OK gujys, I need help on CG
03-31-2013 | 05:42 AM
#26
RE: OK gujys, I need help on CG
When in doubt just use an online CG calculator and be sure you have it right. It only takes a minute to make the measurements you need, and you can be confident that you'll have the plane set up right when you are done.
dce21b - It's always better to start a new thread when you have a new question. But I'll give you the answers you need. First, you have to put the wing on something that will let it pivot freely. A flat piece of wood won't do that. Your fingertips are better, but still not all that precise. I built a small rack with triangular shaped pieces sticking up for the wing to sit on (tops are rounded so they don't poke the covering). That lets me stand back and see how the plane is sitting instead of holding it and trying to figure it out. As for your plane, a dropping tail at low speed normally means a very tail heavy plane. You are lucky you were able to repair it after its first flight, because most wind up all in pieces. As for how nose heavy is too nose heavy, flying the plane tells you that. If you can't hold enough up elevator to keep the plane in its flare when you land, you're too nose heavy. That's enough to know for a trainer, but when you move to aerobats, you'll look at things like inverted flight, behavior in uplines and downlines, which way it pulls in knife edge flight, and so on for setting the CG right. Personally, I've never liked the advice new pilots get to set their trainers nose heavy. Yes, it's better to be nose heavy than tail heavy, but that advice is only good for one flight after which the CG should be moved. What I'll suggest is that you start in the middle of the manufacturer's recommendation, then fly the plane. If you have already soloed, do a few landing passes at 25-30 feet so you can get a feel for how the plane stalls with a little altitude left for recovering. Do the throttle off and fly straight test. When you chop the throttle, assuming your thrustline is set right, you should see the plane continue to fly straight for a few seconds and then gently start dropping its nose. With an elevator throw that is comfortable for flying circuits and doing loops, you'll want to be able to hold the nose up with the throttle off until the plane get so slow that it mushes the nose down. You'll have to be quick on the throttle to recover from that stall, but that's a pretty simple thing. If you can't hold the nose up enough to get the plane to slow down, you are too nose heavy. If the plane flies straight for a few seconds then raises the nose on its own, you are tail heavy (not likely to happen if you use the right tool and follow the manufacturer's recommendation on CG). A neutrally stable plane will let you hold the nose up until it is nearly motionless in the air, which is then followed by a very sudden nose drop. That's not a "bad" CG setting overall, but it can let beginners get into trouble when flaring for landings because it gives you so much elevator control you can force the wing into a bad stall when you'e still too high off the ground. The smooth stall of a slightly nose heavy plane will keep you out of trouble until you have good control over your landings.
dce21b - It's always better to start a new thread when you have a new question. But I'll give you the answers you need. First, you have to put the wing on something that will let it pivot freely. A flat piece of wood won't do that. Your fingertips are better, but still not all that precise. I built a small rack with triangular shaped pieces sticking up for the wing to sit on (tops are rounded so they don't poke the covering). That lets me stand back and see how the plane is sitting instead of holding it and trying to figure it out. As for your plane, a dropping tail at low speed normally means a very tail heavy plane. You are lucky you were able to repair it after its first flight, because most wind up all in pieces. As for how nose heavy is too nose heavy, flying the plane tells you that. If you can't hold enough up elevator to keep the plane in its flare when you land, you're too nose heavy. That's enough to know for a trainer, but when you move to aerobats, you'll look at things like inverted flight, behavior in uplines and downlines, which way it pulls in knife edge flight, and so on for setting the CG right. Personally, I've never liked the advice new pilots get to set their trainers nose heavy. Yes, it's better to be nose heavy than tail heavy, but that advice is only good for one flight after which the CG should be moved. What I'll suggest is that you start in the middle of the manufacturer's recommendation, then fly the plane. If you have already soloed, do a few landing passes at 25-30 feet so you can get a feel for how the plane stalls with a little altitude left for recovering. Do the throttle off and fly straight test. When you chop the throttle, assuming your thrustline is set right, you should see the plane continue to fly straight for a few seconds and then gently start dropping its nose. With an elevator throw that is comfortable for flying circuits and doing loops, you'll want to be able to hold the nose up with the throttle off until the plane get so slow that it mushes the nose down. You'll have to be quick on the throttle to recover from that stall, but that's a pretty simple thing. If you can't hold the nose up enough to get the plane to slow down, you are too nose heavy. If the plane flies straight for a few seconds then raises the nose on its own, you are tail heavy (not likely to happen if you use the right tool and follow the manufacturer's recommendation on CG). A neutrally stable plane will let you hold the nose up until it is nearly motionless in the air, which is then followed by a very sudden nose drop. That's not a "bad" CG setting overall, but it can let beginners get into trouble when flaring for landings because it gives you so much elevator control you can force the wing into a bad stall when you'e still too high off the ground. The smooth stall of a slightly nose heavy plane will keep you out of trouble until you have good control over your landings.
