Rudder Centre of pressure with PMB Foil ??

[jehardiman]

Originally Posted by tspeer

.... The differences in performance of the S25e4 family due to trailing edge angle are all at high angles of attack, where the trailing edge angle affects the onset of separation. And the section that has the "optimum" 38 degree angle is clearly inferior to the one that has the 14 degree angle.

The trailing edge angle didn't change the ranking of which section is better at low Reynolds numbers vs high - the NACA 0020 is still better for low Reynolds numbers and the S25e4 family is better at high Reynolds numbers. The rooftop and its effect on the boundary layer didn't change. But there is a definite improvement at high angles of attack because the onset of separation is delayed. It also brought the hydrodynamic center back to 25% chord.

Trailing edge angle has an effect, but I don't think you can call it the most important factor. And I'd be inclined to go for small trailing edge angles over large ones.

Real data does not support small trailing edge angles for real sized hydro bodies at moderate (5-20 knt) speed, especially at high angles of attack. If you are using Xfoil, at what Rn did you set the turbulent transition for. If you used the default (9x10^6) that is too high by at least a factor of 3 for real applications (and we won't even get into the idea of crossflow inflow caused by waves, roll, etc). There is a real problem of trailing edge rollover if the trailing edge angle is too narrow. This causes extreme flow disruption and oscillating flow/pressure/energy over the body. The EPH shape on the other hand with the bluffer aft body and flat spots (yes, there are designed in flat spots that I've had to go down to the loft and make them put them in) which stablizes the mass flow prior to the trailing edge and actually stablizes the trailing pressure vortex, captures it, binds it stabily to the trailing edge, and bounds it in an energy well, i.e. it doesn't have the energy to cause flow disruption. Remember, in real conditions, the water is not moving and the body is; therefore energy flow in the fluid is in the direction of travel. Real flow is forward and inwards at the trailing edge

Originally Posted by tspeer

:-) You're messin' with me, ain't cha? Like I'm going to seriously take the separated flow behind a bluff body like a circular cylinder with a splitter plate to be relevant to the attached flow at the trailing edge of well-designed section. Good one!

Absolutely not messing with you...the body is actualy "streamlined" by two bound vortices, see Hoerner FDD chpater 3. Now that I have Blevins here go look at Table 10-18, figures 1,3,18,and 19. Correcting for how "D" is measured in each figure and using a real Rn of say 2x10^6 the drag is not as different as you would think.

Here's a good one...make a spade rudder out of an exposed shaft and a flat plate. Shaft diameter is 12% of span located 30% of span back from the leading edge; plate thickness ahead of the shaft is 5% span; plate thickness at the trailing edge is 2% span; normal aspect, sweep, and taper. Now make a NACA 0012 with identical aspect, sweep, and taper. Mr Jackson says that when tested at real Rn's (say 5x10^6) the results between the two will be within experimental error and the results between the two will closer than between the NACA and the Xfoil prediction.