knife edges to regular flight
07-24-2003, 02:47 AM
#1
Senior Member
Thread Starter
Join Date: Aug 2002
Location: St. Petersburg, Fl
Posts: 342
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
Just got a mixing tx and have to ask a question here....hope it's not a dumb one. If when my model is in knife edge and I need some down elevator and right aileron to keep it straight, so I mix that in and my plane does a perfect knife edge...did I just mess up my normal flight characteristics? like when I'm landing cross wind and I need some rudder, will the plane still act right or when I mix that does it screw up any other kind of rudder movement? make sense?
07-24-2003, 05:08 AM
#2
Senior Member
My Feedback: (7)
Join Date: Feb 2002
Location: Duluth, GA
Posts: 907
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
Can you put the mix on a switch? You're mixing some down and right aileron with rudder input, right? Maybe there isn't enough of an effect to cause a problem when you're slipping or crabbing. Check it out and let us know...
I'm no aerodynamics expert, but I've always thought that rudder inputs which produced roll or pitch coupling in knife-edge also had the same effect at any attitude. So IMO the mix will be useful even when you're not doing KE.
I'm no aerodynamics expert, but I've always thought that rudder inputs which produced roll or pitch coupling in knife-edge also had the same effect at any attitude. So IMO the mix will be useful even when you're not doing KE.
07-24-2003, 05:30 AM
#3
Senior Member
My Feedback: (6)
Join Date: Mar 2002
Location: Lincoln,
NE
Posts: 1,815
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
Maybe.
The rudder inputs often produce different coupling at KE compared to level flight.
For example, at KE, the AOA on the wing is probably 0 compared to a bit positive for level flight. This change in wing AOA changes down-wash which in turn changes the AOA on the stab which is one of the causes of coupling. In other words, you need different elevator trim for KE vs level flight, hence the mix.
I've had some planes where the mix really messed with normal rudder inputs, such as stall turns. For that reason, I typically put KE mixes on a switch.
You might try adding rudder in level flight and note what it does. Maybe you want the mix on all the time.
Cheers
The rudder inputs often produce different coupling at KE compared to level flight.
For example, at KE, the AOA on the wing is probably 0 compared to a bit positive for level flight. This change in wing AOA changes down-wash which in turn changes the AOA on the stab which is one of the causes of coupling. In other words, you need different elevator trim for KE vs level flight, hence the mix.
I've had some planes where the mix really messed with normal rudder inputs, such as stall turns. For that reason, I typically put KE mixes on a switch.
You might try adding rudder in level flight and note what it does. Maybe you want the mix on all the time.
Cheers
07-24-2003, 05:37 AM
#4
Senior Member
My Feedback: (5)
Join Date: Jul 2003
Location: Tacoma, WA
Posts: 2,668
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
At KE the plane's not really "flying" as there's no lift off the wings.
At normal flight there is and I'm guessing that has something to do with it.
But alas, I am not an aerodynamics expert, either.
At normal flight there is and I'm guessing that has something to do with it.
But alas, I am not an aerodynamics expert, either.
07-24-2003, 04:37 PM
#5
Senior Member
My Feedback: (23)
knife edges to regular flight
I have a 1/4 scale cap that requires a lot of up mix to the elevator for knife-edge flight.
as the cap comes in to land and slows the nose is high (tail heavy on purpose..... no mix on)
with this mix "on" it seemed to make this problem even worse (obviously).....I changed the mixes to a switch (had role coupling from the rudder also, had that correction on the same switch).
I think it helped with a smoother more predictable landing when the switch was "off".
I also experimented with having this switch "on" or "off" during hovering/torque-role. I have not yet reached a conclusion as to whether it helps with the torque-roll to have the switch "on" or off."
ps....Great Planes position on this issue is to have the corrective mixes ***on at all times***
as the cap comes in to land and slows the nose is high (tail heavy on purpose..... no mix on)
with this mix "on" it seemed to make this problem even worse (obviously).....I changed the mixes to a switch (had role coupling from the rudder also, had that correction on the same switch).
I think it helped with a smoother more predictable landing when the switch was "off".
I also experimented with having this switch "on" or "off" during hovering/torque-role. I have not yet reached a conclusion as to whether it helps with the torque-roll to have the switch "on" or off."
ps....Great Planes position on this issue is to have the corrective mixes ***on at all times***
07-24-2003, 06:42 PM
#7
Senior Member
Join Date: May 2003
Location: Plover,
WI
Posts: 341
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
To a certain percent it will change the normal flight characteristics of rudder inputs, yes. But think about this... depending on how much mix you need, say its 8% right aileron and 10% down elevator for example at full rudder deflection. Now you're coming in for a crosswind landing and you need a little rudder to correct, how often do you have to hold full rudder while shooting an approach? most of the time with all my planes it's less than 25% full rudder travel to keep 'er lined up on final, or takeoff for that matter. Mixing is usually proportional to the master channel meaning if you are using 25% right rudder on your approach that only feeds in 2% right aileron and 2.5% down elevator, the breeze will throw your plane around more than those small percentages of travel most likely. Doing a stall turn those small percentage of inputs coupled to the rudder probably wouldnt effect it much seeing when you kick the rudder in a stall turn the plane has almost no airspeed anyways and that small amount is even less noticeable at 5 mph than it is at 40 mph. Now if you need 25% aileron and 50% elevator to hold it straight on KE you will have much more adverse effects in all other flight aspects, so look at your %'s when you have your KE couple dialed in and consider it using that as your basis.
Just some thoughts and another perspective to look at it from, the only real way to tell is just to try it
I am no aeronautical engineer myself, so take it as you will! Happy Flyin!