03-31-2013 | 06:57 AM
#27
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From: SomewhereNJ
RE: OK gujys, I need help on CG
Thanks for the advice jester... i did get lucky this time i will check the cg by my finger tips on the wings instead of the fuselage. i feel so dumb looking back now.<div>
</div><div>yeah some forums are very anal about posting multiple topics on the same subject. i didnt want to litter the boards so ti speak.</div>
</div><div>yeah some forums are very anal about posting multiple topics on the same subject. i didnt want to litter the boards so ti speak.</div>
03-31-2013 | 08:11 AM
#29
RE: OK gujys, I need help on CG
You want to be 1/3 the the way back from the -center of the leading edge ,at the widest part of the wing , to set the Static CG on the bench
A rule when the CG is not know for a given design
Measure with a tape measure , due to the curve in the airfoil
Do mark the spot on the wing with a marker
The mark will usuall fall about 1/4 to 1/2 an inch just behind the main spar , as a general rule
Most of mine are 1/2 inch in back of the spar
Farther back will make the plane more sensitive to inputs ,more foward less sensitive
Be less sensitive on the first flight
If a low wing , balance the plane inverted
Keep a record of the location ,will come in handy if future repairs are require to rebalance
Balance the plane SLIGHT NOSE down when on the CG
Do not balance it level -or it will just float on the final approach,and not develope the rate of desent as it should
Go fly the plane
I like it when making the turns on the corners , if I hold the up I desire and the plane tracks level through the corners
If the plane drops out at the my desired stick feel ( a 1/4 inch up elorvator on the TX ) , I add 1/2 ounce at a time , to the tail , until it tracks level ,as I desire on the corners
The second test is to go inverted
If the CG is right you should be holding about the same amount of down elevators to fly level inverted
Doing this gives all my planes the same general feel when they fly the left and right corners , a very good thing
Should make for a very good maden flight ,and first few flights
As a rule I work out all bugs in the plane in the first 10 flights
Be it changing the engine for more power or the landing gear to be more stable ETC.
All the above has worked for me for 24 years flying RC
A rule when the CG is not know for a given design
Measure with a tape measure , due to the curve in the airfoil
Do mark the spot on the wing with a marker
The mark will usuall fall about 1/4 to 1/2 an inch just behind the main spar , as a general rule
Most of mine are 1/2 inch in back of the spar
Farther back will make the plane more sensitive to inputs ,more foward less sensitive
Be less sensitive on the first flight
If a low wing , balance the plane inverted
Keep a record of the location ,will come in handy if future repairs are require to rebalance
Balance the plane SLIGHT NOSE down when on the CG
Do not balance it level -or it will just float on the final approach,and not develope the rate of desent as it should
Go fly the plane
I like it when making the turns on the corners , if I hold the up I desire and the plane tracks level through the corners
If the plane drops out at the my desired stick feel ( a 1/4 inch up elorvator on the TX ) , I add 1/2 ounce at a time , to the tail , until it tracks level ,as I desire on the corners
The second test is to go inverted
If the CG is right you should be holding about the same amount of down elevators to fly level inverted
Doing this gives all my planes the same general feel when they fly the left and right corners , a very good thing
Should make for a very good maden flight ,and first few flights
As a rule I work out all bugs in the plane in the first 10 flights
Be it changing the engine for more power or the landing gear to be more stable ETC.
