FAQs


Q1 Is the Greenfinch234 suitable for a beginner to model aircraft construction?

A1 Yes, providing the builder is of a careful and patient disposition, and is reasonably conversant with the use of modelling handtools. Note that this will include sharp modelling knives and various adhesives, so if children (over 12) are involved they should be under supervision.

(Just by the way, the '234' is pronounced Two Thirty Four')

Q2 Is the Greenfinch234 suitable for a beginner to R/C flying?

A2 No! Previous experience is required on a purpose designed trainer aircraft, (plus also if possible experience on a PC model flight simulator), up to at least BMFA 'A' certificate standard. However, for pilots with basic competence the Greenfinch is NOT difficult to fly, can be set up to be very stable indeed with very forgiving stall characteristics, and is also remarkably resilient to accidental (crash) damage. (Proven! - see pdf of construction notes section 7-11, note 11.4).

Q3 Will the Greenfinch234 provide an interesting flight performance for the more experienced model pilot?

A3 The Greenfinch can be set up to be very responsive, and by precise adjustment of the upper wing incidence (provided within the basic design), give a clean entry into spins and flicks. A sensitive and accurate CG measurement device is included which enables the CG to be progressively moved rearwards to suit an expert pilot's preferences.

Q4 What do you recommend for covering?

A4 The Greenfinch is designed as a 'could-be' scale model, to re-create the 'presence' of a full size aircraft, and so using a covering material which simulates the fabric which would have been used on such an aircraft is a very suitable choice. The most well known of these in the UK is Solartex (TM), which is an excellent choice, having very good adhesion and covering properties. This material is usually used on larger models, so some may think this is too heavy for a small model, but this is not the case. Our prototype was covered with this material, and with a high powered motor and battery setup, and plenty of paintwork, still met our design wing loading (14-15oz/sq.ft.) Some parts of the model are best painted rather than covered, such as the spats, cowl, windscreen frames, struts and cabanes, and even the spinner to match the colour scheme. Cellulose sanding sealer is good for filling wood grain, and the grey and white primers and gloss spray paints from Halfords are also ideal for these small areas.

Q5 The structure is too beautiful to cover up - maybe I will just build it to look at, but if I should decide to fly it, what would you suggest?

A5 There are translucent covering films available that will allow the structure to be easily seen, though this does sacrifice some realism since full size aircraft don't usually do this. A good compromise can be to use opaque Solartex on the under surfaces and fuselage, and 'Vintage' Solartex (which is translucent), or translucent film, on the upper surfaces.

Q6 Is the Greenfinch designed only for electric power or can I use glow motors?

A6 This model was designed for both electric and i.c. motors. Nevertheless, we had in mind just 2-stroke glow motors at the time, which are lighter than 4-strokes. However, all our builders who are using i.c. motors have decided to use 30 size 4-strokes, which has both advantages (realistic sound and compact exhaust), and disadvantages - extra weight right up in the nose, making the model nose heavy. Therefore we have introduced an I.C. option, which includes relevant drawings and parts for a custom engine mount, rear mounted battery compartments and a reinforced rear ballast compartment at the extreme rear of the fuselage to offset the weight of the motor. Also available is a specially made 83 degree exhaust adaptor,(which suits OS, SC, and ASP30FS) which keeps the exhaust within the cowl.

Q7 I've decided to use electric power, what size motor and ESC do I need?

A7 This will depend upon your flying style of course, also on the overall efficiency of the power train including the propellor.(The larger the propellor the more efficient it is at converting motor power into thrust). A 150W motor is sufficient, but 200W will provide good take-off performance and gentle aerobatics including loops from level flight - but not large loops. 300 - 350W provides much better vertical performance. Motors should have a kV (ie. rpm/volt) of 900 to 1050 aprox, and with a 3S LiPo battery this will suit propellors from 10x5 to 11x5 and 11x5.5. A 30A ESC is suitable for lower power installations, 40A for sky burners.
Q8 My flying field does not have very short grass, will I have a problem with the spats?

