it is not. i cant get it stable. it starts out small and gets worse and worse. even when i give it full control the opposite way it still does not compensate enough. they gyro helps a little.

yes though, it is a home creation.

I don't think that dihedral will have the desired effect here. In a normal plane, when it rolls a side slip velocity component is added to the free stream which changes the angle of attack of the wings. The upper wing's angle of attack decreases, and the lower wing increases, which changes their respective lift and forms a righting moment. While you will experience some side slip due to force balancing on you saucer, it will not have a stabilizing effect because your lift is not created by airflow over surfaces. Your fans remain a constant distance from the CG and a constant angle relative to the CG, so the moment they create about the CG can only be changed by varying their thrust. Putting in a dihedral angle to their thrust will have no effect, except for requiring more thrust for level hovering due to the thrust not being perpendicular to gravity. I drew up a free body diagram of your craft, and found a very interesting result. As the craft begins to tilt, the moments contributed by gravity out by the fans actually increase the tilting. The lower wing's moment is a function of cos(phi-theta) and the upper cos(phi+theta), where tan(phi) = distance cg is below fans/radius from center of craft to assumed point of gravity acting, and theta equals tilt angle of craft. As can be seen from the two cos expressions, when phi is greater than zero, i.e. the cg is below the fans, a slight tilt increases the downward moment on the lower wing and decreases it on the upper wing, which augments the problem. This seems counterintuitive to me, and it's effect is probably almost completely wiped out by the pendulum effect, especially since a very realistic assumption for your craft is to assume all the weight is clustered right around the CG. If anyone draws up a free body diagram and disagrees please let me know, as I have rechecked mine a few times because it is so counterintuitive, but can't find a problem with it. I agree that your stability issue is much more a controls issue, since changing the thrusts will have a huge effect on angular acceleration of the craft. Why did the Osprey have problems? vertical thrust applications are hard to control well because they don't have much natural stability.
Good luck with your project.

Jamie

Welcome to RCU Jamie. You got me on the fans angle. It won't help. Sorry I suggested it.

As for the other effects you mention, I don't follow. Maybe if I sketched what you are talking about I would, but I don't have time.

jgull,
i think you may be on to something. i have changed some components around so the cg is about 3" from the fan-line, and the fan line is about 2 inches above the bottom of the saucer. (the long flat part, i am not talking about the part that extends downward that holds my radio equipment and battery) so the gc is only about 1" below the bottom. the disk is 36" diameter, and the four fan holes are about 11" diameter each. i am hoping this information will help you a bit. while i hardly understood what you said, i caught a little bit. you said according to your equasion, if phi is the distance the cg is below the saucer, and if the value is greater than 0, a slight tilt will increase the moment on the downward side and decrease it on the other side augmenting the situation. so in my thinking, if you gave phi a negative value, a slight tilt would decrease the downward moment on the lower side and increase it on the other side causing positive stability? or would it just cycle the equasion the other way and create the same result? would the solution be to give it completely neutral static stability?

BTW, it does not jsut tilt over to one side, it tilts over, and then tilts the other way much more severely than the 1st time and by about 4 times it is on it's side, it slips out of the air and falls to the ground. this sounds like what you have discovered to me.

Spaceclam,

I think you got the gist of what I was saying. Looking at my diagram, it seems that making phi negative, i.e. placing the cg slightly above the fans, would have a stabilizing effect. Note, however, that I don't mean above the thrust line, as the thrust line is not what contributes to the stability problem. I looked at your craft in a cross section, and broke it's mass into three parts. The largest, and central part is where you keep the battery and radio and whatnot, and then two smaller outboard masses that approximate the weight of whatever is outboard, i.e. fans, motors, and structure. If you look at the moments around the cg, which is a good way to do it, you can see that the two fans cancel eachother as long as their thrust is the same. If we assume that the main mass, i.e. battery and radio are at the cg, which is not a bad assumption as they are probably much heavier than the rest of the craft, then they contribute no moment. If we assume that the main mass is slightly below the cg, which it would be reality, then it contributes a slight righting moment when the craft is tilted. This will have a stabilizing effect. Looking at the outboard masses, when the craft is tilted, they destabilize it. The easiest way to see this is to draw a tilted T representing a cross section of your craft that has tilted. Make the vertical portion much longer than reality in order to better see the effect. Draw two gravity force vectors at the outer tips, they will be the same magnitude because hopefully you made your craft equally weighted. A trick to see their moment is to extend those lines downward past the cg at the bottom of the T. You can then measure the moment arm of each force as the distance between the downward extending line and the cg where the line you measure is perpendicular to the downward extending line and goes to the cg. It is obvious that the moment arm from the downward tilting side is larger than the upper, and since the force magnitude is the same, the downward portion contributes a larger tilting moment, which augments the problem. However, as I mentioned before, this effect is probably small. The outboard weight will be small compared to the rest, and since in reality your main weight is probably a little below the cg, it may cancel the effect. Further, phi is very small on your craft, I think about 10 degrees or so. It will have some effect though, and moving the cg up towards neutral may help.

I think your main problem is that you have a diverging system. Around neutral it's quite unstable, and needs to tilt a good bit before it's slight stability kicks in to correct. Then it will overcorrect because of its lack of stability around the neutral point, and then this process repeats until, plop. The thrust made by each fan will have a huge effect on the moments of the craft. When these oscillations happen, are you trying to control it? It may be you overcorrecting because it responds too slowly. Maybe give a try and try to be very gentle, letting it correct very slowly, and as soon as it begins to correct taking out your correction.

I saw a demonstration of a similar craft once. They incorporated micro angular sensors and rate gyros and accelerometers in theirs, and I think a small computer chip that kept it stable. When they flew it it was with a RC transmitter, so I'm not sure how much of their electronics was doing the work, if any. The presentation was in German so I have no clue what stage they were in with their development! I guess just keep playing with it, and try to figure out a way to control the fans together.

Good luck,
Jamie

here's the scoop.
it rocks back and forth even more violently when i do not correct. i hung it on a string so i could see what happened, and it was not pretty. it was so violent that i had to stop after about 3 seconds, for fear of ripping the drywall screw out of the foam (that is what it hung from) however, not even full correction can make up for it, and all my travels are set to 100% right now.
it is early in the morning for me, so, what are you saying here? are we saying that i should bring my cg to neutral, or above the of the craft? the battery and radio equipment is currently housed in the lower extremity. the upper one is purely cosmetic. i could easily house my stuff there if need be. if it helps any, the model weighs 2 3/4 lbs.
what i am confuzed about is why it is unstable in the neutral position. it only starts to rock back and forth when my thrust/weight begins to reach 1:1, and it is not ground effect as i have hoisted it well above that and it still rocks. any ideas? i already have 2 peizo gyros onboard.

if you can get the ufo to spin the other way your motors are spinning (like a gyro) I think it will help the stability..just a concept though...

the motors are counter rotating. if i were going to make it spin, it would be very tough to keep orientation.