Lesson Three – Stalling

Learning Outcomes

  • Have an understanding of what a stall is
  • Be able to recognise and react in a stall situation
  • Be able to avoid a stall

stall1 

Understanding the Stall

Look at the pictures above.  In the first image the wing has a smooth flow of air over its upper surface.  This smooth flow of air around the wing is what creates lift and keeps the aircraft in the air.

In the second picture we have started to pull the nose of the aircraft up.  This tilts the wing against the oncoming flow of air.  This is called changing the angle of attack of the wing.  As this happens the air flowing over the upper surface of the wing begins to lose its grip on the surface and does not follow the shape of the wing any more.  You may think this is what happens when we climb, but in actual fact in the climb we also increase thrust so the aircraft still penetrates the air as if it was travelling straight and level – as in the first picture.  A stall occurs when the nose is raised but we have insufficient power to climb.  In this situation the plane continues to fly straight and level only at a slower speed and with the nose raised.

In the third picture the wing is completely stalled.  The angle of attack is so great that air can no longer stick to the top surface of the wing and it breaks away, swirling around in small ‘eddies’.  When this happens the wing is no longer generating any lift and can no longer counteract the force of gravity, so the aircraft falls from the sky!

Reacting to a Stall

To learn to react to a stall your instructor will ask you to fly the following steps:

  1. Starting from a normal circuit, you will create a stall during the upwind leg
  2. At a safe height, close the throttle and start to pull back gently on the elevator to raise the nose a little
  3. Continue to raise the nose as the aircraft slows down
  4. Once a stall occurs the nose of the aircraft will drop sharply, keep the aircraft straight using the ailerons and centre the elevator
  5. Allow the nose to drop and apply full throttle
  6. As the aircraft gains speed, use the elevator to bring the nose up and regain straight and level flight
  7. Climb back to circuit altitude and continue to fly the circuit

This whole process will take no more than a few seconds and the aim is to keep the aircraft flying straight without dropping a wing and losing as little height as possible.

Avoiding the Stall

This lesson becomes important when you are flying slowly and close to the ground, i.e. just after take-off and just before landing.  Practice stalling at altitude so that you can recognise when you model is going to stall and how slowly you can fly before a stall occurs.

If you think a stall is going to happen, the methods to avoid it are simple:

  1. Let the nose drop a little back towards a level attitude
  2. Increase power

Practice flying close to a stall and then pulling out of it before the nose drops.  If you can recognise the signs and react before the stall occurs you should never be in danger of nose diving into the runway!

The Tip-Stall

A tip stall occurs when just one half of the wing stalls, causing the aircraft to tip violently to one side and enter a spiral-dive.  This occurs when an aircraft turns too tightly at too slow a speed, which most often when you are making the final turn towards the runway for landing.  For this reason you must always make sure that your slow turns are made as wide as possible with as little bank in the wing as possible – this will be covered in more detail when you learn to land.

Lesson Two – Figure '8'

Learning Objectives

  • Be able to fly accurate left & right-hand figure ‘8’ patterns
  • Maintain a constant altitude whilst flying in this pattern

figure-8 

Perfecting the Turns

  • Your instructor will handle take-off and landings

The figure ‘8’ pattern is not included in the Bronze Wings test but is a useful training tool as it will allow you to practice left and right-hand turns until you can make precise, constant level manoeuvres in any direction.  This pattern always starts and ends over the centreline of the runway with the crossover point in front of the pilot.

  1. From a normal circuit start an upwind leg over the centreline of the runway
  2. As you begin to fly up the runway, make a 90° turn away from you
  3. Once the aircraft is flying away from you, make a 90° turn back in the opposite direction of the first turn
  4. Continue turning through another 90° until you are flying back towards the end of the runway
  5. As you come around to face the runway continue to turn back onto the runway centreline but facing in the opposite direction to that from which you started the manoeuvre
  6. Instead of flying straight on, carry on turning away from yourself until you cross point 3 once more – completing the first 360° turn
  7. Now make a 180° turn to the left to come around and point back at the other end of the runway
  8. Complete the manoeuvre by flying the final 90° turn back onto the original runway heading
  9. Fly along the runway and continue into a normal circuit pattern

At first this simple pattern will seem nearly impossible to fly smoothly as the turns will be too tight, too loose or you will gain and lose a lot of altitude.  As you spend more time practicing however you will find that you must use a combination of aileron, elevator and throttle to fly an accurate figure ‘8’.

