Setting neutral on flaps and elevators

[Jim Svitko]

Since returning to the hobby years ago I have only built planes that had "flat" flaps and elevators.  That is, they are rectangular in cross section, ignoring the rounded leading and trailing edge.  Setting up neutral was pretty easy.  I would block up the wing so that the tip chord line was horizontal and I could use small bubble levels on the flaps and elevators to get them set at neutral.  I have also used the small digital inclinometer (clinometer?) that is part of the Hangar 9 wing incidence rig.

I have looked at plans of some possible future projects and the flaps and elevators on some are tapered.  That is, they are a trapezoid in cross section.

If the flaps and elevators are tapered, what technique is used to find neutral?  I can think of a few methods but do not have complete confidence in them.  For those of you who have built the tapered control surfaces, how did you determine neutral?

[Brett Buck]

Since returning to the hobby years ago I have only built planes that had "flat" flaps and elevators.  That is, they are rectangular in cross section, ignoring the rounded leading and trailing edge.  Setting up neutral was pretty easy.  I would block up the wing so that the tip chord line was horizontal and I could use small bubble levels on the flaps and elevators to get them set at neutral.  I have also used the small digital inclinometer (clinometer?) that is part of the Hangar 9 wing incidence rig.

I have looked at plans of some possible future projects and the flaps and elevators on some are tapered.  That is, they are a trapezoid in cross section.

If the flaps and elevators are tapered, what technique is used to find neutral?  I can think of a few methods but do not have complete confidence in them.  For those of you who have built the tapered control surfaces, how did you determine neutral?

    I am not sure what the issue is, you move it to center of travel and adjust them to be the same, or whichever way you think they should go. Note that it is not only 0-0, I think the starting point for an Impact is the flap at neutral and the elevator lower surface in line with the flat stab, or about 3/16" down elevator.

   You have to adjust it based on results anyway. It's very rarely correct to have it exactly 0-0. Usually, it winds up with some down elevator with conventional layout with normal prop rotation.

     Brett

[Perry Rose]

On all my builds I extend the wing chord line past the t.e. and make a mark close to the t.e. of the flap. That way I know where zero is and can set the flaps to the mark and the elevators by sight.

[Jim Svitko]

On all my builds I extend the wing chord line past the t.e. and make a mark close to the t.e. of the flap. That way I know where zero is and can set the flaps to the mark and the elevators by sight.

I thought of something like this but was not sure if I could trust eyeballing or extending that chord line accurately enough.  I figured a 0-0 starting point was the way to go and make adjustments from there.  I was looking for a way to start at 0-0 since I could not use a level or inclinometer on a tapered surface.

Maybe I only need to get "very close" since, as mentioned, it is rarely 0-0 and adjustments are likely anyway.

[Dave_Trible]

Jim I use my Robart incidence meter for every step of the process.  The meter never lies.  I wish I could find a different photo with the track bar on the meter straddling the leading edge and flap trailing edge but I think the idea is obvious.  Step one is to temporarily attach an alignment block set at zero to the wing centerline.  Then the motor mounts,  stabilizer and then the flaps zero to the block and elevator zero to the block ( or whatever your preference- I still like 0-0 but my controls aren't super tight).  Obviously everything is on before covering and finishing.  If you mount your flaps and elevators after finishing....

[Avaiojet]

Remember this?

Dead center.

Charles

[Doug Moon]

Hello Jim,

Trust your eye! The human eye is actually a very accurate instrument.  In what we do you only have to be really close to make it work and then final trim it out to get good results.

There are several ways to get to get this alignment correct using various tools as well. I have a protractor that has a notch at 0 degrees.  When the plane is on the bench I can slide the protractor up to the TE of the wing and line up the flap horn to 0 degrees and then at the same time I set up the overall throw as well.  

Check this thread below.

http://stunthanger.com/smf/index.php/topic,29657.msg303041.html#msg303041

Another option, is to NOT taper the flaps.  I quit doing that years ago.  I like the strength of the flat flaps, the ease of getting them dead level and them staying that way while I am building the rest of the plane, and the ease of finish.

