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-   -   Low Wing Center of Gravity (https://www.rcuniverse.com/forum/questions-answers-154/11676168-low-wing-center-gravity.html)

Masterguns 03-08-2020 06:18 PM

Low Wing Center of Gravity
 
Howdy, wardogs!

I passed all of my physics courses, but I prefer to skip long academic minutia of aeronautical engineering discussion in this particular case of inquiry. I want to know how to get a good CG as to not have too many mishaps during my first low wing flight after the transition from high wing examples. I turned the model upside down & it was a bit tail heavy using the manufacturer's suggested point of of measurement using my pointer fingers. If I had tested right side up then the general consensus is that the CG should remain the same-nose down & tail up, correct? Just shift the pack forward or aft or add the least amount of lead to get the suspect side to balance? Dumb it down for me just using the intel I explained. I know it can change from flight to flight, but I'm talking about a first flight baseline. No thread high jacking, please. I thank you in advance for your comment(s).

S/F,
Masterguns

52larry52 03-08-2020 07:36 PM

First off, You are correct to always balance a low wing plane inverted, not right side up. 2nd item, always C/G balance a fuel powered (gas or glow) with the fuel tank empty, and an electric powered plane with the battery in place in the plane. 3rd item, Checking C/G balance with your fingertips is not very accurate. I know.......a lot of guys just grab the plane and hang it on their fingertips, but fact is it not a very accurate way to do it! Use a C/G balancer made for the job. If none is available (poor excuse, buy one!), some guys will balance them on the end of a pencil eraser (two of them and not on the point end, on the eraser end!:rolleyes:). Another good way without a real C/G balancer is to buy some kitchen drawer bumpers (little round clear rubber self adhesive bumps) from a hardware store and stick one on each wing right at the C/G point you have selected for the model being worked on. Stick the bumps on the wing at a reinforced area, over a rib or sheeted area, not just hollow covering. Again, bumps on top of the wing for a low wing, and on the bottom of the wing for a high wing plane. Now you can use your fingertips to balance it at the bumps. Leave the stick on bumps there as they are useful for a quick check of C/G at the field. Now, as to where is that magic spot that is the correct C/G point. The C/G point stated in the airplane instructions is usually a good starting point but be aware that is sometimes WRONG due to language translation or just plain OOPS. The general rule of thumb is you will want the C/G between 25% to 35% of the mean aerodynamic cord of the wing. That simply mean 25-35% back from the leading edge of the wing if it is a simple constant cord wing, not a tapered wing. It is best to stay in the 25 to 29% range to start out. Measure the wing cord with a ruler, 25% of that number is how far aft of the leading edge is the C/G point you want to start at. That is a good safe flyable C/G point, Stay away from the 30 to 35% area untill you are an expert. Enjoy your well balanced new plane.

tedsander 03-08-2020 08:05 PM

52larry52 gave good advice. As to balancing upright vs inverted....it is really only a matter of making it easier on yourself. On a high wing plane, the mass is hanging below the wing, hence finding the CG on the bottom of the wing makes it act like a pendulum. On a low wing plane, doing the same, and the mass above wants to seek a lower level, making it much harder to find the exact balance spot. You will end up finding it either way, and it will be the same. It is just much easier to do it with the bulk of the mass already as low as it can be. Just like balancing a hammer vertically - easier if the head end is down, rather than trying from the handle end.

Masterguns 03-08-2020 09:39 PM

Thanks, guys. I do have a purpose built CG balancer, but for some reason I have more trouble with it than using my pointers. I will put it to use again until I get used to it. So, just clarify, if the nose of the plane rises lowers & the tail lowers then I have a tail heavy plane & the necessary weight shift with the pack or adding a tad of lead in the nose will be required? Or, do I have it backwards? Thanks, again for your help.

S/F,
Masterguns

rgburrill 03-09-2020 04:11 AM

I had a CG balancer and like the OP found it to be a PITA. Since this is his first low wing using his fingertips is just fine - he doesn't need the precision of a competition plane. I've used mine for over 20 years.

speedracerntrixie 03-09-2020 04:23 AM

Agreed, however it would be nice if the OP would let us know what airplane he is dealing with. The upside down/right side up deal can be affected by the amount of dihedral a low wing airplane may have. He should also know that the initial CG position can be changed based on what the airplane does while in flight. Example, if it requires a fair amount of up trim and lands fast the CG needs to be moved back some.

Another thing that I think is important is to know where your thrust angles and incidence angles are and learn to manipulate them. Many poor flying airplanes that guys give up on could have easily been turned into favorites with a few adjustments.

Masterguns 03-09-2020 08:59 AM


Originally Posted by speedracerntrixie (Post 12588211)
Agreed, however it would be nice if the OP would let us know what airplane he is dealing with. The upside down/right side up deal can be affected by the amount of dihedral a low wing airplane may have. He should also know that the initial CG position can be changed based on what the airplane does while in flight. Example, if it requires a fair amount of up trim and lands fast the CG needs to be moved back some.

Another thing that I think is important is to know where your thrust angles and incidence angles are and learn to manipulate them. Many poor flying airplanes that guys give up on could have easily been turned into favorites with a few adjustments.

Sorry about that. This is the Arrows (Hobby Zone . com) Marlin 64mm EDF, No dihedral swept wing & horizontal stab with elevator only. 35.38" wing span, 39.2" length, flying weight - 1050g+/-, motor 2840-KV3150, wing load 66.8g/dm2 (0.15oz), wing area 15.7dm2 (243sq"), recommended battery 4S 2200-2600mah 35c. I'm putting in an E-flite 4S 14.8v 3200mah 30C. It weighs 13.0oz. Turnigy 3000mah 4S 14.8v 20-30C. As I mentioned, I'm aware of possible CG shift on any given flight.

S/F,
Masterguns

Masterguns 03-09-2020 09:53 AM

Got it
 
Balances level at 80mm with Turnigy 3000mah pack all the way to the front. Balances level at 80mm with the E-flight 2300mah pack (0.594lb) slid back 1 1/2" to the rear. Arrows manufacturer recommended 80-90mm. Thanks for the all the help, gents! Happy flying & all the best!

S/F,
Masterguns

jester_s1 03-16-2020 08:08 PM

Do yourself a favor and build a Vanessa CG machine. It doesn't rely on the plane being stable as it teeters on one spot, so it can handle any size or type of plane, upright or inverted.

ETpilot 03-17-2020 02:09 AM

Agreed! Vanessa Rig is the way to go. I use it during the build to try to avoid CG problems later. Just tape things in place and move as needed.


https://cimg7.ibsrv.net/gimg/www.rcu...2ae135465.jpeg
https://cimg8.ibsrv.net/gimg/www.rcu...aab6021da.jpeg



jaka 03-27-2020 02:47 PM

Hi!
I use my fingertips! And have done so on all my planes, racers,sport and scale, since I started 1975. And always with the right side up!
https://cimg0.ibsrv.net/gimg/www.rcu...dd1ba448d7.jpg
https://cimg6.ibsrv.net/gimg/www.rcu...3c5d04e55b.jpg
https://cimg2.ibsrv.net/gimg/www.rcu...e0ef23452a.jpg


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