Glider electric launch conversion

[Mark Hewett]

I have been given an old Multiplex Pilatus B4 glider by a club mate who had started an electric launch conversion and I plan to finish it. It is 2.6m span and will weigh around 2.5kg with electric conversion. It looks like it will be a fast and clean little ship, more of a warmliner than a thermal soarer, probably suited to graceful aerobatics more than soaring. I am quite experienced in ic power flying, but don't know a great deal about electric flight. I have a couple of electric launched gliders (Phoenix 2000 and Vladimir Graphite 3m glass ship) but I bought them as ARTF or second hand and ready set up. The model already has a Jeti 40A Advance 40 Pro ESC, and a hopefully suitable, unknown outrunner motor with a 5mm shaft that I will try to post a picture of. The motor has no identifying markings. Looking at the size of the prop on my Phoenix 2000 (10", unknown pitch), and on my Graphite (14x9.5" or 36x24cm) I am guessing I need a folding prop of 11 or 12" dia, and perhaps 8" pitch? I'll be using 3 cell lipos.  If nobody can identify the motor from the photo, I am willing to punt that it will take 30 amps without emitting smoke, and risk it. I don't have a watt meter, but can possibly borrow one from a clubmate. So I guess I need a folding prop of a suitable size, with a spinner of about 47mm, and a prop adaptor for a 5mm shaft. Any advice will be gratefully received, particularly prop size, where to buy from etc.

[Shaun Walsh]

If you are limiting current to 30 amps on a 3s battery, that's around 330 watts in a 2.5kg model that's around 60 watts per pound so performance will be leisurely.

Regarding the prop size, what are the motor dimensions (can diameter and length) and do you know the Kv rating of the motor?

[Martin Harris - Moderator]

A 5mm shaft suggests a reasonably capable motor.  Power capability of a motor relates closely to heat generation/dissipation.  With no motor information, you could adopt a trial and error process, gradually increasing prop size and/or cell count, monitoring motor temperature after short - then longer - runs.  For safety, the motor shouldn't get hotter than you find comfortable to touch.  Work from behind the prop, as you would with IC and disconnect the battery between "finger" tests...

 

A Wattmeter is invaluable for experimenting with motor and battery combinations so beg, borrow or buy one if at all possible!

[Mark Hewett]

1 hour ago, Shaun Walsh said:

If you are limiting current to 30 amps on a 3s battery, that's around 330 watts in a 2.5kg model that's around 60 watts per pound so performance will be leisurely.

Regarding the prop size, what are the motor dimensions (can diameter and length) and do you know the Kv rating of the motor?

Is the 60 watts per pound a rule of thumb for aeroplanes, or powered gliders? I'm guessing a glider might need something around half the watts per pound due to the clean airframe? The can is 36.4mm diameter and 22mm long. There is no writing or numbers on the motor at all.  

[Mark Hewett]

1 hour ago, Martin Harris - Moderator said:

A 5mm shaft suggests a reasonably capable motor.  Power capability of a motor relates closely to heat generation/dissipation.  With no motor information, you could adopt a trial and error process, gradually increasing prop size and/or cell count, monitoring motor temperature after short - then longer - runs.  For safety, the motor shouldn't get hotter than you find comfortable to touch.  Work from behind the prop, as you would with IC and disconnect the battery between "finger" tests...

 

A Wattmeter is invaluable for experimenting with motor and battery combinations so beg, borrow or buy one if at all possible!

I like your approach - except I don't have a bunch of props to try - and I don't want to buy a bunch coz I'm trying to be poor but retired early. Any guess at a likely size prop to pull 30A with a 3 cell lipo? I realise it can only be a guess with the unknown motor, but I'm not aiming for ultimate performance, and I promise not to moan if smoke does come out...

[Outrunner]

A wattmeter is your friend here, you can run your setup monitoring the wattmeter and if necessary shutdown before the smoke comes out.

