Rocky Mountain Flying Machine Photo Missing


Setting Up Your Airplane

by

Richard Lindberg



 

"Setting Up" usually refers to a new, never flown airplane, but it is equally applicable to any airplane (especially one that doesn’t fly as well as you would like!). There are three aspects of this process to consider: How straight is your airplane (really, two parts–warps and twists, and alignment); how balanced is your radio installation; and where’s the CG? First, let’s talk about your new airplane.

Warps and Twists

As you build (or otherwise assemble) your new airplane, pay particular attention to warps and twists. These are BAD, and you want to avoid them. If you find one or more, try to eliminate them by judicious use of a heat gun, twisting the offending part opposite the warp as you heat it. If it’s the wing itself that’s warped, heat may work (to a limited extent), and sometimes ‘painting’ the raw balsa structure with a 50/50 mixture of ammonia and water, then twisting opposite and heating it to dry it may work. If it’s an ARF and the structure is already covered and heat doesn’t help, you may have to work with your local hobby shop (LHS) and get it replaced.

If it’s the fuselage that’s warped, similar methods may work (it’s worth a try!), but it might be better to cut it, patch it, and re-glue it. If it’s an ARF, see above. If it’s a composite (e.g., fiberglass, moulded, etc.) the best thing to do is work with your LHS and get it replaced.

Alignment

The kit instruction manual that you’re following for your new airplane should tell you how to mount the wing, vertical fin and stab to the fuselage to assure that your bird is straight. If you’re working with an ARF, the manual also should cover this. The following diagram shows the important measurements to be checked. Note that things are symmetrical, and all like-labeled dimensions should be the same. You may have to sand the wing and/or stab saddles on your ARF in order to make things right, and you may have to elongate the holes in the wing for the attachment bolts as well. This is a very important step, so be sure to get it right!

Oh, yes, the vertical fin. This isn’t really shown in the diagram, but its alignment is also important. Be sure that the fin is perpendicular to the wing and stab planforms, and is parallel with the centerline of the fuselage. For a single fin/rudder, it should be centered on that line.

Finally, remember that "an airplane built straight, flies great!"

Radio Installation

OK—you’ve got a straight airplane. Now what? Let’s turn our attention to the radio installation, in particular the servo orientation. If you have a choice of where to put your servos, then there’s a preferred installation orientation, as shown below:

If you’re assembling an ARF, you may not have a choice. If the orientation is ‘bad’, then try to stiffen the installation as best you can.

Now set up your linkages to the control surfaces. The rod, or wire, connecting the servo arm to the control horn should be straight and stiff—you don’t want any flexing. If necessary, brace the rod with guide holes through formers, which you may have to add. The ‘right’ setup for this is shown below:

The dimensions "A" and "B" are determined by what the plans call for with respect to the total movement of the control surface (up/down, left/right, etc.) and how much travel your servo has. Here’s how to do it:

First, set your radio for ‘normal’ throws for all channels. A computer radio will show 100% under "Travel Adjust", or "ATV" or whatever your radio manufacturer calls it. An analog radio might have a set of potentiometers or knobs to adjust total throw in each direction. Don’t forget about those dual rate switches, if you have ‘em—you want to make these adjustments using the ‘high rate’ position.

At the control surface, mount the control horn so that distance "A" is as large as possible—about 1" or so for large aircraft, ¾" or so for smaller airplanes; commensurately larger for giant scale or smaller for park flyers. Now deflect the control surface to the maximum the plans call for, and measure the (horizontal) distance that the hole in the control horn moves. Call this dimension "C".

Now turn on your radio, with the servo plugged into its proper channel, and center the arm, as shown. Run the servo to its extremes and measure the distance some particular hole in the servo arm moves. Choose the hole that is closest to dimension "C" that you measured at the control surface. The distance from the middle of the servo attaching screw to that hole is dimension "B".

Clamp or otherwise fix the control surface in its neutral position, and measure the distance between the hole on the servo arm and the hole in the control horn with the servo in its neutral position. Manufacture a straight rod and attach it to those points. Mount the servo, making sure that the servo arm is perpendicular to the control rod, as shown, and that the control horn is also perpendicular to the rod.

