What angle determines a balanced center of gravity?
10-23-2021, 06:56 PM
#1
What angle determines a balanced center of gravity?
I grew up flying planes and have a couple scratch builds under my wing now. But that was over a decade ago as a teenager. Now that I have big boy money I just finished my first 1/4 scale Phoenix Waco with a 7-cylinder radial engine. Yes, 16 year old me would be VERY proud of future me.
So anyways, I have it all set up for the most part, just need to set aileron weights still but otherwise I am mostly ready to go after breaking the engine in over the summer. Question is, exactly what angle determines a balanced center of gravity? Based on the Phoenix "hanging tool" type center of gravity test, it appears to be SLIGHTLY tail heavy based off of the horizontal stabilizer. The manual is pretty vague. It just says "go for it," basically.... Back in the day I would say close enough, looks good. Like it's not far off at all. But I've flown a tail heavy Sbach recently (after thinking it was close enough) and it was not the most enjoyable experience getting it back to earth safely.
So what angle determines center of gravity? Because looking at the wings you wouldn't know it's not perfect. But looking at the horizontal stab it is still a bit off. I think my brain might be playing tricks on me though. The fact I put a 90cc radial engine on a 60cc plane and it's STILL a tad tail heavy is really messing with my head. So what do you use to balance CG? I have my fuel tank directly behind the firewall and I know that will add some weight to it. And given how slowly this engine sips fuel I know I will never be landing on a completely empty tank, so that is something to consider I think too...
So anyways, I have it all set up for the most part, just need to set aileron weights still but otherwise I am mostly ready to go after breaking the engine in over the summer. Question is, exactly what angle determines a balanced center of gravity? Based on the Phoenix "hanging tool" type center of gravity test, it appears to be SLIGHTLY tail heavy based off of the horizontal stabilizer. The manual is pretty vague. It just says "go for it," basically.... Back in the day I would say close enough, looks good. Like it's not far off at all. But I've flown a tail heavy Sbach recently (after thinking it was close enough) and it was not the most enjoyable experience getting it back to earth safely.
So what angle determines center of gravity? Because looking at the wings you wouldn't know it's not perfect. But looking at the horizontal stab it is still a bit off. I think my brain might be playing tricks on me though. The fact I put a 90cc radial engine on a 60cc plane and it's STILL a tad tail heavy is really messing with my head. So what do you use to balance CG? I have my fuel tank directly behind the firewall and I know that will add some weight to it. And given how slowly this engine sips fuel I know I will never be landing on a completely empty tank, so that is something to consider I think too...
Last edited by RadialWacko; 10-23-2021 at 07:08 PM.
10-23-2021, 07:56 PM
#2
Some planes have incidence in the stab (or wings, or both) that can make it look "off" when just looking at those items. Depending on the plans, there should be a datum line running (usually) from the tip of the spinner straight back. But if yours doesn't have one showing....just get it so at the balance point indicated on the plans, the overall fuselage looks level, or a bit nose down. Then fine tune by testing in the air. The old maxim "Nose heavy may fly poorly, but tail heavy may only fly once" always holds true... so you know which way you want to err.....
10-23-2021, 08:07 PM
#3
Thanks, definitely have heard that many times. So my new question is, if I want to add weight to the front, what should I add?? It seems like with such a relatively small fulcrum I may need some significant weight given the total weight of the plane.
10-24-2021, 04:23 AM
#5
My thought is you should be able to tell if it's nose or tail heavy by eye,, if you think it's a little aft it probably is
Every plane has a CG range not one specific exact point where it has to be balanced at or it going to crash, the larger the plane,, the larger that range is, if your plans/manual only shows one point,, that is in the middle of the range,, A 60cc plane easily has an 1" to 1.25" CG range where the plane will fly fine
You saying the wing looks good, but the tail look tail heavy tells me you're probably in the range, but slightly on the aft side
good luck
Every plane has a CG range not one specific exact point where it has to be balanced at or it going to crash, the larger the plane,, the larger that range is, if your plans/manual only shows one point,, that is in the middle of the range,, A 60cc plane easily has an 1" to 1.25" CG range where the plane will fly fine
You saying the wing looks good, but the tail look tail heavy tells me you're probably in the range, but slightly on the aft side
good luck
10-24-2021, 06:45 AM
#6
To answer your original question, I would make adjustments until the prop ( thrustline ) is strait up and down. Perhaps hang a plumb bob just in front to the prop and then measure the distance between the prop tips and the line.
