Bellcrank placement

[Gary Dowler]

Building a old Sterling PT-19 kit. It was primarily intended for R/C, but had rudimentary instructions for C/L. It is being built for scale, not stunt.
I've figured out that the bellcrank placement shown on the plans was terribly wrong. It was shown far too high, being almost 4" above the bottom of the fuselage (total height 5"), which would require leadouts to enter the upper wing surface about 6" in from the tip in order to exit the tip at or below the vertical centerline. Got the vertical placement sorted out, but this brings the next question.

The plan shows pivot point of the bellcrank as 2" behind the CG. How important is the fore/aft placement in relationship to the CG?  I know it can vary some, but is 2" behind excessive?



Gary

[bill bischoff]

From a physics standpoint, it does not matter where the bellcrank is located, only where the leadout exit is located. That being said, you want the leadouts to pass through the guide without binding or making excessive bends. Since the leadout guide should always be behind the CG, your front leadout should come out practically straight to the guide. The location on the plans should be perfectly suitable.

[BillLee]

Well,..... not quite.

The leadouts at the wingtip guide will want to line up with the CG of the model when in flight.

If the leadouts are flexible, the belcrank location behind the CG will be O.K. (not optimal but O.K.) since the flexible leadouts will more-or-less easily bend to get to the belcrank.

If the leadouts are stiff, like would be with 1/32" wire, they will bind at the tip and you would be better off to put the belcrank as close to the CG as possible.

BTW, this is true for vertical placement as well as horizontal.

[bill bischoff]

Well,..... not quite.

The leadouts at the wingtip guide will want to line up with the CG of the model when in flight.

If the leadouts are flexible, the belcrank location behind the CG will be O.K. (not optimal but O.K.) since the flexible leadouts will more-or-less easily bend to get to the belcrank.

If the leadouts are stiff, like would be with 1/32" wire, they will bind at the tip and you would be better off to put the belcrank as close to the CG as possible.

BTW, this is true for vertical placement as well as horizontal.

Let's assume a 3" bellcrank, with the pivot 2" behind the CG. Let's also assume the average leadout position 1/2" behind the CG. This puts the rear leadout (at the bellcrank) 3" behind the line guide. With a half span of 30", the rear leadout passes through the guide at less than a 6 degree angle, or even less if the leadouts are separated at the guide. Perfectly suitable.

[Tim Wescott]

... which would require leadouts to enter the upper wing surface about 6" in from the tip in order to exit the tip at or below the vertical centerline...

Eh?  I mean -- eh?

The leadout guide wants to be behind the CG a bit, and level with it vertically.  On a bipe, that usually means hanging between the wings (many full-scale biplane designers thoughtfully put interplane struts on their planes, out of courtesy to scale CL designers).  I'm not sure where this entering and exiting the wing surface comes from -- the leadouts should come out of the fuselage and go through the guides, without ever touching the wings.  (Or make the holes in the guides big enough to pass eyelets through, and use buttons on the bellcrank -- you could access them through the cockpits.)

The plan shows pivot point of the bellcrank as 2" behind the CG. How important is the fore/aft placement in relationship to the CG?  I know it can vary some, but is 2" behind excessive?

Not terribly, in my opinion.  As long as the leadouts are smooth through the guides, you're OK.  I could see nudging the bellcrank forward if you were concerned (and if you don't like my button idea).  It won't be as nice as if it's all lined up, but it should be OK for a typical Scale flight.

[Gary Dowler]

Tim, the "enter/exit" the wing would be from the original plan. This is a scale bird, airfoil thickness is barely 1.25", with 12 deg dihedral. Bottom of wing even with bottom of fuselage.  Original plan showed the bellcrank placed 4" above the bottom of the wing.  If built this way the leadouts would have to exit the fuselage side, then enter the upper surface of the wing in order to exit at or just below the tip.
Pardon the crude sketch



Gary

[Target]

IMHO, I would reduce the dihedral to allow the leedouts to ride in a fence on the top of the bottom wing tip.
Seems like maybe the designers were lazy when having a primarily RC design that "could" be used for C/L. It could be that they couldn't be bothered to put an alternate angled dihedral brace in the kit, and that is teh reason for their (seemingly nutty) suggestion.

