Good (Bad) Day at the office

Well I'm not going to say much cept the Zero had a nasty spill on the weekend, in what was otherwise a brilliant day of flying.

Enjoy the youtube vids...

A day in the life of an easystar



Fun at Kambah on Sunday



And from the Mongrel:



and

Electric Magic Numbers

One of my buddies from rcuniverse (you should check out the BBQ and Beers thread - very friendly guys - yours truly is normally nearby) has found an outstanding piece discussing many "magic numbers" for electrics. He didn't write it (he's more of a "try it, if smoke doesn't come out then increase the power and repeat" type) - so this is someone else's work but they posted it on wattflyer so obviously wanted to share it. If you are the owner, drop me a line and I'll put a credit up here for you.

This isn't an article on electrics for beginners - if that's where you are then take a look at:
http://www.ozrcflying.com/2007/01/electrics-of-electric-flight.html

If you are a nitro head thinking about electrics take a look at:
http://www.ozrcflying.com/2007/09/guide-to-electrics-for-nitroglow-heads.html

So, without further ado:

Magic numbers for modelers...

Schoolboys (and girls) know Watts, Ohms and Amperes. Modelers speak of props, lipos and BESC's. Not to forget the mysterious ‘KV’ and the famous ‘C’, which is not the speed of light... But what hides behind these cabbalistic concepts? And how do they relate to the everyday business of flying RC machines? Follow the magic numbers...

Today's pilots understand that you get more from a good electric setup than an average IC engine. Cheap sources of equipment have dramatically changed the price tag on brushless motors, ESC's and batteries. But it is still something akin to the dark arts to select the right combination of these three items in order to fly well. The best way to success is to follow the secret recipe of electrical gurus, based on kilometers of burned windings, and the third law of universal common sense. Enter the hidden side of real-life quantum physics, the one that mixes weight and mass, translates temperature in 'seconds-of-finger-on-the

-motor-bell' and flying times in burned amperes... Who cares for the Science, as long as we can punch holes in the sky

Buy your watts by the kilo (or Pound)

The first magical number tells you how much watts you need to fly your plane. Of course, it works only for decently matched systems. A GWS parkflyer won't fly with a 300gr motor,
however powerful it is...

Foamie, motorglider, Piper Cub: 100watt per kilogram (2lbs)
Trainer: 150 watts per kilo
Warbird, 'sport' aerobat: 200 watts per kilo
Racer, 3D: 300 watts per kilo
EDF Jet: 400 watts per kilo

Examples: a 3kg (6lbs) 150cm (60") Hurricane will fly on a 600watts setup. A 2.5kg Calmato will require 375 watts, etc.

Watt is pushing us forward?

The second magical number gives an idea of how much static thrust you can expect from a good setup. Once again, this is only true for a propulsion system that is performing normally.
These values give a good indication of what is possible... and what is not.

Brushless outrunner: 4gr per watt
EDF: 2gr per watt
Brushless 'inline': 2gr per watt
Brushless 'inline' with gearbox: 5gr per watt

Examples: A warbird with a 1000 watt brushless outrunner will have 4kg static thrust. An EDF jet having a 600 watts power system gives 1200gr thrust on the ground.

Powerful horses...

The third magic number is in fact a magic formula, one that most of us forgot after school...

Watts = Volts x Amperes
Volts = Watts / Amperes
Amperes = Watts / Volts

How does that relate to horses? Easy: you can convert watts to horsepower with the following rule: 1000 watts = 1.34 HP or 1HP = 750 watts.
Example: a Trainer aircraft with a 12 volt battery delivering 40Amps gives 480 watts (or 0.65HP). Note that the same plane having the same performance in IC would use a .40ci engine providing 1HP, which is 750watts... This is because electrics have a better efficiency, with more power at lower rpms. A similar phenomenon applies to diesel vs. petrol cars. The diesel drives better even if both cars have the same 95HP.

One Hot Minute!

E=Mc2 and the planet is warming up, everyone knows that. Electric motors also get warm. To know how much too warm the windings should not glow, here is a rule of thumb that is nothing short of magical...

Prop aircraft: motor weight in grams x 3 = max. watts.
EDF: motor weight in grams x 5 = max. watts.

Example: a 235gr brushless outrunner can dissipate 705 watts for a minute without meltdown.
A 200gr inrunner on an EDF will not die even at 1000watts.
Of course, this is assuming the motor is correctly used and cooled by adequate airflow.
This rule is only true for brushless motors. Old 'can-style' brushed motors like the Speed 600 don't survive more than their weight in watts...

