Issues with the vernable FT Cub

[kilroy07]

@Hai-Lee (AND everyone else of course!) If you have a second, maybe you can take a look at this and offer some help?

Taking your suggestion I added a single Popsicle stick under the leading edge of the wing on my CUB and gave her a go.

Let me be up front and say I am trying a wonky setup with 3 channels but as a Bank and Yank (Ailerons and Elevator.)
It would turn to the left just fine, but get really squirrely trying to turn (roll) right.

In the video around :45 you can see the trouble start, it almost crabs... so I wonder if it's torque or adverse yaw causing the issue?

Thoughts/comments?

Think it's worth modifying it to add a rudder, or should I just scrap them? (I have 3 built...)
(I was going to have my buds and I fly combat with them, but it's hard enough just keeping it in the air...)

 

[Hai-Lee]

Without getting a close look at the plane build and alignment it is a difficult thing to ascertain as to where the problem may be. I will go through a few things I would check for.


First thing is the vertical fin and its alignment. If it is not aligned along the exact centre line of the aircraft it can actually assist in turns in one direction and actually oppose turns in the opposite direction. In the extreme on a "Yank and Bank" it can cause a gentle nose up in the assisting direction and a marked and vicious nose down when turning in the opposite direction. With it opposing the turn in one direction it can even cause the plane to appear to "Crab" just as you would get when using opposite rudder to oppose the ailerons in level flight.

Second thing to check is that the wing incidences are identical. If one wing is of greater incidence angle it can again oppose roll turns in a particular direction. For a "Yank and Bank" setup I would seriously recommend that you consider building a flat, (NO DIHEDRAL) wing if possible in a single piece. This would remove the incidence and similar issues from the equation.

Your video showed the planes behaviour under power and your plane appeared to fly quite fast. Does it do the same when the motor power is lessened or removed from the situation? If the plane behaves perfectly without motor power then the problem may have a thrust angle issue which makes the situation worse.

Final suggestion is to ensure that your fuselage is straight and RIGID. A bent, (banana shaped) fuselage will introduce vertical fin issues as described above as will a tail boom that can flex laterally. You do not need an expensive jig or measuring setup to do adequate measurements. I use a piece of cotton thread or fishing line and attach it to the plane where I am measuring or checking the alignment with a little sticky tape.

For a fuselage straightness test I attach the thread to the under centre of the tail and hold the other end over the centre of the nose and check if the fuselage areas on each side of the thread are the same. A bend if present will easily be seen.

Let us know how you go. I love solving physical aerodynamic performance issues!

Have fun!

 

[Keno]

Take a close look and see if you have a warp in your wing. It does not take much to create a problem. Keep at it you will find your problem.

 

[kilroy07]

Tail is on straight (thank you laser cutter!)

I "think" it may be an incidence with incidence...
In the pic you might be able to see that the left wing does sit higher (I lined the bottom edge of the wing with the elevator/stabilizer and while the right wing is in line, I can see the left having a touch of dihedral. I do seem to recall adding a touch just to "help" stabilize the planes, pretty sure that's where my problem lies now!

Awesome, thanks for the help Guys! Now I just have to rebuild 3 wings and not three new aircraft.

 

[Hai-Lee]

Tail is on straight (thank you laser cutter!)

I "think" it may be an incidence with incidence...
In the pic you might be able to see that the left wing does sit higher (I lined the bottom edge of the wing with the elevator/stabilizer and while the right wing is in line, I can see the left having a touch of dihedral. I do seem to recall adding a touch just to "help" stabilize the planes, pretty sure that's where my problem lies now!

Awesome, thanks for the help Guys! Now I just have to rebuild 3 wings and not three new aircraft.
View attachment 115288

Try building one of them without any dihedral and you will be amazed how stable it remains, as well as how it cleans up its handling!

Have fun!