Free Flight: Scale-Sport

Free Flight: Scale-Sport

Ralph D. Kuenz

In this installment "Iron" Mike Midkiff concludes his two-part presentation on how to get your scale models to fly.

IN LAST MONTH'S article we talked about building in trimability—those changes made to the structure of the model which improve both durability and flyability. In this article we will discuss the step-by-step procedure which has worked for me on flight trimming a rubber scale model. However, before we get into this let me mention a number of points which should be considered before you attempt to trim the model. These are:

1) Leave all of the cheesecake off until the model is fully trimmed. Landing gear covers, pitot tubes, antennas, any other fragile parts don't contribute to the flight trim and are more apt to be damaged; apply after the model is trimmed.

2) Don't attempt first trimming flights in an area that has less than 8 to 12 inches of grass. Even a very light model can fall hard enough to do structural damage if there isn't a cushion of grass to land in.

3) Don't trim a model when there is more than a 5-mph wind. These light, fragile models don't fly fast enough for penetration when there is wind. Also, those first flight trimming attempts will be very erratic and you will have to interpret them to know what to adjust on the model. With the wind blowing, you won't know whether that dive was caused by down trim or a sudden shift in the wind. OK, with that sermon out of the way let's get to it.

The flight trim of the model is divided into two categories: (1) power-off trim (glide), (2) power-on trim (power burst and cruise). Establish power-off trim first. Put together a rubber motor that is slightly longer than the distance from the motor peg to the prop hook and install in the model. Hand wind a number of turns in this motor to take out the slack and hold the prop with a pin so that it doesn't unwind. This will equally distribute the motor inside the model. Now add clay to the inside bottom of the cowl to balance at 1/3 back from the leading edge. (If you have a large front former opening and some sheet wood in this area, there isn't any problem as to where to put that ugly lump of clay.) Make sure of this balance. Don't cheat. A tail-heavy model is difficult to trim and unstable in any gusts. You now have the model's center of gravity established.

Glide Flight Trimming: Hand glide over the grass. This glide should be a gentle push aimed at a point 20 to 30 feet ahead. Carefully observe this glide. Did it stall or dive, or was that a dive after the stall? Be sure! If the model stalled, raise the leading edge of the stab 1/32 of an inch. (Remember that adjusting slot we talked about in the previous article.) Glide it again. If it still stalls, add another 1/32 of an inch under the leading edge until you have a perfectly straight soft glide. Any diving you correct by lowering the stab leading edge a 1/32 inch at a time until a straight glide is achieved.

Now that the glide is long enough and somewhat stable, observe the glide for any pronounced tendency to roll or turn to one side or the other. If this occurs, and is more than a very gradual amount, check by sighting from the rear of the model the under surface of one wing as compared to the other. There may be washin (tip trailing edge down) on the side the model turns away from. Remove by heating with steam, heat, or even a car's exhaust on the field, simultaneously twisting the surface in the opposite direction while the wing cools. Check again by gliding. Don't adjust the rudder at this point, unless it is found to be very warped or way off of the center line of the fuselage. To re-emphasize: adjust only with stab incidence and small amounts of wing washout to get a good, consistent straight glide. Don't attempt power flights until the glide is right!

Powered Flight Trimming: Powered rubber-model flight consists of three stages: 1) power burst, 2) cruise, 3) glide. With the model's center of gravity and flight surfaces pretty well established by our hand-gliding sessions, the model is now ready for power flight. Before actually winding up and launching the model, add some down-thrust. Add shims to the top surface of the front former to produce a down angle on the prop shaft. On a mid- or low-wing model this should be about 2 to 3 degrees. On a biplane or high-wing model this should be 4 to 6 degrees. These degrees of down-thrust are just for starters only. Again, make sure you are trimming in a flying area which has plenty of grass for a cushion. Hand wind about 50 to 60 turns in the motor and launch the model into whatever breeze there may be. Launch the same way you did when you were gliding the model. Observe this first power-glide flight to determine what thrust-line changes may be needed. At this stage, only alter thrust-line angles to get a steady controlled climb.

If the model noses up, add more down-thrust shims to the top of the front former, 1/32 at a time. If the model rolls to the left, add shims to the left side of the front former to produce right-thrust. Now hand wind about 75 turns and launch the same way. If the model turns to the right a little too much, remove some of the right-thrust or add some left-thrust. Continue to test fly the model by adding 50 more winds to the motor and launching and again, then 50 more and launching, until you reach maximum winds for the motor. Observe each flight for the need of still further minor thrust-line changes to produce a wide left-hand nose-up climbing circle. This climb must be at the highest angle of attack which is still under control. No stalling and falling should occur, nor any galloping; either of these conditions wastes power and prevents the model from reaching maximum altitude with a steady, controlled climb.

You may ask why a left-hand power pattern on the model. Without going into a lot of past experience, my models have flown more consistently when flown in a left-hand pattern outdoors. First, a fully wound motor will roll the model slightly to the left during the power burst and, when launched slightly to the right of any wind, the model will maintain a little longer a heading pointed into the wind, which will gain it a little more altitude. Second, the torque will roll the model to the left, but will also keep the nose up without stalling the model. This roll acts as a form of down-thrust which occurs only during the power burst. Conversely, a straight-ahead climb generally requires too much down-thrust with any more than a marginal power. A right-hand climb pattern seems to work well with high-wing, short-nose models, but even though the torque keeps the wings somewhat level, the resultant gyroscopic action wants to spiral in the model after the power burst is over. My choice is a wide left-hand power pattern.

