RSM P-40 & ST 35

[Leester]

In the engine forum I was asking about the ST 35 and wondering how it would do in my RSM profile P-40. I found out today. For 1 lap it was great! Then something broke, I could feel it in the handle. At first I thought a line broke cause it went from level flight to nose in the ground in a heartbeat. I walked the lines to the plane and found them still attached to the leadouts. When I got home I cut into the inboard wing and found the bell crank mount tore through three ribs. The fusalage is cracked just behind the inboard doublers, outboard wing tip and weight box broken off,spars broken. Stab. leading edge broken at the fusalage.The Master airscrew nylon prop broken off at each side of the spinner. Man I wish I knew how to do a pull test at home. The cause was builder error. I just hate learning from my mistakes.

[Oregon_Flyer]

Leester,

Actually I bet you can if you have a scale.  Set up the scale so it is stationary and then by either using an adjustable fixed stop or, rigging an adjustable fixed loop, you can set the
scales adjustment travel to cause the scale to read just a bit over the limit you desire.  By using a short set of joined cables you can now hook up your leadouts to the scale and then you and the plane can perform the dreaded pull test.  I know this will not test the lines but at least  you can take comfort in knowing the airframe meets the spec's.  It works well and just to confirm it I had a test done at the last fun fly and passed   .  I'm betting you can improve on my method by a mile.

Marv

[Leester]

Marv: Thats a good idea, A bait shop or Wally World probobly has a scale for fishing that would work if it goes up that high in weight.

[Greg L Bahrman]

I'm not sure I understand.... Are you saying you would rather pull the bellcrank through the wing at home than at the field???   
What advantage is that      You still end up with junk

There are many tried and proven methods that are commonly used to mount a bellcrank and this is the perfect place to find all the necessary info!!!!      Common guys lets see some pictures to show how it's done.

[Leester]

Yep: I wouldn't have all the other damage to mess with, and It would have been done before it was all finished. This is the first one I have ever screwed up on and I'm not going to let it happen again.

[Bill Little]

Hi Greg,

I think I would rather pull the bellcrank out at home.  The reason is, I will save an engine, and the overall repairs will be less.  Only the areas damaged as the bellcrank fails will need to be fixed.  Just my thoughts.

Bill <><

[Greg L Bahrman]

Yes Bill,  I understand what you are saying etc. Still, technology is such that pulling a bellcrank through a wing should never be an option. Doing a pull test should never be to test the bellcrank mount but to test the leadouts and lines etc. The bellcrank should be securely mounted in such a way that it can not come out.  Did not mean to offend anyone just feel strongly that we have the people here that can steer one in the right direction. It's lucky that it was not a safety issue. I hope the info gets passed on. I know Al Rabe has shown his bellcrank mounts and I just hope others will do the same as there are obviously some of us who still need support in this area........Good Luck and no disrespect intended.    I hope Sparky or you or Randy can shed some light on the different types of mounts that are secure for both Profile and Builtup planes. No one should ever have to spend all that time building a new ship and then have it quickly destroyed after one lap. (IMHO)  I feel the pain !!

[Andrew Hathaway]

I usually use one of two methods of mounting the bellcrank.

First is per the plans for the Sig Skyray 35, with some added structural reinforcement.  The two center ribs are notched for the bellcrank support to key into place.  For added durability I usually use 1/4" square stock glued beneath the bellcrank support against the ribs.  Follow up with 1/4" square braces across the bellcrank support from rib to rib.  Basically creating a frame across the bottom of the bellcrank support.

The other method I typically use is to determine where the bellcrank support needs to sit to be centered in the wing, then glue 1/4" square rails on the two center ribs from front to rear.  When the center section is later sheeted I glue the 1/4" to the sheeting also for even more strength. 

I've never had either configuration pull out.   

[Leester]

Your first method is what I use, and epoxy it to the bottom spar then brace from spar to spar. I use the 30 min epoxy not 5 or 15. I think I left out the epoxy to the spar as there was no pcs. of spar missing or attached to BCM.

