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The Florida Air Museum in Lakeland has a Wittman W-8 Tailwind with four wooden, propeller-like blades on the engine cowling:

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Source: own work

What are the blades for? It's an experimental aircraft so it's possible that they're unique to this specific airframe.

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    $\begingroup$ That Tailwind has several unique features, including retractable landing gear, adjustable air inlets, and a cowl flap. It was built by a guy named Anthony Occhipinti about 1966. I've seen pictures of it in the past but not with the vanes. It definitely is a one-off. $\endgroup$
    – John K
    Commented Oct 31, 2019 at 3:19
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    $\begingroup$ I wonder if the vanes might be intended to serve as "flow straighteners" to reduce the spiral of the propwash-- possibly to increase efficiency and/or to eliminate thrust-related asymmetry in handling (i.e. need for right rudder at high power and low airspeed.) You don't happen to have a picture from the other side to show whether the curved side is on the top or the bottom on those blades do you? On the other hand, a close look suggests the blades are moveable, which may tend to go against this interpretation. $\endgroup$
    – quiet flyer
    Commented Oct 31, 2019 at 4:27
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    $\begingroup$ @quietflyer I won't post it as an answer because I don't know for sure, but I am fairly certain that the owner was experimenting with ways to mitigate P factor and torque effects to gain a few knots. I've been in EAA since the mid 70s and this sort of home brew experimentation was very common back when most homebuilts were made from scratch and individual builders would heavily customize their aircraft and try different things. $\endgroup$
    – John K
    Commented Oct 31, 2019 at 12:15
  • $\begingroup$ I suppose they are probably only ground-adjustable not movable in flight. Sure would be interesting to see a photo from other side to see whether the 2 blades on the right were mounted with the more curved surface of the airfoil facing up or facing down; if were to straighten out the spiral slipstream one would tend to expect the latter. $\endgroup$
    – quiet flyer
    Commented Oct 31, 2019 at 13:14
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    $\begingroup$ It sure looks like right horizontal one is upside-down compared to the left horizontal one, consistent w/ straightening the slipstream. Someone ought to write an actual answer out of all this-- $\endgroup$
    – quiet flyer
    Commented Nov 1, 2019 at 3:31

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After calling the Florida Air Museum, I have discovered that this seems to be a local mystery to the employees at the facility, as well. The museum’s director assumed that they “were an attempt to counteract P-factor or torque. Something to do with balancing required power.”

Since this was little help, I did some more digging and decided that the “blades” on this Wittman seemed to function as adjustable strakes - albeit located far from any control surface and of dubious utility.

Finally, I discovered that there have been numerous experiments performed with ‘Actuated Forebody Strakes’ aimed at controlling forebody vortices as a means of complementing thrust vectoring. Additionally, a NASA study found that a large increase in yawing moment may be brought about through the manipulation of actuated forebody strakes.

In every case that I could find, the tests were concerned with fighter jet aircraft operating at high angles of attack, but it is conceivable that the owner of this Wittman W-8 was attempting to implement some of the same advantages here.

Here are the abstracts to the vortex thrust vectoring and NASA yawing moment studies:

The desirability to enhance the controllability of fighter aircraft at high angles of attack, particularly yaw control, has fostered an interest in both vectored thrust and active control of forebody vortices... The methods investigated include pneumatic or blowing techniques using surface mounted jets and slots, surface suction, variable-height deployable strakes, and rotatable-tip strakes. Flow visualization, and force and moment measurements have shown that all of the methods are effective at manipulating the forebody vortices over a wide range of angles-of-attack and sideslip, primarily through control over flow separation on the surface of the forebody. All are most effective when applied near the forebody tip.

Extensive pressure measurements and off-surface flow visualization were obtained on the forebody and strakes of the NASA F-18 High Alpha Research Vehicle (HARV) equipped with actuated forebody strakes. Forebody yawing moments were obtained by integrating the circumferential pressures on the forebody and strakes. Results show that large yawing moments can be generated with forebody strakes. At angles of attack greater than 40°, deflecting one strake at a time resulted in a forebody yawing moment control reversal for small strake deflection angles. At α = 40° and 50°, deflecting the strakes differentially about a 20° symmetric strake deployment eliminated the control reversal and produced a near linear variation of forebody yawing moment with differential strake deflection. At α = 50° and for 0° and 20° symmetric strake deployments, a larger forebody yawing moment was generated by the forward fuselage (between the radome and the apex of the leading-edge extensions), than on the radome where the actuated forebody strakes were located.

References:

Malcolm, G., & Ng, T. (1990). Forebody Vortex Control as a Complement to Thrust Vectoring. SAE Transactions, 99, 1515-1543. Retrieved August 16, 2020, from Journal of Aerospace

Fischer, D.F., Muriel, D.G., & Lanser, W.R. (1996). Effect of actuated forebody strakes on the forebody aerodynamics of the NASA F-18 HARV. NASA Technical Memorandum 4774. Retrieved from NASA.gov

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    $\begingroup$ Fantastic research, thanks! $\endgroup$
    – Pondlife
    Commented Aug 16, 2020 at 19:30

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