That being said, I am not any kind of expert, nor do I have experience with U/C bi-planes, but, it seems like common sense wise, what I am suggesting would work fine.
I will be interested to see what the experts say.

My first suggestion was going to be to eliminate the dihedral completely and run the leadouts through an extension of the wing-to-wing strut, but, you said it was for scale, and I'm sure that at least having some dihedral in the bottom wing is more scale than no dihedral at all.

Best of luck with your project.

[Steve Helmick]

If those two big dark arrows are intended to mark the CG range, I'd say they are WRONG. CG should be somewhere between the 18%-20% point AVERAGE between the upper and lower wing chords. I'm sure their suggested BC mount location was intended to hide the BC from view through the open cockpits. Also sure that they had no clue what they were doing, regarding converting the beast to UC.

If you need a Saito .80 with throttle for this PT-17, let me know! I really don't know how big it is, me not being a fanboy of Sterling kits, or R/C kits in general.  Steve 

[Gary Dowler]

If those two big dark arrows are intended to mark the CG range, I'd say they are WRONG. CG should be somewhere between the 18%-20% point AVERAGE between the upper and lower wing chords. I'm sure their suggested BC mount location was intended to hide the BC from view through the open cockpits. Also sure that they had no clue what they were doing, regarding converting the beast to UC.

If you need a Saito .80 with throttle for this PT-17, let me know! I really don't know how big it is, me not being a fanboy of Sterling kits, or R/C kits in general.  Steve

Yes, the black arrows are indicated CG positions per Sterling. Forward one is for C/L. This would be about 24% wing chord.
Span is 48". It's getting a OS 40FP for power.  Will have throttle, flaps and brakes.

Also, I misprinted the model, I was trying to figure out why everyone was making comments about biplanes. It's a PT-19, not PT-17.  My bad

Gary

[Steve Helmick]

Oh, now that it's a PT-19, the best plot would be to just mount the bellcrank in the wing and put the leadouts in the tip. Don't forget to put tipweight in the outboard tip!

Are you going to use an R/C system for throttle? Amazingly, that's what I'd do! See, electrons have a good use, after all!  Steve   

[Gary Dowler]

Oh, now that it's a PT-19, the best plot would be to just mount the bellcrank in the wing and put the leadouts in the tip. Don't forget to put tipweight in the outboard tip!

Are you going to use an R/C system for throttle? Amazingly, that's what I'd do! See, electrons have a good use, after all!  Steve   

Yes, r/c for throttle/flaps. It will have brakes, but I want a good feel for them being a tail dragger, so they will operate off the third line , that way I have immediate feedback on it.

Gary

[L0U CRANE]

The lines are not straight from the handle to the leadout guides. They trail in what should be called an accelerated catenary form. A catenary curve represents the sag of a cable between two supports - at uniform cable weight. In addition to the simple weight of our "cables", they are also subject to increasing drag due to their speed through the air.

At the handle, this speed is approximately zero. At the leadout guides, the lines are going as fast as the wing is, at that point. Drag increases as the square of the 'local' velocity.

The curve becomes 'deeper' approaching the leadout guides. Our lines, whether cable or solids, take that curve. As slack 'cables' they can only carry the pull load within their own diameters. They can't push or pull in any other direction!

The crucial thing is to 'aim' the cable pull AT the (fore and aft) CG. Programs, or apps, can calculate this trail angle by length to the guides, velocity, line diameter and number of lines. (the LINE programs.)

IF the pull, aimed by the trail angle at the leadout guides, aims ahead of the CG, it will tend to pull the nose in toward the center of the flight circle. ...And vice versa.

In normal flight, the centrifugal load (I know, it is a centripetal load, but...) is divided equally between the two lines. Their midpoint defines the effect of net line pull. In maneuvers, the 'active' line takes more of the total load (the aim of net pull shifts toward that line and its leadout guide. These are temporary changes, and greatly overcome by inertia and aerodynamic factors, usually, until we resume 'straight' flight with the pull about equal on each line.

NOT trying to show off or anything! These things have been kicked around many times. What I'm trying to do is put them together in a simpler way than many discussions of individual parts have not done. Have I had any success at that?

