Welcome Back, We Missed You: Part Two

Airframes and Engines

By: Frank Granelli

We are glad you returned to model aviation. The sport missed having you as part of us and I am willing to wager that you missed many of its benefits such as relaxation, great friends and the sense of achievement gained as your creation takes to the air.

ARF RTF and BNF?

Let’s talk about that creation and how it has changed in the last twelve years. Remember that twelve years proved to be the average time most returning modelers spent away from model aviation before returning. While many Almost-Ready to-Fly (ARF) “kits” were available in 1997, the subject selection was somewhat limited. Sure, there were a few Mustang ARF kits available, but how many B-25 ARFs were around? Try zero while there are at least two today. The Ready-To-Fly (RTF) concept had not yet truly taken off.

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Also, many ARF airplanes were heavy, had poorly lifting wings with heavier wing loadings and were difficult to repair. Much of the construction was printed paper over thin foam that was wrapped around a heavy wooden structure such as the Zero in photo 1. Ding this type of wing and it could have been a permanent problem.

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Conventionally constructed ARFs, heat shrink covering over wood frames, were just beginning to become available. Even so, colors remained hard to match when performing repairs using the colors available in this country. Try matching the blue color in the mid-90’s Chipmunk pictured above. About the only color that came close was the Oracover® found only in Asia; not even the European colors matched. The covering was also much heavier than that builders used.

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There was another class of ARF aircraft that are very rare today. These were the all soft-foam airplanes sold by several companies. The foam construction was white and the modelers had to either paint them or cover them with a foam-safe, low-temperature covering. These airplanes were somewhat less “ARF” than modern ARFs yet more “ARF” than a wood kit. Note: the foam used was very soft and not the foam used in today’s “Foamies”. The Spitfire’s wing was covered in fiberglass for strength and then painted. The fuselage used tan low-temp heat-shrink covering, sanded and painted olive drab in spots. Very heavy.

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Travel forward to today and wow, have things changed! The ARF selection is huge! Just about any airplane that was ever produced in quantities higher than ten is now available. Maybe that is hyperbole but you get the idea. A quick look trough any manufacturer’s catalog will tell you the ARF selection is nearly unlimited. There must be twenty Mustang versions available in all sizes from small electrics (another new change) to 80-inch+ monsters. Even the 92-inch A-26 Invader in photo 4 is an ARF as is the 76-inch P-6E Hawk (sadly no longer available).

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The Great Planes P-6E Hawk is a good example of another fact of ARF life the returning modeler should know. Unlike the popular wood kits of the past, an ARF’s life span is short. An ARF is manufactured to meet a given demand for that particular model. Once that demand is met, sales for a particular ARF decline rapidly. Unlike a wood kit that takes little warehouse room in its small box, ARF’s take several times the warehousing space and storage cost. Once sales drop, it is no longer feasible to continue selling that particular airframe.

So remember this well if you see an ARF you really covet, GET IT NOW. DON’T WAIT! This is especially true of the less famous airplanes like the JU-87 Stuka (once a Great Planes ARF) or even of the more popular P-47D Thunderbolt in photo 6 (also sadly no longer available).

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Along with the ARF explosion came a new type of aircraft: The Ready-To-Fly (RTF) airframe that has the power and radio systems already installed. One of the key definitions of an RTF airplane is that no glue or tools more sophisticated than a screwdriver or wrench are required. 98% of the work is already factory completed. The Arrow and Alpha 60pictured above are both RTF airplanes (the Alpha 60 is now only available as an ARF). Click on the blued names that are links to the Sport Aviator articles to learn what assembling an RTF airplane is like.

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The famous NexSTAR is another RTF airplane that introduced a three-axis stabilization system that automatically returns the airplane to level flight once the controls are released. The very popular ElectriStar introduced electric power to the full-size RTF realm.

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Both aircraft are still available as RTFs. The same is true of the T-34 Mentor, another electric-powered RTF that has proven valuable as a Basic Trainer.

