“Design-in lightness as standard”

A series of articles on electric flight by Rob Ireland

MAKE A SUCCESSFUL ENTRY INTO ELECTRIC FLIGHT

Electric Starter So…you’re interested in model flying because you want to fly that quarter-scale Spitfire ... etc?
You’ve probably been advised that this type of model will not be suitable to learn to fly with, and guess what, you can’t learn to fly electric this way either!

What we require for learning on is a model type which meets the following criteria;

1) Stable in flight A model which will fly quite happily on a steady course until a course correction is made by the pilot. On neutralising the transmitter sticks, the model should gradually return to a level flying attitude, of its own accord.
2) A minimum of control functions Meaning no flaps, retracts, air-brakes or the likes. A three-function model is the way to go here, either rudder-elevator-throttle as the functions, or aileron-elevator-throttle. Both are equally good.
3) Lighter (rather than heavier) This will allow easier take-offs or launches as the minimum flight speed on a model increases with weight, or more correctly wing-loading. In the other critical phase of a flight – landing!!! – It is even more important.

To assist our flying of this trainer, we need weather conditions which by the nature of these requirements will limit the possible flight-training days;

1) Calm conditions – wind strength up to “light”. This allows the safe flying of our stable, light-weight trainer!
2) No turbulence – (part of 1 above really) If the wind strength is light, there will be a minimum risk of turbulence. Turbulence is to be avoided because it kicks the model unexpectedly, and the beginner will not be able to make the necessary control corrections to avert disaster. It spoils the flying even if you are an experienced pilot! Trees and hedges for example, in the area of the flying strip will cause considerable turbulence depending on the wind direction.
3) A clear sky – and a clear eye will make the flying easier. Excessive cloud cover can add to orientation problems in altitude and distance.
4) A warm air – helps a lot with the enjoyment factor. When the temperature is low, fingers are numb and you’re facing into wind with tears streaming into your ears, you ‘aint gonna learn anything, and you can’t convince me its fun either!

RULES OF THUMB

Models can be suitably categorised (you’ll be glad to hear) into typically, those suitable for beginners, intermediate, and expert? To do so, we apply some numbers to “rule of thumb” calculations which I will describe here.

Parameters required for these calculations are;
Wingspan S (inches) -the wingspan of the model measured tip to tip.
Weight W (ounce) -ready-to-fly weight of the model in ounces.
Wing area A (ft²) -wingspan x average chord (width) in square inches.
Power input P (Watts) -voltage (V) x maximum current (I).

These measured values are then used to derive other useful criteria, the most important to us being;
Wing loading L (oz/ft²)
Power/weight ratio P/W (Watts/lb)
Stall speed Vstall=3.7 x sq root(L) (mph)

(These rules are based on other assumptions including:
Motors are of ferrite magnet types, having efficiencies of about 55%.
Wing section is Clark Y or similar.
Wing span is in the region of 36” to 54”)

As a beginner, you don’t need to worry about these assumptions at all. I will indicate suitable equipment and almost-ready-to-fly models at the end of this section.

The table below illustrates suitable ranges of the main model parameters, for beginners, intermediate (Sunday, sports fliers), and experts (prop-hanging, tail-sliding hooligans)!

Example:

The Electrafly which I fly regularly with trainee pilots has the following parameters;
Wingspan S = 88”
Weight W=47oz
Wing area A=5² feet

ELECTRAFLY 1 (1995)

It is powered by a 7-cell NiCad pack. Under load, this operates at approximately 7 Volts. Current draw (measured) is 16 Amperes.

Power input P=7V x 16A=112Watts

From these figure we can derive;
Wing loading L=W/A=47/5=9.4 oz/ft²
Power/weight ratio P/W=112/(47/16)=38W/lb
Stall speed is approximately 3.7 x sq root(L)=11.3miles per hour

This is an ideal type of model for beginners, as shown in the table above. The added bonus with a glider type like this is it can be flown at speeds only just above the stall condition. So perhaps a minimum flying speed of 14mph?

If we now consider the wing loading on a typical high-winged model, having a span of 50,” chord of 8”, and flying weight of perhaps 4lb?

L=(4x16)/(50x8)x144=23oz/ft²
Stall speed = 3.7 sq root(23) = 17.7mph
Flight speed = 1.5 x 17.7 = 26.6mph

Here the safe flight speed above the stall speed must be maintained because the type of model will not have the same safe margin.

In this case, I would consider the wing loading to be too high. This can cause unexpected “problems” during the take-off and landing phase.
The high wing loading also dictates the minimum cruise speed to be nearly twice that of the powered glider.

The level flight speed by itself is not so much of a problem, but everything is happening too quickly. Banking left or right, making corrections when flying towards you, pulling out of an unexpected dive before hitting the ground etc. It’s a lot easier to learn to fly if the model is capable of a low flight speed giving about twice as much “thinking time” at the transmitter end!

OTHER FACTORS IN MODEL CHOICE

OK, so pull out all the relevant calculations and use the table to select your first model!

Cost is a big issue to fliers, not least to the newcomer to electric flight .........

A word of warning here:
Avoid the “all-in-deal” package advertised by many model shops. They usually comprise, model, motor, prop, servos, speed controller and NiCad, and sometimes transmitter as well.
In a lot of cases with electric models, these “deals” are offered by people who don’t know what they’re talking about!
It all goes wrong with what gets thrown into the box beside the model ...
Motor - often quite the wrong windings for the model and/or prop.
Prop - as the motor above, usually not ideal because the cheapest has been selected for the deal. It needs to be a reasonable “match” to the model and the motor.
Servos - again the cheapest available, which will be the standard sized ones. For a little more money micro servos will be more suitable, saving a lot of weight on the finished model.
Speed controller - again this will be the cheapest available. Always best to select an appropriate unit which has a current capability of about twice the expected current draw of the motor.
Nicad - usually has a couple of things wrong with its choice. The NiCad itself may be of an unsuitable size and weight for the model, and the connectors most widely supplied are usually “Tamiya” types. These are no good at all, as they cannot safely carry the current required!
Transmitter – well, this could be useful if you don’t already have one, but it probably doesn’t come without all the other bits in the “deal”. The standard servos have probable come as part of the radio set anyway, so it’s best to buy the transmitter separately.
Steve Webb Models, for example, supply a range of transmitters sold separately.

Stick with models which fly on 7-cell NiCad packs. This will ensure that the necessary peak-detect type of rapid charger for the NiCads is the simplest and therefore least costly charger available.

For charging sub-C size NiCads, Hitec’s CG315 charger is suitable. Al’s Hobbies (01908 313142) stock this costing £12.95.
Smaller cell sizes and capacities will need a charger with an adjustable charge rate, as the 3.5A of the CG315 will be too high. Ripmax Pro-Peak Xing, £19.95, available from West London Models (020 8897 2326) or similar would be appropriate.

The need for more than one battery pack will become apparent quite soon into the training sessions. Using only 7-cells means that this cost will also be low.

Also, only consider NiCad cells at this stage. Nickel Metal Hydride or Lithium Ion cells are not for you just yet, requiring different types of charger for one thing.

If you are a “builder” then there are plenty of suitable kits for building your first model from. Alternatively, ARTF (almost-ready-to-fly) kits are in abundance for electric models. Building is reduced to a minimum on these kits.

I have produced here a short list of possible models for the beginner to contemplate. They are mostly readily available from model shops advertising in RCM&E or Q&EFI magazines. All of these are suitable for operating from 7-cell NiCad packs.

Note: Numbers in last column refer to photographs below

Next time;
• Some appropriate models and equipment

Pictures of the models referenced above

2

45

68

910

12

1314