All the above has worked for me for 24 years flying RC
03-31-2013 | 08:16 AM
#30
Senior Member
My Feedback: (26)
RE: OK gujys, I need help on CG
ORIGINAL: tony0707
You want to be 1/3 the the way back from the -center of the leading edge ,at the widest part of the wing , to set the Static CG on the bench
A rule when the CG is not know for a given design
Measure with a tape measure , due to the curve in the airfoil
Do mark the spot on the wing with a marker
The mark will usuall fall about 1/4 to 1/2 an inch just behind the main spar , as a general rule
Most of mine are 1/2 inch in back of the spar
Farther back will make the plane more sensitive to inputs ,more foward less sensitive
Be less sensitive on the fist flight
If a low wing , balance the plane inverted
Keep a record of the location ,will come in handy if future repairs are require to rebalance
Balance the plane SLIGHT NOSE down when on the CG
Do not balance it level -or it will just float on the final approach,and not develope the rate of desent as it should
Go fly the plane
I like it when making the turns on the corners if I hold the up I desire and the plane tracks level through the corner
If the plane drops out at the my desired stick location about. ( a 1/4 inch up elorvator on the TX ) , I add 1/2 ounce at a time until it tracks level ,as I desire on the corners
The second test is to go inverted
If the CG is right you should be holding about the same amount of down elevators to fly level inverted
Doing this gives all my planes the same general feel when they fly the left and right corners , and a very good thing
Should make for a very good maden flight ,and first few flights
As a rule I work out all bugs in the plane in the first 10 flights
Be it changing the engine for more power or the landing gear to be more stable ETC.
It
All the above has worked for me for 24 years flying RC
You want to be 1/3 the the way back from the -center of the leading edge ,at the widest part of the wing , to set the Static CG on the bench
A rule when the CG is not know for a given design
Measure with a tape measure , due to the curve in the airfoil
Do mark the spot on the wing with a marker
The mark will usuall fall about 1/4 to 1/2 an inch just behind the main spar , as a general rule
Most of mine are 1/2 inch in back of the spar
Farther back will make the plane more sensitive to inputs ,more foward less sensitive
Be less sensitive on the fist flight
If a low wing , balance the plane inverted
Keep a record of the location ,will come in handy if future repairs are require to rebalance
Balance the plane SLIGHT NOSE down when on the CG
Do not balance it level -or it will just float on the final approach,and not develope the rate of desent as it should
Go fly the plane
I like it when making the turns on the corners if I hold the up I desire and the plane tracks level through the corner
If the plane drops out at the my desired stick location about. ( a 1/4 inch up elorvator on the TX ) , I add 1/2 ounce at a time until it tracks level ,as I desire on the corners
The second test is to go inverted
If the CG is right you should be holding about the same amount of down elevators to fly level inverted
Doing this gives all my planes the same general feel when they fly the left and right corners , and a very good thing
Should make for a very good maden flight ,and first few flights
As a rule I work out all bugs in the plane in the first 10 flights
Be it changing the engine for more power or the landing gear to be more stable ETC.
It
All the above has worked for me for 24 years flying RC
03-31-2013 | 08:34 AM
#31
My Feedback: (6)
RE: OK gujys, I need help on CG
It's really not that complicated. You're probably OK as a starting point with 5 1/4 back from the leading edge where the wing joins the fuselage. If you want to double check, which isn't a bad idea, use an online CG checker. Then adjust as needed, in small increments, after flying.
03-31-2013 | 08:48 AM
#33
My Feedback: (29)
RE: OK gujys, I need help on CG
The CG gremlins are at it again LOL. As I stated previously CGis very often mis understood. Most of us are taught that a nose heavy airplane is more stable. My instructor made sure I learned that. For years I would set up my airplanes slightly nose heavy and fought them just like everyone else. mention the words " Tail heavy " and see the fear in peoples eyes.
The truth is that a nose heavy airplane will just never trim correctly. It will constantly experience pitch trim changes with airspeed changes. The reason for this is that you must carry some up elevator trim to compensate. This trim will become more and less effective with airspeed changes. For a trainer this can be a good thing for a while. When the student gets into trouble the airplane will have some self righting tendancies. For doing aerobatics, it would be a disaster. The airplane would fly strait and level at one airspeed but would pull to the canopy on uplines and knife edge. For a pylon airplane it would require more elevator travel for tight turns which would kill airspeed and could cause a high speed snap. In all cases a nose heavy airplane will need to be landed slightly faster.
