Bob Younts Principals of Flight

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THE DCRC RAIDO CONTROL AIRCRAFTTRAINING PROGRAM

COVERING PRINCIPLES OF FLIGHTEQUIPMENT SELECTIONAND CONSTRUCTION

2003-1April 2003


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INTRODUCTION TO RC AVIATION

THE DISTRICT OF COLUMBIA RADIO CONTROL CLUB (DCRC) WANTS TO HELP YOUENJOY THE PLEASURES OF BUILDING AND/OR FLYING RADIO CONTROLLED

AIRCRAFT.

DCRC HAS BEEN IN CONTINUOUS OPERATION SINCE 1953. THE INITIAL ANDSUSTAINING PURPOSE OF THE ORGANIZATION HAS BEEN TO FOSTER INTERESTIN THE HOBBY AND PROMOTE TECHNICAL ADVANCEMENTS RELATED TO IT.

THIS PROGRAM IS INTENDED TO PROVIDE AN OVERVIEW OF RADIO CONTROLLEDMODEL AVIATION. IT CANNOT, HOWEVER, IMPART THE KNOWLEDGE ACQUIRED BYYEARS OF EXPERIENCE; THAT TAKES TIME. THERE ARE PUBLICATIONS THAT GOINTO GREATER DETAIL ON MANY OF THE SUBJECTS DISCUSSED HEREIN. OURMENTORS AND MOST EXPERIENCED MODELERS ARE HAPPY TO PROVIDEINFORMATION AND ASSISTANCE TO OTHER MODELERS AND FLYERS.

THERE ARE THREE POINTS WHICH MUST BE STRESSED:

1. SAFETY IS OF UTMOST IMPORTANCE BECAUSE IT IS QUITE POSSIBLE TOLOOSE FINGERS, EYES AND EVEN LIFE THROUGH CARELESSNESS.

2. READ INSTRUCTIONS FOR ALL OF YOUR EQUIPMENT. MOSTMANUFATURERS CAN TELL YOU HOW TO GET THE BEST SERVICE FROMTHEIR PRODUCT.

3. YOU CANT TEACH YOURSELF TO FLY. THAT IS WHY DCRC HAS A FLIGHTTRAINING PROGRAM. TAKE ADVANTAGE OF IT!

PREPARED BY BOB YOUNTMember of DCRC Flight Training Staff


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PRINCIPLES OF FLIGHT

There are four fundamental forces acting on an airplane in flight (fig. 1). They are lift, weight,thrust and drag. When these forces are in balance, the plane is in straight and level flight ata constant speed. Changing any one of these forces causes a change in the flight path: if lift

is reduced the plane descends.

Figure 1. Forces in flight

To fly there must be some way to exert a lifting force on the flying object. So, our first orderof business is to examine lift.

Bernoullis principle states that in a fluid (liquid or gas), pressure and velocity are inverselyproportional. In other words he found that the pressure of a fluid decreases at points wherethe speed of the fluid increases (fig. 2).

Figure 2. Flow of air through a Venturi tube.


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Compare the Venturi to the shape of the airfoil (wing cross section shape fig. 3), and thereduced pressure on the top of the wing is apparent. For many years it was taught that thisis what provided enough lift for flight. If Bernoullis principle is what makes planes fly, howdoes an aerobatic plane having a symmetrical airfoil fly?

Figure 3. Airfoil showing Venturi effect.

Angle of attack is also needed to produce sufficient lift for sustained flight (fig. 4).

Figure 4. Differences in pressure produce lift.

Defining angle of attack requires an understanding of the term relative wind. Relative windhas nothing to do with the relationship between the ground and the plane. It is the

relationship between the plane and the flow of air at the wing (fig 5). Relative wind is one ofthe fundamental concepts of flight and control. Without it a plane couldnt pull out of a dive.


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Figure 5. Relative wind.