The most important thing to remember when flying the turns is that it is not simply a case of applying a certain amount of aileron and elevator and holding it there.  You would not turn a corner in your car this way and you do not turn an aircraft this way either!  Turning accurately requires you to master balancing the controls; making constant small adjustments to fly a nice smooth line.

Keep practicing this even once you have passed your Bronze Wings as it is the basis of flying every different type of radio control aircraft and every different kind of aerobatic manoeuvre!

Lesson One – Circuits

Learning Objectives

  • Be able to fly a square left & right-hand circuit of good size, shape and orientation
  • Maintain a constant altitude whilst flying in the circuit
  • Understand how control inputs effect the models flight attitude

Flying the Circuit

  • Your instructor will handle all take-off and landings

A circuit is the most basic pattern of flight used in all forms of aviation.  It is a rectangular flight-path, flown at a constant altitude with the upwind leg flown along the centreline of the runway and into wind.  It is designed and positioned like this so that other aircraft that are taking off or landing are able to do so safely without fear of colliding with another airborne aircraft.
Once your instructor has got the aircraft airborne and flying straight and level in the circuit he will hand you the controls on the downwind leg.  From here you will need to make a series of 90° turns to keep the aircraft in the circuit pattern.
Do not adjust the throttle at this point.  Concentrate on using aileron and elevator to make a smooth turn as follows:

  • Use aileron to bank the aircraft left or right.  Move your right thumb left to bank left and vice versa.
  • Use a small smooth movement to roll the wings about 20-30°, the aircraft will start to turn
  • Now, as the aircraft begins to turn, use small amounts of up elevator to keep the nose from dropping and keep a constant altitude throughout the turn
  • As you reach the end of the turn, use the ailerons to roll the wings level again

The use of elevator is important in the turn to maintain altitude.  Without it the nose will drop and the aircraft will start to dive in a spiral motion towards the ground.  Using the elevator like this is called ‘supporting the nose in the turn’.
Most circuits at the LMMAC field are made in an anti-clockwise direction but we will practice both.

Using the Throttle

As you become more proficient at flying the circuit pattern we will start to introduce throttle to control the altitude of the aircraft.
Most trainers are set up to have ‘positive stability’.  This means they are designed to fly, when correctly trimmed, straight and level at a set speed.  (Your instructor will get the plane flying like this before the lessons begin).  If this straight and level is disturbed by a control input, the aircraft will try to return to this straight and level state.  We can take advantage of this by using the throttle to force the aircraft to climb or descend.
If you feel the aircraft is descending in the circuit.  Move the throttle up by a couple of clicks, this will cause the nose of the aircraft to lift as the wing generates more life and climb slowly.  If the aircraft is climbing too high, reduce the throttle a little and let it descend slowly back down to the desired altitude.
Later on we will use this throttle control to help us climb after take-off and descend to the runway for landing.

i.c. to electric Conversion Table

This is quite simply as it says in the title.  I have put together a little chart that will show you how many watts of power you need from your motor to equal the power of your two-stroke motor…
Remember, to work out the watts we multiply the continuous current of the motor by the number of volts going in.  So when you look at this table, divide the number of watts by the battery you intend to use and that will tell you what sort of amps your motor is going to have to draw to get there.
Clear as mud?  On with the chart then…

2-Stroke Motor Size

Electric Equivalent (watts)

0.20 cu.in

300w

0.35 cu.in

500w

0.40 cu.in

750w

0.60 cu.in

975w

0.90 cu.in

1200w

1.20 cu.in

2250w

50cc

3750w

100cc

7311w

Hopefully this will give you some idea when it comes to choosing an electric motor for your model.

What about the Watts?

