Spiral Instability

[flyguy]

I've done this in a C-150 and in the BC12 that I have now.
I start in low cruise, straight and level stabilized. Bank the airplane about 25 degrees either way and let go of the controls. Don't touch or change anything.
If you do this with a C-150 it will start accelerating and then the nose will go down and the plane will roll more into the bank. The spiral will get tighter and faster until you see that it is going to try and tear the wings off if you don't do something. I may have done it with the 172 I had, but I don't remember. I suspect the results would have been the same.

When I did it with the 12D I have now I went to the left first. It did the usual thing and rolled into the bank until it was about 45* and then to my total surprise it stabilized and wouldn't get any worse. Just went around and around at about 45* or so. I was astounded.
I then tried it to the right and the plane, after reaching about the same 45* bank started hunting up and down in pitch. Fairly mild and controlled at first, like fighting to hang on. Finally it was dropping the nose lower each time and finally it gave up and did the insane spiral thing and I had to recover.

Has anyone tried this with your Tcraft, and if so what were the results.
Darryl

[Hank Jarrett]

Re: Spiral Instability

Haven't done it yet, but plan to. ;-) I want to actually QUANTIFY the stability derivative numbers for the Taylorcraft some day.
What you are playing with is the balance between spiral instability and Dutch Roll instability. All planes have one or the other to some degree. The Taylorcraft, when properly set up is very well balanced between the two. What you want in a plane is VERY MILD Dutch Roll. Spiral instability is great for aerobatics where you are constantly making pilot inputs but a turkey dinner doze off will result in a death spiral. MILD spiral instability is a nice plane to fly because you never even notice you are making corrections, as long as you keep making them. MILD Dutch Roll is nice because the plane averages out to a straight line, but if you tend towards airsickness (or have a family member who does) you are going to need to clean up the plane every once in a while. It is AMAZING how sensitive some people are to Dutch Roll! It is also amazing how much a 6 year old kids stomach can hold and how far they can project it with no warning.
If you like playing with this stuff, try your experiment "Stick Fixed" and "Stick Free". If the controls are free to move it will change the response from if you hold them in neutral once you excite the motion. It is REALLY HARD to hold the controls and not subconsciously move them!
Hank

[flyguy]

Re: Spiral Instability

It is my understanding that there are ways to eliminate the spiral instability problem, such as putting the vertical stabilizer below the centerline of the aircraft. What I have read is that the roll instability is left as is in order to have a better roll rate for maneuvering in that axis.

As for Dutch roll I assume that you are talking about the aircraft wandering in roll back and forth within the stable roll limits in level flight; controlled by wing dihedral. I have never noticed much of that except on V-tailed Bonanzas. The wingtips on them tend to do a little elliptic motion in level flight. My 12D, C172, and C-150s that I have flown, in level flight, will drift left or right on a perfectly calm day, but that is about it.

Seems to me the fixed-stick mode would have the most effect on the pitch stability. I have to try kicking the rudder around and see what the...........man, that is weird.........I just drew a blank on the spelling of aileron, there was absolutely NOTHING there for a minute.....
.......to finish: __and see what the aileron controls do.
DC

[TreeTopFlier]

Re: Spiral Instability

I haven't tried the same test that you have, but I have been really impressed how smoothly the T-craft cuts through the air. The few times I have had the clouds start to close up beneath me, I put the BC12-D into a 110-115 MPH nose down spiral (left or right) to desend through a hole and it just tracks beautifully. When I did this in a cub or a champ, it took a lot of forward pressure on the stick as the speed built up and if I released the stick pressure the nose would "zoom" up. Not so with the Taylorcraft. I believe the airfoil C.G. used is one of the many things that sets this airplane apart from the J4's and the cheifs etc. (a little bit of genius) Even in level flight, flat bottom wings seem to always want to change pitch requiring the pilot to make minor adjustments all the time to maintain altitude. I just don't see much of that in the BC12-D. I only have about 325 hours on my Taylorcraft so far (had it 4 years now).......But I leave the airport smiling after every flight. To be so light, the flight characteristics are just amazing. ......................Don in SC

[astjp2]

Re: Spiral Instability

You ought to try and keep the wings level and kick the rudder hard left or right and make the airplane do a full 360* turn with the wings level. Rattles like crazy but it can be done. Tim

[Hank Jarrett]

Re: Spiral Instability

You ought to try and keep the wings level and kick the rudder hard left or right and make the airplane do a full 360* turn with the wings level. Rattles like crazy but it can be done. Tim

Yea, sounds like an SUV driving down rail road ties! Same with steep slips. Rumbles like mad as the vortices roll over the corners of the fuselage. I LOVE steep slips! I want to see which runs out first in a large rudder T vs a small rudder T. I seem to remember the ailerons hit the locks first in my large rudder 41. Does the small rudder run out first or the ailerons?
Hank

[flyguy]

Re: Spiral Instability

I was hoping that 4 or 5 guys would come in with "yeah, I did that one time," or some similar response and note the results.

That stabilized mode to the left goes against everything that I know about the way a conventional aircraft behaves. There is evidence that my left wing was banged on the tip and repaired and the wash in adjustment is not symmetrical. I thought that might have something to do with it, but it still should not be enough.

The design items that causes a "conventional" aircraft to roll further into the turn are: 1. The vertical stabilizer in a turn tries to act as a horizontal stabilizer and being on one side of the fuselage puts a twisting force on the fuselage, increasing the bank angle. 2. The outside wing is going faster and produces more lift causing additional rolling force into the turn.

