How much justifies obsession?

[Howard Rush]

That is exactly why you carry a further forward CG without any ill effect!!  With the heavier part of the mass being closer to the CG is easier for the elevator to move it and stop it.  It would only make sense that you can run the CG further forward for stability without losing any of the control authority.  

can anyone say concentrated mass (weight) closer to the center of gravity? the weight of my new planes engine is 6.7 oz (fox .35 weight) as opposed to 12.5. out on the end of the stick. Body in motion tends to stay in motion unless acted upon an equal and opposite force. What take less opposite force 6 or 12? Hum no brainier.
True the fuel is a little heaver but is not out on the end of the stick its closer to the CG and the difference is only 4 oz and that can be pulled into the wing if the plane is light enough in gross weight. There is that word again weight. I said this on SSW when I first came back (and boy was I hammered) baseline characteristics are set in weight and most will find this out eventually.

What I guess I am trying to say is its better to have a long nose light weight engine than a short nose heavy engine both planes of the same gross weight. This is just my opinion but who am I.

It would be the cat's pajamas if you could put the battery right on the balance point, but then what would the nose moment look like, and how much ballast would you need to balance?

I'm familiar with the moment of inertia principals...

Really?

Yes, reducing the moment of inertia in the pitch axis makes a airplane dynamically more stable.  However, your intuition is letting you down on how to calculate moment of inertia. Here's how: http://en.wikipedia.org/wiki/Moment_of_inertia .  Look for "Moment of inertia about an axis".  Here is a spreadsheet that will do the calculation for you.  It calculates moment and moment of inertia for a bunch of parts.  You can fiddle with it to see what configuration has what moment of inertia for the same CG.  To compare one configuration to another, fiddle with distances from the CG until both have the same moment: the same contribution to CG.  Then read the moment of inertia.  You will see that Doug's .56 has less moment of inertia that the .72 at the same distance from the CG plus the tail weight it took to balance it (use a negative number for distance behind the CG).  You will also see that having the battery and motor at the same distance from the CG gives you less moment of inertia than having them separated.

The second attachment is a motor-battery example.  Configuration 1 is approximately the motor (part 2) and battery (part 1) locations in my current plane.  Configuration 2 is what would happen if I moved the battery aft 2 inches and moved the motor forward to have the CG come out the same.  

[Pat]

In this thread, I saw someone mention "unobtainium" I build and race front engine dragsters, and it seems that I use a similar alloy we like to call ucantaffordium...

Pat

[Dan McEntee]

   Hi Howard;
     First, let me clarify my statement about me understanding moment of inertia principles, in that I understand them in a basic way. I'm a welder/fabricator/maintenance technician  and math has never been one of my strengths. And I know I'm getting into something that is probably over my head, but I find this forward CG phenomena that Bob has been talking about and what I have read in Paul's thread over on Stuka kind of interesting. I'm not arguing a point, just throwing out some questions as this all relates to my level of understanding.
    From what Paul and Bob are saying, they have been flying with a significantly forward CG that what they have in the past. I don't know the exact parameters of each airplane but Paul's model looks to be what I call a"typical" stunt model for these times so let's use his model. You may know the moment numbers, but I'm just going to shoot from the hip and see if I can get my question out correctly.
    First, I think we need to establish the far end of the nose moment, and I'm going to call that the back of the spinner back plate. And for an I/C engine we'll use the Saito .56 we talked about before, mainly because I just looked up the weight and it's 15 ounces. This engine weighs 15 ounces, and that weight, for calculation purposes, is concentrated in one specific point that I think you would call it's center of mass, correct? And that point is a given distance from the CG when installed in the model, and four ounces of fuel (which I think is typical for a Saito.56) weighs a little over 3 ounces, and it's behind the engine for a total of 18 ounces. It gets lighter as the flight progresses. I'm going to guess that the center of mass of the engine is at least two and a quarter inches behind the spinner back plate and the center of mass of the fuel tank is maybe four inches behind that. How much does that 3 ounces of fuel factor in to the moment of inertia equation as it is burning off? And I guess we had better put a location on the CG, and I would guess that it would be around 28 to 30% of the wing cord?
    Now lets put the electric set up in, and I'll use the numbers in your spread sheet because they are probably closer than what I could guess, and that's 9 ounces for the motor and 13 ounces for the battery. The motor is probably mounted typically of at outrunner, and attached right behind the nose ring and the spinner back plate. It's 9 ounces, and it's center of mass could be an inch to an inch and a quarter in front of the center of mass location for the Saito. And the battery weighs 13 ounces and is located behind the motor, and this is where I hope I can state this correctly. You have the same nose length in either case to put the components in, and to achieve the forward CG location that is being desired, your only option is to put the battery as far forward as necessary to achieve the desire balance point. The balance point is around 25% of the wing cord now, maybe less as Bob has put it "north of that" as he says. I guess you really need battery dimensions to locate it's center of mass so this all may be a worthless discussion, but if you have that 9 ounce motor 1 inch or more further forward than the 15 ounce engine, and the 13 ounce battery potentially further forward than the 3 ounces of fuel that is decreasing as the flight progresses, you have theoretically more total weight in the nose of the electric model? Again, I'm having a hard time explaining what I think I have in my head, but if the CG or balance point is being significantly moved forward, and you haven't moved that back side of the spinner back plate that locates the forward point of the nose moment, and the collective total weight of the electric components being maybe four ounces more than the I/C components, that doesn't affect the moment of inertia? I guess I'm thinking of it as an "average"?
     I know I'm not making this very clear, but I have to make the wild ass guess that the reverse rotation prop is making a significant contribution to the electric set up? I know the moment of inertia is significant, but if Bob and Paul (or anyone else flying an electric set up) were limited to CCW rotating props, would they be getting the same results as far a CG location? Does the motor design have anything to do with it? In other words, would anyone be flying electric if they had to use what is now old fashioned DC can motors with cobalt magnets (read heavy!) to achieve the same power? And not to mention battery technology. The electric set up is truly the sum of it's parts I think, and if one component was not available, would anyone be using it? I just have a feeling that the CCW prop is a significant part of the plan and now that some of you guys have progressed to a certain critical point with it, I would be interested in hearing how your current model flies with a CW turning prop. And if a I/C engine set up with a similar balance point and a CCW turning prop could produce similar results? Maybe not equal, but similar?
    Most of us have been flying CCW and it is the normal mode for a whole lot of us, and stunt model design and power plants and prop design have been dealing with "P" factor and gyroscopic pression  (and all the other effects that are over my head and I get into trouble trying to talk about) for all these years. Has the advances in electrics and prop technology made it possible for us to take advantage of all or some of these properties and effects? And where are lead out locations in all of this compared to before? I don't think I've heard that mentioned or discussed.
    In Doug's Saito .72 powered model, would a 1 inch shorter nose moment and a CCW turning prop made a significant change in it's performance?
    I gotta go to bed, I can't believe how long it took me to think this out and type it!
   Type at you later,
   Dan McEntee

[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,,