A lot of people are confused when they first get involved in electric flight as to which motor, speed controller, battery combination to use to power their model.  This is the first in a series of posts that will look at how to determine and choose the best power train for your electric model…
The first, and most useful, development in the world of electric flight are the manufacturers who actually label their motors with equivalent 2-stroke i.c. motor sizes.  The best know of these are the E-Flight Outrunner series, which stretch all the way from .10 size motors right up to a whopping 1.80 size unit!  They aren’t the cheapest on the market but are very high quality and certainly worth the bucks if you can afford it.  We’ll look at i.c. to electric conversions in another post.
However, if you are looking at building a custom power system, maybe from E-Bay or another online seller, then you will need to work it out for yourself.
The first thing we need to look at is what sort of model you are building and how much power you need to fly it…
With electric models, power is measured in ‘Watts’.  Effectively the more watts a motor can produce, the more thrust is generated at the propeller.  Different types of model have different power requirements, a slow flying trainer needs far less power than a balls-out 3D model.  To give a basic idea we use the following:
Trainer/Sports Model:     90W/lb
Powered Glider:    120W/lb
3D Model:    175-200W/lb
This is just a basic guide, a lot of my models are of the 3D type and so I aim for 200W/lb.  I have designed a couple of smaller sports models that have used 90W/lb and have flown very nicely.
You will notice that the figures given state watts per pound (lb).  You will need to look at the finished flying weight of your model and then work out the power from there.  For example, if your ARTF spitfire weights 3lb 8oz flying weight then you will require (90 x 3.5 =) 315 watts minimum from your motor to get a decent flying performance.
So how do you work out how much power a motor will give?
All motors will give you basic information in their advertisement (if they don’t then don’t go there).  This will hopefully include a ‘Continuous Current’ rating and a ‘Recommended Input Voltage’ – these are the two figures you want to look at.
Quite simply multiply one by the other, for example:
Motor Continous Current = 30A
Recommended Input = 11.1v
Therefore 30 x 11.1 = 333 watts
The 333 watts is what the motor will generate at full throttle and assumes you are using the recommended prop size and your lipo isn’t losing all of its charge when under load.  Therefore, always select your motor to give slightly more power than you need.  You don’t have to fly at full throttle the whole time and flying at lower throttle settings will give you a longer flight.  The only way to know for sure is to use a wattmeter when you have your set-up in the workshop – but that does’t help when you’re buying.
Have fun out there and drop me a line on the contact form if you want to ask an electric flight question…

Safety with LiPo batteries

With more and more people trying electric flight I thought I should include this article on the safe use of lithium polymer batteries…
This article is taken from the British Model Flying Assosciation, which can be found at www.bmfa.org

A guide to safe use of LiPo batteries

from the British Electric Flight Association.
Despite what a number of people may tell you Lithium Polymer (LiPo) batteries are not fundamentally unsafe, but they need to be treated with more care than NiCd or NiMH. If abused sufficiently LiPo cells can catch fire and this fire can be difficult to extinguish. The following precautions should help you enjoy using LiPo batteries without having a major incident.

General precautions:

  • The minimum safe discharge voltage is 2.5V per cell when under load, or 3.0V per cell when not on load.
  • When more than 2 cells in series are used, a controller with an adjustable cutout should be used and it should be set at or above 2.5V/cell.
  • Only charge LiPo batteries on a charger specifically design for LiPo batteries.
  • Always ensure you use the correct charging voltage for the cell count.
  • The maximum charge rate should be 1C, eg. 0.7A for a 700 mAh cell. For best charging, low charge rates should be used where possible.
  • Check the charge voltage (or cell count) and current a second time.
  • Never leave charging LiPo cells unattended (at any charge rate).
  • It is best to charge LiPo cells in an open space on a non-flammable surface (such as a brick or quarry tile) and away from flammable materials.
  • For long term storage it is recommended that cells are fully charged and then discharged to between 50% and 60% of their capacity.
  • Use connectors that can not be short circuited, or use silicon fuel tube to protect exposed connections.
  • Have a dry powder fire extinguisher or a bucket of dry sand within reach.

If a pack is involved in a crash or is otherwise damaged:

  • Remove the pack from the model.
  • Inspect the pack for damage to the wiring or connections.
  • If necessary, disassemble the pack and dispose of any damaged cells.

Disposal of LiPo batteries:

  • Put the pack in a safe open area and connect a moderate resistance across the cell terminals until the cell is completely discharged.
  • CAUTION: The pack may get extremely hot during the discharge.
  • Puncture the plastic envelope and immerse in salt water for several hours.
  • Place in your regular rubbish bin.

By Jan Bassett (BEFA)

Pre-flight Checks

Introduction

Pre-flight checks are designed by the club to ensure that you are ready to fly safely.  We touched on them briefly with the S.M.A.R.T acronym and this lesson will expand on the recommended LMMAC pre-flight check as used in the Bronze Wings test.

