This section is for items of interest to those who want to build and fly models of current or historical aircraft.
If you're interested in the special area of scale modeling related to War Birds, take a look at this forum.
You might be interested in this video published by NorthWest Scale Aero-Modellers. It was sent to us by the Director of their group, Roly Worsfold. His comments in the email were:
"The video was made to inform RC Scale Aero-Modellers about RC Scale Aero-Modelling - focusing on the model Aero-Modeling community with the hopes it will help to promote more RC Scale events in Canada and United States.
Early feedback (before we finished the video content) from novice/experienced modellers and some of their partners is that it removes some of the mysteries and helps them more clearly understand this aspect of the hobby. We hope you find the video similar to yourselves".
If you would like to contact him about this video or their group, Roly's email is: email@example.com
Jim McKeown has suggested a link to this website saying "something useful, it was a help to me".
It is a site that offers a computer product which helps you to determine which electrics to use, including motors, ESC, and batteries.
Here is their site: MotoCalc.
In the past there have been some incidents where electric motors started prematurely and caused severe injuries to our members. I thought that this article from Model Airplane News, July, 2014 and this this addtional web source pdf that I found was a way to improve safety on our electric powered models.
Submitted by Jim Harris, May 2014.
Flying RC model aircraft indoors requires special designs. Because of limited space, walls, ceilings and floors, a proper designed indoor flying machine is required. Outdoor models are flown outdoors and indoor models are flown..........well, they can be flown outdoors too, so they're twice as good. There are exceptions to these rules, that's if your a proficient 3D stick wiggler.
Now, we want to be able to do more than just flying around in circles trying to avoid smacking the wall and if we do, we don't want to spend half the flying session fondling the hot melt glue gun. So, what's needed is an Aerobatic Light Electric machine, or the AL-E principal...... 'floats like a butterfly and stings like a bee'. You need the 'light weight' to fly slowly and the 'punch' to pull out of trouble. A good example of this type of design is my 17" version of the Drenalyn www.lecreusot.net/aeromodelisme/drenalyn at 5.8 ounces and a wing loading of 2.8 ounces per square feet, it was bounced off the walls, ceiling and floor for two indoor sessions and the only minor repair was a failed land gear attachment, which took ten minutes to improve and fix. ( Note: don't use CF rods for LG. ) With a few clicks of trim here and there, it will free flight around in a hover, yet is capable of some extreme 3D. Anyone can fly it, it's just that easy.
If you poke around on the internet, there's endless reading on the subject of what's suited for indoor flying. So, to help you decide, here's what to look for in an indoor design if you want to cruise around, but with aerobatic capabilities........it must be light, with a low wing loading (for it's size) and the power system must be capable of spinning a large diameter prop and generating more thrust in ounces than the flying weight of the model. Low weigh means slow flight and a large diameter prop allows it to fly slow. The model design and the power system must work together.
Your power system should be a 3S setup to keep the amps low when spinning large prop, allowing the use of a small and light battery. What I mean by 3S is that the motor must be wound for three cells with lots of turns, a low Kv and low amp draw, capable of turning big props.
Look for a model design that is simple, light, quick to build, light, easy to repair, light, cheap to build and above all else........LIGHT. When you find that 'perfect' design, not try to improve it by making it stronger, if anything improve it by making it lighter. It will fly much better and less damage will result from impact.
So, next time, leave those outdoor flying machines at home, build a proper indoor model and have a load of fun at the next indoor, with more flyin' than fixin'.
This topic resulted from trying to help those who are new to the hobby of flying electric powered model
aircraft, as well as those who are involved and not seeing any success for their efforts. With all the new
technology and products available to us “E” modelers, such as brushless motors and lipoly batteries, it has become
much easier in the last few years, but you still have to be willing to learn how to apply the principals and the
math to make it all work. I’m not about to explain it all here as that would take up all my hobby time. Besides,
part of the fun is learning new things. So, here are a few guidelines as to where to start learning what you need
To achieve success with electric power, there are three ways to proceed………
(1) Buy something off the shelf that the manufacture has tested and follow the instructions to the letter.
(2) Copy what others have done if they have a model that has the performance you are looking for.
(3) Learn everything about this form of power that’s available and how to apply it, then ‘roll your own’.
Number one does not quarantee success, as there are too many variables that the modeler cannot duplicate exactly and the performance expected usually is overstated in the manufacturer’s advertisements. This is not applicable if you followed rule number three.
Number two does offer success, but only if you copy exactly to the smallest detail, what someone else has done. But, this method will only benefit you if you apply number three and ask questions to find out why this modeler used the items that he did and what part they play in making the system work.
Ah………..I think we’re going somewhere here, yup, # 3, preferred by 10 out of 10 successful “E” pilots the world over.
Number three requires a desire to learn and a willingness to put in the effort. Once you have figured it out and apply your new found knowledge, and see the results in a practical application, the fun and satisfaction level will make you want to feed those grey cells with even more info on electrons.
There’s lots of avenues out there for information: model magazines, the internet - Canadian Electric Flight and e-zone magazine, and asking questions of experienced “E” modelers.
