Notes on Kite Aerial Photography: Equipment

Radio Control Systems
Notes from Peter Bults


For two years, my equipment section menu page promised that a description of radio control devices was being prepared. Peter Bults finally stepped into the breach to provide the introduction to radio control basics contained on this page. I am in his debt. The reader might also search the WWW for other resources describing R/C radios. For instance, the Great Hobbies WWW site provides a brief primer.

The page is illustrated with images from my workbench.

Cris Benton, September 1997

Addendum: In 1999, I received a communication from Steve Kaluf, Technical Director of the Academy of Model Aeronautics (AMA), expressing concern that kite aerial photographers might be using frequencies assigned to model aircraft. The full text of Steve's letter(s) can be found in the KAP Discussion Pages. Now that this concern has been raised, the most prudent course in the USA would be to use the 27Mhz channels per AMA's recommendations. You can find further discussion on the KAP Safety Page.


Peter's Notes:

This page tries to explain the R/C basics to a novice in radio-controlled KAP. I hope it tells you everything you need to know when you start building your first R/C rig.

Radio control

Most KAP-ers use some kind of radio control. It allows you to shoot pictures without taking the camera down every time.

But before thinking about radio control (R/C) take a look at your KAP camera. Has it motordrive? If not, it's hardly worth bothering about R/C. Has it a posibility for an electronic external shutter release? Great.

An Airtronics Vanguard 4-channel radio transmitter (left, 21K jpg) and an Airtronics Infinity 6-channel radio transmitter (right, 16K jpg). The Vanguard is an entry level radio with a very reasonable price - approximately $160 buys the transmitter, the receiver, four servos, ni-cad batteries for receiver and transmitter, battery charger, and assorted accessories. The Infinity radio has computer features for trim, mixing, and the support of multiple models (or in our case camera cradles) and is shown with a flexible "rubber duck" antenna. Standard aircraft radios provide excellent function for kite aerial photograph applications. Their ergonometric design is  suboptimal as they assume two hands on the radio.

What you need is a 4-channel R/C set. If you buy a system with less than 4-channels one day you'll miss something. But the good news is that a simple (cheapest) system works fine for KAP. And don't forget to try to find a second hand one. Real model pilots, sailors and drivers tend to buy a more sophisticated, more computerized system after a while and sell or trade in their "simple" sets. And that's good for us. If you buy second hand or new seperate components it good to know each manufacturer has its own type of connectors. They look comparable but convince yourself the servo connectors fit into the receiver.

Warning: Some manufactures (like Graupner) sell their 2-channel systems as 4-channel. What we need is a system capable of controlling 4 functions (servos). The Graupner 4-channel activates only 2 servo's. But in their way of counting up/down and left/right makes four as well.


A radio set's airborn components. Your camera cradle would carry a battery, a radio receiver, and up to four servos for the functions of plan rotation, tilt, horizontal/vertical format, and shutter release (27K jpg).
When assembling a radio control set your shopping list should look like this:

Note that most new radios are sold as sets that include all of the items on the list.


Batteries:

Batteries are often the most neglected items of radio control. But I advise you to see them as the heart of it all.

The power consumption of RC systems is rather high for batteries. So the use of dry alkaline batteries must be discouraged. They are relatively expensive and not very friendly for the environment.

Examples of battery packs for the airborn receiver. Receivers can operate with either 4.8 or 6.0 volt battery packs. Peter prefers the use of individual batteries in a holder (far left) while I prefer preassembled battery packs like the ones on the right. (23K jpg).

The least expensive, but good and reliable, power supplies for KAP applications are Nickel-Cadmium (Ni-Cd or NiCad) rechargeable penlights. You can charge them with a normal household battery charger. But they take 14 hours to do so for just 4 or 8 batteries!

Buy 12 new penlights and label 4 of them as RX1, RX2, RX3 and RX4 or something like that. Mark the other 8 as TX. The purpose of this is that you use these cells only for KAP and keep the batteries as a receiver set and a transmitter set. Charge the TX cells as soon as the indicator on the transmitter says to do so. The receiver batteries need to be recharged when the airborn system stops responding to the transmitter.

Taking good care of your batteries will that they last long and reliable. If you're in doubt buy new ones and use the other for a less demanding application

The new Ni-MH cells have a greater capacity (1100mAh instead of 600 or 750). And of course they cost more and need a charger that supplies a higher current.

