Park flier electric project, motor spec ?

If I increase the wing length to 60" instead of 41" that should give me approx 2.5 square feet.
Would the consensus be that that would make a more stable, easier to fly model, or would I still need to increase prop diameter as well ?

If you increase wing area by increasing span, you will also need to increase the nose and tail moments, and tail areas, to keep everything in balance.
That would cause the mass of the model to increase, which will need more power to fly it, which will need a more powerful motor to turn a bigger prop. That in turn will need a more powerful speed controller and a bigger battery, which increases the weight, and round we go again! :evil:

Funny thing is I new what was coming next, the minute I posted my comment this afternoon.
Already crossed my mind about increasing the tail area. However, I don't need all the fuselage ? So I can cut a lot of weight there example being the flying stick. I can also increase surface area of the tail section using a lighter design.
Then it occured to me that this aircraft would no longer resemble the original so may aswell of bought a purpose built model to start with, which defeats my original plan to fly this particular model again but with electric power rather than IC.
So back to the prop solution !

By 'increase the nose and tail moment' - I mean make them longer, not thinner!
If you go too thin, you lose torsional stiffness, and bendy fuselages give VERY funny flying characteristics!
There was a well-known manufacturer that produced a foamy electric glider with little or no carbon stiffness.
It flew very nicely, until it got into a dive. Apply up elevator, the fuselage bent down, and you got more DOWN ele as a result, because of tailplane incidence, even though the control surface went up!
A skilled pilot then realized it was going inverted, and automatically pushed DOWN ele, to carry on round, but the fuz then bent the other way, and gave up ele, resulting in a very short pile of bits! [badass]

Swembo comes first, ask me how I know!
I confess I don't have much experience with the smaller electric setups. Having experienced to anguish and Adrenalin of dragging a model at almost stall around the circuit at full throttle I made the decision long ago to go for more powerful setups that could be throttled back, it's less hassle. Having said that I don't use any other battery type than 3s Lipo, because they are so popular the cost per mAh is low compared to any other kind. I don't bother to fit ESCs that have 'headroom' but then I do limit the amount of full throttle (WOT). The advantage of having too much power coming to the fore here! And I always check the temperature of the ESC after landing (easy to do when you reach in to disconnect the battery). I only had trouble with ESCs when I was trying to use can motors in the early days of my return to the hobby. The day I compared the weight of a can motor with that of a 3phase outrunner was the day I chucked the cans in the bin! I don't know what the energy density is like for NiCd or NiMh but I do know that Lipos have 1,000 times the energy density of Pb (Lead acid).
At this point the Americans would say 'go back 5 and punt'. My suggestion, build a smaller model for the setup you have, it won't cost much and you get to have fun. Or, invest in a 300watt setup using a 3s 2200mAh lipo and an 850kV motor as the start point.
When I came back to the hobby I didn't realise how often I was going to crash! I'd go for option 1, you'll have fun and it'll be easy to repair. My personal choice would be something that uses Architects board and 5mm (3/16") dowel. It'll be ugly but you should get 36" span and about 1lb

I contacted the supplier yesterday with respect to the potential lack of power they specified in the motor. Interestingly they accepted my argument and said no problem, keep the motor we will refund you the cost.
So I've ended up just paying for the Prop and ESC, any suggestions for a good basic model I could use them in ?
In the mean time I have not given up on my IC conversion project. I am considering another motor, prop arrangement.
Bigger KV motor to compensate for the weight (that can be throttled back, if necessary ) and a 8X4 prop, using a 3S Lipo pack. Does this sound a more practical option ? Or am I still not grasping the fundamentals.

My error meant to put 8X6 Prop.

My view is it all comes down to your anticipated wing loading. It is very high for what you are after.
So either more wing area (with all the attendant issues) or the keep the same air frame but with less weight to reduce the wing loading.

If you desperately want to fly the existing design un-modified then about the only way to save weight is to use a smaller but very high discharge rate battery as a brushless motor is already pretty 'efficient'. With a correctly matched prop such a set up can get quite close to the power to weight of a RC IC motor, if you include its tank, plumbing and throttle servo, but the downside is the flight time will be short, just a few (3?) minutes.