Resistance is useless...

Gold is a fantastic metal when it comes to moving currents. Unfortunately it is also very heavy. That is the main reason for us to use copper in electrical wires. But then copper transforms some of the current into heat. Not only does it fuel global warming, but it reduces the available in-flight power, which is a real catastrophe!
To avoid this dramatic event, one should always use large enough wires:

Up to 25A: 1.5mm² wire section (15 AWG)
Up to 60A: 2.5mm² wire section (13 AWG)
Up to 100A: 4mm² wire section (11AWG)

Not only wires, but connectors and soldering must be able to handle the current. In this field like in others, bigger is also better...

Round and round

You always wanted to know what the famous 'KV' stands for? This indicates the number of revolutions per volt provided by an electrical motor. It gives us the 'nominal' rpm of a motor on a plane.

rpm = KV x volts x 3/4

Examples: a 1200KV brushless outrunner connected to a 10 volts source will turn at 9000rpm. A 4200KV inrunner on 10 volts will spin at 31500rpm.

Full or empty, that is the question...

The voltage of NiMh cells is said to be 1.2 volts and lipos are sold for 3.7 volts. These 'nominal' values are confusing at best. The real figures depend on what you need. For instance, to know the wattage of a power system, you need to take into account the voltage of the battery at full throttle. But when you need to know if a battery is full, you measure the 'idle' voltage. The values written here give you an idea of typical 'real life' cell voltage.

Lipo in flight (motor full power): 3,3 volts
Lipo fully charged (idle): 4.1 volts
Lipo empty (idle): 3.7 volts

NiMh in flight (motor full power): 1.1 volts
NiMh fully charged (idle): 1.4 volts
NiMh empty (idle): 1.2 volts


Example: In order to get 300watts from a power system, you will need a 3-cell LiPo or a 9-cell NiMh battery and a motor loaded to about 30A.
Here are the (rounded) 'in flight' voltages of typical lipos:
2S = 7 volts, 3S = 10 volts, 4S = 13 volts, 6S = 20 volts, 10S = 33 volts.

Need for speed? Get some serious pitch!

Chosing a prop is not easy. Most people select the right diameter so that the motor doesn't soak too much current . But the pich is often disregarded. Nothing replaces the test flight, but here are some magic numbers to guide you when choosing the pitch of a prop.

Airspeed in kph = pitch (in inches) x rpm / 800
Airspeed in kph = pitch (in cm) x rpm / 2.000

Example: On a big trainer aircraft, a large 14x4" prop spinning at 8000 rpm will get you 40 kph of speed, which is marginal.But a 11x8" at 11000 rpm gives 110 kph which you don't need. The best choice will probably be a 13x6" spinning at 9600 rpm and providing a top speed of 72 kph. This is true for all planes, not only electrics.

Masters of the 'C'

The label on your brand new lipos reads '15-20C', but there is also a '1C' somewhere else on the sticker... WTH???
The '1C' in small letters means the maximum charge current is 1 time the cell's capacity (all lipos charge at '1C'). On the other hand, the '15-20C' note promises you can discharge the battery at 15 times the capacity and even push it briefly to 20 times the capacity without damage. The truth is that most manufacturers are too optimistic, so forget the second number and try to keep the 'peak' discharge current under the first number. A 'realistic' discharge current can be calculated like this:

Max discharge current on the ground = (first number) C x capacity / 1250
Max discharge current during 1 minute = (first number) C x capacity / 1500
Max continuous discharge = (first number) C x capacity / 2000

Example: A 3000mah '20/30C' battery should be able to discharge at 60A during a few seconds. It will survive a take-off at 48A. A whole flight alternating slow passes and full throttle at 40A will be OK. And it could be discharged at 30A continuous without degrading.
Whatever the 'C', remember to provide adequate airflow to cool the battery.

The heat is on!

To cool down an IC engine, you just cut some holes in the motor cowl. For an electric aircraft, you also have to provide cooling for the ESC and batteries. The warm air has to find it's way out of the plane so there have to be additional holes at the rear... But what size of holes do you need to drill?

Air entry surface (cm²) = number of watts / 40
Air exit surface (cm²) = number of watts / 30

Example: a warbird using 1000 watts needs 1000 / 40 = 25cm² of cooling air intake and 33cm² of opening behind the battery to let the warm air exit. The exit MUST be larger than the entry to avoid warm air stagnation which is even worse than too small an air intake.