A8 This is possible - difficulty is getting the model up to sufficient speed for rudder authority while keeping the tail down to prevent the model tripping over it's feet.(This is usual with all tail wheel aircraft of course, particularly small ones). This presents the wing at a high angle of attack, and with max power from the motor the model will try and leave the ground before it has proper flying speed.
Ideally, we want to get the tail lifted before the model reaches flying speed. If you lay down a strip of heavy gauge polythene this will smooth things out sufficiently, but will give you an interesting precision flying exercise when landing back on it! You also have the possibility of hand launching, which can be done by gripping the fuselage below the lower wing, or under-arm launching by supporting the aircraft below the centre of the top wing, and launching as one would if throwing a ball under-arm.
Alternatively, take off the spats - they are designed to be easily removable. If this is not sufficient, then bigger wheels can be fitted. Those with the kits are 1.75" diameter, but 2.25" look fine without spats.

Q9 Do I need a lot of specialised modelling tools?

A9 Some - but not a lot! There is a lot of guidance given in the Construction Notes Section 1 - which you can see in the Manual PDF download section.
Virtually all the parts that need cutting out are already cut with extremely high precision - of course - but there are some parts that need shaping. This is done with modelling knives - Swann Morton scalpel handles are most common, and there are a range of different blades which fit them. Sanding blocks are required,from coarse to fine, small 'radio' pliers, cutting pliers and tweezers. Spring clamps are required to hold some parts together, and some specialist adhesives too, though the most important one is supplied with the kit. Small screwdrivers will be required, both cross head and flat, and also a selection of small files. Specialist jigs are not required, since the structure is self jigging due to the very high accuracy of the cut components. A flat building board is required, but it need be no bigger than an A3 sheet of paper. An A3 cutting mat is also useful to help maintain the sharpness of your cutting blades. When you come to covering the model you will need some specialised equipment for that, and also soldering equipment for electrical connections if you are using an electric motor.

Q10 What extra parts do I need to complete the model?

A10 You will require covering materials, and paint to suit your desired scheme, as well as the radio control equipment, ( receiver and servos), and power train (- if its electric - battery, electronic speed controller [ESC], motor, propellor adaptor, spinner and propellor). Also RC Modellers Glue, Aliphatic resin, and some cyano super glues are useful too. Everything else is included.

Q11 Do you recommend what the extra equipment should be?

A11 In the construction notes, we give general guidance as to what is required, and usually your local model shop will be able to advise. Also the development of equipment for RC models is progressing rapidly, and so specific recommendation could soon become out of date. If in difficulty we can usually suggest a number of options which would be suitable for your particular flying style and experience.

Q12 Are there any other kits in the Finch range available?

A12 At the moment, the Greenfinch234 is the only kit available. However, the structure of the Greenfinch has been designed to be scaleable up to around 3 times the size, or even more. The next Greenfinch in development is the Greenfinch264 (64" wingspan). There are also 3 different monoplane versions planned, the Goldfinch138 (a 38" span two seater tourer/trainer, with similar fuselage and tail as the Greenfinch), the Chaffinch144 (a high aspect ratio wing motor glider) and the Bullfinch138 (wing will be reminiscent of the Red Bull Air racers). And these also are scheduled to be in a range of larger sizes. The Goldfinch138 is in an advanced stage of development, and should be released later in 2011, along with the Greenfinch264.

Q13 Since larger propellors are more efficient, is there any advantage in fitting propellors bigger than 11" diameter when using electric motors in the Greenfinch234?

