Bellcrank stops

[Joe Motta]

I'm doing my first build with a Twister and was told to install bellcrank stops at 45 degrees. This would put max flap deflection at approx 30 degrees. Is this correct?  I'm at covering stage of wing and not feeling 100% sure about the bellcrank . Just double checking

[rich gorrill]

Joe I have never put in bellcrank stops,actually never heard of them. If you put the pushrod in the middle hole of the bellcrank, that should be fine.

Rich

[Dave_Trible]

NEVER use bellcrank stops.  Not nessessary to begin with- most airplanes in the end never over-travel anyway due to hinge line binds or whatever.  You never want to bottom out on feel at the handle but most importantly if you do,  you are flying and loading just one line and the other goes slack...

Dave

[Teodorico Terry]

Dave,

Along the concern of only loading one line when the travel stop has been reached is there a rule of thumb which relates line spacing at the handle with bellcrank size?  The reasoning is similar, with 4" line spacing at the handle you could reach the travel limit on a smaller bellcrank or allowable control surface travel.  I think that the end result is the same where a single line carries all of the load.

Teo

[Dave_Trible]

Dave,

Along the concern of only loading one line when the travel stop has been reached is there a rule of thumb which relates line spacing at the handle with bellcrank size?  The reasoning is similar, with 4" line spacing at the handle you could reach the travel limit on a smaller bellcrank or allowable control surface travel.  I think that the end result is the same where a single line carries all of the load.

Teo

We commonly use handles with adjustable line spacing to adjust control input rates to the airplane control system.  I think in normal flying you wouldn’t encounter a situation where you would ever move the handle and bell rank far enough to cause any overrun issues or bellcrank cam-overs.  If so you’ve got some much larger issues.  Most handle inputs are under 15-20 degrees.  Full panic mode maybe 30 degrees.

[J Motta]

Since my measurement was done with the pushrod at the highest hole of 4 on the horn, I guess I could move the pushrod down to gain more flap. Is that correct?
Joe

[Dave_Trible]

Yes that’s correct.  Always allow the elevators equal to a little more travel than the flaps.  More flap travel is not beneficial.

Dave

[CircuitFlyer]

Bellcrank stops are not about maximum limits to control surface travel. They are used when some jinglenuts walks through the pits and trips over your lines. I prefer to have a known solid end stop on the rotation of the bellcrank before over stressing other components in the control system.

Paul

[J Motta]

Well I feel good now with the way it’s set up and not having to cut my wing up. Actually relieved lol.
Thank you all for your advice both pro and con. Much appreciated.
   Tight lines!
          Joe

[EricV]

Time and poor memory will protect the flyer here, (If I ever knew who's plane this was, and I don't think I really did, I seem to remember he was "visiting" our NAT's) but as far as any kind of control stops go, this has to be the most egregious thing of that sort that I've ever witnessed, and with solid leadouts and the dreaded slider clips no less!

[Howard Rush]

I'm doing my first build with a Twister and was told to install bellcrank stops at 45 degrees. This would put max flap deflection at approx 30 degrees. Is this correct?  I'm at covering stage of wing and not feeling 100% sure about the bellcrank . Just double checking

I'd let the bellcrank move as much as possible for whatever max flap deflection you need.  No stops. That way you'll have the most leverage over control surface hinge moment.  This, of course, ignores the jinglenuts issue, which I should consider because I fly by myself, so everybody on my field falls into that category. 

Combat planes are a different issue.  I set stops to give the fastest loops during panic maneuvers.  Chuck Rudner, a name-brand combat flyer, never used stops.  I found that peculiar, but eventually decided that it's because he's good. 

[Steve Helmick]

IMO, IF you limit your BC to where you protect your flap horn pivot at the TE....and pushrods...if a line breaks during pull test, it's a good thing. I brought this up on SSW forum and a bunch of real Experts said not to do it. I recall Mr. Fancher having some damage to his model at a European WC when a line broke. IIRC, it was during the pull test. As I recall, he also said not to. 

That was the reason I asked, so many years ago, and IF the BC stop isn't too harsh...maybe rubber or springy...at about 45 deg. (at the BC) and the controls are setup so that they can travel that far without damage, it should be a good thing. If it could result in a broken bellcrank...well, then that's not good. I don't think anybody has put any engineering into designing a really good soft stop for BC's, in any case. It should hit the end of the BC, right near the leadout attach point. I was thinking that a slice off of a rubber eraser would be fine, maybe 1/4" thick. Ah, but what kind of glue to use? That should be good for a few pages of posts! 

It's not impossible to track your maximum deflections in the air, and it looks to me like <20 degrees, so if somebody says they want "extra deflection for emergency fubars", then IMO they are dreaming. If you commonly break a line in the air then having the controls deflect LOTS will slow the plane and make the impact (no pun intended) and damage significantly less. Otherwise, you really don't need 45 degrees of up & down.  Steve

[Ken Culbertson]

I was of the "No Stops" school till I had a BC over center and lock on takeoff.  I know that can't happen so the corrective action that I *did not* take was to *not* put stops in at maximum flap deflection.    But it did happen so I now use stops.

ken

[J Motta]

Ken

Did you set your stops at a certain degree of travel?
Joe

[Mike Griffin]

I have never heard of bellcrank stops and it doesnt sound like a good idea to me.

