Notices
O.S. Engines Support Ask Bill Baxter your questions about O.S. Engines.

A Dilemma about Propeller and RPM.

Old 07-07-2015, 05:29 AM
  #1  
gregoryshock
Thread Starter
 
gregoryshock's Avatar
 
Join Date: Mar 2011
Location: Goshen, IN
Posts: 304
Likes: 0
Received 0 Likes on 0 Posts
Default A Dilemma about Propeller and RPM.

I have a dilemma. I bought an OS 75 AX with intention of using it on a Corsair Airplane. For that reason I chose the biggest 3 bladed that I could run on it. After breaking in the engine (I broke it in with the 3 bladed prop) I noticed that the tachometer gave me a reading of 8,500 max RPM. I know that the engine is rated between 2,000 and 16,000 RPM.

I ordered both the Engine and Propeller from Tower Hobbies. For that reason I used the information that was provided to me on their website. But yesterday I was reading the OS Manual, and noticed that the propeller size recommendation isn't the same.

On Tower Hobbies I read the recommended propeller sizes are: 14x6, 14x8, 15x7 But the OS Manual says they are 13x8-10, 14x6-8. The odd thing is Tower Hobbies has a direct link to the OS manual! Why isn't the recommended propeller sizes the same?

Is it ok to run a 15x7 Master Airscrew Propeller on this engine? Should a 3 bladed propeller (14x7) run the same RPM?
Old 07-07-2015, 06:06 AM
  #2  
Bax
My Feedback: (11)
 
Join Date: Apr 2002
Location: Monticello, IL
Posts: 19,483
Likes: 0
Received 6 Likes on 5 Posts
Default

To get optimum power out of your O.S. Max .75AX engine, you'll need to run it near 12,000 RPM or a little more. You have too much propeller on it. The RPM range O.S. states are the limits to RPM that they have gotten during testing. With some propeller, they could idle as low as 2,000 RPM, and with a different propeller could get it up to 16,000 RPM before they decided that more will cause the engine to come apart. Don't expect that RPM range to happen with one propeller. Won't happen.

We'd like to run the engine with a two-blade prop to get good power and efficiency, so a 12 x 8, 13 x 6 or 7 or so. Get the engine between 11,000 and 12,000 RPM for max power with whatever propeller will fly the airplane. Perhaps even running between 10,000 and 11,000 RPM will give you good flight performance. Going below 10,000 RPM will most certainly put you well down on the power curve.

"Scale" propellers are good for display, but almost never good for actual flight. They will be too large for the engine to turn properly.
Old 07-07-2015, 06:51 AM
  #3  
gregoryshock
Thread Starter
 
gregoryshock's Avatar
 
Join Date: Mar 2011
Location: Goshen, IN
Posts: 304
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by Bax
To get optimum power out of your O.S. Max .75AX engine, you'll need to run it near 12,000 RPM or a little more. You have too much propeller on it. The RPM range O.S. states are the limits to RPM that they have gotten during testing. With some propeller, they could idle as low as 2,000 RPM, and with a different propeller could get it up to 16,000 RPM before they decided that more will cause the engine to come apart. Don't expect that RPM range to happen with one propeller. Won't happen.

We'd like to run the engine with a two-blade prop to get good power and efficiency, so a 12 x 8, 13 x 6 or 7 or so. Get the engine between 11,000 and 12,000 RPM for max power with whatever propeller will fly the airplane. Perhaps even running between 10,000 and 11,000 RPM will give you good flight performance. Going below 10,000 RPM will most certainly put you well down on the power curve.

"Scale" propellers are good for display, but almost never good for actual flight. They will be too large for the engine to turn properly.

I understand what your saying...

I want to put the propeller on a corsair 60 by top flite. What size do you think I should get?

Link to kit

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHU80&P=ML

Last edited by gregoryshock; 07-07-2015 at 07:17 AM.
Old 07-08-2015, 05:42 AM
  #4  
Bax
My Feedback: (11)
 
Join Date: Apr 2002
Location: Monticello, IL
Posts: 19,483
Likes: 0
Received 6 Likes on 5 Posts
Default

Start with a 12 x7 or 8 or 13 x 6 or 7.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Manage Preferences - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.