The Profile Brotherhood RC Forum banner
61 - 80 of 81 Posts

·
Registered
Joined
·
6,241 Posts
I’ve triple checked the cg, it’s tail heavy and probably needs about 8 ozs. of nose weight to get the cg it at 4-1/4”.
I've got my Mojo 40 balanced at 6 inches back and it's definitely not tail heavy.

Since I have up elevator trim for level upright flight, when inverted wouldn’t it make the nose pitch down?
Short answer: No.

The excess up elevator trim is the result of a nose heavy plane and made even worse by adding down thrust. The stab and elevator are needing to create a downward aerodynamic force in order to keep the nose from falling.

Once the plane is inverted the nose heavy condition is also trying to drop the nose. The stab and elevator are still creating the same aerodynamic downward force to pull the tail section toward the tailwheel. But now this force is acting to lower the nose even faster.

On a 3D plane that's correctly balanced, the aerodynamic force of the stab and elevator will be close to zero. With the correct CG the plane won't try to dive or climb in hands off upright or inverted level flight, so the stab and elevator don't need to produce any aerodynamic lift to raise or lower the nose.
 

·
Registered
Joined
·
20,069 Posts
It sounds like you need to just go back to liquid testosterone power if you need that motor that far out and still need 8oz.

I never measured my jo40’s CG that I recall.
Do as mcddd said. Take all preconceived notions out of your head and start over with the correct setup.
Ailerons level with tube
Elevator level with h-stab
No up or down thrust in the motor (use a level for this. Set it on a table and put a level on the tube and block up the tail until it’s level. Then take the prop off and add a spinner back plate or something known to be flat up against the prop shaft and put the level against that and make sure it’s “level” (exactly perpendicular to the tube) )
Now fly it! Don’t trim it! FLY IT.
If your cg is correct you won’t need any aileron trim. If it’s off, you’ll know which way immediately. If it dives it’s nose heavy, if it climbs it’s tail heavy. Roll over and make sure.
if it keeps acting weird then you have a warp somewhere. Maybe you screwed up gluing the wings on or the tail wasn’t built square.
you can check both while you’ve got it setup and leveled.Set the level on the stab/elevator and make sure it’s level. Draw a line from the center of the wing LE to the center of the trailing edge and hold the level on that line. If it isn’t level, it’ll never and right. Check for wash in or wash out by looking down the wing from tbe tip to see if there’s any twist at all. If it has none, your wing root is off and you’re screwed.
 

·
Registered
Joined
·
458 Posts
Really surprised CG is a issue on the electric mojo 40. I picked this Jo 40 up from a club member needing to be completely recovered. Yes it was built with the wing upside down haha. It has standard hitec servos in original locations. G32 - 3300 4s and balanced out good at 5.5 with the motor all the way back.
On the primo 40 E , still got away with stock servo locations but used micro instead of standard-
make sure to keep us updated with a flight report and how ya like it!
Aircraft Airplane Monoplane Toy airplane Toy
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #64 ·
Ok, I’ll follow all the suggestions.l you all have given me when I get back home. As far as gluing in the wings, I used the original screws to make sure they were in the same position as before.

I’m perplexed at the cg issue. The cg as is, = 5”+. At 4 to 4.5” the tail falls to the table so I do t see how it’s flying nose heavy??? I’m measuring the cg next to the fuse from the leading edge of the wing. Should I be measuring at the wing tips where the wing angles back???