The information printed in magazines or manufacturers’ web sites can be considered accurate, but be careful of what some ‘so called experts’ post in discussion forums on the net. I have read misinformation on the net that has not been corrected for weeks.
Also, don’t assume that salesman on the other end of the phone at the mail-order hobby shop is steering you in the right direction either. He is a salesman, and is out of stock on the motor you really need, so guess what…………????
So, be prepared to spend many hours reading, pondering, thinking, calculating and eventually it will come to you. Just like that light bulb, all of a sudden it will come on and you will be able to walk the walk and talk the talk.
Asking an experienced modeler at the local flying field has the benefit of seeing your answers demonstrated. You know, a picture is worth……………….
The very first item any electric modeler requires and is considered a definite MUST HAVE, is an Astro Whatmeter or equivalent device, which is required to see the math in action. (amps X volts = watts )
It insures you are that you have achieved the required power needed and are not about to damage any of the components of your drive system, the motor, the speed control or the batteries.
IF YOU ARE USING LIPO BATTERIES, YOU SHOULD READ EVERYTHING YOU CAN FIND ON THEM, GET TO KNOW ALL ABOUT WHAT CELLS YOU HAVE, WHAT THEY CAN/CAN’T DO FOR YOUR SYSTEM AND HOW TO CARE FOR THEM.
While we’re on the subject, make sure you check the individual cell voltages on a regular basis. Why? Well, look it up on the net, it’s very important that you know what to do if they are not balanced and the consequences.
In your quest for knowledge, keep in mind that, with electric, the calculations begin with the prop, as it’s the thrust that flys the model, not the watts or even the RPM. OK, OK, yes, we do want to know the horsepower/watts it will put out, and the RPM may be interesting, but they are only guide lines, and only ‘real world testing’ by flying the model will tell if you have it right. So here’s a list of what to look for……….
How big a prop do I use, and how fast do I want to spin it: do I want speed or thrust?
What kind of power will my motor deliver to turn that prop, and how many amps will it handle?
What KV ( RPM per volt) is my motor, and how does that relate to the prop, gearing and batteries?
If using a gear box, what gear ratio, and how does this affect the amps?
How many amps will my batteries deliver?
Since volts tell the motor how fast to turn, how many volts do I need?
Am I using the proper electronic speed control, and is it programmed properly?
Will my plugs and wires handle the amps?
There’s loads more, but this is a start…………
Now that you have figured all that out, don’t stop now, because there’s more, lots more. Now we get into making your power system fly your model and perform the way you expect. The next important factor is the weight of the model and its wing loading. Never, never, NEVER add weight to an electric model to balance it: move something, even if it requires a rebuild of the nose of the plane. To add weight is to make everything you have learned to this point null and void. Anyways, you will probably get struck by lightning at the flying field by the “E” gods if they catch you with a chunk of lead stuck on your firewall. All that time reading will be wasted, and your widow will sell off your modeling junk for peanuts to get even when she finds out what you really paid for it.
Now, check out Mo Alam’s article on electric flight at Electric Flight - My Key Learnings
Keep in mind these two rules of thumb, and it’s the thrust to weight ratio that’s the most important consideration for a great flying electric model. Also, be prepared to be the proud owner of a very large collection of propellers.
(1) If you are using round cells, to save an ounce is equivalent to adding one cell to your battery pack in performance.
(2) Watts per pound……
50 watts per pound = trainer
75watts per pound = sport aerobatics
100 watts per pound = 3D
The only place that ‘success’ comes before ‘work’, is in the dictionary, so plug away and your efforts will be rewarded with a great flying model airplane powered by electrons.
Personally, I have read hundreds of kit, motor, ESC, gear box and battery reviews. Not that I ever intended to buy all these products, but I was interested in how they worked and why. I have read thousands of posts on the internet, spending many hours at the computer into the wee hours. Yes, it took time, but time well spent, and has made the hobby even more enjoyable now that I actually know what I’m doing, and I’m having the most fun ever in thirty five years of RC modeling. I can even wind my own motors and get it right the first time and, just think, not long ago I thought a watt was an adjective. So there is hope; if I can do it, so can you.
Mo Alam, PDQ Flyers
The popularity and technology of electric radio controlled flight seems to be progressing at an electrifying
rate (pun intended!). It has taken Europe by storm, where it is now the predominant form of r/c flight. In
countries like Germany, Czech Republic and Italy, where glow flying fields are scarce due to them encroaching on
housing, e-flight is now the new normal. E-flight is no longer just indoor and park flyers – people are flying 20
pounds scale airplanes with the same authority as glow, and many of the Pattern and 3D Champions are flying
routines with electric aircraft!
Since there are quite a few PDQ members interested in this sector of our hobby, I felt it might be worthwhile to put down my e-flight learning into an introductory document – hopefully it will serve to demystify electric r/c flight. There is a lot to learn in e-flight and I hope it won’t scare off anyone, but at least some will be fascinated and "charged up" by the renewed opportunity to learn new things.
I am no expert on electric flight, but I have carefully (and seriously) researched this area in the past year and a half. Everything I present in this article is what I have learnt and used, but obviously there is a whole lot more to learn. The more I delve into it, the more I am fascinated by e-flight.
