Force on flap & elevator pushrods.

[Tim Wescott]

Anyone got an idea of the amount of force it takes on a flap horn to make it deflect?  Or how much torque there is at the hinge line?

Based on control geometry and the fact that I can maintain control, I'm guessing that in normal flight, my Atlantis is presenting no more than 16 pounds at the bellcrank's horn -- probably less, because I can maintain reasonable control when it's overhead.

I'm thinking about a design improvement on Paul Walker-style screw-adjustable flaps, but it involves selecting just the right belville washer.  I'm wondering if anyone knows better than my vague musings.

[Howard Rush]

As Gary Letsinger said, maybe quoting somebody else, "Hinge moment calculations are easy to do, but hard to believe. "  I'd like to do some experimental measurements. 

[Tim Wescott]

As Gary Letsinger said, maybe quoting somebody else, "Hinge moment calculations are easy to do, but hard to believe. "  I'd like to do some experimental measurements.

Heh.  I can believe that.  Although I do think you get a valid maximum by (over) estimating the force on the lines and calculating the maximum the surface could be resisting that from the control system geometry.

[Howard Rush]

One way to measure hinge moment is to measure differential line tension at the handle and use my control geometry program to figure mechanical advantage.  I have some of those little strain-gauge luggage scales that I intend to instrument for differential line tension, but I'd welcome a better idea.

You could overestimate by using the amount it takes to break a line and subtract .5mV²/R from it.

[Tim Wescott]

You could overestimate by using the amount it takes to break a line and subtract .5mV²/R from it.

I think just assuming 2 - 4g would be safe enough.  Line-breaking tension would be upward of 10g, and I don't think my plane has ever pulled 40 pounds, even yesterday with the tank problems plus winding up in the wind.

[Igor Burger]

Depends what number you want - if maximum for safety, you must figure from max line tension and all on one line, then like Howard wrote.

If you want in flight load during normal flight, no wind etc, then my teoretical numbers from design for my Max Bee for 3.5m radius is 35N for flap pushrod and 10N for elevator pushrod. My similar model with usual bellcrank had 65N for flaps and 8N for elevator. However speed change will change all with power of 2, so small wind can change it all.

[phil c]

If you go back and look at Bill Netzeband's articles in American Aircraft Modeler he had several graphs for calculating pushrod forces  using some NACA data on the effect of stab/elevator ratio, and the degree of deflection and the pushrod length.   He also had some info on calculating whether or not the pushrod would buckle.

I always have found the results to be usefully accurate.

[Howard Rush]

I saw that data and tracked down where he got it. As I recall, the source said that it appled only to that particular case, and not to generalize it. TE shape, for example, can make a big difference in hinge moment. Wild Bill confidently generalized it.

For Tim’s purpose, he could use Igor’s* or Bill’s numbers and multiply by five or so.

*To use Igor’s, calculate flap hinge moment from his control geometry, then multiply by the square of the ratio of your airspeed to Igor’s, then by the flap area ratio, then by the flap chord ratio.

[Tim Wescott]

Depends what number you want - if maximum for safety, you must figure from max line tension and all on one line, then like Howard wrote.

If you want in flight load during normal flight, no wind etc, then my teoretical numbers from design for my Max Bee for 3.5m radius is 35N for flap pushrod and 10N for elevator pushrod. My similar model with usual bellcrank had 65N for flaps and 8N for elevator. However speed change will change all with power of 2, so small wind can change it all.

Normal load, with some safety factor.

I had an epiphanette* the other day about Paul Walker's flap adjustment dingus.  Most people (as far as I know) implement the adjustable side with a jam nut against the frame, which demands some pretty careful adjustment and some tolerance for slop.  If, on the other hand, you have a belville washer (or other spring washer) in there, then your adjustment screw will always be hard against your elevator rod -- at least until the force gets high enough to compress the spring.

Hence, I wanted to know what force to expect, so I could select the appropriate washer.

* That's like an epiphany, only you're seeing the true nature of something small and unimportant.

[Igor Burger]

Hence, I wanted to know what force to expect, so I could select the appropriate washer.

I expect that one day comes the wind and in some stressy moment close to concrete, I will pull my handle so strong that it will hang with full force on one line :- )))

[Tim Wescott]

I expect that one day comes the wind and in some stressy moment close to concrete, I will pull my handle so strong that it will hang with full force on one line :- )))

I was also planning on putting the adjustable side on the outboard flap, so that in the event that the spring does flex, it'll tend to roll the plane in the direction of more line tension, rather than providing one of those "hey surprise!" moments.

[Igor Burger]

Be carefull, that side with stronger flap can STALL earlier 

[Scientifiction .]

I expect that one day comes the wind and in some stressy moment close to concrete, I will pull my handle so strong that it will hang with full force on one line :- )))

I thought we all did that . 

Flex in the direction of ' more tension ' would be opposite on insides and outsides ??

* That's like an epiphany, only you're seeing the true nature of something small and unimportant.

Could be of slightly more than negligable importance , if it comes loose .

[Paul Smith]

I built a plane with FOUR pullrods.

So both the flap and elevator as basically pulling and somewhat pushing at all times.

Works great, no need for guides or bulky pushrods.

It has an X-crank with outputs on both sides and four 2-56 ball clevisis.