October issue feedback...

[BB]

Mag just popped through the letterbox, had a leaf through and found the free plan.  I like the look of this. Haven't scratch built for a while, but it does look manageable, thanks.

[birdy]

I have read the majority of the publication, and have enjoyed it. Mind you, one thing I am left wondering, is are the A-4's legs retractable? Mr. Carr didn't mention it.

[BB]

They don't look like retracts. If you look at pic of underside of main wing you can see that the leg is fixed with 2 screws and forward of the leg is a fake retracted wheel cover in th open position (p122).  Theres also no slot in the nose for the nose leg, again a fake nose leg door can be seen (p120).

[David Ashby - Moderator]

Bas is right, they're not designed to be retractable out of the box. Builders would have to engineer something themselves.  

[Guest]

david,

the letter on Ohm's Law contains an error in the use of Ohm's Law resulting in an incorrect conclusion re the effect of change in voltage.

see my posting in the thread for Ohm's Law

[birdy]

There is no error in anything said in the letter, only that the writer is "barking up the wrong tree."

[John Bunting]

Well, Kevin says, "In a DC circuit with the same resistance if the voltage (battery) source increases the current used will actually drop". That sounds rather like an error to me.

[birdy]

True. I can only seem to remember parts of it these days. That is definitely wrong though. Sorry.

[Bruce Richards]

I think what the letter says is correct and is made so by the statement  "So, if our required  power were 6 watts". This means that he is considering a constant power system. Thus if V increases I must decrease to keep the power at 6 watts. Howerer, it is confusing.

[Guest]

Kevin clearly states that the para relating Volts, Current (I) and Resistance   by Ohms Law is incorrect.  This is the first time I have heard Ohm's Law challenged. 

He then goes on to use P=V x I  to vary one of the P ,V parameters to calculate the change of current (I)  using the SAME resistance. This is simply not technically correct.

P=V x I is a derived formula and used to calculate the power absorbed by a circuit  where 2 of the 3 parameters are known ( V,I,R )  Furthermore P=V x I is not an expression of Ohms Law - V = I x R is.

If any of these 3 parameters are varied you must go back to the basic Ohms law V=I x R and recalculate the remaining 2 before going back to the power calculation to re-evaluate P

In the last statement - " why larger voltages are used to keep the currents managable " - the fact that the larger motors have larger resistance ( impedence ) appears to have been overlooked.

Using simple resistance to demonstrate

 600 watt motor needs 60 amps at 10 volts to develop the power implying simple resistance of

R = 10/60 = 0.17 ohms resistance

600 watt motor needs 30 amps at 20 volts to develop the power implying a simple resistance of

R =  20/30 = 0.67 ohms resistance.

altho the power is the same , the 2 motors have different windings to absorb the same power from 2 different voltage levels.  So in this example, for the same POWER level all 3 parameters ( V,I,R ) changed.

I hope this example demonstrates that it is not possible to change one parameter ( V ) for the SAME resistance and expect the power to remain the same since the current (I) will also change.

[Myron Beaumont]

Just an interjection here but surely when we talk about AC voltages we mean RMS  (root mean square)  values IE 0.7071 of)max voltage assuming a sinusoidal wave form .Plus aren't we on three phase from an ESC ? Please enlighten me you experts out there ( and yes I understand the differences between resistance and impedance )     Just thought I;d throw my own wobbly in again til we eventually go off thread !       Now then where is my trusty AVO with bags of internal resistance

[Slopetrashuk]

Dear Ed, I was experimenting with Ohms law and let out the magic smoke. Can users advise me on how to get it back inside please?

Andy

[Guest]

andy,

crossing palms with silver or gold  plus suitable incantations in the appropriate direction usually brings results

john

[Peter Beeney]

 David,

I haven't seen the original letter yet, but perhaps there is something missing?

Back in the time of 'Galloping Ghosts', one of my friends and work colleagues always considered that Ohms Law always occurred at around ten to five in the afternoon. Regular. Even a shade earlier if we were lucky.

Could we not we insert this variable into the equation, on the 'watt if'  principle, just to see what happens?  

Pete.

[Erfolg]

With regard to the October issue. I do believe the content is moving in the right direction.

We are starting to see a wider topic coverage. I am a little suprised that there has not been many howls of protest from some IC, absolutists, for that I am grateful.

The RC scene covers  a wide spectrum, from micro (in all its forms) to extremely large. Power sources as diverse as air, methanol, Avgas, petrol, parrafin and elctricity. The same goes for methods of construction, Foam, balsa, ply, Resin matrix systems. So this widening of subject matter reflects a reality. I guess 50% are fixed wing IC fliers, the remaing 50% will fly gliders(slope, flat field), electrics (power assist, DF, fixed wing), witha number of Helicopter fliers of both Electric and IC. So we are really very diverse group

I also see the adverts as an important aspect, I guess the owners feel the same, for differing reasons in some respects. Keep the work up to attract more. Although I know that some will feel differently. 

The magazine does entertain, educate and keep us informed. In other words it is a good read that I believe has improved.

I do like the contributions from Peter Miller, Nigel Hawes, as thier models are the sort I either build or wish to build. There may be some benfit in having some additional contributers who are of a different interest niche than myself.

The discussion on Ohms law is the sort of area which I think is very useful. The intial paragraph was a little skimpy, and was probably intended for those who know a lot regarding the constraints and limits of relationships. The contribution from John Laird, setting out the basic realtionships, the assumptions before moving on, is the way to go in my opinion. I think some articles of this sort would be useful, requiring a page or two, rather than limited to a short paragraph. 

Now what about those covers David!

Erfolg

[David Ashby - Moderator]

Thanks Erfolg, noted re the covers

[Slopetrashuk]

How come you never 'note' me re the covers?

[FLY ELECTRIC]

I've had zillions of E-mails about that letter already, and I think it needs a reply.

It would be easy for a relative Electric Flight beginner reading it to get the impression that he could go from a 3s Li-Po to a 4s pack and see the current actually drop! As most of us know that is the exact opposite of what would happen and on safety grounds alone this should be pointed out.

I wonder how many EScs / Li-Pos / motors have been melted so far...

Whilst it is true that a certain wattage can be achieved using a higher input voltage and a lower current, I think that was sadly lost in the way the letter was written.

And I'm afraid I always roll my eyes when a letter begins "....As a retired electronics engineer..."

Nige.

[Erfolg]

As a retired Engineer I think I have been slighted?

Any way I think it is time to acquaint beginers with Farradays law of Induction as a retired engineer.........

Erfolg

[Slopetrashuk]

How about Brownian Motion?

I'm off for a nice cup of tea......

[FLY ELECTRIC]

I'm sure there are sme very good retired Engineers Erfolg!

Fortunately you don't write in to the letters page and confuse everyone!

Perhaps I should take an electronics engineering course...

 Nige.

[Erfolg]

Just to be pedantic, as an very young student, i was taught that Ohms Law was

I = V/R

and that the resistor was a Ohmic device as the resistance remaind constant over the test range.

But then again we retired engineers know nowt.

Erfolg

[Former Member]

[This posting has been removed]

[FLY ELECTRIC]

Oh Poo, my cover is blown!

[Doug Ireland]

Someone mentioned the output of an ESC being three-phase. Yes it is, it essentially is a Pulse Width Modulated (PWM) variable speed drive. Even though the brushless motors we use in our hobby have permanent magnets fixed inside them they are still (3 phase) AC motors.

I've looked at the waveforms coming out of an ESC while it is driving a load and they are PWM. The whistle you hear from the motor is the carrier frequency the ESC is operating on.

 stands back and waits for the fallout........