Cadet Dutch Roll

[DLC]

This morning I made the first flight of a scale Cadet I built from Earl Stahl plans for a rubber-powered free flight model. WIng span is 32", wing area about 150 sq.in. and ready to fly weight a little under 5.5 oz. I used a GWS LPS motor. In many ways it flew well, more than adequate power, good climb angle, etc. However it has a bad case of Dutch Roll. Gentle turns were O.K., but sharp turns or wind gusts would send it into a wallowing Dutch Roll from which it was hard to recover. The plans showed both a scale vertical tail and a larger vertical tail for the rubber-powered model. I used the larger vertical tail. I used the dihedral from the plan - I don't remember the angle off-hand. Has anybody had any experience with this particular model or can anybody tell me how to overcome Dutch Roll in general?

[jkettu]

Don't know about the Cadet, but dutch roll in general is caused by too small vertical tail, too much dihedral, or a combination of both. Then again, if you have a large fin and/or little dihedral you will get spiral instability, so it is a compromise. Flying speed is also a factor - more speed will cause larger deviations from straight flight. Based on your comments about more than adequate power, I would suggest you try to fly it a little slower. If that doesn't help, then maybe a tad less dihedral. If you are trying to recover by using rudder, you are more than likely to get the timing wrong and actually end up increasing the oscillations. This has nothing to do with pilot skills, it is just a feature of human physiology. Best way to recover is to slow down and not touch the rudder. Dutch roll is not divergent, so you will not lose control even if it is wallowing around quite a bit.

[Ollie]

Dutch roll comes from bad design. Increase the vertical tail area or increase tail moment arm length. Low yaw damping means dutch roll. The yaw damping is proportional to vertical tail area and/or the square of the tail moment arm length.

[Sparky Paul]

And now, the opposite view point..
The larger fin for the rubber-powered plane was required because of the windmilling prop.
For electric, with a folder or a prop braked to a standstill. less vertical area will be OK.

[Marion]

I solved one Dutch Roll problem by taking my hands off the sticks :D

[BMatthews]

What you are describing doesn't really sound like Dutch roll. Dutch roll will show itself in normal flight as well as in turns. As the climb angle gets higher it increases the tendency for the tail to flip back and forth while the yaw causes the wing to roll one way and then the other. In extreme cases a snap roll can occur.

A marginally small fin will let tend to let the tail hang low in a steeper turn. It sounds more like that problem.

Some pics of the model would help.

PS: I just looked at the plans over at www.theplanpage.com and frankly I think the proportions look fine.

Another cause of tail dragging low in the turns is a slightly too far rearward CG. Try shifting it a 1/4 inch forward and fly some more.

And if you want to try adding some tail area just make up and glue on a ridge of balsa that runs around the fin and rudder so that it adds a couple of sq inches more area and try some more flying.

[vintage1]

I agree. Probably a little too much dihedral and too rearward a CG.

[DLC]

Thanks for the help everybody. I had tried moving the C.G. foward in my initial testing, and it helped, up to a point, but didn't cure the problem. My next step will be to follow BMatthews suggestion to increase vertical tail area by adding a strip of balsa around the tail.

[DLC]

Before I could do any test flying of my Cadet, a gust sent it into the ground and broke the wing in two. So I am trying to decide how to rebuild it. I found a method for estimating spiral stability in "Basics of R/C Model Aircraft Design" by Andy Lennon. When I apply it to the Cadet, the results say the Cadet is very spirally unstable. (Using the scale vertical tail size, it comes out in the neutral spiral stability range). So it appears that what I was interpreting as Dutch Roll might really have been a bad case of spiral instability coupled with overcontrolling by a marginally competent pilot. So I am not sure which way to go - do I put in more or less dihedral in the wing when I rebuild it, and should I replace the vertical tail with a smaller one or a larger one? Does anybody have any experience with Lennon's method for estimating spiral stability, or any other analytical method for estimating spiral stability?

[BMatthews]

If it has a tendency to get into a spiral dive and not recover then yeah, that's too big a fin, not enough dihedral, too far rear a CG or too aggresive a climb. These are all factors that will make a model that is spirally unstable go into a terminal spiral dive or suffer from a "close call" where it runs around like a pylon racer.

One of my recent old timer rubber ships suffered from a near case of this. I controlled it by adjusting the rubber motor to one less loop but a longer motor to reduce the climb rate. In my case that was all that was needed. That was fortunate as the design was "fixed" by my wanting to replicate the original design without any changes.

Dutch roll tends to be more of a side to side oscillation of the tail while spiral instability shows up as the model wanting to nose over to the side and run around in a steep, flat turn or even spiral into the ground as the bank angle tightens without any sign of recovery. Often the spiral opens up as the power runs down and the glide can then look fine but disaster was only just narrowly averted in such cases.

[Scaledown]

I've experienced uncontrollable dutch roll on a Fokker DVIII which had no dihedral. I suspect a small tail fin might have been the cause, but I read somewhere that having the wing well above the CG can also contribute. This plane has a deep fuselage. Is the battery pack on the floor perhaps?

[Sparky Paul]

I've experienced uncontrollable dutch roll on a Fokker DVIII which had no dihedral. I suspect a small tail fin might have been the cause, but I read somewhere that having the wing well above the CG can also contribute. This plane has a deep fuselage. Is the battery pack on the floor perhaps?
.
I didn't want to see this! :(
I'm about 1/2 way done with a D-VIII... and comparing it to my Dr-1 and D-VII.. the scale vertical appears inadequate.
I made a larger one which looks better.

[DLC]

It appears that the problem with my Cadet was spiral instability coupled with marginal pilot competence. I flew a revised version yesterday with the smaller 'scale' vertical tail shown on the plans and one degree more dihedral. According to Lennon's method for estimating spiral stability, this should have given the plane slightly positive spiral stability. It was too gusty to really check it out, but the testing I could do indicated that the estimate of slight positive spiral stability is pretty close. The revised version is certainly a lot easier to fly then the first version.