Turn on your radio again and run that channel to its extremes and measure the throws at the edge of the control surface. Now reference the plans for your model. Are the throws too much, too little, or just right? If too much, increase dimension "A" (or decrease "B"). If too little, reduce "A". (By the way, the throws in each direction should be identical. If not, check that the setup is as shown above, with 90º angles at both ends. You may have to adjust the neutral on your servo, or bend the arm at the control horn, but try hard to equalize those throws!)

Do the same for the rest of the control surfaces. We’ll talk about throttle in a bit.

What you’re doing here is maximizing the utilization of your servo’s power. By using the entire rotation of your servo arm to cause the control surface to move its maximum distance, you’ve used what is called the ‘mechanical advantage’ of the servo to its fullest extent—all the power your servo has is being used. If you merely install your servo and rod without regard to the total desired control throws, and the distance "B" at the servo is longer than at the control arm, you will have to reduce the maximum servo arm rotation allowed in order to keep the control surface movement at the correct distance. Result: poor utilization of your servo’s power—you’re only using part of it, and resolution suffers as well. If distance "B" at the servo is shorter than at the control arm, the opposite ensues, and you’ll most likely overdrive the servo, perhaps causing binding or other equally devastating problems, or you won’t get the full throws desired at the control surface.

"Why am I doing this?" you may ask. "What have I achieved, and why is it ‘good’?" Well, what you’ve done is establish a ‘base setting’ for your airplane, in accordance with what the designer/manufacturer found to be the proper settings to achieve the design performance for that airplane. You also set it up for ‘balanced’ throws, so that the airplane would ‘feel’ the same in all orientations of flight.* The base setting allows you to return to a known performance level if you really mess up the subsequent trimming, and the balanced throws make the aircraft fly more smoothly and consistently. And, of course, you’ve set it up to take full advantage of all that your servos can provide (!).

* Commensurate with the overall design, of course. A high wing trainer will perform differently inverted, for instance, than will a high performance aerobatic airplane, but with a balanced setup, you’ll be better able to compensate for different performance, as you’ll see.

Throttle Linkage

A few words about how to set up your throttle linkage is next. First, manually close the throttle barrel on your engine so that no opening is visible. The arm should be leaning back from the vertical at about 50º (most two-strokes) or forward about 50º (most four-strokes). When the throttle barrel is wide open, the arm should be about 45º in the other direction. Now, measure the distance between the middle of the attach point on the throttle barrel and a hole on the throttle arm, and choose a servo arm that will give you (nearly) the same distance at the servo. This is probably going to be a pretty short servo arm, as most throttle arms are short!

Plug in the throttle servo to the receiver and turn on your radio. Pull the throttle stick all the way down, and put the throttle trim in its lowest position. Firmly close the throttle barrel all the way, measure the distance between the hole on the throttle arm to the hole on the servo arm and manufacture a rod to fit. Install it, turn on your radio and check that, when the throttle stick is all the way down, and the throttle trim is also at its lowest position, there is no binding or over-driving of the servo.

Now advance the throttle trim to a middle position; the throttle barrel should be slightly open. Advance the throttle to maximum. Ideally, the barrel should be wide open and the servo just at its maximum position. If this is not the case (it usually isn’t!), then if you have an analog radio you have to adjust the distance at the servo arm. If you are over-driving the servo, reduce the distance. If the throttle barrel is not wide open at the servo’s maximum travel, lengthen the distance. If you have a computer radio, use the throttle end point adjustment to achieve the desired result. In either case, final refinement will be done when you start the engine for the first time. So, now you’re really finished with radio installation!

