4
$\begingroup$

I wasn't able to find a single example of more than two propellers stacked axially in a contra-rotating configuration. There is a huge number of contra-rotating propeller setups, but it is always a combination of two propellers on the same shaft, not four or six.

Why isn't a piston engine used to run more than two standard propellers on a single shaft? Hasn't it been tried (or tested at least) in 100+ years of aviation history?

(This question isn't about turbofans/turbojets, but I also couldn't find anything for ducted fans: two props on a single axis is a maximum.)

Blade-solidity is explained here:
Contra-rotating props explained here:

An increase in a number of blades does seem to reduce the efficiency, whereas an increase in propellers on an axis seems to increase the efficiency.

$\endgroup$
  • $\begingroup$ shaft design. It's much more complicated the more moving shafts you need to embed within each other $\endgroup$ – ratchet freak Jan 23 '17 at 15:34
  • 1
    $\begingroup$ What would be the advantage of this? There are only two directions each propeller can spin, and why would one want to have several propellers on the same axis spinning in the same direction, rather than just one propeller with more blades? $\endgroup$ – Henning Makholm Jan 23 '17 at 16:17
  • $\begingroup$ I've added two references into my original question. Increasing the blade solidity ratio is the worst technique one could employ for packing more thrust into the same space, on the contrary to Contra-roration which seems to give some benefit. $\endgroup$ – FlegmatoidZoid Jan 23 '17 at 17:11
  • 2
    $\begingroup$ I would guess it is diminishing returns. The first CR prop gets you 6-16 improvement, but I doubt the next one gets you that much and the mechanical complexity has to get totally crazy with three or more shafts rotating inside each other. So the guess is that it doesn't help very much and costs a lot. $\endgroup$ – zeta-band Jan 23 '17 at 17:57
  • $\begingroup$ I would think that in a rotary wing aircraft especially, a four rotor stack would not only be obscenely complex, but in terms of the stresses involved on the entire structure it would seem likely that it would have to be a low maneuverability vehicle. Else you would risk fracturing the stack and becoming a rock. $\endgroup$ – NZKshatriya Jan 24 '17 at 15:16
5
$\begingroup$

No, stacking more propellers will reduce efficiency. The case with two is an exception because the second propeller benefits from operating in the slipstream of the first. In total, both propellers accelerate the air backwards without adding swirl losses.

However, propeller efficiency goes down with increasing flow speed ahead of the propeller. This means that every other propeller operating in the accelerated flow of a preceding propeller will have lower efficiency. Generally, it will be better to spread the propellers out sideways than to put them in line.

Note that contra-rotating props are the exclusive realm of very powerful engines with very high disk loading. The most recent designs prefer to use more blades and avoid the heavy gearbox, but the efficiency of such props is poorer than that of propellers with lower disk loading and fewer blades. In a way, the efficiency of the eight-bladed propellers below is similar to that of four two-bladed propellers in sequence, but it is much easier to build with all blades and their pitch mechanism in one hub.

T-56 engines with Hamilton-Sundstrand NP2000 propellers on an LC-130
T-56 engines with Hamilton-Sundstrand NP2000 propellers on an LC-130. Photograph by Robyn Waserman, National Science Foundation (Date Taken: November 21, 2008)

$\endgroup$
  • $\begingroup$ In general, isn't it preferable to have a blown wing, and the more of the wing you can get, the better? The added complexity (and weight) penalties that you get from contrarotating props seem to outweigh the gains, except perhaps in a narrow range of scenarios. $\endgroup$ – egid Jan 23 '17 at 21:52
  • $\begingroup$ @egid: The blown wing only helps during take-off and landing, but is a source of added drag in the time between the two. The higher flow speed and turbulence in the slipstream creates more viscous drag. $\endgroup$ – Peter Kämpf Jan 23 '17 at 22:11
  • $\begingroup$ Ah, interesting. Makes sense. $\endgroup$ – egid Jan 24 '17 at 1:31

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.