The Pre-Flight Check

Check
1. Battery is securely plugged in
2. Receiver securely mounted
3. Servos securely mounted
4. Control rods attached at both ends.
5. Motor not loose on mount.
6. Propeller and spinner secure.
7. Control surfaces and hinges secure
8. Check structures for soundness particularly wing attachment and tail area.
9. Check all servos are connected and secure.
10. ATTACH WING.
11. Check for correct direction of control surface travel.
Stand behind the model and check
12. AILERONS – Stick to right – Right aileron goes up
13. ELEVATORS – Stick forward – Elevator goes down
14. RUDDER – Stick left – Rudder goes left
15. MOTOR – Check if possible (fully cowled motors is a bit hard).
16. Other controls as applicable – e.g. Flaps
17. Check for possible servo stalling. No jittering or fluttering of control surfaces with Rx and Tx switched on.
18. Do a range check with transmitter antenna down. If less than normal find out why.
Do you know what is normal?

FINAL CHECK

Ensure Pilot’s Brain is Engaged!!!

Engines

Introduction

The operation and tuning of radio control model engines is a huge topic and we will not go into all the details here.  This lesson presumes you will have a 2-stroke internal combustion glow engine of around .40 to .60 cubic inch capacity (which is what you should be running in a first aircraft anyway).  This lesson covers engine safety and basic tuning, it does not tell you how to start or run a model engine.  Please refer to the instructions that came with your engine for more details.

Fuelling Safety

The most important things to remember when fuelling your model are:

  • Always refuel away from sources of ignition (cigarettes, hot engines, mobile phones etc.)
  • Always refuel through the correct tube.  This is usually the engine fuel pickup tube or a dedicated third tube from the tank
  • Always use the correct fuel for your engine
  • Never leave fuel sitting in the tank at the end of a day’s flying

Starting Safety

Once an engine is running the only things that will stop it is a fuel flow interruption (caused by incorrect flow or a tuning problem), or something hard – i.e. your fingers – getting in the way of the propeller!
When starting your engine always follow the same safety procedures:

  • Complete all pre-flight checks to ensure engine is mounted correctly and propeller is not damaged
  • Make sure the model is adequately restrained from moving forwards once the engine is running
  • No observers should be near the engine or standing in line with the propeller
  • Always restrain the model with one hand before trying to start the engine
  • Always use an electric starter or ‘chicken stick’ to start the engine
  • Once the engine is running always keep clear of the propeller
  • Always approach a running engine from behind
  • Never reach over a running engine to tune it or remove the glow clip
  • Always stop your engine by closing the throttle completely or disconnecting/pinching the fuel line
  • Never throw anything into the propeller to stop the engine

Basic Engine Tuning

This guide is only intended as a very basic guide.  Refer to the instructions supplied with your engine for more detailed and engine specific guidelines.
Your instructor and other club members are a wealth of information on R/C engines.  Ask for help when starting your engine for the first time.

High Speed Needle Tuning

  1. Set the high speed needle as recommended by the manufacturer for the first time starting
  2. Start the engine and allow it to warm up for a little while
  3. With the model restrained, move the throttle slowly to full power.  Listen to the engine sound:
    1. Lots of smoke and a ‘burbling’ motor – too rich – wind the needle in slowly
    2. Engine ‘screaming’ and no smoke at all – too lean – wind the needle out slowly
  4. The ideal mixture is just before the engine starts to ‘scream’ 100% of the time.  It should be hovering just between the two tones
  5. Now get a helper to hold the aircraft with the nose pointing vertically upwards at full power
  6. The engine should speed up a little and will probably be ‘screaming’ all the time – this indicates how the engine will perform in the air
  7. If the engine stops when vertical the mixture is too lean – open the needle a little

Always make needle movements slowly or a little at a time to get the fine tuning just right.

Low Speed Needle Tuning

  1. Start the engine and run at full throttle for a few seconds
  2. Cut the throttle quickly  and listen to the engine sound:
    1. If the engine speed drops or cuts the mixture is too rich – close the screw a little
    2. If the engine speeds up the mixture is too lean – open the screw a little
  3. When set correctly the engine should idle nice a slowly with no change in speed
  4. Go back and check the high speed needle setting as this can be affected by the low speed setting

Always make tiny adjustments to the low speed needle.  This is often best done with the engine stopped.

