Scale Soaring UK

This seems an opportune moment to describe my method of E-Assist for scale gliders. I would emphasise the word 'my' as there are many and various solutions out there: this one works well for my form of glider construction. The method has evolved through four previous models to my current project, and the mission statement is simple enough. To enable slope flying in light conditions when there is a danger of the model landing out, and to hide the evidence when sustained flight is not necessary. Luckily, this ties in nicely with the current WSRFC rules for electric flight, and many enjoyable flights have been made in situations when coffee consumption would have been the only choice.

Rather than fit the motor to the front of the bulkhead, it is fitted behind to avoid having to ruin a nose profile that is seldom circular. This involves reversing the motor and fitting an extended shaft out of the rear. Needless to say, I would be useless in this situation, and I usually outsource the job to my pal Smallpiece, so it's over to him to explain what needs to done...

Yes, he did warn me, but I am not quite ready yet, so watch this space.....



BC

OK, hope this helps, it makes sense to me. If not just ask.

Looks like the lines of communication to Arlesford-Stan have been re-established

So, first job, make sure F1 is securely tied to the rest of the fuselage, whilst at the same time providing a base upon which to mount the motor.
Polyster resin is simply poured into the front of the fuselage, and the fuselage stood upright as it cures. (You can angle it slightly to accommodate some downthrust, but I tend to let my Taranis take care of that)

Here's a schematic showing the set up in the Super Javelot... (In the event, as E-Assist models tend to be nose heavy, the Lipos had to migrate more to the rear!)

First, the solid nose is made up around 3mm liteply templates. Where normally lead would be screwed to the bulkhead, block balsa is placed instead to cut down the amount (and weight) of filler used...

The filler is applied in instalments and roughly shaped with 40 grit in between applications

chris do you use normal car filer - i haveisopo is that ok ?
i’m fitting motor , and want to say thanks to you and barry for your advice
all clear now
do you always put down and side thrust in if so how much ? have been told about 3degrees which seems about right , but not sure if need down thrust ?
mick

Mick,
As far as I know, no down or side thrust. Not intentionally anyway. They all climb like the proverbial angel, but some down elevator and/or throttle control will hold it. After all, all we want it to do is climb.

Don't over complicate it.

BC

That's right, I don't worry about either side or downthrust...Any variations can be controlled by your thumbs or by the transmitter

Any decent car body filler will do the job

that’s good news
lot easier to set up

Once the nose is more or less complete, it's time to drill the central hole for the shaft. It's best to start with a pilot hole so that any navigational errors can be rectified as you open the hole out. The engine mounting plate is held in place with screws, as the thrust forces push the motor into the bulkhead, and the the rotational forces are easily contained. With the motor screwed in place, the bearing, if one is being used, should be able to slide into place without exerting any bending forces on the shaft. The hole can be enlarged quite easily with a small drum sander in necessary. Once epoxied into place, the front end can be tidied up with a drop more filler. Most of the prop gubbins is now neatly concealed inside the nose...

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?

Geoff Pearce wrote: ↑27 Aug 2018, 13:13 Ok so without being caught, who will try it out on the Mynd?

naughty!

here is my attempt at e-assist

Just perfect Mick well done. I like the threaded recess and nose piece, Brill. Ter.

Well, inspired by this thread, my AV36 has joined the Movember movement. More details at:
http://www.bartonhewsons.uk/home/modelf ... heav3.html

Hi Trevor, super pici's and motor install thread, please keep us informed on its progress and maiden. Ter.

Nice job, Trevor...

Thanks for the kind words guys - and also to Barry C. for his advice and encouragement. I've now ventured outside and run up the motor. Full throttle current draw is around 45amps giving about 500 watts. This works out at around 70w/lb which ought to be okay. It may have to wait until the Spring to find out though - at this time of year I tend to reach for the models that involve little or no slopeside assembly!

Well done Treror, looks nice, look forward to seeing it sometime.
Has it been maidened ?

G

Yes, as a glider it's pretty well sorted so, as long as I've fitted the cg back in the same place and the thrustline isn't too far out, there shouldn't be any surprises on the re-maiden. But you've heard that before

how much downthrust did you go for Trevor?

Be interested to know, given the very short moment of the model.

My Ash, I guestimated 5 degrees then used an inclinometer to set it all up, turned out to be perfect from the first flight, I was very lucky...

My little MPX merlin looks like it has about 10 degrees!

Ell.

Since I didn't have a figure to go on, nor a line to measure it from, I just slid my cardboard template onto the nose, eyed it up from the side and drew round it. It certainly doesn't look like it's got a lot of downthrust but at least I'm satisfied that it's not pointing up! I'm expecting some pitch up under power (the idea is to climb, after all) but if it's too much, hopefully a bit of elevator mixed in with the throttle will tame it.