"Hunting" Thunderbird 1 Revisited

[Skip Chernoff]

You folks may remember last spring I posted about a Thunderbird 1 that I was having trouble getting rid of some serious "hunting". I even posted a little video after I thought I solved the problem by moving the lead outs rearward.I was wrong and I never really solved the issue. I put the model aside thinking I'd get back to it after the contest season.

A number of the experts here suggested that there might be something wrong or hanging up in the control system.So today I took action to really try to find the problem. With surgical precision(hehe) and a set of Brodak plans I was able to very carefully remove the top of the fuselage from forward of the cockpit to just behind the flap horn.(about 10 inches) This action exposed the bellcrank,pushrod from bellcrank to flap horn and the position of the the elevator connection to flap horn. The plane also had a access hatch built into the rear to get to elevator horn and weight box.

I didn't build the plane and got it from a friend in trade for some R/C gear. He didn't build it either. Anyway, it looked very well built and was new when I got it.

Unfortunately the fellow who built the plane had the elevator horn in the "wrong" holes on both elevator and flap horn ,and the flap push rod also in the wrong holes on the flap horn. The flaps had slop while the elevator had none and some down with flaps  in neutral position. Tomorrow I will reset all of the controls to Brodak's specs ,button the bird up and see if these corrections get her flying the way a Thunderbird should.....Stay Tuned....PhillySkip

[Tim Wescott]

The second best thing to not having a problem is to know exactly what the problem is.  Good luck with it.

[Skip Chernoff]

I was able to get the Thunderbird to not hunt in upright level flight. Go inverted and the hunting returned. I moved the lead outs to three different positions. The most forward position is where she stunts the best,as I moved them back I lost line tension. Any other ideas? The "balance" of the plane seems to be good as she neither climbs nor dives when the engine quits. I'm ready to give up on this ship.It's just taking too much time.....PhillySkip

[Mark Scarborough]

move the cg forward slightly, the power off isnt the be all end all of CG determinations.

[Motorman]

Did you try a little down elevator with the flaps at neutral? You coming down to Millville Sunday?


MM

[Skip Chernoff]

The flaps and elevator are at neutral.I didn't try down in the elevator. Sorry but I can't make it on Sunday.I had another event already scheduled.

[Skip Chernoff]

Mark,I've had the CG in many different positions and it doesn't seem to correct the problem. This plane is a real head scratcher.

[Tim Wescott]

Mark,I've had the CG in many different positions and it doesn't seem to correct the problem. This plane is a real head scratcher.

Before or after you got the controls straightened out.  Just because you found one cause doesn't mean there isn't another one, too.

It sounds like a frustrating plane.

[Skip Chernoff]

Ty I checked the incidence of the wing to stab to engine and they are all at zero. I just can't seem to figure this one out. I'm tired of wasting fuel and practice time at the field. I'll use it for a knock around plane in the middle of the winter when it's too cold to fly the good ones. In the meantime I'm getting ready to start an RSM Shark 45.Thanks,Skip

[Larry Fernandez]

Before you throw in the towel, here is my two cents

I built a Thunderbird a few years back that had hunting issues.
Like you, I tried every trim change I possibly could and had no success.

One of the local hotshot flyers in area(I can't remember who, as there are so many around these parts)
Suggested that I put in a tad of down thrust.
I put a washer under the two front mounting holes and low and behold, I had myself a great flying Thunderbird

Larry, Buttafucco Stunt Team

[Skip Chernoff]

Thanks Larry for jumping in .Remember this is a Thunderbird 1 with upright engine so I need to place the washers under the rear mounting holes. Ok ,I'll try it. Thanks,Skip

[Steve Helmick]

Good to see that you're going to try Larry's idea, even if it doesn't work. The way I see it, trimming is a tremendous learning tool, whether the end result is wonderful or not. You try stuff and see what happens. It might not help this plane, but it might help the next one. Somewhere, sometime, that bit of information will be useful to you or somebody you know. Please don't forget to post your results, so we can make fun of Larry and see him pitch a fit!  Steve

[Larry Fernandez]

Please don't forget to post your results, so we can make fun of Larry and see him pitch a fit!   Steve

I'll "Hurl" a Fox,
but I wont "Pitch" a fit.
 
I have been known to "pitch" a tent.

Larry Buttafucco Stunt Team

[Skip Chernoff]

Steve, I promise to share the results.Haven't had time to work on the plane,maybe tomorrow....PhillySkip

[Chris Fretz]

How do you check incidence with the flaps and elevator attached on the airplane?

[Steve Helmick]

How do you check incidence with the flaps and elevator attached on the airplane?

Yeah, that's a problem, and why most the better fliers make the wiggly parts removable from the rigid parts. If you're good with tongue depressors, Popsicle sticks, rubber bands, and clothes pins, you might be able to get useful results with a couple of Robart Incidence Meters. As a career machinist, I would point out that if your LE & TE's aren't consistently shaped, the results still might be less than stellar. If you look at the way they index, you'll see what I mean, just a 90 deg. V against each edge. But then, I don't see any better way to do it, and it does work quite well. Like me, it has limitations!    Steve   

[Skip Chernoff]

Steve, all of the incidences have been checked with my incidence meter. Eveything is per spec. The one positive thing that I am getting out of all of this is experience trying different trimming methods and seeing the result or "not" a result.

