Roll stability

[mcross]

Hi,
I built this micro F-16 http://www.rcgroups.com/forums/showthread.php?t=744291

During the test flight the pilot reported that it wants to roll over when banked. I would like to know if there is anything I can do to reduce this tendancy? The CG was a bit too far forward for the test flight, will this contribute to it wanting to roll over?

Thanks,
Mike

[vintage1]

Mm. anhedral maybe?
That with too much fin or too forward CG might cause that.

[ghoti]

Some might considerate it cheating, but an IR autopilot should fix it. bill

[mcross]

Thanks Vintage.

Hi ghoti, while I wouldn't consider it cheating I don't want to add more weight.
Thanks,
Mike

[BMatthews]

So does the model only do this to one side or equally badly in both directions?

If it's the exact same in both directions then it sounds like it is spirally unstable. If it only does it to one side then the model would appear to be misaligned in some manner.

[mcross]

Hi BMatthews,
I believe it was both ways but I'll have to ask the pilot to be sure.

If it is spirally unstable what can be done about it?

Thanks,
Mike

[Odysis]

Dihedral, high mounted wings, more rudder area above the CoG. Roll stability comes from sideslip, so the way I think of what'll work is imagine the aircraft is slipping left. You want something to make it now roll right.

[JetPlaneFlyer]

If it is spirally unstable what can be done about it?


Adding a little dihedral is the most effective solution. Relocating any heavy items as low as possible in the fuselage may also help a little.

[BMatthews]

Moving the CG lower will help a little but not for the reasons you think. Spiral instability comes from too large a vertical tail area. Or more accurately too large a vertical tail volume coefficient. Moving the heavy stuff lower makes the wing to CG relationship more like a mid or high wing and functionally that's the same as adding a little bit of dihedral.

But if it's not possible to alter the CG in that manner for other reasons then making the vertical tail area a little smaller is the next way to do it. Too small a fin area makes the model skid and side slip in the turns. Too much makes it want to roll into the turns. In extreme cases it'll make it so the model will not even fly in a straight line without wanting to fall into a spiral dive. When it's just right the model will be neutral in the turns and track well when level.

[Brandano]

I wonder, the full size plane has two strakes on the bottom of the tailpipe. Does this model have them as well? How would they affect the flight behavior?

[mcross]

Hi Guys,
I can't lower the CG, there isn't room to move stuff around. I could add dihedral but it won't be easy. I think the vertical stabilizer is the first thing I'll try. I did increase it's size so I'll go back to a more scale size. I would also like to move it back a 1/4 inch or so, will that be moving it's volume coefficient the wrong way?

I don't have the lower fuse strakes on yet. If it is a fin size problem I would think they would make it worse, so I won't use them.

Thanks,
Mike

[BMatthews]

Yes, moving it back means it'll need to be even smaller. But only 1/4 inch isn't anything to worry about.

When I played with a 2 meter aileron glider to experiment with all this I found that a little can make a big difference. You will only need to remove a small amout at a time. For example if you made it 1/8 inch wider all around that is enough to mess things up if the original was on the large side of the acceptable range. Unless you made them a LOT larger I'd suggest go back to scale and move them back if you wish. Or leave them where they are and take more off the leading edge and tips.

Pictures of what we are talking about would be nice too.... hint, hint... :D

[JetPlaneFlyer]

Making the fin smaller can do the trick ... However it only works if the model has positive static lateral stability in the first place. If the model has negative static lateral stability then reducing fin may actually increase the tendancy to spiral :(

Steve

[mcross]

Hi and thanks guys,
Here's a few pics. The plane is 13.5 inch WS, 18 inches long, elervon control and weights 75 grams.
Mike
ETA I just rechecked the rudder size and it is the scale size. I thought I made it a little bigger like I used to do for FF but apparently I didn't. Horizontal stabilizer span was increased by 1 inch and chord increased by 3/8. Wing span was increased by 2 inches and cord increased by 1/2 inch.

[BMatthews]

This doesn't look bad at all. I do notice that I'm not seeing any ailerons. Or is the hinge made by using V slits in the depron? If it's elevons then perhaps there's something going on with the controls rather than this being a fin area problem. The fin IS large but the super short tail moment should be making up for that. But it's only a hunk of depron. Try flying it again to get the feel of it and then slice 1/4 inch off the top of the fin. If it improves but still has a touch of the same roll into the turn then slice of another 1/4 inch. By that time if it was the fin area it should be pretty good and nuetral in the turns. I'd be very surprised if you need to remove a third 1/4 inch.

[mcross]

Thanks,
It doesn't have ailerons, just elevons. I'll try it again when the weather gets better, in the mean time I'll move the rudder back, adjust the CG and paint it.
Mike

[JetPlaneFlyer]

Could there be a little 'stickiness' in the elevon movement which means that the surfaces dont return fully to neutral after an aileron input? This could explain why the model wants to continue to roll into the turn... Just a thought.