SOME STARTING THOUGHTS
Don’t bother to compare electric motors and glow engines – you will tear out your (remaining) hair and get nowhere. An apples-to-apples comparison is impossible! The same electric motor can be set up to work like a 15-size glow engine, and also be set up to work like a 40! As of this moment, you must remember never to ask the question "so what is an equivalent electric motor to a 40 size glow engine"!
Do not be concerned about rpm. This is because the same electric motor can be set up with a propeller to scream at 30,000 rpm, and then be set up to run at a very sedate 7,000 rpm! This means the same electric motor can power a 2lb ducted-fan jet at high speed, yet fly a sedate Piper Cub at about 6lbs! Or anything in between! Just accept that electric motors are a new type of engine, and don’t try to understand them through comparison with what you already know about glow engines.
Besides the familiar receiver and servos, e-flight equipment consists of an electric motor, an electronic speed controller (ESC), and a flight battery pack. For applications between 6 and 12 cell battery packs, the ESC has a "battery eliminator circuit" (BEC) that powers the receiver and servos, without the need for an onboard receiver battery pack. If you are using more than 12 cells in your flight pack, then you will need an onboard receiver battery pack (just like glow), or use something called an "ultimate battery eliminator circuit" (UBEC). The UBEC is only a few grams in weight, and connects to the flight battery pack to power the receiver and servos.
Some ESCs have a receiver/motor switch, but you can run without one also. In e-flight you must always remember to turn on your transmitter first before connecting the flight battery pack – this is to ensure that a nearby radio doesn’t swamp your receiver and turn the motor on.
[SAFETY TIP: Never get close to the propeller when the flight pack is connected to the ESC – a radio problem can start the motor at anytime, with disastrous consequences to you.]
Lets look more closely at the flight equipment.
Most people know how an electric motor works, so I won’t waste time explaining it. There are two types of electric motors – brushed, or brush-less. In a brushed motor, the coils are on the armature that rotates, and the magnets are stationary. The power to the coils is provided through a set of brushes that contact the armature. This is its weak link – the brushes wear out, cause sparking and radio interference, and generate a lot of heat. In a brush-less motor, the magnets are on the armature that rotates, and the coils are stationary. There is no need for brushes. Brush-less motors need little or no maintenance, and are much more efficient, so they produce much more torque. Other than for indoor and park flyers, brush-less motors are the way to go (this is my own opinion of course).
The choice of a motor depends upon the Watts of power that you will need to fly your airplane (see later for a rule of thumb). Respectable brushed motor manufacturers for e-flight are Plettenberg, Kyosho, Kontronik and Graupner – a lot of these motors are badge-engineered by Mabuchi or Sagami. Respectable brush-less motor manufacturers are Plettenberg, Kontronic, Mega, Axi, Astroflight, Aveox, Phasor, Hacker etc.
ELECTRONIC SPEED CONTROLLERS
The ESC provides throttle control by managing the current going to the motor, and can also provide power to your receiver and servos (if you are using between 6 and 12 cells in your flight pack). The ESC connects to your receiver throttle channel (and also provides power to the receiver through the same connection). One end of the ESC connects to the flight battery pack, the other to the motor. ESCs have a voltage cutoff to the motor, to ensure that you still have enough power to operate the receiver and servos once the battery power has been depleted in flight. The voltage cutoff is usually 5V or thereabouts, which means there is plenty of voltage left to allow the receiver to operate while you land dead stick. Like motors there are two types of ESCs – brushed, or brush-less. They cannot be interchanged, i.e., a brushed ESC cannot power a brush-less motor, and vice versa. ESCs are rated for the amperage of current they can handle – e.g., a 40amp ESC can deliver a constant 40 amps of current without overheating. Although ESCs can deliver higher current than their rating for very short periods of time, you do not want to exceed their rating. The choice of ESC depends upon the maximum current you will be delivering to your motor (see later for calculations). Respectable brushed ESCs are Great Planes Electrifly, Castle Creations, Jeti, Kontronic, GWS etc. Respectable brush-less ESCs are Jeti, Kontronic, Hacker, Schulze etc.
FLIGHT BATTERY PACKS
This is the source of power for your motor, receiver and servos. The number of cells determines the maximum voltage available to the motor, and the combination of current and capacity of the cells determines the duration of the flight. There are four major types of cells available right now – Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMh), Lithium Ion (LiIon) and Lithium Polymer (LiPoly). I know very little about the Lithium batteries, so I will concentrate on the NiCd and NiMh batteries.
NiCd and NiMh cells have been the mainstay of electric flight, and are still the easiest, safest and most reliable cells available. LiPoly cells are definitely making significant inroads into electric flight and will one day become the mainstay, but the NiCd and NiMh cells have not come to the end of their life. For a high current application, you still cannot beat NiCd cells.
NiCd and NiMh both have a nominal voltage of 1.2 volts per cell. The cells are connected in series to generate the required voltage. For example, 6 cells will provide 7.2 volts (6 x 1.2v), 8 cells will provide 9.6 volts (8 x 1.2v) and so on. When connected to a "load", the cells deliver less than 1.2v, but for our purposes it’s okay to ignore that for the moment. Battery packs are available in many different sizes and capacities. The cell capacity rating is usually a number like 2400, or 1700 or 600 (many other numbers are available), and it represents the current that the cell can deliver in one hour. For example a 2400 milliamp hour cell should be able to deliver approx. 2400 milliamps (or 2.4 amps) of current for 1 hour before it is discharged completely. In most cases, higher capacity cells mean heavier cells.
