23 '17 at 1:09
  • 1
    $\begingroup$ He addresses several configurations, and they cannot be done justice in an answer here. I suggest you read the dissertation. It is widely available, and he has numerous diagrams, describing hover, translational flight, etc. $\endgroup$ – mongo May 23 '17 at 1:20
  • $\begingroup$ Acknowledged. However, since the OP seems to be asking about a shrouded propeller, it's not wholly clear to me that the answer pertains specifically to that because "rotors" normally refer to propellers in edgewise flight or hover. Could you please clarify what configuration the power/thrust benefits you state pertain to? $\endgroup$ – Marius May 23 '17 at 12:46
  • $\begingroup$ My familiarity with the cited dissertation is general (it's a couple of hundred pages long) but he does cover what happens when the relative wind is changed (as in forward flight) and at different angles (hover, translational, cruise). I believe his quotes statement is for hover, but if you read the paper it also applies to other relative angles and wind (airspeed in a vector). The author does not make a generalized statement akin to what the OP asks, and as I have said before, there are many assumptions involved in generalizing the results, so I decline to do so. $\endgroup$ – mongo May 23 '17 at 19:28
  • 1
    $\begingroup$ @ymb1, I realize that you are trying to clarify the question and get effective answers. But when the question wording changes, etc. it makes answering a moving target. I wasn't trying to roast anyone, just venting a little frustration (grin) which was probably due to nothing here. $\endgroup$ – mongo May 24 '17 at 1:19

That "any prop" part is the killer. Most people get less thrust from trying that:

My experiments suggest only a a max of 20% improvement is possible:

It should spin slower (because more of the prop is producing lift, instead of stuck in the vortex at the tips) for the same power.

The "well designed" part of your question is also non-trivial - there's no such thing if you want the prop to perform optimally across a range of airspeeds - the duct has to be designed for a specific operating condition, so it will always make things worse at "off conditions" - which will usually be more worse than the prop without the duct.

The entire point of the duct is to reduce tip vortices, and the entire point of the tip region of "any propeller" is also to reduce vortices, so using both at once is not going to work properly, if at all (see video above - in real life, it does NOT work). The tips of a duct fan need to be totally different shape to the tips of "any propeller".

I question the accuracy of the paper mentioned: he shows that reduced diffuser angle improves thrust (p. 51 - but totally fails to consider Bernoulli for that, and never even tried angles less than 0 degrees), and his results and diagrams are almost all based on large diffuser angles, which is the opposite to Bernoulli, common sense, and modern practice.

In my relatively vast experience in this field, almost all CFD models and predictions are false (papers are usually written by students, none of whom understand modern ML techniques, especially the "time traveller conundrum" as it applies to CFD validation). Long story short - they do not realize that they're "training" their math to match the observations [almost always of poor experiments, and always of a tiny sample], instead of what the math is supposed to be - a prediction, for more than just the sample observed. Bottom line - if it's not something real that was measured, you can't trust the numbers.

  • $\begingroup$ I guess anything about "vorticies" will pique the interest of many a young engineer. Props generate lift as wings do. Notice many of them are high aspect for the same reasons that wings are, backing off to chunky low AR only when Mach becomes an issue. With shrouds, benefits of prop drag reduction are offset by additional weight and, as airspeed increases, more drag (from the shroud itself). $\endgroup$ – Robert DiGiovanni Mar 2 at 20:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.