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To bec or not to bec.


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Hi Nigel

Not thread divergence at all.

Thing is I still don't know if I will be staying with the glo set up. Testing yesterday showed up a couple of things I would like to improve, but only after that will I decide whether to stay.

Seems a bit pointless to buy 4 gallons, then give 3 1/2 away.

Once I am settled with retaining the glo I can buy in quantity. In fact Weston UK will do 4 gallons free shipping. That would be my way to go.

Jeff

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13 hours ago, Jeffrey Cottrell 2 said:

Hi Peter

 

Agreed, the specs never seem to tell you what you need to know, especially for such a useful gadget.

I got my info from the FlyFun manual, here.

Look on page 2 under 'Programmable Items'.

Seems to me on a lot of esc's you can programme the brake strength, but only to one setting, which may not be appropriate to what you need on a particular flight. The advantage of proportional braking is that you can adjust the brake strength in real time as required, using

0% to 20% on the stick.

Might or might not be an advantage, your call.

Nice motor you linked me to, but this is the one that caught my eye. UK warehouse too.

I know that static current measurements are not that indicative. I do not have telemetry, but I normally fly a mixed flight, but for a specified time, then measure what the charger puts back in.

Then I can work out an average current draw, which relates much better to the real world.

Added bonus is that over a period of time, if a pack starts to take more charge than usual, that's a sure sign it's coming to the end of its life and needs to be retired to less arduous duties.

As regards your book 'how quickly they forget'. I bought it when it first came out, and that's what started this journey down the rocky road.

Having said that, if it all ends in tears you know who I'm going to blame, don't you?

 

Jeff

How quickly they forget! indeed.  I see that you were one of the first to buy the book!  Apologies for not connecting - it's called getting old and senile!  

 

Re average current consumed calculated from what you put back into the pack will not stop you burning out the motor if you have over propped the motor.  If you already have telemetry and that includes the drive motor V and A or just Watts then you have a chance of determining the max current draw or power demand.  Generally, current draw is the one to watch.  If you never see more than the max rated current draw on a full throttle vertical climb then you are safe with that prop.

 

Thanks for the link to the FunFly manual.  I see that as you say there is proportional braking from 20% of throttle stick to fully closed.  I would only suggest that this is a level of difficulty that I would struggle to cope with along with everything else that you are looking to deal with e.g. have you allowed enough for the cross wind, are the wings level is the height right to start the pull out, is the speed right or too slow/fast.  Bottom line is there is an ordinary brake function that operates with a fully closed throttle and that can be adjusted to 7 levels.  That is a great boon as you can just close the throttle and know that your selected brake force will do its bit.  Also, on a 45 deg downline you may not want any brake so just closing and then opening the throttle a tad will stop the braking.  This ESC is the first low cost one that seems to offer this level of brake adjustment.  I should get it.

 

One thing to be aware of.  If you have had to programme in a touch of down elevator to give you a true vertical downline in still air conditions, then as the brake is activated so is the down elevator. Consequently, when looping or bunting and you are on the downside avoid fully closing the throttle to avoid a slight change of model attitude and hence loop/bunt geometry if the down elevator gets triggered.  How do I know?  Guess!

 

It will never end in tears Jeff because you will enjoy flying F3A machines and the constant learning curve that F3A generates that improves both your understanding of what's going on and seeing your skills improve.  At the end of the day, it's the amount of practice you put in that does away with the tears.  

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On 18/07/2021 at 17:22, Peter Jenkins said:

Hi Andy

 

The braking function on the majority of ESCs only comes into play with the throttle fully closed.  

 

So, with part throttle on an electric you have less braking than on an IC.  

 

 

On the majority of ESC's the brake function at throttle closed is merely to stop the prop it won't act as an "airbrake" in fact probably the reverse.
OTOH at part throttle the "airbrake" effect is very effective as witness it's use by electric thermal glider flyers in steepening the glide without an increase in speed in lieu of spoilers.  

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Any windmilling prop provides more drag than a stationary one.  For an IC engine, given the amount of energy required to drive an engine by the prop compared with an electric motor I believe there is more drag generated by an idling IC engine. 

 

The majority of ESCs used for F3A aerobatics have a braking function that is not a simple on/off switch and is extensively used to maintain a constant downline speed that is the equal to the upline and horizontal speed.  An IC engine does this quite well.

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1 hour ago, Peter Jenkins said:

Any windmilling prop provides more drag than a stationary one. 