While each article link reviews assembling that particular RTF airplane, you might want to check out the more generic Sport Aviator article,”How to Assemble Your First RTF Trainer” in the Pri-Fly Section

It may seem counter-intuitive since RTF airplanes are more expensive, but most RTF life spans are longer than that of the average ARF airframe. That is because almost all RTF airplanes are either Basic Trainers or Advanced Trainers. Good Trainers have very long life spans. The 40+ year old SIG Kadet is now a popular ARF as is the Goldberg Falcon 56. Therefore, the RTF trainer you prefer will probably be around for a while should you ever need a replacement during your learning process (hope not). The same is true of ARF Trainers unlike sport ARF airframes.

The T-34 is also an example of a totally new class of RTF aircraft called either Bind N’ Fly or Receiver-Ready. Before Bind N’ Fly, there were the Plug N’ Fly RTFs. The underlying concept is that the airplane assembles as an RTF including the installed servos and power system, but lacks the transmitter. The Bind N’ Fly concept includes a 2.4 GHz receiver that must be bound to your JR or Spektrum transmitter. Receiver Ready (Futaba) aircraft and the Plug N’ Fly aircraft need both your transmitter and receiver but are otherwise RTF airframes.

Since the receiver (sometimes) and transmitter are not furnished, these RTF aircraft are cheaper than a true RTF. Yet, they incorporate the ease of construction of an RTF. In case you are wondering how quickly a full-size RTF can be built, my own record assembly time for a glow-engine RTF is 17 minutes on a friend’s Arrow.

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There are also some Basic Trainers called PTS (more initials?) from Hangar 9 only. PTS stands for Progressive Trainer System trainers. Both the Mustang and F-22 pictured above are PTS trainers as is the T-34. Each RTF airplane is equipped with removable leading edge cuffs that increase lift and/or wing area. Flaps are also available for increased lift and drag to slow and stabilize the airplane. The Mustang even has speed brakes on the landing gear legs.

As your piloting skill improves, each device can be removed in turn increasing the airplane’s speed and reducing the lift. Eventually, the pilot ends up with a true scale airplane that is harder to fly than the trainer version but is still easy on the ole` stick fingers.

Making a Choice?

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Which one should the returning pilot choose? RTF, ARF, other RTF formats? Of course, the answer is that depends. But depends on what? If you have been away for only a few years and have modern equipment, you probably do not really need to start with either a Basic or Advanced Trainer. If your radio is modern and in good shape, go straight to the sport ARF’s as found in Sport Aviator’s On The Flight Line or Park Pilot Sections. The SIG 4-STAR 40 and 60 or the Goldberg Tiger II 40 and 60 are great starting points for you and will probably be the airplane that opens every flying “season” regardless of how advanced your airplanes become in the future.

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If you have been away for more than five years and/or do not have good modern radio systems, or only really just soloed before you had to leave, a good RTF Advanced Trainer like the Pulse XT or the HobbiStar 60 Mark III might be just perfect. The PTS system is also a good fit here. While you are flying and learning on the RTF Advanced Trainer, pick out a good Sport ARF aircraft and build it during your learning flights. The Advanced Trainer, with its symmetrical airfoil and resultant aerobatic capabilities, will stay current for years to come.

If you have really been away for twelve years or more, your radio systems are suspect. Your flying skills are very rusty and today’s airplanes with their more powerful motors/engines will make things happen very quickly. In this case, you might want to consider getting an RTF Basic Trainer like the Alpha 40 with the 2.4 GHz radio system or Tower Hobbies Trainer with the Futaba FASST radio system. The PTS airplanes also fit well here and will grow with the returning pilot.

How Good Are These ARFs?

