Stunt Plane Control Geometry Talk Friday

[Howard Rush]

Need help sleeping?  I'll talk about stunt plane control geometry and how to fix yours tomorrow, Friday, Feb. 2.  I'll go over how to use my Excel spreadsheet to make your airplane more pleasing to fly. 

This will be on Charles Carter's Video Hangout.  He posts a link at about 7:00 PM Pacific time in a message in the Stunthangar At the Bench section.  Click on that link to join the presentation.  The presentation will start at about 7:15 Pacific time.  It will also be available on YouTube, but I don't know how to get at it there.  I think it will also be archived on YouTube. 

Remember: If your bellcrank sees its shadow, it's six more weeks of calculation.

[Charles Carter]

It's going to be good information. See you tonight at around 7:00pm. I plan on posting the link at 6:55pm at "At the Bench". Use link below to find "At the Bench" and look for the link with today's date.


https://stunthanger.com/smf/at-the-bench/

Charles Carter

[Tom Luciano]

Looking forward to it!

[Paul Walker]

Y'all put your engineer hats on for this one!!

[Doug Moon]

DAMN!!  I missed this one. I am about to setup a model for controls and this would have been helpful info.

[Peter in Fairfax, VA]

DAMN!!  I missed this one. I am about to setup a model for controls and this would have been helpful info.

Isn't it tonight?  In that case, still an opportunity.

[Doug Moon]

oh thought it was yesterday. I cant read sometimes

[Doug Moon]

Howard,

I meant to ask you last night why did moving the nobler BC forward make it better?

Thank you for your time last night.

Doug

[Howard Rush]

I meant to ask you last night why did moving the nobler BC forward make it better?

I don’t know. I just messed with the inputs until the response got good. I should go back and try to figure out why, then write it up as if I knew what moving the bellcrank would do.

[Ken Culbertson]

Howard,

I meant to ask you last night why did moving the nobler BC forward make it better?

Thank you for your time last night.

Doug

I want to second that motion.  I have been told over and over that BC position doesn't matter when every gut feeling I have had said it did.   Another follow up I forgot to ask on the 5" bellcrank issue is how does the larger BC cause more leadout clearance issues than a 4".  My drawings say the opposite.  A lot depends on whether you have a straight or angled spar and "C" tube vs "D" tube.  Wing gear mounts get in the way of both if not planned right.

Thoroughly enjoyed the meeting.  I learned some things and confirmed some others.  Do it more often.

Ken

[Howard Rush]

I did some trades awhile back on bellcrank size vs. rotation for the Impact wing I’ve been using. I wanted to minimize the damage done by the swath the leadouts would require. I was going to show the drawing last night, but it’s on an old computer somewhere.  As I recall, the option that took the smallest bite out of the spar was a 5.5” bellcrank that didn’t move through much of an angle (as opposed to a 4” bellcrank that moves +/- 90 degrees). 

[Ken Culbertson]

I did some trades awhile back on bellcrank size vs. rotation for the Impact wing I’ve been using. I wanted to minimize the damage done by the swath the leadouts would require. I was going to show the drawing last night, but it’s on an old computer somewhere.  As I recall, the option that took the smallest bite out of the spar was a 5.5” bellcrank that didn’t move through much of an angle (as opposed to a 4” bellcrank that moves +/- 90 degrees). 

I may be out to lunch, some think I never came back from lunch, but I almost always go with what feels right vs what the math tables tell me until I am proven wrong.  IMHO a 5" bell crank with the same leadout movement as a 4" will provide more force on the flaps at the same deflection which may just move the N-wall a tad.  I am also a believer in restricting bellcrank movement.  So little is gained by the last 25% or so that it is not worth all the lumber you have to remove.  If you reach maximum deflection before the BC arm hits 90 degrees you are putting extreme pressure on hinges, if you reach 90 degrees before full deflection, you are at risk of over-centering.  I know this from bitter experience.  I have a pin just inboard of the bellcrank that hard stops it at 70+- degrees on all of my planes now.  That is about 30 degrees of flap and I have never gone to one line no matter how hard I try to corner.  This conforms to everything I heard last night.

Now,  I am just waiting for someone with a Sh** hot laser or 3d printer to make a rack and pinion bellcrank. 