 

That's why I said that on the majority of ESC's the brake function at throttle closed is merely to stop the prop it won't act as an "airbrake" in fact probably the reverse. In fact not using the brake function would seem more useful in this case.

 

1 hour ago, Peter Jenkins said:

For an IC engine, given the amount of energy required to drive an engine by the prop compared with an electric motor I believe there is more drag generated by an idling IC engine. 

 

I'm not convinced by this supposition. It's the rpm of the prop that matters, I don't see any reason why an engine would require more energy than an electric motor to maintain the same rpm at part throttle. 

Edited by PatMc
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I didn't express myself well.  What I meant was that for the airflow trying to speed up the prop as the aircraft is in a vertical descent, it requires more energy for the airflow to speed up the prop on an IC engine rather than an electric motor.  Hence the need on an electric motor to use the back emf, when it's being driven like a dynamo, to provide the required braking force.

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Hi Guys

Peter
Agree with everything you say, except:
If I do a static test before committing aviation and that shows below the motor max current, I can be reasonably assured it won't go pop in the air.
Not only will the prop unload with airspeed, but most engine issues are to do with overheating, and with thought to cooling air getting in and out, that too will be reduced.The transmitters I use are early model JR ones, the 2610 and 2720, converted to 2.4 with either a FrSky module or the new Lemon one.
Not the most up to date, but so far ultra reliable. Only thing is they won't accept telemetry, so I have to use other means of monitoring battery condition.
btw, did you notice that the motor I linked to has a max current of 90a. If I ever reach that level, something is drastically wrong.
Take your point about Proportional Brake on the FlyFun. I had thought it would be useful, but hadn't considered the increased workload.

PatMc
Generally 100% brake is for folding props, to allow them to fold and reduce drag. With no brake the prop is free to windmill, driven by the airflow.
However, it seems to me that a windmilling prop, with a partial brake set would provide some drag, depending on brake strength.
Sort of like a parachute, with the brake strength being the number of holes the ME109 shot in it.

Meanwhile, while I'm leaning towards the Hobbywing esc, starting to wonder whether they actually exist.
Notice Electric Wingman doesn't list them. Both Aliexpress and Hobbywing USA show them, but when you look at the accompanying picture it clearly shows 6s.
Got a question in to HW USA see if they can resolve this.
Other than that the Plush 32 looks a good option. This is 120a which I initially didn't want. Seemed the ones I looked at charged around an extra quid per amp. However, HK EU warehouse has these at £55.49, so not bad at all.

Final point PatMc, though I think Peter's covered it as well.
On an ic model, the lowest prop speed you can get is tickover, otherwise it's a glider. So, a prop at tickover provides a braking force due to the airflow trying to spin the prop faster than the motor will allow.
So with an electric motor, if you set the brake strength so the prop windmilled at the same speed as an ic motor on tickover, would that not produce the same effect?

Or have I got this horribly wrong?

Jeff

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11 minutes ago, Peter Jenkins said:

I didn't express myself well.  What I meant was that for the airflow trying to speed up the prop as the aircraft is in a vertical descent, it requires more energy for the airflow to speed up the prop on an IC engine rather than an electric motor.  Hence the need on an electric motor to use the back emf, when it's being driven like a dynamo, to provide the required braking force.

That still doesn't explain why an IC engine requires more energy from the airflow to speed up than an electric motor. The back emf of the electric motor is working in opposition to the output from the ESC, the RMS value of this voltage being set by the throttle. 

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Jeff 

For an electric motor, the worst case is when it is working hardest i.e. the aircraft is tethered and max power is being generated.  As the load comes off the prop the power being generated by the motor drops as does the heat being generated.  This is analogous to putting a small diameter or lower pitch prop on to the motor to reduce the current draw.  That's why the worst case in the air is accelerating vertically from a slowish speed.   My telemetry tells me that as soon as I land my electric aircraft and the airflow drops to zero the motor temperature will immediately start to rise and does so by as much as 10-15 deg C above the airborne figure.  I did a trial by taking off with the motor that hot and as soon as I got airborne the motor temperature dropped back to the normal airborne temperature.  I can see the temperature displayed instantaneously on my Tx screen (a JR XG 11 by the way).  