NiCd cells can deliver much higher amperage than NiMh cells, although NiMh cells are lighter, so there is a trade-off. High discharge NiMh cells are also available, but they still don’t seem to put out as strongly as NiCd cells! Generally, for high current delivery (over 35 amps), NiCd cells work better. But you can significantly increase flight times using NiMh because for the same weight, those cells have a lot more capacity. For example, a 7-cell 600mah NiCd pack weighs about the same as a 7-cell 1100mah NiMh pack.
You can fast charge NiCd’s at much higher currents without damaging them. A safe rule of thumb is that you can charge a NiCd pack with an amperage that is equal to 3 times it’s cell capacity rating – e.g., a 2400 milliamp hour (2.4 amp hour) rated NiCd can be charged at 2.4 x 3 = 7.2 amps. For another example, a 600 milliamp hour (0.6 amp hour) rated NiCd pack can be charged safely at 0.6 x 3 = 1.8 amps. A safe rule of thumb for NiMh is that you can charge them at 2 times the cell capacity. [Most people charge NiMh at 1 or 1 ½ times the cell capacity, but I have often read in European magazines that people have been safely charging at twice the cell capacity without any long-term effect. I do this myself.]
With repeated fast charging and discharging of a flight pack, NiCd cells tend to become unmatched, i.e., each cell ends up at a different peak or discharged voltage. Therefore, from time to time they need to be discharged down to about 0.8 volts per cell and then charged back to full capacity to ensure that each cell is brought back upto the same voltage levels. This is called "cycling". IMPORTANT: But NiMh cells should not be discharged fully or they will be damaged, and therefore should not be cycled.
Both NiCd and NiMh cells should be stored fully charged when not in use – that’s because they discharge (leak) over time, and you don’t want them to go below 0.8 volts per cell and be damaged permanently.
There is a common perception that NiCd cells develop something called "memory", that is, they do not provide their full capacity after repeated recharging without a full discharge. In actual fact, this phenomenon is caused more by repeated over charging, which makes the cell peak voltages mismatched, and the cells weaker. We tend to use too high a trickle charge current and often grossly overcharge battery packs. The whole pack then tends to behave the same as it’s weakest cell, and we see a drop in capacity. This is why it is important to use a "peak detect" charger, which ceases charging as soon as the pack is fully charged. If you feel you MUST trickle charge your batteries after they are full, please remember that the proper rate is 1/100th its capacity – for example, 2400mah packs need to be trickled at 24ma or less, and 600mah packs need to be trickled at 6ma or less!
Often, overcharged and unmatched NiCd cells will return to their original capacity if they are cycled a few times, but if the overcharging continues regularly, ultimately they will not return to their capacity and will have to be discarded. If you use a peak detect charger, you will be amazed at how long batteries last – I still have 6-cell packs from my R/C car days (over 10 years ago!) that are in tip top shape, and I’m using them in airplanes! They have never developed "memory", and I regularly recharge them before they have been fully discharged.
I have used NiCd cells that have been "zapped"! This is a process whereby each cell is connected to a very high voltage for a very short period of time. It changes the internal chemistry of the NiCd and lowers its internal resistance. This means that the cells can deliver higher currents. My research indicates there are no long-term effects of zapping – time will tell I guess! Believe me when I say it, zapped NiCd cells dish out huge currents, and the performance boost can actually be perceived in flight.
I could go on and on about the merits and demerits of NiCd and NiMh. The best rule of thumb is this: if you are wanting a high current application (i.e., over 35 amps) then go with NiCd, otherwise go with NiMh. If airframe weight becomes an issue, then opt for NiMh cells of the same capacity, as they will be much lighter.
One other thing to consider is this: if you are a meticulous person and care for your equipment, then NiMh’s will work for you; if not, then NiCd’s are better for you because they take much more abuse and punishment, with respect to charging and storage.
My research indicates that Sanyo makes the best NiCd and NiMh cells, and Panasonic are pretty close behind in the NiMh department. I have heard some really good things about KAN, but have not yet tried them.
Mini and Micro receivers work best for any electric application – there are many 4 to 6 channel, dual conversion, micro receivers available. I use either the Hitec Micro 555 5-channel or the Hitec Electron 6-channel, and highly recommend them. GWS also make micro receivers for small applications, though I don’t have first hand experience with them. I know others have used them with great success. There are quite a few single conversion micro receivers available too, but I haven’t used them. I have recently read that an electronics firm has come up with something called Digital Signal Processing (DSP), and claims to provide glitch free, single conversion performance where all other single conversion receivers have given up the ghost! But who knows!
The only rule of thumb with respect to receivers in electric airplanes is that you should mount the receiver as far away as you can from the flight battery pack and the ESC – both generate radio noise. Do NOT mount your receiver on top of (or right next to) the battery pack or the ESC as you are asking for trouble! A lot of European flyers wrap their receivers in aluminum foil (shiny side outward), and claim they have no electrical interference. I have tried it, but cannot say for certain whether or not it works.
