A Guide To Electrics for Nitro/Glow Heads

This one seems to come up a little bit as Nitro flyers look for a bit of assistance in understanding electrics. So, here's an attempt to provide some assistance. Now please understand - if you are looking for an exact answer to whether a KERJKER34 works with a 384HED you aren't going to find it here - all I'm trying to do is explain the concepts so you "get it".

Now, as a Nitro flyer you know almost everything you need to grasp electric flight - in many ways it isn't that different.

Before starting here go take a look at the Basic Electrics of Electric Flight. It may seem a little like goobeldy gook on the first read, but once you have read that come back here and hopefully by the end some of it will start making sense.

Okay, without further ado lets get into it.

Inter related nature of the Power System

In a nitro/glow model the power system is pretty much defined by one compoment. The engine - once you have defined that most other characteristics naturally flow from it. For example, while varying the fuel payload will change the flight performance of the model somewhat, it wont change the power output of the engine.

This is probably the biggest difference with electric powered flight. It is a combination of the battery, motor and propellor that you choose which defines the power output. So, you can't quite approach the problem from the "this is a .4 size model" perspective anymore (well, you can actually - eflite for example make a range of glow replacement engines - of course you need to choose the correct battery to team with it, otherwise you wont get the correct amount of power output).

How do they interrelate? Well, I'm glad you asked (hello to fans of the Curiosity Show).

The speed the motor revolves at is defined by:

1. The number of volts supplied by the battery.
2. The revs per minute per volt (or kv) rating of the motor.

The amps drawn by the system is defined by:

1. The size of the propellor (bigger prop - more amps, smaller prop - less amps).
2. The speed at which the propellor rotates (which was defined above).

Once you know the amp draw you need to:

1. Make sure every component is rated for at least that many amps.
   

Finally, the total power output of the system is defined by:

1. The input volts from the battery; multiplied by
   
2. The number of amps the propellor draws.

Now - on first glance that might seem a little tricky, but just go back and read through it once more. Each relationships is defined. Hopefully some of the explanations below will help further.

Infinite Possibilities

Okay - infinite is an awfully big number - probably not infinite, but there are a lot variations you can do with electric power. This flexibility often appears to be complexity to those first looking at electric power.

Understanding where a power system delivers

Watts = volts x amps - no doubt you are starting to get sick of seeing this, but you need to understand what this means in application to "get" electric flight.

Why? Why am I laboring the point so much? Because there are many ways in which you can get the power you need to fly your aircraft. And this is where the true elegance of electric powered flight comes through.

There are three ways you can vary the power output of an electric flight system:

If you keep your battery voltage and motor kv constant then:

• Increasing the prop size will deliver more power (by drawing more amps).
• Reducing the prop size will deliver less power (by drawing less amps).

If you keep the motor kv and prop constant then:

• Increasing the battery cell count will deliver more power (by providing more volts and drawing more amps {because the motor is spinning faster with higher voltage}).
• Reducing the battery cell count will deliver less power (by reducing the input volts and drawing less amps {because the motor is spinning slower with lower voltage}).

If you keep the battery and the prop constant then:

• Using a motor with a higher kv will deliver more power (by drawing more amps).
• Using a motor with a lower kv will deliver less power (by drawing less amps).
  

Let's say you decided you needed 150watts to make your plane fly the way you want it to (see the basic electrics of electric flight for info on figuring out the power requirements of your model based on its weight).

So how do you get to your power. The beauty of electric flight is that it can tailor to meet your other requirements with a great deal of flexibility.

For example, let's say you have a scale model and you don't have much room to spin a propellor when she is on her undercarriage. Starting from the prop size as the design input you could choose a motor and battery combination that will deliver the power you need.

Likewise you might have a 3d plane that needs lots of thrust - you will want to spin a big propellor - the biggest one you can. Once again - this can be a design input in choosing the other components.

Or maybe you just already have a battery and motor you would like to use. In this case, you can choose a prop that you know wont overdraw any of the components.

Defining Your Power System

When you are first getting into it, it probably pays not to get too adventurous. Many electric motor manufacturers will tell you a good combination of propeller and battery to use with their motor. Provided that none of their requirements are outside your plane's limits (so you can swing that size prop, the battery will fit etc) then that is probably the best way to start.

If your motor manufacturer doesn't give you the info then use online forums like www.rcuniverse.com or www.rcgroups.com - people here will be all to happy to tell you combos that work.

There some more great info on wattflyer on choosing power systems.

System Limits

Finally - I know it is alluded to elsewhere, but just to touch on it again - you can't just go on drawing more and more amps. Components are usually rated in terms of continuous amps they can provide (for batteries there is a C rating - multiply the battery's capacity by this to get its constant discharge limit - for example 2200mAh 10C battery is 22Amps maximum continuous).