[Gary Dowler]

Made sense to me, thank you for taking the time to post that.

Gary

[Tim Wescott]

Tim, the "enter/exit" the wing would be from the original plan. This is a scale bird, airfoil thickness is barely 1.25", with 12 deg dihedral. Bottom of wing even with bottom of fuselage.  Original plan showed the bellcrank placed 4" above the bottom of the wing.  If built this way the leadouts would have to exit the fuselage side, then enter the upper surface of the wing in order to exit at or just below the tip.
Pardon the crude sketch

Hey Gary:

The sketch gets the idea across just fine.  Looking at 3-view of the plane on Wikipedia, it looks like it has way less dihedral than that.  If the thing was originally designed for free flight, or for no-aileron RC, the designer would have put in about that much dihedral.  Keep in mind, too, that the top wing is going to be a good part of the mass of the airplane, and that will move the CG up.

From the 3-view, the lines should probably come out about 1/3 of the way up from the bottom wing to the top wing.  That's plenty of room.  I'd build the plane, find the up & down position of the leadout guide that makes it hang level, and then nail down the leadout guide.

If I were just building it for fun I'd find a 3-view on the internet (the one on Wikipedia is small, but gets the idea across) and modify the model to fit.  If I were building for competition I'd get a documentation package from someplace, and I might even listen to advise from real scale modelers, at least a bit.

[Target]

Tim-
It is a monoplane, not a biplane.
I agree on the dihedral though!!

R,
Target
Taking a break from shop clean out....

[Tim Wescott]

Tim-
It is a monoplane, not a biplane.
I agree on the dihedral though!!

R,
Target
Taking a break from shop clean out....

GAAAAAAH!  I saw PT-19, but I read PT-17.  In the immortal words of Emily Litella -- that's different.  Never mind.

I'm building an Ercoupe for scale which has similar wing geometry.  I'm putting the bellcrank in the wing, and bringing the leadouts out of the bottom of the inner tip.

Bigger plane though -- dunno if Gary want's to mess with it.

Here's the thread: https://stunthanger.com/smf/scale-models/ercoupe-build-(slow)/msg523811/#msg523811

Gary, you could also just run the leadouts off of the top of the tip.  If you just fly in circles you should be OK that way.

[Steve Helmick]

Vaguely recalling that the Sterling PT-19 was for R.E.T. R/C, so extra dihedral may well have been added. Something to check out and correct for a scale ukie model, for sure.

The one thing I wouldn't do is use a 3rd line for anything, especially something mundane like brakes. If using R/C throttle, just rig the brakes off the throttle OR rig it to work on extreme up elevator. It won't take a lot to stop the model at idle. I wouldn't try to simulate "running up" the engine, but stopping in front of the judges would probably get a few points.  Steve

[Gary Dowler]

Now working off full up elevator is a good idea.  It would eliminate the hassle of a 3 line system.

There is no option to put the bellcrank in the wing on this without a complete redesign of the wing, which I don't have enough time to do.

A brief look of the 'ole interwebs just revealed that the prototype PT-19 had 7.5 deg of dihedral, not the 12 Sterling built into it.  This can probably be changed without considerable effort. I've already had to remake 1/2 of this kit anyway, new center section spars (already remade once, the originals were unimaginably bad) with prototype dihedral wouldn't be hard.

Gary

[Gary Dowler]

Well this was interesting. I had remade the center section spars using the original parts as templates. Wood quality was terrible, and die cutting nearly severed several pieces.
Anyway, after the discussion above I set about redoing them with prototypical dihedral. Something didn't look right.  I was quite surprised to discover, upon closer inspection, that these parts had been made with different dihedral angles on each side!!!
The difference was 3-4 deg!!
That would have made for some funky looking wings.
I've begun redrawing them properly. At every turn this kit has been a pain in my posterior!

Gary

[Target]

It seems the kit is less than sterling, pun intended.
You'll get it whipped into shape. I'm sure it'll be an awesome machine when you're done.

[Tim Wescott]

I've begun redrawing them properly. At every turn this kit has been a pain in my posterior!

This is why I build one or two kits and then swear "never again" and start doing pure scratch builds.  Then, I get tired of making everything by hand and...

