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CG Adjustment

Old 01-11-2012, 07:31 PM
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acdii
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Default CG Adjustment

HiAll, Iam considering modifying my Eflite T-34, and to do so that Ican convert if back to E power, Iwould need to extend the cowl out about 1 1/4". The motor will hang out that much more than the electric one, and the weight will be concentrated at the motor instead of spread back with the battery.

If I were to go this route, would I need to change the balance point on the plane? Would Ineed to move it forward a bit,like more towards the spar?
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Old 01-11-2012, 08:35 PM
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TFF
 
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Default RE: CG Adjustment

The balance point will be the same place. The balance point has to do with the airfoil, size of the tail and distance between the two. If the plane is now nose heavy you have to add weight to the tail to make it fly right.
Old 01-11-2012, 09:19 PM
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Default RE: CG Adjustment

Why not move the firewall back 1-1/4 inches, then if you want to go back to electric, just construct a 1-1/4 in box to move it back to the original position.
Old 01-11-2012, 11:48 PM
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Default RE: CG Adjustment

Regardless whatever you do you will need to do whatever is neccessary to maintain the balance point at the same place that the airplane flew well at before the modifications, Thats a must and TFF is absolutely correct.

Here is an example: I have an airplane that is powered by two OS .25's but this airplanes mission is to also serve as a glider that is towed into the air (an aerotow trainer). Now when the quickly removeable engine nacelles are removed and the airplane becomes severely tailheavy. So, the battery pack which is located clear back under the stabilizer in the rear is now removed from a hatch and replaced in the very tip of the nose section and this compensates for the loss of the engine weights. The process can be quickly reversed also, the idea is the CG must remain the same in either configuration.

John
Old 01-12-2012, 06:14 AM
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acdii
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Default RE: CG Adjustment

Thanks, I have a pretty good idea of CG and what it does, but was concerned that extending the nose would upset it overall, but if its the distance from wing to tail that is critical to it, I am fine. 

As for cutting back that box, two reasons come to mind, which I explained in my Electric to glow thread.  The structure of the plane is such that cutting the box would be difficult, and would also move the nose wheel back 2 inches.  Cutting it would affect the overall structure in that area since the floor extends to the firewall and is all notched together along with the side pieces.  If I cut that back, I would not be able to open the floor up to put a tank in since the floor is part of the stability. 

What I might do is notch the firewall where the motor mounts and just move that part back, leaving the nose wheel and floor intact. I have to study it some more to see if that can be done.  

Originally I was thinking I could take that whole box off, and make a firewall for the glow engine and another for the electric and all I would need to do is bolt it onto the existing structure, but the "firewall" is actually the box, and not the part the cowl attaches to, that is just a thin piece of plywood, and not very structurally sound.  The main structure that takes all the torque and vibrations is the box itself.  What I want to do is to be able to go back to electric if I decide that glow isn't right for the plane without having to buy a whole new fuse. 

If I notch out the box, then I can set it up to be reattached and put the original parts back in with a couple long bolts and blind nuts.   I just need to rotate the motor 90* and see how it fits in the cowl, right now it fits perfect upside down, so I would just need a hole for the glow plug and muffler.  Sideways may need a larger hole, which in that case I would get a second cowl in case I change it back. 
Old 01-12-2012, 07:49 AM
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Default RE: CG Adjustment

Interesting and you obviously thinking outside the box (pun intended) and in my book that is great. Something I encourage in the fellows.

A quick convertable electric/glow Hmm, not my cup of tea but pay no nevermind to that.

Obviously changing nose moments and or tail moments always affect CG and considerations must be made to maintain the CG where it needs to be. Having said that there are other aerodynamic considerations when you alter the tail moment that will affect the flying characteristics beyond the changes in CG. One of these engineering compromises (every engineering consideration in every airplane are always compromises) is something called tail volumn of both the vertical and horizontal surfaces and it is an equation that is area of the surface times the distance.

So Its easy to see that altering the tail moment has additional aerodynamic effects beyond just the change in CG and is more important than the nose moment arm.

Now I definately want to say I am not any kind of engineer I am only just a survivor.

So if you want an electric/glow convertable then go for it but witin the constraints of this Kit Bash (a time honored modelers art) ya really gotta keep the CG where it is in both configurations.