[Martin Harris - Moderator]

There are a lot of variables but could you borrow a few props for testing from fellow club members?  Many long established modellers are likely to have a stash - redundant IC props would give you some ball park indications too.

[Frank Skilbeck]

Putting a small proportion on it and running it at full throttle, checking the rpm and battery volts will give you an indication of the motor kv (kv being the no load rpm per volt), you can then use this to get an idea of what proportion size you'd need. Or just buy a known motor and then you can select the appropriate prop etc

 

Based on an electric glider I've got of a similar size and weight I think a 4s lipo would be better. 

[Mark Hewett]

18 minutes ago, Frank Skilbeck said:

Putting a small proportion on it and running it at full throttle, checking the rpm and battery volts will give you an indication of the motor kv (kv being the no load rpm per volt), you can then use this to get an idea of what proportion size you'd need. Or just buy a known motor and then you can select the appropriate prop etc

 

Based on an electric glider I've got of a similar size and weight I think a 4s lipo would be better. 

That's an idea, I have an RPM meter and some small ic props - I might be able to bodge something up...         I will be going with 3s lipos as I've already got a bunch of them, and the ESC is limited to 3s.          "Or just buy a known motor"  Just buy one! I'm shocked at your profligacy! Shocked I tell you!   :-)

[Mark Hewett]

Hopefully not a daft question......     I have a multimeter that can measure up to 10 amps, before it presumably pops a fuse. Can I place this in series with 1 of the 3 wires to the motor, and thereby measure up to a total motor draw of 30 amps?

[Frank Skilbeck]

9 hours ago, Mark Hewett said:

Hopefully not a daft question......     I have a multimeter that can measure up to 10 amps, before it presumably pops a fuse. Can I place this in series with 1 of the 3 wires to the motor, and thereby measure up to a total motor draw of 30 amps?

No, the feed to the motor is effectively a pulled supply, any readings will be meaningless, ask you clubmates I'm sure one will have a wattmeter or even a dc clamp meter.

Edited March 19 by Frank Skilbeck

[Shaun Walsh]

Looking at the photo's I'm guessing it could be an old 3542 or similar.

Replacements available very cheaply.

https://www.aliexpress.com/item/1005006159885118.html?spm=a2g0o.productlist.main.35.65c041a9ZJTFQt&algo_pvid=7fe7cb32-0758-4eb5-9abd-dbc6643b650b&algo_exp_id=7fe7cb32-0758-4eb5-9abd-dbc6643b650b-17&pdp_ext_f={"order"%3A"109"%2C"eval"%3A"1"}&pdp_npi=4%40dis!GBP!36.24!6.98!!!330.64!63.66!%40211b430817423771072228460e348e!12000036044556520!sea!UK!0!ABX&curPageLogUid=fnikYwgkt3Cc&utparam-url=scene%3Asearch|query_from%3A

[Simon Chaddock]

Mark

The required Watts/lb depends very much on how you want to fly your glider.
You are quite right about the power a glider might require if you just want to cruise about under power for duration but for many the power is set up to give a rapid climb to altitude and then let the glider do its gliding.

Electric is pretty good at doing this delivering high power for a limited time (tens of seconds) and then giving the battery a rest and let the motor cool down.

One advantage of electric is it throttles efficiently so with a suitable setup you can do both in the same flight.

 

Just for info

In a brushless motor the full amps is delivered through each pair of the wires in turn. The unused wire is used for voltage measurement so the speed controller knows when to select the next pair. At 6000 rpm it will be doing this 100 times a second.  The wonders of micro electronics! 

The only way to measure the amps effectively is what the battery is delivering.

Edited March 19 by Simon Chaddock

[Mark Hewett]

Another probably daft question. If I find a setup draws too many amps at full power, why can't I just limit the throttle channel to less than +100 to a setting that draws the maximum amps I want?

[Martin Harris - Moderator]

You can, but it’s more efficient to reduce the load by using either a smaller pitch or diameter, appropriate to the model.