If you can't measure your amp draw you need to be very careful about overtaxing your components. It is best to rely on tables provided by the manufacturer about combinations that work.

In terms of damage - it tends to be a little like this:

• Speed Controllers just tend to cut out when you try and put too many amps through them - the speed controller is safe - the rest of the model is in peril because you have no power.
• Motors will tend to take some abuse, but eventually the high temperatures will damage the magnets that make the motor work, and power versus input volts and amps will drop off.
• Batteries - LiPo and LiIon can be permanently damaged by even one session of overdrawing current from them - they are often the most expensive components and so they are probably the ones to be most careful with. NiMh and NiCd don't suffer from this but have their own issues.
  

Conclusion

There is still quite a bit more to know about this stuff, like going for pitch speed versus thrust etc. On the right hand side the RC articles link has some more stuff you might find handy.

Hopefully it is a little clearer now. If it is please feel free to drop me a line at ozrcboy@gmail.com or post a comment below. If its still all as clear as mud, or (hopefully not) you are more confused than when you started then please don't hesitate to drop me a line or post a comment - the question you ask might be the same one the other glow and nitro heads ask at the end of this - so share the confusion - we will probably all get something out of it.

Cheers,
Oz.

Plane Rescue - Getting your plane out of the tree

It doesn't seem to be that high up, but as you think about what you have in the car/at hand you realise that 5 metres is to far. The days you could scale a tree like that are far behind you, and instead you have found a stick or rock which you have been throwing at the plane in the vain hope of dislodging it.

Yes, after parking my easystar in a tree the other day due to a misjudged approach I was reminded just how hopeless this situation can seem. However, we managed to get the easystar out and I thought I might share how we did it, as well as some of the other useful techniques I've heard used.

First and foremost - don't bother throwing stuff at the plane. Unless you bowl for New South Wales or open the pitching for the Yankees you are not going to be able to generate enough power with accuracy. Even if you do manage to hit it, unless the plane is very precariously held chances are you wont dislodge it, and you may damage the plane to boot.

In general the strategy that seems to work best is shaking the plane out, but obviously you can't shake it out using the trunk - you need to shake a branch (the branch the plane is in ideally) which means you need to get leverage on the branch.

The easiest way to do this is to get some lightweight nylon rope or string over the branch in question. Now - for how to do that there are a few tricks. If the branch is low enough (say up to five metres depending on your arm strength) you can tie the rop around a stick, or metal tool, or the like and try and throw it over the branch (this will be so much easier than trying to actually hit the model). If the plane is higher than that then some other techniques I've seen suggested include using a toy bow and arrow (with the rope tied to the arrow) and using a slingshot (with the rope tied to the projectile). Both of these will give you greater accuracy and power.

Once you get the rope on the right branch (make sure the rope is long enough to reach the ground twice yeah) then you can use it to shake the tree branch and try make the plane fall out.

Now - a couple of things to be careful off:

1. Often planes are most damaged by the fall from the tree to the ground rather than the crash into the tree itself, so if possible get an assistant to do there best to catch the plane.
2. With regard to the assistants safety, be aware that all this branch shaking might dislodge a branch, sticks etc. So be careful.

So - do yourself a favour and buy 20 metres of nylon rope (very lightweight stuff) and keep it in the boot of the car to assist with plane recovery. It is best to have this with you because of course, when a LiPo is stuck in a plane in a tree the Battery Eliminator Circuit continues to draw current which will ruin it once the battery is overdischarged, so to some extent, time is of the essence. We just got my plane out of the tree in time for the LiPo to take a charge.

Here's hoping you don't need to know any of this!

Cheers,
Oz.

The Multi-role Micro Ultra Stick (MMUS)

Well, I'm not quite sure what has possessed me but for some reason I have set out to scratch build a plane. It all kind of started when Paul Daniels from NQRC mentioned to "the crew" over at Aussie BBQ and Beers at rcuniverse that he was starting on version 3 of his easybox balsa kit.

I mentioned it would be great if we could get a version that could be an indoor/outdoor flyer, enough wind penetration and power to fly in some wind, aerobatic, all in at 30" wingspan etc. Yes in other words all things to all people. I made one tiny, and probably in the long run irrelevant concession to reality. Indoors it could fly on 2s LiPo, outdoors on 3s LiPo.

So, rather than encumbering some other poor person with trying to build the unbuildable I decided to have a go at it myself instead. After all, why shouldn't a novice start with a finely balanced compromise between the best attributes of indoor, outdoor and aerobatic flight...

I haven't tried to dream the whole thing up myself, and have instead decided to loosely base the design on the Eflite Mini Ultra Stick, using its shape and proportions of wing span/wing chord/fuse length etc. The airfoil is a rough copy of the Mini Pulse XT (RIP).

