There's a pretty good chance I'm going to need more weight in the tail of my plane, due to larger motor and batteries. The plane has cutouts for side-mounted elevator servos, however my mini-servos are mounted inside the stab. So my options are:

1) Mount the rudder servo in the tail and move the batteries forward, creating more room for adjustment. This eliminates pull-pull and uses a direct push-pull linkage.

2) Use lower capacity batteries (4000mah instead of 5000mah) to lose weight in nose. This will cut down valuable practice time for this noob.

3) Buy bigger servos for the elevator (I have some pretty good Airtronics 94761 mini digitals in there now - don't need a performance upgrade).

3) Add a few grams (<20) of lead to the tail.


Is there any disadvantage to a tail mounted rudder servo with a quality, direct linkage?

Well, start by moving the battery back as far as poss.<div>Then just add some weight until you've found your ideal CG spot.</div><div>As with everyone that's done the great rearward CG experiment, you'll possibly end up a great deal further forward than you original thought best!</div><div>Mine you, it normally takes some years to appreciate that with the slow reduction of the aforementioned tail weight. It's a trade off as you'll discover.</div><div>Keep it simple first, then when happy, do the big engineering. That's my motto.</div><div>
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what great advice! Simple is grace, and saves a lot of work and/ or money.

What David said will get you in the ballpark. You'll need to fly it and trim to your liking in the air, but his method will get you a lot more mileage beforehand.

Consider moments of inertia....would you rather have a single centered mass, or a barbell of equal mass?

Consider the efficiency of a pushrod vs cables, and a long servo lead vs a short one?

Regards,

Dave

Thanks Dave. This is some of what I was trying to figure out.

1) I've heard the 'barbell' analogy before, but to me it almost makes sense to have the weights spread out with the result being increased stability; it would require a more deliberate input to get the plane to pivot. I'm probably wrong and missing something, but it just seems like a centered mass would seem more 'tipsy'.

2) I don't know anything about the efficiency of pull-pull vs. efficiency of a pushrod.

For a given control input or bit of turbulence, the reaction of the barbell will start and stop slower, and the duration of the reaction will be longer. This can have the appearance of being "smooth", but it is also much harder to control as a pilot. Simple test is to put 4 oz on each wingtip and attempt point rolls.....I don't think you'll be happy with the aileron response....especially if you try a quick, compact, 8 point roll.


Exactly! But one contributes to the barbell effect.

Regards,

Dave

Thanks Dave. This is some of what I was trying to figure out.

1) I've heard the 'barbell' analogy before, but to me it almost makes sense to have the weights spread out with the result being increased stability; it would require a more deliberate input to get the plane to pivot. I'm probably wrong and missing something, but it just seems like a centered mass would seem more 'tipsy'.

2) I don't know anything about the efficiency of pull-pull vs. efficiency of a pushrod.
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If one could build a plane with, say, 90% of the weight right on the cg, it would damp faster than anything else. One could do something similar by adding lots of ballast but that kills performance because of the given thrust available. If Unlimited thrust was available, it would be a different conversation.

MOI is one of the main reasons (there are aerodynamic reasons also) I've developed my wing designs around a higher taper ratio than any one else is using. Getting more mass closer to the cg and less at the tips makes MOI sense. And aerodynamically, the design moves the AC closer to the fuse CL, meaning turbulence affects the wings less. Control efficiency is also improved.

I really want to increase taper ratio beyond my current 3:1 but I am hitting a wall on the foam cutting. It isn't insurmountable though. I suppose i could always build one from wood. One of these days I may get around to it. MOI is the reason I build stabs and rudders as light as possible and move the wing forward. The performance envope that these "apparently " small changes result in is quite dramatic

Dave, BTW, just pulled an 18x12 hybrid prop from the husks....50 grams after minor trim. That's for the gasoline engine. Yet another example of reduced MOI

MattK