Next on the agenda of our fundamentals is angle of attack. In figure 5 the wing is alwaysgoing almost straight into the relative wind in level flight. However to initiate a climb ordescent it is necessary to change the relationship of the wing to the relative wind. The anglebetween the wing and the relative wind is the angle of attack (fig. 6).

Figure 6. Angle of attack.

Although figure 6. shows the wing in relation to a horizontal line; it is really in relation to therelative wind. For example 6-5 and 6-6 with high angles of attack are at or approaching aloss of lift or a stall. A stall can be generated when going straight down if the angle of attackbecomes great enough.

Figure 7. Angle of attack is relative to the flight path.


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With an understanding of lift, relative wind and angle of attack we know why a plane can fly.Flying is fine but it is necessary to be able to control the planes direction and altitude. Thisis done with control surfaces: aileron on the wing, elevator and rudder of the tail (fig. 8).

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Figure 8. Control surfaces

These control surfaces can cause the plane to rotate about the three axes of flight (fig. 9),which are pitch, roll and yaw.

Figure 9. Axes of rotation.


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Looking at the control surfaces and the axes of rotation, can you tell which control surfaceseffect which axes of rotation?

Remember, lift is perpendicular to the wing, not to the ground. Look at figure 10. In order toturn and at the same time maintain a constant altitude it is necessary to do something tocorrect for the lost lift.

Figure 10. Lift in a banked turn.

This correction is made by pulling back on the stick to add the necessary amount of upelevator, thereby increasing the angle of attack.

While the subject of different airfoils is complex and fills many a book, some idea of thebasic types is worthwhile knowledge. See figure 11

Figure 11


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RADIO CONTROLLED MODEL AIRCRAFT

THE AIRPLANE

As with full size airplanes, a trainer is used when learning to fly. Flying skills need to bedeveloped and practiced before one becomes a hot shot pilot. Once you are a hot shot,then pick out any airplane you desire, but at the beginning you must start with a trainer. We

will talk about learning to fly later, but for now lets look at types of trainers, materials,electronics and engines.

A recommended minimum wingspan for a trainer is about four feet. Smaller sizes areavailable but they are less stable and therefore more difficult to fly.

There are a number of different trainers and methods of construction. Until the 1980s theonly way to have a new trainer was to build it from a kit. A kit contained most of the partsand materials to build a trainer yourself (fig. 12). Usually glues and covering are separateand sometimes wheels arent included. There a re a number of good trainer kits on themarket; among them are the Goldberg Eagle and the Great Planes PT series. The

advantage of kits is that they are lighter than an ARF (almost ready to fly) and more easilyrepaired because of the knowledge acquired during the building process. If you add up thecost of the kit and the covering material you find it isnt much different from the cost of asimilar ARF. To build a trainer from a kit is going to take many hours for the beginner (80 to100 hours of building time isnt uncommon).

Figure 12. Typical kit parts.

In contrast to the kit is an ARF which has the basic construction and covering completed (fig13). The assembly takes about 12 hours, which is spent on the wing and tail assembly and

installing radio and control equipment.

Extending the ARF trend are new kits that come with radio, servos and engine assembled.Some of these can be ready to fly within one hour of opening the box if the batteries arecharged.


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Figure 13. Typical ARF.

The important characteristics of a good trainer are:A flat bottom wing which provides a little more lift and stability.The wing on the top of the fuselage for stability.Some dihedral in the wing (the tips are higher than the center) for stability.

All three control surfaces as opposed to no ailerons (called rudder only).Sufficient power.

Recognizing that the state of the art is rapidly changing, it is strongly suggested that talkswith the DCRC training staff would be helpful before choosing a model. The people in thehobby shops can also be helpful; just bear in mind that they want to sell what they have instock.

It is difficult if not impossible to learn to fly with a glider. Some type of power will to berequired. Keep in mind the plane and the power must be compatible.

POWER

Internal combustion engine sizes are designated by their cubic inch displacement, 0.40,0.60 etc. Trainers are usually designated the same way. There is nothing wrong with goingslightly oversize on the engine. For example a 0.46 engine could be used in a 0.40 sizeplane with some advantage. There are both two-stroke and four-stroke glow fueled engines(fig. 14). Both are excellent for a trainer, but the four stroke is much more expensive.