Getting there is another topic. Don’t take your fuel into account, always balance with an empty tank. I personally do everything I can to not add dead weight to an airplane. Sometimes it is unavoidable but there are other things to do first. Being that you are using a multi cylinder radial, I’m going to assume that it has an ignition module. The ignition battery should be mounted as far forward as possible. Your RX battery can also go up front as far as possible as well but that tends to bring along a couple concerns. First is having the two that close together. Some feel that the ignition battery can transfer interference to the RX battery. With a 2.4 ghz system I have never had this happen and I personally consider this a non issue. The second concern is the long wire run to the receiver. For this airplane, as long as your wires are at least 20 gauge you should be fine. 2.4 ghz is a bit more voltage sensitive so you do not want the receiver getting less then 5 volts under load ( my philosophy ). That brings up another option to get weight up front. A second RX battery. A second battery will not only provide you with redundant power but will increase available current supply to the receiver and pretty much guarantee you will not experience voltage dips.
You will gain better CG adjustments by removing weight from the tail ( again my philosophy). Looking at the manual I see one thing I would do right off and that would be yo replace the steel rudder and tailwheel cables with Kevlar cord ( Available from Aerospace Composites Products ). That alone would be a big help as I suspect those 4 cables have some weight to them. I would also move the rudder servo forward. The second thing I would do is get rid of the steel elevator pushrods and replace then with carbon fiber rod ( also available at ACP ). That would yield the largest weight reduction off the tail. Doing this would require attaching some Dubro threaded couplers on at least one end of the CF rod with JB Weld. To be honest, I typically just epoxy a clevis on each end with the servo and control surface being centered. A side benefit of getting rid of the steel cables and pushrods is you don’t have long runs of steel that could act as antennas and create RF issues although that is very highly unlikely.
The final thing I noticed in the manual is the aileron linkage and suggested throws. The linkage IMO just plainly sucks. You will never get the same throws on both the upper and lower ailerons with the kit supplied hardware. Unfortunately just about every ARF bipe I have seen uses this system. In order to get equal aileron throw the linkage pivots must be on the ailerons center. As shown in the manual the lower aileron has it above center and the upper aileron has it below center. The bigger issue for me here is that they show equal throw up and down movement. First off the linkage makes that impossible to begin with but you will definitely want some differential in there. If you use the provided linkage, take your measurements from the bottom ailerons and have the travel 20%-25% more upward then down. This will greatly reduce if not entirely eliminate the adverse yaw the stock setup would induce.
Best of luck!
Getting there is another topic. Don’t take your fuel into account, always balance with an empty tank. I personally do everything I can to not add dead weight to an airplane. Sometimes it is unavoidable but there are other things to do first. Being that you are using a multi cylinder radial, I’m going to assume that it has an ignition module. The ignition battery should be mounted as far forward as possible. Your RX battery can also go up front as far as possible as well but that tends to bring along a couple concerns. First is having the two that close together. Some feel that the ignition battery can transfer interference to the RX battery. With a 2.4 ghz system I have never had this happen and I personally consider this a non issue. The second concern is the long wire run to the receiver. For this airplane, as long as your wires are at least 20 gauge you should be fine. 2.4 ghz is a bit more voltage sensitive so you do not want the receiver getting less then 5 volts under load ( my philosophy ). That brings up another option to get weight up front. A second RX battery. A second battery will not only provide you with redundant power but will increase available current supply to the receiver and pretty much guarantee you will not experience voltage dips.