“OK, so these things exist. Just how good are they. Most ARFs I remember were pretty bad flyers and difficult to repair and initially expensive.” Happily, things have changed. Most of today’s ARF and RTF airplanes are built just as most modelers would have built them from wood kits. The fuselage frames are made from strong, light weight plywood. The formers are not solid wood but 1/4-inch plywood framing. The plywood fuselage sides have giant lightening holes.

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The wing structures are balsa or light weight plywood ribs over hardwood spars. Many ARF airplane wings still use the layered plywood spar joiners from the old days. However, increasingly more ARFs and all RTF aircraft use either a steal rod or aluminum tubing instead.

Both the wing and the fuselage are jig built so they are straighter than we could make them. Most use either MonoKote® or UltraCote® coverings that are easily matched. Many ARFs list the covering colors in the photo instruction book just in case any repairs are ever needed.

In short, today’s ARF and RTF airplanes are built better, stronger and lighter than 90% of modelers could do themselves. In addition, they are extremely inexpensive considering the work involved. An RTF Tower Hobbies trainer costs under $300 ready to fly. Other RTF trainers cost less than $400 complete but have either more sophisticated radio or power systems.

The most expensive electric-power trainers like the NexSTAR EP or the T-34 Mentor are just over $500. The NexSTAR EP includes the auto stabilization system and the T-34 can grow with the pilot into a true scale performer. Read the reviews on these airplanes for more complete details

If you have not already, I strongly suggest trying an ARF or RTF airplane depending on your situation. But if you simply must build your new trainer, SIG still offers the LT-40 wood kit. Balsa USA offers the wood kit Swizzle Stik and Goldberg still offers the Eagle trainer. All are great airplanes. But when you build them, be sure to install one of the best improvements in RC low these many years.

Dual Aileron Servos – Don’t Miss Out

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What is that improvement? Remember that center aileron servos used to control both ailerons in all the older airplanes? It worked well when extra servos were both heavy and expensive. But today’s servos are neither.

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Many modern airplanes use two aileron servos. Each controls one aileron. There are several advantages in this system:

1) Any “slop” or play in the linkages is eliminated. Aileron control is precise and aileron trim is exact.

2) Using a “Y” cord, even four-channel transmitters will work with this system.

3) If the servo arm is offset slightly towards the wing’s leading edge, the ailerons will move further in the “up” direction than they will downwards. This helps eliminate adverse yaw, especially on flat-bottom wings.

4) If a computer transmitter is employed, the servo arms can be centered and the transmitter travel adjustments used to create the aileron differential needed to prevent adverse yaw.

5) The same computer transmitter can be set to “drop” the ailerons as flaperons, slowing the landing speeds and making pin-point landings a lot of fun. The flaperons can also be set up as those on a Control Line Stunt airplane making the elevator more effective and allowing tight square loops and awesome inverted spins/blenders.

6) High performance airplanes such as Pattern and IMAC scale aerobatic aircraft benefit from reflexing the ailerons upwards as spoilers in high-wind conditions. These light wing loading performance aircraft become easier to land where and when required using the spoilers.

Wing Improvements

Remember when scale aircraft were heavy and would “snap out” given the slightest excuse? It turns out that using full-scale airfoil information to design model airfoils was not the best thing. The poor flying characteristics that resulted used to be called the “Scale Effect”. Now we know that the Reynolds Number, the number of air molecules that it takes to cover a particular airfoil one layer thick, does make a huge difference.

Airfoils that were gentle when used on full-size aircraft would become real killers when reduced to model size. Several companies and universities have done, or sponsored, research on airfoils in our size range. The result is a whole new series of airfoils that have, among other changes, their center of lift farther forward, remain thicker throughout the wing chord and use blunt trailing edges for stability.

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If you have not flown them yet, you will be amazed at how gentle and forgiving even large scale warbirds are with these revised airfoils. Even sport airplanes have become true performers without any difficult handling problems.