For electric flight, as long as you have balanced your propeller, there is absolutely no need to wrap your receiver in foam; electric motors do not create any vibration as long as the prop is balanced. I always use velcro to secure my receiver in place – it will not come loose in flight unless it’s a big 3D airplane, in which case it’s better to secure it with rubber bands as well.
The only servos I have used for e-flight are the Hitec HS-55, the Hitec HS-81 and the Dymond Mini. I highly recommend them all, although it is now difficult to find the Dymond servos. GWS make Mini, Pico and Naro servos, and I know that many e-flyers use them, though I cannot speak for them from personal experience. For park flyers and airplanes less than 24 oz, the Hitec HS-55 servo is unbeatable. For larger applications, HS-55s can be used for ailerons (one for each aileron). The HS-81 is very light yet has 36 oz in of torque, and can be safely used for elevators and rudders in larger airplane. Larger electrics (5lb’s or higher) should use standard size servos.
COMMONLY USED MATHEMATICAL FORMULAE
The most common mathematical formula you will use is P = I x V, where P is the power in Watts, I is the current in Amps, and V is the voltage in Volts. For example, if an electric motor is drawing 40 amps of current while connected to a 10 volt battery source, then it is developing 40 x 10 = 400 watts.
Another common formula for calculating flight time is T = (60 x C)/I, where T is flight time in Minutes, C is the cell capacity in AmpHour, and I is the current in Amps. For example, if you have a 2000 mah (i.e., 2 AmpHour) battery pack, and you are drawing 40 Amps from it at full throttle, your flight time (at full throttle) will be (60 x 2)/40 = 3 minutes. As another example, if you have a 600mah (i.e., 0.6 AmpHour) battery pack, and you are drawing 8 Amps from it, your full throttle flight time will be (60 x 0.6)/8 = 4.5 minutes. [NOTE: actual flight times will be greater than this, because you will not need to fly at full throttle for the whole time]
Another common formula is L = (144 x W)/A, where L is the wing loading in oz/sq.ft, W is the flying weight of the aircraft in Ounces, and A is the wing area in Square Inches. For example, an airplane with 500 sq.in of wing area with a flying weight of 5 lbs (80 ounces) will have a wing loading of (144 x 80)/500 = 23.04 oz/sq.ft. As another example, an airplane with 200 sq.in of wing area with a flying weight of 1 lb (16 ounces) will have a wing loading of (144 x 16)/200 = 11.52 oz/sq.ft.
Now that we are familiar with 3 important formulae used in electric flight, lets look at e-flight design.
ELECTRIC AIRCRAFT DESIGN PARAMETERS
The principles of aircraft design, whether glow or electric, are the same. What is more complicated in electric flight, is choosing the right motor for the design. Hopefully, the following information will demystify some of it.
Design of an electric aircraft starts with the choice of a target wing loading. The following rule of thumb makes the choice easier:
5 – 10 oz/sq.ft – very light, slow flying, floater, no wind penetration [good for indoor]
11 – 15 oz/sq.ft – light, medium speed, some wind penetration
16 – 25 oz/sq.ft – fast flying, decent wind penetration
26 – 35 oz/sq.ft – heavy, fast, excellent wind penetration
[NOTE: the upper limit for comfortable hand launching is 20 oz/sq.ft]
1. ASSUMPTION: For our purposes, lets decide on a 20 oz/sq.ft wing loading (because we still want to be able to comfortably hand launch the aircraft)
The next step in the design is to pick a wingspan, and calculate out the wing area. This depends, of course, on the size of aircraft you want to build. For a scale airplane this would involve drawing out the plans for the target wingspan and then calculating the wing area.
2. ASSUMPTION: For our purposes, lets decide on a 50 inch wingspan, with a wing area of 500 sq inches
If you look back at our commonly used formulae section, you will find one that says L = (144 x W)/A. We need to adjust this so that we can find the target flight weight of our aircraft, since we have already picked our wing loading and wing area. Our High School Mathematics tells us that the formula above can be re-written as W = (A x L)/144, where W is the weight in ounces, A is the wing area in sq. inches, and L is the wing loading in oz/sq.ft. Using our 500 sq.in area and 20 oz/sq.ft wing loading, our target weight will be (500 x 20)/144 = 69.4 ounces, or approx. 4.3 lbs.
It is now time for another handy rule of thumb with respect to the weight of an aircraft and the Watts needed to fly it:
50 watts per pound - some vertical performance (park flyers, trainers, non-aerobatic scale aircraft)
75 watts per pound - good vertical performance (sport flyers, scale aircraft with some aerobatics)
100 watts per pound - excellent aerobatic performance (precision aerobatics aircraft, scale aerobatic aircraft)
125 watts per pound - unlimited vertical performance (competition precision aerobatics aircraft and 3D aircraft)
150 watts per pound - unlimited 3D performance
[Note: the weight of the airplane is the all up weight, i.e., including motor, batteries and electronics]
3. ASSUMPTION: We would like our aircraft to have very good vertical performance, so we want to achieve, say, 90 watts per pound.