You will gain better CG adjustments by removing weight from the tail ( again my philosophy). Looking at the manual I see one thing I would do right off and that would be yo replace the steel rudder and tailwheel cables with Kevlar cord ( Available from Aerospace Composites Products ). That alone would be a big help as I suspect those 4 cables have some weight to them. I would also move the rudder servo forward. The second thing I would do is get rid of the steel elevator pushrods and replace then with carbon fiber rod ( also available at ACP ). That would yield the largest weight reduction off the tail. Doing this would require attaching some Dubro threaded couplers on at least one end of the CF rod with JB Weld. To be honest, I typically just epoxy a clevis on each end with the servo and control surface being centered. A side benefit of getting rid of the steel cables and pushrods is you don’t have long runs of steel that could act as antennas and create RF issues although that is very highly unlikely.
The final thing I noticed in the manual is the aileron linkage and suggested throws. The linkage IMO just plainly sucks. You will never get the same throws on both the upper and lower ailerons with the kit supplied hardware. Unfortunately just about every ARF bipe I have seen uses this system. In order to get equal aileron throw the linkage pivots must be on the ailerons center. As shown in the manual the lower aileron has it above center and the upper aileron has it below center. The bigger issue for me here is that they show equal throw up and down movement. First off the linkage makes that impossible to begin with but you will definitely want some differential in there. If you use the provided linkage, take your measurements from the bottom ailerons and have the travel 20%-25% more upward then down. This will greatly reduce if not entirely eliminate the adverse yaw the stock setup would induce.
Best of luck!
10-24-2021, 10:54 AM
#7
To answer your original question, I would make adjustments until the prop ( thrustline ) is strait up and down. Perhaps hang a plumb bob just in front to the prop and then measure the distance between the prop tips and the line.
Getting there is another topic. Don’t take your fuel into account, always balance with an empty tank. I personally do everything I can to not add dead weight to an airplane. Sometimes it is unavoidable but there are other things to do first. Being that you are using a multi cylinder radial, I’m going to assume that it has an ignition module. The ignition battery should be mounted as far forward as possible. Your RX battery can also go up front as far as possible as well but that tends to bring along a couple concerns. First is having the two that close together. Some feel that the ignition battery can transfer interference to the RX battery. With a 2.4 ghz system I have never had this happen and I personally consider this a non issue. The second concern is the long wire run to the receiver. For this airplane, as long as your wires are at least 20 gauge you should be fine. 2.4 ghz is a bit more voltage sensitive so you do not want the receiver getting less then 5 volts under load ( my philosophy ). That brings up another option to get weight up front. A second RX battery. A second battery will not only provide you with redundant power but will increase available current supply to the receiver and pretty much guarantee you will not experience voltage dips.
You will gain better CG adjustments by removing weight from the tail ( again my philosophy). Looking at the manual I see one thing I would do right off and that would be yo replace the steel rudder and tailwheel cables with Kevlar cord ( Available from Aerospace Composites Products ). That alone would be a big help as I suspect those 4 cables have some weight to them. I would also move the rudder servo forward. The second thing I would do is get rid of the steel elevator pushrods and replace then with carbon fiber rod ( also available at ACP ). That would yield the largest weight reduction off the tail. Doing this would require attaching some Dubro threaded couplers on at least one end of the CF rod with JB Weld. To be honest, I typically just epoxy a clevis on each end with the servo and control surface being centered. A side benefit of getting rid of the steel cables and pushrods is you don’t have long runs of steel that could act as antennas and create RF issues although that is very highly unlikely.
The final thing I noticed in the manual is the aileron linkage and suggested throws. The linkage IMO just plainly sucks. You will never get the same throws on both the upper and lower ailerons with the kit supplied hardware. Unfortunately just about every ARF bipe I have seen uses this system. In order to get equal aileron throw the linkage pivots must be on the ailerons center. As shown in the manual the lower aileron has it above center and the upper aileron has it below center. The bigger issue for me here is that they show equal throw up and down movement. First off the linkage makes that impossible to begin with but you will definitely want some differential in there. If you use the provided linkage, take your measurements from the bottom ailerons and have the travel 20%-25% more upward then down. This will greatly reduce if not entirely eliminate the adverse yaw the stock setup would induce.
Best of luck!
