Ron,
From previous posts Ive seen of yours, it looks like you have worked alot with airfoils. So please bear with me as I try to pick your brain to figgure this all out as I do not pretend to know alot about this. And Im not trying to second guess or be critical of peoples designs here , only trying to understand what is really going on, on my props as well as others.
For people just trying to make a blade from scratch with a desired (at least approximately close) tsr, without following an established airfoil, similiar to the simple drawing I posted on Dan's post how is it done? In the blades Ive carved the chord (longest line from leading edge to trailing edge) is very close to the same length as the front face of the blade. So I have always calculated the face angle ( angle relative to the wind flow coming in at a 90 degree angle to the face) in the manner shown in my drawing, and thought the approximate TSR was this slope at the tip ( making the assumption the face angle and the chord angle are nearly the same). As I mentioned in my post, The actual RPM I get (no loading) is always close to what I was trying to cut. As best I can measure things -measuring the frequency of the AC output (Hewlett Packard multimeter with frequency setting or using an Oscilloscope)) and adjusting for # of magnet pole pairs I get revs per second which gives me a pretty accurate reading of RPM. Measuring the wind speed is a bit less precise but Ive stood off to the side of a prop that was not too high off the ground with a store bought wind speed meter and also used as many intuitive factors as possible to judge if the meter seems to be correct. Even if I assume the wind speed measurement is +/- 20 % I am still in the ball park when I compare what the RPM should be if I cut the TSR at some number and then compare it to the real world RPM's for a given wind speed. I just finished a 3 blade 4.3 ft prop that I tried to cut 10:1 TSR and when measuring wind speed VS RPM (multiple samples taken then averaged) I am really getting 8.75 :1. Carving error? Some other factor I havent taken into consideration such as how flat do the bladed mount to the rotor hub? All the measurements were made with very little loading on the prop. I have not taken exact measurements to substantiate the following statement; but I notice that as I add loading to the prop it slows down slightly as load is increased up to a point where the loading gets high enough (say equal to about 35 to 40 % of the theroetical max for the given prop size and given wind speed) then it starts slowing down (stalling?) at a much faster rate. Ive always assumed the stalling prop resulted in slower RPM resulting in lower electrical output until the RPM began to stabilize at a lower RPM and a lower power output, and no longer funning at the intended TSR but at something lower for the given wind speed at that instant - essentially slipping relative to what it should be running.
So what errors in approach or calculation or reasoning am I making? Is actual TSR measured at relatively light prop loading or at something greater?
I agree with you that not enough data was given to determine the actual TSR in Dan's report. There was another piece of data given that said he saw 1.5KW at 300 RPM. If I understand how to calculate it correctly, assuming the blade is really a TSR of 6, then the wind speed needed to achieve 300 RPM on a 17 ft prop would be around 30 mph. Was the wind really blowing at 30 MPH? Im not sure, except in all of his report, he mentioned only light winds, and no mention of winspeed guess at this data point, but there was no mention of high wind either and I consider 30 MPH pretty high. As a double check on this reasoning, if he was getting a typical power output from the system of 1.5 KW as calculated using P=0.0012 X Dia^2 X windspeed^3 (mph and ft. as the units) then he could have gotten this amount of power at slightly over 16 mph wind speed, but if the TSR is actually 6:1 this would only be giving him about 160 RPM, yet he stated it was at 300 RPM. Thats one of the things that leads me to think the real TSR is much higher than 6:1, or 7:1. Note to Dan, I realize your annimoter was broken at the time, but what would you guess the windspeed to be when you made this measurement.
Im totally open to comment / correction. Thanks
Johnlm