There are many viable tests to determine the optimum CG. The first step is to make sure all your incidences are correct then plug the numbers into the CG calculator to get a safe starting point. During the maiden take note if you have to give up trim or not. Usually you will have to do this. That up trim is telling you that the airplane is either nose heavy or out of alignment. Thats why it is important to measure and adjust the angles first. For an aerobatic airplane putting the airplane into a knife edge is a good check. With proper CG the airplane will want to pull to the landing gear slightly. I will employ a mix to compensate the final 3% or so. On my pylon airplanes to get a 30 ft radius turn at 130+ MPH requires only 3/16" of elevator travel do to a slighly aft CG position and like I said earlier allows them to slow down nicely for landings.
The truth is that a nose heavy airplane will just never trim correctly. It will constantly experience pitch trim changes with airspeed changes. The reason for this is that you must carry some up elevator trim to compensate. This trim will become more and less effective with airspeed changes. For a trainer this can be a good thing for a while. When the student gets into trouble the airplane will have some self righting tendancies. For doing aerobatics, it would be a disaster. The airplane would fly strait and level at one airspeed but would pull to the canopy on uplines and knife edge. For a pylon airplane it would require more elevator travel for tight turns which would kill airspeed and could cause a high speed snap. In all cases a nose heavy airplane will need to be landed slightly faster.
There are many viable tests to determine the optimum CG. The first step is to make sure all your incidences are correct then plug the numbers into the CG calculator to get a safe starting point. During the maiden take note if you have to give up trim or not. Usually you will have to do this. That up trim is telling you that the airplane is either nose heavy or out of alignment. Thats why it is important to measure and adjust the angles first. For an aerobatic airplane putting the airplane into a knife edge is a good check. With proper CG the airplane will want to pull to the landing gear slightly. I will employ a mix to compensate the final 3% or so. On my pylon airplanes to get a 30 ft radius turn at 130+ MPH requires only 3/16" of elevator travel do to a slighly aft CG position and like I said earlier allows them to slow down nicely for landings.
03-31-2013 | 09:09 AM
#34
Senior Member
My Feedback: (26)
RE: OK gujys, I need help on CG
ORIGINAL: speedracerntrixie
The CG gremlins are at it again LOL. As I stated previously CG is very often mis understood. Most of us are taught that a nose heavy airplane is more stable. My instructor made sure I learned that. For years I would set up my airplanes slightly nose heavy and fought them just like everyone else. mention the words '' Tail heavy '' and see the fear in peoples eyes.
The truth is that a nose heavy airplane will just never trim correctly. It will constantly experience pitch trim changes with airspeed changes. The reason for this is that you must carry some up elevator trim to compensate. This trim will become more and less effective with airspeed changes. For a trainer this can be a good thing for a while. When the student gets into trouble the airplane will have some self righting tendancies. For doing aerobatics, it would be a disaster. The airplane would fly strait and level at one airspeed but would pull to the canopy on uplines and knife edge. For a pylon airplane it would require more elevator travel for tight turns which would kill airspeed and could cause a high speed snap. In all cases a nose heavy airplane will need to be landed slightly faster.
There are many viable tests to determine the optimum CG. The first step is to make sure all your incidences are correct then plug the numbers into the CG calculator to get a safe starting point. During the maiden take note if you have to give up trim or not. Usually you will have to do this. That up trim is telling you that the airplane is either nose heavy or out of alignment. Thats why it is important to measure and adjust the angles first. For an aerobatic airplane putting the airplane into a knife edge is a good check. With proper CG the airplane will want to pull to the landing gear slightly. I will employ a mix to compensate the final 3% or so. On my pylon airplanes to get a 30 ft radius turn at 130+ MPH requires only 3/16'' of elevator travel do to a slighly aft CG position and like I said earlier allows them to slow down nicely for landings.
The CG gremlins are at it again LOL. As I stated previously CG is very often mis understood. Most of us are taught that a nose heavy airplane is more stable. My instructor made sure I learned that. For years I would set up my airplanes slightly nose heavy and fought them just like everyone else. mention the words '' Tail heavy '' and see the fear in peoples eyes.
