Model Forum / Radio Controlled / Air Models / March 2008

Not really needed. If you don't push the motors as hard as the
manufacturers dubious claims,as they are generally in a draught
anyway,and there is usually an airscoop somewhere to caaptialise to move
fresh air over the ESC's. The difference between a little air and no air
is much greater than the difference between a little air and a lot of air.

Not sure what 'S' refers to here...

Basically yes.

Like any wing a prop blade generates lift and creates drag. At a
constant lift to drag ratio, the drag is a fixed proportion of he
thrust. The power loss howeber, is proportional to drag X RPM, so the
lower the RPM the less power is lost for a given thrust..

If going scale, the scale props end up MUCH lower RPM and MUCH steeper
in pitch than 'normal R/C props'

I've seen figure on the Ezone quote as 1:1 pitch to diameter and sub
3000 RPM for some models. Hard to achieve without a reduction drive.

So a typical '40' plane might work nicely on a 10x6 or 11x5 at 10000RPM,
or a 13x10 at 6000 RPM or a 20x20 at 3000 RPM

The first is what the glo engine produces. The second would be typical
for about half the power on a 400W outrunner, and the last might be a
300W or so setup on a geared electric.

Couple that to the freedom from vibration of electric motors in general
and geared electrics even more, and you don't need a massive structure
in the model, so its lighter, stalls slower, looks more realistic etc.
The only downside is wind ..you may need to actively fly the model under
power for landing to keep airspeed up..oh, and torque..you end up with
massive torque just like the full size :-) And need to be careful on
account of that..

My general approach to design is to tie the major stress points..motor,
battery, wing root and undercarriage - together with a ply and liteply
structure, and hang the lightest possible tail assembly and wing tips
on..weight is proportional to the cube of the scale and a good figure of
quality is around 3lb for 60" span, less if you can do it..and at those
sorts of wing loadings a lightplane scale model, or airliner or bomber,
needs only about 40-50W/lb to fly realistically..up to 75W for a fighter
plane is desirable tho.

At 108" that gives me a target weight of 17.5 lb, and that implies
around 700W for a bomber/airliner type plane or around 1500W for a
warbird..well within sensible budgets.

Simple approach is to look here

http://www.modelmotors.cz/index.php?page=61&product=4120&serie=20&line=GOLD

and select a motor that looks right and then contact
www.puffinmodels.com for a quote.. about £170 a pair

Plus a couple of opto controllers at about £100 each.

http://www.puffinmodels.com/product.php?prod=794

That particular motor is good for about 700W easily on a 15x10..

You need about a 5s 10,000mAh pack for a pair of those.

So about £150 plus shipping

http://www.maxamps.com/products.php?cat=24

which ends up as a shade over £500 for your power train. No starter
battery or any field equipment required. Model, charged pack,
transmitter, and switch on and fly..you will need a 12v power supply and
a £100 quid charger or thereabouts back home.

I don't know how that compares with a pair of well sorted 4
strokes..laser etc.plus mounts, throttle servos fuel tanks, etc etc and
full starting kit..

I would estimate around 30 minutes plus cruising time. More if a very
scalelike routine is flown, less with plenty of aerobatics.

If you are like me, than means you take a fully charged pack to teh
site, fly 3 x 8 minute sorties separated by plenty of fat chewing teeth
sucking and watching everybody else fly, and then go home :-)

If you want more, another pack is another £150...

And two packs, if all your planes use them,menas an hours flying on ANY
model between recharges.

Cheaper chinese motors/controllers are available, but you lose the
element of guarantee of quality, and service..