interesting flight condition

[flyguy]

I started to to do my "regular" 1/2 turn precision spin entry when out playing around and checking some temperatures and mixture control and got sloppy on the entry and found a flight "mode" that is new to me, I think.

Had full left rudder at stall speed and just enough aileron to keep the plane from rolling left and it would not stall with the stick all the way back. It just jumped up and down in a slow shake with the nose about level with the horizon. I don't remember for sure how much power I was carrying, but that could have been what made the difference. I usually use about 1500 RPM to get some torque to the left to help the rudder. I think it might have been at idle instead.

This plane is interesting also because it will not go spirally unstable to the left. To the right it will finally give up and head for the ground in a tight fast spiral, but to the left it will just roll into about a 45 degree bank and and stabilize there, going round and round with the nose hunting up and down slightly.

It has been my experience that that is a rather unusual behavior. Most planes will loose it from about a 30 degree bank if you don't touch the controls. That is in either direction.

[Hank Jarrett]

Re: interesting flight condition

When was the last time you trammeled your plane? Sounds almost like you have a yaw correction built in to correct a roll trim condition. If that is what is causing it, getting everything straight will give you a few knots at cruise for free.

Hank

[flyguy]

Re: interesting flight condition

Been there, done that. Chuckle. If it was anything I would say the opposite considering the difference in the adjustments on the struts. Trim tab is only tweaked a little bit and plane flys fine. I tweaked and adjusted every way possible not too long ago. This one is about as good as it is ever going to get without rebuilding the wing. It is very difficult to tell for sure, but the tip that was damaged at some time before I got it and repaired seems to be at a slightly different angle of attack compared to the "undamaged" one. That is the part with the bow at the tip seems to be slightly different. I suspect that is why the strut adjustments are pretty much at opposite limits.

I would think what with the engine torquing that direction that it would be more likely to roll into the turn worse in that direction, but not so. Strange.

You ever play with (any) airplane, checking that spiral instability mode? It is astounding how few pilots (including some instructors) are not even aware of such a thing. Many just hold top aileron in a standard pattern turn and don't even realize what would happen if they let go of everything at that point. It is one of my "pet" airplane subjects.

Darryl

[Guest]

Re: interesting flight condition

I started to to do my "regular" 1/2 turn precision spin entry when out playing around and checking some temperatures and mixture control and got sloppy on the entry and found a flight "mode" that is new to me, I think.

Had full left rudder at stall speed and just enough aileron to keep the plane from rolling left and it would not stall with the stick all the way back. It just jumped up and down in a slow shake with the nose about level with the horizon. I don't remember for sure how much power I was carrying, but that could have been what made the difference. I usually use about 1500 RPM to get some torque to the left to help the rudder. I think it might have been at idle instead.

This plane is interesting also because it will not go spirally unstable to the left. To the right it will finally give up and head for the ground in a tight fast spiral, but to the left it will just roll into about a 45 degree bank and and stabilize there, going round and round with the nose hunting up and down slightly.

It has been my experience that that is a rather unusual behavior. Most planes will loose it from about a 30 degree bank if you don't touch the controls. That is in either direction.

Hi Darryl,

Is that the same as a cross controlled stall? Sounds like it.

I recall doing that in my bc12d and the nose did wallow up and down with some side to side too.

If I held it long enough it eventually got worse and did do a crisp stall and nearly went over on its back.

Seemed like a under damped oscillation kind of event. The a/c was near gross weight.


Dave

[Guest]

Re: interesting flight condition

When was the last time you trammeled your plane? Sounds almost like you have a yaw correction built in to correct a roll trim condition. If that is what is causing it, getting everything straight will give you a few knots at cruise for free.

Hank

I'll bite. How do you do that on an assembled plane?

Dave

[Hank Jarrett]

Re: interesting flight condition

You can ONLY do it on an assembled plane. When I checked mine I built an incidence checker and checked incidence about every second rib. There WILL be a little variance, but a twisted tip is the worst place for it. I am betting that is where the problem is. With changing angles across the span you can trim it out, but ONLY at one airspeed and wing loading, Every other speed and loading will be out of trim. What you are seeing is that out of trim condition at those flight conditions.
There are a series of tests to determine if a plane is spiral unstable or Dutch roll unstable (ALL plane are one or the other, right in the center is the target). Yours sounds like it is one way at one trim speed and the other at another trim speed. That means the plane is crooked. I don't think I would like the instability to come up in the pattern, but ALL planes are crooked in some way or another and to some degree or another. If the tip really is at a different angle, you probably aren't going to be able to correct it without repairing the wing (when it gets recovered). If you use trimming to fix the pattern problem, it will be out of trim at cruise. IF I did 90% of my flying doing touch and go flights, I would fix it for landing trim and live with the cruise trim being off. If you like to GO places, I would just live with the landing trim issues and remember to stay AWARE of the potential danger of a landing departure from controlled flight.