The truth is that a nose heavy airplane will just never trim correctly. It will constantly experience pitch trim changes with airspeed changes. The reason for this is that you must carry some up elevator trim to compensate. This trim will become more and less effective with airspeed changes. For a trainer this can be a good thing for a while. When the student gets into trouble the airplane will have some self righting tendancies. For doing aerobatics, it would be a disaster. The airplane would fly strait and level at one airspeed but would pull to the canopy on uplines and knife edge. For a pylon airplane it would require more elevator travel for tight turns which would kill airspeed and could cause a high speed snap. In all cases a nose heavy airplane will need to be landed slightly faster.
There are many viable tests to determine the optimum CG. The first step is to make sure all your incidences are correct then plug the numbers into the CG calculator to get a safe starting point. During the maiden take note if you have to give up trim or not. Usually you will have to do this. That up trim is telling you that the airplane is either nose heavy or out of alignment. Thats why it is important to measure and adjust the angles first. For an aerobatic airplane putting the airplane into a knife edge is a good check. With proper CG the airplane will want to pull to the landing gear slightly. I will employ a mix to compensate the final 3% or so. On my pylon airplanes to get a 30 ft radius turn at 130+ MPH requires only 3/16'' of elevator travel do to a slighly aft CG position and like I said earlier allows them to slow down nicely for landings.
03-31-2013 | 09:47 AM
#35
My Feedback: (29)
RE: OK gujys, I need help on CG
Flycatch, one thing that I left out is that as you approach the sweet spot on CG your airplane will be affected by airspeed changes less. All my airplanes trim the same from 1/3 power to full power. My 33% Laser actually will trim from 1/4 power to full power. The goal is to get the CG set along with wing tail angles so that the airplane does not require any elevator trim at all. No elevator trim means that there is nothing to become more or less effective with airspeed changes. This will actually lead to the airplane being more stable in windy conditions as well.
03-31-2013 | 12:05 PM
#37
RE: OK gujys, I need help on CG
Here!
Grab a measuring stick, fill in the blank boxes, and figure it out.
http://www.geistware.com/rcmodeling/cg_calc.htm
Speedy-Gonzales
Grab a measuring stick, fill in the blank boxes, and figure it out.
http://www.geistware.com/rcmodeling/cg_calc.htm
Speedy-Gonzales
03-31-2013 | 02:11 PM
#38
RE: OK gujys, I need help on CG
If a plane is set up correctly it will fly on RAILS at any speed you choose to fly it , not just a given speed or range of speeds
Getting a bit off the CG issuebut related to airframe stabiliy at any speed
If you are carring UP or Down elevator after your first flights -your wing incidence is not on the same axis as the stab or the engine is not on the same axis as the wing and stab
All my incidence are set at zero on the benchwing to stab and engine to wing and stab , per my mentor , who was flying for 30 years prior to me ( the pattern boys use a different set of rules regarding there incidence set ups )
Need to have all surfaces pulling on the same axis
IF you are carring UP elevator after the first few flights lets say 1/8 inchthis is telling if you to jack the back of the wing 1/8 inch ( lifting the back of the wing the same exact amount as the EL is UP )
You will be carring the elevator trim at zero after the next flight
IF you are carring Down elevator lets say 1/8 inch you will need to jack up the front of the wing 1/8 inch for you elevator to trim out level on your next flight
Once you are carring a- level elevator you will not have issues with changes in the planes direction with changes in airspeed
Your engine can not be set to pull up or down and must be set at zero or the engines angle of attack will cause you to climb or desend based on how much power you apply or do not apply
My planes require little or no input at times to fly straight and level , keeping my work load regarding TX inputs to a minimum and making flying much more relaxing
You will find you CG will have a direct effect on your rate of decent on the final approach, and determine how quickly the plane drops out OR how well it will glide in to land in front of your nose ( the best place to land -as it is your best look at what the plane is doing to make your corrections)
The engines idle adjustment will have a direct effect on the final as well-needs to be a high enough RPM to fly the plan to the ground
and should be adjusted for the days wind speed down the runway
Just my two cents on the above
Getting a bit off the CG issuebut related to airframe stabiliy at any speed
If you are carring UP or Down elevator after your first flights -your wing incidence is not on the same axis as the stab or the engine is not on the same axis as the wing and stab
