A certificate

[Erfolg]

BEB

I think you are being just a little cavalier with your efficiency.

Being quite conservative with my values , referring to the graphs that were made available some 5-10 years ago. An efficiency of approaching 90% were only achievable often, by not using realistic propellers. The more practical region was typically about 70-75%.

But this ignores switching losses from the MOSFETS and cable losses. although not plentiful as a single value, 88% was achievable although dependant on operation.

Taking the two values together you arrive at a figure that is nearer an overall 60%.

Perhaps far more useful and illustrative (being real values),  are some figures from OS. Which suggest that for a 0.46 2 stroke is 0.75 oz/min. Or a 4 stroke of 0.7 oz/min. These figures for a modest 6 oz tank are 8 minutes and 8.5 minutes.

What size Lipo weights 6 oz or 170 grams, typically this could be a 1500 3s lipo.

Do I need to say more?

There is a real irony here, I really do wish things were different.

Edited By Erfolg on 22/04/2015 15:31:28

Edited By Erfolg on 22/04/2015 15:50:37

[Biggles' Elder Brother - Moderator]

No, no cavelier approach Erf, just the experience of actually doing conversions of IC to electric, weighing the results and comparing the performance - there is no difference! The argument you use above is not a fair one Erf - you can't just compare the weight of the fuel with the weight of the battery! You have to compare the weight of the whole power system.

Ok Erf - you like sums - here we go.

Lets take a OS56a four stroke like this. OS claim 0.99hp (that's 740W near as damn it), and it weighs 461g (without the exhaust - I'll tell you what, lets add 25g for the exhaust, that's very generous of me as its almost certainly heavier than that) so that's 487g.

Let's also say that the weight of the mounting hardware will be the same as for electric (another generous assumption by me as it would almost certainly be heavier in reality).

Examining this site tells us that a 50-size OS four-stroke would use 0.75oz/min of fuel and a 70-size OS about 1oz/min. So from that, by linear interpolation, we can expect our OS56a to use about - 0.825 oz/min. Let's aim for an 8min flight time - to empty tank - so we would need 6.6oz of fuel. That's 187g.

Now for the bits and bobs - the weight of the tank, bung, clunk, tubing etc. Let's say 40g.

So, the weight of the IC power system is 487+187+40=714g total.

Now let's design an equivalent electrical system:

We will use a 5s battery - so that's 18.5v. We need 740W at max throttle - so that means a current of 40A. Now the fuel consumption figure for the IC is not based on constant full throttle - let's assume that a normal flight duty cycle would be at about 50% throttle on average. At 50% throttle the current draw will actually be much lower than half the max current - but let's make the unfavourable assumption and say the average current draw is half of the max current (very pessimistic), so that's 20A for 8mins. which is a capacity of at least 2666mAh.

Now we allowed the IC engine to fly out all of it fuel - which of course you wouldn't do in practice - so to be fair, and compare like performance, we have to allow the electric to do the same - which of course you also wouldn't do, but at least this is fair and equal for both sides. So we need a 2666+ mAh 5s Lipo. The nearest I can find is this - at 2700mAh . This weighs 320g - all-up, including wire and connectors etc.

Next up a motor. It needs to take at least 40A and swing a 12x6 - 12x8 prop (similar to the OS). For the 5s to give us around 10,000rpm at max revs we are looking for around 500kV. So how about this? It weighs 303g (without connectors) and needs a 60A ESC.

Speaking of ESC's we can use this, which weighs in at 61g.

So, electric set up - all up weight is: 320+303+61=684g

So, comparing like for like - IC weighs 714g and the electric set up (for the same power and duration) weighs 684g

There is nothing in it - which exactly matches what I have found in practice!

BEB

Edited By Biggles' Elder Brother - Moderator on 22/04/2015 21:25:44

[Chris Bott - Moderator]

You forgot the IC's throttle servo I think BEB?

