How do i setup a mix for knife edge?
 

Can someone please explain to me how to go about setting up a mix for knife edge flight with my planes? Ive never done it and have a 9cap and would like to learn. I would greatly appreciate it.

RE: How do i setup a mix for knife edge?
 

I presume you are looking for a rudder-to-elevator mix such that you can fly knife-edge without the plane pitching to canopy or landing gear; and a rudder-to-aileron mix such that you can fly knife-edge without the plane rolling.

In my experience, these mixes need to be adjusted separately for both left and right rudder. Only way to get them right is to fly knife edge, see what the plane does, and adjust the mix to correct any roll or pitch. Usually this can only be done for a certain speed and rudder input. If you go faster or slower, you'll need less or more rudder, and the coupling will be different. So you need to settle your CG and basic trimming first, including wing incidences if that's a problem. When you're ready to say your CG is perfect and quiet air trimming is perfect, then start working on knife-edge mixing, taking care to keep a consistent air-speed. That means either flying in still air or flying upwind and downwind at the same throttle setting, which would give different ground speeds.

I've heard stories of whiz kids who can put the plane in the air, punch buttons on their TX and adjust mixes in-flight, and come down after a few minutes with the mixes just where they want them! My own primitive method is to fly a couple passes of KE, assess the coupling, land, adjust the TX, then take off, etc., until I'm done. This is somewhat time consuming, to say the least. I've thought of going up with a buddy on a buddy box, so he can fly the plane while I twiddle the mixes, and save the time of landing and taking off.

So is that your question, or are you looking for step by step instructions on the basics of mixing on the 9C?

RE: How do i setup a mix for knife edge?
 

Thanks that helps! Im also looking for some help on actually setting it up on the 9c...and how to actually implement it in flight. Do I begin the KE and then flip whatever switch Ive assigned the mix to? Should it be one of the toggle switches or one of the trainer typer switches? Sorry for all the dumb questions.. I really appreciate any guidance...

RE: How do i setup a mix for knife edge?
 

Hello guys,
really nice explaination majortom!

As the experts suggest, those mixes should remain ON all the time. Actually, they correct a bad behaviour of the airplane: with them you can reach a almost ideal airplane behaviour - i.e. with rudder input only yaw will be produced, no pitch, no roll. One important thing I learned here on RCU from the great Don Sczcur, is that those mixes are important not only on pure KE flight, but they are needed during almost all the aerobatic manouver. When you have them correctly setup, try to do a slow roll, for example: you will be astonished how simple now it is to perform!!! Also rolling harriers are much more easy, always for the same reason - you give rudder, the plane yaw and doesn't do other bad behaviours.

Majortom said you a very important and not obvious thing, that I learned after many airplane setups :) The mixes are quite speed dependand. This means that you have to concentrate on throttle setting during KE and in general try to fly aerobatics with the same speed. This is not a simple thing - one of the most difficult skill to achieve, in my opinion (I'm still learning it :)

Last thought, as majortom pointed out, is that an aerobatic plane should be trimmed to fly straight following some process, and the mixes are at the end of it. This means that there are other things to setup before, so if you have them "off" you can have hard time to mix the plane! Most important are CG and thrust angles, IMHO. If you are interested, just ask, there are some good infors on the net, fortunately :D

What plane do you fly?

RE: How do i setup a mix for knife edge?
 

As Stek says, these two mixes, rudder-elevator and rudder-ailerons, once settled, should remain on all the time. The way to do that is assign them to any switch, and select the NULL position for activating the mix. Null means no matter what position the switch is in, the mix is on.

Normally, with almost any aerobatic model, flying along at straight & level, if you give rudder in one direction or the other, you will get nose pitching down to some degree, and some degree of roll, which can be proverse (roll in the direction of the rudder turn) or adverse (roll opposite direction of rudder turn).

Ideally, you would like to be able to apply rudder in any attitude at any speed, and the plane would respond with yaw only--no pitch, no roll. There are models that come close to this behavior, but they are not easy to find. Knife edge mix will get you closer to this behavior, which is why it's normally on all the time. I repeat, the model's reaction will be different (usually) for left vs right rudder, and vary with airspeed (and perhaps also with attitude). So mix carefully with trial runs using both left & right rudder, and do it all at normal cruising speed--somewhere between half & 2/3 throttle, depending on how 'overpowered' your model is.

I usually see recommendations to start off with mixes of 2-3% for first run, and adjust from there. But you need to know whether it's + or - some percentage. And the conventions are different from one brand of TX to another. What I normally do for setting up my first approximations is set the mix at 5%, then work the controls on the bench. What I want to see is--with left rudder, the elevators come up a small but visible amount, and the ailerons move a small but visible amount for left roll--left aileron slightly up, right aileron slightly down. Then with right rudder, I want the elevators to do exactly what they did for left--both come up slightly; and ailerons deflect for right roll--right aileron up, left aileron down. Whatever you get for + or - mix indications, don't worry about the signs, just get the right control surface response.

You can do all this with the RX turned on, TX turned on, go to the mixing programs, twirl the knob and see the immediate reaction on the model.

I must repeat emphatically my earlier comment, that there's no point in addressing knife-edge mixes until you've got your CG just where you want it, control surfaces trimmed for neutral behavior in straight & level flight, thrust offsets vertical & horizontal for straight up at WOT, straight down at idle, and no change in pitch with change in throttle. Almost forgot aileron differential, which should also be settled before coupling mixes.

As to trimming procedures, there are pages on the NSRCA website, the mini-IAC website, and a few others. Take your pick. I'm sorry to say there are inaccuracies (most fairly obvious) in just about every one of these I've looked at, but on the whole they are useful, especially to someone who is not an expert (as I'm not). The most important consideration is to do the trims in the proper order; otherwise you'll be making trim changes at step 7 that will throw off your adjustments in step 2, meaning you have to go back and do 2 through 7 all over again. Thrashing around like that, you can spend a year and still not get your plane to fly straight.