If you doubt the effect the new airfoils have had on model aviation, look at photo 20. Click on it to expand it. Notice how high the nose is? This 16 pound plus P-47 is holding a positive attitude of about 25 degrees as it flies by the camera. The airspeed is less than 15 mph. Can you imagine any big scale fighter from the 1980’s or early 1990’s doing this without stalling, hard?

Of course you can’t. But this airplane just cruised by needing nothing but a little “up” elevator and throttle to slowly fly by the camera. The flight path was rock solid and needed no other corrections or control inputs.

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Another change has helped improve our airplane’s flying abilities. Look at the stabilizer area on the Ugly Stik pictured in photo 21. Compared to the wing area, it is fairly small and has a low aspect ratio (span to chord ratio). Note the increased stabilizer area, and higher aspect ratio, on the modern Cherokee in photo 22. Today’s airplanes have much larger stabilizers compared to years ago. The larger stabilizer areas and increased aspect ratios provide increased stability and give even the flat stabilizers a little more lift.

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While increasing a flat stabilizer’s lift is good, using an airfoil stabilizer is even better. An airfoil provides true lift that lowers the wing loading on the entire airframe while increasing stability. Most scale airplanes today use airfoil stabilizers and vertical fins as shown in photo 23. All the older Top Flite fighter designs for example, have been converted from flat tail surfaces to airfoil ones. As a result, their handling has remarkably improved.

The moral of these little adjustments? Don’t be afraid of any scale ARF you want because it may appear to be a daunting task to fly. Airplanes designed in the last 10-12 years fly like pushing a baby carriage. Even the A-26 Invader or the B-25 Mitchell fly like Advanced Trainers. Well, they do as long as both engines are tuned and working.

Engines

We’ll keep this section brief. Today’s glow engines are more powerful and reliable than those of 12-20 years ago. Their idle mixture adjustments are more defined and effective. Properly tuned, a .46 glow engine running on 15% nitromethane and 18-20% oil fuel should idle all day at no more than 2,300 rpm on an 11 x 6 in. propeller. That might have always been the goal, but now that goal is practical.

Power output is also surprising. An O.S. Max 46 AX, running on the fuel detailed above, will reach ~11,700 rpm on an APC 11 x 6 in propeller. Expect above 12,500 on a 10 x 6 in. That is about 1,000 rpm higher than the 1995 O.S. Max FSR version.

Even more surprising is the near absence of 60-size airplanes today. There are a few sport airplanes in that size such as the Pulse 60, the 4-Star 60 and the Tiger 60. These are 70+- inch span airplanes about 10% larger than what were known as 60-size, (64 inch span), airplanes twelve years ago. That is because the engines produce about 10% more power now than then. Have you ever known an airplane designer to not utilize all the available power?

The older 64-inch span airplanes are powered today by the new crop of .52 to .55 sized engines. The good thing about this displacement class is that most of them are packaged in 40-sized crankcases. The power to weight ratio of these engines makes the old style 60-engines nearly obsolete.

Four-Stroke engines are still popular but are more reliable, less trouble prone and just a bit more powerful. They still sound great but use mufflers now to be extra noise-friendly. An extra benefit from their popularity is that just about all fuel manufacturers now make all-synthetic oil fuels designed not to foul these engines. Such fuels usually contain 20% nitromethane. Experience has shown that the 5% nitromethane increase eases transitions from idle to high power and stabilizes idles in the 2,100 rpm range.

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While four-stokes remain popular, small engines in the .20 and under range have all but disappeared. That is due to the rising popularity of electric-power. That is a story unto itself and too long for here. The two airplanes pictured above would have been powered by the Cox TD 0.51 twelve years ago, but not now. Check Sport Aviator’s Flight-Tech section for several articles on electric power. Articles cover motor/ ESC (speed control)/battery selection, battery types and their maintenance

Look over the electric-power airplanes listed in the On The Flight Line and Park Pilot Sections. Each review details the electric power information including motor, propeller, ESC and performances. There are a lot of surprises to, and even more useful information for, a returning modeler about this new form of propulsion.