All my incidence are set at zero on the benchwing to stab and engine to wing and stab , per my mentor , who was flying for 30 years prior to me ( the pattern boys use a different set of rules regarding there incidence set ups )
Need to have all surfaces pulling on the same axis
IF you are carring UP elevator after the first few flights lets say 1/8 inchthis is telling if you to jack the back of the wing 1/8 inch ( lifting the back of the wing the same exact amount as the EL is UP )
You will be carring the elevator trim at zero after the next flight
IF you are carring Down elevator lets say 1/8 inch you will need to jack up the front of the wing 1/8 inch for you elevator to trim out level on your next flight
Once you are carring a- level elevator you will not have issues with changes in the planes direction with changes in airspeed
Your engine can not be set to pull up or down and must be set at zero or the engines angle of attack will cause you to climb or desend based on how much power you apply or do not apply
My planes require little or no input at times to fly straight and level , keeping my work load regarding TX inputs to a minimum and making flying much more relaxing
You will find you CG will have a direct effect on your rate of decent on the final approach, and determine how quickly the plane drops out OR how well it will glide in to land in front of your nose ( the best place to land -as it is your best look at what the plane is doing to make your corrections)
The engines idle adjustment will have a direct effect on the final as well-needs to be a high enough RPM to fly the plan to the ground
and should be adjusted for the days wind speed down the runway
Just my two cents on the above
03-31-2013 | 02:30 PM
#39
RE: OK gujys, I need help on CG
ORIGINAL: tony0707
If a plane is set up correctly it will fly on RAILS at any speed you choose to fly it , not just a given speed or range of speeds
Getting a bit off the CG issuebut related to airframe stabiliy at any speed
If a plane is set up correctly it will fly on RAILS at any speed you choose to fly it , not just a given speed or range of speeds
Getting a bit off the CG issuebut related to airframe stabiliy at any speed
I respectfully disagree. There are too many variables. Airfoil type i.e. Clark-Y, Modified Clark-Y, which generate increasing lift with speed. Semi-symmetrical or 2/3rds symmetrical. Then you have fully symmetrical. You can not use a 0-0-0 type incidence setting on a flat bottom Clark-Y airfoil BUT you can with a fully symmetrical at 0-0-0 BUT there are also variables with the fully symmetrical airfoil. The ideal setup for 0-0-0 is a model with the engine, fully symmetrical wing and stab, that are all centered longitudinally. Any deviation from this "neutral" engine and airframe setup results in engine thrust and wing/stab incidence variables.
The calculated CG in relation to the MAC remains constant. Everything else is a variable.
If all this technical stuff is cloudy and confusing to you ( original post ) then just stick with using the CG Calculator I offered in my previous post and that will get you in the air.
Stick to what is suggested on your plans or instructions concerning engine thrust and wing/stab incidences.
03-31-2013 | 02:55 PM
#41
RE: OK gujys, I need help on CG
I ALWAYS calculate my own CG on a new model before attempting to fly it. I have found many CG's specified on plans and ARF's to be off. Sometimes way off.
03-31-2013 | 03:13 PM
#42
Senior Member
My Feedback: (26)
RE: OK gujys, I need help on CG
I wonder how many modelers fail to realize the CG will change when the fuel is burning off. How many modelers actually place there fuel tanks on the CG? Not to many I figure. I learned this a long time ago running YS engines in pattern planes. A pumped engine does have its' benifits.
03-31-2013 | 03:33 PM
#43
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From: Colorado Springs,
CO
RE: OK gujys, I need help on CG
I fly giant scale so maybe it is different. The cg is always around the wing tube because that is where the load distributes from. Of course that is assuming the plane is designed well.
03-31-2013 | 04:16 PM
#44
RE: OK gujys, I need help on CG
WOW...there are some real "doozies" for answers in these two pages. ( and a few good ones! 
)
Seek answers backed with facts not assumptions and guesswork.
Math and physics do not lie. Not good at either..........use the CG Calculator.
)Seek answers backed with facts not assumptions and guesswork.
Math and physics do not lie. Not good at either..........use the CG Calculator.
03-31-2013 | 04:39 PM
#45
My Feedback: (15)
RE: OK gujys, I need help on CG
ORIGINAL: az3d
I fly giant scale so maybe it is different. The cg is always around the wing tube because that is where the load distributes from. Of course that is assuming the plane is designed well.
I fly giant scale so maybe it is different. The cg is always around the wing tube because that is where the load distributes from. Of course that is assuming the plane is designed well.