 

PatMc

While what you say about back emf is true, the practical situation is that without some form of additional braking an electric aircraft will tend to speed up on the downline.  That is a fact.  So, if you don't want to fly at the speed set by that downline speed you need additional braking.  An IC model does not display the same problem.  An example of what can be done to increase prop braking is to fit a 3 blade prop or use a contra rotating arrangement.   I still use some brake on my contra but not anything like as much as for my single prop machines.  So, rather than a hypothetical argument about which provides better braking, an IC or electric motor, I merely have observed this to be the case and it is not a minor difference.

 

Peter

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Hi Peter

Exactly my point.

If a tethered static test is the worst case, and I do one which shows the current draw still to be within the limits of the motor, highly unlikely I will run into any trouble in the air.

Needless to say, I don't run a static test for more than a few seconds, otherwise heat build up does become an issue.

Changing the subject completely, just done a rough and ready weigh in of the model in ic form.

Got a total weight of just under 9 1/2 lb. Does this sound in the ball park?

Jeff

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Jeff

My experience with a Gangster 75 may be relevant.  The empty weight of the Gangster is 9 1/4 lb.  It has a 12 oz fuel tank so fully fuelled it weighs 10 lb.  The engine is a Super Tigre 90 on a Hanno pipe and turns a 14x10 prop at 9,100 static.  The vertical performance is inadequate for F3A work but as a club aerobat it is perfectly satisfactory.

 

What engine is your 120 pumper a four stroke?  If yes, I would suggest that it will have around the same grunt as my ST90 two stroke.  You can see that with your Excelsior being a tad heavier than my Gangster its performance will be similar i.e. not sufficient for doing proper precision aerobatics but OK as a club aerobat.  

 

My two electric 2 mtr aircraft weigh in at 11 lbs with flight packs and have almost 4 hp available.  The single prop is a 21 x 13 3 blader.  The Contra is turning a 22x18 front and 22x20 rear prop.  Both will accelerate in the vertical upline if too much power is selected and that's not full throttle.

 

By the way, what is the fuselage length for the Excelsior?  It looks like it is a 2 mtr aircraft i.e. fuselage is 2 mtr long and wingspan no more than 2 mtr.  Is it?  If yes, then you need a YS 140 as a minimum to achieve the required vertical performance.  At that point, an electric setup using 10S (2 x 5S in series) will be the way forward and you'd be looking for at least 99 amp ESC and a 21x12 prop.  If the fuselage is around 1.9 mtr then your proposed 8S setup will be fine.

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Hi Peter. Thanks for all your help, much appreciated.

 

Here's the dims. on the Excelsior.

 

Wing Span             185cm   (this at trailing edge to account for tapered tips)

Fuz length, total    157cm   (stern post to F1)

Tail to wing TE      100cm

Wing Chord           42cm

Wing LE to F1        15cm

F1 to prop driver   15cm.

Haven't included the rudder. This is tapered from 19.5cm at the bottom to 11.5cm at the top.

 

My motor is a two stroke, running through a pipe some 57cm long. Prop is 16 x 12 APC.

 

Had quite a bark to it when running, but quite low pitch. Also we were running it inverted without the wing on, so the rear of the pipe was floating. In fact the pipe came adrift a couple of times. Sounded really good on open exhaust!

Didn't have it running long enough to put a tacho on it. Maybe next time.

 

So far, pulled the tank and cleaned it out. Everything seemed ok, apart from the clunk tube looked too long. If the clunk was sucking itself to the tank wall could explain why we could only get a few minutes running. Also replaced all the tubing. The original was probably ok, but it had too many  joins in it for my liking. Anal or what.

In the process of re-doing the throttle linkage. Original used a Hitec 311 servo connected to the carb by a piece of thin bendy wire with no support. Think I can do better than that.

Ordered some support bits from LMS, so all I need to do now is get some fuel, and good to go.

 

So far, looks like I'm talking myself into keeping the ic on it. All depends whether it will hand start. Don't want to carry a starter and its associated battery.

 

Time will tell

 

Jeff

 

P.S. Had a quick look at your Gangster blog. Saw the picture of the completed model. Wish I could decorate that well, very nice job.

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Hi Jeff

 

Guess you must be shorter than me!  Today's designs have a shorter wing span compared with fuselage length which has been driven by the need for good snap or flick roll performance.  That package with a 120 2 stroke on a pipe should do the job!  Don't forget to blow out all the carb jets to make sure they don't store up a problem for you. 

 

As regards starters and batteries, I now have a pair of 3S LiPos that provide either a 12 v start for normal stuff or 24 volt for my big stuff -  that's the 35 cc petrol as I don't really like hand starting that size!  No lead acid cells needed and much lighter.  If I could only just spend the time on it, I'd attach one Lipo to the starter so I don't have to carry it separately.

