Free Flight Wisdom

The enduring magic of trimming a FF model

By Tom Hallman [email protected] Photos by the author except as noted

As seen in the October 2024 issue of Model Aviation.

I returned to the hobby of rubber-powered Free Flight (FF) half a lifetime ago, while in my mid-30s. Before that, as a young teenager, I had built a few rubber-powered models, but I never got them to fly for more than a few seconds. Years later, through the generosity of friends and mentors in the Flying Aces Club (FAC), I’ve learned a few things that have since become firmly rooted, leading to a continually growing fleet of successful FF models.

I’m passionate about building and flying rubber-powered FF models and Towline Gliders. My dad had the bug as a teenager in the 1930s, which made its way to me 50 years later. I’m still in awe of what can be achieved with only a few dollars of balsa, tissue, and rubber. Nothing fancy and nothing high-tech—just cheap thrills using a fairly basic balsa structure covered in tissue with a couple of long, twisted loops of rubber to send the model skyward. It’s the magic of flight, pure and simple.

After a long and productive first day of trim-flying with the Flying Aces Moth, the model flew beautifully at sunset but played a dangerous game with the irrigation ditches.

The author launches his 31-inch Martinsyde S.1, a FF model that was difficult to trim but has flown soundly since being put through a methodical trimming process.Photo by P. Kaiteris.

But it really isn’t magic. To get FF models to succeed takes a heavy dose of patience, planning, and a bit of pondering to make it happen; however, when it all comes together and the model starts to groove through a series of wide, quiet, floaty circles 150 to 200 feet above, those watching from the sidelines will indeed suspect a bit of magic and trickery was involved.

Anyone with the interest and desire can build and fly a rubber-powered model airplane, although the successful trimming of the model begins long before you arrive at a local flying field with your newly built balsa airplane.

Many newcomers to the hobby, young or adult, are easily swayed by the history, beauty, or cool factor of a given full-scale aircraft. It’s powerful to watch a World War II fighter or a Golden Age biplane fly and think, "Look at it go!"

They purchase the kit, built as best as they can, and then it’s off to the ball field using the small propeller and short rubber that came in the box. The rubber is wound by hand to the maximum, but rarely is that first flight as soaring and beautiful as the one in their mind’s eye. More often than not, it leads to a quick, hard up and down, leaving a severely damaged model—leading to a loss of interest and a shift to another hobby.

How do we reverse that outcome and set newcomers on a better path during their first experience, potentially leading to a lifelong interest in FF modeling? It starts with the selection of a basic model airplane—one that is easy to build, with minimal time invested as you go through the process.

As much as we are wowed by a sleek racer, fighter, or biplane, a proven nonscale flier, such as the 24-inch Flying Aces Moth or FF Dart, would be a more productive way to go, leading to success for a first-timer. You can find even simpler designs with a stick fuselage, such as the Peck-Polymers rise-off-ground model, but these hold up very well as a starting point and can give a flier confidence for the eventual building of a simple, scalelike model aircraft.

I won’t go through a step-by-step tutorial on how to build a specific model, but I’ll hit on a few of the basics that will become the standard as you gain experience.

Twisting the wing over hot steam will soften the balsa structure enough to correct the warp. The wrinkled tissue will dry smoothly and drum tight.The wrinkled tissue will dry smoothly and drum tight.

After steaming, shim the LE in the opposite direction to reverse the warp. Allow it to remain pinned down overnight.

A solid building surface can be as simple as a few pieces of dense, inexpensive 1/2-inch thick ceiling tiles. I cut them down to shapes that are slightly larger than the wingspan and fuselage length, or I cut even smaller pieces for the rudder and stabilizer. The benefit of the smaller board sizes is that you can build anywhere with them. Plus, you don’t need to wait until the structures are fully dry before starting on another section. Two or three boards will get you through the project and will last a lifetime.

