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Props and engine mount/propeller selection

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jbhart(at)onlyinternet.ne
Guest

Posted: Wed Mar 28, 2007 5:23 pm Post subject: Props and engine mount/propeller selection

Date: Wed, 28 Mar 2007 12:21:11 -0700

From: Ron <captainron1(at)cox.net>

.................. 

 	  Quote: 	
		   I've wanted a 4 blade prop for some time, I think it probably gives better 

thrust effect (I don't want to start up the old debate of how many blades 

are better). I mostly want to do it for my own gratification and to validate 

my theory. That is why I figure a 68" would be fine. 

............... 

Ron,

In making propeller decisions look at: 

http://www.gylesaero.com/freeware/propcalc.shtml

Using this software I input for a 63 mphi, 2,222 propeller rpm, and 68 inch 

diameter propeller for the FireFly.  Then I changed the pitch for a two 

blade propeller until the engine was putting out 38 hp.  This produced a 

thrust of 166.5 pounds with a propeller efficiency of 73.6%.

Leaving all other inputs constant I changed from a two to a three blade 

propeller and adjusted the pitch until three blade propeller produced the 

same thrust.  Propeller efficiency fell to 69.6% and the engine power to 

produce the same thrust went up to 40.2 hp.  Adding another blade to 

propeller produced an efficiency of 64.3% and an engine hp requirement of 

43.5 hp. 

Then I changed tactics.  I kept the initial conditions, thrust and 

efficiencies constant as I added blades.  To do this you have to shorten the 

blade length and adjust pitch.  What I found was that to maintain 73.6% at 

166.5 pounds of thrust the three blade propeller diameter turned out to be 

57.6 inches and the four blade came out at 52 inches.  

To go the constant 68 inch diameter route, it indicates a 4.1 to 9.3% 

increase in fuel burn.  To go with constant efficiency with no increase in 

fuel burn, one must be prepared to shorten the blades.  If length can absorb 

max engine hp, it is difficult to beat a two blade propeller.  The inertia 

is lower, weighs less, and in most cases is less expensive.

Try the program for your intended setup and see how it comes out.

For what it is worth.

Jack B. Hart FF004

Winchester, IN

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captainron1(at)cox.net
Guest

Posted: Wed Mar 28, 2007 6:50 pm Post subject: Props and engine mount/propeller selection

I have no doubt that Induced drag and Parasitic drag on a 4 blader          will be higher, however I also think that I will get more thrust out          of it with my set up. It is obvious to me and I measured again today          that my max prop can be 68 inches. So working with that, and all the          turbulation that a pusher prop experiences I want as much air          grabbing, thrust sucking, as I can garner out of that 105 hp in back          there. I don't mind being proven wrong on this, I think its fun.              

         Ron (Arizona)              

                       

         ==========================================              

         

         ---- "Jack B. Hart" <jbhart(at)onlyinternet.net> wrote:



=============

-->          Kolb-List message posted by: "Jack B. Hart" <jbhart(at)onlyinternet.net>



Date:          Wed, 28 Mar 2007 12:21:11 -0700

From: Ron <captainron1(at)cox.net>

...................

 	  Quote: 	
		            I've wanted a 4 blade prop for some time, I think it probably gives          better

thrust effect (I don't want to start up the old debate of          how many blades
	


are better). I mostly want to do it for my own          gratification and to validate

my theory. That is why I figure a 68"          would be fine.

................



Ron,



In making          propeller decisions look at:



http://www.gylesaero.com/freeware/propcalc.shtml



Using          this software I input for a 63 mphi, 2,222 propeller rpm, and 68 inch

diameter          propeller for the FireFly. Then I changed the pitch for a two

blade          propeller until the engine was putting out 38 hp. This produced a

thrust          of 166.5 pounds with a propeller efficiency of 73.6%.



Leaving          all other inputs constant I changed from a two to a three blade

propeller          and adjusted the pitch until three blade propeller produced the

same          thrust. Propeller efficiency fell to 69.6% and the engine power to

produce          the same thrust went up to 40.2 hp. Adding another blade to

propeller          produced an efficiency of 64.3% and an engine hp requirement of

43.5          hp.



Then I changed tactics. I kept the initial conditions,          thrust and

efficiencies constant as I added blades. To do this you          have to shorten the

blade length and adjust pitch. What I found was          that to maintain 73.6% at

166.5 pounds of thrust the three blade          propeller diameter turned out to be

57.6 inches and the four blade          came out at 52 inches.



To go the constant 68 inch diameter          route, it indicates a 4.1 to 9.3%

increase in fuel burn. To go with          constant efficiency with no increase in

fuel burn, one must be          prepared to shorten the blades. If length can absorb

max engine hp,          it is difficult to beat a two blade propeller. The inertia

is          lower, weighs less, and in most cases is less expensive.



Try          the program for your intended setup and see how it comes out.



For             

      [quote][b]

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jb92563

Joined: 23 Mar 2007
Posts: 314
Location: Southern California

Posted: Thu Mar 29, 2007 9:59 am Post subject: Re: Props and engine mount/propeller selection

Note: When using that "Prop Selector" program the fields  "Output Power" + "Absorbed Power" must be less than or equal to the max HP your engine puts out at the selected RPM.



If they tally up to more then that, it means in reality your engine can not spin the prop as fast as you selected and/or have over proped the engine.



Try lower pitch/Diameter or RPM until you get the HP your engine is capable of for that RPM.



I designed my own prop selector program as well that automatically reduces the Dia or Pitch for the HP of your engine. 



Unfortunately it does not go past 40" diameter. I may have to haul it out and revise it for bigger diameters.



Ray

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_________________
Ray

Kolb UltraStar (Cuyuna UL-202)
Moni MotorGlider
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