Battery packs are easier to use, but again more expensive and each pack (4.8V and 9.6V) need their own charger.

The simple battery charger included with most radio kits (left, 17K jpg) can simultaneously charge the receiver and transmitter at a relatively slow rate -- around 14 hours. More advanced chargers offer discharging circuits, peak detection, and rapid charging. The field charger shown (right,33Kjpg) can simultaneously charge transmitter and receiver battery packs in only 15 minutes. It is powered by any 12 volt DC source (I use an automobile cigarette lighter socket).


Servos:

Although there are servos available in many variations KAPpers can do all with the "Standard" model. These servos are rather cheap (search for special offers; 3 servo's for the price of ..........). They have enough power (torque) to turn the camera and the rig. Their only disadvantage of standard size servos is weight. They bring 40 to 45 gr to the scale.

Examples of standard size and micro size servos. Servos (for servomechanism) come in an extraordinary range of sizes and torque outputs. A well-balanced rig will need far less torque than one that is out of balance. A poorly balanced rig design will also drain battery packs quite rapidly (19K jpg).

Micro-servos reduce the weight of the rig and your purse. The weigh 17 to 20 gr and cost about twice the price of a Standard-servo. On a well-balanced P&S camera-rig it is worth to use only Micro's. On a SLR-rig I would use the Standard-servo's. It looks better and the relative gain in weight is small. Only the shutter release servo is a Micro in this case.

In model aircraft it is good use to fix the servos more or less flexible into the frame. That why you find some rubber things in the small parts bag. These are not necessary in a decent KAP rig.

For KAP use, of the Standard servos I know of I particularly like the Acoms AS-12. It has 3.2 mm holes allowing you to tighten it to the frame with M3 bolts and nuts. Many others have 4 mm, partly open, holes. Furthermore the screw in the out-going axle is standard M3 instead of some obscure parker thread others tend to use. That makes it easier for me to fix gears to it.


Electronic switches for shutter release:

If your camera has the possibility of connecting a external electric shutter release do use it. The electronic switch is lighter, more reliable and consumes less energy.

Read the camera's manual carefully, or ask an expert, to explain how the external electric release works on your camera. Older (designed between 1985 and 1990) electronic SLR's, like the Ricoh KR-10M have a simple Off/On button. But, I have the impression that on most AutoFocus p&s camera's and most modern auto-everything SLR's you'll find a two-step shutter release. The first step activates the messuring systems, the second step releases the shutter.

Some of these 2-step systems seem to work well triggered by a single step switch. But I prefer to respect the camera design.

Most R/C suppliers can offer you mono, duo, mono memory and duo memory switches. The ones with memory should be avoided by KAPpers. The mono switch works well for cameras like the Ricoh KR-10M. The others will work with the duo type.


Servos for 360 degrees of plan rotation:

This is the most challenging issue for a beginning KAper implementing a radio-controlled rig. This is the only point where you must make modifications to the standard R/C equipment. It is also a point where you have to make an "important" decision.

Normally a servo only turns 90 . For the panning of the rig we need at least 360 .

Now there are two posibilities:

1. Endless rotation
2. To-and-fro rotation

The first option means you have to open and modify the servo. If you don't have the convidence that you can take the operation to a good end chose the second. But there is another reason why I think you should go for the second option. With to-and-fro your joystick will give an indication of the direction the camera is pointing at. That is if you know the direction with the stick in neutral, which can be straight forward as an example. With endless rotation you'll have to look at the rig to find the right direction. And this can difficult with the rig up at 150 feet unless you have very good eyes.

All you need for to-and-fro rotation is set of gears with a ratio of 4:1. Fix the big one on top of the servo while the smaller one should become part of the camera cradle's vertical axis. Notice that this construction speeds up the turning. And a servo is already fast, so move the joystick with care.


Crystals:

These parts of either the transmitter (TX) and the receiver (RX) are important when you want to fly and photograph near other users of RC systems.

Within each radiofrequecy (27mhz, 35mhz, 40 mhz .....) there are several fine-tuned bands. For example in the 40MHz you'll find 40.715, 40.725, 40735 and so on. This allows you and a fellow KAPper to both have a 40MHz TX/RXset without interfering each other's rig. Just make sure you have different bands - or in other words a different set of crystals. If you buy a transmitter/receiver, especially second hand, make both crystals are present and have the same MHz number.



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