• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Mechanically controlled variable pitch props vs the constant-speed variety

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

rdj

Well-Known Member
Supporting Member
Joined
Sep 10, 2009
Messages
387
Location
Northern California
There are a few posts close to this topic, but none that really answer this question: why are there so few manual adjustable pitch prop hubs for smaller engines (Rotax, Jab, VW, etc.)?

All of the bigger engines seem to use constant-speed units, with hydraulic governors and their associated complexity. The smaller engines, however, are almost invariably run with fixed-pitch props, resulting in the inevitable selection of either a cruise prop with poor climb performance or a climb prop with poor cruise performance. This problem was solved almost a century ago with solutions like the Beech-Roby manually-adjustable hub. The design doesn't seem that complicated.

A variation of it can be seen in the manual for the Duc propeller: Duc Hélices (Stupid frame-based site btw--follow the link to 2/3 blade variable pitch, then 'more info'..)
The blades have a finger at their root which engages a slotted disk that can be rotated forward and back by a simple screw mechanism. As the disk moves forward and back, the position of the slot rotates the blade via the finger. The design seems fairly straightforward.

Seems to me, if the Duc mechanism was adjusted such that moving it to one end gave a climb pitch, and the other end a cruise pitch, you wouldn't need much more than a lever in the cockpit labeled 'climb' and 'cruise' to get the best of both worlds.

However, that's not what's out there. There are very few controllable-pitch hubs of any sort for the small engines, while the large engines invariably have the complex constant-speed systems that provide a fixed RPM. The latter seem simpler to operate at first glance, but every time I study them closer I found warnings about don't exceed this manifold pressure or the engine blows up, square-this-n-that, etc. Furthermore, the few products available for the smaller engines are usually the more complicated controllable-pitch solutions as well, such as the Airmaster. Which leaves me wondering, what is it about adjustable pitch that 1) requires constant-speed operation and 2) makes simple manual adjustment undesirable on smaller engines?

Bob
 
Back
Top