Center of Gravity (CG)

The last thing to consider is the Center of Gravity (CG) of your airplane. This is VERY important, so don’t neglect it—if you do, chances are that your new bird won’t fly very well (too nose heavy), or have a very short maiden flight (too tail heavy). The plans for the kit you just built show where the designer put the CG. You should endeavor to have the CG on your rendition in the same spot. If you’re constructing an ARF, the booklet also tells you where to place the CG—usually, so many inches behind the leading edge of the wing. Move the battery and/or the receiver to achieve it, or if necessary, add weight. (Try to avoid adding weight, as a lighter airplane flies better and easier.) Ideally, you took into account the CG location as you constructed your new airplane, placing the servos, etc., in locations so as to achieve the desired result. Right?!?

While you’re at it, lateral balance is also important. Here, you want to balance the airplane about its longitudinal axis, offsetting heavy wings or a heavy muffler setup. Suspend your airplane from the prop shaft and the vertical tail (for a high-winged airplane, such as a trainer) or from the prop shaft and the tail wheel (upside-down, for a low-winged airplane, such as a Warbird). Suspend it such that the attitude is horizontal, with the tail and nose at the same (relative) height. (The pivot point should be right over the CG you determined in the preceding paragraphs—a good check, no?) If the airplane drops a wing, add weight to the lighter one (or reduce weight at the lower one) until the airplane remains in a wings level position. Note that this balancing will be subject to change during your trimming sessions, as some of this unbalance will be affected by aerodynamic forces. For now, get it balanced and things will be better, later!

The Final Result

Congratulations! If you did all that we discussed above, you’ve got a straight, well-balanced airplane ready for its maiden flight. There should be few or no surprises. Your flying sessions should be happy ones, and trimming and flying should be moments of great joy. Let us know how it all turned out!

Older or Pre-Owned Aircraft

Now let’s consider one of your existing airplanes that perhaps doesn’t fly just the way you’d like, or an airplane you recently acquired from someone else. Much of what I discussed above concerning new airplanes is, of course, directly applicable to these cases, but there are a few additional things you have to take into consideration in order to achieve the same results.

If you got an airplane from someone else, chances are that it came without servos (potentially good) or an engine, and no plans (potentially bad). If it’s a "Popular" airplane, you could talk to people in your club who might have owned one, or talk to your LHS about it, or look up information about it on the Internet. In any event, you could probably garner the information you need, e.g., control surface throws, CG location, power requirements, etc. Re-read the above, treating your ‘new’ airplane as an ARF. If you have an older airplane that doesn’t fly great, the simplest thing to do is to also treat it as an ARF and follow the instructions above.

The worst-case scenario is an airplane that you know nothing about, nor does anyone else! What to do? Well, ASK someone! Describe (or show) the airplane and see what that person thinks. Do this with quite a few people. Compare this airplane to similar airplanes about which you know something. After you do this, you’ll have a database of sorts, and a set of starting values for control surface throws and CG location. Start there—you won’t be too far off, the airplane will most likely fly, and you can refine it during subsequent flights.

OK, OK, OK—nobody has ever seen such an aircraft, and you can’t find much to compare it with. Now what? Here are a few hints that might help. First, the CG. Set this at 25% of the Mean Aerodynamic Chord (MAC), or at the spar, or at the high point of

the wing—these three points should be pretty close to one another. (You can find out how to find the MAC by searching the Internet, etc.) At this distance, your airplane might be a little nose heavy, but it should be flyable. You’ll be able to refine this later. Check the lateral balance, too.

Second, set the control surfaces as follows: Ailerons, 15º up and down. Elevators, 18º up and down. Rudder, 30º left and right (or as much as you can get). Set your dual rates at 75% of these values.

Be careful on your maiden flight. Keep the wings level, and climb to a safe (nine mistakes high!) altitude before trimming for straight and level flight. Make all your turns gently! After achieving this, try a loop to test for elevator sensitivity, and left and right rolls to test for aileron sensitivity. If things are a bit sensitive, try the dual rates. You should be able to adequately control it, so bring it in and adjust the throws in accordance with what you found.

There! ANOTHER success!

 

† Of course, you won’t be able to work with your LHS if you have severe warp/twist problems—but you checked that before you bought it, didn’t you?!?

‡ This is only applicable to “standard” planforms–canards, deltas, etc. are excluded. For these, you’re on your own. Sorry