Ken

[wwwarbird]

I don’t know. I just messed with the inputs until the response got good. I should go back and try to figure out why, then write it up as if I knew what moving the bellcrank would do.

 Love it! 

[Howard Rush]

...I almost always go with what feels right vs what the math tables tell me until I am proven wrong.   

Ken is management material!

IMHO a 5" bell crank with the same leadout movement as a 4" will provide more force on the flaps at the same deflection which may just move the N-wall a tad. 

Yep, pretty much.

I am also a believer in restricting bellcrank movement.   

I did this in combat planes so the airplane wouldn't stall when I did a panic turn.   

So little is gained by the last 25% or so that it is not worth all the lumber you have to remove. 

Or the lumber you have to add in a structurally less advantageous place.

If you reach maximum deflection before the BC arm hits 90 degrees you are putting extreme pressure on hinges, 

How's that?

if you reach 90 degrees before full deflection, you are at risk of over-centering.   

Not me.  I read this when you wrote it before and put in a stop on my latest plane a degree or so before the bellcrank went over center. 

I know this from bitter experience.  I have a pin just inboard of the bellcrank that hard stops it at 70+- degrees on all of my planes now.  That is about 30 degrees of flap and I have never gone to one line no matter how hard I try to corner.

The Netzeband wall is squishy.  Your airplane gets hard to fly accurately before you completely run out of control.

And thereby hangs a tale.  Netzeband is a town in Germany, possibly where Wild Bill's ancestors lived.  Joe Gilbert and I were on the US stunt team for the 2022 world champs in Poland.  We had this plan to go to Frankfurt, rent a car or two, attend the Ronneburg contest a week before the Polish events, then drive to Netzeband, look for the Wall, take a picture, then head on to Poland.  I can imagine my wife's reaction when she figured out the plot.

Now,  I am just waiting for someone with a Sh** hot laser or 3d printer to make a rack and pinion bellcrank.

Too ordinary.  How about a self-centering rack and pinion with a Reuleaux triangle pinion? 

[EricV]

<POOF>
...a Reuleaux triangle pinion?

Next best thing to a circle.
EricV

[Ken Culbertson]

How's that?

Howard, I owe you an explanation:

I believe the comment was on pushing a flap past it's maximum deflection.  Most planes don't have flaps that will drop 90 degrees.  45 is about it.  So where does the force from the bellcrank go if it is still pushing/pulling when the flap reaches "maximum deflection"?  The hinges.  I am including the ones that hold the horn in place.  I have seen, or had, a lot of planes where you could pull the leadouts an extra 1/4" after they hit maximum deflection and you would get a little movement.  Something is flexing inside and eventually you are going to get either slop or that "what the F*** was that" sensation at the handle.  Since that would probably happen in the air under a high pull situation or a slack line recovery, you may not even notice it on the ground moving the controls with light pressure.

Or not - Ken

[Howard Rush]

If you can move a bellcrank far enough to get to 90 degrees flap deflection, the pushrod arm length on the bellcrank or flap control horn is way wrong, and the airplane would be hard to fly.  The program starts with max flap deflection: default is 30 degrees.  Then you pick flap control horn length, then the bellcrank output arm radius that gives lots of bellcrank movement.  If you pull on one line as far as it will go, the flaps won't go much beyond 30 degrees.  You can hit max elevator, though, if you pick too big a ratio of flap output control horn radius to elevator control horn radius.  I did that on the program yesterday, looking at different combinations of radius.  The program gives you the finger and aborts. 

[Howard Rush]

I read your piece again, and I see I missed your point.  The only thing between the bellcrank and flaps that could reach maximum travel is the bellcrank output arm.  It could be pointing straight at the flap control horn or diirectly away from it.  The flap control horn or the flap hinges wouldn't be up against any stop, so there wouldn't be any extraordinary force on the pushrod or hinges. 

[Ken Culbertson]

I read your piece again, and I see I missed your point.  The only thing between the bellcrank and flaps that could reach maximum travel is the bellcrank output arm.  It could be pointing straight at the flap control horn or diirectly away from it.  The flap control horn or the flap hinges wouldn't be up against any stop, so there wouldn't be any extraordinary force on the pushrod or hinges. 

