Lots of techie types on here, so this should be fun...

Can anybody relate the CofG position to the MAC (mean aerodynamic chord) of current pattern planes like the Black Magic, Pinnacle, Astral XX, etc.? It used to be the rule of thumb was 25% of MAC as a starting point, but that was a few years ago and planes have changed along the way. I need this to do some weight and balance calcs on a couple of original designs I am drawing up and to (hopefully) ensure they actually sort-of fly OK on the first try.


Thanks,
Mark

I have used the formulae below to calculate the C.of G. (measured along the M.A.C. of the wing) for many O/D pattern planes over the last 18+ years, without any problems----the final position usually ends up about 2/3 of the way back between them --hope it helps!

CG = M.A.C./7 + Tail Area x Moment Arm(forward position)
4x Wing Area


CG = M.A.C./6 + 3x (Tail Areax Moment Arm) (Rearward position)
8x Wing Area


Edited to add: The 4x wing area, and 8xwing area,bits, should be appearing below the underlined bits, but they move between typing and sending the post!

Thanks, that is exactly what I needed.

One question - how are you measuring the tail moment arm?


Mark

[quote]One question - how are you measuring the tail moment arm?
[/quote

From 25% MAC wing to 25% MAC tail.
All measurements in the same units of course!(being ancient, I use inches and square inches)
And just personally, I include the area across the wing and tail seats.

Me too, because my aero prof said to....


Thanks for the help,
Mark

My Pinnacles are flying at about 28% of MAC. I have had 3 of them. Combined with helping setup another 3-4 Pinnacles they all end up about the same spot.

Hope this helps
Troy

For Troy: Do you use the fuselage area between the wing roots (I'm assuming plug in wings) when establishing your MAC, or just the wing panels?

Mike Moritko

I use the effective wing area. You are always suppose to do the wing area in the middle. This is the wing area of the model now the amount of the wing area exposed to drag is a different number that would not include the center fuse portion.

If you were to analyze it like an Engineer you would need to the look at the area of the fuse as well. In this case it would be a top/Bottom view. A free body diagram to include the forces might actually include any lifting done by cheek cowls and landing gear struts. Any fillets would be applied to the lift and drag associated with the model as well as wing thickness and any subfin- dorsal fin appendages would work in the lateral direction.

This is a pretty anal way to look at it but the F-16 has really small stubby wings and one heck of a flying fuse. It can all get really complicated but if you calculate numbers as a reference like moment arms and CG as a percentage of MAC then a bunch of these things all get shuffled into the noise and don't count too much. In the end. The MAC CG reference is nothing more than a reference. It is not a holy grail of getting a model to fly properly and it doesn't carry much weight with me. I look at what the numbers are to tell if I'm close. That is all nothing more. I would say in my experience the number up around 30-33% MAC tend to fly too tail heavy for me. While 25% tends to feel a bit too nose heavy.

I will say that the models I have flown in recent years all ended up about at this 27-28% number and flew well there. If I was building a model from the ground up this is what I would shoot for. If I had a model already built tested and produced as a kit like my Pinnacle, the Astral XX, or the Zeque I would use the CG recommended by the designer and change it to my preference. In most cases I have found that I like my models much more nose heavy than the designer suggests for sequences. This results usually in the need for a little more positive in the wing than they call for. This also means that my models need rudder on knife edge and they need down elevator inverted too even on 45 deg lines they need down elevator inverted and rudder on the knifes.

On helping lots of guys I find less experienced guys try to run the CG aft to get the model to snap better and roll better. This is usually the case because they have not learned the coordination needed to rudder the rolls. Its a tough skill but reality is once you learn it the models fly so much better. I actually have trouble with models so balanced that they fly on knife with no rudder. The indoor foamies are like this the Sypher I flew at the ETOC and the Tensor are the same. Put it on knife and it stays there. Throttle up to climb and throttle down to descend really a weird feeling especially trying to fly precision and not 3D.



Troy

I agree, I just need a ballpark figure to allow me to get the weight & balance calcs in the right area so I don't have a battery pack strapped under the tail or similar. The flying tells the tale after it is all said and done.


Thanks for the help,
Mark