Other essentials include X-Acto knives with new, sharp blades, sanding blocks and emery boards, pins for holding down the balsa pieces, glue sticks and rubbing alcohol for attaching the tissue, small brushes, and various adhesives, such as Titebond and CA glue. You will also need a rubber winder with a 5:1, 10:1, or 20:1 winding ratio, along with a stooge that holds the model while you wind. Check the article’s "Sources" for links to companies that offer these and other FF items.

The .032 sheet aluminum thrust plate and wood screw gives you total control of the thrust setting, a most important adjustment for any new rubber-powered model.

A successful trim session begins by having a wellbuilt model that is free of warps. It’s also important to have a nose block (the removable part connected to the propeller that plugs into the fuselage) that won’t wobble and is large enough to easily install your motor. A 5/8-inch, hard balsa cube is a good place to start.

Wobble pegs are aluminum sleeves that slip over the motor peg, preventing the rubber from bunching in the rear and disrupting the CG.

The nose should be faced with 1/32-inch plywood using a brass, four-prong dome nailhead as the nose button, which is first drilled to fit the propeller shaft wire then anchored into the plywood with CA glue. The block should also have a 1/32-inch thick sheet-aluminum thrust plate on the back. This combo allows you to easily set and adjust any amount of thrust angle, which is critical through the trimming process.

I typically start with a two-loop rubber motor of FAI/Tan, which comes in widths of 1/16, 3/32, 1/8, 3/16, and 1/4 inch. The motor should weigh between 30% to 35% of the empty model and be two to three times the length of the propeller hook to the rear peg.

This gives you a motor that will supply the model with a long power run and enough torque to get it up into the blue at a reasonable pace. A slower model is safer and easier to trim than one that is racing wildly through the sky because of a short, thick motor.

The motor should be braided, which is a technique that creates an even tension of the rubber between the ends. This limits any shift in the motor’s position, which could otherwise disrupt the center of gravity (CG) that is the main force that stabilizes the model in the air.

You should also add a wobble peg to the rear of the motor. It’s merely an oversized aluminum tube with flanged ends that fits over the rear peg tubing. (My "Free Flight Basics" #1 episode ["FFB"-1] on my YouTube channel details this process. See "Sources.") The free movement of the wobble peg helps prevent the natural bunching of the rubber in the rear, which again limits any shift in the CG.

One reality of rubber power is that the motor wants to creep forward and wrap itself around the propeller shaft hook, ultimately preventing the propeller from spinning. You can limit this tendency by easily making a "reverse S-hook" with a pair of pliers and stock propeller-shaft wire, usually .032 or .047 in diameter. (See "FFB"-3 for a video tutorial on making two versions.) All of this preparation and we haven’t even been to the field yet! Not to worry; it is time well spent! Allow these introductory steps to become your standard. Here is where the trimming story of my own Flying Aces Moth from late last year begins. You can follow along flight by flight on my "FFB"-1 YouTube video.

These are various sizes of thrust plates that have been drilled and cut from .032 aluminum sheet. They are held in place on the rear of the nose block with a small wood screw.

Start by test-gliding the model to find its CG. I like to glide the model without the motor or propeller attached because it makes the airplane lighter and less prone to damage. The CG mark will remain constant throughout the life of the model, with or without the motor and propeller attached.

Check for the CG mark on the plans, which is typically one-third of the distance from the leading edge (LE) to the trailing edge (TE). When balancing the wingtips with your fingers, add nose or tail weight (ballast) with soft modeling clay until the model is level. Over soft grass, glide the model with a gentle, straight, level motion and observe, then do it again. If it dives, remove some of the nose clay. If it stalls, add more to the nose until you get a level glide.

The 24-inch Moth was designed in 1941 by Herb Spatz. It’s a simple structure with solid flying characteristics, making it a good choice for a first rubberpowered model.

After the glide is smooth, mark the CG location, even if it has changed slightly from what the plans show. Eventually, if the model stalls at the powered launch, you will be adding downthrust to the propeller shaft and not adding weight to bring the nose down because we already know that the model will glide smoothly from these backyard tests.

