I had dug an older pattern model (Otop) out of the attic to give to a friend to introduce him to bigger electric models, he only has a 6S setup which gives 1850watts on a 17x10APC. I was doubtful that I could get the CG to work without adding noseweight which I did not want to do, and , after pushing everthing as far forward as I could I got the CG at 32% of MAC ( Mean average chord) this was about 10mm behind the wing tube on the Otop where I had previously flown it at the centre of the wing tube using a 10S setup. I thought this would be a bit to far back and decided to check the CG on my Wind 110 just to compare the two.

I was very surprised to find the CG on the Wind to be at 37% of MAC (210mm behind the leading edge of the wing) It does not feel unstable or tail heavy at this position.

Question, are the CG characteristics of every model different or can we assume that pattern models being mostly of broadly similiar design will share similiar tolerance to CG position, or are there, as I suspect, other factors to be considered here?

John

Hi John, Yes, you are correct, the location of the CG is different on every pattern plane. And again yes, there are a number of other factors that impact the location of the CG. Notably the size/weight of the electric motor, location of the battery tray and whether or not you use two small elevator servos in the rear of the fuse or stab versus an elevator pushrod assembly/DEPS with servo near the rudder servo mid-ship. Note, the Otop was not a true 2m plane, it was somewhat shorter in length. The best I can recall, the Otop flew best with the CG approx 5mm in front of the wing tube. Also, let me add the capacity/mahs/c ratings which ultimately determines the total weight of the batteries and whether you use 8s or 10s packs in series that you obviously know about. John, I have a question for you. From my calculations his 6s set-up is pulling approx 75 amps, right? At 1850 watts and based on my past experience, I really question this Otop's performance. Is he going to try to fly this combination in a pattern contest? The reason I ask is that I feel he might be disappointed due to it being underpowered and very tail heavy. Think back about the total weight and the overall performance of this plane when you flew it with a 10s set-up. Remember what the performance felt like pulling approx 2500-3000 watts? I hope I have been able to help in a small way, Everette

Everette

Thanks for taking the trouble to reply. Yes, I understand that the location of the main components affects the Cg location but my question really related to the ability to run with what seem like a rearward CG location on a pattern model. I.e. my Wind 110 is at 37% of MAC and is very stable, there is no way I would try to fly an Extra or similiar at that percentage, does the long tail moment on a pattern model have an effect on the ability to run with a rearward CG? I suppose at the end of the day it doesn't really matter where the CG is as long as it works!

As to the Otop I know it will not bore holes in the cloud base at 1850 watts, but I have pared the weight down to 9lbs 2oz RTF which means I have 200watts per pound which should give reasonable aerobatic performance with limited vertical.
I previously had it at 2400 watts on my Axi 5330/18 with a RTF weight of 11lbs which was 220 watts per pound and it flew well. BTW I have done some more research on the CG and the reccomended location for the Otop is 175-195mm behind the leading edge, I have it now at 195 so I will give it a try next week if the weather permits and let you know how it works. My friend will not be flying competition aerobatics so I am hoping he will just have a nice smooth flying model.

John

Hi John, now I understand what you are looking for. There was a time in pattern when it was "vogue", for some, to run a rearward CG and like you said or asked, it was possible because the long tail moment on a pattern plane made it more possible, IMHO. Keep in mind, my knowledge base comes from practical application based on 25 years of building pattern planes and not a degree in aeronautical engineering. My placement of the CG of approx 5mm in front of the CG suited my needs based on how I set-up my planes and personal preferences and feel. Also note, beginning with that particular plane, I've used small elevator servos in the side of all my fuses, under the stab, instead of using a DEPS type arrangement ever since. You are right, in the final analysis, the placement of the CG starts with a reference number provided by the designer or manufacturer and you determine what suits your personal taste or feel to achieve the flight characteristics you desire by changing the location of different components. Relative to your placement of the CG at 195mm behind the leading edge, where does that put my 5mm in front of the wing tube? Just curious because that was several years ago and I had given the plane to a flying buddy who then "strained it thru the trees"! BTW, I had an AXI 5330 F3A motor in mine and it was STRONG! You guys will do fine. Enjoy the flight, Everette

Everette

I originally flew the Otop with the CG on the front of the wing tube this is 160mm behind the LE. At 195 it is about 5mm behind the wing tube.

John

There are a few things that can affect where the balance point of the plane is/should be.

1) The MAC is not necessarily in line with the wing root, depending on the taper of the wings.
2) The shape of the airfoil, and the location of the pressure gradients (the Wind has a very thin and wide wing, so the diffusion is slow and taking place over a very long distance which may reduce the sensitivity to CG location).
3) Like you said, a pattern plane with a long tail and small precise elevator controls will counteract the sensitivity that comes with moving a CG rearward.

These are just some thoughts off the top of my head... but yes, I would expect the tolerance of CG to be different on different planes, and the little pictures in my head seem to tell me the biggest factor has something to do with a ratio of the thickness:length, where a fat wing with a smaller chord would be much more sensitive to adjustments.

Will you have a chance to test fly it before handing it off? Can always add a little weight just for the purpose of honing in on the balance point - better to play it safe and slowly remove weight to see where the range is...

Joe

Yes, I am going to test fly it before handing to my friend. There is a CG calculator available at "The R-C Aircraft Proving Grounds - Centre of Gravity" which seems to have all of the variables such as wing sweep built in to the calculator.

John