[Steve Helmick]

Bad kits (Sterling, Berkley) are definitely a PITA. I would suggest redesigning the kit wing. It's wrong already, so it's almost always easier to start from fresh scratch.  Steve

[Skip Chernoff]

Guys let's not  be so tough on Sterling Kits. Agreed that at times the wood was rock hard and the parts die "crushed". We can't compare what is available now to what was available back then. Sterling manufactured kits to fit a "price point". They also had something for everyone ,Scale,combat,stunt,sport,carrier....you name it.

Another thing, the plane in question was primarily designed for  R/C or Free Flight not C/L. I'm certain that the designers knew that anyone who would convert it to C/L would have to do some figuring stuff out on their own.

About a year ago one of the designers from Sterling was moving from the Philly area out West. He asked us if we would like some old kits he had sitting around.Naturally, we said yes.We ended up with some Prototype stuff,and a few  Southwick "Skylark" kits. The stuff was just beautiful. Not "Die Crushed", light, straight wood.
The top blocks on  these Skylarks was feather light. I don't think these were "hand picked" either.

Just remember that many of these Sterling Kits helped us get our start in the enjoyment of building and flying model airplanes so cut them a break!.....Cheers,Skip

[Gary Dowler]

Skip, overall I will agree with you. Any hole in the ground can turn up a gold nugget, right?  I understand completely about the market Sterling was catering to, and they did make a lot of great aircraft. I have 4 of them. But the average quality was poor.
As mentioned, heavy wood (which could be an asset for inexperienced fliers), parts more crushed out than cut, etc.  I tried to build a Flying Fool 2 years ago. Ended up using it as nothing but a pattern to make a larger version of because of wood quality.  I have a profile P-51 with flaps. Many of the parts, especially ribs, are cut so badly they are scarcely recognizable. I put it away.   I have a Corsair that I have not dug into, it's an unknown at this point. Though I did build one many years ago that I recall as being ok.  And I have the PT-19 I'm building now. I've had to replace 1/2 the wood thus far because of terrible die cutting. The LE material is so heavy/hard I think I could beat the dog to death with it, and the plywood parts are simply inexcusable. Wood quality is terrible, die stamping that nearly severed several spars and the dihedral on these parts is 3-4 deg different one side to the other.

The result is extra cost, a lot of extra time and even some redesign of the parts to simply make them work.   I like Sterling designs, but to market a kit to beginners and those on tight budgets they really should have had better quality control. Their customer base would have been far better served and been happier, and more loyal.

Gary

[Serge_Krauss]

If those two big dark arrows are intended to mark the CG range, I'd say they are WRONG. CG should be somewhere between the 18%-20% point...

Good suggestion. In using on-line c.g. calculators put up by RC clubs and observing c.g. placement in general aviation and FF, I have found that RC, full-sized planes, and especially FF models have significantly smaller static margins than CL models. For whatever reason, we seem to need less sensitivity in CL. So our c.g.'s need to be further forward of R/C suggestions. The c.g. range given may have suited R/C and FF applications of the model. By all means, move the c.g. into that more CL-appropriate range, especially for a scale application.

[Steve Helmick]

At the time I made the post about the dark arrows and CG location, we were still under the erroneous assumption that it really was a PT-17 (staggered wing biplane) per the original post title, and not the PT-19 (monoplane) as the thread evolved. Not being fully informed (the LE of the wing wasn't shown in the plans picture), I plead lucky as charged!  Steve

[Robert Whitley]

In defense of the Sterling model kits of our youth I have to agree with an earlier post that they reflected the standards of their time.

Some were somewhat substandard with regard to wood and cut quality but others were actually quite good.

I have an RC SE5a that I started back in the seventies for control line scale and stored away with all my other modelling stuff until recently. This thread encouraged me to dig it out and see what it’s quality is. The framed up kit actually has very good wood, diecutting and engineering.

I look forward to eventually getting back to it and finishing it as a detailed fun scale model.

We need to evaluate models for what times they represent and not our current expectations.

Wishing you great success and enjoyment of this classic.

[TigreST]

Think Mr Al Rabe put a bunch of words together related to bellcrank location and wing dihedra and such.  Very enlightening commentary from more then a few years ago.  Maybe worth a google and look see.

T.