ohnow

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Hi all, first post here. When Ifly a Nextar .40 orTower Trainer .40 and use the rudder at maximum the planes dive. When Ifly my Somethin' Extra .40 it would take forever to final dive. When I fly my new .61 Chipmunk and use the rudder to go left it SIGNIFICANTLY dives and if Iuse right rudder it climbs. I'm mostly interested in getting the Chipmunk improved but I don't understand what's happening. The CG and lateral balances are good. I've reduced the rudder throw significantly (much less than the instructions called for) and although it helped it also doesn't give the response I would like. Can anyone point me in the right direction (pun intended )?

ro347

My Feedback: (13)

I believe it know as "yaw".... You need to compensate with aileron in the opposite direction. Even my aerobatic planes do this to some degree. It seemed more extreme on my trainers.

Now as far as the Chipmunk climbing when given rudder to the right - STUMPED!

MinnFlyer

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The question is, why are you turning with the rudder?

hugger-4641

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What kind of radio are you using? Sounds like you may have some kind of mix happening in the radio (maybe a v-tail setup?) or maybe the rudder and elevator pushrods or linkages are in contact with each other somewhere?

oldvet70

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Thats my question too. It sounds as if you are not flying the planes properly.

ohnow

My Feedback: (1)

Tx for the responses. In the case of the Chipmunk Iwas testing the response of the rudder and the dive (and climb if you use right rudder) was horrible. In other cases I fly with the rudder to "slide" in circles (I'm not sure if this is an official maneuver or just something fun). Ialso like to fly rudder and elevator only as some of my older sailplanes have no ailerons and a stick I'm working on is the same way. Also when I'm inverted and just want to change my yaw I'll use rudder. I use rudder to turn for landings or probably 1% of the time in normal flying just for the previous reasons but normally I fly with normal ailerons. My Somethin' Extra flies like a laser even though it's had it's incidents with gravity several times. Yet when I fly the Somethin' Extra and use rudder it reacts like Iwould expect. . . the pitch changes very, very little and just the yaw changes.

I followed mfg specs for throws on both the Somethin' Extra and the Chipmunk and actually reduced the Chipmunk a lot. I'm stumped why the Chipmunk would dive so drastically and especially stumped as to why it would climb going right and dive going left. Besides it snapping from tight elevator rolls (fixed by reducing throw) the model flies great.

ohnow

My Feedback: (1)

Good thought on the mix. I've never used mix on my radio but I did look through the instructions and verified there were no mixes. I thought of using a mix to compensate but since it would need elevator up going left and down going right I'm not sure Ican do that.

TFF

I had a Kaos that would dive with rudder but mine did it both ways. I believe what happens is that there is not enough dihedral for the design coupled with not a lot of fuselage side area to help the yaw. The plane falls off because it cant make a true yaw by acclerating the outboard wing and decelerate the inboard. Why it climbs I dont know; may be p factor in the mix? Before computer radios, the pattern guys would cut their wings in half and change the dihedral to get it neutral in yaw. Your SSE has a center wing with close to equal amounts of fuse above and below to make it equal in yaw.

dignlivn

Are your wings level when you input the rudder ?


Bob

hugger-4641

My Feedback: (6)

You should be able to program a mix for this, but I wouldn't do it either. There's bound to be something out of whack that is causing the problem. Maybe something is causing the elevator to pull or flex partially when you give right rudder? Maybe the wing itself is flexing or is not level with the stab? Maybe you could get someone to film some close passes with a good digital camera so you could slow it down and analyze the control surfaces in flight. Keep digin, you'll get to the root of the problem sooner or later.

da Rock

What you're experiencing is well known, it's called rudder couple. It happens with MANY airplanes. There are two basic types: pitch and roll.

Precision aerobatics modelers used to hack up their models and "adjust" things like dihedral and the height of the horizontal stabilizer and such. Nowadays they use the mixing capabilities of their TXs. If they have a pitch rudder couple (rudder causes either nose up or tuck) they mix rudder/elevator with the rudder as master. They adjust the amount of elevator trim needed in either direction (it's not always the same amount from left as from right rudder). If their rudder couple causes roll they mix rudder/ailerons much the same as the former.