[mcross]

Thanks JetPlaneFlyer, I'll check that out.

[Thomas B]

My little Ikarus profile Su-27 would act like to wanted to roll deeper into the turns when banked as well....but only when it was too nose heavy.

It was a little like it was having bottom rudder added in the banked turn..only it did not have a functioning rudder...:)

With the Cg in the right spot, this tendancy went away.

[mcross]

Hi guys,
It's been a while since since you all helped me with this plane. I want to thank you, I incorporated most of the advice and was rewarded with a very nice flying plane. I moved the rudder back about a 1/4 inch, added about a 1/8 inch of dihedral to each wing tip and moved the CG back. I found it had some down thrust and removed it. Painted it's up to 80 grams and has about that much thrust. It flies great! It has just the slightest tendency to roll level from a bank. Tracks straight as an arrow and is very fast.
Only one very minor thing I don't like but I doubt there's anything that can be done, the nose drops more than I'd like in a turn. It seems to take more pull on the elevator to keep it level than it should.
Thanks again for the help,
Mike

[RCAV8R13]

OK here's my two cents.
The vertical fin is too high above the CG. When an airplane is in a turn air travels around the fuse in a curved fashion and pressure will build up on the OUTSIDE of the vertical tail. This is know as "Long tail slip". It causes an airplane to yaw in the opposite direction of a turn. The longer the tail, the stronger the yaw angle. When this yaw is combined with dihedral the plane rolls back to level. If there is insufficient dihedral, the pressure on the outside of the fin (way above the CG) will roll the plane into the turn. I have personally experienced this phenomenon first hand flying UAVs at work. We built an airplane with two vertical stabilizers with all their area above the CG. It was horribly spirally unstable. The cure was to add rudder in the direction of the turn, just like full scale pilots do, all the way around the turn. This transferred the pressure on the fins from being outside to the inside, keeping the plane from rolling. I proved this by flying straight and level and applying ruder. The plane rolls in the opposite direction of the rudder. There is no dihedral effect and the pressure above the CG causes adverse roll with rudder.
I also made a hand toss model of our plane and moved the tails up and down. Up it was unstable, down it was stable.

If you add the bottom fins you will lower the overall vertical area and improve the spiral stability. It works on the real thing.
RCA
BTW When will my motor be available. I want the hot one.

[mcross]

Hi RCA,
Thanks for your input! I'll have to read it over when I have more time to understand it better. Any thoughts on how to keep the nose up in a bank, I think it just needs the CG moved back a little. I'll do some more work on it after NEAT.

Sorry the motors are taking so long, NEAT kind of snuck up on me and I had to stop work on the motors to get ready. On a good note, the most recent batch of parts from GWS look a lot better. I didn't spin one up yet but they look centered where to older batches were visibly off center.
Mike

[Texas Buzzard]

OK here's my two cents.
The vertical fin is too high above the CG. When an airplane is in a turn air travels around the fuse in a curved fashion and pressure will build up on the OUTSIDE of the vertical tail. This is know as "Long tail slip". It causes an airplane to yaw in the opposite direction of a turn. The longer the tail, the stronger the yaw angle. When this yaw is combined with dihedral the plane rolls back to level. If there is insufficient dihedral, the pressure on the outside of the fin (way above the CG) will roll the plane into the turn. I have personally experienced this phenomenon first hand flying UAVs at work. We built an airplane with two vertical stabilizers with all their area above the CG. It was horribly spirally unstable. The cure was to add rudder in the direction of the turn, just like full scale pilots do, all the way around the turn. This transferred the pressure on the fins from being outside to the inside, keeping the plane from rolling. I proved this by flying straight and level and applying ruder. The plane rolls in the opposite direction of the rudder. There is no dihedral effect and the pressure above the CG causes adverse roll with rudder.
I also made a hand toss model of our plane and moved the tails up and down. Up it was unstable, down it was stable.

If you add the bottom fins you will lower the overall vertical area and improve the spiral stability. It works on the real thing.
RCA
BTW When will my motor be available. I want the hot one.
.................................................. ..............................................
RCAV8 is Corrct! I agree with his analysis. His practical work IS important.

I have said that too large a fin/rudder reduces spiral stability. I said to build a simple glider with an enormous fin. (vert. stab.) ...trim it the best you can for straight ahead glide. Then tweek the fin to cause a slight left or right turn. Once in a turn the turn should gradually steepen with that large fin. The large fin will not let it slip into the turn.
Then start to cut away some of the fin a little at a time. As the fin area is reduced the glider/plane will be allowed to slip into the turn. This causes the dihedral to be effective and the spiral is stopped when the fin is small enough.
If you keep cutting away the fin the plane will experience "Duth Roll". That will be a "huntig" back and forth from left to right. Just add a little more area to the fin to stop the Dutch Roll and you have done all you can except increasing the dihedreal a bit. Reducing the fin area is easiest. Have fun.