First flights are fraught with nerves and unanswered questions. Let us approaeh the problem logically and set up some rules for pre-flight inspection to remove some of the first flight buts. Since design problems will be covered later, we'll assume a kit model or a debugged design is to be checked out.
Preflight inspection should include: i-Balance; 2-Control freedom; 3-Wheel tracking and freedom; 4-Warps; 5-Engine and accessories; 6-General airworthiness.
Landing gear position is important far goad take-affs and landings. Simple-lined, attractive.
Balance should be according to the designer's recommendations. Conventional ships should balance forward of a point one-quarter of the distance from the leading edge to the trailing edge. An example would be: for a wing ten inches wide, balance should be less than 2.5 inches from the leading edge. Notice we said ndthing about the leadouts. They don't affect the balance point as such. Suspend the model by wing tips, indoors out of the wind, and see where it remains level in flight position. If your arms are too short, get a helper. If ship balances aft of quarter chord, add weight to nose until balance moves forward of same. If it is way up forward don't worry about it. You're safe.
Controls should he free of drag and rough spots in travel. At times during flight you will have less than one pound of tug and will need free controls. (Monoline users; be sure your elevator pops back to neutral when torque is released.) Controls should be free enough that the elevator will droop from its own weight. Use a fairlead or bushing every 6 inches on your pushrod to prevent buckling and loss of control.
When fixing your wheels on their axles use nylon or ordinary wheel collars or solder a washer on the end. Avoid use of acid core solder to prevent corrosion which results in wheel freezing. Use a good soldering paste, a hot iron and wash joint thoroughly aftcr hardening, with lacquer thinner. Oil immediately and periodically. A surprising number of flights are botched from stuck wheels. Roll the airplane forward gently on a smooth surface. It should move straight ahead, whether tricycle or conventional. If not, bend tail skid or nose wheel until it does. Same goes for speed dollies. Panted wheels should be checked carefully for sticking. Remember you have to take off before you can fly.
A warp in a wing results from uneven shrinkage of covering or poor alinement during construction. Warps are dangerous, but can be cured. They will cause one panel to have a different angle of attack from the other, resulting in different amounts of lift and a banked altitude in flight. For instance: Outboard panel warped down at the rear; model will bank into the circle causing a decrease in line tension and during a tight maneuver possibly complete loss of control. On a stunt ship any warp is bad since it will get you upright or inverted. An illustration shows how to view airplane to look for warps. Line your nose up on center line of the fuselage and keep both eyes open. If no warps are present the trailing edge will split the upper and lower contour of the wing. Warps may be corrected rather simply. To soften the covering and structure, steam affected panel thoroughly using a vaporizer or tear kettle on both sides until covering becomes slightly loose to touch. Sit down somewhere and warp surface in opposite direction and hold for at least 15 minutes. When released it will spring back a little, so go past the place you want it to end up. Let it set up overnight so that all will be shipshape before flying.
Run your engine at home, during the day, to cheek out tank fittings and location. Try her inverted too. No need to fling the plane around madly during this check since you prove nothing. A final check for loose joints, etc., will finish up your preflight check. Don't forget to take a rag with you to the field. Always clean model after flyii~g. Before flight, lay out your lines and make sure that neutral elevator corresponds to your neutral at the handle. The author washed out a good ship for not checking this, so it can happen. Pull test lines to check fittings and lines. First take off is best accomplished downwind to give you a half lap to catch up if anything goes wrong. No matter how carefully designed arid built, a model airplane must still prove itself in flight. Here are some characteristics to look for on the first flight.
In ihese four pictures, left to right, the four positions in loop. The climb. Wing warps and incorrect wing weight have bad effects.
Inverted at the top. Flap angle reveals reversed controls applied. Dive at end of the maneuver. Kits not hard to test. Dream Ships !?
Assume the model is trimmed too nose heavy. The take off will be slow and will require an excessive amount of up elevator to break free. Maneuvers may be small enough but you'll have to ride the controls hard to do them. Level flight will be beautiful and level. Normally you'll have lots of tug in this condition, but this is not a criterion. The plane will resist all attempts to make it turn quickly. The landing will be rather hot and abrupt as the engine quits. All in all, this is the best condition for a first flight, since the ship will be completely stable and you'll have the best chance of making a second flight the same day.
If your ship should be tail heavy, beware! You are in for a real thrill. Take off will probably be instantaneous and plane will undoubtedly climb right up to the top. Controls will be quite sensitive. When you try to bring her down to low altitudes she'll try to tuck the nose under and up control wfll cause bucking. If you get hold of one of these monsters, let her find her own altitude. If only mildly tail heavy she may fly alright, but condition will show up in extreme reactions to minor changes of controls and loops wiii tend to tighten up. A loop with a tail-heavv ship will generally end at a much higher altitude than it started. Tug will probably be light. Your feeling will be, that the plane is flying you. This coedition should be corrected with lead in the nose before trying that second flight.