A13 - part 1. Potentially yes, in as much as greater efficiency means that less power is used, (20% plus/minus 5% for the 12x6 propellor subject of the following tests below), giving a longer flying time per charge, and you'll have more thrust available through the discharge range of the battery. But that isn't the whole story
A 12" diameter propellor looks a little large on the nose of the Greenfinch234. Also, there is a much greater risk of overloading the motor with a freshly charged LiPo, particularly with the new high C rated batteries (which will have a voltage over 11V even under load).
As the voltage drops to around 10V, this is less likely, and you can enjoy the extra performance available. It is, however, very important to check with every battery/ESC/motor/propellor combination that at full power, across the charge range of the battery, that the motor (and obviously the ESC also) are not overloaded, and that good cooling is available for both the motor and the ESC.
If ground tests show that a 12x6 propellor overloads your motor - particularly with a fresh LiPo, very great care must be taken, either to use suitably reduced power (say max 2/3 throttle) for the first half of the flight time, use a smaller propellor, or fit a higher power motor. However, all this should be kept in proportion - full power of the orders of which we are discussing with the Greenfinch will give it a ridiculously high flying speed, and therefore would only actually be used in vertical maneouvres, where full power would only be used for a few seconds. (Prop hangers be warned!).

A13 - part 2. The following are detailed results from testing with a Hacker A16M and an Overlander Thumper T3536/8, with 11x5 and 12x6 propellors, with particular reference to the Greenfinch234. These are included for information and interest ONLY - note the warnings! Also note the max thrust performance figures for the Hacker and 11x5 XOAR in the last paragraph.

We are using static thrust to give a guide of performance, and although there are limitations with this particularly for coarse pitch props, it is a reasonable method of comparison for propellors with relatively large diameters compared to the pitch. (Coarser pitches tend to stall statically at high power inputs, and only unstall once the propellor is moving through the air, which effectively reduces the angle of attack on the propellor blade and unstalls it). For relatively slow flying aircraft like the Greenfinch, static comparisons are a reasonable guide (but a guide only) to actual flying performance. In-flight data logging and now telemetry will enable us to see what's really happening. However, in the meantime ...

Previous tests have shown that measuring static thrust against input power with both an APCE11x5.5 and the electric wooden Overlander XOAR 11x5, there was very little difference between the two propellors, and also little difference between the Overlander ThumperT3536/8 (kV=1000) and the HackerA16M (kV=1060) up to around 1kg static thrust, though both extend to 1200g and more with fresh batteries.

The next stage was to explore the maximum useable performance from these two motors by trying a bigger propellor such as the 12x6 APCE. We would expect the bigger prop to be more efficient, (ie less power for the same thrust) and so it was. However, the Hacker is a nominal 350W motor (max 15secs) on 3S (350/10.5V=33A), and the Thumper 450W on 4S (450/14V=32A), so we should be careful not to exceed 30A for more than a few seconds.

We should also look at how much extra power a 12x6 requires to turn at the same speed as an 11x5. Power is proportional to the 4th power of the prop diameter, and directly as the pitch. (also to the square (ie 2nd power) of the rpm, but we're considering the same rpm here). 12" is 9% bigger than 11", so power will increase by 1.09x1.09x1.09x1.09 = 1.41, ie 41%. 6" is 20% bigger than 5", so power due to the pitch increase will go up by 20%. Putting both together we have 1.41 x 1.2 = 1.69, so 69% more power required to turn at the same speed. But the motor does not have the torque to produce this power at that speed, so it will only be able to turn this propellor at a reduced speed. As the speed reduces, the power required drops by the square of the speed, and at some point, a balance will be found, but we need to make sure that that balance does not involve excessive current being drawn by the motor. The motor manufacturers will, or should, have already done this of course, hence their recommendations. Nevertheless, a little careful experimentation could be interesting, and highlight performance issues which could affect our equipment.

Kicking off with the Thumper, the 12x6 figures are once again a fairly straight line between 300grm up to around 1000grm, but offset (ie less power for a given thrust), relative to the 11x5 figures. Hence at static thrust 500grm, the 11x5 required close to 100W (Thrust [gram]/Power [Watt] ratio g/W= 5:1) which is good, but the 12x6 required only 77W, the g/W ratio now an even better 6.5:1. At 400grm, the equivalent figures are 77W (g/W 5.2) and 58W (g/W 6.9). Below 300grm the two curves get closer together, nevertheless at 300grm the respective powers are 51W (5.9) and 40W (7.5), and at 200grm - 30W (6.7), and 26W (7.7).

