That was my feeling also. Attached is a e-mail to Tarver and his reply.
I am not a Prop Engr, so hate to screw up something that someone more knowledgeable than me has designed.
----- Original Message -----
From: Roger Peterson
To:
info@aeromatic.com
Sent: Wednesday, November 25, 2009 7:35 PM
Subject: Prop for my PA11
Enjoyed talking with you about a prop for my PA11. I will be ordering it next week, but still trying to decide if I would be better off with 74" or 76" blades.
I have a souped up 0-200 continental that is developing between 115 and 120 HP. I have the engine balanced the same as the ones used in the cafe races and it is very smooth and I have run is for a couple of hours at 3500 RPM
I am now using a 76AK2-40 Sensenich and my static RPM is just under 2500 RPM
At WOT, level flight, I can reach 3,000 RPM but normally cruse at about 2400 RPM which gives me a speed of about 83 mph.
I spend 5 month each summer in Canada on floats, but it would sure be nice to have a faster cruse as the flight back and forth to Texas gets long.
I have been playing with the tip speed calculator on P Ponk Web Site
http://www.pponk.com/HTML PAGES/propcalc.html and if I stay with 2750 as red line, it looks like 76" might be better, but I know there is more to it than that. I have flown in Super Cubs using the Bore 80" props and they really pull.
Roger
Just a few problems I see with the tip calculator. You have to add the forward speed of the airplane to the tip speed. The calculator sayd that a 74" prop doing 2800 rpm, the tip speed is 616.4 mph and that equals mach .86 at 0 deg. At 100 deg 616.4 mph equals .78 mach.
Add cruise speed of 100 mph to the 616.4 = 716.4 mph which = approx mach .94.
Density altitude is used to calculate mach number and DA not only takes into account temperature but also the specific weight of air at the altitude of operation. The thickness of the blade at the tip also affects what tip speed you can go. A real thin blade can go closer to mach 1 without mach buffet than a thicker one. The effeciency curve is not linear. You could have air going over mach 1 when the tip speed is less than mach 1.
At the flight levels that we fly at, usually vfom 0 to 10K' the temp from summer flying to winter flying is far greater difference than the elaps rate of temp due to altitude change. Where I am I could be flying in the winter at 0 degrees and as high as 110 deg in summer. That's a big difference, 110 deg. The temp gradent from 0 to 10,000' on a given standard day is only 35.7 deg.
So you can see that there is more to it that the calculator deals with.
Happy Thanksgiving.
Kent