Electric Engines

It is possible to learn to fly and take your Bronze Wings with a suitable electric aircraft.  If you intend to do this you should make the club aware so that you can learn to fly with an instructor skilled in electric flight.
The parts of the Bronze Wings tests that focus on glow-engine safety will be replaced with items specific to electric models such as battery safety, ESC arming procedures and propeller safety.
The most important things to remember with electric engines are:

  • Always switch your transmitter and receiver (if RX battery used) on before connecting the flight battery
  • Always use the correct motor, battery, ESC, propeller combination to reduce the risk of overload and fire
  • Once the battery is connected and armed the engine should be considered live and dangerous, even if the propeller is not spinning

Radio safety

Introduction

Radio Safety is the most important part of flying a radio control model aircraft.  If a student pilot cannot demonstrate an understanding of safe radio operating procedures then they would never be allowed to switch their aircraft on, let alone fly it!

The Frequency Key System

Most model clubs adopt some sort of frequency peg, or key, system to identify which transmitter frequencies are in use and which pilot’s are using them.  LMMAC is no different operating a ‘keyboard’ style system to indicate which frequencies are in use.
Your transmitter will be set to, or contain a crystal that allows it transmit on a certain frequency within the 36 megahertz band.  A sticker on the back of your transmitter will show the frequency displayed as something like 36.470.  This number tells us the transmitting channel, in this case channel 647.
When you want to switch on your transmitter you must place your frequency key into the relevant slot in the keyboard before switching on your transmitter.  Your key then stays in the board until after your transmitter is switched off and placed back into the compound.
When not using your transmitter, make sure you return it to the shelves below the keyboard and hang your key over your aerial.

NB:

It is important to get transmitters certified to ensure their safety.  The club should already have provided instructions on how to do this.  Certified transmitters use a narrow yellow key, uncertified a wider blue key to ensure a wider safety margin.  These keys will be provided by the club.

Radio Installation

As you are building your first trainer aircraft, your instructor will be able to help you with installing the radio gear correctly to ensure trouble free operation.

Servos

Use all four screws provided with standard servos and attach the rubber anti-vibration grommets correctly.  Usually this involves placing a rubber grommet over each servo mounting hole and pushing in the small metal sleeve from the underside.  Your instructor will help to explain this to you.

Linkages

Check all linkages from servo to pushrod and pushrod to control horn are secure.  Short lengths of silicone tubing should be used to ensure clevises do not vibrate loose in flight.

Receiver & Battery

Plenty of foam should be used to ensure the receiver and flight battery are safe from vibration.  Make sure they are fixed down and cannot move around in flight.

Aerial Routing

Receiver aerials should be routed outside or inside the fuselage and stretched out to their full length.  They should not be in contact with servo leads or pushrods as this can cause interference.  Aerials should be taped or fixed in place so that they cannot swing forward and foul the propeller.

Switch Mounting

If mounting your receiver switch outside the aircraft, ensure to locate it on the opposite side to the engine exhaust.

The Range Check

Once your engine is started and before the first flight of the day it is important to do a range check.
With the aerial on your transmitter down, walk backwards from the pits to the edge of the runway all the time watching the control surfaces on the aircraft.  You are looking for any unexplained movement not originating from a control input.  Any movement might be a sign of a ‘glitch’, or interference in your receiver.  Once at full distance, check full throttle response and movement of your controls before coming back.  It sometimes helps to have a helper stand next to your model to give you the ‘thumbs up’ if everything is okay.

S.M.A.R.T

S.M.A.R.T is a useful acronym to remember.  It stands for:

  • SWITCH – Is everything switched on?
  • METER – Are your batteries fully charged?
  • AERIAL – Is your transmitter aerial fully extended?
  • RATES – Do you need high or low rates selected (if applicable) for this model?
  • TRIMS – Is the model trimmed?  Are all the controls moving freely and in the correct direction?
Do these checks before you take off – every time you fly!

2.4GHz Radio

The new 2.4GHz radio technology allows a greater safety margin than 36MHz as it seeks and locks onto a free channel before it will work.  LMMAC still requires pilots using 2.4GHz sets to place a key in a different board so that other pilots are aware a 2.4GHz set is in use.

Always switch on your transmitter BEFORE you switch on your receiver!

Welcome to flight school!

Welcome to this series of flying lessons designed to get the novice pilot from that first flight all the way to the Bronze Wings test.
These written lessons are intended to be used whilst learning to fly your radio control aircraft with a qualified instructor. They will help you to understand what is happening at various stages during the flight and give you an idea of what will happen next during your lessons. Once all these lessons have been completed to the satisfaction of your instructor you should be confident and able to pass your bronze wings.
These lessons WILL NOT teach you to fly on your own without the aid of an instructor. To try to do so is irresponsible and dangerous!