I’m not sure if the plans call for it, but there is a 1/4”sq. wood support under the rear stab and 2 screws on each side into them from the top. I’m not sure why the screws were added vs. just glue. I’ll remove the screws and see if it’s glued as well, assuming it is. The servos are full size hitec’s.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #65 ·
Really surprised CG is a issue on the electric mojo 40. I picked this Jo 40 up from a club member needing to be completely recovered. Yes it was built with the wing upside down haha. It has standard hitec servos in original locations. G32 - 3300 4s and balanced out good at 5.5 with the motor all the way back.
On the primo 40 E , still got away with stock servo locations but used micro instead of standard-
make sure to keep us updated with a flight report and how ya like it! View attachment 145837
Really surprised CG is a issue on the electric mojo 40. I picked this Jo 40 up from a club member needing to be completely recovered. Yes it was built with the wing upside down haha. It has standard hitec servos in original locations. G32 - 3300 4s and balanced out good at 5.5 with the motor all the way back.
On the primo 40 E , still got away with stock servo locations but used micro instead of standard-
make sure to keep us updated with a flight report and how ya like it! View attachment 145837
I like the covering scheme on your Mojo. Mine was built with the removable wing option so the wing itself is probably heavier since it has a wing tube and some reinforcement to the middle spar/fuse area. I read the cg should be 4-4.5”. My Mojo balances at 5-5.5” right now with the motor all the way forward. At 4-4.5” the tail doesn’t come off the table. My newest Lipos are lighter so that isn’t helping my tail heavy bird. I know some have said it it dives at 1/2 throttle it’s nose heavy with the elevator even, but I’m thinking it has to do with the motor being picked down. I’ll check all the surfaces again and remount the motor and see what happens. After I trimmed up the elevator foe level flight, it flew good, but when I went inverted it dove like it was nose heavy but I think it was the motor pitch. We shall see. I bought a PA Addiction also used and it flew perfectly. With a used bird, you just never know.
 

·
Registered
Joined
·
6,241 Posts
I’m perplexed at the cg issue. The cg as is, = 5”+. At 4 to 4.5” the tail falls to the table so I do t see how it’s flying nose heavy??? I’m measuring the cg next to the fuse from the leading edge of the wing. Should I be measuring at the wing tips where the wing angles back???
The 4 to 4.5" recommended CG was done on purpose by Swany in order to give non-3D flyers a better chance of successfully flying his Mojo 40 design. While it works OK for someone who doesn't fly 3D, the 4 to 4.5" recommended CG is actually way too nose heavy for someone who does fly 3D. Most 3D pilots would prefer a CG at about 5.5 to 6.0". If your Mojo 40 is balancing level at around 5" measured back from the leading edge at the fuse, then you are still a little too nose heavy for 3D flying.

The best way to determine the ACTUAL best CG for your plane is to trim it to fly hands off level upright, then check to see if it also flies hands off level inverted. If it dives when inverted, then the plane is still too nose heavy for good 3D flight. If it climbs when inverted, then the plane is tail heavy. However being tail heavy can increase the elevator effectiveness and make the plane more responsive, which is sometimes desirable.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #67 ·
The 4 to 4.5" recommended CG was done on purpose by Swany in order to give non-3D flyers a better chance of successfully flying his Mojo 40 design. While it works OK for someone who doesn't fly 3D, the 4 to 4.5" recommended CG is actually way too nose heavy for someone who does fly 3D. Most 3D pilots would prefer a CG at about 5.5 to 6.0". If your Mojo 40 is balancing level at around 5" measured back from the leading edge at the fuse, then you are still a little too nose heavy for 3D flying.

The best way to determine the ACTUAL best CG for your plane is to trim it to fly hands off level upright, then check to see if it also flies hands off level inverted. If it dives when inverted, then the plane is still too nose heavy for good 3D flight. If it climbs when inverted, then the plane is tail heavy. However being tail heavy can increase the elevator effectiveness and make the plane more responsive, which is sometimes desirable.
Thanks, your experience with the Mojo helps me understand that the CG is a much broader range than I thought. Based on that I’m probably good where the CG is at, no need to add weight (I was worried about turning it into a dog). I’ll re-mount the motor with zero down when I get back. I did find in the Swanny section a pdf of the plans. On the plans he listed 3* right and 0* down for the motor. Some here say profiles don’t need any right offset. I’ll leave it at about 2* for the 1st flight since it will be easy to remove a washer to get it back to 0* if necessary. I’m more encouraged now based on everyone’s input.
 

·
Registered
Joined
·
2,925 Posts
Might be time to bring up jeremy chin trim guide,a good read anytime. I like checking planes in a vice,i use a indence meter to level wing,then check every surface to that. I also like the pointer stick on wing tips,makes setting the al easy. All this help you got to win.
 

