ted fancher imitation

[Tim Wescott]

It currently has roller bearings, which might be good enough.

I thought you didn't like roller bearings?  Or is that just ball bearings?

A note to others, because Brett probably already knows this: you can get different grades of ball bearings, including different preload levels.  I'm guessing that for a CL bellcrank you want bearings with no preload

[Tim Wescott]

  BTW, I think the lack of vibration is THE critical issue with electrics, but not necessarily for aerodynamic reasons. I have flown two electric airplanes that had *severe* hunting, really, nearly hopeless. In both cases I could move the handle and see the lines sag differently, but *nothing*, repeat, *nothing* happened to the flaps/elevator and the airplane continued on its track. Then a little more, and it broke loose, moved to far, then stuck again. As far as I can tell, this is solely and entirely a matter of stiction in the controls. The controls on both airplanes were reasonably "free" with no load but appeared to be glued in place in flight.  I am more-or-less certain that the issue is lack of vibration to keep the control stiction broken.

     This is what is holding up my electric airplane. I have cut the wing in half twice now to replace the bellcrank pivot, and I really don't know how I should do it. It currently has roller bearings, which might be good enough.

Are you sure it's the bellcrank pivot and not some other part of the control system that's stiffening up under load?  I know that different brands of ball link -- and different runs within a given brand -- can range from altogether floppy to pretty damned stiff.

And -- I thought you didn't like roller bearings?  Or is it just ball bearings that you don't like?

I'm assuming that Brett knows this, but you can get ball bearings in various fits -- for a control line bellcrank assembly you probably want a set that has a fairly loose fit, and you probably want to either assemble it with light or no lubrication, or you want to build the wing with access to the bearings so that you can renew the lubrication from time to time.  Oil tends to harden up with time, unless you use the really expensive stuff.  It's best to ask someone who's been there -- which I guess means Brett, in a few years.

[roy cherry]

hi ted the fuse has been built from a niece piece of dense half inch then sanded into an egg shape i did not do a fuse cut out then it was coverd in . 6 glass cloth sanded and then filled with resin  till compleatly filled and the coverd in lite wheght tissue applied with yellow coloured dope  i will paint in the fuse cut out     cherrs for now   roy

[Ted Fancher]

Those are very interesting plans.   What is the theory behind cutting a huge hole in the aft part of the fuselage (for weight reduction), and then adding a weight box at the tail of the airplane?   Wouldn't it be better to leave the structure in place and try to minimize the extra dead weight at the rear?

Hi Pat.

Good question.  The Imitation was designed and built as a test bed for both aerodynamic experimentation and for testing of the many (at the time) new and mostly heavier ball bearing equipped schnerle ported engines intended primarily for RC by the MFGRs.   I wanted to insure that I would be able to adapt the CG easily with engines with a wide weight range.  Thus, the plans show not only the weight box aft but also the three firewall spacers (1/4", 1/2" and 3/4") shown near the firewall sideview on the plans.  I used all of the spacers at one time or another for various weight engines but only required the weight box when I switched to the heavier four stroke Enya.  The tail moment arm on the Imitation and Excitation designs were significantly longer than the "standard Nobleresque" w.a.g. that was the norm on most "original" stunt designs for several decades.  The cut-out in the tail on the original (as well as stringered fuse side cut-outs on the Excitation) were pre-emptive design tools to minimize the need for knives, glue and labor to meet trim requirements.

With the heavier modern powerplants this concern is pretty much a non-issue and the longer tail moments have pretty much become ubiquitous in most "modern" era designs...and tail ends on a diet pretty much passe.

Ted

P.s.  Strictly from an overall weight consideration a single hunk of lead at the extreme aft end would add less weight than retaining the removed chunks of fuse closer to the desired CG...mass X distance, etc.  As I've never been in search of the zero ounce stunter a consideration like that isn't high on my list however.   Again, primarily providing a simple way to maintain the desired CG with powerplants of variable mass.