I think you have got it!  It takes a special situation to cause an over center.  First you have to have enough flap movement to allow the pushrod to not meet any resistance, second the bellcrank center needs to be behind the upline (conventional position in back) leadout position so that the bellcrank can rotate past 90 and finally the plane has to be going slow enough that the wall is not a factor.  It can happen, and it has happened to me.  I didn't invent this, I discovered it!  I was doing my initial pre-finish test flights.  I had put in three 2 min flights and had what I needed, BUT it was a nice day and I did have one more battery.
In my rush to get into the air I fumbled with the beloved wrist thong and I didn't recenter the controls before the motor started.  New ESC and not set to slow startup it literally did a backflip and spalt.  Autopsy showed the BC over-centered and the controls locked on full up.
In flight the down line would have pulled it back and you would just have one of those "WTF was that"  moments.  Ironically, that flight was to be the "where will it stall" test where I take it to one line both directions.   

Moral of story - one more flight is not a good idea.

Ken

[Howard Rush]

I read what you wrote then and put a bellcrank stop a degree or so before it went overcenter.  Thanks.

[Keith Renecle]

Hi Howard,

This is really interesting. I'd love to see the talk. Is it up on YouTube yet? Please post the link if it is. Thanks.

Keith R

[John Skukalek]

Keith you can find it on Stunthangar YouTube.

[Dave_Trible]

I think you have got it!  It takes a special situation to cause an over center.  First you have to have enough flap movement to allow the pushrod to not meet any resistance, second the bellcrank center needs to be behind the upline (conventional position in back) leadout position so that the bellcrank can rotate past 90 and finally the plane has to be going slow enough that the wall is not a factor.  It can happen, and it has happened to me.  I didn't invent this, I discovered it!  I was doing my initial pre-finish test flights.  I had put in three 2 min flights and had what I needed, BUT it was a nice day and I did have one more battery.
In my rush to get into the air I fumbled with the beloved wrist thong and I didn't recenter the controls before the motor started.  New ESC and not set to slow startup it literally did a backflip and spalt.  Autopsy showed the BC over-centered and the controls locked on full up.
In flight the down line would have pulled it back and you would just have one of those "WTF was that"  moments.  Ironically, that flight was to be the "where will it stall" test where I take it to one line both directions.   

Moral of story - one more flight is not a good idea.

Ken

Ken I'm just wondering if your crank went over center 'as a result' of the crash rather than beforehand.  I know in my planes the crank,  once connected to rest of the control system can't move more than 40-45 degrees each way simply because the the flaps and elevator bind up on themselves to prevent any further travel.  It would seem the surfaces would have to go close to near 90 degrees each way to get close to a cam over of the crank.  Than seems impossible under normal conditions......but i'm not seeing your control throws/high of your horns, etc.  I've been happy for decades with a saying about my set up:  3/4" to 3/4"- 1" to 1" (or sometimes 1" to 15/16" to move the elevator slightly faster than the flaps.)  Never had any disagreeable hinge moment issues even with 75 ounce,  .76 powered large flapped airplanes.  Maybe that is actually helping to overcome hinge moment problems vs. a really light airplane........(pulling harder on the lines).  In any event I have never found any need to go taller on the horns.  That may create a need to swing the bell crank so far.  My handle spacing will vary to airplane some but is less than 4" on a 4" bell crank.

Dave

[Ken Culbertson]

Ken I'm just wondering if your crank went over center 'as a result' of the crash rather than beforehand. 

It was before.  What you are saying is true but this setup allowed the bellcrank to rotate the full 90 degrees without meeting flap or elevator resistance.  If it was not over centered it would not have crashed.  As free as the controls were they would have self centered as soon as it started rolling.  I probably should have noticed it on the bench but how many times to you pull just one leadout tight and leave it that way.  Live and learn - Ken   

[Howard Rush]

In any event I have never found any need to go taller on the horns.  That may create a need to swing the bell crank so far.  My handle spacing will vary to airplane some but is less than 4" on a 4" bell crank.

I'd recommend a video for you to watch if I knew where it is.

[Dennis Nunes]

The YouTube video of Howard's presentation during the Stunt Hangar Video Hangout can be found here:



The presentation starts around the 23:30 mark.


Enjoy,
Dennis

[Howard Rush]

Thanks, Dennis.  Program gets underway at 24:30. 

[Keith Renecle]

Thanks very much Dennis!