·
Registered
Joined
·
6,241 Posts
The right thrust on a Mojo 40 will make it easier to maneuver and harrier in upright 3D flight. But the right thrust will make it more difficult to maneuver and harrier in inverted 3D flight.

So if the right thrust helps you out, then use it. You can always reduce or remove it later on when you begin to explore inverted 3D maneuvers.
 
  • Like
Reactions: Heinz57

·
Registered
Joined
·
8,450 Posts
NEVER down thrust a symmetrical wing. Fix that first.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #71 ·
The right thrust on a Mojo 40 will make it easier to maneuver and harrier in upright 3D flight. But the right thrust will make it more difficult to maneuver and harrier in inverted 3D flight.

So if the right thrust helps you out, then use it. You can always reduce or remove it later on when you begin to explore inverted 3D maneuvers.
Thanks for the info. I plan on trying right thrust 1st, and as you said it’s easy to remove a washer if I want to get rid of it.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #72 ·
NEVER down thrust a symmetrical wing. Fix that first.
Yep, I see that now. I found a pdf of the original plans and Swanny called for 0* down thrust & 3* right. Easy fix to re-drill the motor mounts. I’m out of town, but will get on it when I return.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #73 ·
I finally got out today to see how she flys with zero degrees down and right on the motor. Much better but still nose heavy. I removed the previous weights and it improved more. Just for kicks I added about 1/2 oz. to the elevator and it fly real nice. Now I’m at just a click or two of up elevator and inverted at 1/2 throttle is finally flat. My Ch is now about 6” from the leading edge but it flys and handled the 17 mph gust with no problem. Since it flys good at a 6” cg. debating on moving the motor back to where I started. I think a combo of the motor offset and thinking I needed to be at a 4-4-1/2” CG had me screwed up. I ran 4 Lipos through it and beginning to have a blast. Currently running a 13-5 woody, but just ordered a 14/4 & 14-6 to try.
 

·
Registered
Joined
·
2,925 Posts
Man thats good news,the cg do make or break a plane. Plus,look at all you learned. How many planes i built and did not understand cg,just put it where plans said. Now,because of the bros i got a clue of setting up a plane,thanks guys,wish i had met you guys sooner. A great flying plane is a joy,a few tweaks is all it takes sometimes.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #76 ·
Yes move that motor back! It’ll be a different plane with the correct nose moment
Yes, I plan to do that soon. I’ll probably fly it a few
more weeks just to have some fun, but will definitely move it and see what happens. Thanks to everyone who responded with all the suggestions. It was really helpful! I’ve always gone with the plans cg recommendation so this was a real eye opener for me.
 

·
Registered
Joined
·
2,628 Posts
It will become a much more lively machine if you move it back--WAAY better for tumbles and such :)
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #78 ·
Ok, I will. Any issue with a 7” or more CG, or should I target to maintain 6” which is were I’m at with motor out front and 1/2 oz. of weight on the elevator? I’m comfortable with flying it at a 6” cg. Also, as I move the motor back, the Lipo will move back accordingly, so I may not be able to move it much without it becoming super tail heavy.
 

·
Registered
Joined
·
6,241 Posts
Before you move the motor back, I'd suggest trying different CG locations by adding weights to the tail around 1/2 ounce at a time. Once you find the right amount of weights to get a neutral CG location (hands off upright, hands off inverted), measure exactly where the plane balances at that time. Then remove the tail weights and move the motor back and adjust the battery location until you reach that exact same CG location.

Moving the CG behind this point will make the elevator even more responsive. It's great for getting the plane to loop, flip, and tumble inside a smaller area. But it's not so good for trying to keep the plane going in a straight line without constantly wandering off course.
 

·
Registered
Joined
·
182 Posts
Discussion Starter · #80 ·
I’m pretty much neutral right now with a 1/4 oz. Weight on each side of the elevator. I’m thinking another 1/2 oz. total will do it. I’ll give it a shot the next time out. Sounds like a good method. I appreciate all the input, I think I’m getting close to a Pro Bro setup….finally!
 
61 - 80 of 81 Posts
Top