Roll Instead of Flat Spin
 

Hi

I've just scratch built a fun-fly plane (40-size).

When I apply full-rudder on leveled upright flight, the plane started to flat-spin a little but then quickly go into a roll!

What makes a 3D plane flat spin?

Appreciate any tip!

Thanks.

Re: Roll Instead of Flat Spin
 

Flat spin . .


Get into a good spin (that is, one that started with the wing stalled and the engine idling) using a bit of aileron in the same direction as the full rudder (and full up elevator). Slowly move the aileron in the opposite direction to the rudder, slowly opening the throttle, while it's spinning like crazy.

You should find the spin flattens out, if it's capable of it,.

An inverted flat spin is the same, but with full down elevator.

Mind the ground!!

-David C.

Flat Spin
 

Thanks David.

I'm not sure if the term flat-spin is used correctly!

I've flow several fun-fly (ARF) planes that does not roll when full rudder is applied in straight upright flight. Instead, it slows turns around (laterally). By holding the rudder long enough, I may get the plane to make a u-turn.

Several 3D models in FMS do this too!

Any idea what makes the plane does that instead of rolling?

Thanks.

Roll Instead of Flat Spin
 

The three axes of motion are controlled in this way:-

Pitch - Elevator
Roll - Ailerons
Yaw - Rudder.

In other words, the rudder isn't there to create roll (that's the aileron's job) but to initiate turns to the left or right.

I say 'initiate' because a turn to, say, the left makes the right wing go faster, and therefore creates more lift on that side, so the rudder also creates a banking effect in the direction of yaw. If this persists, (especially if altitude is maintained with a bit of up elevator) then the aircraft will eventually make a U turn as you described.

It's actually more complicated than that, but that'll do for the purposes of this explanation!

If you want to roll, (that is, roll around the axis that runs from the nose to the tail )ailerons will do the job.

A spin (often confused with a roll) is entirely different. It's the result of losing lift from the wing, and letting the control surfaces on the tail take over.

In a full size aircraft, spins can be dangerous, in that there can be a maximum number the aircraft is capable of before it naturally flattens out. You've heard of the expression 'in a flat spin' meaning the person has lost control? Some aircraft, once they are in a flat spin cannot get out of it, as all the control surfaces are in a stalled condition and the pilot simply has no control.
I have seen a Tiger Moth flat spin all the way to the ground, but, as the vertical speed was so slow, the pilot happily survived, as did most of the structure of the aircraft! It came down like a sycamore leaf!!

A 'snap' is really the initial phase of a spin, in that it's caused by loss of lift as the wing starts to stall.

Hope that helped!

Happy flying!

-David C.

Thanks
 

Hi David

Thanks for the detailed explaination. It gives me a better understanding of roll and spin.

The plane I built has the following dimensions:
a) 40" wing span
b) 42" fuselage length
c) 12" chord, 16% thick symmetrical airfoil
d) CG at about 1/3 of the Chord (LE)

When full rudder is applied, the plane will yaw a little before one of the wing tip drops, resulting in a bank. In my other ARF fun-fly plane, the yaw is continous (without any wing tip dropping) and ended with a complete U-turn of the plane.

I wonder if the length of the fuselage is causing this? My ARF fun-fly plane has a shorter fuselage (as compared to wing span).

Anyway, I manage to reduce the problem by adding some aileron in the opposite direction of the rudder!

Thanks again!