[Simon Chaddock]

I mentioned motor cooling.

In your glider the motor is completely buried in the fuselage and sitting behind a close fitting spinner so there is very little airflow through to the motor. Brushless motors are efficient but at least 10% of the input electrical energy comes out as heat. In addition a brushless motor is very compact so it needs a positive airflow to effectively carry the heat away,

Just a heads up. 

[Frank Skilbeck]

You can get spinners with inbuilt cooling ducts, you just need to put in exit holes to allow the warm air out.

[Martin Harris - Moderator]

Try to get cold airflow over the ESC as well.  Make a small air scoop if necessary - and make sure there’s a larger exit - ensuring a good path over the ESC’s heat sink. 

[Peter Beeney]

 Hi Mark,

Picking up on your original post I’m very much in agreement with you, I think your warmliner observation is spot on. From the model info you mention I'm guessing that the wing loading might well be around 18-20oz/sqft, or perhaps even a shade more, if so then that could make it just a little bit slippy, in my view possibly about right for those graceful aerobatics! But it is all just a guess, some hard figures would be nice!  Relating to the motor, as Martin said it is a nice chunky lump, if it were mine I would rather think that a 30A current flow probably wouldn't really bother it that much but what I would personally consider to be the first action would be to establish it’s kV. Generally speaking, I’m of the opinion that a model aeroplane can only ever fly as fast as the prop can turn. So with that in mind for sure the very first thing that I would do would be to check the unloaded revs per minute, once I've just got the necessary little bits of kit together to do that it only takes a few moments. In this case I would very much like to see a figure of at least 1,000rpm per volt, and I wouldn't be the least bit unhappy if it was indeed slightly higher. 

 

 I hope that as a starting point this may be of some interest. If not then there is no harm done. It’s a very brief sort of starting point, maybe just a ‘pocket précis’ so to speak…  I do have to be fairly careful when posting; I’ve inadvertently got into full sized mega trouble in the past when posting what I thought at the time were just perfectly innocuous remarks about motors and watt meters etc so I don’t bother too much these days…. Please accept this disclaimer,  my ramblings are definitely not any form of instruction on how to assemble or operate your glider….

 

The very best of luck with it all.

 

PB

[Jolly Roger]

Hi Mark,

I know you’ve had lots of good advice already but thought I’d add a few comments because I’ve just been through an almost identical experience.

Similar to your (utterly classic) Multiplex Pilatus, I just finished motorising a 2.7m span, 2.5kg warmliner - a Simprop Solution.  
Like your Pilatus it has a fibreglass fuselage and veneered foam wings.  Also like you I wanted to run it on 3s LiPos because I have plenty and also I don’t aim for higher power in a glider. 

I juggled around with motor kV and prop diameter/pitch on ecalc and also got advice for this set up from George at 4max:

35/47, 1190kV motor 

11x6 folding prop

60A ESC

This delivers 520W, or 95W/lb (funny the mixed metric/imperial units we modellers settle on!) 😆

To be honest I’m used to much less power, happily hand-launching 13lb models (at 20oz/ft^2) on less than 60W/lb, but it’s quite fun to have near-vertical performance for once.

Doing it again, I’d drop to 11x5 to lower the current. Even so, my motor and esc are only radiator-warm after the 30 second bursts I typically use. 
As others have said, finding the kV of your motor is a key step that will help determine your prop.  Hopefully it’s around 1000-1200 kV.🤞
Hope this helps and have fun!! 😃

Rog

 

[Mark Hewett]

Well, the clubmate that gave me the Pilatus, found the original prop, a 12x6", that he had used, and lent me his watt meter. The setup gave the following results :  24amps, 225W, 9,150RPM. It looks like I have the option of fitting a bigger or coarser prop if I want to, without anything melting.

 

I flew it several times today and it is still in the same number of pieces that Multiplex intended - just. The wind was about 15 gusting 20mph.