Just some thoughts and another perspective to look at it from, the only real way to tell is just to try it
I am no aeronautical engineer myself, so take it as you will! Happy Flyin!
07-24-2003, 10:50 PM
#8
Join Date: Oct 2002
Location: Darien, IL
Posts: 626
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
Can you use the offset function of your computer radio so the mix only starts after the rudder is deflected a certain amount. Trying to learm more about offset now!
Mark
Mark
07-25-2003, 02:24 AM
#9
Senior Member
Join Date: May 2003
Location: Plover,
WI
Posts: 341
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
That much I dont know... every radio is different and I only have had one I havent used half the stuff on it yet so I still have some learning to do myself in that aspect.
I havent used half the stuff on it yet so I still have some learning to do myself in that aspect.
07-25-2003, 05:37 AM
#11
Senior Member
My Feedback: (5)
Join Date: Jul 2003
Location: Tacoma, WA
Posts: 2,668
Likes: 0
Received 0 Likes
on
0 Posts
knife edges to regular flight
ML - I know the Futaba 9C can do that. It stands to reason that the better one, 9Z, could too. Not sure about any others...
07-25-2003, 02:51 PM
#13
Leave it on.
If the airplane is demonstrating coupling in knife-edge flight, then the same coupling will be present with rudder deflections in horizontal flight. Try a flat rudder turn without mixing. Does the airplane pitch and try and roll out. Turn on your knife-edge mix and fly at about the same throttle setting as you do in knife-edge. Most likely the airplane will behave better in the rudder turn. Rudder coupling has the same effect regarding coupling regardless of what the airplane's attitude is, level, climbing, inverted, or knife-edge. When trimming an aerobatic aircraft, the object is to remove that coupling in all flight regimes.
To answer your question directly, when you have mixed out the coupling in your knife-edge, you have mixed out rudder coupling in all flight regimes at the same rudder/throttle/airspeed settings which is a very good thing. As the airplane hopefully has some amount of linearity in it's behavior, the mix should be effective at a wide variety of rudder deflections and airspeeds. The mix, however, loses it's effectiveness at excessive rudder throws such as in knife-edge loops, where strange angular and aerodynamic forces come in and the fuselage becomes stalled and buffeted. This is where having a mutipoint curve in the mixing becomes handy. Difficult to program and takes lots of trial and error testing, but can really open the flight envelope by creating a very linearly flying airplane.
F3A airplanes are designed with as little coupling as possible, but most still need some mixing to get them perfectly de-coupled. In an ideal situation, the airplane will only yaw with rudder, pitch with elevator, and roll with aileron.
Normally, the mixes are small enough that at slower speeds, the slight mixed surfaces will not have much if any response, such as in 3D flight. In the 3D flght envelope, control effectiveness requires very large amounts of throw and a few % of mixing is not going to have any effect.
The one mix which I feel needs to be switchable is if I have a rudder-slaved mix (rudder slaved to the throttle or elevator). In 3D manuvers such as waterfalls, torque rolls, etc. I want the rudder to remain perfectly centered to keep the slow maneuvers straight. In precision flight, slaving the rudder to elevator or throttle as required can remove an airplanes tendency to yaw on pushes or pulls, and to correct any slight thrust offsets at different throttle settings.
A carefully trimmed out airplane, where any negative tendency is removed, creates a wonderful flying machine and the pilot gets to concentrate on placement of maneuvers, geometry, and style.
I know I have moved a little beyond your question, but I hope the additional information gives some food for thought. Most folks use very little of the radio's capacity. Computer radios are designed to allow the pilot to regulate any decent airplane to fly as perfectly as possible, not just to double the throws and go crazy.
Mark
To answer your question directly, when you have mixed out the coupling in your knife-edge, you have mixed out rudder coupling in all flight regimes at the same rudder/throttle/airspeed settings which is a very good thing. As the airplane hopefully has some amount of linearity in it's behavior, the mix should be effective at a wide variety of rudder deflections and airspeeds. The mix, however, loses it's effectiveness at excessive rudder throws such as in knife-edge loops, where strange angular and aerodynamic forces come in and the fuselage becomes stalled and buffeted. This is where having a mutipoint curve in the mixing becomes handy. Difficult to program and takes lots of trial and error testing, but can really open the flight envelope by creating a very linearly flying airplane.
F3A airplanes are designed with as little coupling as possible, but most still need some mixing to get them perfectly de-coupled. In an ideal situation, the airplane will only yaw with rudder, pitch with elevator, and roll with aileron.
Normally, the mixes are small enough that at slower speeds, the slight mixed surfaces will not have much if any response, such as in 3D flight. In the 3D flght envelope, control effectiveness requires very large amounts of throw and a few % of mixing is not going to have any effect.
The one mix which I feel needs to be switchable is if I have a rudder-slaved mix (rudder slaved to the throttle or elevator). In 3D manuvers such as waterfalls, torque rolls, etc. I want the rudder to remain perfectly centered to keep the slow maneuvers straight. In precision flight, slaving the rudder to elevator or throttle as required can remove an airplanes tendency to yaw on pushes or pulls, and to correct any slight thrust offsets at different throttle settings.
A carefully trimmed out airplane, where any negative tendency is removed, creates a wonderful flying machine and the pilot gets to concentrate on placement of maneuvers, geometry, and style.
I know I have moved a little beyond your question, but I hope the additional information gives some food for thought. Most folks use very little of the radio's capacity. Computer radios are designed to allow the pilot to regulate any decent airplane to fly as perfectly as possible, not just to double the throws and go crazy.
Mark