 

I'm getting quite excited to hear what happens next!

 

Yes, the Gangster colour scheme was pinched off one of my earlier electric 2 mtr birds.  The underside was just to keep it simple but effective at making sure I could tell which way up it was!  It took a bit of time to get the fuselage and fin done!  The wings were a piece of cake in comparison.

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Hi Peter

 

Shorter than the average bear, boo boo. God, that dates me!

Actually we've touched on this before.

If you recall I posted a picture of my Smart Move after I removed the damaged wing centre section. Your comment was that it still looked in proportion, but going from 60" span down to 52" had made it look more modern.

With my lack of experience with glo motors, going to leave work on the fuel system until I'm really sure it needs it. When I got the model home I had to wipe off some fuel from the cowl, and this looked fresh, so my guess is it had been run fairly recently.

If it ain't broke don't fix it.

Funnily enough, my clubmate remarked that the battery on his starter was getting flat, so close of play for that day. Then he idly flicked it and it started up straight away.

Interested myself to get it in the air, but it might be a while. So many projects have taken a backseat because of this one, really need to catch up on some of those.

Just one quick question on starters.

Notice you use 24v to start your big engines. Is this with the same starter? I have a couple of big helicopters I will be breaking for spares so that frees up a number of 5000ma 4s packs. Could I run a starter on those?

Fought shy of treating myself at the moment. One more expense I may not need if I go electric, but if I can use my existing packs then I might be tempted.

 

We'll see

 

Jeff

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Hi Jeff

 

I have a Sullivan Dynatron that is quite a beast.  It is rated for 24 volts but will also start up to 90 engines on 12 v.  I originally got it to start a Saito 180 that was in my first big aerobatic bird, the Capiche 140.  The power with 24 volts behind it is quite something.  There is also a geared starter that is sold by Just Engines that will also start bigger motors with ease.

If you have a starter that will take a fully charged 4S, that is 16.8 v, then you can use one.  Most standard starters will only take 13 or so volts being the 12 V nominal.  Used for a short time, I doubt that the 4S would kill your starter but for continual use I would stick with a 3S.  Sullivan Dynatrons seem to hold their price so even a second hand one, assuming its working won't be cheap.

 

Peter

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On 16/07/2021 at 23:29, Jeffrey Cottrell 2 said:

Hi guys

Apologies to the Bard for taking liberties with his words.

Here's the deal:

Looking for an esc, around 60-70a, but for an 8s set up, which, I believe, means opto. Trouble is I can't find anything below 120-150a and a similar price tag in pounds.

Any suggestions?

Other than that, my current (ahem) understanding is that anything above 6s, the bec can't cope with higher voltage. So, what would happen if I used a 6s esc but disconnected the bec. Would this let the magic smoke out?

Your thoughts?

Jeff

 

Just my 2d-worth on HV electric setups:  I've got a couple of models running on 12S, which is two 6S packs in series.  The 12S ESCs are BEC-less (but not opto), and I supply power to the Rx and servos from a stand-alone BEC connected to only one of the packs, so it's running within it's voltage capability.  With 8S you're probably looking at two 4S packs, if only because many chargers can't handle 8S, so a similar setup could be used.

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Hi Alan

Thanks for that, good thoughts.

I used to run a couple of big helicopters on 8s (2 x 4s in series) but since my charger will charge 8s I made up an adaptor lead so I could balance charge as a single 8s pack.

Only problem I can see with your system, and it's only minor, would be that one pack would be slightly lower charge than the other. Would still balance charge ok, but might take a little longer.

Mind you, if I did go for a stand-alone bec, this would be my weapon of choice. This one is 12s capable, so you could run from positive on one pack, and negative on the other, and keep them both at similar states of discharge.

Just my thoughts

Jeff

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Slightly different discharge of the packs doesn't concern me because (a) it's only a small difference and (b) they're balance-charged individually.  I don't think that 12S BEC was around when I assembled my 12S setups (conversion from OS120 glow) about 10 years ago, but it could have simplified things a little.