[Ted Fancher]

Re the chat about electrics and hunting/vibration.  I don't understand the apparent emphasis on the bellcrank pivot bearings (and, probably, the identical need on the horn bearings if modest degrees of rotational friction is the issue).  It seems to me that those interfacing surfaces would have a very low degree of "sticktion" due to lack of vebration compared to the control lines once they start getting twists in them.  The counter argument, of course, would be that the hunting issue would only surface upon arriving in inverted level flight as we mostly take off with the control lines well separated.

What am I missing???

Ted

[Tim Wescott]

Re the chat about electrics and hunting/vibration.  I don't understand the apparent emphasis on the bellcrank pivot bearings (and, probably, the identical need on the horn bearings if modest degrees of rotational friction is the issue).  It seems to me that those interfacing surfaces would have a very low degree of "sticktion" due to lack of vebration compared to the control lines once they start getting twists in them.  The counter argument, of course, would be that the hunting issue would only surface upon arriving in inverted level flight as we mostly take off with the control lines well separated.

What am I missing???

I'm guessing that Brett feels that, with the load of a honkin' big stunter pulling on the lines, the bellcrank pivot will experience more friction than will the flap and elevator pivots, which are only supporting aerodynamic loads.

I've pulled PFH* on a bellcrank mounted to my workbench, and not noted significant stiction.  But then, I wasn't trying it on 65' lines.

* pretty hard

[Crist Rigotti]

Could the problem be with the way we tape the hinge gaps causing some stiction?

[Dave_Trible]

I'm not sure I see the bellcrank issue- but not doubting.  To me the issue is the zero- clearance ball link horns.  There's no slack there to allow for anything like small misalignments or a mis-shaped part or any tiny hand movement, etc.  I think this is mostly where 'stiction' occurs in the controls.

Dave

[Brett Buck]

Re the chat about electrics and hunting/vibration.  I don't understand the apparent emphasis on the bellcrank pivot bearings (and, probably, the identical need on the horn bearings if modest degrees of rotational friction is the issue).  It seems to me that those interfacing surfaces would have a very low degree of "sticktion" due to lack of vebration compared to the control lines once they start getting twists in them.  The counter argument, of course, would be that the hunting issue would only surface upon arriving in inverted level flight as we mostly take off with the control lines well separated.

What am I missing???

   I was just flying around in level flight with no twists in the lines right after takeoff, and I could move the handle maybe 1/4" before the controls moved. The same airplane had only a very minor problem when it had a PA65 in it.

     The reason I am concerned about the bellcrank bearing is that it's the one part I cannot get to when it is completed, and, it can't afford to tilt on the shaft at all. The current airplane has a brass tube about 1" long running on a titanium pivot rod. That has too much stiction for my tastes.  The electric has the bellcrank on an axle, suspended from bearings in the spar, and about 2" between the bearings. That's so I can run very loose bearing tolerances without incurring a lot of tilt.

      Brett

[Dave_Trible]

My light bulb was a little slow to come on but I do also see what Ted was talking about-  the line friction could also contribute to stiction ( boy spell check loves that word). Vibration would help with that too.  I can feel that sometimes with my big IC jobs if I don't wipe the lines down.  I'm cleaning every flight with alcohol these days and it does matter.

Dave

[Brett Buck]

My light bulb was a little slow to come on but I do also see what Ted was talking about-  the line friction could also contribute to stiction ( boy spell check loves that word). Vibration would help with that too.  I can feel that sometimes with my big IC jobs if I don't wipe the lines down.  I'm cleaning every flight with alcohol these days and it does matter.

    Right - but we know for certain that the line drag is not an issue, otherwise they would ALL do it. The controls felt like they were welded in place, and this is on two separate airplanes, one of which was very competitive with a PA65 and showed little tendency to hunt.

      Control drag or stiction in the airplane has always caused hunting, and when you fix it, it goes away, whether the lines are twisted or not. It's just that the bar has been raised (lowered) on how much tolerance there is for it.

