why is this thing stable in roll? it flies, and with enough control to fly circuits around the yard, I bet a roll or loop is not out of the question.

[link=http://www.rcuniverse.com/forum/m_8222254/tm.htm]http://www.rcuniverse.com/forum/m_8222254/tm.htm[/link]

I am completely baffled, my only guess is the chines have some dihedral effect. but the center of mass is way above the center of lift (the sled)..

They act like poly dihedral

A roll would still be a coordination challenge - up elevator - rudder - down elevator - neutral

I really don't think it's so much the chines as the big fin sticking up and combined with the extreme angle of attack along with the extreme short aspect ratio. When there's so many "extremes" linked to any design the results defy "normal" logic. In this case the strong vortices off the very short "span" combined with the high location of the fuselage or center pod along with the fin and rudder are providing the stability in roll. Like you say though it's about as delicate as a soap bubble and any sort of more extreme maneuvers, especially at this low an altitude would likely ruin the balance. A loop should be not only possible but probably not hard with more power. But the final pull out is going to use a LOT of altitude. So to avoid the dreaded Figure 9 be sure to keep it up high. As for rolls? I wouldn't.

after some testing today it rolls fine, but it's bit underpowered for a decent run on a loop.. it needs some "jenny craig" action.. too much bidi tape

My Flying Lawnmower did a decent roll. Not sure how to catagorize the stability of the roll. It would lose a lot of altitude.

did your lawnmower have ailerons?

Hi cncswiss1
Yes, but they are elevons since they also handled the elevator function. Flew it for about 4-5 years and always had a ball with it. one day it dived into the ground for no reason I could find. My guess is bad switch. Decided not to rebuild it so I could build my SPAD biplane.

the ability to do rudder only roll is what baffles me, I have no ailerons at all, RET only.

Roll stability can be caused by having more side surface area above the vertical C.G.. Judging from the pictures of your craft, the vertical C.G. is just above the chines by maybe an inch. Most of the area of your rudder and v. stab. are above the vertical C.G.. So, when your craft rolls a bit and side-slips in that direction the greater surface area above the C.G offers more resistance to the side-slipping than the area below the C.G. thus righting the craft automatically. If you want more roll stability, add more area above the vert. C.G., but keep that added vertical area aft of the longitudinal C.G so you don't sacrifice yaw stability in the process. The chines themselves are probably adding to the roll stability too because they are angled out like wings with extreme dihedral.

From the video it looks like your craft flies a lot like a "pizza box flyer". PBFs are not too different than flying lawn mowers. My PBF was very stable at high angles of attacks (45+ degrees) too and roll-stable at all AoAs. Although it had elevons, I found that I could steer it around quite well with rudder and elevator input only because of this roll stability. In other words, aileron input wasn't necessary to fly it around. Aileron input was only necessary for performing certain aerobatics.

I don't have any video of my PBF, but here are a couple of pictures.

my PBF needs opposite aileron to keep flat with rudder turns

You've thrown me for a loop. I'm not sure what your point is there. Maybe you are confusing rudder turns with flat turns? Your snow craft isn't doing flat turns as it is banking in the same direction that it is turning. My PBF banks the same as your snow craft with rudder input. For example, left rudder causes left banking. Up elevator is then needed to keep from losing altitude because of the left bank induced side slip to the left. The result is a left banked turn not a flat turn. Flat turns are specific maneuvers that require the negation of any roll coupling that the rudder input causes in order to make a turn with the wings level and with fuselage side lift only. All flying craft with positive roll stability will need opposite aileron to maintain a flat turn. Your PBF should behave just like my PBF and your snow craft with rudder input as your PBF needs opposite aileron to maintain a flat turn.

Also, we all know that duct tape is the handy man's secret weapon, but as Red Green says "nothing says 'poor craftsmanship' like wrinkles in your duct tape." ;-)

Or was that "Nothing says 'unprofessional job' like wrinkles in duct tape." I couldn't track down the exact quote.

gotcha, if I fly it sans opposite aileron it'll roll into the turn as in normal flight, the ole bank and yank

Yes, a friend gave me a rudder and elevator only plane a while back (it wasn't a PBF but had lots of dihedral like a trainer for lots of roll stability and thus, banking with rudder input the same as with my PBF). I set it up and flew it with, of course, the rudder servo plugged into the rudder spot on the receiver. I was not used to using the rudder stick for anything other than knife edge flight or specific aerobatic maneuvers. It was disconcerting for me to fly without aileron control, so after I landed, I plugged the rudder servo into the aileron spot on the receiver. The next flight I felt right at home controlling the rudder with the aileron stick because the rudder had so much influence over the roll control on this plane.

The rudder's yaw/roll coupling is caused by the dihedral effect of the greater, total, side area above the center of mass than bellow the center of mass. For example, left rudder input causes left yaw and positive air pressure on the right side of the fuse and v. stab. The greater total side area above the center of mass offers greater resistance to this air pressure from the right and rolls the craft to the left.