If you were designing a model from scratch for each power system, you'd design a lighter one for electric too..

[Erfolg]

BEB

You are throwing numbers around like confetti.

Using OS values 6 oz of fuel = 8 mins. for an engine of approx 45 size

What size of = 1kg aircraft can you power with a 1500 3s Lipo and for how long?

I am sure you know the true answer. If you cannot see it, well we will just see things differently and I did time the duration of models before I made my decision, along with the duration of a simulated "A" test. Be sure, my decision was not one taken on lightly, as I did not want to use an IC model. The answer was conclusive, ther was a step change in duration. Also I had tried two seperate (electric) models (of my own) to be sure, the answer was so overwhelminingly in favour of the IC models, I bit the bullet. I never had a duration issue and that was without managing the throttle.

[Dave Hopkin]

Posted by Erfolg on 22/04/2015 21:46:28:

BEB

You are throwing numbers around like confetti.

Using OS values 6 oz of fuel = 8 mins. for an engine of approx 45 size

What size of = 1kg aircraft can you power with a 1500 3s Lipo and for how long?

I dunno - but 1500 3s usually weigh in at around 120g which is roughly 4oz - for 6oz I would be looking at the 2200mAh mark......

As for the aircraft of 1kg, Bixler 3 (948g) I can get 12 mins out of mine on a 2200mah and thats coming in with around 30% left.... if I ran it to deadstick around 16mins........

[Geoff S]

I flew my Ezee Pezee yesterday which is specified for an up to 60 size engine. I fly it with a 5055 890kv motor on 4S LiPo. In order to make it balance on the CoG I've had to fit 2 x 3600 4S packs (7200 4P, effectively) which gives it an easy 15 minute duration of mildly aerobatic flight. That's far longer than the 10 mintes or so I usually stay up for and also demonstrates how much lighter the electric set up is than the specified glow.

Sure it would be easy to fit a huge fuel tank (there's plenty of room) but why would you?

Geoff

[john stones 1 - Moderator]

I would think so you could have longer flights Geoff

John

[Biggles' Elder Brother - Moderator]

Posted by Erfolg on 22/04/2015 21:46:28:

BEB

You are throwing numbers around like confetti.

Using OS values 6 oz of fuel = 8 mins. for an engine of approx 45 size

No - I have been very careful to ensure that every number there is backed with hard data - links to actual real components - I'm not making this up Erf - as you would discover if you actually read it! Or if you did an actual conversion of an IC model to electric power.

BEB

[Erfolg]

Geoff

The Turnigy & Zippy1500 3s weights in @ circa 170g all or most 2200 are heavier than 170g. The Turnigy 2200 weighs in at 248g, & 227g. yes their is some variation weights all in excess of 170g and may be the odd much lighter than most, but not the norm..

The question then becomes one of what configuration of model will give you the 15 minutes (again which was measured) necessary to complete one part of the "A" test as run by my club. Pulling 200w how long will a a 2000mah battery last? would it be say 10 minutes to be completely used or maybe 5 to 6 minutes if run to say about 50%.

Then consider what size model could you sensibly fly on 200w, just about 1kg and it should have a UC.

My 2200 *2 weigh in at approx 600g or about 22oz or 30 minutes of flight of a OS 45 against my 8 minutes.

Although preferably a electric flyer, I will conceded, the unthinkable, that for duration per gram of fuel IC models still trump electric models. Yes, you can always make reference to the exception, for me it is the general case that matters.

I can see that it unacceptable that electric models are at a disadvantage to IC models for duration. Yes I concede that the world is flat, and that the planets do resolve around the Earth..

[Biggles' Elder Brother - Moderator]

I give up! "There are none so blind as those that will not see"!

BEB

[bouncebounce crunch]

Get a grip you lot.

in Oz as from our forthcoming financial year. July...

club Solo flyer is fine if club rules allow and flyer is competent.