[Jim Svitko]

Doug, if you had my eyes, you would not trust them!  I need trifocals.  Nevertheless, I agree--at times the eye is right but I prefer a measurement to verify.

I think leaving the flaps flat is probably the way to go.  I doubt if there is much difference in performance with tapered flaps, or elevators for that matter.  However, I think the taper looks a bit better.

I can live with the tapered elevators as I only need to get close (as you mention) and then adjust a clevis as necessary.

Thanks for all the suggestions.  I figured there were several ways to do this I only need to choose what I think is the easiest yet most accurate method.

[peabody]

Noel Drindak did a great piece in Stunt News about setting the horns to zero.....he recommended sliding a long tube that was a precise fit over the horn(s). The longer the tube, the more the alignment differences appear.
Have fun!

[Doug Moon]

Noel Drindak did a great piece in Stunt News about setting the horns to zero.....he recommended sliding a long tube that was a precise fit over the horn(s). The longer the tube, the more the alignment differences appear.
Have fun!

You mean like this...??

[Pat Chewning]

When I cut the ribs with the laser cutter, I cut a template (anti-rib) with an extended cut for the chord line.  This helps align the flaps into the neutral position.   (NOTE:  I do this for asymmetrical undercambered glider wings.)  It would be even easier for convex symmetrical airfoils as used in CL models.

Here is a very crude drawing (if I can figure out how to attach)   [Template in red, wing with flap in blue]

It works best if you have both a top and a bottom template, clamp in place and "voila" -- aligned control surface to main wing surface.

You get this automatically if you cut the wings from foam and use the shucks to align the surfaces.... the shucks acting just like the templates.

[Steve Helmick]

I'd vote for using (at least) 12" pieces of aluminum tubing on both flap and elevator horns. Why? I've seen Howard do this at the field when "flap tweeking". No, not "twerking".

Many Expert class fliers make their flaps and elevators (also rudder, if any) removable...remove the hinge gap sealing tape, pull the single hinge pin, off she comes. IMO, this is an excellent way to go if you're at all serious about making your airplane work as well as possible. If you want to try smaller flaps? Easy. Try bigger elevators? Easy. Try tapered flaps and un-tapered flaps? Use flaps or elevators from a crashed plane on your latest one? Easy!  Second most importantly, when you do a "flap tweek", you aren't stressing the flap itself, and you have the ability to add those tubes to the flap horn legs to apply the eyeball to. Most important, if you are so unlucky as to break the flap horn while tweeking, it'd be way easier to put in a new one.  Steve

[Bill Johnson]

Not that I have much experience at this but I do it the same I do it on full size aircraft: Clamp 2 pieces of wood or aluminum angle, one on top, one on bottom, parallel to the chord. I used two tongue depressors, aligned over the rib just outboard of the control horn, and clamped the pieces at/on the left flap. That holds that flap perfectly in line with the rib or at 0 angle. That's how you zero an inclinometer to check control surface travel. In this case, I downloaded an inclinometer app to my android phone for free, zero'd it on the left flap/rib then tweaked the outboard flap to match that angle.

Without an inclinometer, you can easily clamp up the outboard flap and you'll see it deflect up or down when you clamp it if it's not zero'd.

Does that make sense?

Then, of course, you may have to tweak it as others said based on test flight.

[Trostle]

(Clip)

 what technique is used to find neutral?

Well, I think the question is how to set "neutral on flaps and elevators".  (Adjusting the flaps to correct one or the other tip being high is another question for which there are other solutions).

My first recommendation is to read Brett Buck's response in this thread.  You should be able to get close to zero flap position with the bellcrank in the neutral position along with the elevator as Brett explains.  That will get you in the ballpark for the model to fly and get through the pattern.  Then, as Brett further explains, your model trim will probably need to have the elevator "neutral" adjusted up or down depending on a number of factors based on how your model performs.  The simple method to adjust the elevator trim with respect to the flap is simply to use an adjustable pushrod.  There are any number of methods to accomplish this.  This will give you minute adjustment capability and will be able to accurately adjust and repeat the adjustments to your heart's desire.  I would not ever consider building any stunt ship, whether my design or a design from another, without the ability to adjust the elevator pushrod length.  First thing though is to read and understand what Brett explained.

Keith