How much justifies obsession?

[Bob Hudak]

Dan,
 I started reading your post and realized you're over analyzing forward c.g. Think simple ! If a plane with a IC motor balanced 2.75" behind the leading edge is converted to electric then you will find it will probably fly better maybe 1/8" to 1/4" forward of the IC setup.Test flights and experimenting will tell what the airplane wants. As far as a pusher props go, the only difference I feel is less tendency to come in on me in on me at the top of a vertical 8 and overhead.
                              Bob

[RC Storick]

whats really over looked is the fact if you fuel your IC pane up and balance it it will be close to the same spot as the electric plane.

[Mark Scarborough]

Dan,
 I started reading your post and realized you're over analyzing forward c.g. Think simple ! If a plane with a IC motor balanced 2.75" behind the leading edge is converted to electric then you will find it will probably fly better maybe 1/8" to 1/4" forward of the IC setup.Test flights and experimenting will tell what the airplane wants. As far as a pusher props go, the only difference I feel is less tendency to come in on me in on me at the top of a vertical 8 and overhead.
                              Bob

Bob, this has not been my experience at all,, I have found that the CG wants to be way farther forward,, even if you take into account a full tank of fuel on an IC airframe,, the CG for electrics wants to be forward of that,, but the best part is that it still maintains a good solid corner with a locked in level flight,,, My last electric,, may it rest in peices,, was balanced almost an inch and a half farther forward than calculations whould have shown an IC airplane to be,, and that just got it close to where it wanted to be..granted it was really close by the time It met momma earth,,

[Howard Rush]

Bob, this has not been my experience at all...

Mine, neither.  I did the calculation to make the electric rig have the same CG as the IC rig with half a tank of fuel.  Then I had to shove the battery as far forward as possible and add about 2 oz. of lead to the nose.  I had to move the battery up, too.  Paul wrote recently on SSW that his CG came out more than an inch forward of where it did with the IC engine.  I should have listened to him sooner. 

[Mike Haverly]

I might as well pile on.  I just walked in from the shop where I have doing some "off season" trimming, which would be permanently affixing nose weight that I was previously reattaching from flight to flight.  There has never been a time where I thought the ariplane as nose heavy.

[Howard Rush]

Dan, I was just responding to folks' misconception that putting the battery on the CG and mounting the motor way forward would result in less moment of inertia than having the battery and motor as close together as possible.  This assumed that both alternatives have the same CG.    

Your long post above covers a lot more ground and raises some interesting questions.  You mentioned taking a typical stunter, keeping its nose the same length, and replacing the IC stuff with electric stuff.  The CG would--in the case of the sets of electric and IC stuff I used, anyhow-- come out farther forward with the electric stuff, as you say.  On my first electric airplane, I shortened the nose so that the CG would come out the same as it did on my IC plane.  It did, but it turned out that the airplane was happier with the CG farther forward.

After experimenting to find the best-flying CG for electric planes and the best-flying CG for IC planes, it seems that everybody is finding that electric airplanes work better with a farther forward CG than IC planes.  Beats me as to why.  I don't think it's prop rotation direction: both Paul Walker and Igor Burger are using props that go the traditional direction (CCW looking from the front, CW looking from the back).  It would be interesting to get some matched pairs of optimized props and see what the rotation direction effects really are.

Leadout position on electric planes is another mystery.  It turns out that my plane flies best with the leadouts way behind where you'd think they ought to go.  

You suggested that Doug move his .72 back.  Given the same CG, of the three configurations (.56, .72 in the same place as the .56 + tail weight, .72 moved aft), the one with the .72 moved aft until it balances in the same place without tail ballast would have the least moment of inertia.  

 

[Randy Powell]

Which is why my new ship has a pretty long nose and lots of room for battery position adjustment.

[Dan McEntee]

Dan,
 I started reading your post and realized you're over analyzing forward c.g. Think simple ! If a plane with a IC motor balanced 2.75" behind the leading edge is converted to electric then you will find it will probably fly better maybe 1/8" to 1/4" forward of the IC setup.Test flights and experimenting will tell what the airplane wants. As far as a pusher props go, the only difference I feel is less tendency to come in on me in on me at the top of a vertical 8 and overhead.
                              Bob

       Yeah, I've definitely been guilty of that a lot. But if it were only 1/8 to 1/4 inches, that wouldn't wake me up. The kind of results that Howard and Mark are presenting is what gets me. How in the H-E-Double Toothpicks does an air frame know what it's method of propulsion is?  Howard's example is to the one end of the extreme that I was trying to explain. I saw Paul's photo of the bottom of his new model with the balance point marked and that is what got me thinking about it, and the plane has a left hand prop on it in the photos. And then Howard tells us where his lead outs are!
    What props are you guys using, and has anyone taken a tach reading at full power just before take off? I'm curious about that also. Don't know what it could mean but I would like to know.
    The closest thing in an I/C set up that I could think of, is a set up like on my Score, with a Saito .56 in it. It's a fairly short nose moment, and because of the fuel economy of the breed, I typically only have about 3 1/4  ounces of fuel on board at take off. And due to the short nose, that weight is closer to the balance point than a typical .60 or larger two stroke application. I don't know where it balances at and I'll have to check. I bought the model as it is and Crist Rigotti built it and set the engine and controls up. Once I got the engine figured out and consistent, I fell in love with it, and I have not felt the need to make any changes with it's trim. With my job and other things in life, I've been kind  of stuck on a plateau and haven't been able to build models and practice up enough to get to the next level, and with this airplane, it's one of those situations where it probably flies better that I fly it. But when real flying weather gets here and I get it down off the wall, I will be looking at the CG of it and maybe move it around a little. And try a left hand prop on it and see what that is all about.
   Type at you later,
    Dan McEntee

[Mark Scarborough]

Dan, I dont know why either,, I know that part of the forward CG I refer to is compensated for by the empty/full fuel tank situation,, but its still farther forward of that,, I cant explain why,,, I could fantasize about some reason but I could not back it up,, but like was mentioned,, you fly it,, adjust it,, fly it,, adjust it, until it works to your expectations ( or hopefully better than)
as to the leadouts, this was something I just heard about so I really cannot speak to that as I have not tried it,, except that through lack of focus, when I was trying to get my electrajet to fly well, I kept adding nose weight, but neglected to follow that with leadout adjustments,, it still flew well with ultimatly around 7 ounces of lead on the nose, and the leadouts in the initial position,,

but then I am still a bit ham handed so It takes awhile to figure out trim before I adjust,,

PS the lead was added to find CG before I ultimatly reconstructed the nose to allow the battery to move forward farther at which time I was able to remove the lead....
PSS for the record, it flew exceptionally even with the lead,, which had it at about 72 ounces all up. It was the most solid above 45 degrees of any airplane I ever had,, the down legs were much more managable,,