Bronze wings is recognition proves pilot is capable to fly a set amount of manoeuvers with models under 2 kilos electric and foam included.

Silver wings over 2 kilos ic and electric

Gold wings for advanced manoeuvers

glider rules with or without power vary slightly.

extra --- no assistance from stabilizing receivers is allowed, it is piloting skills that are being tested not electronic saviours.

Wings Accreditation gives you the options of competition and display day flying. so go your A or B gentlemen.

 

 

Edited By bouncebounce crunch on 23/04/2015 06:44:27

[The Wright Stuff]

BEB, Erf, I'm not sure that it's as simple as picking a single example set of numbers and then generalizing from it. What I interpret from BEB's numbers is that the initial weight penalty of IC is greater than leccy, but that electric power then has a greater marginal cost in terms of weight gain per additional minute flight time, to use terminology from economics!

In that case, one could argue that electric and IC are fundamentally similar in terms of performance in any given model, as BEB points out, but by design, one usually adds more flight time to an IC model, compared to when designing a leccy model, because the weight penalty of putting in a bigger tank is less, per extra minute of flight time, than putting in a larger capacity Lipo. I therefore agree with Erf in the sense that when generalized to all of the available models, plans and designs, the IC duration is more-often-than-not longer than the electric. But this is by design, not a law of physics!

Of course there are lots of other factors. If changing a Lipo is perceived to be less effort than stopping, refueling, and then restarting a IC engine, then the time penalty is more acceptable, so we choose to put smaller batteries in. If I was cynical, I could also say that selling RTFs with smaller batteries also increases sales of additional battery packs. I would say that economics plays at least a big a part as physics in deciding which type stays up for longer!

Aside, I do have a bugbear about Lipo capacity. If you can only use 60% of a 2200 mAh pack, why not just call it a 1320 mAh pack and have done with it. If I buy a pint of beer I expect to be able to drink all of it!!!

[Chris Bott - Moderator]

Posted by Erfolg on 18/04/2015 10:44:16:

My clubs requirements are much higher than the "A" test, in demonstrating control, in circumstances which are quite demanding. In this case IC models have the edge, in that their duration is much higher and the added weight improves the dead stick landing, in windy conditions, the model will penetrate much easier.

Back to the question in hand, which originally was what is suitable for an A test. I think Erfolg is generalising that electric models are light and have short duration.

Well, earlier in the year, a member that I'd been teaching passed his test with an electric converted Arising Star. Same model, same weight as an IC version, and it had plenty of duration.

Last year sometime, another friend passed his A test with a 6lb electric model of mine. That's an off the shelf electric model that comes with all the servos, the esc and the motor installed, not a conversion at all.

Here is the telemetry plot from the actual A test. Note: He used 2700mAh from a 4000mAh battery and passed with flying colours.

[andyh]

I passed my (BMFA) A-test with a wot4 foam-e weighing 1.1kg, with a 3S 2200mAh battery

more than ample duration for a few circuits & fig-8's

[graeme jones]

I was puzzled by some comments above re. flight duration of IC and electric and the time taken to fly the A and B schedules, so I flew both earlier today. The A flights took a total of 4 mins 10 secs with a Super 60. The B flight took 5 mins 25 secs with a HK Rainbow, including an extra circuit due to an aborted landing. I would suggest that if the flights take much longer than that you are doing something wrong.

Graeme

[andyh]

the A-test Erf is talking about is his club A-test, which is considerably more involved than the BMFA A-test

[graeme jones]

But the OP was about the suitability of a particular model.

Which reminds me, I can't find anything above regarding the fact that there are at least FIVE DIFFERENT A TESTS. If I turned up with, say, an Easy Pigeon, I would be perfectly entitled to take the A Certificate (silent flight - electric) test. Or a Hirobo Shuttle to take the A (Heli) test. Neither of the clubs to which I belong specify which test, just that an A cert must be held to fly solo.

Graeme