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Figure 14. Two stroke OS 46 FX engine and OS four stroke

Glow fuel is a mixture of lubricant, nitromethane and alcohol. A 10% fuel is 10% nitro, 18%to 20% lubricant (synthetic oil and/or castor oil), with the balance being alcohol.

There are several factors to consider in selecting a power plant. ABC (aluminum, brass andchrome). The piston is aluminum and the cylinder sleeve is chrome-plated brass. Theseengines dont have rings and break in faster, plus they are usually a bit more powerful. Most

ABC engines have ball bearings (BB) rather than sleeve or bushing bearings. As usual,better costs a bit more.

Electric power is becoming more popular as the battery technology improves, but at this

writing electrics dont have the needed flight time to shoot many approaches when learningto land. New polymer technology may improve electric flight time. Repeated flights requiremore than one set of batteries. Fly on one set while the other is charging from the carbattery.

Some of the newer engines have the high-speed needle valve located toward the rear of theengine, which keeps the fingers out of the propeller. The OS brand is proven, and a specialnod goes to the FX versions, but there are other good brands. Soliciting comments fromflyers cant hurt in making a selection.

MATERIALS

Most models are constructed with a combination of balsa (a soft lightwood), plywood(different thickness and quality), and hardwoods (spruce and pine). If building from a kit, youwill find these materials in the box, but with ARFs, they are already in the constructed plane.

Whether a kit or an ARF, you will need different glues. Epoxy comes with different workingtimes (5, 15, 30 minute for example). Generally the longer the working time the stronger thebond. CA (cyanoacrylate) is like crazy glue, very fast curing and very strong. CA comes inseveral thicknesses for different applications. Whatever type plane you select, you will need


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both epoxy and CA, if not for building, for repair. Experienced builders may use ambroid,casine and other specialty glues. It is a good idea to have debonder around when using CA.

COVERING

Any trainer will be covered with some kind of film, and with ARFs this can be important. The

most common coverings are MonoKoteand Ultracoat. These are Mylar materials with a

heat sensitive adhesive on the back and a special iron may be used to attach the material.Both are also heat shrinkable, that is the material will shrink, eliminating wrinkles, when aheat iron or heatgun is used.

Figure 15. Covering tools.

About the coverings for ARFs, some use a material that neither MonoKotenor Ultracoatwill adhere to. Consequently, if the covering tears it can only be patched with clear tape, orworse, stripped and recovered.

Some more advanced planes may be sheeted with balsa, and then covered or painted.There are also aircraft made from foam which has the color built in.

ELECTRONICS

The radio control system consists of a transmitter (fig. 16), receiver, batteries, servos, switchand possibly a charging jack. The transmitter sends a signal to the receiver which interpretsthe signal and sends inputs to the servos (servomotors) that cause the controls to move.Power to operate the receiver and servos comes from a battery pack that is carried in theplane. The transmitter, in most cases, has two control sticks to cause the signals to be sent.The right hand stick controls the aileron and elevator while the left hand stick controls therudder and throttle. There are also trim adjustments next to each of the sticks to make slightadjustments to the controls.

The various parts of the radio system are tied together with connectors, male and female,that are keyed so that you wont make improper connections. The receiver antenna can bepulled from the receiver if too much pull is exerted, and the antenna on the transmitter canbe bent.

Take care of the radio equipment, it lets you keep the plane in the air.


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Figure 16. Four channel transmitter with reversing switches on the front.

For a beginner, a most important selection factor is the need for your radio to be compatiblewith the instructors buddy box. A buddy box provides for a dual control system thatenables the instructor to take control and avert an impending crash. Here again, it isnecessary to talk to instruction personnel to get up to date information.

Obviously the onboard battery is being used up while in flight. To make a number of flights,it is a good idea to have a charging jack for recharging without taking the wing off.