Check the internal resistance

Modern batteries provide tremendous performance thanks to a very low internal resistance ('Ri'). But all batteries are not equal. To compare two brands or to know if an older pack is still fit-to-fly, you must measure the Ri. All you need is a voltmeter and an (cl)amp meter (or a wattmeter that combines both functions).

Measure the voltage 'V1' during a discharge at a current 'A1' corresponding to ±1C
Measure the voltage 'V2' during a discharge at a current 'A2' corresponding to ±10C

Ri = (V1 - V2) / (A2 - A1)

Example: on a brand new 3-cell 2200mAh lipo you measure 11.4 volts at 2.2A discharge and 10.5 volts at 22A discharge. The Ri of the pack is (11.4 - 10.5) / (22 - 2.2) = 0.045Ω. This means a single cell Ri of 0.015Ω.
Several month later, your plane doesn't fly like it used to do. You measure Ri again with 11.2 volts at 2.2A and 9.5 volts at 22A, which gives 0,086Ω. This means that the battery has lost half its performance...
To be meaningful, Ri must be measured in 'standard' conditions. Ambient temperature, cells temperature and state of discharge have a direct impact on the results. The easiest is to always measure Ri on a freshly charged pack at ambient temp.

What goes up...
...Must come down. But when? Follow these magic formulas to estimate how long you can fly using a specific battery:

Contest or 'full throttle': Seconds = capacity (mAh) x 4.2 / max current on the ground
Aerobatics: Seconds = capacity (mAh) x 7 / max current on the ground
'No-stress' flight: Seconds = capacity (mAh) x 11 / max current on the ground

Examples:
FunJet race using a 2.400mAh battery discharging at 42A Max: 2400 x 4.2 / 42 = 240 seconds, or 4 minutes.
F3A aerobatics using a 4100mAh battery discharging at 52A Max: 4100 x 7 / 52 = 552 seconds, or 9 minutes.
Piper Cub flight using a 3000mAh pack at 34A Max: 3000 x 11 / 34 = 970 seconds, or 16 minutes.

Fly longer: add a cell!
The last magical number gives you an estimate of how much energy a battery stores:

E = capacity (in Ah) x voltage

For instance, did you know you can fly longer with a 3S 1000mAh lipo than with a 2S 1300mAh...? Indeed, to get the same flying style, the 2S at 7.4volts needs to discharge at 13.5A for 100 watts of power. The 3S needs giving only 9A for the same power. Using the time formula, we get a 'No Stress' flying time of 20 minutes for the 3S vs 18 minutes for the 2S. As a bonus, the lower discharge 'C' rate on the 3S battery means it will last longer.
The magic number tells the same story:

Energy in the 2S: 1.3 x 7.4 = 9.62
Energy in the 3S: 1 x 11.1 = 11.1

Some will say that a lower voltage usually means a larger prop and better efficiency. True, but the higher 'C' discharge and current on the motor cause losses that cancel the expected benefits.

Demonstration on my P-40 Svenson (170cm span, 4kg AUW, Motor HXT50-55)
The motor uses 51A Max current on a 6S lipo. The voltage magic number predicts ±20 volts so we can estimate the power: 20v x 51A = 1020watts or 1.36HP. This plane has more than 250 watts per kilogram, it is powerful and climbs vertically, just like the magic formula says: 4gr x 1020 = 4080gr thrust. But beware of the excess heat buildup because the motor weights only 320gr! In theory, it should not be used above 320 x 3 = 960watts. However, on this plane the 26 cm² air intake and 34 cm² air exit provide optimal cooling.
2.5mm² power cables are used for efficient current transfer. The motor has a KV of 500, it runs at 20 x 500 x 3/4 = ±7500 rpm. The prop is a 15x8", which gives a max speed of 8 x 7500 / 800 = 75kph, which is ideal for this warbird. I use a 4400mAh battery, So I can fly for 4.400mAh x 11 / 51A = 949 seconds or about 16 minutes of cruising 'No Stress' performance.
the battery is sold for '20/30C' and could deliver a maximum of 20 x 4400 / 1250 = 70.4A peak and 20 x 4400 / 1500 = 58A during one minute. I must avoid flying continuously at full throttle because I would discharge the pack above its safety limit: 20 x 4400 / 2000 = 44A...

Aerodynamics Basics to Improve Your Flying

This short piece is to explain some of the aerodynamic forces that affect your flying and what they mean. Yes - it is theory but I think it will be worthwhile anyway.

Turning

Why does an aircraft turned when it is banked? Why does an aircraft lose altitude when it is banked?

The answer to both of these questions is the same and the diagram on the right illustrates it.

