I was flying my new Venus II last weekend (second time out with it since building it new) and while flying knife edge from right to left the plane sort of porpoised twice with no change of input. In other words, it was holding the line at first for say 75' then the nose dipped down, then back up to normal, then down, then back up to normal. The first time this happened I practically suffered cardiac arrest because I have never had a plane act like this in knife edge flight so I thought for sure I was going to lose total control and crash. But I leveled it out, brought back around and tried some other maneuvers that went fine and then tried knife edge t again, and wham, the exact same result, two sort of porpoise or bobbings (if you will) in a row, really really strange. I tried it a third time after yelling at everyone to watch and they all said "wow", I thought you were doing that with the sticks...but I was not. The plane seems to fly great every other way that I have tried out so far...e.g. inverted, loop, rolls..etc, also engine (a YS110-S) was really really running well so that can be ruled out.

I landed it and checked the stiffness of the rudder horn, connection rod and servo and they check out great. Also I am using a new hi-torqe Futaba I think 3020(?) BB servo (I can never seem to remember the servo numbers anymore). I was wondering if anyone might know what may be causing this? Note: I thought about trying knife edge in the other direction to see what happened but did not get a chance before I packed it up and called it a day. thanks in advance

Could be a number of things....

What type of RX battery are you using?

What is the torque rating on the rudder servo? It may not have enough torque or the servo could be defective. It sounds like the servo is losing power and then getting it back again. It could also be overheating under load.

You could also be getting radio interference. What frequency are you on?

Also, I see you're using a connection rod between the servo and rudder. Is the rod fully supported? It could be bending/bowing under load inside the fuse.

Cheers
Jason.

Due to the predictability you can pretty much rule out interference. Sounds to me like the characteristics of a stall. I don't know enough about the Venus to know if it can stall on knife-edge, but that just sounds like a stall behavior. It's also possible that the rudder servo is putting too much demand on the system and causing a temporary low voltage condition that can cause problems depending on what type of system you're on (PPM, PCM, or Spread Spectrum).

Sierra, is your rudder servo mounted in the tail or inside the fuse? If it is inside the fuse, I would wonder if it is the control rod flexing under load.

I had the Venus II with the same engine; the plane should be able to hold knife edge all day and all night. I had the JR DS821 for all servos on a 6 volt power system - that's 88 oz-in of torque. It was enough to fly the plane though I'm not sure what would happen with less power on the rudder. The Futaba S3010 is the standard, hi-torque, ball bearing servo and has 72/90 oz-in depending on battery.

What are you using for a prop?

Here's some more information to gather: Have you tried KE rolling in each direction so you have used both right rudder and left rudder? Could the pushrod be buckling in the pushing direction and work fine when it is pulling? How does the servo/pushrod (rudder)surface behave if you (on the ground) move the stick and resist the movement of the surface with your hand (especially in the direction you have had the excitement)?

I'd be intested in knowing what you find!
Tom

THANK YOU ALL FOR ALL THE RESPONSES. It took me a while to get back on this thread due to tax day among other things so thanks for your patience. Battery is 6V 1600mAH. The direction of KE right to left has the push rod in tension so it can't be buckling. I guess it could be stalling, I never thought of that, I thought I had it running full throttle, so a stall would be a surprise but I guess it is not out of the question. I will take it out this weekend and KE it the other direction (left to right) to see what happens and report back. I am thinking it could be a voltage drop so I will install my 2000mAH 6V battery to see if that changes anything. Yes, the rudder servo is brand new Futaba 3010 and when I put it under load on the ground the rudder, rod, clevis and servo all hold well.

Prop is 14X10 and the Rudder servo is mounted on the side of the fuse near the tail with about a 7 " long rod, whatever rod is the next larger than the 2-56 (there goes my memory again!) is the one I used, pretty heavy stuff and the control horns are all the heavy duty type. Radio is Spread Spectrum Futaba 8 channel

Sounds like you are OK with the prop, the servo, and the linkage, as long as it is all working properly. The YS 110 and a 14x10 prop should give you all the power and speed you need - I had that combo for a bit and was satisfied.

Maybe one thing to check is your rudder throw rate - too much throw would cause a stall, or maybe some other odd behavior. It doesn't take too much rudder with that plane to hold a KE.

Keep us posted.

Unable to get out last weekend due to poor weather. They say it is supposed to rain this weekend too.

rain sucks.

I did get a chance to go out early Saturday before the rain came but I did a maiden on my P-47 and man did that thing fly nice after the first trim out and throw adjustment flight. So many planes, so little time (and weather)!

Without knowing the exact specs of the servo i'm speculating but my theory is that the servo is fading out due to heat and load from pressure on the rudder doing the maneuver over an extended period of time.

You said the plane maintained a good knife edge for some distance before it happened the first time, so perhaps the pressure on the servo and strain caused it to heat up and fade.

On the 2nd and 3rd attempts it probably happened earlier in the maneuver due to the latent heat remaining from the first instance.

I have a Venus II and use a Futaba 9151 Digital F3A rudder servo with 132 oz-in. torque and use a regulated 5.5V power source. My Venus will knife edge the length of our 1/4 mile strip without a problem at all.

great info, thanks! Sounds like the most plausible explanation

weather was AAA+ today and I flew the Venus and is knife edged all day. I flew knife edge circles and it was on a rail. I don't know what was causing it but it did not happen today, it flew great!