What Makes A Great Tow Plane?

by Len Buffinton

In the sailplane world, tow planes are commonly referred to as TUGS. The primary job for this aircraft is to tow, or tug, their long winged silent partners into the skies above. Without the usefulness of these servants, there would be no “towing” in the phrase “aerotowing”

On the radio control market, there are many planes to choose from, therefore, one needs to be able to wade through all the possibilities to arrive at the perfect Tug.

What to consider?
Flying Characteristics

Size of the Tug, 1/5 scale, 1/4 scale 1/3 scale 1/3.5 scale and everything in between or even a

larger
Power for the Tug: Gas, Glow fuel, Electric.

What about the style?

• High Wing

• Mid Wing

• Low Wing

• Shoulder Wing

• ARF (Almost ready to Fly) or Kit

• Tail Dragger or Nose Wheel

• Scale or Non Scale appearance

These are all factors in deciding which plane will become your TUG. Let’s take a look at these considerations and see how they affect your choice.

Flying characteristics,

In reality, you could tow a sailplane into the air with just about anything with a motor, but you might not want to.

You could hook up a tow release onto the turtle deck of a Pitts biplane for example. But again, you might not want to.

1) It will require a long fast takeoff roll,
2) It will fly fast and be very sensitive in flight
3) The sailplane pilot will have a difficult time staying in position behind the plane.

Due to the flight Characteristics of that plane, you may want to choose a more stable platform. When looking for a Tug, think about what you are doing with that plane. Your primary JOB, is to pull the sailplane up to altitude. A strong motor, slow speeds and flying characteristics with stability in pitch and roll are features you are looking for.

After getting to altitude, your next concern is getting it back down to the runway, fast, after all, there is another sailplane waiting to be towed. Having flaps on the Tug is a useful option, you would drop the flaps to full deflection upon release, and then point the nose of the Tug toward the ground and dive for the runway.

Size or Scale
Model airplanes are scaled to a size, 1/5 1/4 1/3 etc.
Quarter Scale ( 1/4 ) is a very common size today. This means the plane is one forth the size of the real plane. The wingspan of the model, if multiplied by 4, would be the size of the real plane. Of coarse, you can reverse that formula and divide the real plane’s wingspan by 4 to get the size the model would be.

What size do you need……. or Want?

The most common mistake made in aerotowing is under-sizing the Tug. If you plan to only tow foam sailplanes and 2-meter gliders, then a Senior Telemaster type model is a perfect choice. But if you plan to tow larger sailplanes, possibly 4 meters or more, you will need a larger, more powerful tug.
There is a saying in models that bigger flies better. This is true, and therefore the tug you choose should be as large as you can accommodate. Transportation ability and budget will help determine the size you choose. A plane with a larger wingspan and overall size is easier to see up high.

Wingspan
As mentioned above, the longer the wingspan, the easier it will be to see up high, after all, that’s where the sailplanes want to go.
The type of plane is a personal choice. You can choose between high wing, low wing or mid wing style planes. Over the last couple years, most of the tugs I’ve seen at the various aerotows are high wing type.
Some of the advantages of a high wing design are:
1) Stability of the tug.
2) Many different models available
3) Easy setup at the field.
4) Flaps are very effective.

 

Low wing aircraft also work very well, but are typically flown in a more scale fashion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mid wing/shoulder wing designs are another choice, these are usually very powerful, workhorse type planes, somewhat faster and less stable than the others.

Once you have chosen your plane, its time to choose your propulsion.

What type of motor ?

This is an important decision you have to make. Think about where you fly, can you use gas and glow, or is it an electric only field?
Advantages and disadvantages of each type:

Gas
Advantages:
Cheap to operate, Clean, easy to start, no glow starter or field box required, strong and powerful motors available, many size options, simple to repair, dependable, can be muffled to operate quietly and they don’t need a rest or recharging.
Disadvantages:
Can be expensive to purchase, depending on the brand and size, Usually a little heavier than glow motors, vibration, noise if not properly muffled, can not be used in some locations due to noise and gas exhaust on the lawn ( sod farms )

Glow Fuel Motor
Advantages:
Most R/C pilots have at least one glow powered plane, they are familiar with how they operate and have all the field equipment already. There are many sizes and styles available, 2 stroke, 4 stroke, even Radials and Twins. They produce very little vibration and require little maintenance.
Disadvantages:
Glow fuel is expensive. The cost to operate the motor for a full day of aerotowing will be high. The motors are not as dependable as Gas or Electric, there is equipment needed to start them, very messy and need to be muffled well.