So, for our target weight of 4.3 lbs., it looks like we will need 4.3 x 90 watts = 387 watts.
Going back to the "commonly used formulae" section, we now need to look at the one that relates power to current and voltage. P = I x V, that is Power in watts = current in amps x voltage in volts. Using this formula, and our target of 387 watts power, we can make this with many different combinations of current and voltage, as follows:
54 amps x 7.2 volts (6 cell pack)
46 amps x 8.4 volts (7 cell pack)
40 amps x 9.6 volts (8 cell pack)
32 amps x 12 volts (10 cell pack)
27 amps x 14.4 volts (12 cell pack)
4. ASSUMPTION: We would like our aircraft to provide full throttle flight for 4.5 to 5 minutes (note that this means as much as 8 minute flights, as full throttle will only be used on vertical aerobatics).
Going back to our commonly used formulae section, the flight time formula is T = (60 x C)/I, where T is time in minutes, C is cell capacity in amphours, and I is current in amps. Using this formula and a 1700mah cell capacity (1.7amp hours), our 6 to 12 cell options shown above provide the following full throttle flight times:
54 amps give us (60 x 1.7)/54 = 1.9 minutes
46 amps give us (60 x 1.7)/46 = 2.2 minutes
40 amps give us (60 x 1.7)/40 = 2.6 minutes
32 amps give us (60 x 1.7)/32 = 3.2 minutes
27 amps give us (60 x 1.7)/27 = 3.8 minutes
Looks like 1700 mah cells are not going to do it for us, so we will have to try 2000 mah cells. As it turns out, that will not work either, so we’ll have to go to 2400 cells. Reworking the above equation for 2400 mah cells (2.4 amp hour) gives us:
54 amps give us (60 x 2.4)/54 = 2.7 minutes
46 amps give us (60 x 2.4)/46 = 3.1 minutes
40 amps give us (60 x 2.4)/40 = 3.6 minutes
32 amps give us (60 x 2.4)/32 = 4.5 minutes
27 amps give us (60 x 2.4)/27 = 5.3 minutes
Looks like we have two possible combinations that will work for us – a 10 cell 2400 pack, or a 12 cell 2400 pack. The final choice will depend upon how good we are doing in the airframe weight, so we’ll leave the choice till much later. Since our two combinations are less than 35amps current, we can also use NiMh cells if we want. That comes later!
We have now reached our toughest decision, and that is, what motor to use. In my opinion, brush-less is the only way to go if you are to have the same performance characteristics as glow engines. Even so, there are a huge number of brush-less choices, and this is where it becomes difficult. To complicate matters, you can use gear drives to drive a larger diameter and pitch propeller, trading rpm for torque. At this point, I will keep things simple and choose a direct drive scenario (i.e., no gearbox).
Time for a bit of theory! Electric motors can handle a wide range of Volts and Amps. However, the manufacturer states the current range within which a motor is the most efficient (which means that it produces the best torque and power for the current being sent through it’s coils). For our purposes (27 amps and 32 amps), we are looking for a brush-less motor that is the most efficient at a current range of 30 to 35 amps, and can take 10, 12 or more cells. The best way to get this info is to get to the manufacturers (or retailers) web site and read their specs – look for Plettenberg, Hacker, Mega, Axi, Phasor, Astroflight, Aveox etc. Axi makes an "outrunner" brush-less motor in which the outer can rotates with the magnets attached to the inside. The can acts like a flywheel and generates very high torque. In my opinion, Mega also has a fine line of brush-less motors that will fit the bill, and their website provides all the data needed. They are easily the best value for money – they are a low buck brush-less motor being developed in the Czech Republic, and are of excellent quality. If you want to go Cadillac, then Plettenberg or Hacker would be your choices. Astroflight and Aveox also make good brush-less motors. You can also find the website of Icare (an electric flight retailer based in Montreal) and their website provides data for Plettenberg, Mega, Axi etc. The basic information you are looking for are: how many cells can the motor handle, what is its maximum current rating, what is its most efficient current value or range, what sort of propeller sizes are recommended, and what is its weight.
Not to complicate matters, there is one other bit of information that is important for brush-less motors – the number of winds in the coil. They range from 1 wind per coil to 5 or more winds. The rule of thumb is that for high-speed applications (e.g., ducted fans, or racing applications) you need a smaller number of winds, and for higher torque applications (e.g gearing and large airplanes) you need higher winds. A good compromise is to buy a 2 or 3 wind motor, so we can use it for both applications and be more flexible.
So lets say we have chosen the motor based on the max. efficiency current of 30 to 35 amps; the important task now is to record it’s weight (again, the website provides this information). Now we need to choose an ESC. This is much easier! We need to find an ESC that will handle 10 or 12 cells (those were our two possibilities in the power calculations), provide a BEC, and be rated at a bit higher than the 35 amps we will put through it. In fact, 40 amps is a standard size, so that’s what we should choose. It must be a brush-less ESC for our brush-less motor – Jeti, Hacker, Castle Creations all manufacture good brush-less ESCs. I think Jeti is a good value for money. Again, the websites will give us the weight of the ESC we have chosen, which we need to record.
