Thanks Grayhawk.   I do think about more cylinders.    It doesn't seem fair that the old pistons made thousands of HP and we are stuck at 350.   A 500hp turbo-charged piston would cruise in turbine territory but have much better range.    ...and if it used some of the exotic metallurgy current in turbines it could be lighter than what we have today I expect.

On Jul 1, 2014, at 5:00 PM, Sky2high@aol.com wrote:

Colyn,

 

As the 2 blade prop reaches horizontal, the descending blade has a higher AOA and the descending blade has a lower AOA with respect to the relative airflow.  In climb there are two airflow vectors to consider - vertical relative to climb rate and horizontal relative to forward speed.  The higher AOA creates more lift - that is why you hold right rudder in the climb with a clockwise prop rotation.

 

With a three blade prop and when one blade is descending through the horizontal, the other two are ascending, not at the opposing horizontal minimum AOA.  Thus, the thrust is more even, the blades are usually shorter and the tip vortice induced drag may be less because of reduced tip speed.  When a blade is ascending and at the horizontal, the other two are descending, but not at max lift AOA.  

 

It seems that 3 blades are smoother and a good match for 6 cylinder engines when the prop is properly indexed.  That is the engine power pulses are more even and the three blade lift curve is also smoother - even in cruise.  With modern prop airfoils, the loss in cruise may be very small.

 

Now you can think about 4 or 5 blades in climb and perhaps eight or twelve cylinders or even two rows of 9 cylinders in a radial.

 

Grayhawk

 

In a message dated 6/30/2014 5:34:25 P.M. Central Daylight Time, colyncase@earthlink.net writes:

Grayhawk,  could you please expand on that climb performance argument a little?  

On Jun 30, 2014, at 11:13 AM, Sky2high@aol.com wrote:

Hmmmm.......  See http://en.wikipedia.org/wiki/Single-blade_propeller

 

Here is more to think about (rather than just efficiency).

 

Blades > 2 = better climb performance - consider the relative air (AOA) to the prop chord for both the ascending and descending blade for a 2 blade versus longer arcs, better bites for more than 2 blades.  Don't confuse  this with level flight where all blades see the same AOA.

 

Blades > 2 can produce the same thrust as Blades = 2 but the prop diameter for more blades can be smaller, thus allowing for higher rpm whilst still avoiding the tips going supersonic.  I.E. The further the tip from the hub the faster the tip is moving at a fixed rpm. 

 

Momentarily consider the weird 2-blade Hartzell CS prop for the 320 - an 84 inch diameter prop cut down to 70 inches.  Most props deliver max thrust about 2/3 out from the hub.  What did that mean for the enormous chord and pitch for that prop?

 

Finally, consider the corkscrew path of each blade tip and its path separation (interference) based on airspeed.  You'll be surprised - odds are the bird will hit the windshield and not a prop blade at cruise speed.

 

Hmmmm.....

 

Grayhawk

 

PS Computations left to the reader and EXCEL. 

 

In a message dated 6/30/2014 9:39:14 A.M. Central Daylight Time, stevens5@swiftdsl.com.au writes:

I am interested in this subject, because I purchased a partly built kit some years ago, (which I am still building!) which came with an MT 3 bladed constant speed prop. In my conversation with a builder in South Australia recently, he mentioned that he had swapped out his 3 Blade MT prop for a 2 blade prop, and increase his cruise speed by 7 knots. (Can’t recall if that was 7 Kts indicated, or 7 Kts TAS.) Not sure how this fits with the graphs which indicate the 3 blades are more efficient! It was my understanding that because a 3 bladed prop generates 3 tip vortices against only two on a 2 bladed prop, it is less efficient.

 

 

Rob Stevens

Perth, Western Australia.

 

 

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Charles Brown
Sent: Monday, 30 June 2014 7:38 PM
To: Lancair Mailing List
Subject: [LML] Re: Two-blade or three-blade prop for I-550

 

I've enclosed the spreadsheet provided to me by Les Doud of Hartzell.  In 2009 his phone number was Phone: 937-778-4262 .  He believes that the 3-blade is more effiicient than the two blade even in cruise.  I have a hard time believing that, and my airplane with 3 blades has not lived up to the prototype Legacy's performance with the 2-blade as documented in the CAFE report; and the 3-blade is heavier and more expensive to buy and overhaul.  Of course, my airplane is probably not as clean as the prototype.

 

But the 3-blade prop sure looks cool. 

 

=

=