John[8D]
Old 01-12-2012, 07:50 AM
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jester_s1
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Default RE: CG Adjustment

I'd worry about that box withstanding the vibration from a glow engine. Even if you prop and spinner is balanced right, glow power makes one power pulse per revolution instead of providing the near constant torque of an electric motor. If you compare the front end construction of glow planes and electric planes, the electrics are always built lighter because they don't need to be as strong.
Old 01-12-2012, 05:59 PM
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Default RE: CG Adjustment


ORIGINAL: acdii
Originally I was thinking I could take that whole box off, and make a firewall for the glow engine and another for the electric and all I would need to do is bolt it onto the existing structure, but the ''firewall'' is actually the box, and not the part the cowl attaches to, that is just a thin piece of plywood, and not very structurally sound.
The new box is structurally solid itself; however, no chain is stronger that its weakest link.

Consider running long bolts from the engine's mount through the box and "firewall".

One part that is glued over a piece of plywood is actually glued to only one of the layers of that plywood.

Also consider that one of the reasons for structures supporting reciprocating engines to be heavy is to use their inertia to "absorb" the vibrations.

Good project, acdii
Old 01-12-2012, 08:02 PM
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acdii
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Default RE: CG Adjustment

This is why I am really thinking things through instead of sawing and gluing in a piece.   One other thing I didn't consider, if I move the box back a bit, then I can no longer put the battery pack in, it would push the CG towards the tail.  Going to give it some more thought to being convertible, yet strong enough for the 4 stroke.

OTOH, I could always get a second fuse and bash that one instead since nothing is installed on it.  Just swap wing and tail.
Old 01-12-2012, 11:10 PM
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jester_s1
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Default RE: CG Adjustment

a 4 stroke beats up your airframe even worse than a 2 stroke does. With that box sticking out there, you're giving your engine extra leverage to shake it loose.
Old 01-13-2012, 05:59 AM
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acdii
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Default RE: CG Adjustment

I think I have a solution to this problem. Completely remove the box all the way back to the CG mark. Build a new stronger box that slides in where the original box goes and bolt it in place to new plywood sides, and notch where the original floor sits.  I can make two boxes, one for glow, which includes the tank, and one for electric that has the battery tray.  The Glow one would include a plate that covers the bulkhead that the cowl attaches to and have a few bolts lock it all in place, so rotational torque from the motor is handled as well as strengthening the fuse up to the CG point.   I can then fuel proof the box and seal the tank inside it.  This should make it cleaner and less likely to look like a hack job. 

I can leave the original box sides in, there is at least a 1/2" clearance with the current battery so I can make the box out of 1/4" ply.  Some spruce corner blocks epoxied in at the front should help strengthen it up too. 
Old 01-15-2012, 08:58 AM
  #12  
acdii
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Default RE: CG Adjustment

After giving everything a once over and determining what it would take to make it work I decided it isn't worth the time or the money. A replacement fuse alone is over $100, so one mistake would be very expensive.  I would basically need to rebuild the entire forward section, and I am unable to locate a tank that would fit that would be large enough for any flights longer than a few minutes, which defeats the idea behind the conversion in the first place. About the only size tanks I could find that would fit are 4-6 ounces.

The floor of the battery tray is about 1/2" from the wing, and the battery is about an inch thick, and sits just below the canopy, so I couldn't find a tank 1 1/2" thick that was narrow enough to fit.

Add in how much weight would be added with the additional plywood that I would need to add to beef it up, so going to keep it electric and just use a bigger pack.
Old 01-15-2012, 02:02 PM
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Default RE: CG Adjustment

I still don't think you have CG down pat yet.....CG center of gravity depends on a few things....there will be a range that you can operate in.....the further aft CG is the more sensitive in pitch the airplane will be....keep adding weight to the tail and it will get to the point the airplane just can't be controlled...you moved the CG aft by adding weight to the tail....or lighting the nose...On the other hand moving CG forward the airplane becomes more stable....but on a nose heavy airplane you'll find the airplane doesn't want to slow down on approach...or will not flair enough or depending on what you are used to flair late......in this case you're changing the engine and that will effect CG.....your new engine will probably weigh more so you are moving the CG forward.....you will have to add weight to the tail to bring the CG back in the operating range established in the instructions.....
Since you're going from electric to glow You'll set CG with everything in the airplane for flight except fuel......
Old 01-15-2012, 08:07 PM
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acdii
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Default RE: CG Adjustment

I decided not to convert it, it would add too much weight up front, which I would need to counter by adding weight to the tail, making the plane even heavier.  Right now the CG is perfect, the first flight was tail heavy and very pitchy. I added an ounce to the nose to balance it out and make it just a touch nose heavy, 1/8" forward of CG which made the plane more docile, and helped in shorten the landing, this plane likes to float with the droops on.  Once I get to the point I no longer need the droops I will remove the weight to put CG right where it is called for in the book.   I may even need to pull some weight if I get larger packs for it, depends on where the weight lands, as it depends on just how far forward I can get the pack in.

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