Kurt
03-31-2013 | 04:42 PM
#46
My Feedback: (15)
RE: OK gujys, I need help on CG
ORIGINAL: Speedy-Gonzales
WOW...there are some real ''doozies'' for answers in these two pages. ( and a few good ones! )
Seek answers backed with facts not assumptions and guesswork.
Math and physics do not lie. Not good at either..........use the CG Calculator.
WOW...there are some real ''doozies'' for answers in these two pages. ( and a few good ones!
)Seek answers backed with facts not assumptions and guesswork.
Math and physics do not lie. Not good at either..........use the CG Calculator.
I think it would be nice if a beginner could recognize bogus calculator results (perhaps from inputting errors). Understanding the basics would help.
Kurt
03-31-2013 | 05:51 PM
#47
RE: OK gujys, I need help on CG
Kurt,
I'll go along with that! Bogus calculator inputs would be a problem for a beginner.
Explaining the basics is a great idea. You go first. I will not interrupt.
I'll go along with that! Bogus calculator inputs would be a problem for a beginner.
Explaining the basics is a great idea. You go first. I will not interrupt.
03-31-2013 | 05:52 PM
#48
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Joined: Jun 2010
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From: Sydney, AUSTRALIA
RE: OK gujys, I need help on CG
ORIGINAL: goirish
Finishing my G-Shark with a dle-55. The instructions say 5 1/4" back from the leading edge at the middle of the wing. Does that mean from root to tip on one side of the fuselage go halfway and that is the CG. At that point the wing is only approx 7". Of of this because I didn't finish the 3rd. grade.[&o][
]
Finishing my G-Shark with a dle-55. The instructions say 5 1/4" back from the leading edge at the middle of the wing. Does that mean from root to tip on one side of the fuselage go halfway and that is the CG. At that point the wing is only approx 7". Of of this because I didn't finish the 3rd. grade.[&o][
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Based on the root chord dimension you specified, the 5 1/4 position is about 30% back from the leading edge at the wing root.. sounds about right.. Adjusting the CG forward will make the aircraft more stable, adjusting it rearward (only to a point) will make the aircraft more manoeuvrable in pitch IE it will loop better..
Don't get bogged down with terms like MAC (mean aerodynamic chord) this is usually a formula to calculate the chord of a swept and tapered wing for an equal sized rectangular wing.. In these situations CG is given as a percentage of MAC... http://aviationglossary.com/mean-aerodynamic-chord
5/14 inches will work great..
03-31-2013 | 06:04 PM
#49
RE: OK gujys, I need help on CG
ORIGINAL: FallDownGoBoom
Generally it means 5 1/4'' back from the LE of the wing at the root chord. Or think of it this way: draw a line down the middle of the bottom of the fuselage lenthwise from the motor to the tail, then measure 5 1/4'' back from the LE of the wing. That's the starting CG point, and you'll probably end up adjusting it to personal taste 1/4'' either way.
Generally it means 5 1/4'' back from the LE of the wing at the root chord. Or think of it this way: draw a line down the middle of the bottom of the fuselage lenthwise from the motor to the tail, then measure 5 1/4'' back from the LE of the wing. That's the starting CG point, and you'll probably end up adjusting it to personal taste 1/4'' either way.
It doesn't get any simpler than that.
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03-31-2013 | 07:41 PM
#50
RE: OK gujys, I need help on CG
ORIGINAL: flycatch
I wonder how many modelers fail to realize the CG will change when the fuel is burning off. How many modelers actually place there fuel tanks on the CG? Not to many I figure. I learned this a long time ago running YS engines in pattern planes. A pumped engine does have its' benifits.
I wonder how many modelers fail to realize the CG will change when the fuel is burning off. How many modelers actually place there fuel tanks on the CG? Not to many I figure. I learned this a long time ago running YS engines in pattern planes. A pumped engine does have its' benifits.
Need to get a quick start up in combat events
I am 68 years old
THe idea is a real good one , to increase stability of the airframe throughout the flight ,with the a constantly changing decrease in fuel weight ,as the flight proceeds , the change in the weight at the CG 's will have less direct effect on the planes balance , than if the tank is located any place else in the fuse
Need to run two cycle glo engines rich for the same reason , fuel burning off in the tank will cause the engine to -lean out as the fuel in the tank decreases ( does not apply to a pumped engine )
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