You removed the visible warps before flight, didn't you? But let's say your ship was shy on tug inverted. Your sharp eyed helper observed that the plane was flying with the outboard wing lower than the inboard in upright level flight, but when inverted the opposite was true. The outboard trailing edge is warped up or perhaps the inboard trailing edge is down. Not so good. This can get you in trouble during a maneuver when the wing is really working hard. Slight warps may be controlled by tabs, either soft aluminum or a triangular cut piece of balsa. Place tab on outboard panel since it's more effective there, and bend opposite to warp. Experiment until wings ride level in both upright nnd inverted flight. If flaps are used, slight warps may be corrected by bending flaps opposite to warp.
The matter of wing tip weight comes up immediately. Why the weight? The control line model must support not only itself but the lines. Lateral position of the center of gravity of model must be displaced toward the outside of the circle an amount sufficient to cause equilibrium to occur during a tight pullout. Huh? Flight characteristics will look like this. insufficient weight: Model will have light tug, and tend to get lighter on sharp pullouts, both upright and inverted. Too much weight will cause just the opposite, including tendency to drop outboard wing in a glide. Not off, just sag a little. Proper weight will keep wings level through all maneuvers.
If your model balanced properly and retrimming did not correct sluggishness, or unusually sensitive response, your controls are probably to blame. Insufficient response would then mean not enough motion of control surfaces. An illustiration shows points where control sensitivity can be adjusted. Roughly works this way. Increase of dimensions 1, 2 or 6 will increase ratio of elevator to handle movement. Decrease of 4 or 5 holding others constant will increase motion. Generally 1 and 3 are fixed and 2 is inaccessible, so the easiest place is 4. Assuming no flaps for present, sluggishness maybe corrected by moving hole in elevator horn closer to elevator. If you've already got 45 degrees up and down, forget it. You need a bigger elevator. A flat plate will generate its maximum lift at 45 degrees. Any increase in angle will increase the drag hut decrease the lift. Therefore, if 45 degrees doesn't do the job, it must be made larger. Many fliers don't seem to realize this fact. My own preference is a 4 inch spacing at handle, a 3 inch bellcrank using the center hole for pushrod, and a 0.75 inch horn. This gives me roughly same elevator motion as hand motion. With the CG located just forward of quarter-chord, 30 degrees elevator motion is usually sufficient.
In general, the farther forward the landing gear is placed the harder it will be to raise the tail for take off and landings will be tougher. Ideal location allows full control of take off and landing attitudes. Too far aft will naturally nose you over on landings and perhaps take offs.
If line tension is too light and warps and wing weight are not to blame, easiest correction is more engine offset. Rudder offset has little effect on tug. believe it or not. There are other ways which will be covered.
The vertical location of the tank is extremely important to the stunt model. Assuming you have a good commercial tank or one of sound design, errors in location will show up as follows: If center line of tank is higher than the center line of the needle valve, even 1/16 inch engine will richen up during inside loops and other upright maneuvers. Engine will get lean inverted and leaner on outside maneuvers. This is due to fact that in a loop, up to 20g's of centrifugal force may be built up. This increases or decreases the pressure head of the fuel in the tank causing it to flow faster or slower. Vents may also affect this condition. The Darwin or similar tanks using 2 vents should have pieces of fuel line with one end cut off at a 45 degrees angle, slipped over vent tubes. Face open end of tube forward to prevent syphoning during flight. Most modern stunt engines have adequate fuel suction, at least 15 inches, so troubles with fuel flow will usually be the tank or dirt.
To summarize briefly, we've found several symptoms which may have separate causes. For instance, nose heaviness, and insufficient control motion look somewhat the same, but there are subtle differences which can be recognized. There are some key maneuvers which will Isolate trouble. With experience it is possible to completely analyze a ship in one flight.
Our pet procedure goes something like this: Peak the engine and then back the needle valve out slowly so it just runs 4 cycle. Will develop almost as much power as peaked and prevent a lean run. Take off is made with neutral or slight up elevator. Rate of climb is observed to check trim. The ship is gradually worked to the top to check tug and warps. If it doesn't get too light we try a sharp climb. A warp or light tip weight will show up quickly here. A sharp pull out is next for same reason. Run through a few inside loops and see how she turns and how easily it responds to minor motions of handle. If nothing shows up yet we lay it over on its back and run through the same procedure. Following this come the square loop and as many of the eights as we have fuel for. And finally the glide, approach and landing get close observation for trim and balance. By working through this sequence of maneuvers you can do a full pattern first flight, IF, she checks out on each stage. Don't, however, try sharp pull ups, etc., if she's riding light or if she looks tail heavy. We were also listening to the engine and making sure tank and needle valve settings were right.
With original designs the problems are multiplied. What looks like nose-heavy trim might be insufficient wing area, too little elevator area or motion, or wrong angular setup of wing and stab. Any number of things can go sour with a dream ship. About the best advice is to try to analyze what it should do and see if it does it.
See you next month.