It's fairly well known and not really unique.

da Rock

Yes, most radios that give you mixing capability give the ability to do exactly that.

It's actually not very risky to do at all. First off, while sitting on the ground you can move the rudder stick and watch the other surfaces to see if they're moving the way they should. Simple. Then in the air, you test gently at first to see how the plane reacts. Since most modelers don't use the rudder much, it's not going to be a hardship to land and readjust if the changes are too much or even if they're backward.

Pattern guys do it all the time.

ohnow

My Feedback: (1)

Thanks for the heads up on the Kaos. I just finished one for a friend last night will will test it next week!

ohnow

My Feedback: (1)

Thank you all for the posts! To answer a question, Yes the wings are level and I'm in level flight when this happens. As another note, as you would expect, the dive/climb is less at lower speeds. I'm not sure that comment really means anything. Also regarding if Ihave mix set-up or not I also did a ground check of moving the rudder only and no other surfaces moved.

Regarding the rudder couple, would that explain the dive to the left and climb to the right?

I've inspected the wings and did not notice any warping (and do not notice any adverse flying habits normally) also from eyeballing the wings seem to parallel the stab like it should and the rudder seems at 45 degrees, but I will check that with a triangle. I did not build the plane. Just a shot in the dark here, the .61 is side mounted and the head is sticking out of the cowling probably 1/4" on the right. Could that and/or propwash be a contributor? Would a different prop cause different affects?

da Rock

A number of things. For example, most models have right thrust built in. Sometimes it contributes and sometimes it's not the proper amount and that contributes.


You didn't build it, right. So check the rigging to the rudder. Matter of fact, check all the rigging. Every pushrod connection to the servo should make a right angle. The pushrod and servo arm should make a 90degree angle. If not, then that servo moves the rod farther one direction than the other. And that's just on the servo side of the pushrod. The pushrods should also make a 90degree from the point they connect at a horn to the surface's hinge line.

The head sticking out isn't usually much of a problem. What you're seeing is what the plane does, and many, many planes do for other modelers. They haven't found heads sticking out to be a problem.

Propwash is normally a contributor to lots of asymmetric things. It spirals around the fuselage and hits the fin from one side. Unfortunately, there's nothing to do there that's reasonable. And the prop's contribution to rudder couple is zip except in the asymmetry. But good thing about our hobby, you can test another prop in a heartbeat. But expect the difference to be somewhere other than in the rudder couple effect. Slower landing speed, better vertical, stuff like that. The rudder couple is coming from the layout of your airplane.

gyrocptr

Two possible contributors:
(1) P-effect (propeller-effect), or P-factor;
http://en.wikipedia.org/wiki/Propeller_walk.
(2) tilted stabilizer.

Lnewqban

I would do the following:

1) Verify symetrical throw of the rudder.

2) Place the plane on the balancer, balance it on the CG, and then, command full left and right rudder and observe if the balance changes. (A very heavy pushrod moving back and forth inside the fuse may change the CG enough).

3) Verify identical incidence of both sides of the stab-elevator assembly with an incidence meter, as well as identical semi-spans and areas. (When yawing, the fuse blocks the flow of air over one half of the stab-elevator, making the effect of the other one dominant).

4) Observe if the same happens when flying inverted.

sukhoi_sammy

I read your inital post. When you are flying your trainer and trimmed for level flight you apply rudder. As the airplane starts to bank the rudder angle departs vertical and starts to roll to horizontal. The more it becomes horizontal the more the rudder will act like an elevator and cause the airplane to dive. It is more prevalent in a dihedral wing than in your stright wing of the somthing extra because of how the dihedral wing presents itself to the oncoming air in a bank. When you flew gliders you didnt notice this as much because they had polyhedral wings. The outer wing panel (inside of the turn) of a polyhedral wing is flat in the turn and produces lift to counter the adverse yaw the rudder makes in a bank. When making a rudder turn you always need to add up elevator to maintain altitude in the turn. Not so (much) with a poly wing glider. To make your turn "flat" (flat turn) you will apply opposite aileron in the turn to keep the wings level countering the yaw of the rudder, this is referred to as cross controlling. You also need to apply elevator and power to maintain altitude. Why your chipmunk in a right rudder commanded turn does not dive is unusual. There could be some effect of rotational tork or propeller airflow from the engine but that would not be enought to completly overcome the effects of the rudder. If its not a radio programming issue or linkages acting on one another I would investigate the incidinces of the flight surfaces.

