Mailing List lml@lancaironline.net Message #70331
From: Jim Nordin <panelmaker@earthlink.net>
Sender: <marv@lancaironline.net>
Subject: Two-blade or three-blade prop for I-550
Date: Mon, 30 Jun 2014 16:11:28 -0400
To: <lml@lancaironline.net>

Hummmm … when I see the military planes with props like the E2C Hawkeye with more than 7 blades I have to think more is better for some. When the hp is high and clearance is tight, more blades is obviously good. Same for our lawndarts – so why not an 8 blade for the LNC2? It’d be short likely - $$$$. No need to be that short.

The Navy’s stuff:


“In 2004, the E-2C's propeller system was changed; a new eight-bladed propeller system named NP2000 was developed by the Hamilton-Sundstrand company to replace the old four-bladed design. Improvements included better fuel economy as a result of increased efficiency, reduced vibrations and better maintainability as a result of the ability to remove prop blades individually instead of having to remove the entire prop and hub assembly.

http://en.wikipedia.org/wiki/E-2_Hawkeye

 

Increased efficiency? Who needs that? Petrol is only $7/gallon. Cheap if you say it fast.

 

Now what to believe is best for your rocketship or which is most efficient, effective, I dunno cause I ain’t no aerodynamicist or “cool” appraiser. Over my pea head. I paddle when things get tough, flap and other stuff.

Jim

 


From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Sky2high@aol.com
Sent: Monday, June 30, 2014 10:14 AM
To: Lancair Mailing List
Subject: [LML] Re: Two-blade or three-blade prop for I-550

 

 

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. 

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