Hank

[Guest]

Re: interesting flight condition

You can ONLY do it on an assembled plane. When I checked mine I built an incidence checker and checked incidence about every second rib. There WILL be a little variance, but a twisted tip is the worst place for it. I am betting that is where the problem is. With changing angles across the span you can trim it out, but ONLY at one airspeed and wing loading, Every other speed and loading will be out of trim. What you are seeing is that out of trim condition at those flight conditions.
There are a series of tests to determine if a plane is spiral unstable or Dutch roll unstable (ALL plane are one or the other, right in the center is the target). Yours sounds like it is one way at one trim speed and the other at another trim speed. That means the plane is crooked. I don't think I would like the instability to come up in the pattern, but ALL planes are crooked in some way or another and to some degree or another. If the tip really is at a different angle, you probably aren't going to be able to correct it without repairing the wing (when it gets recovered). If you use trimming to fix the pattern problem, it will be out of trim at cruise. IF I did 90% of my flying doing touch and go flights, I would fix it for landing trim and live with the cruise trim being off. If you like to GO places, I would just live with the landing trim issues and remember to stay AWARE of the potential danger of a landing departure from controlled flight.

Hank

You are describing rigging the wings using the T-craft rigging procedure and drawing, correct?

Setting the the angle of incidence / wash in at the 2nd rib from the tip.

No trammel bar involved there. Usually the trammel bar implies squaring up the sections of the spar and the compression ribs, drag / anti drag wires. That's what I thought you were describing by "trammeling".

Dave

[flyguy]

Re: interesting flight condition

Yeah Dave, definitely a cross control stall. Now that I think about it I remember more than one plane doing similar behavior when doing accelerated stalls. My other Tcraft and an Aerobat that I used to fly would sometimes jump up and down at the limit when trying to get them to pop over the top in an accelerated stall.

I guess I have somehow given the impression that my plane flies "funny." Actually it flies almost perfect under normal conditions ie, landing, take off, climb, cruise, and normal turns. That is after a lot of tweaking.

Unless one has adjustable rudder trim a plane is going to only trim out at one speed. Mine is at 2050 RPM and is OK up to 2150, which is where I cruise on trips. I use 2000 when I am just out for on a local flight.

According to a pretty good bit of info that I read a long time ago the reason "conventional" aircraft (and that virtually 100% of GA planes) are spirally unstable is because the vertical stabilizer starts trying to act like a horizontal stabilizer in the turn and as it pushes down on the tail it torques the airframe into the turn. In addition the outside wing is passing through the air at a faster rate than the inside wing and it's added lift adds to the roll into the turn effect.

There are some aircraft out there that have tails that are tilted down (ala upside down Bonanza style), or have a vertical stabilizer pointed down. Check a photo of the big drone that we use to waste the bad guys in the Middle East.

Don't mean to do aerodynamics 101 here, but I have been fascinated by the subject since I was a little guy flying model airplanes.

Darryl

[Hank Jarrett]

Re: interesting flight condition

What I am talking about is trammeling the whole plane, not just the wing. When you trammel a wing your are making the spars parallel and straight. You can align the ribs as you build the wing by shifting them up or down slightly on the spars so the airfoil is straight and even across the span. Once the plane is together you need to make sure the whole plane is true and square. The wings need to be perpendicular to the center line (or at least swept the same). The dihedral needs to be the same on both wings and the twist needs to be the same at every span wise station. The sailplane guys get totally ANAL about this with contour gauges and incidence measurements to a gnats ass every few inches. Then again, they have a glide ratio over 50:1 and we are down under 16:1 or so. We are only required to be the same at the root and the second rib from the tip. (We are leaving out all the tail trammeling here since I think your problem is probably in the wing, and is being fixed by un-trimming the tail).
On your plane you mentioned some damage repair on one tip. I would bet that the wing may be fine at the normal measure points, but is NOT symmetric in other places. By checking the angle at ribs at the same station on both sides I think you will find some of them are off. It is actually academic because you can only move the wing by twisting it with the struts and if it is twisted between stations moving one in moves the others out. You can only really trim up the whole wing and live with the off trim speed rigging. As long as you are used to how YOUR plane reacts, it isn't that big a deal (as long as it isn't so bad you have control loss problems, especially a surprise turning on to final a little late!) Just know when your plane will bite and don't go there. The next time you recover you cn level all the ribs and straighten it out, but be ready, you will notice it flys different! Don't want it to bite you on a RH pattern!
I WOULD be interested in hearing how much variance there is rib to rib on the incidence angle. I don't thing the average pilot even notices what you are asking about. Aerobatic pilots, test pilots and anal retentive engineers usually notice these things.

Hank

You are describing rigging the wings using the T-craft rigging procedure and drawing, correct?

Setting the the angle of incidence / wash in at the 2nd rib from the tip.

No trammel bar involved there. Usually the trammel bar implies squaring up the sections of the spar and the compression ribs, drag / anti drag wires. That's what I thought you were describing by "trammeling".

Dave