Help !!! problem with new plane

[sajeev]

Hi All !!

THis is the plane i built:

http://www.rcgroups.com/forums/showthread.php?t=384335&goto=newpost

Problem with it is it wont do axial rolls. does a kind of circular /roll/nose going in one direction/tail in another.
second problem : at high speeds the tail end of the plane wobbles. not much but its noticible from the ground.

Any help is greately appriciated.

~ Sajeev

[JMP_blackfoot]

A cause of tail wobbling at high speed is turbulence generated at the wing to fuselage junction. It used to be said that adding fairings over the wing roots, in the Spitfire manner would cure this problem.

[sajeev]

"It used to be said that adding fairings over the wing roots"

What are fairings ??

[JMP_blackfoot]

to fair : to join so that the external surfaces blend smoothly

fairing : a member or structure whose primary function is to produce a smooth outline and to reduce drag (as on an airplane)

You can see them between the wing and the fuselage, mostly in the trailing edge area.

[sajeev]

aaahhhh!!! ok . gotcha !!! thanks JMP for that. will try to somehow fair it. its a removable wing. so dont have ideas on how to do it. anyways will give it a try :)

any ideas as to why it wont do axial rolls ?

[jkettu]

Can't see anything fundamentally wrong - a mid wing would be marginally better than low wing, but your design shouldn't be too bad either. You might try more aileron differential or, if you can afford it stability-wise, more rearward CG. If nothing else helps, your only chance is to learn to use rudder and elevator during rolls.

[Guille]

Sajeev; you have a summ of problems: 1- Dihedral 2- Low wing 3- Lack of differential in ailerons...
Do as JKETTU says; add aileron diff (try to get more "up" than "down" by changong the servo hole or bending a little bit the rods) and roll will be more centered. But; do it step by step and go trying...

[sajeev]

hi Guille and jkettu, i do understand now that aileron differential is required. could you please tell me how much is required for my setup.

thanks,

sajeev

[Ollie]

How much aileron differential is required depends on airfoil, airspeed, throttle setting, etc. You find how much adjusting by flying just like a test pilot.

[sajeev]

:o sorry but i am new to this :confused: what does it mean to fly like a test pilot ?

[Sparky Paul]

To "fly like a test pilot"..
You must be able to evaluate exactly what the plane is doing that you like or don't like, and decide what to change to improve the condition.
Change ONE thing, and one thing only, go fly, to see if that thing makes a difference.
If you've changed 2 or more at a time, you can't know which made a difference, if any.
For aileron differential, try 25%. Fly the plane... better? Reduce it 5% at a time, until the results aren't good.
Worse? Add 5% at a time, until the result are good. or you're out of adjustment capabilities.
It shouldn't take a lot in any event.

[Guille]

Sajeev, do as paul says... do one change at time and give you some time to evaluate properly the results.
For adding differential to the ailerons, you can also put two servos (one each aileron, use an Y-wire). By setting the servo wheels you'll get the so deisred differential (let's start with 20%).
Good luck!!!

[BMatthews]

Sajeev, there's a reply with a sketch of differential control inputs due to poor control installations on your other thread now. Hope this helps.

In your case I suspect you want to remove any differential. From your pictures the model appears to have a symetrical airfoil or very near to it. Such an airfoil will actually work best for aerobatics with either very little or NO differential. I think your problems with the poor rolls are actually caused by accidental differential due to the control installation. You may even have opposite differential that is compounding the problem.

You should also make sure that your ailerons are centered along the centerline of the airfoil. If they are drooped or lifted so that one side or the other is even with the top or bottom surface then that will also make the model roll funny. Here's what I mean. All the options shown here are supposed to be with the ailerons at neutral.

[Guille]

Bruce made a smart obsevation: he differential we are talking about is related to MOVEMENT, not in static position. so you have to focus on MOVEMENT diff. instead of static diff (mean; check the aileron position is as in the "good" sketches that Bruce sent).

[sajeev]

ok i checked that. its a bit high near the horn and low near the end....probably because i used tape for the hinges
is there something stronger that i can use ?
have a question tho...i used about 2 degrees of dihedral....will that affect the roll ?

[Guille]

well; yes... use plastic hinges. Seems that ailerons are twisted... hmmm. Perhaps you may think on replace them... also the hinges...
do this and then let's face the other changes, agree?

[sajeev]

ok Guille :) ! Agreed.

I checked the Aileron. its very straight. SO i think i need to replace the hinges. dont know if we get plastic hinges here in India :( . let me try my luck.

[Guille]

Call this guys... I don't know if they are near you but... just try!!

Suntech Systems Inc.
Mr. Sharad Goyal
Tel: +91-11-981-0156-637
sunhobby@hotmail.com

[gymguy]

ok i checked that. its a bit high near the horn and low near the end....probably because i used tape for the hinges
is there something stronger that i can use ?
have a question tho...i used about 2 degrees of dihedral....will that affect the roll ?

I'm wondering if you have too much CA on the hinges or the ailerons near the wing tips. Check the flexibility of the hinges near wing tips as compared to those near fuselage. If the outer hinges are stiff or the ailerons are stuck with CA they won't bend as freely and the ailerons will twist.