OK, your model is flying pretty well on the original motor at maximum winds. Remove this motor and install a fresh one. Go immediately to near maximum winds. Observe how the model climbs with this fresh power. Make any corrections to the thrust line only to achieve a consistent, wide, left-hand climb. Watch the cruise at altitude after the power burst is complete. Does the model want to fly away in a straight line down wind, or does it tighten up into a right- or left-hand spiralled descent? In either case, make adjustments to the rudder to maintain a wide left-hand cruise and glide. This becomes difficult, since you must trim with the rudder which will change your power burst pattern.

Here is how you would do it: The model climbs well to the left and gains good altitude, but then flies down wind without turning. Slightly off-set the rudder trailing edge to the left and add a touch of right-thrust to compensate. This may be only the thickness of card stock. What trim is necessary to get a good, wide, left-hand cruise and glide must be compensated for by opposite thrust-line changes or the power-burst climb pattern will be destroyed. Suppose the model climbs out well in a left-hand pattern, but the cruise and glide gradually tighten up into a left spiral dive. Add right trim to the rudder, just enough to maintain a wide left-hand cruise and glide, but you will have to add some left-thrust, or take out some right-thrust to maintain your original climb pattern.

You can easily see the time and effort that must go into trimming your model to fly right during the power burst, cruise, and glide. Half a dozen random flights are nowhere near enough to trim a model properly. One thing I can't believe happens when a guy comes to a contest with a new model and expects to trim it with a few flights and luck out with a two-minute flight. It doesn't happen that way. First of all, have you ever seen a single contest day that wasn't windy or gusty? You will never get it trimmed properly in that hurricane. Secondly, the concentration that must go into the systematic effort of making a miniature aircraft fly as good as it looks will never happen at a contest with all the activity that is going on. As with anything in life, if you want to get anything good out of it, you have to put effort into it. Systematically build your model right and trim it right. There is nothing as

Free Flight: Scale-Sport

Ralph D. Kuenz

In last month's article I talked about building and trimability — those changes made to the structure of the model improve both durability and flyability. This article will discuss the step-by-step procedure that has worked for flight trimming rubber scale models. However, before we get to it, let me mention a number of points that should be considered before attempting to trim a model:

1. Leave cheesecake off until model is fully trimmed. Landing gear covers, pitot tubes, antennas and other fragile parts don't contribute to flight trim and are apt to be damaged — apply after model is trimmed.

1. Don't attempt first trimming flights in an area that has less than 8–12 inches of grass. A very light model can fall hard enough to cause structural damage; turf isn't much of a cushion.

1. Don't trim a model in a 5-mph wind. Light fragile models don't fly fast enough for penetration in wind. Also, first flight trimming attempts will be very erratic; you will have to interpret and know what to adjust. If the wind is blowing you won't know whether a dive was caused by down trim or a sudden shift in the wind.

OK, sermon out of the way — let's get to flight trim. Models are divided into two categories:

1. Power-off trim (glide)
2. Power-on trim (power burst, cruise)

Establish power-off trim first. Put together rubber motor slightly longer than the distance from motor peg to prop hook and install in model. Hand-wind the number of turns, take out the slack and hold the prop pin so the motor doesn't unwind — this will equally distribute the motor on each side of the model. Now add clay inside the bottom cowl to balance. If the leading edge has a large front former opening or some sheeting area, it isn't a problem; put an ugly lump of clay. Make sure it's balanced. Don't cheat — a tail-heavy model is difficult to trim and unstable in gusts. Now you have the model's center of gravity established.

Glide Flight Trimming

Hand-glide over grass. The glide should be a gentle push aimed at a point 20–30 feet ahead. Carefully observe the glide. Did it stall? If it dives after a stall, be sure the model stalled; raise the leading edge of the stab 1/32 inch. Remember the adjusting slot talked about in the previous article. Glide again; if it still stalls add another 1/32 inch under the leading edge until you have a perfectly straight, soft glide. Any diving is corrected by lowering the stab leading edge 1/32 inch at a time until a straight glide is achieved.

Now glide long enough to be somewhat stable and observe. If a pronounced tendency to roll or turn to one side or the other occurs in a very gradual amount, check by sighting under the rear of the model and compare one wing surface with the other — there may be wash-in at the tip, trailing edge down on the side the model turns away from. Remove the warp by heating (steam, heat from a car's exhaust) while simultaneously twisting the surface the opposite direction; as the wing cools check again by gliding. Don't adjust the rudder point unless you find the rudder very warped or way off the center line of the fuselage. Re-emphasize: adjust stab incidence in small amounts. Work out any wing washout to get a good, consistent straight glide. Don't attempt power flights until the glide is right.

Powered Flight Trimming

A powered rubber-model flight consists of three stages: 1. power burst, 2. cruise, and 3. glide. With the model's center of gravity and flight surfaces pretty well established from hand-gliding sessions, the model is now ready for power flight.

Transcribed from original scans by AI. Minor OCR errors may remain.