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Different rudder needed on different knife edge directions ?

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Different rudder needed on different knife edge directions ?

Old 11-14-2009, 05:04 AM
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gauravag
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Default Different rudder needed on different knife edge directions ?

Hello everyone,
I am flying my Javelin 90 airplane and while practising knife edge, notice, that the when i roll to the left, i need to give a less amount of right rudder, but when i roll to the right, i need a lot more left rudder.
On the ground, i measured that the deflections are the same, no expo tuned in . The control linkage has no slop , in fact when doing the left rudder, the rudder is pulled towards the servo, so no chance of the slop in the pushrod.
In straight flight the rudder is also neutral.. Servo is hi torque Futaba digital S3050.

Thanks for sharing your views.
Gaurav
Old 11-14-2009, 05:27 AM
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ntsmith
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Default RE: Different rudder needed on different knife edge directions ?

Torque and gyroscopic precession. Any greater mind than mind care to elaborate?
Old 11-14-2009, 08:08 AM
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Default RE: Different rudder needed on different knife edge directions ?

The engine is mounted with some right thrust, no?

Surely not the only ingredient in the aerodynamic soup, but a contributor.
Old 11-14-2009, 09:18 AM
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ntsmith
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Default RE: Different rudder needed on different knife edge directions ?

The right thrust will be a factor of correction at a "specific speed" and will vary depending on many factors.
If you were to give right rudder to an aircraft flying straight and level the airplane probably would want to pitch nose UP do to gyroscopic precession and pitch DOWN when left rudder is kicked in. (We are looking from the cockpit forward) This alone is quite a significant reason. However we also have the torque of the engine which will provide a different roll speed left and right as well as that sidethrust. the rudder will have a significant difference in its ability to turn right and left. Most pattern aircraft end up with a differential on the rudder. The first step is to make certain it flies equally to the right as the left. Once in KE flight then the effects seem to be amplified. In KE flight therefore you will need A/different amounts of rudder in different directions as well as B/different amount of elevator.
Nothing in flying planes ever has a singular effect. . if only it did
Old 11-16-2009, 07:32 AM
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gauravag
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Default RE: Different rudder needed on different knife edge directions ?

nsmith,
You were very correct. I noted that when in knife edge while holding the right rudder the plane tracked straight, but on the other side, ie by holding left rudder, the plane pitched to belly .So i had to program up elevator and it tracked straight then.
Buy the differential is still there. I still need to give a lot more left rudder to hold a straight knife edge than right rudder.
I understand i need to program that out. How do you do that ? By increaseing the left rudder's end point ? I have a Futaba 10C , so i am sure its got the capability. Just need to dial it in.
Old 11-16-2009, 09:23 AM
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ntsmith
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Default RE: Different rudder needed on different knife edge directions ?

There will be no problem doing that with the Futaba 10C as its a superb radio.
You have end points which set the overall surface throw ability and many people set this to the maximaum to get the highest resolution from there servos.
One would assume that you may have dual rates being used so you have a couple of options.
Since ke flight, spins and stall turns require more rudder than other manouvres I choose to switch the high rates in for those seperately. Under the dual rate settings you can give more throw one way or the other so just dial in more to the side that needs the most moverment.
Under the prog mix 1-8 mixes 1234 give linear mixes and 567 and 8 give a curve mix.
Another mix I normally use is throttle to rudder as the sidethrust will never be the same for all speeds and hence can be dialed in to compensate.
By the use of these mixes it is possible (never perfectly as their are too many variables) to set things so you actualy feel no difference in flying ke to the right and ke to the left.
The first thing is fly around and get the sort of throws you need to feel happy with and then throw iot around performing ke loops and similar high control input manouvres to get the high rates and get them set in the dual rate setttings so it "feels" the same both ways. I then normally set the throttle to rudder. A nice extended vertical climb will show the amount necessary to compensate. then follows the tuning for elevator and aileron(try flat turns and see how much opposite aileron and elevator is required and dial those in)
More than likely in the first stages of tuning more aileron one way will be required and I usually like more down than up. However, you can use the same amount of elevator up and down but still get the feeling that you have more down than up by adjusting the expo to be less on down elevator than the up elevator.
Personally, and it is a personal preference, I find about 35% expo gives a nice feel but remember it is NEGATIVE on a Futaba radio. JR didnt seem to do their homework right and they use POSITIVE. Not sure what school their engineers went to
To start with though I find a couple of dozen flights just "flying" the plane without any compensation seems to give alot more help when you switch in the mixes.
Dont forget one thing at a timeis the golden rule and one thing allways effects another. Also give yourself a switch to switch the mixes in and out to see what the effect is.
PS Another common mix is low throttle to elevator so in a down line their is the mearest whif of down elevator which prevents most planes natural tendency to pull to the canopy.
Above all dont forget this is meant to be fun so go out and enjoy flying.
Old 11-19-2009, 01:54 AM
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Andy P.
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Default RE: Different rudder needed on different knife edge directions ?


ORIGINAL: ntsmith

I find about 35% expo gives a nice feel but remember it is NEGATIVE on a Futaba radio. JR didnt seem to do their homework right and they use POSITIVE. Not sure what school their engineers went to

Simple, your "adding" expo not taking it away, hence +'ive value........................................but only a Fruitybar user would stumble on this , of course, you are forgiven and now educated to JR standards
Old 11-19-2009, 05:24 AM
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Default RE: Different rudder needed on different knife edge directions ?

