I would like to know what the effect of mounting the battery under or buried in the bottom side of the wing is.

I would like to know what the effect of mounting the battery under or buried in the bottom side of the wing is.

The effect on CG is just a formula, but there are structural and dynamics issues, too.  What airplane, and why would you put the battery there?

turbulator ON LEADING EDGE (not on high point) will do similar effect

So that's why you don't clean the bugs off!

The reason I ask is that it has become somewhat obvious as more and more models are converted to electric power when we start the trimming process that the C/G inevitably gets moved back from the location shown on the glow power plan. S

Honestly,, this is the first time I have heard of the CG moving back,,,
mine have all been farther forward than I calculated based upon Glow "standards"

My Impact , while not as far forward as Pauls,, is definetly forward of the glow standard location,,

...how to calculate the effect on the C/G of placing a lower weight power unit on the front of the fuselage and locating the much heavier battery somewhere behind it.  It would be nice to have some way of determining exactly where that heavy battery should go as it would save a lot of time and shifting back and forth to get the darned C/G in the right place from the start.

Here's a calculator and an example.  All the tedious algebra is invisible.  List all the stuff that's in the first configuration, but not in the second configuration, in the yellow cells of Configuration 1.  Put the weight of the first airplane (including however much fuel you want to consider) in the green cell.  List all the stuff that's in the second configuration, but not in the first configuration, in the yellow cells of Configuration 2.  The weight and CG changes of Configuration 2 appear at the bottom.  Units are given as ounces and inches, but you can use any weight (or mass) units and any distance units, as long as you are consistent.  The answers will come out in the units you used. 

To find where to put the battery, try different locations until the Configuration-2 CG comes out where you want it.

Guys, check to see that I did this right.

[quote author=Igor Burger
3/ this is questionable, but can play role on some models - vibrating wing and uneven angular speed of prop can keep turbulence and thus attached airflow on wing surface what makes stronger stabilizing moment on wing at high AoA (due to separation on TE especially at hingeline) - turbulator ON LEADING EDGE (not on high point) will do similar effect
[/quote]

Does this somehow support my findings when entering electrics?

I have first built and/or converted four airplanes in a row all of which had silkspan covered wings. All of them did show more or less unstable level and inverted flight and/or poor tracking. I've tried, with limited success, turbulators, too, but not on leading edge.

Then I've built/converted another three airplanes with solid (foam or fully sheeted) wings. They all track nice w/o instability in level/inverted.



 

Does this somehow support my findings when entering electrics?

I have first built and/or converted four airplanes in a row all of which had silkspan covered wings. All of them did show more or less unstable level and inverted flight and/or poor tracking. I've tried, with limited success, turbulators, too, but not on leading edge.

Then I've built/converted another three airplanes with solid (foam or fully sheeted) wings. They all track nice w/o instability in level/inverted.
 

I still think that stability problem on electric is more trim problem then some kind of vibration, for example I had that problem myself on begin of my eflighs, until I found I must really move CG forward. But also 1g difference in tip weight can cause problems or misaligned handle etc. However if you have consistent records with 3 models, it could be signifficant. I do not use open bays construction anymore just to keep proper airfoil shape. And I would say that if there is a reason, then it is more kink on airfoil in place where LE sheeting converts to open bay. But I am only guessing, I never tested anything regarding this problem, I simply converted all my designs to hard wings and problem solved :- ))

Hi Gang,
For starters, it sure does seem that having taken data from a large numbers of flyers, that E-powered versions of the same ship either want or tolerate more forward CGs.
The question of want or tolerate depends on the flyer and his pre-existing tendencies to balance nose heavy (like me) or more tail-heavy for that power-steering feel.
My less than exhaustively explored conjecture is this: tightly governed E-setups lose less RPM in the hard corners, and as a result, the airplane finishes the corner with more thrust applied.
Thrust from a nose-mounted propeller DESTABILIZES the airplane, so greater instantaneous thrust makes the plane more lively and more "sort of tail-heavy" at the corner exit.

Now somebody is going to suggest that I am crazy for saying that a front-mounted prop is a destabilizing device ... Howard, do you want to explain it to them, or shall I?

later,
  Dean P.

So, stability is good, and a front-mounted prop reduces stability, and -- duuuuude!  I'm gonna go build the World's Best Stunter!

Thank you for all of the replies. You are all of course correct that the "effective" C/G or to be more accurate the flight balance point moves forward when you go electric powered. I misspoke in my initial post (everyone tells I that I do not know if I am going forward or backwards. Maybe they are right) LOL. What I meant to say was that the balance point moved forward of the published C/G.

I find the mention of the destabilization very interesting as I have observed this myself on many electric powered models. I did not know what caused this but Igor's and Dean's theories may have explained that. Of even more importance is Igor's comment on the way he has corrected this (I am sure most of us have noticed how darned smart and practical he is - No wonder he's the World Champ!). I think that I will experiment with sheeting the entire wing as I for one am more interested in correcting the problem than in proving or disproving the theory.

I may have to change the way I think about covering the ribs. Darn it all, now I have to order in a bunch of contest grade 1/32 balsa and check my supply of carbon fiber tissue. If sheeting the wing solves the problem I'll also have to find some space in the garage to store all of the silk, polyspan and Monocoat I will not need. Maybe there enough space next to the fuel proof paint and glow engines I no longer have a use for. (LOL)

I now have two identical Predators. Sort of. One has a foam wing and the other is built up. The built up is also a take apart. They are within 3/4 of an ounce of each other. They both fly very well. I have to give the nod to the built up as being a better overall flier. However I used the foamie this year, as I was thinking I might need the TA in Australia. Didn't need to put it at risk!

One thing that I noticed this year in working with them was that the built up wing responded to more vortex generators better than the foamie. I found that just two per panel (8 total for the wing) got the job done on the foamie, and more made no additional improvement. However, on the built up, the improvement continued with more and more VG's. It ended up with 24 total on the wing.

This may support Igor's thoughts on maintaining a constant airfoil shape.

I still think that most of the issues related to IC's converted to electric are related to trim issues, of some nature. Most of my electric fleet is no different than the IC fleet other than power system. I have conversions that work fine, and one that needs some trimming.......

Not sure if it has been mentioned yet....
Every I/c model airplane I have seen become More tail heavy from the start of the flight to it's landing.
Since the fuel is always forward of the c/g.

Sorry if I am missing it but I've read and re-read all the posts and do not see any mention of i/c models c/g during a flight....not in relation to electric which does not change from start of flight to end of flight....

Sorry if I am missing it but I've read and re-read all the posts and do not see any mention of i/c models c/g during a flight....not in relation to electric which does not change from start of flight to end of flight....

The guys who responded to Eric's original question are pretty much hard core stunt people who are familiar with the fuel CG shift.  We all considered that and calculated it when we went electric and figured out where to put the electric stuff.  For example, CG shift on my IC airplane was .477 inches between a full tank and an empty tank.  I located the CG on my first electric plane .23" forward of where it was on the empty IC airplane to account for half a tank of fuel.  Pretty much all of us discovered that the CG on the electric airplane needed to be another inch forward of that.   I could have saved myself some ballast if I'd paid attention to what others had warned me.  Now we're conjecturing why the CG ends up another inch forward.