StampingKid wrote:Sarcasm is lost on scientists. Or maybe not? <SNIP>
Ah the challenges of trying to be amusing on the internet, especially risky to the humor challenged like myself.
OK, back to serious answers.
StampingKid wrote:But back to the discussion. Things like glues and razor blades have inherent hazards but so does stepping out of the shower. We though still bathe, hopefully. I appreciate the little tips as I have been frustrated more than once at crushing a piece with an X-acto blade.
An alternative to razor blades is real scalpel blades. They are just as sharp as the old carbon double edged razors, slightly thicker so a little stiffer, but not so much as to excessively crush balsa. And they have standard handles with blade covers. I actually use and a generic X-acto holder for both my broken razor bits and scalpel blades. I'm more used to the feel in my hand than a standard scalpel. Slightly pricier than double edged for the cheap among us. Won't slice the lightest balsa as clean as double edged, on the other hand they will cut thicker/heavier balsa better than razors.
StampingKid wrote:I did want to ask about something I read in Ron Williams book about using lighter pieces at the tips of the wing and on the stabilizer so that it is easier for the plane to self correct. i assume that is because the center of gravity is for lack of a better term "centered". My question is: how much lighter? I guess this is the next step in my education from weighing every piece.
Generic warning, the following discussion is one of those WAY out there in degree of importance to getting high flight times (however useful it might be in many other areas). If you are flying state or national winning times, it might be useful. If not, worry about more important things like building the plane light, making sure it ways exactly at the minimum, losing weight, building straight, and LOTS of time spent winding, flying and trimming.
With that caveat out of the way, into the meat.
I think carneyf1d has given you the high level treatment about stability from an aerodynamic sense dealing with all the forces. But that's not what Ron Williams is talking about.
You actually got it pretty close when you mentioned better centered cg. The technical term is moment of inertia. Just like mass is a measure of an objects inertia or resistance to change of linear motion, moment of inertia measures to an objects resistance to changes in spin.
Think of an ice skater spinning. As they move arms in and out the speed changes. Its because their moment of inertia changes while their overall mass is constant.
An object with all its mass very near its center of gravity (skaters arms and legs in tight) has a very low moment of inertia and a low resistance to changes in spin. An object of the same mass , but located far from the center of gravity (skaters arms and legs out wide), has a higher relative moment of inertia and more resistance to changes in spin.
Note, the change in speed has to do with conservation of angular momentum. No forces act while the skater spins so the angular momentum must be constant. Angular momentum is rotation speed times moment of inertia. Arms in, low moment of inertia times high rate of spin equals arms out, high moment of inertia, low rate of spin.
OK, translated to planes. If you make the extremes lighter you reduce the moment of inertia. Those corrective forces carneyf1d mentioned have an easier time correcting the flight path back to nominal. Said another way, you can use smaller control inputs to get stable. Smaller generally means less drag. Of course gusts or bumps have an easier time too. But in most tradeoffs while flying, lower drag almost always beats out any other benefit or risk.
SIMPLEX airfoils. Higher simplex typically has higher lift, but also higher drag. Lower simplex has lower lift, but less drag.
Side comment to make sure we are all on same page when we say higher or lower. What we are talking about is the ratio of the height of the curve above a line connecting its ends to the length of that line connecting the ends of the curve. To a first approximation, airfoils with a higher ratio of height to length (typically expressed in percent) have a higher lift and associated higher drag.
In a low site, you need to keep speed down to stay out of the rafters so need a higher lift wing and the low speed keeps the drag in control.
In higher sites you need all the torque and turns you can get into the motor, so you will be flying faster, and the lower drag of the lower simplex can be the right trade.
As to why cut off the rear end. Well, that's why us lazy modelers like the simplex curve. It looks like an airfoil and is mathematically derived to give the same height of the curve to length from front to back so long as all changes in length happen at the back. If you use a simple radius, each change in chord length drives a change in radius to keep a constant ratio of height to length.
Oh, and the exact relation of lift to drag for the airfoils we use (ie is there objective evidence that a simplex is better or worse than a simple radius) at the very slow sizes and speeds we fly is very poorly defined. Two reasons. One, its actually VERY hard to get good data as the noise is often larger than the signal. Two, historically most of the money has been aimed at manned flight where the profit is, so little scholarly work at model speeds and sizes. And for a number of reasons the data doesn't scale worth a darn. But due to the military's need for small, covert, unmanned devices, this is one of the hot areas of aerodynamic research currently.
Jeff Anderson
Livonia, MI
)