Lets not confuse the gentleman. On a Futaba system you apply NEGATIVE expo to soften the centres. On most other makes it is POSITIVE to soften the centres.
I just want that point absolutely clear. Put the wrong value in and you could have a very twitchy airplane, or at the worst a crashed one.
Old 11-19-2009, 08:45 AM
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Default RE: Different rudder needed on different knife edge directions ?

He he... and due to the same effect you'll find that your plane rolls nicer in one sense that the other...
Mixing solves almost everything (or just put counterspining props)
Cheers
Old 11-19-2009, 09:15 AM
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ntsmith
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Default RE: Different rudder needed on different knife edge directions ?

Wasn't counterspinning props Sebastiano silvestri's answer
Old 11-21-2009, 02:31 AM
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Ryan Smith
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Default RE: Different rudder needed on different knife edge directions ?

Do you have a lot of sub trim on your rudder?
Old 11-21-2009, 11:48 PM
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Default RE: Different rudder needed on different knife edge directions ?

Nigel,

Allow me to submit the following for your consideration –

Once the airplane is in steady state Knife Edge, gyroscopics are not longer in play. Right thrust of the motor/engine can be addressed by differential rudder throw (more left than right). However, the “perfect” plane (one that is geometrically accurate in all dimensions) will relatively pitch to the belly with left rudder, and to the canopy with right rudder. I submit the cause for this is spiral airflow from the propeller. The solution is to increase the incidence of the left stab while decreasing the incidence of the right stab an equal amount. The stab opposite the inputted rudder is dominant in yawed (knife edge) flight. The amount of bias is dependant on the airplane design, but could be as much as 1.5 degrees difference, with the bias being correct when the pitching behavior in knife edge is equal with left and right rudder. Ill effects of the biased stab are nill……as the biased stab while not geometrically accurate, is aerodynamically accurate to the relative airflow (which is twisted from the spiral airflow of the propeller).

Regards,

Dave
Old 11-22-2009, 01:59 AM
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Default RE: Different rudder needed on different knife edge directions ?

The solution is to increase the incidence of the left stab while decreasing the incidence of the right stab an equal amount. The stab opposite the inputted rudder is dominant in yawed (knife edge) flight.
Fascinating!
I see that you have (added?) flow straighteners to some of your latest pattern planes.
From what you're saying, it should follow that rudder-to-elevator pitch coupling should be reduced by these devices?
I am playing with one on my Gaudius(which already has a very powerful rudder),and,in just a few flights so far, have noted the expected increase in powerbut also a change in rudder-elevator coupling.
It's gone from a slight up-mix requirement on both directions (JR figures,4% each way), to a similar down mix(approx 6% R,2%L) with the flow straightener.
Old 11-22-2009, 09:18 AM
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Default RE: Different rudder needed on different knife edge directions ?

F3a05,

I originally added the vertical fin in the hopes it would cure a small imbalance between fuselage efficiency in KE flight during rolling circles. Higher fuse AOA was needed in the portion of circles that used up elevator, compared to portions of circles that needed down elevator. The vertical fin was effective in this regard. I also found, as you have, that the vertical fin effects the KE pitch coupling. After adjusting the incidence of the stabs (twisting them) to yield the same pitching characteristics in KE, the vertical fin size can be altered to eliminate the pitch mix in KE flight. I've used the vertical fin on several designs now, and the effect can be very powerful on some planes.

Regards,

Dave




ORIGINAL: f3a05

The solution is to increase the incidence of the left stab while decreasing the incidence of the right stab an equal amount. The stab opposite the inputted rudder is dominant in yawed (knife edge) flight.
Fascinating!
I see that you have (added?) flow straighteners to some of your latest pattern planes.
From what you're saying, it should follow that rudder-to-elevator pitch coupling should be reduced by these devices?
I am playing with one on my Gaudius(which already has a very powerful rudder),and,in just a few flights so far, have noted the expected increase in powerbut also a change in rudder-elevator coupling.
It's gone from a slight up-mix requirement on both directions (JR figures,4% each way), to a similar down mix(approx 6% R,2%L) with the flow straightener.
Old 11-22-2009, 02:26 PM
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Default RE: Different rudder needed on different knife edge directions ?


ORIGINAL: DaveL322

Nigel,

Allow me to submit the following for your consideration –

Once the airplane is in steady state Knife Edge, gyroscopics are not longer in play. Right thrust of the motor/engine can be addressed by differential rudder throw (more left than right). However, the “perfect” plane (one that is geometrically accurate in all dimensions) will relatively pitch to the belly with left rudder, and to the canopy with right rudder. I submit the cause for this is spiral airflow from the propeller. The solution is to increase the incidence of the left stab while decreasing the incidence of the right stab an equal amount. The stab opposite the inputted rudder is dominant in yawed (knife edge) flight. The amount of bias is dependant on the airplane design, but could be as much as 1.5 degrees difference, with the bias being correct when the pitching behavior in knife edge is equal with left and right rudder. Ill effects of the biased stab are nill……as the biased stab while not geometrically accurate, is aerodynamically accurate to the relative airflow (which is twisted from the spiral airflow of the propeller).

Regards,

Dave
Dave, could you get the same effect from tweeking the elevator halves?

Volkert
Old 11-22-2009, 06:52 PM
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Default RE: Different rudder needed on different knife edge directions ?

Volkert,

Absolutely. I've used the elevator halves tweek on several planes (most recently an Abbra).

Regards,

Dave

[/quote]

Dave, could you get the same effect from tweeking the elevator halves?

Volkert

[/quote]

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