 

On the first flight it needed a lot of forward stick to prevent severe pitching up at full power, the power is just enough for a moderate climb rate, and at times I failed to prevent the nose pitching up excessively leading to nasty tip stalls and at one point an unintentional loop! Full down trim was not nearly enough, but once I reduced, and then cut power, I was able to keep control, and start breathing again. With power on it was nearly uncontrollable. With power off it felt like a heavy model with the CofG too far back - no longitudinal stability - and very happy to viciously drop a wing with a height loss of 100ft or so in the recovery. But the CofG is about on or if anything slightly forward of Multiplexes 80mm recommendation. Setting up an approach with gentle turns, and keeping the speed up for fear of the tip stall, resulted in a very long landing, but I was just relieved to get it back on the ground.

 

The next attempt was with the elevator adjusted down and trim re-centered, and a throttle/elevator mix to prevent the excessive pitch up under power. This, and subsequent minor adjustments, were a great improvement. The model though was still challenging to fly, and especially to land with no flaps or spoilers.  It seems very aerodynamically clean with an excellent lift/drag ratio and a lot of momentum - nearly like a molded glass ship. Setting up a long low approach with gentle turns, washing off as much speed as I dared, and dragging the model in with small amounts of power, closing the throttle over the threshold, seemed a good technique.

 

A power off dive test did show some positive longitudinal stability and I believe the CofG is about right, but the model still flies like a heavy thing with a too rearward  CofG. I suspect the problem may be slop in the elevator control setup. I noticed this while checking over and setting up the model and thought it might be a problem. Although the elevator centres well with no load on it, applying just light pressure moves it off centre. With control inputs against air loads, I suspect that the elevator doesn't return to centre, mimicking the effect of a lack of longitudinal stability caused by a rearward CofG. With the tail off, I can just see that the slop is caused by a Z bend moving in the bellcrank under load. I guess I'm going to have to cut a large access hole in the fibreglass fin and try to solder a metal clevis to the end of the rod instead 😞 

 

 

[Mark Hewett]

I cut the access hole and found both holes in the bellcrank were oversize for the clevis pin, and wire rod Z bend. The clevis I was able to move to a different bellcrank hole. For the Z bend slop I used a drop of medium cyano, this certainly works but I'm not sure how long it will last, so I've left the access hole hatch just tacked in place with hot glue. I also added turbulator tape at 20-25% chord to the outer 25% of wing span - hopefully this will reduce the tip stalling. We'll try again tomorrow!

[Frank Skilbeck]

Have you tried adding some reflex on the ailerons for landing, will kill some lift and reduce the tendency to tip stall. 

 

How much differential have you got on the ailerons, I have flown one 4 meter scale glider that was very prone to tip stalling and was much improved with 2:1 differential

[PatMc]

You can also leave the motor running at it's lowest practical speed during the approach. This causes the prop to be windmilling & adding drag rather than thrust, the result being a steeper descent. 

If using "spoilerons" as mentioned by Frank, I usually combine this with 50% (aprox) rudder to aileron mix as the ailerons will be less effective than normal.   

Also I find for the throttle - elevator mix that it's a good idea to have a few seconds delay (If your Tx has the facility) in the elevator's return to normal when the power is cut. This allows the model to slow down as it settle to it's glide angle & speed. It helps prevent loss of altitude due to inadverant stall at this change over point.The actual time delay has to be reached by trial & error over a few flights.

[Mark Hewett]

2 hours ago, Frank Skilbeck said:

Have you tried adding some reflex on the ailerons for landing, will kill some lift and reduce the tendency to tip stall. 

 

How much differential have you got on the ailerons, I have flown one 4 meter scale glider that was very prone to tip stalling and was much improved with 2:1 differential

Aileron reflex at what sort of angle? A few degrees would make the wing faster - the last thing I want. Too much would leave little roll control. About 20 degrees or so?

 

There is some aileron differential but not much as I remember. Yes, it might be good to increase the amount.