A minimum of four channels are required to control aileron, elevator, rudder and throttle.Additional channels and sophisticated programming capabilities arent needed for a trainer,but could be used in the future.

Batteries which are NiCads (nickel cadmium) and a charger will come with a new radiosystem. Of course the batteries must be fully charged when you set out to fly. Generally thisis achieved by overnight charging using the charger that came with the radio system.Battery capacity is labeled in milliampere hours, (ma) also known as C. The onboardbattery has a capacity of 500 or 600 ma at 4.8 volts (1.2 volts per cell for NiCads).

With proper care you can expect long battery life. The charger that came with radio will

charge at a rate of C/10; at this rate the battery wont be overcharged even if left on for acouple of days. Other chargers using rates in the area of C/4 will charge in a few hours, butcant be left on for more than what it takes to reach full charge. These chargers should havethe capability of detecting peak charge and then switching automatically to trickle.

Brand new batteries usually wont reach full charge until they have been cycled two or threetimes. There are chargers on the market that cycle the batteries by draining the to about 1.1volts per cell and then switch to charge mode.


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To determine the condition of your batteries, an ESV (expanded scale voltmeter) is usedand is an excellent piece of equipment to have in your field box.

CONSTRUCTION

READ THE INSTRUCTIONSthat came with the plane; then READ THE INSTRUCTIONSAGAIN.With a kit or ARF you should be thoroughly aware of what you are going to dobefore starting.

Starting with an ARF, these are the things to do, but the order should be as listed in theinstructions.

Assemble the wing halves.Glue, or screw, on the tail assembly.Install engine.Install control servos.

Install control linkage.Install the radio receiver but hold the battery out.Balance to place CG properly.Set control throws.

WING

The center joint of the two halves of the wing is subject to a lot of stress when pulling out ofa dive, which will happen at some time or another. One type of wing joint consists of awooden, or wood metal laminate, dihedral brace inserted about 4 into each wing half usingan epoxy glue. If that is the case, it is strongly suggested that a 2 piece of fiberglass tapebe epoxyed around this joint. ARFs that have a long metal tube which inserts into each winghalf wont require the fiberglass treatment.

TAIL

In attaching the tail assembly care must be taken to insure proper alignment. Theinstructions should tell how to do what is required, if not get help.

TANK

When assembling the fuel tank, dont let the pickup line weight (clunk) touch the back of thetank. Make sure the fittings and top of the tank are tight. You will never get the engine to runright if there are leaks in the fuel system.

At some point before adding the fuel tank and engine mount, fuel proofing should beconsidered. Polyurethane, clear or colored, spray paint works well to protect the firewall andsome people use it in the area of the fuel tank so spilled glow fuel doesnt eat up the plane.

CONTROLS

Several methods of fastening the control surfaces to the wings and stab are available, butwhatever hinges are used, a few fundamentals apply. Hinges:

1. Must be aligned properly.2. Must be securely attached.


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3. Must be free of interference.4. The gap between the control and fixed surface should be kept to a minimum.

There are several different types of hinges used (fig. 16).

Figure 16. Hinges

SERVOS

When securing the servos to their mount, be careful that the starting holes for the mountingscrews are not too large. Since the servos transmit power to the control surfaces, theyshould be securely attached. Also care must be taken to make sure there is not any


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interference of the motion of the servo arms. A final care is to make sure the motion of theservo arm is perpendicular to the direction of throw, otherwise the motion of the controlsurface wont be the same in both directions.

CONTROL HOOKUP

The installation of the connectors and linkage between the servos and the control surfaces

(fig. 17) will take a bit of time to get right, but time well spent. Whatever connections areused, they must be secure to prevent loss of control of the plane.

Figure 17. Typical radio and servo installation.

Some of the various clevises, connectors and linkages are shown in figure 18.

Figure 18. Connectors and linkage.

For methods of connecting the control surfaces to the servos see figure 19.


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Figure 19. Control connections.

