1. There are two tails. In the side view we only see one. For a back-of-the-napkin Cp, stack the second tail on top of the first one and then look at it.
Be careful in your assumptions of the second wheel cover. CP is aerodynamic pressure. If your car yaws, one of the wheel covers is in the shadow of the other. It is highly probable that turbulence tumbling over and around one cover and the wing, will significantly reduce the aerodynamics of the other cover. The area of the second cover you are summing for a CP probably should be significantly reduced or, conservatively, maybe not be counted.
Well, let's take a look at that: It would take 20 degrees or more of spin before the nose even comes into line with the "shadowed" tail and about 60 degrees before the the nose of the leading tail comes into line with the trailing edge of the "shadowed" tail. If it's gone around half of that I'll take John's advice and pull the chute!
Teague reported to his crew that he could only get a few degrees (single digits) out of line at high speed because of the stabilizing effect of the rear wheel fairings. If Kent wants to ignore Al's 30+ years of experience and success, who am I to argue?
Blue, as I mentioned in another thread, I’m trying to understand your understanding of the CP. I have always used this simple method (see second paragraph) to determine CP. http://exploration.grc.nasa.gov/education/rocket/rktcp.html
Please note it uses the projected (i.e., the flat plate) area of the part. However, from your posting above you consider the part’s shape to arrive at a modified area. For example, while I would consider the full area (as seen from the side) of the body you decrease “effective” area due to the rounded shape. Since we are concerned with dynamic pressure that makes sense. But do you see anything inherently unsafe with using projected area? All it would do is move the CG and CP farther apart, no?
Yes, projected area is a radical over-simplification; especially for round bodies. Go to the next page where they start to talk about how stability analysis is done. At the bottom of the page is this:
<<When computing the stability of a rocket, we usually apply the aerodynamic forces at the aerodynamic center of airfoils and compute the center of pressure of the vehicle as an area-weighted average of the centers of the components. >>
Each component has to be analyzed separately for its individual contribution to stability. Why? The area of a fin creates a lift slope vs. angle of attack based on its shape, especially its aspect ratio (reference span squared divided by reference area). A tall fin (like Speed Demon) is 3 to 5 times as effective as the same area of a "blade" tail (like Nebulous). Multiply the lift-slope derivative by the length from CG (tail moment arm) and that is the stability contribution of that component.
Bodies (like most LSR, including this design) are more complicated and we usually chop them up into sections and calculate the effective lift-slope for each section, multiply that section by its area, then add all of this up. Pile it up on a spreadsheet and float the CG around until the sum of all of the lift slopes is zero and that's the yaw-neutral point, or, "Cp". This is called "second order" analysis, doesn't require any calculus, and is over 95% accurate for low to moderate yaw angles. Farther than that and we're pulling a chute anyway.
Sharp edges on the top or bottom of a body or fin increase the lift-slope, round edge decrease it. So the round top on Costella's designs improves the stability of the body forward of the CG and hurts it aft. Conversely, the sharp lower edge forward hurts the stability while it helps the tail. Jack has enough "effective" tail aft far enough in his designs to be stable. I'd like to see less sharp edges forward, we disagree, I still respect him.
For CAD simplicity, Rob has rounded the tip and bottom of the fins in the published figures. When I fab them, I'll be putting a specific shape on the top and bottom that has been proven in air racing to be much more effective than any other tip design to date. Far from taking credit for the 747, I actually adapted Boeing raked wingtip design rules to straight wing airplanes and we went faster. We modified 7 L-29's over the last two years and they go far faster on less power than our competition. Right now, I have modified 4 aircraft to go from the 300 mph class to the 400 and 3 more to go from the 400 to the 500 mph class. 3 of these unfortunately don't race, they are used as military targets in cruise missile and counter-air intercept simulation. Like LSR, it's the kind of thing that's only impressive to the people inside the field.
Next mod gets tested Monday, looking for another 30 mph on a 500 mph airplane. I'm 13 for 13 so far with 17 separate mods, making ever aircraft I touch faster with every mod I've made.