When the wing is parallel to the ground all the lift acts directly against gravity. However, when the plane is banked the lift doesn't fight gravity directly. It pulls up perpendicular to the wing, but that is no longer straight up.

We can think of this lifting force as acting in two directions. Some acts against gravity (less however, than when in level flight, so the plane may lose altitude). Some of the lift acts in the direction of the horizontal, pulling the aircraft into the turn.

Stalling

Many of us think of stalling as simply that speed where we lose lift and the plane noses down. Of coures, the whole story is a little more complicated.

Stalls happen when the wind stream striking the wing passes the critical angle of attack (see wikipedia for more detail if you want). This is normally 14-16 degrees.

The picture on the right tries to illustrate this (click on it to see a blown up version).

As you pass the critical angle of attack you get what is called a separated air stream (go and look at wikipedia if you want more details). This separated air stream (where air passing over the top of the wing doesn't meet air going under the wing) causes turbulence, and destroys the low pressure system above the wing that normally gives lift. That's why you stall.

Given how difficult it is to measure true airspeed in our models anyway, the more reliable indicator of stalling is to watch the way the aircraft behaves in the air. If at neutral attitude she can't maintain altitude then you are potentially heading for a stall (of course you may also be descending to land under power...).

Adverse Yaw

There's a pretty good explanation of this one on wikipedia which is worth a look which explains the forces involved. I'm going to put it slightly differently because I've never seen adverse yaw that prevents an aircraft entering a bank (I'm not saying it doesn't exist) - adverse yaw is a yawing force (so a force that rotates the plane around its CoG on an axis drawn from the top to the bottom of the plane {what the hell do I mean? Think of your plane cartwheeling - that's the axis of rotation "yaw" describes}) that causes the nose of the aircraft to fail to track true when you bank the aircraft using the ailerons.

So if you roll the aircraft and it tends to go nose high or nose low (so if you from level flight you roll to perpendicular and the fuse is not parallel to the horizon) you are getting some adverse yaw.

Now - if you've never really noticed this before chances are you don't get it, or don't get it enough to make a difference.

The correct way to fix adverse yaw is with differential ailerons as the first option (so the aileron that comes up comes up further than the one that goes down goes down - there's a good pic on the wikipedia page - check it out). If you've got a computer radio and your ailerons on different channels then chances are you have a differential function you can use. If you don't the same thing can be achieved by carefully adjusting the neutral position of the servo arm (assuming ailerons are on different servos).

Although your servos hopefully throw the same arc by changing the neutral position in that arc you can make the push/pull motion in one direction larger than the other. Set them up so that in the direction you want the most throw the servo travels through the point perpendicular to the servo. Hopefully the picture on the right makes this all clear.

If you only have one servo to control both ailerons then I guess you are going to learn how to use the rudder to compensate - have fun with that.

Induced Lift and Aerodynamic Lift

Aerodynamic lift is the low pressure system created above an airfoil as a result of the Bernoulli Principle. You need something shaped like an airfoil to produce it. Just as an interesting aside, it's the same effect which allows sailing boats to sail into the wind. While they are sailing into the wind the sails take a shape similar to an airfoil. However, rather than trying to create a low pressure system on the outside of the surface of the sail, the goal is to create a low pressure system just in front of the leading edge of the sail, to pull the boat forward. Just like an airfoil they also have a critical angle of attack which if you exceed (by trying to sail too close to the wind) you will take away the "lift" and the wind will spill from the sales.

Aerodynamic lift is the one that fails when you stall. However, you get more lift for less drag from aerodynamic lift than from induced lift.

Induced lift is the equal and opposite reaction when an airstream strikes an object and bounces off. A simple cross kite is kept in the sky by induced lift - the airstem hits the kite and bounces down. As a result an equal an opposite force pushes the kite up.

Flatties (models made from depron etc with flat wings) fly only on induced lift. They are light enough that they do not need much lift (and half the time they have their nose in the air hovering anyway, so who needs a wing?).

Planes with a "built up" wing (an airfoil) use a combination of aerodynamic and induced lift to fly. For example, when a powered plane approaches to lands with its nose slightly up it is using induced lift (as well as aerodynamic lift) to achieve a very slow but very stable flight. However, the amount of thrust required to hold the aircraft in this attitude is surprisingly high (given how low its airspeed is). That's because the induced lift comes at the expense of large induced drag. Likewise when the aircraft climbs out. It is getting lift from both aerodynamic and induced sources. However, as the plane transitions to stable level flight the induced lift all but dissapears (as does the drag it creates) and the aircraft enters its most effecient flight mode.