CONTROL HORNS

Control horns are mounted on the elevator, rudder and sometimes on the wing. It isimportant that the holes in the horn are even with the hinge line, otherwise unequal throw iscreated.


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LANDING GEAR

Install landing gear in accordance with instructions, both when and how. If you will be flyingoff a paved runway, you dont want a lot of throw at the nose or tail wheel.

ENGINE

Generally engine mounting per the instructions is fine. If, however, the engine is to be

mounted to its mount with screws, as opposed to bolts, a change in procedure is suggested.Drill the mount through and use bolts for engine attachment. The use of either lock washersor locking nuts that have a plastic insert is recommended. This is a good time to think abouthow to prevent nuts and bolts from loosening from the vibration produced by the engine.

The use of Locktiteon all metal fasteners will keep things together, but use the one withthe blue, not red, fluid. This certainly applies to the muffler and its attachment bolts.

RADIO

The radio receiver installation isnt difficult, but it should be protected with foam and securedin position. Dont, however, put in the battery at this time. We will use it in the next step tohelp get the center of gravity (CG) in the right place.

PROPELLORS

Different size engines require different propellers (props) which are designated by twonumbers, diameter and pitch. A typical prop for a 46FX might be a 10x8. The 10 is thediameter and the 8 is the pitch. Diameter is straight forward and pitch indicates how far theprop would advance in one turn if it were a screw.BALANCE

When checking the CG, the fuel tank should be empty. For a trainer a little forward CG ispart of the design to provide for stability. A plane with a rearward, or aft, CG will be veryunstable and difficult to fly. It is critical that the CG be located where indicated by the kitmanufacturer. This is usually between 25% and 33% of the chord length for a non-tapered

wing. At this point the plane is complete and the wing is attached. Put the battery on top ofthe wing, and using a finger under each wing at the designated balance point (fig 20), lift theplane slightly. If it isnt level, move the battery around and see if it can be balanced that way.If that makes it balance, try to locate the battery in the fuselage exactly below where it was

Figure 20. Checking the CG


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on top of the wing. If unable to balance the plane by adjusting the battery location, it will benecessary to add weight somewhere to get it balanced. Just remember, the longer themoment arm the less weight that will be needed

.Now with the battery in the plane and the radio on, it is time to check both the direction andthe amount of throw at each control surface. Follow those instructions again. If the direction

of throw is backward, there is a reversing switch somewhere on the transmitter that willsolve that problem.

All of the proceeding applies to both ARFs and kits, but when building a kit many moresteps are involved. Careful adherence to the kits instructions will work for the most part.Covering, however, requires some tools, a special covering heat iron and heat gun.Covering is an acquired skill. So, if you cant get help, practice on mockups or somethingbefore attacking your plane.

GROUND SUPPORT EQUIPMENT

When the plane is finished, it is time to head out for flight instructions, and there are things

you need to have at the field:

Mandatory itemsFuel (10% suggested) is needed.Fuel pump to get fuel from bottle to plane.Glow plug igniter.Chicken stick or electric starter.Rubber bands (for most trainers)

Suggested items:Spare prop(s).Spare glow plugTools to fit all fasteners on the plane plus #1 philips and small straightscrewdriver. Also, glow plug wrench, prop nut wrench.

Other useful items are ESV meter, field charger, 12v power and panel, electric starter, glues

and a sharp blade such as an Xactoknife.

During the course of breaking-in and using an engine problems may be encountered. Thechart on the following page is provided to assist trouble shooting and correcting problems.The chart assumes the plane has be fueled and the glow plug igniter functions.

POST FLIGHT

All fuel should be removed from the plane and run out of the engine before taking it home.Also, many people find the use of some type of after run oil helps preserve the engine. Onechoice is pneumatic tool oil, but there are others such as Marvel Mystery oil. After flying theplane will be oily and dirty. It is a good idea to clean the plane with paper towels and asolvent such as Windex, or some other cleaner..


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Bob Younts Principals of Flight