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In the following situation with a slipstream pylon for a self-launching glider, what are the tradeoffs in sticking the propeller before vs. after the pylon?

Tractor Pusher

The wind turbine industry has the same conceptual problem, and their consensus is that the turbine is better with the blades in front of the support tower. However, their support tower is circular and so induces unacceptable turbulence and pressure pulses in the turbine blades. Their system is also designed to operate for decades.

Contrast this to the case in question, where the pylon is slipstreamed and so has significantly lower drag and wake turbulence. My hunch, then is that it seems that behind the pylon is better because the airspeed across the pylon is much lower.

However, I am loathe to reject the wind turbine industry findings with some hand waviness about "slipstreamed" profile.

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If it's for something like an airplane or powered glider, for best thrust efficiency and noise level, you want the prop on the front. If behind, the turbulence from the pylon causes both efficiency loss and a loud buzz as the blades pass through the pylon's wake. This is a bigger deal than the effect of the prop's wake hitting the pylon when mounted forward. The characteristic buzz sound that the Lake Buccaneer makes is from having the prop behind the pylon.

For an engine on skinny pylon with little frontal area, the difference between pusher and tractor is not that much, so noise effects are probably a bigger issue. The efficiency difference grows when the body in front of or behind the prop gets large. From an article by designer Peter Garrison in FLYING:

Now, how to compare this with the efficiency efficiency of a pusher propeller? For this I'm obliged to turn to books, since it's not possible to do a rule-of-thumb calculation from basic physical principles, as it was for the tractor. In Roskam and Lan, Airplane Aerodynamics and Performance, there is a chart showing the effect of what you might call the occlusion ratio -- the ratio of fuselage diameter to propeller diameter -- on the efficiencies of tractor and pusher propellers. The efficiency of the pusher prop is lower than that of the tractor, but negligibly so, until the fuselage diameter reaches half the prop diameter, at which point the two curves begin to diverge. When the fuselage diameter is 60 percent of the prop diameter the pusher has dropped behind by about 3 percent; by the time the fuselage diameter is 70 percent of the prop diameter, the decrement is 8 percent. An equation is provided (it is also found in Dan Raymer's Airplane Design: A Conceptual Approach) that yields roughly the same result.

The Volmer Amphibian is a good illustration of this. It had the option of doing either; you could design the mount to point the engine forward or backward. Part of the attraction of the tractor option was it didn't limit the engine choices (you need an engine with thrust bearings for pusher installation if you want to install it that way, as well as a prop designed for pushers).

Having the prop forward is widely seen as more efficient, with better thrust for the same horsepower and a simpler installation. I would say most Volmers were built as pushers however, mostly due to discomfort with having a propeller blade tip passing a foot and a half behind the pilot's head (I'd not be too happy having to put the thing into trees with that head chopper right behind my head, and if I was building one, there is no way I'd have the prop on the front).

The consensus however, seems to be that the tractor arrangement is superior performance wise, while the pusher is safer.

In the case of an electric motor glider (which I assume is what you are contemplating) there is another factor, which is the desirability of an aft folding propeller if the mount is fixed.

Recognizing that the efficiency difference may not be enough to worry about although the noise effects could be significant, if the pylon retracts, I'd put the prop on front. If the pylon is fixed and the prop feathers, I'd also put it on the front. If the pylon is fixed and the prop folds, then it goes on the back.

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  • $\begingroup$ Good stuff! What kind of quantitative difference is there for the fore vs aft mounting? $\endgroup$ – Kenn Sebesta Feb 12 at 20:27
  • $\begingroup$ Wouldn't the prop slipstream cause more drag on the pylon in the tractor arrangement? The noise argument is convincing (and easy to check) but the performance claims need more than just some handwaving. $\endgroup$ – Peter Kämpf Feb 12 at 21:30
  • $\begingroup$ @PeterKämpf I've been reading Peter Garrison's columns since the 70s and found the column where he talks about it. Made some edits for better clarity and less hand waving. $\endgroup$ – John K Feb 12 at 23:05
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The airflow downwind of the propeller will be moving significantly faster than upwind, since the incoming air is drawn over a wider angle than the outgoing air, which is more or less a cylinder. Also bear in mind that the outgoing airflow will be rotating in a helix and so will impart a sideways force to the pylon. If the pylon isn’t circular in cross-section then it should be designed to take account of the airflow direction (which may be best established experimentally. Don’t be surprised if the airflow is at 45 degrees to the direction of travel.

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  • $\begingroup$ That's a really good point about the airfoil orientation. $\endgroup$ – Kenn Sebesta Feb 12 at 20:26
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    $\begingroup$ If you can find a video of the ‘flying cars’ episode of Scrapheap Challenge (UK, c.2001) you’ll see that one team came totally unstuck by not taking this into account :-) $\endgroup$ – Frog Feb 13 at 0:06

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