What factors makes a plane turn well?

[Zamzara]

As the title suggests, I seem to have problems with this in my designs.

I can modify an existing plane design so that it balances longitudinally, flies in a nice straight line, and pitches up and down perfectly well.

But when it comes to turning back round, my own designs seem to have an annoying tendency to either keep going or to fall out of the sky.

What should I be looking at for? Aileron shape, tail shape, fin size?

 

[JasonK]

Depends on how they are falling out of the sky and how your trying to turn them.

Do you have any pictures of your designs and video of the turning failures?

Are you flying elevator/rudder? Elevator/Aileron? Elevator/Aileron/Rudder? or something else?

 

[JasonK]

if your design is a 'traditional' wing and tail, this post has some good starting points for the relative sizes of your surfaces:
https://forum.flitetest.com/index.php?threads/determining-control-surface-area.62991/#post-565944

 

[quorneng]

Zamara
As jasonK suggests there is insufficient evidence to make a meaningful recommendation.
A picture would be a great help with some idea of it size and weight.

Just to show the range of possible things that might or might not create a 'good flier' this is my own design.

50" (1.33m) span and weighs 26.2 oz (743g)
It looks pretty basic with a 'plank' one piece wing with what appear to be small ailerons and a V tail however its movable tail surfaces are only connected as an elevator so there is no rudder. It is flown 'bank and yank'.
There are arguments to say such a layout could have some serious short comings however it is aerodynamically pretty efficient, stable and controllable to fly. It has more than enough power (350W) for its weight so cruises on half that. It can handle a bit of wind with ease and it can & does thermal, power off, as a glider pretty well too.
Yes I expected it to fly but I am amazed at how well it does.

 

[Merv]

...What should I be looking at for?...

Make sure the up ailerons go up more than the down ailerons goes down

 

[Flightspeed]

Make sure the up ailerons go up more than the down ailerons goes down

I’m just curious, why? It almost seems like it should be the other way around, because doesn’t the air flow faster under the wing? I feel like this is some info that could be extremely useful.

 

[JasonK]

I’m just curious, why? It almost seems like it should be the other way around, because doesn’t the air flow faster under the wing? I feel like this is some info that could be extremely useful.

because you want more drag on the wing going down to create a coordinated turn, otherwise the drag induced yaw because adverse yaw.
many planes it is fine for them to be matched.

 

[Merv]

I’m just curious, why?....

As Jason said, it’s adverse yaw.
If the down aileron has more drag, that is it goes down more, it will keep the plane from turning. If the adverse yaw is bad enough, the plane will actually turn the opposite way. I have had planes that bank left but yaw right. If this happens to you, use the rudder with the aileron to control the plane until you can land and fix the problem.

You want the drag to help your turn not fight against the turn.

In most transmitters can be programmed to correct the adverse yaw. The old school way relies on which tooth the aileron servo horn is in. If your servos come out the top of the wing, you want the servo horn one tooth to the aft of the plane. If they come out the bottom, one tooth towards the leading edge. A very slight change in aileron travel can make a huge difference in adverse yaw.

 

[Zamzara]

Thanks for the replies so far.

On the current model (with ailerons) I have identified the problem now. The tail was flappy at the point shown, which was causing the rudder to be pushed to the side by the pushrod trying to keep the same length. This is based on a Baron fuselage with a more glider-like wing, and with the tail reinforced it now flies pretty well. Perhaps just a slight tendency to change pitch during a bank.

Another thing I've learned is that the ailerons must go to the wingtip. Inboard ailerons don't appear to work correctly.

I'd still be interested to know if there are other general principles. Especially for a rudder-only model, as it's not something I've been able to make work (prototypes now scrapped).

 

[quorneng]

You have to remember that in normal flight the rudder is really a secondary turning control. Moving the rudder makes the plane yaw. When it is yawing other aerodynamic effects cause it to bank which then makes it fly round in a turn. The main aerodynamic banking effect is wing dihedral. The greater the dihedral the greater is the tendency to bank under the effect of rudder induced yaw.
You may need a lot of rudder to get the plane to bank quickly but full rudder is likely to be more powerful than the dihedral banking effect so the amount of rudder will likely be reduced or even briefly centred to keep a constant bank & turn.
Dihedral is also self righting so once the rudder is centred the plane will return to straight an level flight although a short application of opposite rudder will cause the plane to straighten up the plane more quickly.
There are secondary effect caused by the rudder. During the act of yawing the plane one wing is speeded up and the other slowed down. This in itself can create a modest bank effect which may be adequate for a gentle turn but if the plane is flying close to its stall speed the yaw process may actually slow one wing to the stall point. The drag from that wing rapidly rises and the lift decreases. The result is a violent bank, turn and the nose drops. It then becomes a question of having sufficient height to recover.
The effect of a control surface is the product of its area and the distance it is from the planes centre of gravity. In general a smaller surface working at a greater distance will be more controllable than a bigger one at a shorter distance.

 

[ThatPolishHotdog]

Thanks for the replies so far.

On the current model (with ailerons) I have identified the problem now. The tail was flappy at the point shown, which was causing the rudder to be pushed to the side by the pushrod trying to keep the same length. This is based on a Baron fuselage with a more glider-like wing, and with the tail reinforced it now flies pretty well. Perhaps just a slight tendency to change pitch during a bank.

Another thing I've learned is that the ailerons must go to the wingtip. Inboard ailerons don't appear to work correctly.

I'd still be interested to know if there are other general principles. Especially for a rudder-only model, as it's not something I've been able to make work (prototypes now scrapped).

One thing to consider, and sorry if this comes across as insensitive, but as I see you are a new member, maybe you are holding the right stick too long? It seems like you have figured it out though, but I am just curious because a lot of people I trained thought that you needed to hold the stick right in order to keep the plane turning, and thus, they would lose control as the plane dived toward the ground.