This is where you get out your gram scale (AN ESSENTIAL TOOL IF YOU WANT TO DO ANY SERIOUS ELECTRIC FLIGHT) and weigh all the components – battery cells or whole pack, receiver, servos, motor, ESC etc. By the way, a digital gram/ounce scale is the best of course. According to my scale, our airborne equipment will be as follows:
2 HS-81 servos (elevator and rudder) at 0.6 oz each = 1.2 oz
2 HS-55 servos (ailerons) at 0.2 oz each = 0.4 oz
Hitec Electron 6-channel receiver = 0.8 oz
Jeti 40 amp controller = 1.5 oz
Motor = 7.6 oz (Mega brush-less)
Prop/spinner = 2 oz
This makes a total of 13.5 oz.
Now lets get to the battery. A 10-cell pack of Sanyo CP2400SCR NiCd cells weigh 21 oz. A 10-cell pack of Sanyo GP3300 NiMh cells weighs 22.8 oz. Obviously, it’s worth going for the NiMh cells for the additional flight time they will provide, as there is only a 1.8oz weight penalty. Just to verify, using our flight time formulae, the 2400 NiCd will provide (60 x 2.4)/32 = 4.5 minutes, whereas the 3300 NiMh will provide (60 x 3.3)/32 = 6.2 minutes!
So now our airborne equipment weight has reached 13.5 + 22.8 = 36.3 oz. Since our target weight was 69.4 oz, this leaves us 33.1 oz for the airframe and finishing. That’s a bit over 2 lbs for a target airframe weight.
So our challenge is to build a 50 inch wing span, 500 square inch wing area airplane which will have an empty weight (including finishing) of 33.1oz. Perfect time to discuss building techniques for electric flight!
BUILDING TECHNIQUES FOR ELECTRIC AIRPLANES
In building airframes for glow engines, we have always been cognizant of engine vibrations. A lot of plywood is used to ensure the model does not deteriorate from the effects of vibration. People tend to correlate strength with weight, so their quest to build a strong airframe ends up in grossly overweight airframes. Building for vibration is completely unnecessary in electric flight, because there is no vibration (as long as you balance your prop of course). Because of this lack of vibration it is also possible to make use of foam parts that are amazingly strong yet incredibly light. You don’t need to have fiberglass cowls, since ABS cowls will easily withstand the test of time without any vibration, and think of the weight that it can save!
The keyword in electric building is lightness and engineered strength – remember light doesn’t have to mean flimsy! Lets be clear that strength does not necessarily mean weight! For example, two balsa sheets laminated with their grain perpendicular to each other is stronger than an equivalent size and thickness of light ply, yet half the weight! Ribs can be the same strength yet half the weight, if there are circular lightening holes cut out from them.
Firewalls for electric motors need only be made from 1/8 inch. plywood (and at most ¼ in plywood for large airplanes) and you don’t need heavy motor mounts or bearers to mount them.
Finishing foam or sheeted parts with ¾ oz glass cloth and thinned down epoxy with a spray coat of acrylic paint is pretty light (remember, heavy epoxy paints are not needed in electric flight, because there is no glow fuel to attack it!). And of course the old faithful Monokote always works! The makers of Solarfilm are now making an iron-on covering that is ½ the weight of Solarfilm or Monokote!
For electric airplanes, thin and medium cyanoacrylate glue are the only adhesives required – due to the lack of vibration, medium zap is more than adequate for firewalls. Foam parts are best adhered with Weldbond polyurethane glue which is incredibly strong and incredibly light – this works well for foam-to-foam as well as wood-to-foam joints. A lot of ARF electric kits use fiberglass fuselages – they are strong and incredibly light.
Enough said on building airframes; a 2 lb airframe for a 50in. span, 500 sq in wing area airplane should be easily achievable, without sacrificing any strength at all.
Back to our aircraft design….
To complete our discussion on design parameters, if it were impossible to meet the airframe weight goal of 2 lbs for a 50in span 500sq in wing area airplane, then we would have to go back to our original assumption #2 and repeat our calculations for a larger wing area. An increase in wing area adds very little airframe weight, but allows a higher target weight with the same wing loading as before. Since our airborne components are still the same weight, we end up with a higher airframe target weight, which is easier to achieve.
We finally come to the last part in the process… propellers. Electric propellers are very different from glow propellers. Since electric motors don’t have comparable torque to glow engines, the propellers have to be designed for much higher efficiency – as a result they are much lighter, have more accurate airfoil cross sections, and often have tips designed to reduce drag. Most electric propellers are not the same pitch throughout – i.e., they change pitch the further out you go towards the tip. Electric and glow propellers are not interchangeable – a glow propeller mounted on an electric motor will be substantially underpowered, as they are much heavier, and the electric motor draws a lot more current to turn such a heavy load. An electric propeller mounted on a glow motor could be dangerous, as the lighter prop might flex too much and disintegrate!
Electric propellers come in two varieties – fixed or folding. Folding propellers were designed because a lot of electric airplanes do not have landing gear, and land on their belly. This can sometimes break the propeller, or much worse it can bend the shaft on the smaller motors. Graupner folding propellers are universally considered the best. Graupner also make a full line of fixed propellers and precision spinners. APC and Master Airscrew also make electric props.