Getting there is another topic. Don’t take your fuel into account, always balance with an empty tank. I personally do everything I can to not add dead weight to an airplane. Sometimes it is unavoidable but there are other things to do first. Being that you are using a multi cylinder radial, I’m going to assume that it has an ignition module. The ignition battery should be mounted as far forward as possible. Your RX battery can also go up front as far as possible as well but that tends to bring along a couple concerns. First is having the two that close together. Some feel that the ignition battery can transfer interference to the RX battery. With a 2.4 ghz system I have never had this happen and I personally consider this a non issue. The second concern is the long wire run to the receiver. For this airplane, as long as your wires are at least 20 gauge you should be fine. 2.4 ghz is a bit more voltage sensitive so you do not want the receiver getting less then 5 volts under load ( my philosophy ). That brings up another option to get weight up front. A second RX battery. A second battery will not only provide you with redundant power but will increase available current supply to the receiver and pretty much guarantee you will not experience voltage dips.
You will gain better CG adjustments by removing weight from the tail ( again my philosophy). Looking at the manual I see one thing I would do right off and that would be yo replace the steel rudder and tailwheel cables with Kevlar cord ( Available from Aerospace Composites Products ). That alone would be a big help as I suspect those 4 cables have some weight to them. I would also move the rudder servo forward. The second thing I would do is get rid of the steel elevator pushrods and replace then with carbon fiber rod ( also available at ACP ). That would yield the largest weight reduction off the tail. Doing this would require attaching some Dubro threaded couplers on at least one end of the CF rod with JB Weld. To be honest, I typically just epoxy a clevis on each end with the servo and control surface being centered. A side benefit of getting rid of the steel cables and pushrods is you don’t have long runs of steel that could act as antennas and create RF issues although that is very highly unlikely.
The final thing I noticed in the manual is the aileron linkage and suggested throws. The linkage IMO just plainly sucks. You will never get the same throws on both the upper and lower ailerons with the kit supplied hardware. Unfortunately just about every ARF bipe I have seen uses this system. In order to get equal aileron throw the linkage pivots must be on the ailerons center. As shown in the manual the lower aileron has it above center and the upper aileron has it below center. The bigger issue for me here is that they show equal throw up and down movement. First off the linkage makes that impossible to begin with but you will definitely want some differential in there. If you use the provided linkage, take your measurements from the bottom ailerons and have the travel 20%-25% more upward then down. This will greatly reduce if not entirely eliminate the adverse yaw the stock setup would induce.
Best of luck!
I haven't done the CG with the batteries yet. My RX does have two battery ports too, so I will add a second RX battery for some weight in the front. I think that should do it, cause it's not far off at all. This is my first Rx with the XT-30 connectors, I really like them over the JR connectors.
Batteries next to each other transfering interference? Huuuh? I am no RF expert, but I do have my general HAM radio license and intuitively that just sounds... wrong. But who knows... I have the Frsky SR10 with dual frequencies which is nice to have (plus gyro!)
I really don't think the steel cables hardly weigh anything at all, they are so incredibly thin. But replacing the steel rods is a good idea, I think I will do that!
Hmm well that's funny you say that, because when I epoxied the bottom aileron control arms on, I actually accidentally did it backwards. So the arms point forwards instead of backwards (more of an upward angle). So I think that might be a happy mistake! I just did it without even really looking at the manual because it made more sense in my head that if the top wing is forward of it, the control horns point more forwards than back this way... But sounds like that "mistake" will pay off!!
Thanks for all the great info.
Last edited by RadialWacko; 10-24-2021 at 11:00 AM.
10-24-2021, 03:29 PM
#8
No, batteries together isn't a problem, he means back in the days before 2.4 mhz radios, you wanted to keep all your radio equipment away from your ignition module or magneto systems, typically 8"-10" if possible, those could cause interference with AM and FM systems, this forced people to mount stuff further aft, with 2.4 it's a non issue, I have several planes where the batteries are right up there under the cowl, not an issue these days
good luck
good luck
Last edited by scale only 4 me; 10-24-2021 at 03:33 PM.
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