David Bathe

Obviously he wants to move out from the sports flyers.

ohnow

My Feedback: (1)

I don't think I could move away from sport flying even if Iwanted to Although Ienjoy good combat when I can find it and my combat planes don't have rudders

David Bathe

Pitching, positive and negative and rolling with rudder is a typical problem.
It's due to any number of numerous things, dihedral, wing placement, stab placement, GC, rudder shape etc etc.
Every aircraft, as you're discovering, is different. It's an easy fix though and done, albeit to a lesser degree even on competition aerobatice machines.
Start with eliminating some of the contributors. Rudder shape, dihedral, wing and stab position obviously can't be changed. Set the CG in place that where ever you feel most comfortable and then start using mixes to flatten out the problems.
Start with an aileron mix to eliminate the roll, then elevator (yeap, it's normal for some aircraft to pitch in different directions) to cure the pos/neg pitch.
Best to to do all the above via knife edge passes as they show up a great deal more easliy.
Don't be alarmed if you require a significent amount... it's a scale aircraft after all, thus far from neutral.
My clipped wing Cub required enormouse amounts of mix to get it to fly straight. Now will KN pass totally straight with only rudder held in.
Regarding flying with rudder... use it! Most people don't and they're missing out on so much.
Flying a model with neutral controls, ie the aileron only roll, elevators only pitches and the rudder only yaws the aircraft is a great experience compared to say one that rolls (and in your case pitches)when all you really wanted it to do is yaw.

ohnow

My Feedback: (1)

Thanks Dave and everyone! I've learned a lot and feel a lot better knowing it's not my imagination. I'll play around with the ideas people have given and hopefully will get it dialed in. The climbing to the right and diving to the left still has me stumped but I'll see if Ican come up with something there. Thank again everyone!

David Bathe

It's due to the spiralling airstream from the propeller and it's effect on the fin and rudder.
What happen under all normal operations is that the propeller slip stream spirals it's way around the aircraft hitting the vertical fin from the left hand side as viewed from the back.
The result of this is the typical yaw to the left during take off and a drift to the left during a vertical up line. The normal cure for these being, a little right rudder and engine right thrust respectively to cure this natural tendency to yaw left.

What's happening as you turn with rudder is the following:
As you input left rudder, the aircraft rolls slightly (because it isn't neutral as discussed above) to the left and the spiralling slipstream hits the fin as usual, but because of the roll, it translated into a left yaw downwards. Add to this the fact that the left rudder is indirect contact the the slipstream thus starts to act like down elevator. Thus the slight dive.

When you turn to the right, the slim stream to hits the rudder from the left (causing a slight left yaw) and because the aircraft is banken to the right, that left yaw translates into a slight pitching up of the nose. Thus the climb. During a right yaw the right rudder input is sheilded by the fin and plays no real elevator effect.
It's a s#it load easier to show you, than write it down... but that's basically it.
Hope this helps.

sukhoi_sammy

After reading what dave said I would investigate the engine offset (right thrust). Remove the propeller and set the airplane on its side. Put an incidence meter on the vertical fin. Use something to ensure the rudder is inline with the stab. Some wood on either side taped snugly together. Zero the incidence meter and secure the airplane in place, use some tape to do this. Then remove the incidence meter and place it flush on the thrust washer of the engine. It should give you about 2 or 3 degrees of right thrust. If it is zero then I think you are seeing what Dave explained. Also check to see if the engine is centered on the firewall left to right. Sometimes the engine will have some right thrust and the engine offset to the left to center the prop shaft coming out of the cowl.