Gurney flaps are thin strips of balsa that allow you to quickly trim a model on the field. In this case, the model needed a bit more right turn on the rudder.

After covering my model with Japanese tissue ("FFB"-2), I noticed a wing warp that had twisted one wingtip in the opposite direction of the other. Not surprisingly, my first test glide sent the model diving to the left, so I couldn’t continue until I had reversed the warp.

Using a saucepan on a stove top with 1/4 inch of boiling water, I was able to steam the warped wing panel, gently twisting and holding it for 15 to 20 seconds in the opposite direction of the warp. I pinned it down to a building board with shims that continued to reverse the warp and let it dry overnight.

After removing the wing the next day, the wingtip had relaxed to a more flattened position, which matched the angle of the other wingtip. Perfect!

Returning to my backyard test grounds, affectionately known as Hallman’s Meadow, I gave the Moth a few tosses, which were straight and level. Oh, the power of a steaming saucepan!

The green dots show that the wingtips are now identical after using the steaming process described in the article to reverse the warp.

The following week, I took the Moth to my favorite flying field, a sod farm in Wawayanda, New York. A portion of the field is left to harvest hay, so I used that acreage of higher grass for the trim session and the promise of soft landings.

This model came in at 31 grams without rubber, so I suggest a motor that’s two loops of 1/8-inch rubber, with each loop measuring 30 inches long. With the hook-to-rear-peg length being 14 inches, the motor is slightly more than twice the hook-to-peg length.

The motor weighs 10.5 grams, which is 34% of the model’s empty weight. This motor is slightly shorter than what I’ll use for the trim session, but it’s a good place to start because it falls into the 30%- to 35%-weight sweet zone. It will be strong enough to get you flying, allowing roughly 1,600 turns for higher torque power.

For this trimming tutorial, I used a longer motor because I like to push the limits from the start. My 1/8-inch loops will be 38 inches long, weighing 13.25 grams, making it 43% the weight of the empty model. Maximum winds are closer to 2,100 for a longer motor run.

For the initial trim session with the Flying Aces Moth, I first braided the motor using the simple technique of winding 200 forward turns into the single 60-inch loop of rubber before folding it over into two loops, each 30 inches long.

The motor braids itself as you fold it over and hold the ends together with two small dental rubber bands. This is when you also add the aluminum wobble peg to the rear end of the motor, being sure to snug up the dental band to hold the peg in place.

After installing the motor with a stuffer stick, I put 100 turns into the rubber. I released the propeller while still holding the model to watch the rubber relax, ensuring that the amount of braids would do the job. The motor should be suspended between the two ends or just barely touching the floor of the fuselage. It should not be too tight, which could prevent the free-wheeler on the propeller from working.

I also reballasted the model, adding nose clay to match the original CG. Additionally, to keep the nose light, I had carved an 8.5-inch propeller from light balsa. A plastic propeller will work as well, but it will require some tail weight to match the CG.

Flight #1: I started with 150 turns as a powered glide, merely to see whether the model was stable. It easily left my hand and felt airworthy.

Flight #2: I was up to 300 turns. The Moth flew a 1/4 circle to the right but felt like it was diving and picking up speed. I removed some of the downthrust by adjusting the thrust plate, which at this early stage was 4º of right thrust and 4º of downthrust.

The plate is held in place by friction and the strength of the flathead screw ("FFB"-4). As I increase the winds and torque, I’ll be forced to place a small drop of super glue on the edge of the plate to secure its position.

Flight #3: I used 400 turns. The flight gave me a half circle, but I again felt as though it was speeding up while going downhill. It came down with turns left, so I added a bead of clay to the tail skid and tossed it to see if the nose came up, which it did.

Inexpensive, fourprong dome nailheads are perfect nose buttons, allowing free movement of the propeller shaft with washers when trimming a model. Simply drill the hole with a pin vise.

