Flap Differential ?

[Robin_Holden]

Good morning fellas from France.

May I ask : What is the 'ideal' flap differentioal please ?

How many degrees should the flaps have? How many degrees should the elevators have ?

Is there an ideal combination to aim for ?

Much obliged ,

Robin.[ ex-pat Brit' in the Charente full of ex-pat Brits' ] .

[john e. holliday]

It will take a book if it could hold all the info on the subject.  It all depends on plane design.  Wing area and layout.  Stab/elevator layout.   Fuselage and power train.  I think the original Green Box Nobler said 45 degrees both flaps and elevator both ways.  Can't remember what the Smoothie or Thunderbird by Veco said if they did.  You just used their controls and it came out about the same.  Equal flap and elevator movement.   

[Larry Cunningham]

There is no correct answer. It is worth noting, however, that the linearity of your control can be compromised by different radii on the control horn drives.. It may not matter much; lots of people use elevator horns with sliders for adjusting the drive radius.

You can configure neutral offsets on your control horns for a particular setup to minimize distortion, however unless your control horns also have adjustable neutral offset angles, changing the elevator horn drive radius will distort things.

I'm sure some people have some spreadsheets for farting around with such parameters; the flap and control horn linkage operates in a plane (2D), so it isn't terribly difficult to model, just average trig. When you add the bellcrank operating in an orthogonal plane, the problem is 3D and that complicates things for simple calculations.

In the end, I tend to think of the adjustable drive radius slider on the elevator as a practical and reasonable trimming feature.

L.

"Seek the lofty by reading, hearing and seeing great work at some moment every day." -Thornton Wilder

[PJ Rowland]

I have an adjustable slide mechinism - coupled with several preset holes - so I can move up and down laterially in 2.5 mm incremients. I can then use the slider to get more a more finite adjustment within each of the 2.5mm hole spacings. So I could increase the throw by .02 m if I choose to. or as much as 5mm


Its about finding the corner that best suits your model, and flying skill / style.  Dial that in.

As others have said there is no " magic formular " thats why we all build in adjustment.

[Larry Cunningham]

I'm sorry, I hope no one feels that they "got Cunningham started" again.

I'd just like to point out that the question of flap/elevator throws and ratios is a prime example of a practical trimming issue. And our conventional mechanical bellcrank-rod-horn-rod-horn control system linkage, while mechanically simple, has geometric difficulties when it comes to ease of adjustment and experimentation.

Again speaking of elegant solutions, I bring up the downright beauty of an electronic servo driven system, for pure adjustability during trimming. This is not some exotic notion of real time navigation and autonomous control by a microprocessor! It's a practical approach to flight trim.

Face it, no two <at least non-ready to fly> CL stunt models are precisely identical and certainly each human pilot is unique. Even casual flyers have their preferences, while seriously competitive one are likely to be obsessed with trimming. And when a variable environment is factored in the value of programmability is even clearer.

An electronic controller to "read" a bellcrank transducer, look up flap and elevator deflection values from a table, and issue them to drive servos is trivial. And it is easy to implement the look up table in non-volatile "flash" memory with a simple interface so a laptop can rewrite it as desired. The value of a programmable electronic control system is so obvious to me.

I'm sure some very serious tip-top competitors agree that continuous fine trimming is necessary but see electronic control systems as overkill. Practical questions about weight, batteries, installation, reliability, response rate and general "feel" arise. Finally are notions that electronic control systems on CL stunters simply violate the spirit of our event. These are all worthy concerns.

L.

"When I read about the evils of drinking, I  gave up reading." -Paul Horning

[Dennis Moritz]

Perhaps electronic tuning would work. Why wouldn't it. Computers micro and macro improve many operations. They're pervasive, in almost everything. CL, IMHO, is defined, however, by the straightforward operation of our familiar mechanisms. Like rowing a skull. Or track and field. Perhaps electronic programing could devise the optimum tilt of an oar stroke, or instantly vary the flex of a pole in a pole vault, but that is not the same athletic concept.