Going up from 700g static, respective figures are 152 (4.6) and 126W (5.6), and at 1kg 228W (4.4) and 195W (5.1). Max grunt with the Thumper, the 12x6 and a freshly charged Overlander Extreme 40C 2.6Ah was about 1.4kg (350W, 32A@11.0V, g/W=4:1), but of course this reduces with decreasing available voltage, nevertheless 1150grm was still available with 2.2Ah out of the 2.6Ah used up.

Those who have invested in the Hacker would be looking at a very similar performance to the Thumper, but would have to use caution with full throttle on a fresh high capacity LiPo, since I recorded a max static of 1.75kg, power 425W (38A@11.2V, 4.1g/W). For this test, the motor was uncowled and very well cooled, and was held only for a few seconds, so 38A was safe in the circumstances, but this is obviously NOT a recommended flying procedure. What is interesting here is that providing restraint is used when the battery is fresh, we can potentially benefit from the better efficiency further on into the flight time. For example, 1.5kg at 354W - 34A@10.4V - (T/P ratio still a good 4.2:1), was still available with 1.75Ah gone out of 2.6Ah. This is of course beyond 33A, but the prop would certainly unload in flight down to safer levels. So if you really wanted to blitz some vertical rolls well into your sortie, you probably could, providing you've satisfied yourself that you are still operating within safe parameters.

Though the 12x6 performance figures indicate longer flight times, and greater max performance, when used with caution as mentioned above, an electric 12x5 would be a better match - if it existed ...

Since I think most Greenfinch builders will fit the wooden 11x5, I thought I would recheck max grunt with that prop, the Hacker A16M and the Overlander 40C battery mentioned above. From freshly charged, there's 1.5kg static for about 10 secs, reducing to 1.4kg (370W @ 32.5A, 11.4V - T/P=3.8g/W) for about another 15 secs or so. Note that even with this prop, we are touching max current levels for the motor, and would have to rely on early restraint and in-flight unloading to operate at safe current levels. However with 2Ah gone (of average load - 10 to 15A)) from the 2.6Ah battery, there was still 1200g static available at 275W (27A@10.2V, 4.4g/W). Bearing in mind a fully loaded Finch is about 1100 gram, those with a penchant for verticals shouldn't feel short-changed.

Q14 What capacity of battery should I use for the Greenfinch234. I'm looking for about ten min flight time. Is there an easy way to judge this requirement?

A14 Your best bet for batteries is a 3S (ie 3 cells in series) with capacity around 2200mAh.
You'll get better life from your LiPos if you only use roughly 80%~ 90% of the capacity for each flight, and I've found that an 8-10 minute flight with a battery of this size tends to leave me with 10 ~ 20% left. Depends very much on how you fly of course - gently cruising around will probably give you about 12 to 15 minutes.

There is a way of getting a rough estimate. There is an approximate rule of thumb which says that for general sport flying, you need about 50W/lb installed power, which for the Greenfinch at 2lb7oz is about 120W. At 11V, this will be about 11A. (We wouldn't advise installing a motor rated at only 120W, since you may well need a fair bit more for windy days, taking off from long grass, getting a really solid hand launch, or amusing yourself with some impressive verticals - nevertheless this is still a good guide for general current consumption). Taking an average of 11A from a 2Ah capacity, will take a time of 2/11 of an hour which is about 11 minutes. If you start throwing it around a bit, this will probably drop to about 9 minutes. And if you just cruise gently about, you might well get close to 14 minutes or more.

Another way:
The Greenfinch weighs about 1100g, so to fly it around with gentle aeros, you'll need an average thrust of about 1/2 of that, say 500g. Depending on your propellor and the efficiency of your drive system (see Q13 on the FAQ section of our website), you'll probably need about 75-100watts. So at 10.5 ~11 volts, this will be aound 8 - 10amps. Drawing 10amps from say 2Ah capacity will take 2/10 hour =12 minutes. Because of higher power required for take off, and other times when you lean on the throttle a bit, you'll probably only get about 80% of that, which is about 10minutes. Bear in mind that the 2200mAh battery will still leave you with about 200mAh left, which is a good thing.

