Jim,
First I want to warn everyone that the advance ratio (AR) is a measured quantity found by measuring the airspeed of the plane and the prop rpm. The AR, or forward motion per prop revolution, is the airspeed divided by the rpm (the airspeed is in inches per minute - multiply by 60 for airspeed in inches per second). The helical block upon which one makes the blade does not need be the same pitch as the AR. For most it is not. Previously there was a poll in this group concerning which pitch block to use for forming the initial prop for an F1D. The answers were all over the place. I just chose to have the AR and pitch block match. Remember, too, that just because a blade is formed on pitch block of a certain pitch, that does not mean the formed blade will have that pitch as the balsa usually relaxes and changes shape after removal from the block. Many people fiddle around with the blades after forming, or do a second forming, to get the desired pitch distribution.
As stated above, the airspeed of the plane does not alone determine the AR as the prop rpm matters. The AR on my current F1L is higher than most because the prop rpm is lower not because the plane flies slower than most. Brett Sanborn's F1L at this year's Nats had about the same airspeed and prop rpm as mine. Thus, it had the same AR. Bill Gowen says that my F1L gets away with having a higher pitch than usual because it has symmetrical prop blades (less flare) and stiff blades. Thus, the blades do not change pitch during the flight. This means that the blade can have a higher static pitch without stalling at high torques when the blade on other planes flare.
So, for a flaring blade, the initial AoA is less than what I use so that the flared AoA does not get to high. This can be an advantage as this setup allows a lower pitch when the motor runs out or torque. This is how Brett's plane may work. Bill Gowen's F1L may work in a similar way but Bill's F1L flies heavier (and faster) than many others because Bill uses heavier (longer) motors. Bill's style of flying works as he has won the Nats at least twice in F1L (Brett is another Nats winner in F1L).
Mike Kirda also has a point that the pitch distributions are similar no matter what pitch block one starts with. So, one can look at props as part art and part science. But, to answer your question, I think that most good F1L props fly with an AoA of around 7 degrees or a bit more and a prop rpm of 96-100 rpm at cruise. The good fliers do not worry much about this but rather adjust and trim their planes to fly the longest and that includes playing around with the pitch settings. That is why we bring pitch gauges with us to contests.
Leo
--- In Indoor_Construction@yahoogroups.com, <indoor_construction@yahoogroups.com> wrote:
Hi Leo,
Just read over your response a couple more times. I think I am catching on. Am I correct to say that most of the best F1L fliers mold their blades on pitch blocks about 7.5 inches less pitch than their actual blade setting will be? If so, why is that more efficient than using a pitch block with the same pitch as the setting used to attach the blades? Less drag? More thrust? Better washout? This is all very fascinating to me.
Thanks Leo,
James Alderson
Sent from my iPhone
On Sep 8, 2013, at 5:10 PM, leop@... wrote:
Jim,
A plane that advances or goes forward 22.5" for each revolution of the prop is said to have a prop with a 22.5" advance ratio (AR). If an EZB prop is built on a 22.5" helical pitch block, has an advance ratio of 22.5" (the as built helical pitch and the AR often do not come out to be the same), and is set at a pitch of 26" at a 5" radius, the prop blade will have an angle of attack (AoA) of 4 degrees along its whole length. How does one get the 4 degrees? The angle of a blade set at 22.5" pitch is 35.6 degrees at a 5" radius. The angle if the blade is set at 26" pitch at a 5" radius is 39.6 degrees. The difference of 4 degrees is the AoA of the blade.
An EZB is often a much lighter and slower plane compared to an F1L. The as flown AR's may not be the same so a good EZB prop design may not work well for an F1L. As a point of comparison, my current F1L flies with an advance ratio of about 23" (this is by design and I built the prop on a 23" helical block). I set my blades with an AoA of 7.5 to 8.5 degrees (or a 30" to 31" pitch at a 5" radius). The prop has a 15" diameter. This AoA (or pitch) is similar to many other F1L's, w hile the AR of my F1L is more than usual. Stan Chliton's Spirit F1L has an AR of just over 20" and a pitch of 26.5" (I do not know the exact radius but somewhere between 4.5" and 5") for his 15" prop. This gives an AoA of about 7-7.5 degrees. The same AR, AoA, and pitch are found on Laurie Barr's CargoLifter F1L (14.7" prop diameter. Both of these planes are among the best F1L's and one cannot go very wrong copying their design parameters.
Leo
--- In
Indoor_Construction@yahoogroups.com, <
indoor_construction@yahoogroups.com> wrote:
I am becoming more and more interested in propeller design and construction. Recently I read the following specifications found on Bernard Hunt's EZB plan from the early 1990's. Blades molded on 22.5" pitch block/ Blades rotated to 26" pitch at 5" radius when attached to spar. Am I right in thinking that this would produce a prop with more blade twist that a standard blade formed on a 26" block? More angle of attack at the root and less at the tip? (but standard 26" pitch angle at the 5" radius station?)
I'd like to know how I can learn more about these types of adjustments and what they produce.
Thanks,
James Alderson