Mike

[Dan McEntee]

   I think if you search the forums you will find stories of stops or limiter causing problems and crashes. I have built a lit of control lie models in my time, but never had a situation where I was concerned about a bell crank "going over center." If it does that, I think something is wrong somewhere. If the bell crank goes over center, your elevator is probably at 90 degrees to the stab.  And if you give a rapid movement to the handle, and it hits a stop, it will change the direction of flight, and the result will be at least momentary loss of all tension. If you must have a stop, a single post in line with the bell crank center about an inch or so off the pivot towards the  inboard wing tip will keep it from going all the way around. But like I sad, if your controls can go all the way around like that with everything hooked up, sumptin ain't right!
   Type at you later,
    Dan McEntee

[Ken Culbertson]

   I think if you search the forums you will find stories of stops or limiter causing problems and crashes. I have built a lit of control lie models in my time, but never had a situation where I was concerned about a bell crank "going over center." If it does that, I think something is wrong somewhere. If the bell crank goes over center, your elevator is probably at 90 degrees to the stab.  And if you give a rapid movement to the handle, and it hits a stop, it will change the direction of flight, and the result will be at least momentary loss of all tension. If you must have a stop, a single post in line with the bell crank center about an inch or so off the pivot towards the  inboard wing tip will keep it from going all the way around. But like I sad, if your controls can go all the way around like that with everything hooked up, sumptin ain't right!
   Type at you later,
    Dan McEntee

It is not only possible, it is quite easy if your aft up line  leadout guide is ahead of the bellcrank pivot any significant amount or attachment points for the control lines are behind the pivot.  In most cases the limits of the flap or elevator horn stops the rotation well before over centering and it cannot happen if the bellcrank hole is further from the pivot than the one on the flap like most, but, on my last ship I connected the bellcrank to the flap horn using the lower hole and didn't discover that the controls were way too fast till the first trim flight.  Being electric that trim flight was without finish so I took off the top and reversed them.  Now I had the speed I wanted.  What I couldn't see inside was that the bellcrank no longer maxed out the horn movement and being floating there was nothing to stop it.  When I moved the leadouts forward to test the trim it over-centered while flexing the controls right before take off and I didn't have time to react when it locked on full up.  It was a short flight.  Controls were over-centered.  I was able to duplicate it later in the shop.   From that point forward I always put a single post just in front of the pivot to prevent it from happening again.

Personally I don't see the downside of a stop as long as it is well past any amount of control you would normally give.  IMHO it is better to put the full force of a one line episode on the stop rather than the flap horn but that is just me.   

Ken

[Brett Buck]

I'm doing my first build with a Twister and was told to install bellcrank stops at 45 degrees. This would put max flap deflection at approx 30 degrees. Is this correct?  I'm at covering stage of wing and not feeling 100% sure about the bellcrank . Just double checking

   No stops!  You don't want a hard limit on the control travel, you want to make sure that can get adequate control motion with full handle motion, which is more about the rate of motion with respect to your handle than it is about the total motion end to end.

     Brett

[Chuck_Smith]

If you can move a 4" bellcrank so far that you need stops your wrist is much more flexible than mine.

I like to have enough travel to address an "Uh oh" situation. Yeah, you'll waffle the corner and ruin the maneuver, but low points on one figure is better than no points on it and all the subsequent tricks. At least to me.


Howard, interesting comment about the combat ships. I always thought of stops on a combat ship as more of a speed control. I never used them because when I got into an extended tight turning situation I could overcontrol and slow the plane down for really tight reversals. Back in the SuperTigre day we had enough power to recover speed so quickly. Miss those days. Modern combat is a bit more refined but misses that savage and aggressive attitude of the 70's.  I love the smell of nitro and laminating film in the morning.

[Ken Culbertson]

   No stops!  You don't want a hard limit on the control travel, you want to make sure that can get adequate control motion with full handle motion, which is more about the rate of motion with respect to your handle than it is about the total motion end to end.

I think we are talking two things here.  Stops that limit control are not a good idea.  Stops that prevent damage are.  Nobody gets 90 degrees of flap or elevator travel.  There is a point where the hinges and horn mounts stop movement.  If you allow the bellcrank to exceed those limits you can damage them.  That is the point where I think a stop is useful.

Most over center situations are never detected because once you put pressure on the other line it reverses and you may not even know that you broke the flap horn loose.  It is those times where you lose all line tension preventing the controls from self centering that you are screwed.

Ken

[Ken Culbertson]

If you can move a 4" bellcrank so far that you need stops your wrist is much more flexible than mine.

It must be, because I can easily get 2" if I use my wrist.  Most of my travel comes from fingers and a pretty wide line spacing. Except for corners, and not much of that, I don't think my wrist moves hardly at all.

Could be wrong though.  When you are flying you don't think about it and when you are thinking about it you are not flying.  Next time I video I think I will do a close up of just me.  Oh, the vanity....

You were lucky to have Nitro and Laminating film in the 70's.  I had gasoline and rubber bands.

Ken