Q15 I have often heard comments about the relevance of the 25C/30C figures on battery labels --- what do they signify and isn't there a link between the amp/hr level and the max charging amp rate?

A15 If the battery has two 'C' levels quoted, the first will be a measure of how much current the battery can supply constantly i.e. without overheating. The second will be a measure of the maximum current that can be safely taken from it for a strictly limited period, usually referred to as the 'burst' current.

If a battery has a capacity of 2200mAh, this means it can deliver 2.2Amp for 1 hour. In this example the 'C' rating is '2.2'. So 25C would be 55Amps. The Overlander Sport range has ratings of 25C/35C. So the burst current would be 35x2.2=77A. The Overlander 'Extreme' range is rated 40C/60C, and the Thunder Power 2250mAh ProPower is rated 45C/90C - (that's 101A/202A !!). To estimate the time the battery will last, divide the 'C' rating by the average current taken during a flight, and this gives you the answer as a fraction of an hour, so multiply by 60 for the time in minutes. For example, taking 100A from a nominal 2A battery will only be possible for a maximum theoretical time of 2/100 hour = 1.2minutes, though at this discharge rate the time in reality would be somewhat less than the theoretical.

It isn't generally expected that these highly rated batteries would be treated so harshly of course, but the significance of the high C rating is that the higher the C rating, the lower is the internal resistance of the battery. The lower the internal resistance, the less energy is dissipated, the battery performs more efficiently in turning chemical energy into electrical power, and the battery will run cooler for any given current. It also means of course, that for any given current - particularly noticeable at higher currents - the voltage at the terminals will be higher, which in turn can drive more current through the motor. This double advantage is why such batteries are referred to as 'high power' batteries. However, there is no escaping the fact that the battery still has a finite capacity, and so drawing more current obviously reduces the time over which it can be delivered.

The maximum charge rate is also referred to in terms of the 'C' rating - hence 1C, (taking about an hour to charge), 2C (half an hour) and lately even 5C. So a large capacity battery will have a commensurately large C rating, and therefore would be charged with more amps than a smaller battery, so long as the battery is charged at the recommended C rate. So a 2Ah battery would be charged at 2A at 1C, (taking approximately 1 hour of course), 4A at 2C. A 5Ah battery could be charged at 10A so long as it is specified as being capable of being charged at the 2C rate, and so long as your charger has the power to do so at whatever the voltage of the battery is. These days, most modern LiPo batteries can be charged at 2C, though it is generally considered beneficial for the battery to be charged at only 1C if you are not too pressed for time. The benefits could be a longer battery cycle life. Some batteries state that they can be charged at 5C, if there is urgent need, but the charger must be equipped with a balancing circuit for each cell. These days, most modern chargers are so equipped, besides it is well advised to use the facility no matter at what rate the battery is being charged.

For example, one cell of a pack can become low on voltage, and if you are NOT using a system which monitors the condition (voltage) of each cell, this can result in the other cells receiving excessive charge. This can be dangerous. Consider a 2S pack. LiPo cells are limited to a maximum voltage of 4.2V per cell, so the charger would be limited to 8.4V. If one cell - for whatever reason - is 0.2V below the other, the other will receive 0.2V more, and would receive 4.4V. This could begin the breakdown of the chemistry within the cell, and it is important to be aware that this may not become noticeable until up to 15 minutes has passed. 'Noticeable' means swelling of the pack and may be followed by bursting into flames. So - ALWAYS use the balance connections. ALSO - if you drop the battery, or the battery is involved in a crash, or the battery is accidentally short circuited - even briefly, so you thought you got away with it - take the battery to a safe non-flammable place and leave it for AT LEAST 15 minutes.

After charging, or before using a battery, its good practice to use one of the modern LiPo checkers which plug in to the balancing circuit of the battery, and give you an instant read-out of the voltage of each cell in the pack. The cell voltages should be the same, or very nearly the same, ideally within 0.01volt. Some checkers have a balancing circuit which will discharge the higher voltage cells until they are all the same. Some Wattmeters have this facility built in as well.