I thought I had it when I got rid of the upright hunting and the plane would still turn a corner,but the moment I went inverted and she started hunting again my bubble burst one more time. I try the down thrust and see what happens...PhillySkip

[john e. holliday]

What is the line spacing on your handle?   Of the four Ringmasters I have one would not fly level as in hunting.   I was using the hard point Fancher Handle.  Went to the old easy just 4 inch handle and the problem was solved.  Just to make sure I tried the hard point again.   Now I have one Ringmaster that does not like the hard point.   Don't what bellcrank is in it as I didn't build it.

[Skip Chernoff]

John I've tried a number of different handles with different line spacing,and overhang....doesn't seem to make any difference.

[Howard Rush]

I didn't notice any mention of control system friction.  It's the first thing to suspect. 

Can you tell whether the problem you're seeing is a limit cycle or instability?  A limit cycle occurs when you give it a little control and not much happens, then you give it a little more control and a lot happens, then you repeat in the other direction.  It is typically caused by friction in the system, but can also be caused by an aerodynamic perversion.  If the controls are super free and the flaps have no play in them, then you might suspect an aerodynamic perversion.  Sealing the flaps would be the next step.  Plain old instability is like balancing a broomstick on your nose. It happens when the CG is too far aft. Old airplanes with too-small tails (I don't know if the Thunderbird is in this category) are sensitive to CG location.  You could go by the CG on the plans, but it might be better to find somebody who has successfully operated this particular Thunderbird version to see where he or she put the CG. 

[Tim Wescott]

Can you tell whether the problem you're seeing is a limit cycle or instability?  A limit cycle occurs when you give it a little control and not much happens, then you give it a little more control and a lot happens, then you repeat in the other direction.

Hm.  Interesting divergence in vocabulary, there.  Wikipedia's definition of the term "limit cycle" matches what I know from nonlinear system dynamics.  By that definition your "limit cycle" and your "plain old instability" are both limit cycles.  For that matter, they're both soft limit cycles (i.e., they'll start up on their own, where a hard limit cycle is exemplified by a pendulum clock which won't start until you give it a big-enough nudge).

[jfv]

For what it's worth, I have an electric plane that hunts and while the problem is not completely solved, when I switched from a Reyco hard point handle to a Reyco adjustable cable handle, it was significantly better.

[Skip Chernoff]

Howard what you are describing as "limited cycle" response is EXACTLY what the plane does. Early on in this discussion excessive friction in the controls was mentioned, so I cut into the plane to check everything out and other than the ratios being not what the plan showed the system was very free and smooth operating. I haven't been able to get to the field to test fly the TBird ,but I will tape seal the flap hinge line before trying a bit of down thrust as was suggested. Thanks,PhillySkip

[Mark Scarborough]

For what it's worth, I have an electric plane that hunts and while the problem is not completely solved, when I switched from a Reyco hard point handle to a Reyco adjustable cable handle, it was significantly better.

my opinion,,
this is indicating the CG is to far aft to me, the cable handle allows some give and mush in the control input by its nature, which dampens the control input/response ot the airplane.
Electrics tend to want a much further forward cg as documented by many on here ( most of whom are better competition pilots than me)
I suggest you move the CG forward, way forward of the suggested IC CG and try again with the hardpoint handle,,

[Fredvon4]

Skip I have exactly zero reason to risk a opinion here, but do have an experience that may be worth checking

The plane flew reasonably well but seemed to have, as describe above...a tendency to need more input than reasonable then require opposite to correct

It was suggested for me to check the freedom of controls... on the bench they seemed just fine, not sloppy, no sticking, to me very smooth... a Guy who is much better than I flew it and claimed the controls were sticking/binding

I re-investigated and found the original Bell crank was installed with a threaded through bolt and the Bell crank...UNDER TENSION was digging into the threads and causing a bind that required a lot of input but then went too far....Yadda Yadda

Put a handle to your lead outs and try it UNDER Tension to see if they are truly free with no "limit cycle"

[Tim Wescott]

Put a handle to your lead outs and try it UNDER Tension to see if they are truly free with no "limit cycle"

+1.  I was going to suggest that, but Fred beat me to it.

[Skip Chernoff]

Fred I'll try your suggestion as well. Early on in the trimming process I tried more nose weight and the plane was so sluggish it would hardly loop. I'm certain that we'll find the cause of this problem.

[Skip Chernoff]

Eureka! Today we got to the field with the "Hunting TBird" .Conditions were perfect. First I sealed the flap hinge line ,and flew the plane.  No dice, the plane was still hunting especially inverted. Then I placed a washer under the engine rear lug to give down thrust. Still no luck. So we looked closer at the lead outs and when viewing carefully into the inboard wing slot we discovered that the builder of the model had a balsa cross member running chord wise below the up line (front) and the down line (rear).Under tension
 both lines was rubbing on this unwanted and unneeded piece of balsa. What was the builder thinking?
Anyway ,once we punched this "crossmember" out of the way the plane flew just fine. Not a contest plane,but still fun to fly.So to all who said "your controls are binding....you were right! Thanks ALL for jumping in to this thread....PhillySkip

[Pat Chewning]

Well I've been flying control line for 1 year and 2 months now.  The 1st plane I built (Akromaster) I did what I thought was a "good" job of installing the bellcrank and the leadouts.   After a crash, I discovered that I could have done better to prevent the arms of the bellcrank and the attached lead-out wires from binding.

Then I build a Twister and did what I thought was a "great" job on the control system.  After a near-crash flight, I dug into the wing and found I could have done a better job securing the nut and screw for the pivot!

Next build I guess I'll do a "super-duper triple-checked excellent" installation and I just might not have to cut into the wing.

Glad you found your problem before the plane crashed.