So - without any further ado here are some planned vital stats:

Wing Span: 760mm
Wing Chord: 200mm
Type of Wing: Flat, built up, constant chord balsa wing.
Length: 660mm
Type of Fuse: Box fuse sheeted balsa
Wing Area: 1520 sqcm / 235 sq inches
Power System: GWS 2205 Outrunner and 2s/3s 450mAh Elegance LiPos.
Power Output: 45 watts (2s) / 70watts (3s)
Target AUW: 230g (8.2 oz)
Wing Loading: 4.6 (2s) / 4.9 (3s) (oz/sq inch)
Power to Weight: 100 watts/pound (2s), 135 watts/pound (3s)

My apologies for mixing my units between SI and Imperial - still those that work with SI are used to having to cope with non SI.


The Prototype Wing

Well I have started. The first prototype wing has been built. At the stage of these photos the main structure is done, although I have added some wing tips since this.

Already I have figured out some improvements - namely making the secondary wing spar (the vertical bit at the top of this photo) once continous piece and keying the longer ribs into it (in the prototype it is three pieces). Also, by making the wing spars a fraction longer it is possible to build the necessary support for the wing tips (not shown in these photos) into the spar.


The prototype wing took about an evening and a half of cutting out balsa and putting together. Because the wing is small I was able to do most of this sitting in front of the telly.

The hardest bit is definitely cutting the ribs. The spars are dead simple to make using a balsa stripper (see your LHS). For the ribs I cut one of each type (there are two types of rib) and used that one as a template to cut the rest finally holding them all next to each other and squaring them off with one another as best as possible. Some scratch builders have suggested to me making a template out of plywood - I'll have to look into exactly how they do that.

There's not much to say about assembly (I think). Start by keying all the ribs into the slots in the main wing spar (and secondary spar). Make some approximate 100mm marks on the leading edge block, leading edge support, tubular spars and the unkeyed ribs on the second wing spar. Start with the leading edge support and tack glue the ribs into the correct place. Then secure the back of the short ribs in the correct location along the secondary wing spar. Push fit the tubular spars (lining up as best you can) then tack glue the leading edge block. Tack glue tubular spars to ribs and glue ribs into main wing spar. If you are lucky it will look vaguely straight - if not - don't panic covering can fix many problems.

Fuselage

Watch this space. Will be a boxed up fuse (1.5mm balsa), probably nothing on the bottom. There may be some 3mm stuff just to provide some reinforcing in some places. Getting CoG on target will be vital if I am to be able to easily swap out 3s and 2s LiPos to reduce wing loading for indoor flight.

CAD Files

Anyway, I've done some CAD files - one that shows the wing ribs (there are two types) that need to be cut, and the second with a full plan of the built up wing. Watch this space for details coming soon on how the fuse is put together.

Such as these CAD files are of any use to anyone, you are free to download and modify to your hearts content. You may share them with others, host them, etc, but may not charge a fee, except reasonable costs associated with sharing them. Also you must transfer these same rights to any person you pass the plans onto.

Death of the MPXT

Well, sooner or later it was due to happen. I've lost one of my balsa planes to interference.

I was flying at glitch park (Kambah), and had foolishly let my Mini Pulse XT get low and slow in one of the worst parts of the field.

When I suddenly came to my senses and realised where I was I started advancing the throttle for power to climb away from the ground. At just that moment she started to buck violently upwards from a glitch. I went full noise. As I started correcting that she inverted from another glitch (no - not a stall or snap roll - she had enough speed). I started to correct that, but by now she was only two metres from the ground. She took another hit and went in at full throttle. The result of this is unfortunately pretty much what you would expect. The wreckage of the crash was strewn over 5-6 metres.

The plane was destroyed with no realistic prospect of rebuild. Even the wing was badly damaged (you can't see it from the photo but significant parts of the Leading Edge were crushed by the impact).

So, is there anything to learn from this - well, it was wet and I knew I was taking a risk by flying (wet ground causes reflections of radio waves creating glitches - particularly as you get close to the ground). I was also in the part of the field were glitches are worst even when it is dry.

Might be time to think about that 2.4Ghz gear again. As for the MPXT, this was her second flight of the day, and her 51st flight overall, so she just made her half century. I certainly would have liked to have had her for longer (had done two knife edge circuits that morning and was feeling well pleased with myself), but heh 50 flights - it could have been worse.

I haven't figured out yet whether all my components okay - my main concern is the Eflite 480 brushless - it looks like it took quite an impact. And what to do next - I don't know. Maybe an Eflite Mini Ultra Stick - but maybe I'll just hold off for a bit.

BTW - I've started a scratch building project - the MMUS - Multi-role Micro Ultra Stick - built along the same lines and proportions (but smaller) of the Eflite MUS, but with the idea of being an indoor flyer, but with enough wind penetration to fly outdoors. More details in the next post.