Scale Soaring UK

A really helpful thread this Chris. I’ve noted the rather neat metallic ‘button’ on the front of some of your models but never really worked out exactly what you have done. Having had a first look at the feasibility of adding an occasional use pusher motor to my AV36, there are a few issues that have made me re-think the idea. Since the model is based on the AV36 at Old Warden, I don’t want to do anything that makes it look like a halfway house to an AV45 so am now thinking of putting a motor in the nose instead, just as you describe.

What sort of power loading do you aim for? I imagine that something like 30 Watts/lb would be enough for sustainer duties. The AV36 weighs in at around 7lb so that would equate to 210w. The model already has a 3s 2200mah LiPo on board at present (powering the radio and servos through a uBEC) so on the face of it that would be enough, with a setup pulling 20-30 amps at full throttle.

this is how i have to put mine in , as cant get a long shaft sorted , just means all weight up the front and a few holes for cooling

Sorry Trev, I don't use arithmetic to calculate power to weight ratio, as it makes my head hurt. The Turnigy G series motors are helpfully rated against an equivalent sized glow motor, and as an oldie, I can relate to that. As a comparison, my 5th scale Duster, weighing 4lbs, runs on a single 3s Lipo and is just right, but you wouldn't call it over-powered, so at 9lbs I think you might want a bit more in the battery department...

Trev,
9Lbs at 75 Watts/Lb is 675 Watts (Just under one HP)

This motor:- https://hobbyking.com/en_us/turnigy-g32 ... -glow.html

With a 4S Battery will give that at about 45 Amps. RPM will be about 8500.

For best results, use a 65C Graphine battery, and a 60 Amp ESC.

Hope that helps.

BC

Now it's time to think of cooling. Although E-Assist motor runs are typically around 7-10 seconds, some cooling is essential. So, back to drilling pilot holes. One each side of the noseblock seems to do the job, aiming to exit somewhere near the central aperture. For neatness, the holes are lined near the entrance with brass or Ali tube, and then, once again tidied up with filler...

Trev, I agree with Barry, you should work on a minimum of 75watts a pound of models final weight (your first thoughts of 30 watts a pound is far too low and would be severely under powered). 75 watts will give you a decent climb rate without seeming to be a speed freak. Regards Ter.

Sometimes, it's even scale! (Full size Dart 17R)

terry white wrote: ↑26 Aug 2018, 12:37 Trev, I agree with Barry, you should work on a minimum of 75watts a pound of models final weight (your first thoughts of 30 watts a pound is far too low and would be severely under powered). 75 watts will give you a decent climb rate without seeming to be a speed freak. Regards Ter.

Really loving this thread and learning lots.

I agree 75W/lb (static) is a good target starting point.

I tend to use less power; I happily hand-launch a 10lb glider on 52W/lb, 13lb glider on 58W/lb and my 28lb Cub jumps into the air on 65W/lb.

To sustain level flight in no-lift conditions I only need about 20-25W/lb for my gliders and not much more for the Cub.

I only mention this so you know how little power we actually need to launch and sustain flight, not to suggest that you start with such a low power set up. If you aim for 75W/lb you can always drop a prop size later. Better too much power than too little.

Just to chip in a quick point on electrics... Remember that power is voltage x current? Go big on the volts not the amps! Higher voltage (and therefore lower current) - it's electrically way more efficient as it reduces heating losses. So for example move up from 3s to 4s. All the components in your powertrain will thank you for the reduced current. Only downside is that if you're thinking of moving above 6s, everything suddenly gets very expensive.

Hope you don't mind the interruption.

As you were....

Thanks for all the responses. I must admit I did wonder whether to use 30 or 50w/lb in my original posting. Back in the 80s (you know, the time when many of the modern writers seem to think that electric flight wasn't possible) 50w/lb was the holy grail. This enabled even draggy models like my Stearman to take off from grass and fly a realistic aerobatic routine. My suggestion of 30w/lb as being adequate to sustain flight on a relatively efficient machine such a glider was based on this experience so it was admittedly a bit of a leap.

My other point of reference is my T61 motor glider. This was set up to draw 45a off five cells, giving about 75w/lb (now there's a coincidence!) This positively leaps into the air compared with the full size and has a very respectable rate of climb - certainly more than 'sustaining' power.

Anyway, rather than disrupting this thread further, when I finally get round to starting this project, I'll revive the build thread ( viewtopic.php?f=12&t=1219&hilit=av36 ) and share the experience - and ask more questions! - there.

Trevor

Ok so without being caught, who will try it out on the Mynd?