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Hi Guys

Well, looks like I'm going the ic route with the Excelsior. Got the motor running. Not perfect, but encouraging. So, got most of the support gear I need, but most significant, bought some fuel.
'I love the smell of methanol in the morning'.
Having done all that, seems a waste to not at least try the smelly stuff.
So, on to the rest of the model.
Got a question about servos. Bit of a thread drift, I know, but Peter lurks on here, and I think he might be the best person to answer.
If need be I'll start a new thread.
So, model currently has Futaba 3003's all round. Probably perfectly serviceable, but I think I can do better.
Got a couple of old helicopters to dismantle. All equipped with metal gear digitals, mostly JX ones.
Two spring to mind for the Excelsior ailerons.
First is a PDI 4409 MG. 7.8 Kg torque at 0.13 sec. Second is a Hyperion DS20GMD. Lower torque at
3.2 Kg but very fast at 0.07 sec.
So, which would be better?
Bit of an oddity with the JX ones. On test they seemed to be relatively slow, more so that the transit time would indicate, but also did not do 60 deg each way as they should, more like 45. Never noticed that on the heli but then again control movement is at the swash, not the servo arm.
Having said that, I am using one of my JR tx's which has atv adjustment up to 150%. Winding that up to max, not only did I get the throw I was expecting, but the transit speed was noticeably quicker.
I read somewhere that these servos were programmable, but I've never done so. So, could this be the issue?
Specs here, but not really clear.

Your thoughts?

Jeff

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On 19/07/2021 at 10:26, Nigel R said:

 

A bit of thread divergence, but, I wouldn't view 4 gallons as "large" quantities when you're playing with a 20cc size glow. Even with an economical four stroke instead of a thirsty two stroke, I'd imagine you'll burn 12oz a flight.

 

IIRC there is ~150oz in an imperial gallon.

 

4x gallon is thus 600 fl oz.

 

600 / 12 = 50 flights.

 

 

Hi Nigel

 

Just a follow up to something you posted earlier.

Not sure what it is in oz, but I measured the size of the tank and it comes out to some 490cc, which translates as 0.86 pint. (actually turns out to be 16.5 oz).

Anyway, I ordered 4 x 1/2 gallon of 5% from Weston UK. This on the basis that they would ship in 1/2 gal for a very reasonable £4.50.

So, if the tank takes 0.86 pint and I use a full tank each flight, I would get just less than 5 flights out of each bottle, or 20 for the whole lot.

Bear in mind that, although I would love to get this old bird in the air, it will never be my go-to model each session.

Could well be that 20 flights might be a whole season's worth.

So, while my results are broadly in line with yours, don't think that's much of a drawback.

Just my thoughts

Jeff

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2 hours ago, Jeffrey Cottrell 2 said:

Hi Guys

Well, looks like I'm going the ic route with the Excelsior. Got the motor running. Not perfect, but encouraging. So, got most of the support gear I need, but most significant, bought some fuel.
'I love the smell of methanol in the morning'.
Having done all that, seems a waste to not at least try the smelly stuff.
So, on to the rest of the model.
Got a question about servos. Bit of a thread drift, I know, but Peter lurks on here, and I think he might be the best person to answer.
If need be I'll start a new thread.
So, model currently has Futaba 3003's all round. Probably perfectly serviceable, but I think I can do better.
Got a couple of old helicopters to dismantle. All equipped with metal gear digitals, mostly JX ones.
Two spring to mind for the Excelsior ailerons.
First is a PDI 4409 MG. 7.8 Kg torque at 0.13 sec. Second is a Hyperion DS20GMD. Lower torque at
3.2 Kg but very fast at 0.07 sec.
So, which would be better?
Bit of an oddity with the JX ones. On test they seemed to be relatively slow, more so that the transit time would indicate, but also did not do 60 deg each way as they should, more like 45. Never noticed that on the heli but then again control movement is at the swash, not the servo arm.
Having said that, I am using one of my JR tx's which has atv adjustment up to 150%. Winding that up to max, not only did I get the throw I was expecting, but the transit speed was noticeably quicker.
I read somewhere that these servos were programmable, but I've never done so. So, could this be the issue?
Specs here, but not really clear.

Your thoughts?

Jeff

Hi Jeff

 

I think the 3003s are marginal for a 120 aerobat.  Also being analogue they will not reliably hold their position for a lkngis period of time.  A min of 5 Kg cm should be adequate.  We want an accurately centreing servo and a medium speed is fine.  High speed servos are unnecessary.

 

Mount a large pointer on the servo arm and deflect the servo one way and then the other and mark the pointer position when the servo has centred. If it is identical then its a good servo to use.  If the centre is different when returning from left and right then its best not to use it.  A wandering centre will mean the aircraft cannot be trimmed accurately the prerequisite for accurate flying.

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