By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

October issue feedback...

thoughts and commentary...

All Topics | Latest Posts

Search for:  in Thread Title in  
BB26/08/2008 12:30:00
avatar
1182 forum posts
32 photos

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.

birdy26/08/2008 17:15:00
avatar
1423 forum posts
110 photos
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.
BB26/08/2008 18:14:00
avatar
1182 forum posts
32 photos

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 - Moderator26/08/2008 18:53:00
avatar
Moderator
11035 forum posts
1723 photos
617 articles

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

John Laird26/08/2008 21:38:00
avatar
442 forum posts
57 photos

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

birdy26/08/2008 22:28:00
avatar
1423 forum posts
110 photos
There is no error in anything said in the letter, only that the writer is "barking up the wrong tree."
John Bunting26/08/2008 23:19:00
105 forum posts
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.
birdy27/08/2008 12:36:00
avatar
1423 forum posts
110 photos
True. I can only seem to remember parts of it these days. That is definitely wrong though. Sorry.
Bruce Richards27/08/2008 13:18:00
avatar
1849 forum posts
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.
John Laird27/08/2008 15:32:00
avatar
442 forum posts
57 photos

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 Beaumont27/08/2008 19:06:00
avatar
5797 forum posts
51 photos
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
Slopetrashuk28/08/2008 00:16:00
avatar
1083 forum posts
25 photos
8 articles

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

John Laird28/08/2008 09:17:00
avatar
442 forum posts
57 photos

andy,

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

john

Peter Beeney28/08/2008 11:00:00
1593 forum posts
59 photos

 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.

Erfolg01/09/2008 13:02:00
avatar
11781 forum posts
1340 photos

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 - Moderator01/09/2008 19:21:00
avatar
Moderator
11035 forum posts
1723 photos
617 articles
Thanks Erfolg, noted re the covers
Slopetrashuk01/09/2008 19:27:00
avatar
1083 forum posts
25 photos
8 articles
How come you never 'note' me re the covers?
FLY ELECTRIC01/09/2008 22:27:00
77 forum posts
7 photos
5 articles

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.

Erfolg02/09/2008 12:54:00
avatar
11781 forum posts
1340 photos

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

Slopetrashuk02/09/2008 12:59:00
avatar
1083 forum posts
25 photos
8 articles

How about Brownian Motion?

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

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
Sussex Model Centre
CML
electricwingman 2017
Slec
Sarik
Advertise With Us
Latest "For Sale" Ads
Has home isolation prompted you to start trad' building?
Q: The effects of Coronavirus

 Yes - for the first time
 Yes - but Ive bashed balsa before
 No - Ive existing projects on the bench
 No - Im strictly an ARTF person

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E!