Flight #4: This test was with 720 turns. It flew with a better attitude, nose slightly up, and gave me one and a half circles to the right. This felt pretty good, so I continued to the next flight with more turns but not before removing even more of the downthrust. I left the clay on the tail. My flight time was 18 seconds.

Flight #5: This flight was 840 turns. It gave me a gentle, level circle and a half, but I felt the need for more power, so I bumped up the turns more than before. The flight time was 18 seconds.

Flight #6: With 1,200 turns, there were no trim changes. The little Moth gave me six circles, nose-up slightly for a floaty glide path. The circles were tighter than I liked because it can prevent the model from climbing, so I adjusted the thrust plate for a bit less right thrust on the next flight. The flight time was 60 seconds.

FF rubberpowered models of many shapes and sizes can be taught how to fly by using a slow and precise method of trimming them. Here are the Babcock, Clipped-Wing Cub, and Mureaux Fighter.

Flight #7: This was with 1,400 turns or two-thirds power. The flight looked reasonably stable, and had a wider circle and a more powerful climb. There was a slight stall throughout, but that was cleared up by removing a pinch of the tail clay before the next flight. The flight time was 80 seconds.

This 24-inch FF model of the 1941 Flying Aces Moth is the perfect choice for a first rubberpowered model for young or adult newcomers. The model was built by the author in 2023.

Flight #8: The flight was on 1,600 turns. This flight was also good, but it didn’t climb as well, so I decided to put the bit of clay back on the tail to lift the nose for a better climb. The flight time was 87 seconds.

Flight #9: I put in 2,000 turns. The breeze was up a bit, so I activated the dethermalizer (DT) for a 90-second release. A DT is the mechanism built into a model that includes a slow-moving viscous timer, combined with a spring and line that attaches to the hinged stabilizer, which pivots to a 45º angle when released.

This action changes the model’s aerodynamics and usually breaks it free from a thermal ride or an out-of-sight flight. The Moth was beautiful as it rose high over the field toward the distant tree line. Fortunately, the DT did its job and brought the model down before it drifted too far. The flight time was 120 seconds.

Epilogue

Later that day, during the golden hour, I gave the Moth one more flight, this time with 2,150 turns. It rose high into the cool evening, drifting slowly toward the sun through a kaleidoscope of colors. It was possibly the most beautiful flight of one of my models on its first day. The Moth finally came down after nearly two minutes of slow, lazy circles, landing inches from the water on the thorny embankment that borders one of the field’s irrigation ditches.

I couldn’t have asked for more from this great, little airplane. It flew better than most on their initial trim sessions. I didn’t need to dive too deeply into my trimming bag of tricks. If I had, I might have considered using one of my favorite tools, the Gurney flap, which is a thin, 1/16-inch wide strip of stick balsa, one to two inches long, and only 1/32- to 1/16-inch thick. These are easily added or repositioned on the wing or tail using a glue stick. Rubbing alcohol softens the glue if you need to reposition the stick. They can be used to gently push a model left or right by positioning one on the side of the rudder’s TE, or they can be used to create up- or down-elevator on the top or bottom of the stabilizer’s TE.

The author noticed a severe wingtip warp during the Moth’s test glides. The warp was easily reversed by using water and the steaming saucepan method.

You can also effectively use them on the wing, placing them midway or toward the tip on the TE, which acts like an aileron to tilt the model one way or another. This is a useful adjustment to make in order to prevent the model from spiraling.

Be sure to check out my 14-part "Free-Flight Basics" series and the four-part "Free-Flight Trimming Basics" series on the MaxFliArt YouTube channel. Also check out the "Anatomy of a Trim Session" videos. The visuals should help you to understand some of the mystery that surrounds trimming various types of FF rubber-powered models.

That noted, nothing works better than simply getting into the game, building a few models, and gaining experience through each trim session. At some point, the needed adjustments become intuitive and the models begin to soar. Each model is different and each comes with its own issues and concerns. The fun is finding out what each of them needs.