Electric
Advantages:
Quiet and not damaging to the surroundings. Many sizes available, Powerful, Clean, Cheap to operate.
Disadvantages:
Expensive to purchase, requires large batteries, which are also expensive to purchase and can only get a limited number of tows before having to re-charge the batteries.

If you have the option to use a Gas powered plane to tow, that would be the most practical choice. You set it up, get it running and forget about it.. The motor will run all day on a gallon of fuel, requires no rest, and provided smooth dependable operation. Once properly adjusted and broken in, a gas motor will start on a flip or two.

If you have field restrictions, electric may be the way to go. The advance of electric flight has been nothing short of astounding in the recent years. Battery technology has produced lighter and more powerful batteries that can be charged in a fraction of the time once required. Motors are being made which are in some cases, more powerful than some of the best gas motors. BUT, and there is always a BUT, the fact remains, they are limited on the number of tows before recharging. This is the main drawback of electric powered TUGS. If you only plan to do a few tows, then this is a viable option. On the other hand, if you plan to host an aerotow event and have pilots come from all around the area to fly at your field, you better have a gas tug. The quickest way to take the fun out of flying sailplanes is to have a line of eager pilots waiting for a tow while you’re charging the batteries, or for that matter, even changing out the batteries every 5 flights.

Whichever you choose. You need to put in more power than you would for just sport flying. You may not need the extra power on every tow, but there will be a time when you do, so you better have it. If a particular plane calls for a .60-.90 size motor, you will want to put a 1.20 in it for towing. Remember, its not only the TUG that’s at risk if you don’t have enough power to get the sailplane up, its also the sailplane.

Once you have decided on the plane you want as your tug, you need to outfit it with a tow release.

A tow release consists of mechanism which the tow line will be attached to. This mechanism will have a servo attached to it, which when activated, will pull a “pin” and release the towline. These releases are available commercially or you can build one yourself. Here are some typical commercially available tow release’s. The Tug release is much different that the one for a sailplane.

The sailplane release will normally be fitted into the nose of the glider, where as the Tug release will be placed on top of the fuselage, near the trailing edge of the wing.
It is most common for the tow release function to be operate from either the retract gear switch on the transmitter or a push button switch near the same location. This switch needs to be easily reached and activated, therefore, it’s not advisable for the tow release to be assigned to a “slider switch”.

Both the glider and the tug will have a release. Each pilot is responsible for their respective planes. Its up to the tug pilot to protect the tug, and get it back in one piece, and the sailplane pilot is responsible for his sailplane. With each pilot having a release on their plane, if something goes wrong, either pilot can “dump” or release the towline.

Although the only modification which is mandatory for turning a plane into a TUG is the tow release. the following is a list of options you may want to consider to make your plane an effective and trouble free towing machine.

Larger fuel tank

Due to the larger motor being installed, you will want to increase the size of the fuel tank. If you are towing multiple times in a row, you will not want to keep stopping to fuel. Put in a larger tank and keep the glider pilots happy.

Strengthen the landing gear supports
Your tug will take-off and land hundreds of times, far more than was ever intended in the design of the plane.  Very few airplanes are sold with the understanding they will be used for aerotowing. Because of this, the landing gear area should be strengthened. The typical landing gear supports would be perfectly fine for every day sport flying, which might see a half dozen landing a weekend. Over time this area will cause problems if not reinforced. Remember, in aerotowing, take off and landings are pretty much all you do. Having a little more strength in this area now will save you time and aggravation later, especially when something happens in the middle of the day, with guys waiting to be towed.

 

Larger Wheels

Scale in this meaning, refers to the fact that it resembles a real airplane. A model of a Wilga, for example,  is referred to as a Scale plane.
Most sailplanes which are aerotowed are Scale sailplanes, copies of the real plane. With the tug, you have the choice of getting a scale tug or a plane designed to just tow sailplanes. (picture of Peg) These purpose designed tugs are very effective, with all the options and modifications already considered in the design. They will have a large engine mount, heavy gear support, large tank area, flaps, and be built to take the pounding given over the use of the plane. Most of these designs require you to build them from a kit or more commonly, scratch built from plans.

Now it’s time to do it. Weigh the options, choose the plane, and create a tug.

See you at the field.