Wright Stuff B

[carneyf1d]

a plane has 3 directions of stability. pitch, yaw, and roll. Pitch is basically nose up and nose down. Yaw is nose left, nose right. Roll is bank left or bank right. Pitch stability: To keep a plane from stalling or nose diving after it hits the rafters you can 1.) have a good amount of decalage. Decalage is defined as = wing angle of attack - stabilizer angle of attack. Usually ranges from 2 to 5 degrees. 5 being more pitch stable. 2.) Build a swept back leading edge on the stabilizer. Instead of having a straight spar for the front of the stabilizer, you can sweep the sides back, forming a V shape leading edge. The sweep doesn't need to be big. 1/2 inch to 3/4 on each side should be more than enough.
Yaw stability is controlled by the dihedral of the wing. Tip plates can also be used instead of dihedral.
Roll stability is controlled by the rudder. All the roll of a free flight plane is caused by the propeller. All the planes turn left because this counteracts the rightward angular momentum of the propeller. Now you know why you've always built your planes with left turn. If you build a decent sized fin or tip plates on the stabilizer, you should have enough roll stability. If you have massive propellers, and roll becomes a major issue, some people like to offset the wing, so more wing area is over the left side of the plane and actually gets lift because of the rolling force.
Hope this clarifies. Let me know

[karatekid44]

thank you that is much clearer.

[jander14indoor]

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.

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.

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

[carneyf1d]

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.

I think everything imaginable about generic building/flying/trimming has already been mentioned in previous posts. All that's left is the little tips and tricks. Unless you can think of something that we've never covered.

[jander14indoor]

No, basics are pretty well covered, but new folks read the list and may not have digested all 900 or so previous posts. I don't want them thinking that you have to worry about things like airfoil shape or moment of inertia to get started.

Jeff Anderson
Livonia, MI

[karatekid44]

so then basics are covered

[StampingKid]

Speaking of the history of our discussions, can we count on this remaining an active board as long as Wright Stuff is an event? I think I would have a panic attack if I woke one morning to find it gone. I have cut and pasted some discussions in the past for future reference but as the discussion progresses more becomes relevant.

[andrewwski]

All threads are archived at the end of the season and we start fresh for the next year. But the threads are never actually gone - they are still there for you to read and reference.

[StampingKid]

Where will we find the archived threads?

[Dark Sabre]

It will probably show up between the team forums and the administrative forums, when we make it visible to everyone.

As for the longevity of the forum...you can still access the Wright Stuff threads from 2003 through 2008 here: OBB archive (the forum prior to the switch to phpBB3).
Specifically: the 2008 WS forum (you will see a lot of familiar faces here ), 05 and 06 WS forums, All event threads from 06 to 03, including some Wright Stuff threads. We plan to be around

Going to be trying to make the archives on phpBB3 a little more organized