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CG point for FT Sportster

nhk750

Aviation Enthusiast
#1
Can anyone tell (or show with a picture) me exactly where the CG point is for setting up the FT Sportster? I'm having some issues with it being tail heavy. The build video and plans dont really show where it is on the wing. All it takes, when checking the cg with the plane upside down, is a few fractions of an inch to completely change the cg when putting my fingers on the recomended 2 inch from the leading edge area. I already added 14 grams of weight to the nose and it still seems tail heavy when checking cg. If I move my fingers just 1/8" towards the tail, then it balances out ok.

I built this one a little on the heavy side, almost 1,000grams AUP and the tail has some extra weight from reinforcement and a plastic tail skid. It has unbelivable power from my upgraded power pack, but I'm not sure the design like all the weight...
 
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Hai-Lee

Old and Bold RC PILOT
#2
You could keep adding extra weight to get the balance but a better solution might be to remove the motor and put a half inch plywood pacer to bring the motor forward. The motor is the second heaviest item in the plane and already is as far forward as it can go so just make it go further.

This is similar to the simple technique used when converting a gas plane to electric. You may even find that you can remove some of the additional weights already added.
 

JimCR120

Site Moderator
#3
I would echo HL. Some things go where they must as a part of design. Other things can just go where it's most convenient or efficient. I like having my plane mostly balanced without the LiPo so I can have that be the last adjustment done as needed. Adding weight can be a good thing but it also means you'll be expending more energy to propel you craft and more energy expended means less potential flight time.
 

nhk750

Aviation Enthusiast
#4
My problem is finding the exact cg point. Its very sensitive on this design and I'm not sure I'm measuring at the correct position. Then I can mess with weight shift. I think when I mount my cameras it will balance out a little better. But, I don't always fly with cameras on board. So far, I like my Storch way more than this plane, but I do need to give it a little more time as its my first low wing. My Storch has a semi dihedral wing (built accidently that way) and is an aerobatic wonder!
 

Hai-Lee

Old and Bold RC PILOT
#5
The exact CG point can vary from plane to plane and even build to build. Ti ascertain the exact CG I start at the nominal 25% of cord or whatever the CG calculator dictates. From there I do the test flight and trim the plane for steady level flight at minimal throttle.

Upon Landing I check the positions of the control surfaces and note any deflection from the design neutral position,

CG setting can be read directly from the elevator. If the elevator is deflected downwards then the plane is flying tail heavy and conversely if the elevator is deflected upwards then the plane is excessively nose heavy.

The use of minimal throttle is because of thrust angle adjustments which are adjusted after the second test flight or if the thrust line is severely incorrect and obviously a serious issue some shimming would occur before the second flight to make it more easy to get a more precise test on the second flight.

It works for me!
 

nhk750

Aviation Enthusiast
#6
Thanks, that helps me get a grasp on fine tuning this thing. I just checked the cg again and it is really close, so once again I wait for the weather, wind, and the wife to cooperate so I can fly again.
 

nhk750

Aviation Enthusiast
#8
Sounds good, probably in a few weekends. We have a wedding to go to this weekend and it also wont stop raining, arggg...
 

TexMechsRobot

Posted a thousand or more times
#10
And when it does stop raining it's usually too windy to enjoy. I feel like I'm fighting just to keep it in the air and not really enjoying the time. Towards the end of April or May the wind starts to cooperate and then July, August, and September are just perfect for flying.
 

nhk750

Aviation Enthusiast
#11
Mine balances perfect at the second ridge of the wing...
Thats where I thought it was, thanks! The Build directions were weak on explaing this. This means I'm still slightly tail heavy. I dont feel like messing with the motor mounts so I'm going to add a little more nose weight or possibly a heavier spinner.
 

Hai-Lee

Old and Bold RC PILOT
#12
On the matter of adding nose weights, I have found that a few heavy washers in the spinner between the prop and the locking nut are equal to almost twice as much weight in lead in the power pod on short nose designs.
 

nhk750

Aviation Enthusiast
#13
And when it does stop raining it's usually too windy to enjoy. I feel like I'm fighting just to keep it in the air and not really enjoying the time. Towards the end of April or May the wind starts to cooperate and then July, August, and September are just perfect for flying.
Yes that is true, it has been windy when nice, but that being said, I have gotten a lot of flying in this winter, probably every other week a battery or two in between rain storms. My Tiny Trainer is perfect for going out in the football field near my house for a quick spin now and then.
 

nhk750

Aviation Enthusiast
#14
On the matter of adding nose weights, I have found that a few heavy washers in the spinner between the prop and the locking nut are equal to almost twice as much weight in lead in the power pod on short nose designs.
Nice idea! I will try that first. I think I just need a little to put me over the top.
 

nhk750

Aviation Enthusiast
#17
FIXED CG!

It took a month to get this 3$ (5$ With shipping) part from China through Hobby King, but it was worth the wait! Nice quality 30mm spinner that added enough weight to the nose to finally fix my cg issue. I now know why my tail is so heavy too. I used oversized bbq skewers and also added a tail skid. So now my CG is perfect, according to the build specs. I just need a day without wind, rain, and honey doos to fly. I bet this Sporster will handle nice now, especially with all that power.

IMG_2674.JPG

IMG_2675.JPG
 

nhk750

Aviation Enthusiast
#18
Finally got to fly it today and perfect cg! It fly's really good now, but I hit a gas pipe on landing at our local club (old landfill has pipes sticking out of ground), so I packing taped up the wing tip and it fly's great again. Maybe it is because this is my first low wing, but I think I have more fun with high wings. This Sportster does have a good roll rate though. I am still not sure I like this airframe design that much though, maybe it will grow on me.

With my Aerodrive motor upgrade it has unlimited vertical and hovers (I cant hover very good though) and also performs a knife edge pretty good. I am sure a pro could pull of any aerobatics with it now and it is a good aerobatic trainer for me.