Fundamentally electric is heavier than IC but much smoother which has advantages in prop efficiency (and is far less messy!) so the best solution is to make full use of these attributes by reducing the air frame weight to compensate for the heavy battery. Modern materials (eg foam and /or carbon fibre) make this process easier to the extent that electric is now preferred for some fully aerobatic classes. In the same way 'jets' are now either electric fans or real turbines.

I can fully understand the interest in converting an old IC design to electric but its not so easy to do successfully whereas a plane designed for electric has significant 'spare' performance.

Which way you go is up to you.

Thanks that makes a lot of sense.
So to keep it as it is appearance wise, I need to significantly reduce my wing loading without compromising the structural integrity of the model. I then need to optimise my power by using Lipo rather Nimh batteries.
By eliminating the IC engine, fuel tank, linkages, servo, RX battery etc I lose weight. But ultimately gain it back with the Lipo battery, motor and ESC.
So the more weight I can trim from the air-frame the better with respect to longer flight time/power to weight ratio.
If I managed to get it to fly for 3 to 5 minutes I would be happy (obviously the 5 min versus the 3 would be better).
Changing the size of the model or it's design defeats the original object of the exercise. It's a decent little trainer which was built to take an O.S. Max 10 engine with a 7x4 prop and flew well, if memory serves me right.
I am not giving up on IC engines all together, just thought this was a nice little conversion project, how wrong I was.
Would be nice to see it flying again without the hassle of glow fuel etc.
Thanks again for all the help and support its very much appreciated.

Ian
The OS Max 10 'bare' weighs 3.95 oz (112 g) so say 6 oz (170 g) with tank, throttle servo and Rx battery.
I expect your motor weighs 40 g and a 30A ESC another 30 g leaving 100g for the battery.
A 1300 mAh 3s 30c LiPo weighs 107 g so giving a total weight very close to the original OS 10 installation.

A 1450 kV motor on a 3s will spin at about 14000 rpm driving a 7x5 "e" prop (the "e" is important as they are lighter and more efficient than an IC one) again not too far removed from the OS 10 performance.

Finally the 1300 mAh battery will provide full power (16 A) for nearly 5 minutes, however the big advantage with electric is any time you reduce the throttle the flight duration goes up in more or less a direct proportion.

It can be done.

That's the best news I have heard might be in with a chance of flying the old girl yet.
I think it's time to end this thread I will keep you posted on developments.
Just got my Golberg Electra all back together, finished checking everything today so that's ready for a test flight, just waiting for some decent weather now.
Thanks again for all your help it's been much appreciated and I have gained a lot of knowledge in a short space of time. Hopefully I can put it all to good use on my next project.

Simon said 'A 1450 kV motor on a 3s will spin at about 14000 rpm driving a 7x5 "e" prop (the "e" is important as they are lighter and more efficient than an IC one) again not too far removed from the OS 10 performance.'
Is that by measurement Simon? The math says 16,965rpm.

Klippy
16,965rpm.
Are you forgetting that a brushless motor is not "synchronous". If I remember my electric motor theory correctly the applied volts times kV is only theoretical. It can never be achieved as at that rpm the motor back emf would exactly match the applied voltage so no current would flow.

The motor has to run at less than the kV rpm so the back emf generated is sufficiently below the applied volts that the required current flows. To make things a bit more complicated that current also reduces the actual voltage that reaches at the motor.

So to arrive at the rpm I "guestimated" a 10% reduction in the theoretical rpm when at full power current and the 3s battery voltage dropping to 10.7 V at the motor thus giving "about 14000 rpm".

I've never bothered measuring the rpm's my motor/prop/battery combo's achieve.
If the Watts are sensible, and the model goes vertically (upwards!) on full draft, that's enough! [badass]
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I was going to measure one, once, out of curiosity, set up everything, then found my tacho battery had died, so I never did! :oops:

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Don't forget - in these cold air conditions, a lipo does not deliver full power, they prefer it a bit warmer!

About 35^oC is optimal I understand. You're absolutely right Simon, I was playing devils advocate![badass]
Interesting Eric, I've never had a model with a setup that would go unlimited vertical and not for the lack of trying!