I have this curse of a curiosity that movitivates me to understand everthing about the way the physical world works. I think I also have a little bit of a would be test pilot syndrome.
DC

[Pearson]

Re: Spiral Instability

.

When I did it with the 12D I have now I went to the left first. It did the usual thing and rolled into the bank until it was about 45* and then to my total surprise it stabilized and wouldn't get any worse. Just went around and around at about 45* or so. I was astounded.
I then tried it to the right and the plane, after reaching about the same 45* bank started hunting up and down in pitch. Fairly mild and controlled at first, like fighting to hang on. Finally it was dropping the nose lower each time and finally it gave up and did the insane spiral thing and I had to recover.

Darryl,

Don't forget that the prop does not blow the air straight back. It imparts a spiraling motion to it, which causes a force on the left side of the vertical stabilizer. Do you think it may be possible that this very slight force might help keep the bank from increasing to the left and aggravate it to the right? Also, in one of your later posts you mentioned that the outside wing is travelling faster than the inside wing. What is that in relation to? That may be true in relation to the Earth, but not to the wind, unless you are skidding. Have you considered that your slip indicator may be inaccurate? Have you done a "Falling Leaf" in your plane? If you can get into a stall and keep it there for awhile without falling off into a spin, try to see how long the wings will stay level with NO AILERON input. If the ball stays centered for a few seconds while the wings are level, your plane is good. But from what you describe, I suspect that you will find that it will fall off to the right each time, or your ball will be off to the left slightly.

[flyguy]

Re: Spiral Instability

Yep that is exactly what I thought. Problem is, at higher bank angles the pressure you mention from the rotating slipstream is both rolling the airplane to the right and at the same time dropping the nose. Which is the biggest effect? I don't know. I think it is likely the wing rigging on mine.

As for the air, we are probably about to get into the down wind turn insanity here, Chuckle.

The air is standing still remember, and you are in effect swinging the airplane around on a string (cable if you like.) The tip of the outside wing is going through the STILL air faster than the tip on the inside of the circle. It is the main reason for holding top aileron in a turn.

None the less, it is my understanding that "conventional" design airplanes are spiral/or roll, unstable. Mine (and I suspect other 12Ds) are unusual even if they simply RESIST loosing it, as the worse (right) side does.

Would be nice if someone else would try it with a 12D. Frankly I thought several guys might come on and say, "Yeah, Yeah, we all knew about this 50 years ago. It's because the whatsit and the zatzifraz is designed different on Tcrafts."

On subject, but off the line of thinking is what I told a RV4 flying friend of mine. I said that if I could have sat with Ted Kennedy in his plane for 5 minutes I could have for sure told him something that would have saved his life. "Just take your hands off and center the turn indicator with your feet. Pitch will take care of itself." My friend replied that if he was that bad of a pilot nothing I could have told him would have helped. Maybe so, bit cynical I think. No horizon and no lights at night have wasted a lot of guys.

Darryl

[Mark Julicher]

Re: Spiral Instability

Darryl,

Also, in one of your later posts you mentioned that the outside wing is travelling faster than the inside wing. What is that in relation to? That may be true in relation to the Earth, but not to the wind, unless you are skidding.

Yes, in relation to the earth and also the wind. Imagine a plane in a steep bank and there is a string tied to the inside wing tip and the string is tied to a pivot point on the ground. As the plane turns around the point the outside wing must travel faster. The degenerate case would be where the wing tip is touching the ground as the plane flies tight circles around it. In that case it is easy to see how the outside wing is traveling faster.

[Pearson]

Re: Spiral Instability

Well, we are getting off point here, but I still say that in a non skidding or slipping turn both wings are travelling at the same speed in relation to the air. If one wing were tied to a pivot point on the ground and turning about that point, the plane would be in a spin. I am talking about the situation that Darryl originally described, where he is in a normal turn. Other than the brief period where an aircraft is entering or exiting a slip or skid, all the parts of the plane are travelling at the same speed. As a plane enters or exits a slip or skid, one wing will travel just slightly faster than the other. But even then, after the plane is established in the slip or skid, all the parts are travelling at the same speed again relative to the air. If you are in an established coordinated turn, the air is flowing over the wings at the same speed, otherwise one wing would be creating more lift than the other and require some other force to hold the established bank angle. Look at it this way; if you were in a ninety degree banked turn which wing would be travelling faster in relation to the air?

[flyguy]

Re: Spiral Instability

Please define the aeronautical term "top aileron" as used by thousands of C-150 instructors.
DC

[Pearson]

Re: Spiral Instability

Top aileron would the one that is farthest from the ground.

[flyguy]

Re: Spiral Instability

And no one has ever mentioned to you the reason why a pilot must hold a slight down pressure on the top aileron in a coordinated turn?
DC

[Pearson]

Re: Spiral Instability

Darryl,

It sounds like you are saying that you have to hold your top wing down when you are in a turn. Is that why you were surprized when you were doing your test. I don't have to do that in mine, at least not that I am aware of. I have not done these sort of test like you are describing. I usually just enjoy the fun of being up in the air. Next time I am up I will do some of this stuff you are talking about. I am surprized that there has not been more input from others here. I would be curious to hear if others have to hold their top wing down in a turn.