[Dennis Moritz]

Using a suspended crank (Tom Morris or equivalents) allows bracing top and bottom. I always tie bellcrank into the main spars with plywood and epoxy. Additionally I add a triangular brace that extends the connection to the first bay after the crank. Guess I'm paranoid or have a vivid imagination. Actually it's the pull test I fear even more than the flying. Been to contests where the puller often gives a sudden vicious yank. The number may be right but it seems to me the yanking adds to the stress on the controls. Also it's important to hold the model supporting the wingtip, putting a hand near or between the control wires , supporting the load there. I had an ARF Streak fuse crack when the person doing the test held the fuse instead of supporting the airplane via the wingtip. The controls in the wing were fine. But after a few flights the nose fell off. (During the pull test I heard a creaky crack, but chose to ignore it. Not smart.) My sympathy. Hate when a mistake makes for some splinters. Hope you get it back together. Doesn't sound too bad. Reinforce the broken spars and leading edges with ply and epoxy. Epoxy seems to take loads better than CA. Learned all that the hard way. It takes crashing and re crashing the same plane. The CA joints failled, whereas the epoxy joints held (breaking wood past the joint.)

Over the past year I've taken to trying to think through the cause of my crashes. Was I tired at the end of the flying day? Was the wind too weird for flying my usual tricks? Should I have flown out a bad engine run and been conservative etc. etc. Did I inspect my controls sometime in the last year? etc. etc. The bright and obvious thought occurred to me one day. Less time spent on crash repairs means more time building something new. Also, flying a plane again and again in a given configuration allows me to gain a better feel for its flight.

[Bill Little]

(SNIP)

I hope Sparky or you or Randy can shed some light on the different types of mounts that are secure for both Profile and Builtup planes. No one should ever have to spend all that time building a new ship and then have it quickly destroyed after one lap. (IMHO)  I feel the pain !!

Hi Greg,

No offense, and there are several great ways to mount a bellcrank that should never give any problems.

For several years I have been using suspended bellcranks.
I use "torsion spars" mounted the the rear of the main upper and lower spars.  I have been making these from spruce or bass, usually.  They end up looking like very elongated triangles.  When these are glued, top and bottom, to the respectrive spars and then the center sheeting glued over that, you will have to totally destroy the wing to pull out that bellcrank. 

There are other ways that will work as well.  I have abandoned the plywood floor and the bellcrank screwed to it.  But in defense of that method, I have a Smoothie that is 43 years old built that way and it is still quite functional.

There are soo many ways to do the things we do! 

Thanks
Bill <><

[john e. holliday]

I have used suspended bellcranks with plywood on outer surfaces.  But, on conventional bellcrank mountings I cut another plywood plate to fit over the bolt on the other side of the bellcrank.  Also as much as I hate to, I agree with Andrew about 1/4 sq balsa to reinforce the rib.  Now about pull testing.  The person holding the plane is the one who is supposed to be flying it.  I always put an hand behind and infront of the wing.  Have never broke a fuselage yet.  The reason is I had seen too many people going for first aid after a bellcrank came  out thru the wing tip.  Later,  DOC Holliday

[Dennis Moritz]

Yep. The person flying is suppose to hold the plane. In the case I was pull tester and flyer and I made the mistake of having someone else hold for me, while I pull tested. Guess there could have been an issue with that situation from many standpoints. The fuse was held in front of the wing. It cracked. The Streak Arfs have hollow fuselages and fairly light construction up front. Has caused problems in a number of cases I know of. In another case a honking forty simply flew off an ARF Streak flown for a season, the nose breaking off in flight. Much as mine did. Good idea to rebuild the nose in different ways before assembly. When I rebuilt the front end I used carbon arrow shafts embedded in the hollows above and bellow the wing. These were epoxied in with microballoons. The front end of the shafts were sunk into a drilled out conventional ply, balsa, maple nose. Dan Rutherford has recommended this fix. It works.

[Leester]

Well better late than never, here's some pics

[Leester]

and some ugly ones

[Leester]

more uglyness

[Dennis Moritz]

When I've used single platform mounts in the past, I've run 2 triangle ply plates in line with the plywood floor going to the next bay, tying these into the lower main spar. I've even at times cut out a long rectangle in the supporting rib so that the ply butts up against ply. Everything in the joint is epoxied. No quarter inch rib is then needed. A suspended crank is a better bet though in many respects. Centering the bellcrank minimizes interference with ribs and allows for a stronger installation. Shame about the nice looking profile. Definitely fixable.

[phil c]

If you still want to use a plywood plate bellcrank mount, probably the best way is to use half inch thick center rib.  Slot the BC mount through it and then pin the mount into the rib with a couple pieces of 1/8 in. dowel vertically through the rib.  Quick and easy and strong enough to snap the lines in a pull test if you pull too hard.