     Brett

[Lauri Malila]

I don't like the idea of having titanium as a sliding surface, especially with as big as 1/4" diameter and against brass. Stiction and other surface issues are well known with Ti. But how big issue it is in bellcrank, propably not very big. A good grease will take care of the problem but still I would like to be able to inspect it every now and then.
I see no issues with the usual 3mm/1/8" hard steel wire, as long as it does not flex under load as it usually does. The point is to make the bushing so rigid that the forces don't load the center of the pin but near the spars.
So a bush body from good hard aluminium screwed/glued/riveted to the bellcrank, with at least 3/8" o.d, filled with some low friction plastic, like Kel-f, Vespel or even Delrin would be my choise. L

[Howard Rush]

How much would a 3mm steel rod deflect?  I support mine just above and below the bellcrank, and the bellcrank force should be vertically distributed over the bushing, so I'd think the rod would be mostly in shear.

[Brett Buck]

I don't like the idea of having titanium as a sliding surface, especially with as big as 1/4" diameter and against brass. Stiction and other surface issues are well known with Ti. But how big issue it is in bellcrank, propably not very big. A good grease will take care of the problem but still I would like to be able to inspect it every now and then.
I see no issues with the usual 3mm/1/8" hard steel wire, as long as it does not flex under load as it usually does. The point is to make the bushing so rigid that the forces don't load the center of the pin but near the spars.
So a bush body from good hard aluminium screwed/glued/riveted to the bellcrank, with at least 3/8" o.d, filled with some low friction plastic, like Kel-f, Vespel or even Delrin would be my choise. L

     I am not too worried about wearing it out, because the pressure is so low. I used the lightest of light sewing machine oil (with a lubricant access hole). Grease of any type completely gummed it up and caused tremendous drag. I also polished it from the rather rough mill finish. It's not going to wear out. Break from hydrogen or chlorine embrittlement, maybe.

      The reason it's 3/16 titanium is to prevent binding as the rod deflects. 1/8 music wire bound up under 20 lb load with my test piece.  3/16 steel was OK, but it was too heavy.

     The newer version (electric version) is the same axle but glued directly to the bellcrank, with the bearings at the ends (separated by about 2"). Originally, they were as above, then after I realized that control drag was causing the severe hunting in other cases, I cut it apart and redid it with ball bearings at the ends of the shaft. After talking with Paul I realized that the loads on the 2 balls that were ever going to be loaded were probably too high, and cut it open again to switch to rollers, and it has been sitting there while I consider the issue. Delrin, delrin impregnated with Teflon, or Howard's magic plastic is the next likely step, with a fair bit of slop to guard against any future swelling. Even substantial slop doesn't lead to much tilt since the distance between is 2".

     Brett

[Gerald Schamp]

To the point of the Imitation fuse twisting. A very good and simple solution, and I've done this on two profiles. Make the balsa fuse outline from 1/2 inch balsa, with kind of a geodetic structure between the balsa fuse outline. Normal 1/2" balsa nose section, back to the high point of the wing. Install the 3/8" x 1/2" motor mounts, then sheet both sides of fuse with 1/64" plywood. Use a full sheet of 1/64" ply, both sides with grain running nose to tail. do not make the plywood diagonal, as this lets the fuse flex. Make a second 1/32" plywood doubler for both sides of the nose section back to the high point of the wing. Sheet the fuse then with 3/32" balsa both sides, from the high point of the wing to tail. Make 1/4" balsa doublers for the nose section back to the high point of the wing, both sides.  On the tank side, cut out where the tank will be mounted. I made a filler on the front of the fuse to blend into the spinner, like the Pathfinder has. Sand the fuse round top and bottom. This makes a very, very stiff fuse and the total weight is satisfactory for our needs, 7 to 8.5 ounces. Normal silksapn and dope to finish. If you have seen any of Don Hutchinson plans, I pretty much copied his idea, only with the addition of the 1/64" ply.  I think Tom Morris does that also on his profiles. Anyhow, just another thought to keep the fuse from twisting, especially with the low aspect ratio stab/elevator of the Imitation.