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Goals for propulsion efficiency: thrust, HP, speed or ????

Posted By:
Ried Jacobsen
#1 Posted: 1/10/2010 21:17:44

Hi all.  Since the post on "Streamlining " seems to also be diverging into prop design, selection, and theory, I thought a new post might be in order.

I am looking for opinions, advice and feed back from people more intelligent than me (which should be around 90-95% of you reading this post!)  I want to know what is out there and what might be best bet for low noise and decent efficiency and able to handle a wide range of operational requirements on the same flight.

The last 100 years props for lower speed applications have been two blade, fixed pitch, relatively low rpm.  The last 20 years alternatives have been starting to make in roads, such as three and more blades, impellers, pusher configurations, composite construction, ground adjustable, flight adjustable, P-tips, Q-tips, etc.  What is the state of the propeller marketplace?

For my particular motorglider application, I would like to use around a 75HPengine.  I want light weight, and would like to avoid using any type of speed-reduction unit (but might consider arguments in favor of speed reduction).  I would like to feather the prop to minimize drag during soaring conditions, flat pitch for takeoff, and higher pitch for efficient cruising.

What choices and/or compromises should I consider?

BTW, I am also watching the "Streamling" post, so look that over and comment on that also.  I am appreciating the exchange of ideas on these items!

Ried Jacobsen

Ried Jacobsen
#2 Posted: 1/10/2010 22:17:55

Past thinking for propellors has considers the prop as a disk that accelerates the air when the combination of rpm and pitch exceeded air speed, is a source of drag if the combination of rpm and pitch is less than air speed, and is neutral if the rpm and pitch were equal to air speed.

If the "impeller" or propeller is placed or used differently could drag at slow rpm be reduced while thrust increased during high rpm operations?

Can prop tip noise be reduced in other methods besides reducing tip speed.

Can 5, 7 or 9 blade composite propellors be developed to reduce prop tip noise with higher rpms but lower tip speed due to smaller diameter propellors?

Howard Handelman
Homebuilder or Craftsman
#3 Posted: 1/21/2010 18:03:44

Reid, if you have not already done so, buy and read Jack Norris's  book:  "Propellers, The First, and Final Explanation", described extensively and available through his website . Unfortunately, you cannot yet buy a prop of his design for the performance spec's you have in mind (although you can buy one for the RV-8).  You may be able to buy one from either Lonnie Prince or Craig Catto in the not too distant future.

The subject is much more complicated and difficult than most folks realize and there are some hold-outs which are way off course. For instance, compare Jack's designs to the square-tipped prop from McCauley on the C-162.  Jack proved that was wrong in 1948.

Paul Lipps has a related, but dissimilar design and he may be able to contribute to your quest, too.


Michael Crowder
Homebuilder or Craftsman
#4 Posted: 1/26/2010 09:49:31 Modified: 1/30/2010 10:25:29

After reading most of Jack's book, I too was surprised when I saw the Square tipped propeller on the Cessna C-162 SkyCatcher...



One of the things that has I've battled with is cooling air into the engine.  I read with interest  of Paul Lipps' description in a Contact Magazine article about improved engine cooling with his propeller design.


I mentioned in the streamlining thread that I'm running a Prince P-Tip and have been fairly happy with the aircraft performance.  A few Sonex flyers with the Sensenich propellers seem to have better cooling with the same engine and cooling baffles.  So if Lonnie Prince is looking at improving his propeller designs I'm in!

There's also an active propeller design group on Yahoo!Groups


Other useful links....








Sonex N293SX


Michael Crowder Sonex N293SX TriGear, Dual-Controls, Jabiru 3300
Ried Jacobsen
#5 Posted: 1/29/2010 13:21:20

 I have attached a couple of PDFs from my trusty Fan Engineering handbook by Buffalo Forge.  For long vehicle tunnels, enclosed axial flow fans are used more frequently to generate thrust than open propllers.

On various occasions ducted fans have been attempted on smaller aircraft.  What have been the pitfalls that have kept them from being accepted.

Similarly, mixed flow sections were used for early gas turbine compressor sections.  Is this an avenue worth looking into for generating thrust?

Thanks for all the responses so far, and the recommended books for further research.  I appreciate the time you guys took to reply to my questions.


Files Attachment(s):
fan types Scanned Document.pdf (38561 bytes)
Fan Engr Hndbk Scanned Document.pdf (94860 bytes)
Tom Hackel
Homebuilder or Craftsman
#6 Posted: 1/29/2010 18:30:30

Ried, am I correct in thinking your fan is a part time operator? Just to get you altitude then glide until you need a boost again or are you looking to have 100% run time?


Ried Jacobsen
#7 Posted: 1/29/2010 19:11:09

It depends on my particular mood on the day I am flying.  Some days I will want to soar and play with the hawks (gliding) and other days I will want to get some place ($100 hamburger) and motor on full time. 

Basically I want the best of both worlds!