The topic is a little obscure. But I have been working on knife edges, and I find my planes very tricky to hold in a good knife edge. I also have been practicing on a flight sim and the big difference is how a plane is balanced up/down.

With landing gear hanging down, it seems almost impossible to create a 0 up/down balance. The net effect of this is that once you put the plane in a knife edge, you not only have to keep feeding it rudder, but also a slight amount of aileron, because it keeps wanting to right itself. Combine this with a little elevator to actually get the plane to fly where you want it, and I have my hands full.

So my basic question is, does anyone else worry about this stuff, and balance a plane up/down? Or do you take the easy way out and rip off the landing gear? Or do you do the hard thing and get good at the sticks?

Larry

What plane are you talking about here? From your description it sounds like your model has a high profile rudder or perhaps a large counter balance on top. That will cause the plane to roll in KE, wheels or no wheels. If its a tall rudder or large volume on top, when applying rudder it will tend drop the opposite wing. Right rudder will drop the left wing and vise versa. Although dropping the opposite wing would roll the plane on its back, not right itself. As far as coupling goes, when it pulls to the wheels and elevator is needed that is different. I have one that goes to the wheels on rudder and havn't solved that as of yet. For me, it was best to mix it out on my radio. Slave the elevator to the rudder if your radio is capable. Its also a speed sensitive situation. Best bet is to fly straight and level and apply rudder. However the plane reacts it will duplicate that in KE. Good luck, Joe

The plane I am currently working with is the Hellraiser. It has a wonderful ability to knife edge, however, it always tries to right itself. Thus the question. However, while I didn't understand all of your answer, I was wondering if people are mixing a little aileron in with opposite rudder. This would also create a very flat rudder turn.

Larry

Biplanes has a tendency to roll with rudder application due to the top wing. All of my Ultimates do this. Many planes also have a tendency to pull to the bottom or the top when flying in kinfe edge. You either have to fly it out or mix it out in the radio. I' be willing to bet your plane won't make a flat turn either exhibiting the same characteristics as in knife edge flight. I've had lots of bipes an this is common. A lot of mono wing planes do this as well, some less some more.

This rolling tendency with rudder use also enables bipes to do snap rolls faster. My DPM Ultimate will do outrageously violent and fast snap rolls as will my Eflite ultimate.

Sorry, just thought it was a mono. Joe

My I suggest finding an aerobatic flight trimming guide. I have seen a few different ones on the www, but I cannot remember specifically where. They will tell you a series of basic flight tests that allow you to figure what the airplane is doing, why it is unbalanced, and how to fix it. Sometimes the plane needs adjusting, sometimes a radio mix is needed. Unless you don't need them, I would not rip off the landing gear until all else fails. I have not flown the type of plane you have, so I don't really know. All of a sudden the website r/c airplane (or aircraft) proving ground comes to mind. Part of the problem may be the location of pushrod linkage connections in the servo and control horn which make for a overly sensitive plane. Select the holes that match full servo throw to full throw desired from the control surfaces. If you have the end points of the transmitter too low you lose control resolution (feel/sensitivity). There is a magazine out called 3-D Flyer (.com) where Chip Hyde is doing a series of articles explaining all of this. I just ordered all the back issues because they have such great information.

For the vertical placement of the CG, I would base that on hovering (assuming no up/down thrust) because KE coupling arises from many many other things. Most planes you'll find you have to hold a little up elevator in a hover, or at least input up elevator more often than down elevator, largely due to the vertical CG.

Vertical cg?
that is a puzzle -
When we do setup hover tests on foamie electric monoplanes -which typically have 0-thrust and common thrust/wing /stab c/l, we find the model does pitch down -as noted .
however the vertical cg is just below the wing.
If we hold the model right on the le of the wing - , it typically takes a couple of degrees of up trim and right rudder to get a "balanced " hover --against the fingers.
It almost sems that good old prop spiral flow is at work here - what do you think?
This has held true on dozens of these little buggers we have done.
Flight trimming is another thing altogether and that sorts out easily.
But - this static condition is interesting

Hey Dick,

I'm sure some of the spiraling airflow could be blocked by the vertical fin, resulting in more force UP on the left stab than there is force DOWN on the right stab, resulting in a pitch down moment. I wouldn't think there was much force from that though, although maybe enough for a click or two of trim like you said. Obviously in your tests, you were careful to place the CG where it needed to be, but most people are fairly oblivious to this fact... people just put all the components below the wing because it's the norm and it looks a little better from the top. Throw in landing gear and it's pretty difficult to get the CG high enough. A few clicks like you experienced is nothing compared to some planes I've flown that require up elevator all the time in a hover.

Another technical consideration would be downwash from the wing. The downwash in level flight is probably inducing an angle of attack on the stab (a negative AoA = positive pitching moment), but then in a hover the wing isn't creating downwash like it is in level flight. Therefore, if you trim for level flight, it may be like being trimmed down elevator in a hover. More to think about... bottom line is you want to get as close as possible, because these little nuances have much less affect than having the CG an inch or two below the thrustline

Baron

I am going to add above the wing battery mounts ( Velcro ) and double check latest planes.
The most noticable change is back to the old standard - lower wing loading .
In the following pics - the two models are identical in size but one is 3mm and the other is 6mm Depron (both braced with small cf rods )
(both with the new Hacker outrunners -small 34 and large 22L)
The white one with red yellow blue tip splashes is 7.7 AUW
The red black white one will be 11 -12 ozs.
We found that the light one at 300 squares (wl of 3.8 ozs /sq ft!) beats everything we have tried so far -so I duplicated it except for weight and power .
I expect to see faster flying - but I want to see what else really changes -good or bad.
This business of whacking things from foam then test flying the same / next day - is incredible.
I have learned more with this stuff in a few months than I could have done in a year with the gas stuff.
One noticable thing is getting aileron diff correct - the single servo for aileron -can easily produce too much down to up aileron and the rollers try to pull to the down aileron side!
weird.

Hi Larry

As you probably know I also have the Hell Raiser biplane. And as you know from the aerobatics forum, I've got mine pretty well trimmed out for knife edge. My HR doesn't want to roll out of the knife edge unless I enter the knife edge from a nose high (high alpha) start. But if I'm flying with a mostly level attitude before rolling into KE, the plane doesn't seem to want to roll out. I have no clue what the aerodynamic reason for this is. It might even be more of a speed issue than an AOA issue. Hard to tell. But I have seen this behavior and it seems to be avoidable if you enter the KE in certain ways.

Clark