So, how does one decide what propeller to use? The starting point should be what the manufacturer recommends for the motor – this data is usually available in the instruction sheets that come with the motor, or are available on the manufacturer web sites. Since the manufacturer does not know what application you are using the motor in, it is only a starting guideline. To get it right, you MUST have a device called an Astro Whatt Meter (yes, it’s called Whatt meter, not Watt meter!). THIS IS AN ESSENTIAL PIECE OF EQUIPMENT IF YOU ARE GOING TO DO ANY SERIOUS ELECTRIC FLIGHT. The Whatt meter installs between your flight battery pack and your ESC, and displays the following information in real time – the actual voltage that is going to the motor, the current that is being drawn by the motor, the input power (in watts) that is being generated, and the amount of milliamp hours that has been expended from the battery.
The only way to find the right propeller for your setup is to fully charge your flight pack, hook up the Whatt meter, run the motor at full throttle, and check the current that is being drawn. Remember back in the motor selection section we talked about the maximum efficiency current of the motor? Well, the bottom line is that you have to find the right diameter and pitch propeller that will draw the maximum efficiency current for the motor. In our example above, we had chosen a motor whose maximum efficiency current was between 30 and 35 amps. So, we need to find a propeller that will draw approximately 32 amps at full throttle (split the difference between 30 and 35 amps), at the start of the fully charged flight pack.
The process to follow is to start with the manufacturers recommended prop, determine the current that is flowing at full throttle using the Whatt meter, with a fully charged flight battery. In our example, if it’s less than 32 amps, then you should either increase the diameter or the pitch so that the motor is forced to draw more current to drive the heavier loading. In our example, if the motor is drawing more than 32 amps, then either the diameter or the pitch must be reduced to reduce the current draw. Of course, the same rules as glow props apply, that is, bigger diameter gives you more power and vertical performance, and a higher pitch gives you more speed.
Gearing allows a motor to draw the same current, yet turn a much larger diameter and larger pitch propeller. Put another way, gearing allows the motor to trade rpm for torque. As a result, the motor can still be running at it’s maximum efficiency rpm (thus producing peak power) yet can turn a much larger propeller. This lets us use a small motor designed to operate efficiently at a high rpm, to power a large aircraft.
As an example, a motor might be turning a 7 x 5 propeller in direct drive mode, and drawing 30 amps to do it. If it were geared, it would be able to turn a 10 x 8 propeller and still be drawing 30 amps. A larger diameter propeller means more thrust, which means better performance for the same aircraft, OR being able to fly a heavier airplane!
Alternatively, with suitable gearing we can equip a small airplane with a very large propeller that produces high thrust at low speeds, which is ideal for 3D aerobatics, or indoor flight.
Gearing adds very little weight to the system, and increases thrust dramatically. Not sure why gearing didn’t catch on in glow engines, but it is pretty much the norm for larger electric airplanes.
There are many different gearing ratio’s – e.g., 2 to 1, 3 to 1, 5 to 1 and so on. So, for a 2 to 1 ratio it means that for every 2 revolutions of the motor, the propeller turns one revolution. I haven’t yet figured out if this means that the torque is doubled, but I guess I have some more learning to do! I know that if I am to progress into larger, heavier airplanes (that is, greater than 4 or 5 pounds) then I will have to get very friendly with someone who knows about gearing! I also know that geared electric motors are now powering airplanes over 20 pounds, so the sky is the limit.
I hope this article has given you some insight into one more facet of our wonderful hobby.
Interested in RC helicopters and want to learn how to fly them, but the cost and potential for crashing your new model is making you cautious.
An RC helicopter flight simulator is a program that simulates the flight of a real RC helicopter, from the controls to the physics of flight, to weather conditions. running from $130 to $200. The price you pay for the simulator will be less than the cost to fix your model after your first crash.
However, with a simulator, the inevitable crashes will not cost you time or money, unlike with a real model - you will be back flying again, in the time it takes to load a screen. This means that the flight simulators are an excellent place to gain initial flying orientation, try complicated flying moves, and gain confidence, without the worry of damaging an expensive model.
The outlay for a simulator can in fact save you money over time.
The most common flight simulators are:
RealFlight Generation 3 RC Flight Simulator, which even comes with a controller that models a real radio so you can get a total immersive experience. Models are modeled after real one but the scenery’s are more like a video game, you can buy expansions to add more models and realistic scenery’s at extra cost. This cost around $225 and is available from local hobby shops.
Reflex XTR This one does NOT come with a controller, there for you use your actual controller that you are going to be flying with, this is a good choice if you have already purchased a controller. Models are modeled after real ones and free to download, scenery’s are modeled after real ones also free to download. This cost around $225 and is available from local hobby shops.
Before you buy your first helicopter and gear it is a good idea to come out to the field and talk to some of the members. They can tell you what a good buy is and what to stay away from as they have already been there.
One of the best things that you can do is buy brands that you can get help with, if the members in the club mainly use a certain brand of radio or helicopter, then you know that someone will be able to help you with your setup. You can also get tips and pointers on what support equipment you will need as well as what to spend a bit more on for the quality or convince of use.